Planetary and other terrestrial worlds

Chapter 9

• Geology is the that deal with Earth’s physical structure, it history and processes that act on it. • An extension of this science is Planetary geology, the extension of the study to other solid bodies in the What are the terrestrial like inside?

Most of the terrestrial interiors are divided into three layers: • Core Mainly consist of high density material such as and • This is the layer that surrounds the core. It is rocky material that consist of that contains silicon, oxygen and other elements • This is the outer most layer. It is lowest-density rocky material such as granite and basalt ( a common form of volcanic ) The interior of the planets are layered because the material was melted. The heavier material sank towards the interior. Gravity was the force that drag the heavy material and left the lighter material to remain at the top. This process is called differentiation All the terrestrial planets went trough the process of differentiation The Earth’s metallic core consist of two distinctive regions: A solid inner core and a molten (liquid) outer core

Interior structure of the terrestrial planets Why is the Earth Differentiated?

• Begins by 1,500 K – 4.6 Billion years ago (age of ) • Differentiation - The Earth melts – 4.5 Billion years ago • Crustal Formation – 3.7 Billion years ago 2,000 – 2,500 K Melt iron

1/3 mass of Earth falls to the center in the form of molten iron  5000 K (molten Earth) What causes differentiation and geological activity

• Heat of accretion As collide and form a planet, the kinetic energy from the collision is transformed in heat which add to the thermal energy of the planet • Heat from differentiation In the process of differentiation, the dense material sink and the lighter material rises. The gravitational potential energy is transformed into thermal energy by friction • Heat from radioactive decay The material that built the planets contain radioactive isotopes of elements such as uranium, potassium and thorium. These elements decay into lighter elements releasing energetic subatomic particles that collide with the material heating it.

Heating mechanisms for a Cooling mechanisms for the interior of a terrestrial planet

Convection. Hot material expands and rises while cooler material contract and falls

Conduction. Transfer of heat from hot material to cooler material through contact

Radiation. A planet will lose heat to space through thermal radiation. A planet acts like a black body and emit radiation () . Because of their low temperature, planets radiate primarily in IR How do we know what is inside a planet?

The only planet for which we have information about the interior is the Earth. But the information does not come from direct sampling of the interior of the Earth Earth’s radius is 6,378 km. Drilling only can go not more than a few km (16 km). It is possible to sample only a small upper layer of the crust! For Earth, much of the information about the interior comes from seismic waves (waves generated during an earthquake) Seismic waves have been used to study the interior of the using monitoring stations left by the Apollo on the surface Seeing inside the Earth

• Drill a hole – Petroleum usually drill to ~ 6 km – Deepest hole: Kola Super deep borehole (~ 12 km) – Deepest we can go ~16 km P (pressure) and S (shear) Wave analogy How do we know anything about the interior of the Earth?

Seismology!

• Earth’s interior structure is probed by studying how seismic waves travel through it (we can only drill so far! – 16 km).

• Earthquakes generate seismic waves.

•P waves (pressure) can travel through the liquid core but they are deflected by the core

•S waves (shear) travel in the mantle but not through the core

• Waves are reflected and refracted by different materials and travel through these materials at different speeds (higher density material →faster speed). The Earth interior Using seismic wave and computer models it is possible to model the interior Earth radius = 6,378 km

• Mantle - 3000 km thick (80% of planet volume).

• Crust - 15 km thick (8 km under - 20-50 km under .

• Density and temperature increase with depth.

• High central density suggests the core is mostly nickel and iron.

• There is a “jump” in density between the mantle and the core caused by different materials.

• No jump in density between inner and outer core because material is the same and just goes from liquid to solid.

• The temperature in the core is about 5,000K and the density about 12 g/cm^3

•(Reference: Density of water is 1 g/cm^3 or 1000 kg/m^3) The surface area-to-volume ratio Mathematical Insight 9.1 For a spherical object (planet), r is the radius of the sphere:

Surface area = 4 π r²

Volume = 4/3 π r³

Surface area-to-volume ratio = Surface area/ volume = (4 π r²) / (4/3 π r³) = 3/r

The radius appears in the denominator. Larger bodies have a lower surface area- to-volume ratios.

