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Theses and Dissertations

2018-11-01

Writing and Designing a Chapter on and for the Textbook Exploring the (explanet.info)

Braxton Clark Spilker Brigham Young University

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This Thesis is brought to you for free and open access by BYU ScholarsArchive. It has been accepted for inclusion in Theses and Dissertations by an authorized administrator of BYU ScholarsArchive. For more information, please contact [email protected], [email protected]. Writing and Designing a Chapter on Mercury and Pluto for the Textbook

Exploring the Planets (explanet.info)

Braxton Clark Spilker

A thesis submitted to the faculty of Brigham Young University in partial fulfillment of the requirements for the degree of

Master of Science

Eric H Christiansen, Chair Jani Radebaugh Bart J. Kowallis

Department of Geological Sciences

Brigham Young University

Copyright © 2019 Braxton Clark Spilker

All Rights Reserved ABSTRACT

Writing and Designing a Chapter on Mercury and Pluto for the Textbook Exploring the Planets (explanet.info)

Braxton Clark Spilker Department of Geological Sciences, BYU Master of Science

Exploring the Planets (http://explanet.info) is a free online college textbook covering the basic concepts of emphasizing the character and evolution of the planetary bodies in the . The latest edition (3rd edition) was published online in 2007 by Eric H Christiansen. Since the release of the third edition, two important planetary missions have been completed: MESSENGER (to Mercury) and (to Pluto). These missions provided new information and fundamental insights into these planetary bodies, which have not yet been included in Exploring the Planets. The modern results based on recent investigations of Mercury and Pluto are critical for our understanding of the nature and history of these bodies and the Solar System and build upon the previous information on Mercury and Pluto gained from (1974-1975) and the , respectively. These two planetary bodies are end members in a spectrum of objects in the Solar System. Mercury is small, hot, dense, and a silicate metal rich end member of the planets, helping scientists understand the thermal and accretionary evolution of the terrestrial planets. Pluto is cold, icy, distant from the , and a representative object of the vast , and is thus another end member among planetary bodies. These two bodies refine models of how different planets will evolve over time, and how our Solar System has evolved. For these reasons, it is important to update Exploring the Planets to summarize the current understanding of the of Mercury and Pluto. This way, students can better understand their formation and evolution and the implications for the evolution of our Solar System.

Keywords: Mercury, Pluto, , Exploring the Planets ACKNOWLEDGEMENTS I would like to thank Dr. Eric H Christiansen for giving me the opportunity to work on this book, and the countless hours he has spent instructing, reviewing my work, and making my thesis the best it can be. I would also like to thank my other committee members, Dr. Jani

Radebaugh and Dr. Bart J. Kowallis, for their helpful comments and suggestions. My parents,

Kim and Brian Spilker also deserve my thanks, for supporting me in my graduate work. TABLE OF CONTENTS

TABLE OF CONTENTS ...... iv

LIST OF TABLES ...... vii

LIST OF FIGURES ...... viii

INTRODUCTION ...... 1

CHAPTER 5. Mercury ...... 4

5.0 Introduction ...... 4

5.1 Major Concepts ...... 5

5.2 The Mercury ...... 6

5.3 Major Geologic Provinces ...... 7

5.3.1 Cratered ...... 8

5.3.2 Caloris Basin ...... 8

5.3.3 Smooth Plains ...... 10

5.4 Impact Craters and Basins ...... 10

5.4.1 Province Ages ...... 11

5.4.2 Impact Cratering: Versus Mercury ...... 12

5.4.3 Impact Basins on Mercury ...... 13

5.4.4 Crater Degradation ...... 14

5.5 Magmatism on Mercury ...... 15

5.5.1 Formation of Magma on Mercury ...... 15

5.5.2 Types of Magmatism ...... 16

5.5.2.1 Effusive ...... 16

5.5.2.2 Pyroclastic Volcanism ...... 18

5.5.2.3 Intrusive Magmatism ...... 22

5.6 Tectonic Features ...... 23

5.6.1 Contractional Faults ...... 23

5.6.2 Extensional Faults ...... 25

5.6.3 Tectonic History ...... 27

iv 5.7 ...... 27

5.7.1 Extrinsic Volatiles ...... 28

5.7.2 Intrinsic Volatiles ...... 28

5.8 Mercury’s Interior ...... 30

5.8.1 Core ...... 30

5.8.2 ...... 31

5.8.3 ...... 32

5.8.4 Exosphere ...... 34

5.8.5 Magnetosphere ...... 35

5.9 Geologic Evolution of Mercury ...... 35

5.10 Conclusions ...... 41

5.11 Review Questions ...... 42

5.12 Important Terms ...... 44

Additional Reading ...... 45

REFERENCES USED: MERCURY ...... 81

CHAPTER 13. Pluto and the Kuiper Belt ...... 89

13.0 Introduction ...... 89

13.1 Major Concepts ...... 89

13.2 The Planet Pluto and the Kuiper Belt ...... 90

13.3 Pluto’s Spin and Climate ...... 92

13.4 Pluto’s ...... 93

13.5 Atmosphere ...... 94

13.6 Geological Provinces ...... 95

13.6.1 Circum- ...... 95

13.6.2 : Pluto’s Heart ...... 96

13.6.2.1 Sputnik Planitia ...... 96

13.6.2.2 The Pitted Highlands: Eastern Tombaugh Regio ...... 98

13.6.3 Bladed : Tartarus Dorsa ...... 99

v 13.6.4 The Macula: The Dark ...... 99

13.6.5 Polar Regions ...... 100

13.7 Impact Craters ...... 100

13.8 Tectonic Features ...... 101

13.9 Volcanic Features ...... 102

13.10 Internal Structure ...... 103

13.11 The of Pluto ...... 105

13.12 Geologic Evolution of Pluto (and ) ...... 107

13.13 Conclusions ...... 108

13.14 Review Questions ...... 109

13.15 Key Terms ...... 110

Additional Readings ...... 111

REFERENCES USED: PLUTO ...... 135

vi LIST OF TABLES

Table 5.1 Physical and Orbital Characteristics of Mercury ...... 47

Table 13.1 Physical and Orbital Characteristics of Pluto and Charon ...... 112

Table 13.2 Physical Characteristics of Pluto's Ices ...... 113

vii LIST OF FIGURES

Figure 5.1a The Surface of Mercury ...... 48

Figure 5.1b Shaded-relief map of Mercury...... 49

Figure 5.2 Geologic map of Mercury ...... 50

Figure 5.3 Heavily cratered plains ...... 51

Figure 5.4 False color image of Mercury ...... 52

Figure 5.5 Caloris Basin ...... 53

Figure 5.6 Caloris Montes ...... 54

Figure 5.7 Antipode of Caloris Basin ...... 55

Figure 5.8 The Smooth Plains ...... 56

Figure 5.9 Crater frequency curves...... 57

Figure 5.10 trajectories ...... 58

Figure 5.11 Crater diameter and morphology ...... 59

Figure 5.12 Crater morphology ...... 60

Figure 5.13 basins ...... 61

Figure 5.14 Crater degradation ...... 62

Figure 5.15 Magma formation ...... 63

Figure 5.16 flow features ...... 64

Figure 5.17 Pyroclastic volcanoes ...... 65

Figure 5.18 Volatile concentration in magma...... 66

Figure 5.19 Cinder cones ...... 67

Figure 5.20 Pits ...... 68

Figure 5.21 Lobate scarps ...... 69

Figure 5.22 Enterprise ...... 70

Figure 5.23 Global map of lobate scarps ...... 71

Figure 5.24 Mercury's geologic timescale ...... 72

Figure 5.25 ...... 73

Figure 5.26 Radial graben formation ...... 74

viii Figure 5.27 Extensional grabens ...... 75

Figure 5.28 of Mercury ...... 76

Figure 5.29 Mercury's internal structure ...... 77

Figure 5.30 Mercury's zone of ...... 78

Figure 5.31 Thermal history of Mercury ...... 79

Figure 5.32 Geologic history of Mercury ...... 80

Figure 13.1 Pluto as imaged by New Horizons ...... 114

Figure 13.2 Pluto's Orbit ...... 115

Figure 13.3 Pluto's ...... 116

Figure 13.4 Pluto's cryosphere ...... 117

Figure 13.5 Morphology of eroded ...... 118

Figure 13.6 Sputnik Planitia ...... 119

Figure 13.7 Processes of Sputnik Planitia ...... 120

Figure 13.8 al-Idrisi Montes ...... 121

Figure 13.9 The Pitted Uplands ...... 122

Figure 13.10 Sublimation pits ...... 123

Figure 13.11 Tartarus Dorsa ...... 124

Figure 13.12 Atmospheric haze layers ...... 125

Figure 13.13 Map of impact craters ...... 126

Figure 13.14 Extensional tectonic features ...... 127

Figure 13.15 Wright Mons ...... 128

Figure 13.16 Internal structure of Pluto ...... 129

Figure 13.17 Pluto's moons ...... 130

Figure 13.18 Charon ...... 131

Figure 13.19 Kubrick Mons ...... 132

Figure 13.20 Thermal history of Pluto ...... 133

Figure 13.21 Geologic history of Pluto ...... 134

ix INTRODUCTION

Exploring the Planets (http://explanet.info) is a free online college textbook covering the basic concepts of planetary science including the character and evolution of the planetary bodies in the Solar System (the planets, important moons, , and Kuiper Belt Objects). The latest edition (3rd edition) was published online in 2007 by Eric H Christiansen. Earlier paper editions

were published by Prentice Hall in 1990 and 1995.

Exploring the Planets approaches an introductory study of the solar system mainly through basic geological principles. Compared with other introductory planetary geology texts, such as

Planetary Sciences by de Pater and Lissauer (2010), Introduction to Planetary Science: the

Geologic Perspective by Faure and Mensing (2007), The New Solar System by Beatty, Petersen, and Chaikin (1999), , Evolution of a Habitable World by Lunine (2013), An Introduction to the Solar System by Rothery and McBride (2018), or Planetary Geology by Rossi and Gasselt

(2018), this book is one of the few with a basic geology approach and has a unique ordering of chapters, unlike any other textbook. It is intended to be used as a primary or supplementary source in introductory science courses (geology or astronomy).

Exploring the Planets is divided into three sections. The first section gives a broad overview of the Solar System and an introduction to planetary science. This section helps the reader develop the geological background required to understand the processes that have shaped the planets, and to begin thinking like a planetary scientist. The second section discusses the planetary bodies within the inner Solar System (from Mercury to the belt). This section is organized by the size and complexity of the planetary bodies; beginning with the smallest and simplest (meteorites and asteroids) and progressing to the largest and complex (Earth). The progression from

1 simple to complex helps students to develop the ability to comprehend increasingly complex concepts. The third section discusses planetary bodies beyond the . Unlike the previous section, these chapters are organized by increasing distance from the Sun (starting with and moving out to the Kuiper Belt). This order emphasizes the effects of distance from the Sun on the size and composition of the planetary bodies. A final chapter compares the planets to further help students organize their ideas and develop a better understanding of the planets and planetary processes. No other planetary science textbook (including those that discuss geology) are organized in this way. The other books are either organized by planetary process, increasing distance from the Sun, or a combination of both.

Another unique aspect of Exploring the Planets is its availability to the public. Many planetary science texts are only available as hard copies. Furthermore, these other texts are often written from a more advanced perspective, so the concepts may not be as consistently written in simple and easy to understand terms. Exploring the Planets helps surmount both problems.

Being online gives the public information about the solar system and helps educate college students who will use this resource. An online platform also makes it easier to update and keep current—no expensive and time-consuming effort to print the book on paper is required. A digital text also allows us to include or link to videos and animations that help bring to the still images and static descriptions.

Since the release of the third edition, in 2007, of Exploring the Planets, two important planetary missions have been completed: MESSENGER (to Mercury) and New Horizons (to

Pluto). These missions provided new information and fundamental insights into these planetary bodies, which have not yet been included in Exploring the Planets. Other missions, such as

2 Cassini to , Dawn to and Vesta, and Juno to Jupiter, have also yielded important new information about planetary bodies during this time frame, but I focused on Mercury and

Pluto for this project. I discuss Mercury first and Pluto second, with the figures for each chapter shown after the chapter’s text.

The modern results based on recent investigations of Mercury and Pluto are critical for our understanding of the nature and history of these bodies and the Solar System as a whole. These two planetary bodies are end members in a spectrum of objects in the Solar System. Mercury is small, hot, dense, and silicate and metal rich end member of the planets, helping scientists understand the thermal and accretionary evolution of the terrestrial planets. Pluto is cold, icy, distant from the Sun, and a representative object of the vast Kuiper Belt, and is thus another end member among planetary bodies. These two bodies refine models of how different planets evolved over time, and how our Solar System formed.

For these reasons, it is important to update Exploring the Planets to summarize the current understanding of the and Pluto. This way, students can better understand their formation and evolution and the implications for the evolution of our Solar System.

I have produced two, pedagogically and scientifically sound chapters (both developed for the web), for Exploring the Planets, the first on Mercury (http://explanet.info/Chapter05.htm) and the second on Pluto (http://explanet.info/Chapter13.htm). The finished chapters are comprehensive in scope, accurate, easy to understand, and contain helpful figures and animations to facilitate learning. Instructors and students use this book as a resource in introductory college courses for non-science majors, and the new chapters (on Mercury and Pluto) are being used this semester (2018) in BYU’s online class Geology of the Planets (Geol 109). The next edition of

3 Exploring the Planets will be freely available at explanet.info. Anyone who desires to learn more

about the planets of the Solar System will find this book helpful.

In order to produce instructive chapters on Mercury and Pluto, I used effective pedagogical

principles of transfer, cognitive learning, and cognitive apprenticeship to organize, construct

figures, and write text. I have implement these principles to help students recall previous

information, present new information, guide students in developing critical thinking skills, and

elicit and assess performance.

Exploring the Planets is a valuable textbook for explaining basic geological and planetary

concepts to college students and so, it is important to keep the textbook updated. To provide a

complete up-to-date and free resource on the planets, I have updated the chapters on Mercury and Pluto by summarizing the new scientific literature on these planets by rewriting the chapters and introducing new images and figures. Other chapters will be updated as new scientific information and assessments from students are acquired.

CHAPTER 5. Mercury

5.0 Introduction

In our quest to examine objects of the inner solar system in order of size, Mercury

(named after the god of the Romans) is next, having a diameter about 1.5 times that of the Moon. Moreover, Mercury is the closest planet to the Sun. It has unique compositional and physical characteristics and consequently provides exceptional insights into planetary formation and evolution near the Sun.

Most of what we know about Mercury comes from two NASA missions. The first was

Mariner 10 in 1974-1975 Mariner 10 flew by Mercury three times but did not go into orbit

4 around it and only imaged half of the entire planet. Decades later, MESSENGER (MErcury

Surface, Space ENvironment, , and Ranging ; Mercury being the

messenger of the gods) performed 3 flybys while orbiting the Sun and then in 2011 dove into an

orbit around Mercury, imaging the planet at high resolution revealing new details about how

Mercury fits into the overall picture of the solar system. MESSENGER carried 11 scientific

instruments to record and process data, including surface compositions, exosphere composition,

images, and magnetic field strength. As a result of the flood of new information associated with

this mission, a more detailed picture of the nature of the small planet closest to the Sun is

emerging. MESSENGER sought to answer several questions prompted by Mariner 10and ground

based observations, such as: What led to the large core in Mercury? What are the nature and origin of the magnetic field? What is the composition of the crust and what are the important volatiles and their sources and sinks? How did Mercury evolve with time?

5.1 Major Concepts

1. Mercury is larger than the Moon, but the processes that shaped its surface features were

remarkably similar to those that shaped the Moon. The major landforms are: (a)

intercrater plains, (b) the Caloris Basin, and (c) sparsely cratered younger lava flows

called smooth plains.

2. Impact craters range in age from old, highly degraded features to young, rayed craters

surrounded with halos of bright ejecta and prominent systems of secondary craters.

3. There are two generations of plains on Mercury, both of which are lava flows. Deposits of

pyroclastic volcanoes, produced by high concentrations of some volatile elements, dot

Mercury’s surface.

5 4. Prominent fault scarps extending across the surface are a result of the planet decreasing its

spin rate, or “despinning”, coupled with global contraction as the planet cooled. Grabens

are rare and found only within some of the flooded impact basins.

5. Mercury has a large metallic core compared to its size with an outer core that is thought to be

partly molten and convecting, so its internal structure differs significantly from that of the

Moon. The convecting, outer core is the source of Mercury’s weak magnetic field (1%

that of Earth).

6. The major events in the history of Mercury include: (a) accretion in the nebular disk around

the ancient Sun and, a densely cratered primordial surface formed; (b) differentiation

separated large amounts of iron to the core; (c) the was

accompanied by significant volcanism created the intercrater plains concurrently with

planetary contraction; (d) the Caloris impact formed a giant multiring basin; (e) eruption

of formed the smooth plains; (f) continuation of planetary contraction created

lobate fault scarps on the plains; and (g) Today, Mercury’s lithosphere is thick and

immobile. Moreover, since it lacks surface and atmosphere and fluids, Mercury has

changed little in the last billion years.

5.2 The Planet Mercury

Several of Mercury's physical features distinguish it from the other planets as summarized in the table at the beginning of the chapter. Depending on how one defines what a planet is, icy Pluto is the only planet smaller than Mercury. But the Solar System contains many objects that are much smaller (asteroids and planetary satellites, for example).

Mercury spins once on its axis every 59 terrestrial days, and a year (its orbital period) is

6 88 days long. This represents a 2:3 ratio, wherein there are 2 mercurian years to exactly 3 mercurian days. Such coincidence is called spin-orbit coupling and probably evolved during the early history of the planet as a result of the constant tidal tug of the Sun on the closely-orbiting planet. The Moon’s spin-orbit coupling is even tighter, with a 1:1 ratio.

