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Chapter 5 : Planetary Surfaces

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Planetary Surfaces : Topics to be covered

• Mineralogy and Petrology (5.1)• Surface Morphology (5.3)• Impact Cratering (5.4)

– 5.4.1, 5.4.2, 5.4.4.1• Surface Geology of Individual Bodies (5 5 to 5 5 4• Surface Geology of Individual Bodies (5.5 to 5.5.4

inclusive)

Agenda:• Part I : Rocks and Magma• Part II : Planetary Surfaces – Qualitative

Descriptions• Part III : Impact Cratering 2

I. Rocks and Magma

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Solid Surfaces• We concentrate on the solid surfaces of the smaller

bodies in the solar system: asteroids, terrestrial planets, moons.

• We detect: surface structure, albedo variations, fractures mountains volcanoes canyonsfractures, mountains, volcanoes, canyons

• What processes could be in common?– Plate tectonics– Volcanism– Impact cratering– Atmospheric weatheringFirst we will talk about the surface material (rocks).Then we’ll talk about the processes that affect the rocks.

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Primary materials

• The primary building blocks of the planets are metals (ρ~ 8 g/cm3), silicates (ρ ~ 3 g/cm3), ices (ρ ~ 1 g/cm3) and H & He (ρ low, but varies with pressure)

• The bulk densities and other important properties of the planets are a direct result of the fact that materials with higher condensation temperatures were able to condense closer to the Sun when the planets were forming. 5

Mineralogy and Petrology• Petrology is the study of rocks, their structure,

composition and origin.• Mineralogy is the study of minerals, the solid chemical

compounds that occur naturally. Each mineral is characterized by a particular chemical composition and crystalline architecture.

• There are different minerals which have the same chemical composition but different structure (eg diamond and graphite).

• Minerals can be identified in the field with a combination of hardness, cleavage (breakage along planes), density, colour, luster and streak (colour of the powder that comes off when scraped). 6

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Chemical classes of minerals

• Minerals are often classified by which ions are present.

• For example:H lid (Cl F B I )– Halides (Cl-, F-, Br-, or I-)

– Carbonates (CO32-)

– Phosphates (PO43-)

– Sulfides (S2-)– Hydroxides (OH-)

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Silicates

• The crust and mantle of the Earth consist primarily of silicates, containing primarily silicon and oxygen.

• Examples:– quartz SiO2

– Olivine (Fe,Mg)2SiO4. For olivine, Fe and Mg can be substituted for each other.

– Feldspar (K,Na)AlSi3O8 or CaAl2Si2O8.

– Pyroxene (Fe,Mg)2Si2O6

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Silicates• Feldspars make up 60% of surface rocks on Earth.• They are low density (2.7 g/cm3) and float upwards in

magma• Potassium rich feldspars are orthoclase, while feldspars

rich in sodium and/or calcium are called plagioclaserich in sodium and/or calcium are called plagioclase.• Quartz has a similar density, and is likewise common in

surface rocks.• Pyroxenes make up 10% of the Earth’s crust. They

contain Mg and Fe, and have densities between 2.8 and 3.7 g/cm3.

• Olivine is the densest of the silicates and is an important constituent of rocks formed at depth.

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Oxides• After silicates, the next most common minerals are

oxides: metals and oxygen.• Magnetite: Fe3O4 (black metallic or reddish)• Hematite: Fe2O3 (reddish brown or steely gray)

G thit HF O ( ll i h b t t )• Goethite: HFeO2 (yellowish brown to rusty)• Limonite: FeO(OH)·H2O (orange brown)These rusts are probably responsible for Mars’s red colour.

• Ilemenite (Fe, Mg) TiO3 (black, opaque) and Spinel MgAl2O4 provide most of the opacity of the Moon’s maria.

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Other common minerals• Pyrite (FeS2) (Fool’s gold)• Triolite (FeS)These are high density (5 g/cm3), and probably abundant in

planetary interiors

• Clay minerals are hydrous aluminum silicates. They can contain a lot of water.

• These exist on Mars and on Earth and have been found on carbonaceous asteroids.

• Clay minerals can behave as catalysts for biological molecules

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Ices

• On Earth, we tend to neglect ices as minerals, but they are important constituents of bodies in the outer solar system.– Water ice: H2O– Water ice: H2O– Carbon dioxide: CO2

– Methane CH4

– Ammonia NH3

• Hydrate minerals (like clays) can also contain water• Water ice can exist in clathrates, where a guest molecule is caged in the water ice lattice.

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Rocks

• Assemblages of different minerals are called rocks

• Rocks are classified by their mineral content and formation historycontent and formation history.

• We will discuss four types: primitive, igneous, metamorphic andsedimentary.

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Primitive Rocks

• Primitive rocks formed directly from the protosolar nebula.

• Elemental ratios are the same as the solar abundances.

• The only samples we have of primitive rocks are chondritic meteorites.

• We’ll discuss these when we cover asteroids and meteorites.