If two objects start with the same internal temperature, the larger body cool off slower than the smaller body

Parameters that affect the geological history of a planet Shaping the

Process that shape a planetary surface: • Impact cratering Creation of bowl-shape impact craters by and striking the surface • Molten rocks or coming from the plantet’s interior in an eruption • Tectonics The disruption of the planet’s surface by the internals stresses • Wearing down or building up of features by effect of wind, rain, water and

Shaping the crust: Impacts on Earth

Barringer Located near Winslow,

The best preserved Diameter 1.2 km Age ~ 50,000 yr

Estimated impactor size 30-50 m

Estimated speed of impactor ~ 26,000 miles/hr Shaping the crust: Volcanoes

Mt. St Helen before and after 1980 eruption

Mauna Loa – 1984 Length ~ 75 miles Covers ~ half Big Island of Hawaii 33 eruptions since 1850s

Shaping the crust: Cordon Caulle (Chile). June, 2011 Shaping the crust: Cordon Caulle volcano (Chile). June 2011 Shaping the Crust: Tectonics

• The ocean floors are continually moving, spreading from the center, sinking at the edges, and being regenerated. • currents beneath the plates move the crustal plates in different directions. • The source of heat driving the convection currents is radioactivity deep in the mantle Shaping the Crust: Erosion and Deposition

• Erosion (breaking down) and deposition (building up) require the presence of a fluid (gas or liquid) • Water, rain, wind cause erosion Deposition from the Mississippi River erosion created the river created Louisiana Grand Canyon wetlands. and Mercury’s Surface

• Surface similar to the moon, large number of impact craters! •Old surface •No indication of •Craters have a flat bottom and have thinner ejecta rims than lunar craters due to higher gravity on Mercury than the moon •Craters not as dense as on the moon - filled by volcanic activity • Not dark features like the “maria” on the Moon • Caloris Basin, evidence of a large impact crater. It is the largest crater on the 10 image (Flyby in 1974- 1975) planet, about 1/2 Mercury radius Mercury, an image of half of Caloris Basin Image taken by Mercury’s Surface • Cliffs are seen on the surface •This features are not seen on the moon •They appear to be about 4 billion years old •They are not the result of plate tectonics •Probably the result of the surface cooling, shrinking and splitting at this time •Some are several hundred km long and has high as 3 km high •Cliffs may have formed when tectonic forces compressed the crust Mercury A recent image (false colors) taken by the Messenger spacecraft

The Messenger spacecraft is at the present in orbit around Mercury. It went into orbit in 2011 It is the first and only spacecraft to orbit this planet A high resolution image of Mercury taken by the Messenger mission A detailed view of craters in high resolution image of Mercury taken by the Messenger mission Lunar Geological History Moon surface

Energy from the formation caused at least the outer few kilometers to melt (deep ocean of molten rock) This took place about 3-4 billions years ago. Molten rocks flooded the largest impact craters. Maria (singular mare) are large impact craters flooded by lava. The dark color comes from dense, iron rich basalt that rose from the lunar mantle Lunar Surface , large scale features Lack of and water preserves surface features Maria, singular Mare (younger) – Highlands (older) – crust material mantle material (maria means • Elevated many km above maria “’) • Aluminum rich, low density (2,9 • Maria - darker areas resulting g/cm3). from earlier lava flow • Basaltic, relatively iron rich, high density (3,3 g/cm3). Lunar Highlands

Highlands - light, rough, heavily cratered Older areas compared to maria Heavy cratered compared with maria

Lunar Lowlands

Lowlands – dark, smooth Maria Basalt – fine grained dark rich in iron and magnesium (stuff that sank in magma ocean) Few hundred meters thick Younger than the high Less craters compared with highlands The formation of Mare Humorum Lunar maria and mountain ranges