The surface environment of Mercury is harsh. With essentially no atmosphere to moderate them, temperatures rise to almost 700 K (430°C or 806°F) during the day; if pure lead or tin were on the surface they would be molten during the day. At night, temperatures drop to less than 100 K (-170 °C or -274°F). Some areas near the poles get little or no sunlight; they are permanently shadowed, always frigid, and amazingly contain water ice!

As the planet closest to the Sun, Mercury is a body of extremes and represents a unique planetary "end-member" with a small size and an iron-rich make-up. By studying the compositions of its rocks, important information about the general chemical components and early differentiation of the inner solar system can be learned. The concepts of planetary evolution developed earlier for the Moon will be tested here and new principles will be developed, which can then be applied to larger, more complex planets such as .

5.3 Major Geologic Provinces

Mercury features broad tracts of heavily cratered terrain that contrast with large areas of lightly cratered plains and large regions of high and low topography. These and other similarities with the Moon are immediately obvious from the photos of Mercury (Figure 5.1a and b and link). Indeed, it is difficult for many nonspecialists to tell the surfaces of the Moon and Mercury apart. The two types of plains are called the cratered plains and the smooth plains (Figure 5.2).

Of course, there are abundant impact craters, the largest of which is Caloris Basin.

7

5.3.1 Cratered Plains

To the naked eye, Mercury is a grayish brown world with abundant impact craters. The

heavily cratered terrains are similar to their lunar counterparts, but the heavily cratered regions of

Mercury include broad areas of gently rolling plains between the craters and basins. These are

the cratered plains and they are the most widespread type of terrain on Mercury (Figure 5.3).

(Specialists call them “intercrater plains.”) Craters of all sizes are apparent, and clusters of

impact craters are common with crater rims overlapping older craters. A few long, bright streaks,

perhaps rays from young craters, extend for thousands of kilometers (Figure 5.3). The bright

color comes from fresh, ground-up thrown out during impact. Locally, the cratered plains

are transected by high arcuate cliffs, known as lobate scarps. Finally, the cratered plains are

somewhat darker than other parts of Mercury, because of their -rich component. In

exaggerated color, carbon-rich crust is blue, and may be areas where Mercury’s primary crust

has been exposed by impacts (Figure 5.4).

5.3.2 Caloris Basin

Caloris Basin is one of the largest impact basins in the Solar System, measuring 1550 km

from rim to rim (Figure 5.5). Like the lunar basins, it was created by the impact of an asteroid-

sized object early in Mercury's development. Caloris is larger than Imbrium Basin (1250 km

diameter) on the Moon. Morphologically, Caloris and Imbrium are very similar. The interior part

of the basin is covered by lightly—cratered, smooth plains, and is known as —.

The Caloris plains are extensively ridged and fractured and are unique among the planets--- similar features have been found in other impact basins on Mercury, but not on the Moon, Mars,

8

or the satellites of the outer planets. These plains appear to be volcanic in origin based on their

smooth surfaces, embayment of older craters, and the presence of a few flow features. The

interior rings have been buried by these plains but are apparent as low, concentric ridges. Other

ridges are similar to lunar wrinkle ridges, and most likely formed by a similar, contractional

process. Shorter arcuate ridges probably formed above lava-flooded craters on the floor of the

basin (ghost craters as seen on the Moon).

Near the center of Caloris Basin lies a spectacular feature called Pantheon Fossae, which

is a system of extensional valleys (Figures 5.5, also see Figure 5.25). The rim of Caloris Basin is defined by Caloris Montes, a series of 2 km high rugged ridges that surround the basin interior

(Figures 5.5 and 5.6). Caloris Montes are scoured and rough in appearance and are made up of ejecta deposited from the Caloris impact. The hummocky plains between the blocks of Caloris

Montes are probably ejecta that filled in the topographic lows.

The plains around Caloris are known as the Odin Formation (Figure 5.2), which appears to be ejecta partially buried by younger lava. A lineated zone extends from the Caloris rim out past the Odin Formation to a distance roughly equal to the diameter of the basin. The area is dominated by long, radial ridges and valleys (Figure 5.5). This lineated terrain is best expressed to the northeast of Caloris Basin and is possibly the most rugged topography on Mercury. Similar ridges are apparent around Imbrium and both the lunar and mercurian terrains probably formed in the same way, by catastrophic deposition and of ejecta thrown from the vast crater. An extensive field of secondary crater chains and clusters (groups of closely spaced craters) has been mapped beyond the lineated ejecta showing the large effect the Caloris impact had on the mercurian crust.

9 Apparently, the impact that formed Caloris Basin was so great that the effects were

transferred to the antipode, or position on the opposite side of the planet as well. A peculiar terrain of hills and linear valleys (Figure 5.7) occupies a region more than 500 km across, centered on the exact opposite side of the planet from Caloris. Perhaps the preexisting crust was broken up by focused seismic waves originating from the impact site. At the point directly opposite the basin, vertical movements of several kilometers were possible from the event.

Smooth plains have partially buried this “weird” terrain and are therefore younger.

5.3.3 Smooth Plains

Another major geologic terrain found in several areas scattered around Mercury are smooth plains that resemble the lunar maria (Figures 5.2 and 5.8). This type of terrain covers about 25 percent of the planet and is distinct in that it is very smooth, only sparsely cratered, and is tannish-yellow in exaggerated color (Figure 5.4). The largest area of smooth plains lies near the north pole, possibly reflecting a global asymmetry similar to the asymmetric distribution of maria on the Moon. Numerous other smaller patches are scattered across the planet and can be seen on the geologic map of Mercury (Figure 5.2). Impact structures larger than 10 km in diameter are rare in the smooth plains. The plains are quite level and often fill major depressions, such as impact craters like Caloris. Mare-like wrinkle ridges and lava flow fronts are common within the smooth plains.

5.4 Impact Craters and Basins

The landscape of Mercury is dominated by impact craters of all sizes and states of

degradation. These craters constrain the age of major geologic provinces and range in size from

small pits to large basins (up to the 1550 km diameter Caloris Basin). The craters have various

10 ages from old, highly degraded depressions to young, fresh, bowl-shaped craters surrounded by halos of bright ejecta and extensive ray systems. In many ways, these impact features are similar to those found on the Moon and asteroids. However; close examination reveals that the mercurian craters differ from lunar craters in several important aspects.

5.4.1 Province Ages

As discussed in the chapter on the Moon, crater density indicates the age of a planetary body and its different areas. The crater density on Mercury’s cratered plains is less than on the

Moon’s heavily cratered highlands (Figure 5.9). Also, for any crater diameter, there are fewer craters on the mercurian cratered plains than on the lunar highlands. This difference is especially strong for craters greater than 100 km across. So, the rolling plains between the craters hint at what careful crater counts show about Mercury’s cratered plains; even though the plains are the oldest surfaces on Mercury, they do not date back to the accretion of the planet like the heavily cratered highlands of the Moon. Based on crater density, the cratered plains are 4.1 to 4.0 billion years old; the lunar highlands date back to 4.4 billion years ago.

Why aren’t the cratered plains of Mercury as old as on the Moon? Let’s examine some of the facts. In addition to having fewer craters, the cratered plains vary in age across Mercury. The younger age and broad age distribution were probably caused by the same thing--active volcanism during the intense bombardment that followed accretion. Many of the early impact craters were buried and eventually erased by eruptions of voluminous lava flows. As a result, the oldest craters were completely destroyed. The craters apparent today on the cratered plains must have formed during and after the Late Heavy Bombardment, which started about 4.1 billion years ago—apparently when the orbits of the outer planets were rearranged and a shower of

11

asteroidal bodies peppered the planets of the inner solar system.

As shown by the stratigraphic relations between the smooth plains and the cratered

plains, the smooth plains are the youngest major terrain on Mercury. This conclusion is, of

course, supported by its sparse crater population (Figure 5.9). The emplacement of the smooth

plains, after the cratered plains formation, buried craters dotting the cratered plains and

smoothing out the surface. In many instances, the smooth plains were not thick enough to

completely bury large craters leaving smoothed out, but still visible craters.

5.4.2 Impact Cratering: Moon Versus Mercury

A major physical difference between the Moon and Mercury that affects crater

morphology is their surface , Mercury’s is twice that of the Moon. Mercury is both larger

and denser than the Moon and its gravitational field is thus stronger. On Mercury, the distances

that ejecta and secondary craters travel from the primary crater are systematically shorter for a

given crater size on Mercury than on the Moon because of Mercury’s greater gravity (Figure

5.10). Greater gravity pulls ejecta down to the surface in a shorter amount of time, and thus at a

shorter distance from the source. This mechanism also explains why secondary craters are more

clustered around mercurian craters than lunar craters. Since ejecta is spread out over a smaller

area on Mercury, the ejecta blankets must be thicker and will have an increased ability to

degrade or bury nearby craters. The zone of secondary craters is often marked by long, linear

grooves that radiate away from the crater. The grooves are produced by the impact of closely

spaced ejecta fragments that fall closer to the crater, and are more pronounced than their lunar

counterparts. Mercury’s greater gravitational pull likewise gives ejected blocks higher velocities.

The higher speed produces larger, more prominent secondary craters upon impact on Mercury

than the Moon.

12

The transition and progression of impact craters from simple to complex occurs at smaller diameters on Mercury than the Moon (Figure 5.11). This difference has been attributed to higher speeds (42 m/s, compared to 11 m/s for the Moon) of impactors because of Mercury’s proximity to the Sun, and not the gravity of Mercury. Other factors, such as impactor and target rock strength and porosity, may affect crater morphology progression in smaller ways.

Even though complex craters start to form at smaller diameters on Mercury, the types of craters that form are the same as those on the Moon. Small craters on Mercury are simple and bowl shaped. With increasing size, terraces on the crater walls become apparent and central peaks develop, then irregular clusters of peaks appear. The largest impact features are basins with inner rings (Figure 5.12). Apparently, neither gravity nor impact velocity change the forms or the progression order of craters. Although the resultant feature is slightly different, the cratering process is fundamentally the same on both planetary bodies.

5.4.3 Impact Basins on Mercury

Large, multiple--ring basins, similar to those on the Moon, are also found on Mercury.

The most common are relatively small, ranging from 200 to 600 km in diameter. These craters usually have an irregular inner ring, an outer terraced rim, and an ejecta blanket with numerous secondary craters. Such peak ring basins are more common on Mercury (Figure 5.13) than on the Moon (110 on Mercury vs. only 7 on the Moon). Typically, the peak rings are prominent and fresh. In other cases, the rings are partially or completely flooded by volcanic material.

Compared to the Moon, large multiring basins are far less common on Mercury (there is only one - Caloris). One way to explain this apparent lack of large basins centers on an observation regarding the state of isostatic adjustment of old basins where the lithosphere rises because crustal material has been removed above it. A different explanation for the small number 13 of mercurian basins was alluded to in the description of the intercrater plains. These volcanic plains appear to have formed during the Late Heavy Bombardment, and their emplacement may have destroyed many older impact basins.

5.4.4 Crater Degradation

As on the Moon, the dominant erosional processes on Mercury are caused by impact cratering; subordinate processes include tectonism, space (removal of elements by light particles or meteor impacts), volcanic burial, and topographic relaxation (isostatic adjustment). Degradational sequences have been established for mercurian craters showing the morphologic changes with increasing age (Figure 5.14). The freshest craters are young, with well-defined rims, hummocky ejecta blankets, and systems of bright rays comparable to

Copernicus and Tycho on the Moon. Numerous rayed craters from 1 to 50 km in diameter dot

Mercury’s surface (Figure 5.1). Subsequent bombardment breaks down the crater rim and churns up the ejecta blanket or completely buries it beneath other ejecta deposits. Ultimately, the crater is transformed into a low-rimmed depression with large numbers of younger, superposed impact features. Many of the original crater features become completely obliterated or barely recognizable. Degradation of mercurian craters by impact from secondary fragments does not occur as far from the primary crater as on the Moon because of shorter ballistic ranges.

Another degradational process centers on the state of isostatic (gravitational) adjustment of old basins, wherein a crater tries to flatten itself out in the same way that a pit in viscous tar slowly disappears as material flows in to remove the cavity. Of course, silicate rocks flow much slower than tar and may not flow at all if the lithosphere is cold and strong. Perhaps Mercury’s crust cooled more slowly than the Moon's, remained pliable and “plastic” longer, and was able to adjust more rapidly to erase the signatures of ancient impact basins.

14

In summary, mercurian impact features differ from lunar craters and basins in three

important ways. First, the ejecta thrown out of craters on Mercury does not travel as far as on the

Moon. Considering the larger strength of the mercurian gravitational field (almost twice the

Moon's) this is logical. Second, even the most densely cratered terrain is not saturated with

craters and is younger than the lunar highlands. Apparently, the oldest impact structures were

removed by some process, probably due to viscous relaxation or volcanism before and during the

Late Heavy Bombardment. Third, many of the ancient mercurian basins are very shallow and ill-

defined as a result of the relaxation and volcanism.

5.5 Magmatism on Mercury

Magmatism and volcanism have been important parts of Mercury’s geologic history, and volcanic rocks are a significant component of its surface. Two types of volcanism have occurred

on Mercury: effusive and pyroclastic volcanism. Effusive volcanism produces lava flows, such

as those on the lunar maria or on the island of Hawaii today, and can cover large areas (link).

Pyroclastic volcanism (link) occurs when bubbles of volatile separate from molten magma

and explosively expand and pop. Some magma has been trapped beneath Mercury’s surface to

form intrusions.

5.5.1 Formation of Magma on Mercury

The processes by which magma forms and erupts on Mercury are the same as those that operated on the Moon. Magma forms by partial melting of the mantle, producing a mix of

crystals, dissolved gas, and collectively called magma (Figure 5.15). Magma formed by partial melting deep inside a planet typically has a density less than that of the overlying rock.

This contrast in density produces a positive buoyancy force on the magma and it rises upwards.

15 As magma accumulates into larger amounts, it may exert pressure on the overlying rocks and cause fractures to develop. These magmatically induced fractures provide conduits through which magma reaches the surface. The upward movement of magma will continue until (a) it erupts on the surface, (b) stalls because it intrudes rocks with a lower density than the magma or

(c) stalls because the strength of the rock is greater than the buoyancy force. If the magma stalls, it can pool to make a large magma reservoir, which may then cool and crystallize as an intrusion or pluton (refer to figure 2.18).

5.5.2 Types of Magmatism

5.5.2.1 Effusive Volcanism

Before the MESSENGER mission, multiple suggestions for the origin of the mercurian smooth plains were given, including formation by ballistic erosion and deposition of ejecta associated with the formation of major impact basins, notably Caloris. According to this hypothesis, as impact-energized debris surges (high velocity movement of material) moved away from Caloris, they may have ponded in depressions, creating smooth plains in the same way that the lunar highland plains (like those that fill Ptolemaeus, Figure 3.19) formed. But most of the mercurian plains are younger than Caloris, the youngest impact basin of an appropriate size, which is a weakness of this explanation. Small patches of smooth plains within craters may also arise by mass wasting from the walls.

On the other hand, many planetary scientists believed the smooth plains formed by extrusive outpouring of fluid mafic (iron or magnesium-rich) , much like those that formed the lunar maria. With higher resolution images from MESSENGER, more volcanic features were observed on the smooth plains, favoring a volcanic origin. The evidence supporting a volcanic

16 origin includes: (1) the large volume of material that accumulated to form smooth surfaces, (2) differences in the volume of plains material in craters and basins of the same size, (3) the striking similarity in morphology and distribution of the smooth plains and the lunar maria, (4) the age differences between the smooth plains and the basins they occupy, (5) the presence of lava channels, (6) buried impact craters, (7) lack of clear association with impact craters of the right age, (8) lobate flow similar to volcanic flow fronts on other planetary surfaces, (9) high potassium concentrations that can be associated with volcanism, (10) differences in geochemistry between the crated plains and ejecta of craters. These observations show the majority of the smooth plains are volcanic and not impact ejecta deposits.

Throughout large areas, the plains material is thought to be approximately the same thickness (1-1.5 km) as the mare basalts. In many places, however, the smooth plains do not completely bury the older impact craters and their rims, which are mostly lower than 1.5 km and protrude through the cover of lava. Thus, on a regional scale, the young volcanic cover is incomplete and discontinuous.

The lava flows that make up the volcanic cover bury or embay preexisting impact craters, lowering the density and producing a smooth, and younger surface. The impact crater density of the northern plains suggests a formation age between 3.7 and 3.9 billion years, before the planet cooled enough to create a thick lithosphere that magma could not penetrate (see

Figure 5.31). There is some debate on the ages of the lava flows in basin; some suggest these flows may be as young as 1 billion years old. However, it is difficult to tell, because it is a small area and crater density calculations are less reliable over small areas.

The majority of Mercury’s volcanic eruptions appear to have formed by a process similar

17 to that which formed the lunar maria ---quiet fissure eruptions of fluid, basaltic magma. These materials ponded in depressions, covering most of the vents through which lava rose to the surface and preventing the development of steep-sided volcanoes like those on Earth. The mafic

(Figure 2.16) composition of these lavas was determined by x-ray spectrometric measurements

made from orbit that showed these lavas have uncommonly low concentrations of iron and high

concentrations of magnesium. This high magnesium content and their high temperatures (1300

°C; mafic magmas with high magnesium content typically have hotter temperatures) made the

lavas very fluid, so they could flow long distances and pond over their vents. They probably had

the approximate consistency of motor oil.

While most of the effusive volcanism produced smooth plains with no lava flow features,

there is one area on the eastern edge of the northern smooth plains that does (Figure 5.16). Two

large impact craters have been filled by lava and are connected by a wide that, at one time, was filled with a flood of flowing lava 10 to 20 km across. These lava channels are similar to the

sinuous rilles on the Moon’s volcanic maria. The floods of lava thermally eroded (melted rocks

on the floor of the channel) pre-existing fractures into wide lava channels. Tear-drop shaped

“islands” were left as erosional remnants of the cratered plains. The channels lead into other

impact craters that flooded, and in some instances overflowed, with lava. To create such features,

the lavas must have been very fluid and erupted at exceptionally high rates.

