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Planetary Geological Processes See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/301371064 Planetary geological processes Conference Paper · November 2014 DOI: 10.1063/1.4902843 CITATIONS READS 0 2,860 2 authors: Rosaly M. Lopes Anezina Solomonidou NASA European Space Agency 405 PUBLICATIONS 7,240 CITATIONS 103 PUBLICATIONS 399 CITATIONS SEE PROFILE SEE PROFILE Some of the authors of this publication are also working on these related projects: Thermophysical, Rheological, and Mechanical, Property Measurements on Icy Compositions with Application to Solar System Ices View project Titan alluvial and fluvial fans View project All content following this page was uploaded by Rosaly M. Lopes on 22 June 2016. The user has requested enhancement of the downloaded file. Planetary Geological Processes Rosaly M.C. Lopesa and Anezina Solomonidoua,b aJet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109 bLESIA - Observatoire de Paris, CNRS, UPMC Univ. Paris 06, Univ. Paris-Diderot – Meudon, 92195 Meudon Cedex, France Abstract. In this introduction to planetary geology, we review the major geologic processes affecting the solid bodies of the solar system, namely volcanism, tectonism, impact cratering, and erosion. We illustrate the interplay of these processes in different worlds, briefly reviewing how they affect the surfaces of the Earth’s Moon, Mercury, Venus and Mars, then focusing on two very different worlds: Jupiter’s moon Io, the most volcanically active object in the solar system, and Saturn’s moon Titan, where the interaction between a dense atmosphere and the surface make for remarkably earth-like landscapes despite the great differences in surface temperature and composition. Keywords: impact cratering, volcanism, tectonism, erosion, terrestrial planets, Io, Titan PACS: 96 INTRODUCTION The solid bodies of the solar system have different surface appearances because of the existence and relative importance of the major planetary geologic processes. The surfaces we see today have been shaped by the interplay of endogenic (volcanism, tectonism) and exogenic (impact cratering, erosion and surficial) processes. Understanding the distribution and interplay of endogenic and exogenic processes on a planet is important for constraining models of the interior, surface-atmosphere interactions and climate evolution. This chapter will review the major types of solid bodies in the solar system in terms of comparing the role of these major geologic processes. The surfaces of the Moon and Mercury are characterized by numerous impact craters, while the surface of Venus and Io are dominated by volcanism, with impact craters being totally absent on Io. Saturn’s moon Titan presents, like the Earth, significant effects from erosion by liquids and wind. To understanding the evolution of these different worlds, we need to understand how planetary geologic processes operate. The chapter aims to give an overall introduction to the very complex field of planetary geology. Our Geologically Diverse Solar System Studies of planetary geology can greatly help us to understand how the planets and moons of the solar system were formed, and how their current surfaces came to be. Analyses of rocks, either in-situ or brought to Earth via meteorites or space missions can help determine the composition of the planet or moon as a whole and from this to infer the composition of the material from which those bodies were originally formed. Studies of the geology of other planetary bodies largely rely on studies of the geology of our own planet. However, there are major differences. The Earth’s surface is largely dominated by plate tectonics, in which large plates of the crust move and create mountain chains (where plates collide), subduction zones (where one plate dives under another) and spreading ridges (where plates are moving apart). No other body in the solar system is known to have plate tectonics. One consequence of plate tectonics is the destruction of old crust. If we compare the Earth’s surface with, for example, the Moon’s, we immediately see that much of the Moon’s surface is covered by impact craters, while few impact craters can still be found on the Earth. Other than plate tectonics, the other destructive process on Earth is erosion. The Earth is the only planet with the right combination of atmospheric surface pressure and temperature to allow liquid water to exist and to cover such a large fraction (~70%) of the surface. Impact craters are erased relatively rapidly on Earth by the action of plate tectonics and erosion due to weather, as well as volcanism. The Earth’s atmosphere does offer our planet some protection from small meteorite impacts and, for this reason alone, would have a different crater size and frequency distribution from that of an airless body. The planets of the solar system can be divided into two major compositional groups. Mercury, Venus, Earth, and Mars are known as the terrestrial (Earth-like) planets