Caucasus Mountain ↓

← Mare Serenitatis

← Mare Tranquillitatis Moon Volcanism: Maria (Dark Areas)

Mare Imbrium SW Mare Imbrium The volcanism took placeafter the impacts – most 3 – 1 billion years ago

Aristarchus Marius Hills Mountains

Montes Tenerife

Montes Appeninus (Mons 5.5 km)

Montes Alpes- Mons Blanc 3.6 km The Alpine is the long feature to the left of the image Lunar Erosion Lunar Craters - caused by impacts

• Pressure to the lunar surface heats the rock and deforms the ground. • Explosion pushes rock layers up and out. • The ejecta blanket surrounds the crater •It forms radial features around some craters. This are called rayed craters • In some cases, the material compressed bounces back and form a crater with a central peak

• Craters can be up to 100 km in diameter • A new 10 km crater is formed every 10 million years • A new 1m crater is formed each month • A new 1cm crater is formed every few minutes!

Cratering on the Moon

• The rate of cratering on the moon is determined from the known ages of the highland and maria regions. • The Moon (and solar system?) experienced a sharp drop in the rate of meteoritic bombardment ~ 3.9 billion years ago. • The rate of cratering has been roughly constant since that time. • Only a few craters appear on the maria, showing that they are younger •The highlands have a large concentration of craters Tycho Crater ~ 100 MY old Rays Fresh rays, young feature 85 kilometers across

Wall Floor Rim

Central Peak Lunar Impact Basins

Imbrium Rim Orientale Basin (Far side of the Moon) Big, frequent impacts until 3.8 billion years ago Impact events continue on all and planets today but at a much lower rate! Geology of

• geological features • Evidence of flow of water Major Martian geographical feature • Valley that extends one-fifth of the way around the planet(!) at the equator Bulge •Up to 120 km across and 7 km deep •Roughly the size of North America •The Grand Canyon would fit into one of its side "tributary" cracks •Lies on the equator - 10km high •NOT caused by water - probably due to •Less heavily cratered = Young (2-3 stretching and cracking when Tharsis bulge billion yrs) formed

Map from - 2001 Valles Marineris 120 km across and at least 2,000 km long Mars - Telescopic Exploration Giovanni Schiaparelli (1835-1910) •Mapped bright and dark regions •Saw polar caps which changed with seasons •Surface colors appeared to change - plant life? •Identified long narrow features he called them canali (grooves, channels). It was translated as canals… Percival (1855 -1916) •Built an observatory in 1894 in Flagstaff AZ to study Mars (now Lowell Observatory) •He purchase and used a 24” Alvan refractor for his observations • He thought that canals were used by a civilization to bring water from the poles to the equatorial desert . He argued that canal were the work of an intelligent civilization on Mars •Images taken by the Mariner and Viking space craft about a century later (1960”s) prove that there were no canals Map of Mars by Schiaparelli (1988)

• The largest volcanoes in the solar system are found in Mars •Shield volcanoes •None are known to be currently active but eruptions occurred 100 million years ago

Olympus Mons on Tharsis slope Its base is about 600 km across. Its peak stands about 26 km above the average surface level (about three time as high as Mount Everest)

Mars has a only 40 percent that of Earth, and its volcanoes rise roughly 2.5 times as high because of this. Liquid ? About 4 billion years ago Mars had a thicker atmosphere, warmer surface, and liquid Yes – Massive flow long ago water. Some recent evidence of minor water flow Runoff channels •Found in southern highlands •Extensive river systems (like Earth) •Carried water from highland to valleys

Mars Earth

Outflow channels •Caused by flooding •Found at the equator •Formed about 3 billions year ago A high resolution image of a Mars runoff channel Evidence of flow of liquid water Ridges of crater suggest the impact debris was muddy. This crater was probably made by an impact on icy ground More evidence of flow of liquid water on Mars Another image showing the erosion caused by flow of water in Mars.