5.5.2.2 Pyroclastic Volcanism

One of the more exciting discoveries made by the MESSENGER mission was the

identification of many small pyroclastic volcanoes scattered across the intercrater plains.

Pyroclastic eruptions are explosive; lava and rock fragments are violently thrown outward from

18 the volcanic vent. Pyroclastic deposits have been found on the Moon but they are uncommon.

These types of eruptions result when volatiles within the magma separate (exsolve) from the magma to form bubbles that then expand and rupture explosively at low pressure. The volatiles

in magmas are typically water, but may include (CO), carbon dioxide (CO2),

sulfur (S in various forms), or (CH4). The Earth experiences very violent pyroclastic

eruptions (e.g., the stratovolcano of Mount Saint Helens in Washington or the large calderas at

Yellowstone National Park in Wyoming) because terrestrial magmas are different—they have

high water content and are rich in silica. Silica-rich magmas are “stronger” so that when bubbles

explode, they do so very energetically. Such water-rich silicic magmas are rare on other planets

—including Mercury.

The volcanoes produced by pyroclastic eruptions on Mercury (Figure 5.17) have irregularly shaped pit craters surrounded by diffuse and bright haloes. The circularity of these

haloes is one evidence that the volcanoes are largely shaped by explosive eruptions ejected out of

a single vent, instead of a multitude of fluid eruptions of different volumes and durations that

would have led to irregularly shaped lava flows of different lengths. The bright, diffuse deposits

have an average radius of 25 km. The centrally located pit craters are tens of km across (similar in size to Crater Lake in Oregon), and reach depths of as much as 4 km, with no estimates on rim

height. The irregular pit shape is probably the result of multiple eruptions at the same location,

widening and reshaping the pit with each eruption. In other instances, these pits might result

from collapse of a magma chamber after eruption and magma withdrawal.

These pyroclastic volcanoes are spread throughout the intercrater plains, the edges of the

smooth plains, and around Caloris Basin’s rim (and other crater rims). Pyroclastic eruptions

began around 3.9 billion years ago and appear to have continued until relatively recently (about 1 19 billion years ago). However, dating the pyroclastic volcanoes is difficult at best. Because of their small sizes and thicknesses, it is difficult to obtain accurate impact crater frequencies. Since many of these volcanoes are inside impact craters, the age of the crater constrains the age of the . This technique does not give precise ages, but because the volcano must be younger than the impact crater, it gives the upper limit of the volcano’s age.

For pyroclastic eruptions to occur, the mercurian magma must have volatiles—on the order of a few thousand parts per million or more— and a pathway to the surface. High volatile concentrations like this can result from several processes (Figure 5.18). 1) The interior of

Mercury might have high concentrations of volatiles, and therefore a magma derived from it will have a high concentration of volatiles as well. 2) A dry magma could intrude into a volatile-rich layer and assimilate (melt and incorporate) the volatiles or explode as it reacts with them. 3)

Even if a magma initially is volatile-poor, the volatile concentration can increase as the magma crystallizes minerals that do not contain volatiles, leaving the volatiles in the residual melt, and thereby increasing their concentrations. In reality, the concentration of volatiles in mercurian magma is most likely a combination of all these processes. Based on the volcanoes and size of the deposits, Mercury’s pyroclastic eruptions were less violent than Earth’s (more like fire fountains on Hawaii or a cinder cone like Mexico’s Paricutin volcano (Figure 5.19)), probably because of the planet’s lower volatile content (~2 weight percent in mercurian magmas) and the more fluid, mafic composition of its lavas. Keep in mind that many silicic, explosive terrestrial magmas have volatile concentrations exceeding 10,000 parts per million (as much as 7 weight percent (70,000 ppm) water plus other volatiles).

In addition to having volatile enrichment, a mechanism for the magma to reach the surface needs to exist. This would have been easy early on, when the lithosphere was thin, and

20 extensional fractures readily developed. However, a billion years into Mercury’s history, the lithosphere would have been too thick for magma to easily traverse the crust. Moreover, as

Mercury cooled and its lithosphere thickened, it started to contract, making it even more difficult for magma to fracture its way to the surface. The association of volcanic craters with impact craters (for example, those concentrated along Caloris Basin’s rim) provides an important clue to how the magmas reached the surface. Impact fractures penetrate deep into the crust, providing weaknesses or pathways for magma to rise. As magma rose along the fractures, the volatiles could separate at increasingly lower pressures, preparing for explosion. (The release of volatile at low pressure is the same process that occurs when bubbles of carbon dioxide form and pop when a can of shaken soda is opened.) The volatiles separate and then expand in an explosive manner, blasting rock and magma fragments into ballistic trajectories. The pyroclasts land on the ground in a circle around the vent, much like the eruptions that formed the dark halo deposits on the Moon (Figure 4.34) and which form cinder cones on Earth (Figure 5.19). But because of the low gravity, particles were thrown farther from the vents than on Earth and the pyroclastic cones are not as high.

The presence of pyroclastic deposits on Mercury was unexpected because of its proximity to the Sun. Anciently, when materials condensed around the forming Sun, minerals with volatiles like water and carbon dioxide were not stable at the high temperature found there. Consequently, some scientists predicted that Mercury would be volatile poor. However, the presence of pyroclastic volcanoes, and some other features described below, indicate Mercury is more volatile rich than previously thought.

21 5.5.2.3 Intrusive Magmatism

On the way to surface eruptions, magma intrudes the crust. Sometimes, the magma does

not erupt, but is trapped and solidifies underground. Since volcanism occurred on Mercury, we

are quite certain that intrusive rocks (plutons) formed as well. Uplift and erosion may not have

exposed once buried intrusions as has happened on Earth, but there are a few pieces of evidence

of intrusions in the form of extensional features at Mercury’s surface. Grabens form, as

explained above, when magma is injected into the crust; this causes stretching and expansion

over the roof of the dike or intrusion, and the crust can fracture. The manifestation of such

underground fractures are narrow grabens at the surface.

One of the most prominent impact craters with evidence for intrusive magma

emplacement is . It has concentric grabens within its peak-ring (Figure 5.13). The

grabens probably formed by emplacement of dikes in the shape of cones emanating from a

central magma chamber (Figure 2.18) creating circular grabens on the surface. Such craters with

subsurface intrusions are called floor-fractured craters. Pantheon Fossae (Figures 5.5) may

have been formed like this.

Irregular pits (not of pyroclastic origin) have been identified on Mercury (Figure 5.20).

These isolated pits have steep sides, are rimless, not surrounded by ejecta or lava flows, are

irregularly shaped (many parallel the crater rim), and may be structurally controlled by the host

crater. They occur near the center of impact craters and are usually superposed on volcanic smooth plains deposits. One idea is that the pits form in lava-filled craters where magma in

underlying magma chambers withdrew, either by eruption somewhere else or draining back

22

down the conduit system. After the magma has left, the fractured crust collapses into the void left

by the magma, leaving behind pits outlining the past extent of the subcircular dikes.

5.6 Tectonic Features

Mercury and the Moon share similar tectonic features. Both bodies have undergone

extension and contraction, resulting in distinctive tectonic landforms. Unlike the Moon, only

small areas of Mercury were stretched by extensional faults, probably as a result of local, not

global stresses.

5.6.1 Contractional Faults

Contractional faults on Mercury are broken up into two categories: lobate scarps and

wrinkle ridges. Both are interpreted to be the surface expression of thrust faults. Lobate scarps

are arcuate and are located in the intercrater plains. They have steep scarp faces and gently

dipping back slopes (Figure 5.21). Lobate scarps can be hundreds of kilometers long and have

large offsets of 1-3 km. On the other hand, wrinkle ridges are generally smaller, low-relief arches

that form by folding and faulting and are restricted to the smooth plains. The biggest wrinkle

ridges are a couple of hundred kilometers long and the average length is approximately 94 km

(Figure 5.8).

Many wrinkle ridges and lobate scarps are revealed in MESSENGER images. One of the

most prominent lobate scarps is Enterprise Rupes (Figure 5.22). It is a long, arcuate scarp that cuts across crater and offsets the crater rim by a few kilometers.

These two varieties of thrust faults are found at all latitudes on Mercury and their

orientations change with latitude (Figure 5.23). As you may remember, the orientation of a thrust

23 fault system is perpendicular to the direction of contraction. You can demonstrate this to yourself by pressing inward on both sides of a piece of fabric or paper. An elongate ridge or ridges form in the middle of the paper at right angles to the direction you push. Using this principle, we can conclude that the direction of contraction across Mercury changes with latitude. At low to mid- latitudes (between 60°N and 60°S), most thrust faults are oriented N-S, and therefore, contraction occurred in the E-W direction. Around 60°N and 40°-60°S the orientation of the faults changes from NW-SE or NE-SW. Above 80°N or S the faults are oriented E-W, which means they underwent N-S contraction.

The contraction and thrust faulting that formed the large lobate scarps are probably a result of two different processes acting in conjunction with one another. First, Mercury experienced the collapse of an equatorial bulge during despinning. Early in Mercury’s history the planet would have been spinning more rapidly than today’s very slow spin rate. The rapid spinning, coupled with the Sun tugging on the body near its middle, would give the planet an equatorial bulge, so the diameter between the poles was slightly shorter than at the equator. As

Mercury lost , the planet’s rotational speed decreased, and the bulge would have collapsed, allowing the formation of a more spherical planet--the diameter between the poles and at the equator became the same. The collapse of the bulge could produce thrust faults with the orientations observed today. However, this model, or the idea of the thrusts faults formed, predicts extensional faults at the poles that are not seen.

Subsequent to despinning, Mercury cooled. As Mercury’s initially hot, molten core and mantle cooled and solidified and the materials became denser, with the solid portion taking up less space than the liquid it crystallized from. This caused Mercury to contract, forming the lobate scarps at the surface—much like wrinkles form on a raisin when a grape dries out and

24 shrinks. This interior cooling model for the formation of thrust faults predicts fault orientations would be random, unlike the pattern seen on Mercury. Perhaps despinning created weak zones on which the younger contractional faults formed, preserving the orientation of the old faults, and thereby using portions of both models for the formation of the thrust faults.

Wrinkle ridges are smaller than lobate scarps and occur only in the volcanic smooth plains (Figure 5.8). The orientations of the wrinkle ridges do not conform to the same global pattern as the lobate scarps. Apparently, the orientations of wrinkle ridges are produced dominantly by local stresses. An example are the systems around the ghost craters in Figure 5.8. The compressional stresses that acted on this region focused on the buried rim of the craters to create nearly circular wrinkle ridges that outline the buried craters. Wrinkle ridges also formed in the flooded Caloris basin (Figure 5.5). The wrinkle ridges in Caloris are concentric to the basin rim and may be a result of local compressional forces instigated by subsidence of the heavy lava flows rather than global compression.

The cross-cutting relationships of the lobate scarps with impact craters of different ages show that contraction began near the end of the Late Heavy Bombardment (4-4.1 billion years ago; Pre-Calorian). Lobate scarps were the first thrust faults to form; the wrinkle ridges on the smooth plains formed later. Contraction may even continue today (Figure 5.24).

5.6.2 Extensional Faults

Even though Mercury is a planet dominated by contraction, there are extensional faults within some flooded impact craters. These extensional faults are expressed as grabens that appear as tears or splits in the crust. The largest grabens are in the centers of Caloris and

Raditladi basins.

25

Grabens in Caloris basin (Figure 5.25) form a radiating pattern discussed previously as

Pantheon Fossae (it can be difficult for some to see in images what are rises and what are depressions. If this is the case, look at the image upside down your eyes will correct). These grabens originate at the center of the basin and radiate outward. Each of the grabens are a couple of kilometers wide, a few hundred meters deep, and increase in width towards the center of the basin. About 200 km from the center, the orientation of grabens changes from radial to concentric and parallel the basin rim. Three competing hypotheses have been suggested for

Pantheon Fossae’s formation. The first hypothesis is that the impact (that lies near the center of Pantheon Fossae) produced the large fractures, but Apollodorus’ ejecta partially buried the grabens, making it appear that the crater is younger and unrelated to the radial faults.

The second hypothesis suggests that the grabens are a surficial expression of the emplacement of subsurface dikes, or magma-filled fractures. The third hypothesis links the radial grabens to lithospheric doming, most likely related to the emplacement of a shallow magma chamber soon after the Caloris impact (Figure 5.26). As magma to rose up to a shallow depth, intruding into the crust just below the crater floor, it domed and stretched the roof to make room for the intruding magma. When comparing the deformation predicted by each hypothesis to the actual graben orientations and locations, lithospheric doming is the best explanation.

Extensional faults have also formed within other large impact basin and smaller craters.

The grabens in Raditladi basin (Figure 5.13) are all concentrically oriented within the peak ring.

Similar concentric grabens appear in the Rachmaninoff basins (Figure 5.27a and b). Even a small crater buried by lava flows within Goethe basin has a polygonal pattern of intersecting grabens bound within concentric grabens (Figure 5.27c). The wreath of circular grabens must have developed over the buried crater rim. Unlike those in Pantheon Fossae, these grabens probably

26 formed by the cooling and shrinkage of thick volcanic units inside impact craters. As the lavas

crystallized and cooled, they contracted and fractured, much like the much smaller polygonal

joints that form in basaltic flows on Earth. On Mercury, where large amounts of lava were

emplaced, the fracturing occurred on a much larger scale producing grabens.

5.6.3 Tectonic History

Tectonism on Mercury began before the end of the Late Heavy Bombardment (~3.9

billion years ago) and continued until 1 billion years ago. Early on, Mercury was rotating so

quickly that an equatorial bulge formed, but as Mercury’s rotation slowed, the bulge collapsed

and produced N-S thrust faults at low latitudes and E-W thrust faults at high latitudes. Over time, these faults were buried by ejecta from impacts and lava flows. As Mercury’s interior began to cool, the planet contracted and reactivated earlier thrust faults. The crust deformed along these lobate fault scarps. Subsequently, wrinkle ridges formed in the lavas of the younger smooth plains by cooling and contraction of the lavas. Tectonism was an important process on Mercury and it ceased once the lithosphere became too thick to deform.

5.7 Volatiles

As paradoxical as it might seem, the planet closest to the Sun actually has water ice on its

surface. It is not found everywhere, only at the poles on the floors and walls of impact craters.

These regions are covered in shadows—at all times producing the freezing conditions necessary

for ice to remain. If any ice is exposed to the Sun’s rays, it warms up and rapidly sublimes to

vapor. So where did this water come from?

27

5.7.1 Extrinsic Volatiles

One possible source of water is from that hit Mercury. Comets, formed in the

outer reaches of the solar system, are rich in water-ice and other volatile materials. When a is diverted to the inner Solar System and strikes Mercury, as the ice vaporizes. If some of the vapor makes it to a cold, permanently shadowed region in a crater before it escapes to space, it will condense back to ice. After multiple comet impacts over billions of years, ice layers could build up within the shadowed craters. These are called extrinsic volatiles because they come from outside of Mercury and were not incorporated in Mercury when it accreted.

5.7.2 Intrinsic Volatiles

Intrinsic volatiles, in contrast to extrinsic volatiles, come from inside Mercury. Because

Mercury formed close to the Sun, scientists initially thought Mercury accreted from volatile-poor materials, and the T-Tauri winds stripped away any gravitationally trapped gases. However, data from MESSENGER showed certain volatile materials are not completely absent and instead were accreted to Mercury during the planet’s formation. Thus, it is possible that not all volatiles that were deposited on Mercury were from outside sources. Some of the intrinsic volatiles that have been detected in relatively high concentrations are potassium, , sulfur, and carbon.

They have been detected by spectroscopic methods (e.g., MESSENGER’s XRS and GRS instruments) and inferred from surface landforms.

One such type of landform that is evidence of intrinsic volatiles are Mercury pyroclastic volcanoes. While the exact volatile responsible for the pyroclastic volcanism is unknown, some have speculated sulfur is the culprit. At low pressures, sulfur becomes insoluble in magmas,

28 coming out of the melt in bubbles that can pop at the surface and cause explosions. It is unlikely that the explosive volatile was water, as it is on Earth; Mercury is probably very water poor.

Another feature that is produced by and evidence of intrinsic volatiles are Mercury’s hollows (Figure 5.28). The hollows are irregularly shaped depressions in otherwise smooth, sheet-like deposits. They typically occur in clusters and are 10s of meters deep and 100s of meters across. They have steep sides and commonly coalesce to leave a series of residual knobs.

The sheets have no superposed impact craters, suggesting they are young. Most hollows are on the floors of impact craters but can be found on crater walls and peak rings. Mercurian hollows are also associated with carbon enriched (dark or low reflectance) material, and they are confined to within 60° of the equator; none are found in the polar regions. The hollows appear to form from sheets of volatile-rich material (probably carbon-rich material brought to the surface by impacts) that degrade as the volatiles sublime away as vapor. This causes the surface to collapse and form a pit, leaving residual knobs of more resistant material between the pits. Sublimation can continue along the edges, widening the pit. Since sulfur, sodium, carbon, chlorine, and potassium have been identified and can vaporize at the surface temperatures found on Mercury, one of these could be the hollow forming volatile. One unsolved problem associated with the hollows is their age. The hollows appear to be the youngest surface features on Mercury; their ages do not correlate with the age of the host crater. If the volatile-bearing substance is excavated by an impact and deposited on the surface, the hollows should be older in old craters and younger in young craters, as impacts have occurred over a range of times. Thus, there is no adequate explanation for either the responsible volatile or the age of the hollows. You might ponder this question in your spare moments.

29 5.8 Mercury’s Interior

Unlike the Moon and Earth, Mercury has no seismometers, and therefore we have no means to image its internal structure. Luckily, geologists have other tools to constrain the internal structure such as the planet’s moment of inertia, gravity, magnetic field, surface composition, and bulk density. By using these tools, along with the current understanding of terrestrial planets, scientists have created a picture of Mercury’s internal structure (Figure 5.29).