and are characterized by silicate compositions and iron cores. They formed much closer to the Sun than the outer planets, in the part of the solar nebula which was too warm for ices to condense. The terrestrial planets all have solid surfaces where the major geologic processes have operated. Mercury is heavily cratered, its proximity to the Sun allows for impactors to have high encounter velocities. The lack of any significant atmosphere on Mercury also means that, not only it does not have any protection from oncoming objects, but also it has had no erosion by weather. Similarly, the lack of plate tectonics means that old crust was largely preserved. If we consider impact craters, knowing that the oldest surfaces of the solar system will have the largest impact scars and the largest numbers of craters (1), then next to Mercury is Mars, with also a considerable number of craters, largely because of its proximity to the asteroid belt. However, the surface of Mars shows considerable signs of active geology such as volcanism, tectonism, and erosion. The surface of Venus has fewer impact craters than Mercury or Mars, and is dominated by volcanism. The dense Venusian atmosphere (with a surface pressure of 94 bar) is capable of protecting the surface from some impactors, which break up in the atmosphere. Also, volcanic activity has resurfaced large areas of the planet, erasing the evidence of craters. The small number of impact craters recognized on the Earth’s surface or in the oceans (184 according to (2)) attest to how geologically young the surface of our planet is. FIGURE 1. Montage of images of planets in the solar system (not to scale). At the top is an image of Mercury, showing its old cratered surface imaged by Mariner 10. Next down is Venus, its volcanic surface revealed by the radar instrument on the Magellan spacecraft. The Earth and the Moon, imaged by the Galileo spacecraft, further illustrate the diversity of geology, the ancient surface of the Moon contrasting with the young surface of the Earth. Mars (imaged by Mars Global Surveyor) shows both ancient cratered surfaces and younger terrains created by volcanism and erosional processes. The Jovian planets (Jupiter imaged by the Cassini spacecraft and Saturn, Uranus, and Neptune imaged by Voyager) are gas giants with no solid surfaces, but have a plethora of moons showing yet more of our solar system’s diverse geology. The Jovian (also called Jupiter-like or gas giants) planets are Jupiter, Saturn, Uranus, and Neptune. Because of their primarily gaseous compositions, they do not have solid surfaces where the geologic processes described above operate, thus they are thought to have silicate-iron cores. Of particular interest to geologic studies are their moons. In fact, the moons of our solar system display remarkable geologic diversity. The Moon is the best studied extra- terrestrial body so far, and the only one for which we have collected samples and brought them back to Earth for study. Our Moon has a silicate composition similar to the Earth’s mantle, and a small iron core. Mars has two moons, Phobos and Deimos, both are small and irregularly shaped. Their surface compositions appear to be similar to carbonaceous chondrite meteorites in composition. These moons may be captured asteroids and may have formed elsewhere in the solar system. Venus and Mercury have no moons. In contrast, the gas giants offer a large number of moons that, even within one system, show remarkable diversity in the surface geology. Jupiter’s four major satellites are a good example of such diversity, ranging from Io’s volcanically dominated surface to Europa’s icy young surface, Ganymede’s tectonic activity and evidence of resurfacing at some time in the past, to Callisto’s heavily cratered, ancient surface. Saturn’s largest moon, Titan, is the only moon in the solar system with a substantial atmosphere, allowing for erosion processes to be rampant. Tiny Enceladus ejects volcanic plumes from its southern polar region, our only uncontested example so far of cryovolcanism, a type of volcanism in which water, rather than molten rock, is the magma (3). Uranus and Neptune have many icy satellites, with Miranda being particularly unusual because of its complex surface geology while Triton shows evidence of recent cryovolcanism, as well as plumes probably caused by solar heating. Many other small worlds in the solar system display interesting geology, and we look forward to finding out what the surface of Pluto will be like, as the New Horizons spacecraft approaches. FIGURE 2. The four largest moons of Jupiter, known as the Galilean satellites, show how different geologic processes have shaped their present surfaces. These moons were first seen by Galileo Galilei in 1610. Left to right and in increasing distance from Jupiter are Io, Europa, Ganymede, and Callisto. The distances of these moons from Jupiter help explain some of the differences in their composition and geology.
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