The Oraibis crater is about 32 km across. The bottom is filled with . Image was taken by the Mars Express in May, 2011 What happened to the water?

• Liquid water (from runoff channels) froze into permafrost (water ice just below surface) and polar caps about 4 billion years ago • After ~1 billion years, volcanic activity heated the surface and melted permafrost • Flash floods created • Volcanic activity slowed after that and the liquid water refroze • Mars Global Surveyor in 2000 revealed “gullies” along the insides of craters - evidence for more recent existence of liquid water? Evidence of salty water running down the slopes of rims of craters (gullies). Salty water may stay in liquid phase for a short time due to its lower freezing point Polar Phoenix (Phoenix landed on Mars on May 25, 2008) More evidence of frozen water under the surface

A trench carved by the scoop of Phoenix lander (about 10 cm deep). The white material inside the trench is water ice that melted (or sublimated) soon after being exposed Mars most recent mission: rover Curiosity

•Curiosity landed on Mars on August 5th, 2012 inside the crater •The area may have been flooded by fast- moving water •A good site to look for possible organic material which may be indication of fossil bacterial life An image send by Curiosity (left) showing evidence of flowing of liquid water. Comparison with deposit of rocks in a dry river bed on Earth The Face on Mars In 1976, Viking orbiter 1 images revealed a mountain that looked somewhat like a face •Commented on in a NASA press release •Immediately seized by tabloids •Taken by some as proof that an advanced civilization existed on Mars •More likely just how our minds find familiar patterns •Below is a high resolution image taken a few years ago by the Mars Orbiter spacecraft Geology of The thick cloud cover of the planet completely hide the surface of Venus. No visual images from the surface of the planet are available. All the images shown here (except for the image from ) are images . Radar: Radio waves are emitted towards the surface , they bounce back and are received by the spacecraft spacecraft (1990-1994) used radar to map the surface of Venus. This were the first look at the surface of the planet.

Major geological features . Impact craters Venus has only a few impact craters. Most of the ancient craters have been erased. The few craters are large, there are no small craters suggesting that small bodies may burn in the thick atmosphere before they reach the surface.

Geology of Venus Major geological features . Volcanic and tectonic features Evidence of volcanism is clear. Several volcanoes were imaged by the radar. There is evidence of lava flow, lava plains and volcanic mountains. The radar images do not show active volcanoes or eruptions. The presence of sulfuric acid clouds suggest that outgassing is replenishing the atmosphere of . is removed from the atmosphere by chemical reaction with rock. This suggest that outgassing must still occur at least in geological time scales (millions of years) . Weak erosion The images returned by the Venera mission does not show evidence of erosion. Since the temperature is high, there is no precipitation. Wind must be slow at the surface. . No evidence of plate tectonics. All the surface seems to have the same age everywhere. Radar maps of the surface doesn’t show the types of plates we find on the Earth.

Venus’s Surface

Radar (radio waves) echoes reveal the surface topology (Magellan spacecraft) There are no direct images of the surface. The planets is always covered by thick clouds. Only radio waves can penetrate the clouds •Elevated “continents” make up 8% of the surface (25% on Earth) •Mostly rolling plains with some mountains (up to 14 km) •No indication of plate tectonics •Buckled and fractured crust with numerous lava flows Venus’s Surface: Volcanoes and Craters Images from the Magellan spacecraft (1990-1994)

•Volcanoes resurface the planet every ~300 million years •Shield volcanoes are the most common (like Hawaiian Islands) • A caldera (crater) is formed at the summit when the underlying lava withdraws

