Earth and the Geology of the Terrestrial Planets (Bennett et al. Ch. 9) Major Ideas In This Chapter ● Terrestrial planets looked (largely) the same when they were formed. Differences due to geological processes. ● Geological activity is driven by internal heat ● Planetary size plays a large role in retaining heat ● Distance from the Sun, rotation affects erosion ● Crater density can indicate surface age ● Earth has a unique geology Terrestrial Planets ● Compared to Jovian planets: – Smaller size/mass – Large “core” to atmosphere ratio – Higher density – Closer to Sun and closer together – Warmer – Few or no moons – No rings (NASA) Planetary Surfaces and Interiors ● Terrestrial planets + Moon were similar when young – Subjected to heavy bombardment – Differences due to processes that occurred after formation ● Understanding the surface features: planetary geology ● Processes in the interior drive activity at the surface Your book uses “terrestrial worlds” to refer to the terrestrial planets + the Moon. (from Bennett et al.) (from Bennett et al.) How Do We Learn About Planetary Interiors? ● Average density determinations ● Local gravity variations as measured with artificial satellites ● Magnetic fields: molten core/convection ● Lava flow: internal composition ● Earthquakes: internal structure Earthquakes: Seismic Waves ● Earthquakes generate vibrations – Typical wavelength ~ several km – Reconstruct interior ● Two types of waves: – P-waves: compressional waves – S-waves: shear waves ● S-waves cannot pass through liquid (from Bennett et al.) – Earth's interior has liquid layer ● Monitoring also done on the Moon (from Morrison and Owen) Interior Structure of Terrestrial Planets crust ● Density stratification – Core ● iron, nickel ● Earth has liquid outer core – Mantle core ● Rocky layer (minerals with silicon, oxygen, ...) – Crust mantle ● granite, basalt lithosphere Interior Strength crust ● Most of earth's interior: solid rock – Rock varies in strength – Can deform and flow ● Lithosphere ● Below lithosphere: higher T → core rock flows easier ● Lithosphere “floats” on the soft rock below mantle ● Thickness important lithosphere How does lithosphere thickness affect volcanic eruptions/mountain formation? Why Layering? ● Differentiation – Gravitational separation of materials with different densities ● Interiors were hot initially → rock/metal molten Why are planets round? Phobos and Deimos (the moons of Mars) (NASA) Planetary Interiors (from Bennett et al.) ● We expect smaller planets to have smaller cores – Mercury? – Moon? ● Small planets = thicker lithospheres What Drives Geological Activity? ● Heat – In general: bigger = more heat ● How do we heat? – Accretion – Differentiation – Radioactivity Which of these processes is still taking place in terrestrial planets? What about sunlight? These processes result in the core/mantle/crust structure What Drives Geological Activity? ● How do we cool? – Convection – Conduction – Radiation ● Example: Earth: – Convection in interior (flowing solid rock) – Above lithosphere, too rigid to flow—conduction takes over – At surface: radiation What Drives Geological Activity? (from Bennett et al.) Planetary Size ● Larger planets remain hotter longer ● Mercury/Moon – Cooled quickly (~ 1 billion years) – Lithosphere thickens, mantle convection stops – Geologically dead ● Venus – Similar in size to Earth, so probably still active ● Mars – Cooled more—unclear if the deep interior is still convecting Cooling Terrestrial Planets Interiors Total store of heat is proportion to the planet's volume, Energy is only lost through the surface—rate of energy loss is proportion to the surface area of the planet, Cooling time is related to the total amount of heat/energy stored / rate of energy loss (volume to surface ratio) Planetary Cores and Magnetic Fields ● Magnetic fields are generated in some planets ● What is needed to generate a magnetic field? B-field why? Mercury yes large metal core (despite slow rotation) Venus no rotation too slow Earth yes molten rock Moon no cooled off Mars no no metallic core or cooled Planetary Cores and Magnetic Fields (from Bennett et al.) Shaping Surfaces ● Impact Cratering – More small than large craters – All terrestrial planets had impacts – Impact at 40,000 to 250,000 km/h ● Craters are circular ● D ~ 10x impactor size ● Depth ~ 10-20% diameter ● Sometimes: central peak Tycho crater on the Moon ( NASA) (from Bennett et al.) Impact Craters Shaping Surfaces ● Volcanism – The eruption of molten lava onto surface – Magma rises: lower density / trapped gases / squeezed – Result depends on how easily lava flows (from Bennett et al.) Shaping Surfaces ● Volcanism (cont.) – Volcanic plains and shield volcanoes made of