5.8.1 Core

The high overall density (5.4 g/cm3) of this small planet indicates the presence of a large and iron-rich core. The radius of the core is estimated to be 2030 km, 83% of Mercury’s radius

(2440 km). This core would be larger than the entire Moon and would occupy the largest fraction of any planetary volume; some have even called Mercury the “Iron Planet.” The presence of a weak magnetic field and calculations of the thermal evolution indicate the core of Mercury is divided into a convecting, liquid outer core and a solid inner core. While the size of the entire core is well constrained, the radius of the inner core is not, but is speculated to be between 1000 km and 1500 km. Thus, there may be a layer of molten iron to be 530 to 1030 km thick (Figure

5.29).

Our understanding of terrestrial planetary evolution and Mercury’s density indicates the composition of Mercury’s core is mostly iron. But calculations of the cooling rate show that, based on the planet’s size, the core should be completely solidified today, if the core was 100% iron. Since a liquid, convecting core seems to be necessary to make a magnetic field, part of the core must be molten. An easy way to enable core to remain molten for a longer period of time is to add a light element. This would lower the melting point of the mixture and extend the lifetime

30 of the liquid core and the magnetic field. Both silicon and sulfur are relatively light elements that fit these criteria. A mixture of iron with silicon, sulfur, or both may have kept the melting point lower and prevented total solidification of the core. Moreover, the less dense, light elements would also tend to concentrate in the molten outer core as the inner core crystallized and became more iron-rich. This could keep the outer core liquid at the same temperature where a pure iron core would be solid.

Mercury’s magnetic field is relatively small—the strength is only 1% of Earth's—but extremely important for what it tells us about the interior of the planet and how it protects the surface from cosmic radiation and the . Just like magnetic fields produced by the movement of electrons, the origin and generation of planetary magnetic fields is a result of moving charged particles—a geodynamo. This mechanism requires a material that conducts electricity (iron), of the conductor (liquid iron outer core), and planetary rotation. The weakness of Mercury’s magnetic field is attributed to this liquid layer being thin, restricting the range of motion of convection. If the outer core were larger, the greater distances traversed by the convective fluid would result in a stronger field. If the core is indeed made of metallic iron

(plus or minus sulfur or silicon) it should be a very good conductor of electricity.

5.8.2 Mantle

Compared to the crust and core, little is known about the mercurian mantle. The only information comes from indirect observations about the crust since the genesis and evolution of the crust and mantle are tied together.

Compared to other planetary mantles (composed of the lithosphere and asthenosphere),

Mercury’s mantle is very thin, only about 400 km or about 17% of the radius. The Moon’s

31

mantle is 63% of its radius (because the Moon is entirely frozen, an unusual circumstance);

Earth’s mantle occupies about 46%. Mercury’s mantle is thought to be mostly rigid, beginning

near the surface, just below the thin crust, with only a thin asthenosphere at its base, near 400 km

depth. Initially, Mercury was so hot that the lithosphere was nonexistent, and a magma

existed, similar to the Moon’s. As Mercury cooled, a primary crust formed. However, unlike the

Moon’s magma ocean, plagioclase did not crystallize; instead the element carbon crystallized in

the form of graphite. Like plagioclase, the graphite floated on top of the ocean as Mercury’s first

crust. In addition to the formation of the primary crust, cooling of Mercury caused the asthenosphere to freeze, creating and thickening the lithosphere as Mercury continues to cool.

Compositionally, the mantle is thought to be magnesium-rich and iron-poor, based on observations by MESSENGER of the crustal composition. The negligible amount of mantle iron

points to highly reducing conditions, which means that there was very little , and thus

oxidation in the minerals, inside Mercury when the core formed. This caused most of the iron to

be in its native metallic state (Fe0) and to congregate in the core, leaving behind an iron-depleted

mantle. In its more oxidized state, like other bodies, iron (Fe2+) can enter silicate minerals like

olivine and pyroxene.

5.8.3 Crust

Mercury’s crust is thought to range in thickness from ~30 to ~100 km, with an average of

35 km, and has a unique bulk, or overall composition as well as variation across compositional

terrains as determined by MESSENGER’s spectrometers. The bulk composition of the crust

gives important clues into the planet’s differentiation history. And the varying composition of

different terrains show the numerous processes, such as cratering, tectonics and volcanism that

have acted on Mercury’s surface. 32 The MESSENGER spacecraft produced some unexpected results concerning the

composition of Mercury’s surface. The crust is enriched in the “volatile” elements potassium,

sodium, and chlorine. Mercury’s sodium and chlorine concentrations are higher than those of

Moon. However, Mercury’s potassium concentration is similar to that of the KREEP lunar

regions, even though this is a result not of magma differentiation, as on the Moon, but its high

bulk content. Although Mercury is described as being paradoxically enriched in “volatile”,

elements there is more to the story here than meets the eye. In most settings, these elements are

volatile; however, oxygen-poor environments can change the volatility of elements. Since

Mercury accreted in the O2-poor zone of the innermost accretionary disk these elements behave

as refractory and have remained on Mercury’s surface.

In contrast to being enriched in volatile elements, Mercury’s surface is depleted in iron,

with little to no iron (as well as aluminum) observed anywhere on the surface. However,

Mercury has high concentrations of magnesium. The high magnesium plus the low aluminum

suggest there is very little plagioclase on the surface, and instead the surface contains

magnesium-rich pyroxene and possibly magnesium-rich olivine. A mineral assemblage of

pyroxene, olivine, and minor to no plagioclase indicates the smooth plains are made of a rock

type ranging from ultramafic (known on Earth as komatiite) to basalt (Figure 2.16). These observations show that Mercury underwent a slightly different style of differentiation than the

Moon, with a graphite crust (instead of plagioclase) forming on a magma ocean, followed by

basaltic volcanism.

As mentioned previously, in the Major Geologic Provinces section, the majority of

Mercury’s surface is broken up into two distinct terrains: the smooth plains and the cratered

plains. Each of these terrains has a unique composition and morphology. The distinct chemistry

33 of each terrain could be a result of a heterogeneous (varied) mantle, or the magmas that formed

each terrain underwent different igneous evolutions. Compared to the cratered plains, the smooth

plains have higher concentrations of volatile elements: potassium, sodium, and chlorine, and

lower concentrations of magnesium, aluminum, calcium, and sulfur.

The internal structure and chemistry of Mercury have some significant differences from

the other terrestrial planets. These include the origin of Mercury’s anomalously large core, the

low iron in rocks, in the crust, and the presence of volatiles like graphite. These are outstanding

questions in planetary science that have yet to be answered.

5.8.4 Exosphere

Mercury has a tenuous envelope of volatiles known as an exosphere that can only be detected by sensitive instruments. In a true atmosphere, gas molecules routinely collide with one another—convecting and swirling around in winds generated by heating the gas and surface. But, in an exosphere, the density of molecules is too low for them to collide with one another and behave like a gas. Instead of being dominated by molecules of carbon dioxide or , less volatile elements such as sodium and calcium make up Mercury’s exosphere. They are derived from the solid surface by sublimation or sputtering by energetic particles from the Sun. The atoms are ejected off of the surface into elliptical trajectories around Mercury until they collide with the surface again. Some atoms leave the exosphere and escape completely from the planet.

The solar wind directly ejects some atoms while it ionizes others, removing electrons and producing a charge that allows them to be entrained and removed via the magnetic field.

34

5.8.5 Magnetosphere

The very outermost part of Mercury is its magnetosphere—the envelope of space dominated by charged ions high above the neutral atoms and molecules in the exosphere. The magnetic field of a planet determines how these ions move. A magnetosphere can block the effects of some types of cosmic radiation and can protect an atmosphere (if there is one) from being stripped away by the solar wind. The magnetosphere is distorted by the solar wind into a tear-drop shape elongated away from the Sun.

Since Mercury’s magnetic field is weak—1% that of Earth—it is small and does not extend far above the surface. Even as weak as it is, Mercury’s magnetic field protects the surface, so that charged particles in the solar wind are not able to bombard surface materials as much as they are on the Moon.

5.9 Geologic Evolution of Mercury

The geologic history of a planet depends on many factors, including its size (mass and radius) and its chemical composition (determined by its position in the solar nebula). As it ages, each planet passes through three general stages after accretion: (1) a highly active period of crustal formation and mobility; (2) a volcanic stage accompanying a thickening lithosphere; and

(3) a terminal quiescent state when the lithosphere is too thick to move laterally or to allow magma to puncture it. The rate at which a planet evolves through these steps depends on how quickly it cools, which in turn depends on the planet's size and composition. In this sense,

Mercury provides geologists with an important reference marker in several ways. First, it is the planet closest to the Sun and thus it has an "extreme" chemical composition rich in metallic iron

(Figure 5.30). Second, Mercury is larger than the Moon and evolved at a slightly different tempo.

35

Lastly, although smaller in radius, Mercury has nearly the same mass and surface gravity as

Mars and almost the same bulk density as Earth, thereby showing how differences in these qualities affect a planet's subsequent development.

Using the same methods and techniques as those used to study the Moon, geologists have been able to establish a geologic time scale and a general view of the geologic evolution of

Mercury. The large impact basins, such as Caloris Basin, like Imbrium on the Moon, provide useful references for the major geologic events. These periods of time have been given formal names taken from prominent craters of various ages. From oldest to youngest they are: pre-

Tolstojan, Tolstojan, Calorian, Mansurian, and Kuiperian. Comparisons of the crater densities on these terrains with those on the Moon can be used to estimate the absolute ages of these stages: pre-Tolstojan (4.6 to 3.9 billion years ago), Tolstojan (3.9 to 3.7 billion years), Calorian (3.7 to

3.1 billion years), Mansurian (1.7 billion to 280 million years ago; and Kuiperian (280 to 0 million years ago). Of course, you can gain a sound understanding of the planet’s history without memorizing these unfamiliar names, but we use them to emphasize how geologic time can be broken down into specific periods. Careful study of the figures in this chapter will reveal these fundamental relative age relationships. Figures 5.31 and 5.32 show schematically how the interior and surface of Mercury may have evolved.

Stage I: Accretion. Mercury probably accreted from oxygen-poor, carbon-rich materials that condensed at high temperature from the nebula that gave birth to the Sun and the rest of the planets. As a result of the unique reducing conditions from which Mercury accreted, the, usually volatile, elements sulfur, carbon, potassium, and chlorine did not behave as volatiles and became unexpectedly enriched on Mercury. Where Mercury formed, much of the iron was in a metallic state---not in silicate minerals as it was farther out where the other inner planets formed (Figure

36 5.30). Thus, the high density of Mercury could be explained by the simple incorporation of a large proportion of this metallic iron. Moreover, Mercury is apparently water-poor. Silicate condensates that contain volatile water formed farther from the Sun where the temperature was low, likely in the vicinity of the asteroid belt and beyond. Mercury grew to its final size by accretion of the condensed solids made of metallic iron, silicon, magnesium, and minor volatiles sulfur, carbon, potassium, and chlorine.

Stage II: Differentiation. Heat deposited by accretionary impacts and drove Mercury’s internal differentiation. By analogy with the Moon, much of the outer part of

Mercury probably began to melt during its accretion about 4.5 billion years ago.

Light silicate minerals eventually crystallized and formed the crust. Carbon may have crystallized as dark graphite, floated upward, and accumulated in the ancient crust. Denser silicates accumulated to make the mantle. This silicate shell (the crust and mantle) may be only

~400 km thick.

Mobile crustal plate interactions may have been limited to this early period of crustal formation. A rigid lithosphere, consisting of the crust and upper mantle, must have developed well before the end of heavy bombardment, but no impact craters formed during this episode are preserved. As heat was radiated from the planet into space, the depth of the molten zone increased and the rigid lithosphere formed above it.

During this epoch the core was formed, as a "rain" of metallic droplets moved through the mantle and concentrated in the center of the planet. This redistribution of mass from its initially more homogeneous state released more heat. This may have aided in forming magmas that extruded onto the surface, burying the most ancient impact basins and creating the

37 intercrater plains. This ancient volcanism may be one of the most important differences between

Mercury and the Moon because it explains why the intercrater plains have fewer impact craters

than the lunar highlands. When the much smaller core formed on the Moon, the amount of

energy released would have been small by comparison with Mercury.

Geologic evidence suggests that Mercury never contained abundant carbon dioxide or

water and probably never outgassed an atmosphere or hydrosphere as the interior differentiated.

Mercury has sufficient gravity to retain an atmosphere at least as thick as Mars, but its exosphere

of sodium vapor pales by comparison to Mars’, nor is evidence of a past eolian regime visible in

MESSENGER photos.

Another hypothesis that might explain Mercury's high density and lack of a thick atmosphere appeals to the possibility of a massive hit-and-run impact during this stage of its early evolution. A large, perhaps Moon-sized, body, may have collided with Mercury after its core formed. If the impactor was large enough, at least 20% of Mercury's mass, it could have stripped away the outer shell of less dense silicates leaving Mercury smaller and richer in dense iron. Thus, the high density of Mercury could reflect its accretion history and not necessarily in an oxygen poor part of the disk. Such a large collision may have purged the volatiles from Mercury's outer portions making the formation of a significant secondary atmosphere less likely.

Stage III: Late Heavy Bombardment and Formation of Intercrater Plains (pre-

Tolstojan and Tolstojan). A period of intense bombardment is recorded by the large craters and basins. There is no evidence preserved of planetary wide expansion, which presumably would have accompanied this thermal event.

38 During this period, the spin rate of Mercury slowed and reached the current 2:3 ratio between its year and day. Despinning could have initiated the development of thrust faults and lobate scarps as an early tidal bulge collapsed. Subsequently, the lithosphere cooled, thickened, and in time contracted. Mercury's radius may have decreased by 7 km. This contraction accentuated global thrust faulting, producing the lobate scarps so characteristic of the mercurian cratered plains. Many lobate scarps appear to have formed before Caloris Basin formed.

Stage IV: Formation of Caloris Basin (Early Calorian Period). The formation of the large multiring Caloris Basin was a major event in the geologic development of Mercury. Ejecta from this basin extends more than 1000 km away from the rim. The excavation of the basin modified the landscape over a large area, forming thick ejecta deposits and radial ridges and valleys far beyond the outer ring of . Hilly and lineated terrain on the opposite side of the planet probably formed as a result of this tremendous impact. If a lunar analogy can be drawn, Caloris probably formed about 4 billion years ago during the Late Heavy Bombardment.

Stage V: Formation of Smooth Plains (Middle to Late Calorian Period). The smooth plains that fill Caloris Basin and cover much of the northern hemisphere began at a time when the rate of impact had greatly decreased. The plains are mostly of volcanic origin and formed as flood lavas. Small variations in crater densities between different areas imply that the period during which flooding occurred was relatively short. The smooth plains were emplaced as the final product of the volcanic stage of Mercury's evolution, probably by 3.7 to 3.9 billion years ago and may have already begun just before the Caloris Basin impact. Even though the lithosphere was thickening, magmas were apparently still able to reach the surface. The timing of the volcanic events may approximately coincide with similar events on the Moon.

39 Structural modification of the smooth plains produced long wrinkle ridges and open

graben that may be related to isostatic adjustment of the basin's interior or local intrusions of

magma.

Stage VI: Light Cratering (Mansurian and Kuiperian Periods). After the period of

intense volcanism that created the smooth plains, the surface of Mercury was subjected to light

cratering, which formed the bright-rayed craters we see today. The density, distribution, and

morphology of these craters resembles the post-mare cratering on the Moon with slightly degraded, but still relatively fresh craters formed during the Mansurian Period and rayed craters formed during the Kuiperian Period. Magmatism greatly diminished and the only volcanic activity during this time made young pyroclastic volcanoes.

The absence of subsequent modification of the surface of Mercury by tectonism, volcanic activity, or atmospheric processes is significant because it indicates that after the period of basin flooding, the geochemical and tectonic evolution of Mercury were essentially complete. The extrusion of the lavas of the smooth plains was apparently the end of Mercury's dynamic history.

Mercury's lithosphere may even be rigid all the way to its core with no intervening asthenosphere. Nonetheless, Mercury appears to have remained warm enough to maintain a liquid outer core. The only processes available to modify Mercury after the end of its final period of volcanic activity are degradation of slopes by gravity-driven mass movement, the occasional impact by objects ranging from small meteorites through micrometeorites and solar wind particles and cosmic rays, volatile loss from the interior to create the young hollows scattered across the surface, and water ice deposition in polar craters.

40 5.10 Conclusions

The cratered surface of Mercury is strikingly similar to that of the Moon, and attests to the importance of meteorite impact as a general process in the solar system. The largest impact structure photographed on Mercury is the multiring Caloris Basin, similar in form, and probably in age, to the Moon's Imbrium Basin. This large basin is younger than the otherwise heavily cratered terrain that is similar in many ways to the lunar highlands, though more sparsely cratered. Still younger volcanic plains fill the Caloris cavity and are found scattered across the rest of the planet but concentrated in the northern hemisphere. Even older volcanism probably buried many of Mercury’s original craters, explaining why its heavily cratered areas have fewer craters than those on the Moon. This reemphasizes the importance of volcanism in the development of the planets. Mercury is also transected by distinctly mercurian lobate scarps that appear to be thrust faults created as the planet cooled and contracted.

The most significant differences between the Moon and Mercury are the result of

Mercury's larger size and enrichment in iron. Impact crater ejecta are distributed closer to the craters than on the Moon. Perhaps more important, Mercury appears to have cooled more slowly so that plains-producing volcanic activity during the period of intense bombardment was more long-lived and perhaps more vigorous than on the Moon. Moreover, the interior must be relatively hot to this day, because Mercury has a magnetic field thought to be generated by convection within a molten metallic core. The Moon’s small metallic core is probably solid—its magnetic field disappeared long ago. Mercury's iron-rich composition and large core may be the result of condensation and accretion of its constituents in an oxygen-poor zone near the forming

Sun. As a consequence, metallic iron, rather than iron oxide, and carbon, rather than carbon dioxide, are found in greater abundance in and on Mercury than the other inner planets. The

41 absence of a significant atmosphere or any surface fluids on Mercury was predetermined by its conception in this water- and carbon dioxide-poor part of the ancient solar system.