•Largest volcanic structures are called coronae - upwelling in the mantle which causes the surface to bulge out - not a full- fledged volcano. •Usually surrounded by other volcanoes •Venus is thought to still be volcanically active today. Magellan radar images did not show any active volcano. Radar image of a volcano with lava flow taken by Magellan spacecraft A 3-D view of three volcanoes taken by Magellan spacecraft (radar images) Radar images taken by Magellan of “pancake” volcanoes An image taken by the soviet spacecraft Venera. The spacecrafts Venera landed on the surface of Venus in 1970s • The spacecraft had a very short life time on the surface, survived only an hour before the heat damaged the equipment. •No other spacecraft have landed on the surface of Venus after the Venera •Little evidence of erosion - young surface •Rocks are basaltic and granite • The thick cloud cover makes Venus seem like a heavily overcast day on Earth all the time! The unique geology of Earth • Plate tectonics • How was the earth surface shaped by plate tectonics? Overview of the Earth

• Dense rocky composition • Evolving surface < 600 Myr old • Atmosphere mostly Nitrogen and oxygen • of Water •

Mass 6 x 1024 kg Radius 6400 km Density 5.5 g/cm3 Surface gravity 9.8 m/s2 Escape Speed 11 km/s The Earth Interior

• The Earth is very dense: 5.5 g/cm3 – Reference: water 1 g/cm3 – Normal Rocks 2-4 g/cm3 – Pure Iron 7.8 g/cm3

The Earth’s interior is differentiated (distinct layers – densest sink, lightest floats) . At some point in our past, Earth melted . bombardment of debris . radioactivity The Earth Interior

Layer Comp Type Thickness Temp Core Iron & Nickel Solid/liquid 3500 km >6000 K Mantle Plastic 2900 km 1300 – 3800 K Crust Granite & Basalt Solid 10-1100 km <1000 K Plate boundaries The motion of the Earth surface

• The plate tectonic theory explain how the plates moves and drift as they “float” on top of the mantle () • The theory was proposed in 1912 by the German . The idea was that continents drift across the surface of the Earth. • But at that time, nobody knew about a mechanism that will allow the continents to move. • But today there is more evidence to back up the theory and it is widely accepted. • The discovery of mid-ocean ridges and the seafloor spreading in the 1950’s provided more evidence. • Today the continental motion is explained by convection, driven by heat released by the Earth interior Seafloor spreading

Convection pushed the material up into the ridges in the ocean floor separating the two plates pushes denser seafloor crust material under layers of less dense continental crust California’s San Andreas

• The San Andreas fault is the boundary where two plates are sliding sideways • It is very active • The dates mark the most recent earthquakes • The lines in the inset show how far the two sides of the fault shift moved in an earthquake Where do we find volcanoes? An example are the Hawaiian islands

Shield volcanoes are formed by “Hot Spots”. As the plate moves over the “hot spot” multiple volcanoes are formed.

Stratovolcanoes typically where one plate descends beneath an adjacent plate. Hawaiian Island hot spot Geological history of the terrestrial planets Continental Drift Past present and future of Earth’s continents About 200 millions years ago all the continents were together in a single “supercontinent” called Pangaea Production of magnetic fields in planets

Creation of magnetic field in planets: .An interior of electrically conducting fluid (Liquid). Charged particles move with the molten or conductive material .Convection in that layer .Rapid rotation

.The Earth meet all the requirements .Venus has a very slow rotation rate (-243 days), no magnetic field .Mars may retain enough heat but not enough to have convection .Mercury is a mystery: It has some magnetic field but has a slow rotation (58.6 days). It is possible that it has a molten core with convection (or the magnetic field could be relics of ancient magnetic field frozen in the core?) How are the planetary magnetic fields generated? – metallic liquid regions plus rotation of planet – & : liquid metallic – & : near surface convecting ? Earth Magnetic Field

• Earth’s magnetic field protect us from energetic particles in the solar wind and cosmic rays. • These particles do not strike directly the surface, they are trapped in the magnetic field lines. Auroras • Some charged particles from the solar wind get trapped in the Earth’s magnetic field lines. They will spiral toward the magnetic poles and precipitate in the atmosphere releasing energy . • That energy excite the atoms in the upper part of the atmosphere and they emit light. The emission is line emission. The colors of the aurora depends on the atoms being excited.