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Igneous rocks• Igneous rocks form from cooling magma.• Intrusive or plutonic rocks form underground.• Extrusive or volcanic rocks form near or above ground.• Temperature, pressure and cooling time control the

f t lli tiprocess of crystallization.• Intrusive rocks tend to be coarse grained (large crystals)

from slow cooling.• Volcanic rocks tend to be fine grained, even glassy for

very rapid cooling (no crystals, amorphous structure).• The texture depends on the cooling time: the

composition depends on the order in which crystals come out of solution. 17 18

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Basalts and Granites• Igneous rocks are classified by silicate content.• Basalts have a low silica abundance (40 – 50%

by weight). They also tend to contain heavy minerals such as pyroxenes and olivines. They are basic (Mg) or mafic (Fe). They are usually dark coloured, and common on the Moon.

• Granites contain ~70% silica by weight: they contain feldspars and quartz. They can be felsic(more feldspar) or silicic. They are lighter in colour, and rare except on Earth. 19

Magmas in Volcanoes• Felsic lava is more viscous than basaltic lava.• Felsic volcanoes tend to be explosive.• Basaltic lavas have low viscosity and tend to fill in

lowlands: ex Hawaiian volcanoes, lunar maria.Th t t d li t f th l l• The gas content and cooling rate of the lava can also determine the type of volcanic rocks associated with a volcano.

• Pumice is formed by rapid cooling of gas rich lava. More common in felsic lava where gas cannot escape.

• When blasted pieces of rock (pyroclasts) fall, the rock can be cemented together, forming volcanic tuff and breccias (conglomerates of broken rock)

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Metamorphic rocks

• Rocks that are transported to depths, heated and kept under pressure, undergo chemical and crystalline transformation. In general crystals become largergeneral, crystals become larger.

• Similar transformation could take place as a result of impact-induced shocks at impact sites. “Shocked quartz” has been found at impact sites.

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Sedimentary rocks

• Material transported by winds or water will eventually become rock.

• Minerals and grains can be sorted by size and weight.• Sandstone makes up 20% of sedimentary rocks on EarthSandstone makes up 20% of sedimentary rocks on Earth.• Shale, made of clays and silt, makes up 70%.• Remaining 10% is limestone, derived from the interaction

of dissolved CO2 with silicates to form calcium carbonate or calcite.

• On Earth, most limestones are biological, fossils of animal shells.

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Chemical:Limestone

Clastic:Sandstone

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Sedimentary rock on MarsLayered Martian sedimentary rock in some locations is expressed as alternating bands of light- and dark-toned material. Outcrops of this type are common in Mars Global Surveyor (MGS)Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC) images of Holden Crater (26.5°S, 33.9 °W).

The bands might have resulted from deposition of sediment in a lake that would have occupied Holden Crater. Alternatively, there could be materials deposited by falling out of the air, the same way that volcanic ash is deposited on Earth.

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The Rock Cycle

-Melting & Intrusion-Solidification of melt-Mountain BuildingUplift & Exposure-Uplift & Exposure

-Weathering-Erosion & Transport-Deposition & Burial-Metamorphism-Melting & Intrusion

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II. Planetary Surface Morphology – Qualitative descriptions for

Solar System BodiesSolar System Bodies

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Surface morphology

• A variety of features are observed on solid bodies: basins, craters, faults, mountains, volcanoes, lava lakes, scarps

• Endogenic processes (from the body itself)• Endogenic processes (from the body itself)• Exogenic processes (from outside)

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Surface/atmosphere

• The surface and atmosphere are related: e.g. CO2 cycle on Earth

• Volcanism on Io dominates the atmosphereatmosphere

• Volcanism and tectonics can reform the surface.

• The surface can be dated based on morphology (craters are removed, lines are broken).

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Convection cells in the mantle transport heat from deep inside the Earth, and ultimately drive plate movement.

Begin with the Earth:

The creation of plates at ocean ridges, and eventual subductionpushes and drags the plate, producing the motion we observe.

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• Earth’s crust is broken into a dozen distinct plates which “float” on the denser mantle underneath.

• Where two oceanic plates move apart, sea-flooring spreading occurs and new crust is created.

• Where two continental plates collide, material “piles up” and mountain ranges form.

• Denser ocean plates sink under lighter continental crust. 31

• The plates move about one cm/year, and over time continental landmasses merge to form large supercontinents, and then separate again.

• Where plates collide/sink friction creates heat (Volcanos) and sliding plates cause Earthquakes.

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Tvashtar volcano on Jupiter’s moon Io

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Infrared view of Io going into Jupiter’s shadow

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Europa’s ice flows

Spreading ridges

Few craters

Young ridges: possible cryovolcanism or eruption of ice and gases. Galileo, NASA

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Venusian faults

Radar image of impact crater cut by a later normal fault. Image is 100 km wide, in the rift ,valley of DevanaChasma. From the pressures needed to form faults like this, Venusian faults must be stronger than those on Earth.