The geology of Mercury reinforces the notion that the tectonic and volcanic activity on a planet depend on the internal temperature. Since most planets were initially quite hot as a result of their accretion, much of their thermal history is dominated by cooling. Small planets, like

Mercury, with a surface area to mass ratio, cool rapidly and have short thermal histories.

Nonetheless, Mercury, with a surface/mass ratio higher than Mars and lower than the Moon, may have had a thermal history intermediate to these planets. And as it cooled and shrank, long lobate thrust faults and wrinkle ridges formed by contraction.

In short, the history of Mercury produced a Moon-like planet whose development was modified in pace and tenor by the distinctive properties of this, the innermost of the planets. We look forward to learning more about the “Iron Planet” when BepiColombo, launched by the

European Space Agency in 2018, arrives at Mercury in 2025.

5.11 Review Questions

1. Compare and contrast the surface of Mercury with the Moon.

2. In what ways do the impact craters on Mercury differ from those on the Moon? Why do

they differ?

3. Is it possible to determine the absolute ages of surfaces and features on Mercury with the

information in our possession?

4. Why do surface temperatures on Mercury range from very hot to very cold?

5. Why is spin-orbit coupling a common phenomenon in the solar system?

42 6. How do the smooth plains on Mercury differ from the lunar maria? What was their

probable mode of origin and age?

7. How does water ice persist at the poles of Mercury?

8. Where did the water ice come from that is now located at the poles of Mercury?

9. Identify the main differences between a lava flow versus a pyroclastic eruption.

10. What is the principal evidence that Mercury experienced global contraction during its

history? When did this happen---early or late? Why did Mercury contract rather than

expand?

11. How does the interior of Mercury differ from the interior of the Moon? Is there any

evidence that the interior is still molten? If so, what is the evidence?

12. Mercury formed very near the early Sun. What is the evidence that Mercury formed in a

compositionally distinctive part of the nebular disk? Could this explain its metallic iron-

rich composition?

13. Outline the major events in the history of Mercury and compare them to the major events

in the Moon's history.

14. How did magma form on Mercury? And how did it reach the surface?

15. Your job is to make recommendations for an astronaut mission to Mercury. Where should

the space ship land to obtain the most information about Mercury? What tasks should the

astronauts perform? What instruments should they take with them? What should they

bring back with them? Assume the astronauts have a small "rover" and will be on the

planet for two weeks.

43 5.12 Important Terms

Antipode

Basalt

Caloris Basin

Despinning

Erosion

Exsolve

Effusive eruption

Fissure eruption

Floor-fractured crater

Graben

Hollow

Intrusive magmatism

Intercrater Plains

Irregular pit

44 Late Heavy Bombardment

Lava channel

Lobate scarp

Multiring basin

Pantheon Fossae

Pyroclastic eruption

Regolith

Secondary crater

Sublimation

Smooth plains

Spin-orbit coupling

Thrust fault

Volatile

Volcanic vent

Wrinkle ridge

Additional Reading

Strom, R. G. 1987. Mercury: The Elusive Planet, Washington, DC, Smithsonian Institution

Press.

45

Rothery, D.A., 2015, Planet Mercury: From Pale Pink Dot to Dynamic World: Switzerland,

Springer, 179 p. https://en.wikipedia.org/wiki/Mercury_(planet) https://en.wikipedia.org/wiki/Mariner_10 https://en.wikipedia.org/wiki/MESSENGER

46 5.1 Physical and Orbital Characteristics of Mercury Mean Distance from Earth (Earth = 1) 0.387 Period of Revolution about the Sun 88 d Period of Rotation 59 d Inclination of Axis 2° Equatorial Diameter 4880 km Mass (Earth = 1) 0.055 Volume (Earth = 1) 0.06 Density 5.44 g/cm3 Exosphere (main components) O, Na, K (thin) Surface Temperature 100 (-173) to 700 K (427 °C) Surface Pressure (Earth = 1) 10-14 bars Surface Gravity (Earth = 1) 0.37 Magnetic Field (Earth = 1) 0.01 Surface Area/Mass 23 x 10-11 m2/kg

47 Caloris Basin

Figure 5.1 (a) Mercury is heavily cratered and is similar to the Moon. The circular feature at the top of the left image is Caloris Basin. It was formed by a giant impact and was subsequently filled and surrounded by smooth material. The majority of Mercury’s surface is peppered with impact craters and is called the intercrater plains.

(NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington).

48 Caloris Basin

Low High

Figure 5.1 (b) This colorized shaded-relief map of Mercury shows low topography is located in the polar regions

(blues and greens), while high topography is in a central band between 30° N and 30° S (yellows, browns, and reds).

Interestingly, Caloris Basin’s interior is just as high, or higher than its rim. (USGS).

49 180° 135°W 90°W 45°W 0° 45°E 90°E 135°E 180°

45°N

Caloris Basin

0° Odin Fm

45°S

180° 135°W 90°W 45°W 0° 45°E 90°E 135°E 180° Map Key Figure 5.2 A geologic map of Mercury shows the three main units are Caloris Basin and its ejecta, the smooth Crater Smooth Plains plains, and the heavily cratered terrain. The heavily cratered terrain formed first and have been pummeled by

Crater 1 Crater 4 impacts, the eruptions of the smooth plains followed, and while ongoing, the Caloris Basin formed by a large

Crater 2 impact, subsequently smooth plain volcanism shut off. The terrain and craters and listed by relative age from young Terrain Heavily Cratered Crater 3 Old Craters (crater 1) to old (heavily cratered terrain). (Modified from Prockter, L.M., Head, J.W., Byrne P.K.., Denevi, B.W..,

Kinczyk, M.J., Fassett, C., Whitten J.L., Thomas, R., and Ernst, C.M. 2016 and Kinczyk, M.J., Prockter, L.M.,

Denevi, B.W., Ostrach, L.R., Skinner, J.A. 2018). 50 Figure 5.3 The intercrater plains are the most heavily cratered terrain on Mercury. The craters are numerous and of variable sizes. Many craters overlap the margins of older craters, and young impacts may have completely destroyed (or buried) older craters. Most craters are very degraded, and in some instances, only subtle hints of the previous craters remain. In between craters are many small hills and patches of plains. Bright streaks are rays from a young impact. (NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington).

51 Figure 5.4 Mercury’s surface can be divided into three major crustal types on this false color image that reveals compositional differences across the planet. 1. The dark blue regions are the cratered plains that are enriched in carbon (graphite). 2. The yellowish-brown smooth plains, prominent in the northern hemisphere and in Caloris

Basin, are mostly younger lava flows. 3. The light blue-to-white streaks and spots are young impact craters and ejecta. (NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington).

52 Figure 5.6

Caloris Montes

Caloris Plantia Apollodorus Crater

Pantheon Fossae

Ghost crater

Wrinkle ridges

500 km

Figure 5.5 Caloris Basin is one of the largest impact features in the Solar System. It is 1500 km in diameter and in many ways is comparable to the Imbrium Basin on the Moon. The basin is filled by younger lava plains (Caloris

Planitia) and is highly ridged and fractured. Wrinkle ridges mark buried, impact-generated rings. Pantheon Fossae is a younger tectonic feature that domed the smooth plains; Apollodorus is an impact crater near the center of the dome. Ridged ejecta deposits (Caloris Montes) and smooth volcanic plains surround the basin. The rectangle shows the location of Figure 5.6. (NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of

Washington). 53 Caloris Montes

Caloris Basin

50 km

Figure 5.6 Caloris Montes is a ring of mountainous peaks that surrounds Caloris Basin. The Montes formed from the ejecta thrown out by impact. Younger ridged smooth plains (on the left) buried most of the crater floor.

(NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington).

54 Surface waves A B

Caloris impact

Shock waves

Core

Surface waves

Surface disruption at antipode

Figure 5.7 The “weird terrain” at the antipode of Caloris Basin is one of the most peculiar areas on Mercury.

(A) This region is broken into valleys and hills up to 2 km high that are interspersed with smooth plains. (B) It is

believed that this terrain is the result of focused seismic waves caused by the impact that formed Caloris Basin.

(Modified from The New Solar System).

55 100 km

Figure 5.8 The northern smooth plains closely resemble the lunar maria. Eruption of fluid mafic or ultramafic lavas formed the smooth plains; they flooded the surface and buried older craters and faults producing a young surface. Their surfaces are only sparsely cratered and are commonly deformed by compressional stress creating wrinkle ridges (white arrows)—some of them developed over buried crater rims (red arrows). (NASA/Johns

Hopkins University Applied Physics Laboratory/Southwest Research Institute).

56 10

10

4 x 10 y

Crater saturation 10

10 y

10

Lunar higlands 2 10 Smooth plains 10 y

10 imbrium 10 y Heavily cratered terrain

10 Number of craters/km of Number

10 y

10

10 y 10

10

0.062 0.25 1 4 16 64 256

Diameter (km)

Figure 5.9 The impact crater frequency curves of the lunar and mercurian terrains show the Mercury’s intercrater plains are slightly younger than lunar highlands. The smooth plains are intermediate in age between the mare and lunar highlands.

57 a. Mercury b. Lunar Ejecta Trajectories Ejecta Trajectories

Figure 5.10 The greater gravitational force of Mercury causes impact ejecta to be deposited nearer to the source than on the Moon. This concept can be illustrated by imagining the same meteor impacting on both Mercury and the

Moon and comparing the effects. Thus, secondary craters are closer to the primary and ejecta blankets are thicker near the primary impact site than on the Moon.

58 6

Complex 3

1 5 km

0.5 Simple Craters

Crater depth (km) Crater 25 km

0.1 Simple Craters Complex Craters

0.05 Peak Ring Craters

Lunar Crater Transition

0.01 0.1 0.5 1 5 10 50 100 200 Crater diameter (km)

Figure 5.11 The morphology of impact craters changes systematically with increasing diameter from simple bowl-shaped craters at small diameters to complex craters at larger diameters with flat-floors, terraced walls, and central peaks. At diameters of about 10 km, mercurian craters transition from simple to complex. Interestingly, at this juncture the crater depth does not increase as much as diameter. On the Moon, the transition from simple to complex craters occurs at a larger diameter of 15 km denoted by the vertical, black line, because of the slower impactor speeds. Examples of simple and complex craters are shown in images within the corresponding sections of the graphs. The green square represents the peak ring crater . Eminescu has the smallest diameter (125 km) of any peak ring crater and so exemplifies the onset diameter of peak ring craters. (Modified from Schon, S.C. and others, 2011 and NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute).

59 Ringed Peak-Cluster Basin Complex Central Peak Cluster Simple Prominent Central Peak

Smooth Floor

Ringed Peak Cluster Protobasin Peak-ring Basin Multi-ring Basin Prominent Small Central Peak Peak Ring

Small Peak Ring

Figure 5.12 With increasing diameter, impact craters become increasingly complex. Small craters are bowl shaped and progress up to peak-ring basins. Peak-ring basins start to form in craters that are greater than 125 km across, much smaller than on the Moon (340 km in diameter) because of the higher velocity of the impactors, and multi-ring basins are the largest type of basin seen. (Baker and others, 2011).

60 100 km

Figure 5.13 Peak ring basins are represented here by Raditaldi, a young and well-preserved example. The central ring formed as a central peak explosively expanded outward; the missing segments of the inner ring were buried by younger lava flows. The rim of the crater slumped inward, creating terraces that are especially evident in the northwest region. The area around the basin has radial lineations formed by secondary impacts that extend for hundreds of kilometers. (NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of

Washington).

61 Figure 5.14 Craters become more degraded over time as more impacts and erosional processes occur, subduing features formed by the original impact. The relative ages of craters can be determined by observing how degraded the crater is; the more degraded the crater the older it is. Crater in (A) is highly degraded and probably formed between 4.6-3.9 billion of years ago. The topographically subdued rim is the only visible crater feature. Crater (B) is also highly degraded with no apparent ejecta deposits, with superposed craters and subdued topography. The crater is younger than (A) and formed 3.9-3.7 billion years ago. Crater (C) is 3.7-3.1 billion years old. The terraces on the rim are more apparent than in the older craters. Crater (D) is 1.7 billion to 280 million years old and is relatively well preserved. The crater is not dotted by superposed craters, has a well-defined crater rim, an obvious central peak, and ejecta deposits surround the craters perimeter. Crater (E) is less than 280 million years old, and is a small bowl-shaped crater; it has a well preserved morphology, and some bright rays emanate from the crater (Barnouin and others, 2012; APL).

62 Lava ow

Fractures 0

15 Dike 30

Crust Depth (km) 45 2 3 4 1 60

Mantle

Magma body partially melted mantle

Figure 5.15 Magma formation and evolution begins with partial melting of the mantle and accumulation into a magma body composed of melt, crystals, and gases. 1) The low-density magma rises toward the surface through the denser mantle and subsequently into the crust through narrow cracks called dikes. 2) The force of the magma fractures the crust, creating a crack and providing a pathway for the magma to move upward through the crust toward the surface. The fracture that the magma travels through, forms ahead of the ascending magma, allowing cracks to form at the surface before lava erupts. The pressure of the magma in the dike may result in other dikes off shooting from the main one. 3) Eventually, magma reaches the surface and erupts from the fractures. 4) Over time, the subterranean reservoir empties and long lava flows develop. (Modified from Head, 2009).

63 2

Secondary Channel

1

40 km

Figure 5.16 Wide lava channels cross the Northern Smooth Plains of Mercury. The two flooded impact craters

(blue) are connected by a lava channel (green); the channel originated as crater 1 overflowed with lava and the lava moved down into crater 2. Crater 1 was not completely buried by lava, as evidenced by the peak ring (red) remnants that poke above the smooth plains. The channels are thought to initially have been fractures produced by the Caloris impact. Lava flowed along the fractures, widening and deepening them by thermal and mechanical erosion. Near the end of the channel are small ridges of high ground the lava flowed around. (NASA/Johns Hopkins University

Applied Physics Laboratory/Carnegie Institution of Washington).

64 Wrinkle ridge

Vent

Pyroclastic deposit

50 km

Figure 5.17 A small pyroclastic vent is ringed by deposits of bright material with diffuse (not sharp) boundaries in Caloris basin. (A) The bright material was explosively erupted from the 2 irregular scalloped depressions which are interpreted as the source vents. B) The deposits boundaries (red), vents (blue), and are shown on a sketch map

(Modified from Kerber, 2009).

65 Pyroclastic Eruption

Dike

Crust Crust Volatile-rich Crust

Mantle Magma body Volatile-rich Partially melted mantle Mantle

Volatile-rich Melt

Mineral

Crust Crystallization of magma

Mantle

Figure 5.18 Volatile concentration in a magma can increase in three principal ways, as shown in these simple cross sections. A) The magma is generated from a volatile-rich mantle. B) A magma body assimilates volatile-rich crustal material. C) Volatile concentration increases in residual melt as magma crystallizes volatile-poor minerals.

In all cases, when the magma is reaches low pressures near the surface, the volatiles rush out (exsolve) of the melt, to make gas bubbles, which expand explosively to power pyroclastic eruptions.

66 Figure 5.19 Paricutin is a cinder cone in southwestern Mexico. This volcanic eruption is a good analog for how pyroclastic eruptions occurred on Mercury. The still hot and molten pyroclasts were ejected by the violent popping of water bubbles that separated from the magma at low pressure. Note the lines of ejected pyroclasts falling back to the Earth and then flowing or rolling down the flanks of the cone. On Mercury, the pyroclasts would travel farther and create a lower broader cone because of the lower gravity (Photo credit: U.S. ).

67 100 km

Figure 5.20 Multiple and interconnected pit craters formed by the withdrawal of magma created a prominent arcuate chain in crater (center). The rimless pits (inset) formed by collapse when magma drained back into the subsurface (NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington).

68 Inner Core

Mercury contracts as it cools

Outer Core

Lobate scarp Mantle

Thrust fault

Figure 5.21 Lobate scarps are surface manifestations of thrust faults. The tops of the scarps are between 1-3 km in height and have steeply dipping scarp faces and gently dipping back slopes. The fault forms by contraction, allowing the movement of material above the fault over the material below the fault. The scarps are evidence that

Mercury cooled and contracted with its diameter shrinking by almost 7 km.

69 Rembrandt

200 km

Figure 5.22 Enterprise Rupes, a lobate scarp, is 820 km long (marked by white arrows) and cuts across

Rembrandt basin (715 km diameter). The thrust sheet advanced to the southeast by a few kilometers and offset two smaller craters and the rim of Rembrandt basin rim. Other lobate scarps (the NW corner of the image) cut the intercrater plains. (NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of

Washington).

70

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180° 135°W 90°W 45°W 0° 45°E 90°E 135°E 180°

Figure 5.23 A global map of lobate scarps on Mercury shows most are concentrated between 60°N and 60°S.

Very few lobate scarps developed in the smooth plains (tan). Perhaps the older lobate scarps were buried by the

younger lava flows the smooth plains. There are three main latitudinal bands of lobate scarps with the boundaries

marked by red lines. In between 60 N and 60 S, the orientation of the faults is close to N-S. North and south of these

latitudes the orientation changes to E-W. This dichotomy in orientation suggests mechanisms in addition to

contraction due to interior cooling are responsible for compression on Mercury. (Modified from Watters, 2015).

71 Mercury’s Geologic Timescale 0 Ga

Kuiperian Period

1

Planetary contraction

2

Mansurian Period

3

Calorian Period Smooth plains Caloris basin impact Tolstojan Period 4 Heavily frequencey Cratered Crater Pre- plains Tolstojan Period

Figure 5.24 The periods in Mercury’s geologic time scale have been named after the most prominent impacts of that time frame. Starting at the oldest (Pre-Tolstojan) and extending to the present (Kuiperian). Planetary contraction (blue line) began about 4 billion years ago and has decreased in intensity with time. Subsequent to both the onset of contraction and the Caloris impact, the eruption of the smooth plains began at about 3.9 billion years ago. This pulse of volcanism was short lived, ending 3.7 Ga. (Modified from http://luna1.diviner.ucla.edu/~jpierre/mercury/posters/Poster-09/poster-09.html).