basalt (high density, but runny) ● all terrestrial planets and some Jovian moons show volcanic plains or shield volcanoes—basalt common – Stratovolcanoes made of lower-density rock—rare outside of Earth. – Volcanoes outgas atmospheres ● Atmospheres of Venus, Earth, and Mars, and Earth's oceans came from outgassing Solar System Volcanos The Culann Patera volcano on Jupiter's moon Io (Galileo Project, JPL, NASA) Olympus Mons on Mars—the largest volcano in the solar system (Mars Global Surveyor Project, MSSS, JPL, NASA) Shaping Surfaces ● Tectonics – Surface changes due to forces acting on lithosphere – Most tectonic features arise from mantle convection ● Compression features ● Cracks and valleys – Fractured lithosphere → plate tectonics (from Bennett et al.) Shaping Surfaces ● Erosion – Breakdown/transport of rock ● Glaciers ● Rivers ● Wind ● ... – Erosion can build (sand dunes, river deltas, ...) – Erosion makes sedimentary rock Effect of Planetary Properties ● Volcanism/Tectonics – Requires internal heat → planetary size matters ● Which planets had volcanism/tectonics initially? – Moon/Mercury already cooled – Earth large → still active – Venus similar to Earth → still active? – Mars should be cooler inside, much less activity than past Effect of Planetary Properties ● Erosion – Requires weather (wind, rain, ...) – How does planetary size affect an atmosphere? – Distance from Sun (how does this affect things?) – Rotation (why?) – Moon/Mercury: no atmosphere → no erosion – Mars: thin atmosphere → little erosion – Venus/Earth: thick atmospheres ● Earth cooler: oceans form. Still lots of erosion. ● Venus slow rotator: little erosion (from Bennett et al.) (from Bennett et al.) Impact Craters and Age maria highlands ● All planets impacted during heavy bombardment – Old surface = high crater density ● Lunar highlands – Age ~ 4.4 billion years ● Maria – Age ~ 3.0 – 3.9 billion years ● Heavy bombardment ended ~4 billion years ago ● Impact history on moon applies to other planets Apollo 16 image of the moon (mostly far ● Crater counts → geological age side) (NASA) Geology of the Moon and Mercury MESSENGER image of Mercury Apollo 16 image of the moon http://messenger.jhuapl.edu/gallery/sciencePhotos/image.php?page=1 &gallery_id=2&image_id=143 (mostly far side) (NASA) Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington ● Many craters ● Cooled long ago—little recent tectonic/volcanic activity ● No atmospheres—no erosion ● Ancient volcanic features—active when young Terrestrial Planets (from Bennett et al.) Mass Radius Density Mercury 0.055 M 0.382 R 5.43 g cm-3 Venus 0.815 M 0.949 R 5.25 g cm-3 Earth 1.0 M 1.0 R 5.52 g cm-3 Moon 0.012 M 0.272 R 3.34 g cm-3 Mars 0.107 M 0.533 R 3.93 g cm-3 Overview of the Moon ● Smallest of the terrestrial worlds ● Heavily cratered highlands ● Smooth maria: lava plains ● Some tectonic features ● No erosion ● Geologically dead today. Crater counts, calibrated on the Moon, allow us to determine geological age Apollo 15 image of Mare Imbrium (NASA; http://sse.jpl.nasa.gov/multimedia/display.cfm?IM_ID=863) Geology of Moon ● Highlands: bright, heavily cratered ● Maria: smooth, dark regions ● Craters should be roughly uniform—what happened in Maria? ● Lava very runny—lack of water/trapped gases (NASA/Apollo 17) (Luc Viatour/Wikipedia) Geology of Moon (from Bennett et al.) Geology of Moon ● Tectonic features found in maria – Contraction during cooling (graben) Apollo 15 image of Mare Imbrium (NASA; http://sse.jpl.nasa.gov/multimedia/display.cfm?IM_ID=863) Source: Clementine Project database High iron content in the Maria High in iron the content Maria are composed of basalt. Lunar basalt has a higher iron content than on earth. on than content iron has a higher basalt of basalt. Lunar composed are Maria (NASA/LPI) Lunar far side is at higher elevation than Earth-facing side. Why? Today's Moon ● No geological activity ● Major impacts infrequent ● No wind/weather ● Micrometeorite impacts break up surface rock into powder Apollo 11 footprint. (NASA; Apollo 11, AS11-40-5878) Overview of Mercury ● Lots of craters (lower density than Moon) ● Volcanic resurfacing – Small lava plains ● Cliffs and shrinking of planet ● Geologically dead MESSENGER image of Mercury http://messenger.jhuapl.edu/gallery/sciencePhotos/image .php?page=1&gallery_id=2&image_id=143 Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington Scarp radial to Caloris Basin (NASA/Mariner 10) Mercury ● Innermost planet ● No activity ● No atmosphere ● Hot on day side, cold on night side (100 K) ● Rotates 3 times for every two orbits ● Unusually high density Distance from Sun: 0.39 AU radius: 0.38 R ⊕ mass 0.055 M Mercury as imaged by the MESSENGER ⊕ -3 spacecraft average density: 5.43 g