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Venusian volcanism

• Hundreds of thousands of volcanoes• Volcanism is widespread and planetwide• Vast “mare-like” lava plains dominate• Many individual cones and mountains• Also evidence for subsurface volcanism• No direct evidence for ongoing volcanism

(?)

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Surface expression of upwelling p p gmagma in coronae (top). Low density magma reaching near the surfaces pushed upward on overlying crust creating a dome structure

Mountain ridges of various scales are present on Venus (left). These may reflect crustal shrinkage due to cooling or upwelling tectonic processes

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Continent-like highlands – lower density rock?

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Martian Surface Features

• Largest volcanoes (Tharsis rise, Olympus Mons) largely confined to one region

• Impact craters (with obvious weathering) with highImpact craters (with obvious weathering) with high number density in Southern “highlands” and much lower in northern hemisphere

• Valleys and Canyons, some of which show evidence for running water

• No mountain chains, only shield volcanos (associated with mantle hotspots): no active plate tectonics on Mars. 40

Mars: Topography• Crustal dichotomy: North is low; South is high• Highest point: +27 km (Olympus \Mons)• Lowest point: -8 km (Hellas Basin)

(referenced to 6.1 mbar atmospheric pressure)

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Solar system’s largest volcano:Olympus Mons (shield

l )volcano) on Mars

Mars Global Surveyor46

NASA’s Mars program

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Water on Mars

• Geomorphology Evidence:– Small valley networks– Outflow channels

Gullies– Gullies– Paleolakes/seas & Paleoshorelines (?)

• Other Evidence– Martian meteorites– Hematite-rich evaporitic sulfate deposits– D/H: 5x Earth’s ratio

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“Splash” craters : mud-flow appears to surround formation of a new (large) crater. Water-ice under the surface (permafrost) may be quickly released due to heating by impacts, thus producing underground or near surface mud flows at the time of crater formation. 49

Do features seen in this image look like typical seashore landforms found on Earth?--i.e., are these beaches, windblown coastal dunes, or even wave-cut cliffs??

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• MER Odyssey Water Evidence– hydrated iron sulfate mineral

called jarosite– presence of sulfates– rocks' physical appearance,

such as niches (vugs) where crystals grew

– spherules and crossbeddingHematite in “blueberries”– Hematite in blueberries

• Interpretation : Rocks have originally been laid down by minerals precipitating out of solution at the bottom of a salty lake or sea

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Gamma-ray emission from Martian poles shows strong hydrogen line; only plausible source is water ice.

Very similar to Gamma-ray emission seen by Lunar Prospector at lunar poles

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Mercury

Similar to Moon, except:•Smooth areas•Higher albedo•ScarpsL d•Large radar

reflection from poles•Shallower craters•High mean density•Secondary craters and ejecta curtains are closer to main crater rim

Messenger55

• Small intercrater plains may represent surface lava-flows from the time of mercurian formation

• Lightly cratered surface+very flat-terrain argue for melting early in Mercurian history

• Surface ridges (scarps) attest to global shrinkage due to planetary-scale cooling

Mercury Surface Morphology

cooling• Probably a differentiated body• Reflectance spectra show no indication

of iron in minerals on surface

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Caloris basin in Mercury

Formation likely same as maria on moon – giant impact near the end of the LHB

Seismic waves from impact affected the entire planet –antipodal region of Mercury shows hilly and lineated terrain caused by seismic focussing from impact

False colour image, NASA - Messenger 57

Rough terrain on MercuryMessenger image of jumbled, chaotic terrain exactly opposite the Caloris Basin. It’s believed that seismic waves generated by the impact were focused to this point.

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Lunar highlands

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Lunar SurfaceMorphology - I

• Heavily cratered regions – Lunar Highlands– primitive surfaces showing late-heavy bombardment– Primary mineral : anorthosite (related to feldspars –

lightest crystals formed from olivine-like melts)– Likely represents floating feldsic “rockbergs” in the

primitive lunar magma ocean at the time of the moon’s formation

– Deep craters in the highlands imply this anorthosite crust must be deep (tens of km at least)

– Implies that large fraction of the moon was once entirely melted, perhaps during formation in grazing impact with the Earth?

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Mare• Lava-covered plains, few craters therefore younger• Dark colour is due to basalt (igneous rock)• Moon mare are exceptionally flat because magma was

especially hot (1400-1600 K) and therefore very fluid.

For comparison: lunar highlands

Mare Crisium on the Moon62

Lunar SurfaceMorphology - II

• Lunar lowlands – Maria – Younger surfaces than highlands (fewer craters) with rock

ages between 3.2 and 3.9 Ga– Rich in Basalts and very flat (basaltic lavas)y ( )– Result of lava outflows after either large impacts several

hundred Ma after lunar formation (about 10-40 m deep) or buildup of delayed localized heating by radioactive decay

– Elemental composition of maria basalts are depleted in siderophile (iron-dissolving) elements and enriched in lithophiles (which tend to rise to the top of melts)

– Maria rock further suggests moon has been melted and that the magma is from deeper in the lunar mantle than rock from the highlands 63

From Hartmann, 1999

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