72 Apollodorus Graben

20 km

Figure 5.25 Pantheon Fossae, at the center of Caloris Basin, consists of a series of radiating extensional grabens. Apollodorus impact crater is near the center of the Fossae but occurred after and did not cause it.

Instead, the grabens probably formed by domal uplift above a magmatic intrusion. Doming stretched the surface and caused extension. The magma may have flowed along the fractures forming subsurface dikes. (NASA/Johns

Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington)

73 Upper Crust

Magma Chamber

Lower Crust

Mantle

Figure 5.26 The radial grabens of Pantheon Fossae probably formed by the emplacement of a subsurface magma chamber. This displaced the overlying rock and domed the surface. The doming and formed large extensional faults in the crust.

74 A B

100 km 100 km

C

Figure 5.27 Extensional grabens on the floors of impact craters on Mercury. (A) Circumferential grabens inside the peak rings of (A) Raditaldi and (B) Rachmaninoff basins. (c) This buried crater within Goethe Basin has grabens that intersect polygonal fractures. In all three, circumferential grabens are most common. These grabens are thought to form when hot, thick lava flows cool, contract, and crack. (NASA/Johns Hopkins University Applied

Physics Laboratory/Carnegie Institution of Washington).

75 10 km

Figure 5.28 The hollows in Kerstv crater are shallow pits in a sheet of dark (low reflectance) volatile material originally brought to the surface by impacts and then deposited on the crater floor. Hollows appear to grow by sublimation of volatiles in the sheet, Small pits form and then widen as more volatiles escape leaving behind residual knobs. These are among Mercury’s youngest features. (NASA/Johns Hopkins University Applied Physics

Laboratory/Carnegie Institution of Washington).

76 Mechanical Chemical properties properties Magnesium-rich olivine silicates& pyroxene Asthenosphere

Outer core sulfur-rich iron convecting liquid Crust

Inner core sulfur-poor

Exosphere sodium (Na) and calcium(Ca)

Figure 5.29 Mercury’s internal structure is divided into layers based on chemical and mechanical properties and is similar to the structure of the other terrestrial planets. Chemically, Mercury differentiated into a very large iron- rich core with a radius of ~2000 km, a ~400 km thick mantle made of iron-poor and magnesium-rich silicates, and a thin low density silica-richer crust with an average thickness of ~35 km. Mechanically, Mercury’s core is divided into a solid inner core with a radius between 1000 km and 1500 km (shown). The outer core is probably a convecting liquid of iron-sulfide that creates the magnetic field. Above the core, lies a thin, weak asthenosphere and thicker rigid lithosphere. Mercury has a tenuous exosphere made of ions temporarily removed from solids at the surface.

77 Disk richer in oxygen Oxygen poor envelope iron oxide (FeO) rich silicates carbon and metallic iron rich fewer silicates

Earth

Mercury Venus

Mars

Snowline

Figure 5.30 Mercury accreted near the Sun within an oxygen-poor envelope, which was rich in iron and molecular carbon. Under these circumstances iron was in its native metallic (Fe0) state, which may help explain why Mercury has such a large iron core and a thin mantle of iron-poor silicates. Outside of the oxygen-poor envelope, the iron was oxidized (Fe2+) and was incorporated in silicate minerals. This reduced the amount of iron available to sink into planetary cores and increased the proportion of silicate minerals in Venus, Earth, and Mars.

This figure shows the Sun (yellow), Mercury (gray), Venus (orange), Earth (blue), Mars (red), and the planet orbits.

Beyond the “snowline water ice was stable, and cores of the giant planets could accrete.

78 Thrust faults Crust 0

Lithosphere Asthenosphere Asthenosphere Magma ocean

1000 Molten outer iron core Iron draining to form core Depth (km)

Solid inner iron core 2000

4 3 2 1 0 Billions of years before present

Figure 5.31 A graphic representation of Mercury's thermal history shows that a massive core must have formed early during the period of accretionary heating. At the same time, much of the mantle probably melted.

Rapid cooling later produced a thick, rigid lithosphere and resulted in contraction of the planet and cessation of volcanism. The core was initially molten, but as the planet cooled it crystallized, and today only a thin shell of liquid persists as the outer core.

79 Figure 5.32 The surface and interior of Mercury have evolved dramatically over the course of its history as illustrated in this sequence of diagrams.

Stage I. Accretion in a carbon-rich and oxygen poor Stage II. Differentiation allowed separation of a zone near the forming Sun. This set the stage for a crust, mantle and a large iron core which was initially planet with a large iron core and a thin mantle of molten. Dark graphite formed and migrated upward to silicate minerals. At the end of this stage, Mercury make a carbon rich ancient crust. was densely cratered.

Stage III. The Late Heavy Bombardment created new Stage IV. Excavation of Caloris Basin and formation impact basins and craters. Core separation may have helped of the associated hilly and lineated terrain on the power volcanism that buried the ancient heavily cratered opposite side of the planet. The weak asthenosphere terrain. Ongoing volcanism during this period of cratering and the molten outer ore became thicker as Mercury created the intercrater plains. lost its internal heat. During this stage, the planet had cooled sufficiently to start global contraction, resulting in compressive stress and thrust faulting at the surface.

Stage V. Formation of the smooth plains from volcanic Stage VI. Light cratering. Following smooth plains extrusions during declining impact rates. volcanism, Mercury became tectonically inactive as the lithosphere thickened. Only sparsely distributed pyroclastic volcanoes erupted. The only other process to significantly modify the surface was the occasional impact of meteorites, which created craters with bright rays.

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CHAPTER 13. Pluto and the Kuiper Belt

13.0 Introduction

Pluto, named after the Roman god of the underworld, is the largest member of the group of icy outer Solar System bodies that make up the distant Kuiper Belt (Figure 13.1). This asteroid-belt-like collection of objects lies beyond the orbit of . In the late 1800’s and early 1900’s, astronomers predicted a ninth planet beyond Neptune. discovered

Pluto on February 18, 1930, by observing movement of the object across pairs of images taken with an Arizona telescope. Upon discovery, Pluto was called the ninth planet. Since 2006, considerable debate about whether Pluto is a real planet has ensued.

Until recently, very little was known about tiny Pluto except what could be determined from telescopic observations, with the best images coming from the Hubble Space Telescope. In

July of 2015 that changed in dramatic fashion. The New Horizons spacecraft raced past Pluto, imaging one hemisphere in detail. The New Horizons images revealed a beautiful and geologically diverse world—to the complete surprise of all. They will fuel the study of Pluto for years to come and lead us to a better understanding of Pluto, other planets and moons, and the icy bodies of the outer Solar System.

13.1 Major Concepts

1. Pluto is the smallest, coldest, and outermost planet(?). Some call it a , along

with the asteroid Ceres and other Kuiper Belt Objects.

2. Pluto and its largest moon Charon form a double-planet system with a strongly elliptical

and highly inclined orbit. Pluto has four other small icy moons.

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3. Pluto is a geologically complex with several major terrains: (a) Tombaugh Regio, (b)

Bladed terrain, (c) Upland terrains, (d) The Macula, and (e) Polar Regions.

4. Pluto’s surface is dominated by volatile ices of nitrogen and methane and a very thin

atmosphere formed by the sublimation of these ices. The cycling of nitrogen and methane

from the surface to the atmosphere and back forms a rudimentary volatile cycle.

5. Pluto terrains range from heavily cratered (4 billion years old) to no craters (less than 10

million years old). It is also scarred by large, tectonically produced extensional fractures.

6. Pluto has a large basin, perhaps impact in origin now filled with nitrogen ice that flows

inward as . Within the basin, this glacial ice sheet undergoes solid state

convection from solar heating.

7. Pluto is a member of the Kuiper Belt, which consists of hundreds of small, icy objects

orbiting between 35 and 50 AU in a flattened disk. It is similar to an outer solar system

asteroid belt.

13.2 The Planet Pluto and the Kuiper Belt

Pluto’s physical characteristics and features are different from any of the other planets we

have seen so far (Table 13.1). Pluto is small, with a diameter (2,377 km) only 70% that of

Earth’s Moon. It is also 40 times farther from the Sun than the Earth. Pluto’s orbit is highly

elliptical and inclined to the ecliptic by 17° (Figure 13.2 and video). Only asteroids and comets

have similarly inclined orbits. Moreover, Pluto’s orbit is in a 2:3 resonance with Neptune. Pluto takes 248 Earth years to complete one orbit, and one day on Pluto is 6.4 Earth days. Pluto’s spin axis is strongly tilted (120°); basically, it rolls along its long orbit around the Sun like (as seen in this video). The severe tilt can cause extreme seasonal changes, especially at the poles,

which can experience arctic winter and arctic summer. Pluto’s surface is covered by highly

90 volatile ices of nitrogen, methane (like Neptune’s moon ) and carbon monoxide; these ices

sublime (vaporize) to form a thin atmosphere. The density of Pluto is 1.9 g/cm3, a result of the

large proportions of ices that were present in the ancient this far out from the Sun.

Pluto’s surface temperature ranges from 55 K (-218 °C) down to 32 K (-240 °C), or 32° above

absolute zero – a very cold environment, indeed.

Pluto and three other “dwarf planets” (Eris, Haumea, and Makemake, with several

others proposed) are located in an area of the outer Solar System known as the Kuiper Belt. This

is a disk that spans from 30 AU (just beyond Neptune’s orbit) to 50 AU from the Sun and is

reminiscent of the asteroid belt, in that it contains small bodies of leftover material from the solar

nebula. Because of its great distance from the Sun, the Kuiper belt contains planetary bodies rich in the volatile ices--water, nitrogen, methane, ammonia, and carbon monoxide, instead of being

metal- and rock-rich like the objects in the asteroid belt. Study of Pluto gives us insight into the

origin and evolution of Kuiper Belt Objects (KBOs).

Pluto’s classification as a planet was questioned after the discovery of multiple Pluto-like bodies in the outer Solar System. This elicited the following “problem:” Either there are many more planets than the traditional 9, or Pluto (and the newly discovered bodies) are not to be

considered true planets. In August 2006, the International Astronomical Union (IAU) revised the

to exclude Pluto, reclassifying Pluto as a “dwarf planet,” by saying that a planet needs to a) be round, b) orbit the Sun and c) clear its orbit. Although Pluto is round and orbits the Sun, it has not swept up all of the orbiting “debris” in its path. There are many icy

objects in the Kuiper belt with about the same size as Pluto (some may be even larger). However,

others disagree with this reclassification a dwarf planet. Led by (who headed the New

Horizons mission), a group of planetary scientists created their own definition: “A planet is a

91 sub-stellar-mass body that has never undergone nuclear fusion and that has sufficient self- gravitation to assume a spheroidal shape… regardless of its orbital parameters.” This definition indicates that a body is a planet if it is round and doesn’t make its own internal energy. Debate of the definition of a planet, and Pluto’s status as a planet, is still ongoing.

Pluto’s small size and distance from the Sun has made it difficult to study. The arrival of

New Horizons meant that our understanding went from a body of about 20 pixels across to a completely fleshed out picture of a planetary body with recent processes and geologic history.

The New Horizons findings have helped us understand the formation and subsequent evolution of small, distant, icy bodies, which can be directly contrasted with the terrestrial planets.

13.3 Pluto’s Spin and Climate

Like the Earth, Pluto experiences seasons because of planet’s axial tilt. Since Pluto has a much greater axial tilt (120°) than the Earth’s (24°), the arctic and antarctic circles are much larger on Pluto, extending from nearly 30° north and south of the equator all the way to the pole, compared with just 66° on Earth (Figure 13.3). Strangely, at the height of Pluto’s summer, the most direct sunlight is located within the arctic circle, not the mid-latitudes like we experience.

Currently, Pluto’s northern hemisphere is in the summer season, while the southern hemisphere is in winter and complete darkness. Presently, the direct and continuous sunlight on the northern hemisphere is causing nitrogen ice to sublime from the north pole and enter the atmosphere or freeze out on the cold south pole. The amount of sunlight each area on Pluto is receiving is a major contributor to the distribution of volatile ices on Pluto’s surface.

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13.4 Pluto’s Ices

Pluto’s surface is composed of four primary ices. The most prevalent ice is water ice

(with traces of mixed in ammonia). Water ice was detected on the surface or just below (covered

by a thin blanket of more volatile ices) by New Horizons in all major regions on Pluto, with the exceptions of Sputnik Planitia and the northern polar region (Figure 13.4). Similar to the icy satellites of Jupiter and Saturn, water ice forms the lithosphere and “bedrock” of Pluto. It is by far the most abundant of all the ices, and based on Pluto’s density, probably makes up 49% of the planet by volume. Of the four ices, only water ice is strong enough to build up topography without collapsing under its own weight. This means that all rugged mountains and chains on Pluto are water ice, even if covered by a thin layer of volatile ice. Just imagine looking out your window to stare at mountains 4-5 km high that are made completely of water ice instead of rock.

The remaining ices are nitrogen (N2), carbon monoxide (CO), and methane (CH4), listed

in order of decreasing volatility. Some other physical characteristics of these ices are shown in

Table 13.2. Consider how cold it must be on Pluto to have – the main atmospheric

gas found on Earth and is essentially frozen onto the surface as an ice. The volatile ices

cover most of the surface, so that the water ice bedrock is not able to visibly peak through in

many areas. Sputnik Planitia is a region wherein a deep basin contains high concentrations of all

ices, especially nitrogen. Its location near the equator, facing generally away from the Sun,

means that temperatures are low, stabilizing nitrogen. In contrast, the north polar region has high

amounts of methane present and low amounts of nitrogen because it is more often directly facing

the Sun, causing the more volatile ice (nitrogen) to sublime. Western Tombaugh Regio also has

high concentrations of methane that have not been covered by , or organic-rich material,

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that have fallen out of the atmosphere, most likely because it is colder at these higher elevations.

Carbon monoxide is not very prevalent on Pluto, and only found in semi-high concentrations within Sputnik Planitia (Figure 13.4). These ices are surficial and are not a major part of Pluto’s

bulk composition.

13.5 Atmosphere

We have mentioned the “atmosphere” of Pluto multiple times, but it is a very, very thin

atmosphere with a pressure of less than 10 microbars (0.1% that of Earth’s). Even at that low

pressure, the molecules interact by colliding with one another, creating a gas that can flow in

response to pressure and temperature differences—unlike the isolated molecules in the even

more tenuous exospheres of the Galilean satellites or Mercury. Unexpectedly, there is evidence

the is dense enough to transport ice particles and deposit them in sand dunes

(Figure 13.7). Apparently, wind blows from the topographic high of the al-Idrisi Montes down into the basin of Sputnik Planitia, carrying some of a dusting of methane “snow” that has fallen

or condensed at the top of the montes. The methane sediment is then deposited as transverse

dunes on Sputnik Planitia.

The composition of Pluto’s atmosphere is like the composition of the surface veneer. The

atmosphere contains a lot of nitrogen with lesser methane, and some other hydrocarbons as well.

The similarity in composition between the surface veneer and the atmosphere is a result of

sublimation of the volatile ices. The surface temperature is so close to their sublimation point

that small seasonal oscillations in temperature can cause the ices to sublime in one place and

condense in others. Thus, these ices escape easily into the atmosphere that reflects the

94 composition of the solid surface. This is quite different from Earth and many other planets where

the volatile envelope bears little resemblance to the composition of the solids on the surface.

13.6 Geological Provinces

13.6.1 Circum-Sputnik Planitia

The surface of Pluto is covered by volatile ices that are deposited, eroded, and

transported. Erosion of these ices has produced different provinces that most likely have formed by deposition and erosion of ice (including glacial flow) with small variations that create

different morphologies (Figure 13.5).

The washboard terrain lies directly west of al-Idrisi Montes (see Figure 13.1). It is made

of parallel ridges and grooves with crest to crest distances of about 1 km. The ridges and grooves

run from NE to SW; they are superposed on top of all impact craters showing they are quite

young. The washboard appears to be a thin, regional deposit perhaps similar to dunes, or it could

be the partially eroded remnants of a volatile ice deposit or features formed by tectonic or

landslide processes.

One of the most amazing terrains on Pluto is the dissected terrain (Figure 13.5 bottom

row). This terrain of sinuous and dendritic valleys was completely unexpected on this world

where everything was thought to be frozen solid. Each valley is oriented downhill, suggesting

something flowing incised into the ice. However, the valleys terminate into depressions with no

obvious deposits. Here on Earth, even the ends of dry rivers have alluvial fan deposits. Did the material sublimate after being deposited? In some instances, smaller valleys merge into a larger

valley, similar to feeder systems of rivers and glaciers on Earth. These valleys look to have been

95 formed by erosive glacial activity of nitrogen ice, similar to the active valley glaciers seen in the

Tombaugh Regio.

13.6.2 Tombaugh Regio: Pluto’s Heart Named after the discoverer of Pluto, Tombaugh Regio is a unique, heart-shaped region

mostly north of Pluto’s equator (Figure 13.1). The western half is a large basin, filled with a

smooth, impact crater-free, nitrogen-ice ice sheet known as Sputnik Planitia. The eastern half is

an upland of methane ice pitted by sublimation and cut by nitrogen glaciers.

13.6.2.1 Sputnik Planitia

Sputnik Planitia is a broad basin 3-4 km deep and a thousand km across that forms the

western half of Pluto’s heart (Figure 13.6). The original depression may have formed by a major

impact in Pluto’s distant geologic past. Today, the basin is filled by a sheet of young, nitrogen-

rich ice (see Figure 13.4). The bright ice sheet is broken up into polygonal shapes separated by

100 m deep trenches (Figure 13.6). The polygons are 20 to 40 km across, with the centers raised

~50 m above their margins. Within some of the polygons and along the margins, blocks of water

ice appear to “floating” in the nitrogen ice. The diameter of the cells and the dimensions of the

floating, water-ice mountains indicate the ice sheet is about 10 km thick. Below the ice, lies a

thin region of the water ice lithosphere and perhaps an uplifted section of a liquid water layer.

These polygonal shapes were perhaps the most exciting discovery on Pluto. They are reminiscent of the tops of convection cells, such as might form on a boiling pot of thick tomato soup, but this sheet is solid (but weak), not liquid. There may be enough radiogenic heat to sustain solid state convection in this weak layer of nitrogen, or convection could be driven by cooling from a previously warmer climate. This convection produces the polygonal cells by

96 warmer ice rising in the middle part of the cell (elevating the central cell) and cold ice sinking at

the cell boundary troughs (Figure 13.7). There is nothing else like this in the entire solar system.

Imagine standing on icy terrain convecting and spreading beneath your feet —hundreds of tiny

“plates” growing and then “subducting” at the margins. Mathematical models conclude that each

cell might overturn in 100,000 to 1,000,000 years—so some polygons might be spreading at the

same rate as Earth’s (much larger) tectonic plates move.

Sputnik Planitia’s western and southern regions are surrounded by Montes (mountains) rich in methane (CH4) ice, quite different from the nitrogen ice that fills the basin (Figure 13.1

and 13.4). However, on Pluto, solid methane is weak and unable to support the topography of a

mountain without flowing and deforming under its own weight. On the other hand, water ice is

common on Pluto, as indicated by its density, and is strong enough to support topography at such

low temperatures; at Pluto’s low surface temperature water ice is basically as strong as silicate

rock on Earth. Consequently, the methane is most likely a thin veneer covering water ice

mountains. One of the most notable mountain regions is the al-Idrisi Montes, on the northwestern edge of Sputnik Planitia (Figure 13.1). It has randomly oriented blocks of ice that are an amazing

5 km high and 40 km across. They form a rough chain of mountains extending for 100s of km

(Figure 13.8). For perspective, Earth’s Tibetan Plateau rises about 5 km above the plains of India and Valles Marineris on Mars is about 7 km deep. The blocks of al-Idrisi have flat or gently sloping upper surfaces with a series of irregular knobs that are like features in the surrounding terrain. This suggests the blocks broke away from the highlands to the west and slid chaotically into the lower basin.

97 13.6.2.2 The Pitted Highlands: Eastern Tombaugh Regio

The eastern pitted highlands extend 4 km above Sputnik Planitia and are very different

from the western lobe of Tombaugh Regio. It is a heavily pitted upland or highland terrain

(Figure 13.9). The highlands composition is different than Sputnik’s; methane is the dominant

ice with lesser nitrogen and carbon monoxide (Figure 13.4). Individual pits cover the highlands

and range in diameter from a few km to 25 km across. The floors of the pits may be smooth soft,

deformable nitrogen ice. Smooth plains stretch between multiple pits and are up to 50 km wide.

The uplands may have originally been a thick smooth deposit of methane ice mixed with

nitrogen. As the more volatile nitrogen vaporized it eroded the highlands during the last/current

“interglacial period” (warmer climate) in the region.

The expectation that Pluto’s surface was geologically “dead” was shattered by the

discovery of active glaciation around and within Sputnik Planitia. Valley glaciers originate in the

uplands, west of Sputnik Planitia, and flow down into the basin enlarging some of the troughs

formed by sublimation. The tongues of glacial ice are outlined by medial and marginal moraines

that show up as darker streaks in the elongate lobes—just like terrestrial glaciers (Figure 13.9).

But these glaciers are not flowing masses of water ice—instead they are made of nitrogen ice which is weak and can flow even at the low temperature of Pluto’s surface.

Sublimation is a commonly occurring process on Pluto that drives the formation of innumerable observed pits. These pits can be seen across most of Pluto's surface, with obvious

pits in the pitted uplands (previously described) and on the plains of Sputnik Planitia (Figure

13.10). They form in nitrogen-rich ice, the most volatile and therefore the easiest to sublimate.

We can best envision how they form by examining those on Sputnik Planitia, where the pits are

98 small and shallow, only 10s of meters deep. In some locations, the patterns of pits show they

form along anisotropies in the ice (flow margins, fractures, or compositional differences in the

ice). As sublimation continues, the pits grow and merge leaving rough residual highlands

between troughs. In the pitted uplands, the pits are much deeper (up to 2 km), irregular, and

large than on Sputnik Planitia (Figure 13.9). They appear to have grown and merged with one

another much more extensively than on Sputnik, implying the process has been going on for a

much longer time than on the youthful Sputnik Planitia.

13.6.3 Bladed Terrain: Tartarus Dorsa

East of the pitted uplands is Tartarus Dorsa (Figure 13.1), which is made of several broad

swells that are covered with N-S oriented, blade-shaped ridges with long narrow valleys between

them (Figure 13.11). This region has been named Tartarus Dorsa. (In Greek mythology, tartarus

was the deep abyss used as a dungeon for tormenting the wicked and a prison for the Titans; dorsa means back or top.) The steep-sided ridges are several hundred meters high and 5-10 km

apart. These blades are located near the equator and are made mostly of methane ice. The blades

(called penitentes on Earth where they form in snow) are probably a result of sublimation and

wall collapse, forming elongate depressions that face the Sun and spires of non-sublimated ice,

but some have suggested formation by preferential deposition of ice. The consistent orientation of the blades suggests there is a factor that determines the orientation, such as solar illumination,

wind direction, or a regional fracture network.

13.6.4 The Macula: The Dark Terrains

The large dark equatorial region on Pluto is named Macula (Figure 13.1). An

underlying topography can be seen beneath the dark reddish material indicating the reddish

99 coloration comes from a thin, laterally extensive deposit on a pre-existing cratered landscape.

The thin material is most likely a result of deposition. We first encountered tholins on

Saturn’s large moon, Titan. They are aggregates of small, solid, organic (carbon-rich) particles precipitated in the atmosphere after photochemical reactions 500 km above the surface split apart

methane. Aggregates of these reddish tholins produce thin layers of haze that extend up to 200

km into Pluto’s atmosphere (Figure 13.12). The tholins then fall to the surface and have mantled

Cthulhu Macula. Tholins are relatively refractory (compared to the volatile ices) and are

concentrated along Pluto’s equator where solar insolation is higher, resulting in instability and

sublimation of volatile ices. Tholins precipitate globally on Pluto, but are diluted by the

deposition of nitrogen, methane, and/or carbon monoxide ice everywhere but the equator (Figure

13.1 and 13.4).

13.6.5 Polar Regions

The northern polar region of Pluto has been named Lowell Regio (Figure 13.1). Because

of its location, Lowell Regio experiences long periods of darkness (winter) and light (summer).

These long periods of continual sunlight (or absence of it) cause fluctuations in the temperature

at this location that determine if there will be net deposition or sublimation of volatiles. During the winter volatile ices are deposited and during the polar summer the ices sublimate. This

creates a seasonal transfer of volatiles and ice back and forth from pole to pole.

13.7 Impact Craters

Even though New Horizons only mapped about 40 percent of Pluto’s surface, over 5000

impact craters have been identified (Figure 13.13). However, impact crater frequency varies

greatly across the surface. Some areas have crater abundances consistent with formation before

100 the Late Heavy Bombardment, while other areas have no identifiable impact craters, suggesting

the surface is younger than 10 million years old. The upland plains north and west of Sputnik

Planitia are quite heavily cratered and are the oldest terrains of Pluto, with an age of about 4

billion years. In contrast, Sputnik Planitia is the youngest terrain seen on Pluto. No identifiable

impact craters are observed on the ice sheet, probably because of the quick overturn of the

convection cells. Any crater that impacts the nitrogen ice sheet would be obliterated within

500,000 years. While much of the surface is ancient (around 4 billion years old), Pluto also has

enough heat to be geologically active in some places.

The morphologies of impact craters on Pluto are very similar to those on other icy

planetary bodies; with small and simple craters being bowl-shaped, and larger craters having

central uplifts. The largest impact crater, Burney crater (named after the 11-year-old schoolgirl who suggested the name “Pluto” for the planet Clyde Tombaugh discovered), is 220 km across.

No large multiring features like those on Ganymede or Callisto have been found on the imaged hemisphere. The ejecta of plutonian craters is hard to discern because it is mantled by ice deposits. Many of the crater floors are dark red, which could be tholins, once buried by nitrogen and methane ice, which have been excavated by impact.

13.8 Tectonic Features

Pluto’s surface is riddled with extensional tectonic features (Figure 13.14). These include numerous belts of aligned and arcuate troughs that are several hundred kilometers long and a few kilometers deep. They are interpreted to be grabens or rifts bounded by steep normal faults.

These fractures branch from and into others, cutting across preexisting landforms and terrains.

The tectonic features are variably degraded, indicating deformation occurred over a long period

101 of time. For example, Virgil Fossa contains unbroken rift segments up to 200 km long; the eastern end cuts across a large impact crater, while the western edge branches out into many segments (Figure 13.14c). In many ways, the extensional features are similar to those on the and Saturn, but they are not as complex as the grooved terrain of Ganymede or highly fractured . Importantly, no compressional tectonic features have been identified on

Pluto.

The mechanism of formation for these extensional features is conceptually simple. Early in Pluto’s history, most of the water inside Pluto was liquid. As time passed, Pluto lost its internal heat leading to the freezing of the liquid water into ice. Freezing occurred from the top downward. The phase change from liquid water to ice water decreases its density causing expansion during freezing. On a global scale, this freezing and expansion of the subsurface ocean produced extensional stresses at the surface of Pluto and created extensional fractures. The lack of contractional features shows that Pluto did not contract during cooling like the silicate dominated inner planets (for example Mercury).

13.9 Volcanic Features

Because volcanoes are windows into a planet’s deep interior that reveal how hot it might be at different times in a planet’s history, it is important to find and study volcanic features on other planets. Two possible cryovolcanoes have been identified in the southern hemisphere of

Pluto and may give a glimpse into Pluto’s interior. In general, these features are large mounds with central depressions. Wright Mons is most morphologically like a (Figure

13.15). It is 3-4 km high and 150 km in diameter and has a central depression that is 5 km deep, with concentric rings surrounding the depression. This feature appears to be constructional, made

102 of a series of flows followed by retreat of magma and collapse at the vent. There are very few impact craters on this mound, indicating a young eruption age. The height suggests that it must be made out of materials stronger than nitrogen or methane ice, which flow readily. Most likely, the construct is made of water ice coated by thin deposits other ices. Maybe cryomagma formed in an anomalously warm region of the interior and rose through fractures in the water-ice lithosphere to erupt on the surface. Alternatively, ammonia-rich ice melts at a lower temperature than water ice and could have allowed the creation of cryomagma. But why would it be warmer or compositionally different just in a few places? Do icy plumes reflecting subsurface convection play a role here?

13.10 Internal Structure

The internal structure of Pluto is similar to the icy moons of the outer planets and probably other Kuiper Belt Objects as well. Pluto’s density is 1.9 g/cm3 indicating it is a mix of dense silicates and metals as well as low density ice. It is most likely partially differentiated with a rocky core, a liquid water layer, a water-ice mantle, and a surface veneer of volatile ices

(Figure 13.16).

Pluto’s core is thought to be 1700 km across, about 70% of Pluto’s diameter. The core is probably made of dense silicate rock mixed with some metal. An iron core is unlikely because there may not have been enough refractory metal or enough early accretionary heat to differentiate a metal core from a rocky mantle.

Pluto probably has lower concentrations of radioactive elements than the terrestrial planets; these elements (potassium, uranium, and thorium) condense with silicates rather than in volatile ices. But Pluto must have some radioactive elements concentrated within its rocky core.

103

Perhaps the decay of these elements, or tidal heating, produced enough heat to allow tectonic features to form in a thin, weak lithosphere, form the young cryovolcanoes, and maintain a liquid water layer in contact with the core.

The location of Sputnik Planitia, in regard to its axial tilt, and computer models show

Pluto’s rocky core is probably surrounded by a layer of liquid water at least 100 km thick below an outer layer of solid water ice. Early in Pluto’s history, the liquid water layer was most likely thicker and as Pluto cooled, the liquid water froze, creating a water ice lithosphere that increased in thickness at the expense of the liquid layer. At some point in the future, all the liquid water will be completely frozen.

The outer 200 km are made of a water-ice crust (essentially a rigid lithosphere) capped by the highly volatile surface ices of varying thicknesses. The veneer of ices is composed mostly of nitrogen and methane with lesser amounts of carbon monoxide. Nitrogen is the most volatile ice and tends to concentrate in the coldest locations, such as Sputnik Planitia and the pole experiencing winter (the south pole currently; but we have no images from the south--it not illuminated when New Horizons flew by in 2015; Figures 13.1). Methane is widely distributed, concentrated in the Lowell Regio and the equator (especially in Tartarus Dorsa and the tops of al-Idrisi Montes), but is patchy at the mid-northern latitudes (Figure 13.4). Carbon monoxide is not as common on Pluto as the other ices but is found in high concentrations in the low elevation

Sputnik Planitia, along with the other ices. In fact, the Sputnik Planitia basin appears to be a cold trap, becoming a storage location of all the ices as vapors move to low elevations in the thin atmosphere and then condense as solids (Figure 13.4).

104 Pluto has a low pressure (0.1% of Earth’s), nitrogen and methane-rich atmosphere

(reminiscent of the volatile surface ices). This atmosphere is sustained by the sublimation of

nitrogen and methane ice. In this way, the evolution of the atmosphere and surface ices are tied

together.

13.11 The

Pluto has 5 moons that have been discovered thus far (2018): Charon, , ,

Kerberos, and (Figure 13.17). Charon (named after Pluto’s ferryman) is the largest and

closest of the 5 moons. Fortunately, New Horizons was able to collect high-resolution images

and other data for Charon, and some things were also learned about the four smaller moons.

Charon is 1212 km in diameter (about half the size of Pluto) with a slightly lower density than Pluto of 1.7 g/cm3 because of Charon’s thick water ice mantle and presence of ammonia ice

at the surface. The moon is divided into three main regions that are the result of different

processes (Figure 13.18). Charon’s northern and southern hemispheres are divided by large,

equatorial chasms that span the globe, made of two interconnected rifts. Serenity Chasma is 50

km wide and 5 km deep and Mandjet Chasma has the same width but is 2 km deeper (7 km).

Another, separate chasm, Argo Chasma, is 690 km long and 5 km deep. North of the first two

chasms lies Oz Terra, a tectonically disrupted terrain with scarps, angular fault-bounded crustal blocks, depressions, and ridges. The crater frequency shows the surface is more than about 4

billion years old. These extensional tectonic features must have formed by the same mechanism

as those on Pluto--the freezing of an interior liquid layer causing expansion of the globe.

Mordor Macula, Charon’s north polar region, is covered with dark red material, similar to

Cthulhu Macula on Pluto (Mordor was the Dark Land in ’s The Lord of the Rings.) The

105 material gets darker with increasing latitude, presumably because the deposit gets thicker. The

polar location and dark red color indicate emplacement by seasonal deposition/trapping and then

alteration of methane in what may originally have been a large impact basin. Right now, the

north polar region of Charon is in the middle of a 150-year-long summer season, while the south

pole is in winter (Charon faces Pluto and shares the same orbital tilt). Amazingly, the red material may not come from Charon at all, but instead it traversed thousands of kilometers from

Pluto to fall on Charon. These volatiles are probably derived from Pluto by sublimation of methane ice that then escapes from Pluto’s atmosphere and makes its way to Charon. The methane is then deposited on the pole experiencing winter. As the pole moves into the Sun the methane reacts with the radiation and becomes refractory, making tholins in the atmospheres of

Pluto and Titan.

The southern hemisphere, called , is not as broken up as the northern hemisphere. It is made of smoother plains with fields of small hills 2-3 km in width, and moated mountains, which may be of cryovolcanic origin (Figure 13.19). The crater density of Vulcan

Planum is slightly less than Oz Terra indicating a younger surface that may have been cryovolcanically resurfaced.

Not much is known about the other moons of Pluto. They orbit Pluto at a greater distance than Charon and are much smaller, ranging from 10 to 37 km across. They are bright, porous, undifferentiated bodies that did not become large enough to become spherical. They all have nearly circular orbits but chaotic, nonsynchronous rotations; they are not tidally locked with one face toward Pluto. Because of these irregularities, it is speculated that they are giant hunks of ice ejected from Pluto in some past , perhaps during the event that formed the Pluto-

Charon system.

106

13.12 Geologic Evolution of Pluto (and Charon)

Pluto condensed far from the Sun where nitrogen, methane, and carbon monoxide were stable, and it accreted from these ices and silicates; however, there was not enough material to create a large planetary body like Saturn or even Neptune. Charon and Pluto’s other moons may have accreted from material that was orbiting what would soon become Pluto. Others suggest a more dramatic event that created the moons. Soon after accretion, when proto-Pluto had begun to differentiate, a large body impacted Pluto. This scattered proto-Pluto and the impacting object into many objects that then reaccreted to form Pluto, Charon, and the other small moons, similar to the impact that formed the Earth’s Moon.

Just as interesting as the formation of Pluto’s moons is the evolution of this planet’s orbit.

Initially the orbit and axial tilt of the plutonian system may have looked like any other . However, the bullies changed Pluto’s orbit forever. As the Gas Giants migrated outward from the Sun (the event that initiated the Late Heavy Bombardment) they gravitationally pushed Pluto onto its side and forced Pluto to roll around the Sun in an elongated and inclined orbit. Or perhaps the collisional event that formed Pluto also imparted a skewed inclination to the planetary bodies.

Regardless of how Pluto and its moons formed, the thermal and geologic history of Pluto is similar to other icy bodies (Figures 13.20 and 13.21). Pluto differentiated quickly, with rocky material sinking to the center of the planet, within thousands of years. Soon after differentiation, the outermost part of Pluto began to freeze as water ice. Concurrent with cooling, Pluto degassed, releasing volatile gases that formed an atmosphere. Some of these volatiles (nitrogen, methane, and carbon dioxide) condensed onto the surface as ices. As Pluto continued to a cool

107 and the lithosphere continued to grow, a large impactor hit Pluto, forming the basin called

Sputnik Planitia. A high concentration of volatiles accumulated in the low, cold basin, freezing out to form a vast, nitrogen ice sheet with peripheral glaciers. As Pluto outgassed and impacts occurred the planet continued to cool, its water ice lithosphere growing at the expense of a subsurface liquid water layer. With continual freezing, the water ice lithosphere continued to expand, producing long extensional tectonic grabens and fractures. However, Pluto is not completely frozen yet, and still has enough heat to sustain a 100 km thick liquid water layer, local cryovolcanoes, convection in Sputnik Planitia, and tectonic activity. One day the heat will run out and Pluto will be a completely frozen planet, like Charon. Then only geologic processes related to the surface, atmosphere (dunes and bladed terrain), sunlight (sublimation, deposition, tholin production), and light impact cratering will occur.

13.13 Conclusions

The expectation of Pluto being geologically dead was summarized nicely by Nimmo and

Spencer (2015): “Pluto does not experience significant tidal heating and is predicted to show no signs of recent resurfacing”. However, New Horizons showed an active Pluto with many geologic features we have seen on other planets and icy moons—especially Neptune’s Triton and

Saturn’s Titan. These include impact craters, tectonic scarps, and cryovolcanoes. But the variety of volatile ices (methane, nitrogen, and carbon monoxide) leads to the most rapid surface changes and produces many familiar and unfamiliar, even exotic surface features.

Pluto has a rocky inner core that makes up 2/3 of the planet’s diameter, surrounded by a liquid water layer, a water-ice mantle, and a dusting of nitrogen and methane ices. Most of

Pluto’s surface is dotted with impact craters and surfaces that date back to 4 billion years ago.

108

However, the basin of Sputnik Planitia is very young with its convecting nitrogen-rich ice sheet.

Active glacial flow from the pitted uplands into Sputnik Planitia occurs today and valleys show evidence of past glaciation in other locations.

Sublimation and deposition of volatile ices and wind-blown deposition are the dominant geologic process on Pluto today, forming pits, penitentes, and dunes across the surface.

Extensional features show rift tectonics dominated Pluto’s history penultimate history, most likely caused by the freezing of a liquid water layer. Constructional mounds in the southern hemisphere may be evidence for recent past cryovolcanism where water lava erupted onto the surface. The unexpected geologic activity on Pluto has important implications for how long it takes icy planetary bodies to cool, how the Sun can affect planetary surfaces, and how long bodies are geologically active. Continued study of Pluto and the geologic processes acting on the planetary body will give more insight into how icy planets form and evolve and will show important contrasts with terrestrial planetary evolution.

13.14 Review Questions

1. Why was Pluto demoted from the class of planets by the International Astronomical

Union? On the other hand, can you justify reclassifying Pluto as a planet again?

2. What are the unique aspects of Pluto's orbit?

3. Outline the major differences and similarities between Pluto and Neptune's moon Triton.

Consider their sizes, atmospheric compositions, internal structures, surface features,

relative ages, surface temperatures, and distances from the Sun

4. What does the density of Pluto imply about its composition? What are the major ices that

have been identified on its surface?

109

5. What kind of an atmosphere does Pluto have?

6. Why isn't Pluto just a cratered ball of ice like many of the icy satellites of the outer

planets?

7. Explain the composition and formation of the white plains of Sputnik Planum. Contrast

its composition with that of the mountains of Pluto.

8. What types of tectonic features can you find on Pluto? Or on Charon?

9. How do the distinctive dark regions on Charon's poles form?

10. Make a graph to compare the diameters and densities of Pluto and Charon to other

planetary bodies in the solar system. Include the terrestrial planets, satellites of Jupiter,

Saturn, Uranus, Neptune, and a few asteroids on your graph. Explain any apparent

groups.

11. What is the Kuiper Belt? What kinds of bodies have we found there?

13.15 Key Terms

Barycenter

Chasmata

Cryovolcano

Dwarf planet

Glacier

Haze

Insolation

Kuiper Belt

Moraine

Montes

110 Penitente

Solid State Convection

Sublimation

Tholin

Additional Readings

Olkin, C.B., Ennico, K, and Spencer, J., 2017, The Pluto system after the New Horizons flyby:

Nature Astronomy, v. 1, p. 663-670.

Stern, S.A., Grundy, W.M., McKinnon, W.B., Weaver, H.A., and Young, L.A., , 2018, The Pluto

system after New Horizons: Annual Reviews of Astronomy and Astrophysics, v. 56, p.

357-392.

111

Table 13.1 Physical and Orbital Charactristics of Pluto and Charon

Pluto Charon Mean Diststance from Sun (Earth = 1) 39.5 Period of Revolution 248 y 6.4 d Period of Rotation 6.4 d 6.4 d Inclination of Axis 120° 120° Equatorial Diameter 2,377 km 1,212 km Mass (Earth = 1) 0.0022 0.0003 3 3 Density 1.9 g/cm 1.7 g/cm

Atmosphere (main components) Nitrogen (N2), Methane (CH4) None Surface Pressure 1 microbars Surface Temperature 40 K (-233 °C) Known Satellites 5

112 Table 13.2 Physical Characteristics of Pluto’s Ices

Ice Composition Vaporization Temperature (K) Condensation Temperature (K)

Nitrogen N2 63 32

Methane CH4 91 45 Carbon monoxide CO 52

Water H2O 273 200

113 A B North pole Lowell x Regio

Ivanov Vallis Pioneer Venera Terra Terra Voyager Hayabusa Terra Terra Mwindo Kupe Vallis Fossae Sleipnir Piri Fossa al-Idrisi Planitia Montes Sputnik Piri Rupes Planitia Tartarus Zheng-he Dorsa Montes

Viking Terra Bare Montes Equator Tombaugh Regio Cthulhu Macula Hillary Montes

Tenzing Montes

Wright Mons Piccard Mons

Figure 13.1 Pluto as imaged by New Horizons at its closest approach in July of 2015. (A) A true color image of

how Pluto would look to the naked eye. (B) A color enhanced image reveals a geologically diverse planet that has

undergone many geologic processes, with some still continuing today. Perhaps the most spectacular part of the

image is Sputnik Planitia, a basin filled with a large ice sheet at the heart of Pluto. This image shows the location of

the areas discussed in this chapter. (NASA/ Johns Hopkins University Applied Physics Laboratory/Southwest

Research Institute). y axis

Orbit of Mars Saturn Jupiter

Uranus

Neptune

Pluto

Spin axis

x axis

Figure 13.2 Pluto’s orbit around the Sun is highly elliptical; for about 20 Earth years, Pluto is actually closer to the

Sun than Neptune. The orbit is also highly inclined (17 degrees). Note the tilt of Pluto’s spin axis, which creates strong seasonal changes. The sizes of the planets are not to scale, and Pluto is extra-large to show its spin axis.

(Modified from Brown and Braselton, 2006).

115 Earth Pluto Night North Pole Arctic 23° Circle

120° South Pole Night Side

Sun Equator North Pole

Equator

South Pole Day Arctic Circle

Figure 13.3 Because of Pluto’s extreme tilt, Pluto’s arctic circle (above this latitude the Sun will remain above the horizon, during the summer, as well as below the horizon during the winter, for months at a time) extends much farther southward than on Earth. Presently, the north pole is facing the Sun. This drives sublimation of nitrogen ice from the north pole to the atmosphere, until the nitrogen moves and recondenses on the cold, south pole. (Modified from MIT/Alissa Earle).

116 Methane Nitrogen Carbon Monoxide

Figure 13.4 The cryosphere of Pluto is dominated by two main volatile ices with a third minor component represented by different colors on this map. Methane, the second most common ice, and is abundant everywhere except in the dark equatorial maculas. Nitrogen is mostly concentrated in the mid-latitudes and at low elevations where it is colder today. (Currently, the north pole is in its warmer summer season—if -218° C can be called warm).

The minor component carbon monoxide dusts the entire surface lightly. All three ices are in high concentrations within the cold basin Sputnik Planitia. Water ice underlies these more volatile icy deposits. (Gladstone and others,

2016; NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute).

117 Figure 13.5 Geologic processes on Pluto’s ices have made various morphological land forms including (A) the washboard terrain and (B) various valley indented regions. The washboard terrain seems to be a deposit of volatile ice covering the surface; they may be wind-oriented dunes or tectonic landforms. The dissected ices are the result of past valley formation by glacial erosion or, perhaps, even temporary flows of liquid nitrogen. (Moore and others,

2016).

118 Figure 13.6 Sputnik Planitia comprises the western half of heart-shaped Tombaugh Regio. It is a deep basin partially filled with a nitrogen ice sheet ~10 kilometers thick. The sheet is broken up into many separate polygons that appear to be the surface expressions of separate convection cells. The western margin of Sputnik Planitia is surrounded by water ice montes, and east part is bordered by the bright and irregular pitted uplands. (NASA/Johns

Hopkins University Applied Physics Laboratory/Southwest Research Institute).

119 al-Idrisi Montes

20 km Roots of al-Idrisi Montes

Water ice lithosphere

Figure 13.7 On Sputnik Planitia, each polygonal cell forms by convection as warm ice rises at the center, reaches the surface, and flows laterally as it cools, ultimately returning to the dark interior of the ice sheet from whence it came. The energy that drives this convection is thought be radiogenic heat or solar heating. The black lines mark some of the many, unexpected, transverse dunes on Sputnik Planitia. The source of the “sand” is probably methane ice that precipitates on top of al-Idrisi Montes and is then blown off the mountain, and on to Sputnik Planitia’s floor.

These dunes were, in part, discovered by BYU geologist Dr. Jani Radebaugh. (NASA/Johns Hopkins University

Applied Physics Laboratory/Southwest Research Institute).

120 100 km

Figure 13.8 al-Idrisi Montes is a jumbled mass of water ice blocks on the edge of Sputnik Planum. These blocks appear to have slid from the west as large landslides, down into the basin. The blocks are currently covered with a dusting of methane ice and tholins. (NASA/Johns Hopkins University Applied Physics Laboratory/Southwest

Research Institute).

121 A Pitted Uplands

Valley glaciers

50 km

B

Glacier margin

100 km

Figure 13.9 The Pitted Uplands are the eastern part of Tombaugh Regio. They have a large number of pits and sublimation features (on the right of each image) etched into nitrogen ice as well as active glacial systems made of flowing nitrogen ice. The glaciers are outlined by dark sinuous lines (moraines), and originate from the Pitted

Uplands and flow west into Sputnik Planitia. Image (A) shows glacial valleys with flowing nitrogen ice. Image (B) shows a glacier flowing out onto the basin. The black arrows show the glacier’s flow direction. (NASA/Johns

Hopkins University Applied Physics Laboratory/Southwest Research Institute).

122 Figure 13.10 Sublimation pits are a common feature on the surface of Pluto. These pits appear in nitrogen-rich ice as it sublimes into the atmosphere. The pits vary greatly in diameter and morphology. Some are small (a couple kilometers across) others are large (upwards of 25 km across). Some pits are nearly circular, while others are elongate, and some form as individual pits but many form long chains of pits. Look at the image carefully to see that these are pits and not domes and ridges. The Sun is shining from the upper left, so the dark shadows are on the left side of the pits. (NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute).

123 Figure 13.11 Tartarus Dorsa, also known as the bladed terrain, has several large swells with blade-like ridges on top. These blades are penitentes (elongate and thin blades of hardened snow), but much larger than Earth’s. The blades are oriented N-S, shaped by solar insolation and atmospheric circulation. The blades form by preferential deposition of methane ice or sublimation of thick methane ice deposits. (NASA/Johns Hopkins University Applied

Physics Laboratory/Southwest Research Institute).

124 100 km

Figure 13.12 Haze layers extend 200 km above the surface of Pluto. The haze is made of tiny solid particles called tholins that form by photochemical changes in gaseous methane and nitrogen. These tholins fall down to the surface and collect in a dark reddish equatorial band, including Cthulhu Macula (Figure 13.1). (NASA/Johns Hopkins

University Applied Physics Laboratory/Southwest Research Institute).

125 Figure 13.13 Impact crater frequency varies with location on Pluto. Most of Pluto is very heavily cratered and indicates a surface age of 3.9 billion years old. (Mid latitude areas with low crater abundance were not imaged at a resolution sufficient to see craters. And the projection has “spread out” the craters near the north pole so that they seem farther apart than they really are. Sputnik Planitia is an exception, with no identified impact craters marring the surface, indicating a very young age, and possibly, presently active processes. (NASA/Johns Hopkins University

Applied Physics Laboratory/Southwest Research Institute).

126 Figure 13.14 Extensional tectonic features are seen all over Pluto's surface. The features can extend for hundreds of kilometers and have a few kilometers of relief. Many cut across impact craters while others have craters superposed on the fractures, indicating the wide range of time that Pluto has been undergoing tectonic stresses. All of the fractures are extensional, and no contractional tectonic features have been seen on Pluto, consistent with the freezing and expansion of a subsurface ocean (Moore and others, 2016; NASA/Johns Hopkins University Applied

Physics Laboratory/Southwest Research Institute).

127 100 km

Figure 13.15 Wright Mons is a large mound with a central depression that has been interpreted as a cryovolcano.

The sparsity of impact craters suggests a young age. Possible liquid water lava flowed from multiple vents in producing ridges and lobes. The summit crater is rounded by concentric fractures and probably formed by collapse as cryomagma was withdrawn (NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research

Institute PIA20155).

128 Pluto Charon Veneer of nitrogen, Water-ice methane, and carbon monoxide lithosphere 200 km thick Veneer of ammonia Water-ice lithosphere

Rocky core

Rocky core diameter 1700 km

Atmosphere of nitrogen and methane (temporary and tenuous)

Liquid water layer 100 km thick

Figure 13.16 The internal structure of Pluto is dominated by a large rocky core surrounded by a subsurface ocean of liquid water, and a thick water-ice mantle. Charon has a similar internal structure with the absence of a subsurface ocean. Neither world is thought to have a metal core. Pluto is so small accretion did not produce enough heat to melt refractory metal and allow it to separate from silicates, but it did produce enough heat to melt ice and allow the melt to separate from ice. (Modified from Johns Hopkins University Applied Physics Laboratory). 129 Figure 13.17 Pluto has five known moons, the four smallest are shown here. The moons are small, porous, and have elongate shapes. The moons are only a few tens of kilometers across and not large enough to become spherical by gravitational deformation. Pluto’s moons are named after Styx (the river of the underworld), Nyx (greek goddess of the night), (the hound of Pluto), and Hydra (the many headed serpent). (Weaver and others, 2016 and

NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute).

130 Modor Macula

Argo Chasma

Oz Terra

Serenity Chasma

Vulcan Planum

Mandjet Chasma

Figure 13.18 Charon, Pluto’s largest moon, is not as geologically diverse as its parent, but had a geologically active past. Oz Terra is very broken up by tectonic features. Large fault-bounded chasms encircle the equator, produced by extensional tectonism. Vulcan Planum is younger than Oz Terra and was probably resurfaced by cryovolcanic activity. The color of reddish Modor Macula is thought to be due to tholins produced on Pluto which then escaped and aggregated onto Charon’s polar regions. (NASA/Johns Hopkins University Applied Physics

Laboratory/Southwest Research Institute PIA19968).

131 Moated mountain

Figure 13.19 Kubrick Mons is a mountain encircled by a narrow moat on Charon’s Vulcan Planum. It is 20 km in diameter, 3 km high, and surrounded by a 2 km deep moat. The moated mountains are thought to be cryovolcanic in origin and are part of the thermal event that resurfaced Vulcan Planum (NASA/Johns Hopkins University Applied

Physics Laboratory/Southwest Research Institute).

132 Water ice and rock mixture Water ice Contraction(?) Extension

0 Age (billions of years) Liquid water Rock

500 Water

Depth (km) Rocky core 1000

4 3 2 1 Billions of years before present

Figure 13.20 The thermal history of Pluto starts with accretion of frigid ice (water, methane, and nitrogen) and silicate-dominated materials in the distant reaches of the disk of debris around the ancient Sun. Early on, accretionary and radiogenic heat allowed dense silicate rock to separate from ice and sink to the core. Part of the water ice mantle also melted and creating a layer of liquid water above the rocky core. The creation of this liquid water may have led to contractional tectonics (since liquid water is denser than water ice) though there are not contractional features on the surface (perhaps erased by later cryovolcanism and extensional tectonics). As Pluto lost its heat, the liquid water began to freeze forming a thicker icy lithosphere and leading to a change in tectonic regime.

As the ice froze and contracted, extensional tectonism dominated. Today, Pluto may still have a 100 km thick liquid water layer, and as it continues to freeze, Pluto will keep experiencing extensional tectonics.

133 A B Liquid Water

Equator

Tholin deposition Water ice C Water ice D on montes Sputnik Planitia thickening continues impact thickening Sputnik Planitia Tholin Deposition

Figure 13.21 (A) Pluto formed by accretion of silicate- and ice-rich materials (gray and blue respectively) in the cold outer Solar System. (B) The density differences of these materials led to differentiation, with silicates sinking to the center and liquid water rising to the top and freezing from the surface downward. Outgassing of Pluto’s volatile elements accompanied this differentiation and covering the surface with nitrogen and methane ice. With the formation of an atmosphere the nitrogen and methane reacted with sunlight producing tholins that create the reddish band along Pluto’s equator. (C) After differentiation a large object impacted Pluto, producing an enormous basin

Sputnik Planitia. Concurrently, the liquid water froze, thickening Pluto’s water ice lithosphere and expanding

Pluto’s surface generating extensional fractures. (D) Presently, Sputnik Planitia has filled with volatile ices producing a sheet of ice. Pluto’s water ice lithosphere continues to grow at the expense of liquid water, elongating and forming extensional fractures across the surface. Within a billion years or so, all of Pluto’s liquid water will be exhausted and tectonics will cease (Modified from James T. Keane, 2016, Nature). 134 REFERENCES USED: PLUTO

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