Geology of the Moon • Smooth, dark lunar maria are less heavily cratered than lunar highlands • Maria were made by flood of runny lava • Wrinkles arise from cooling and contraction of lava flood • heavily cratered, no atmosphere (too small), geologically inactive Geology of Mercury • A mixture of heavily cratered and smooth regions like the Moon • Smooth regions are likely ancient lava flows • Tectonics: Long cliffs indicate that Mercury shrank early in its history • No atmosphere (too small), geologically dead 1 Geology of Mars • mountains & canyons • Valles Marineris • volcanoes • thin atmosphere (CO2) • no plate tectonics • evidence for water erosion Cratering on Mars • Amount of cratering differs greatly across surface • Many early craters have been erased 2 Volcanism on Mars • Mars has many large shield volcanoes • Olympus Mons is largest volcano in solar system Tectonics on Mars • System of valleys known as Valles Marineris thought to originate from tectonics 3 Dry Riverbeds? • Close-up photos of Mars show what appear to be dried- up riverbeds • Details of some craters suggest they were once filled with water • Mars rovers have found rocks that appear to have formed in water Water on Mars? • Some craters have frozen water exposed for short period • There is evidence for (frozen) water within 1 meter under the surface • this underground water may be all over the planet 4 Ancient Water on Mars • Liquid water can not exist on Mars today. • temperatures below freezing • air pressure too low • Dry river channels in southern highlands • heavily cratered terrain (> 3 billion years old) • Some surface features are eroded. • implies running water, rain • crater lakes • Mars was warm & wet over 3 billion years ago. Recent Water on Mars? • Liquid water could exist temporarily with today’s temperatures and air pressures…in a flash flood! • Underground water seeps out to form erosion gullies • these gullies were observed on a crater wall • at their size, sandstorms would cover them in few million yrs • such floods have occurred within the last few million years 5 Martian Weather Today • Seasons on Mars are more extreme than on Earth • CO2 condenses & sublimes at opposite poles • changes in atmospheric pressure drive pole-to-pole winds • sometimes cause huge dust storms Martian Weather: N Polar Ice Cap & Dust Storm 6 Climate Change on Mars • Mars has not had widespread surface water for 3 billion years • Greenhouse effect probably kept surface warmer before that • Somehow Mars lost most of its atmosphere Climate Change on Mars • Magnetic field may have preserved early Martian atmosphere • Solar wind may have stripped atmosphere after field decreased because of interior cooling 7 Geology of Venus • Has a thick, cloudy atmosphere -- you can not visually see the surface • we must image the surface using radar • smooth plains with few mountain ranges • few craters • many volcanoes and domes of lava (corona) • Venus is very active with tectonics & volcanism Volcanism & Tectonics on Venus • Impact craters are randomly spread over Venusian surface. • implies that the planet’s entire surface is the same age • crater counting suggests an age of 0.5 billion years old • Volcanism “paved over” the surface 1 billion years ago. 8 Erosion on Venus • Photos of rocks show little erosion. This means no wind, rain, or ice on the surface. • The surface of Venus is very hot (430 C)… too hot for liquid or ice to exist • Venus rotates very slowly (P = 243 days), so no wind is generated Does Venus have plate tectonics? • Most of Earth’s major geological features can be attributed to plate tectonics, which gradually remakes Earth’s surface • Venus does not appear to have plate tectonics, but entire surface seems to have been “repaved” 750 million years ago • Possibly this is a cyclic process that cools interior 9 Atmosphere of Venus • Thick carbon dioxide atmosphere with produces a strong greenhouse effect • Earth escapes this fate because most of its carbon and water is in rocks and oceans • Surface pressure 90 times Earth’s • Slow rotation produces very little weather • Reflective clouds contain droplets of sulphuric acid Climate History of Venus • Venus should have outgassed as much H2O as Earth. • Early on, when the Sun was dimmer, Venus may have had oceans of water • Venus’ proximity to the Sun caused all H2O to evaporate. • H2O contributed to runaway greenhouse effect • surface heated to extreme temperature • UV photons from Sun dissociate H2O; H2 escaped, O stripped 10 Summary • What are terrestrial planets like on the inside? – Core, mantle, crust structure – Denser material is found deeper inside • How do interiors get hot or cool down? – Radioactive decay is currently main heat source – Cooling occurs due to convection, conduction and radiation – Interior heat drives geological activity • Why do some planetary interiors create magnetic fields? – Requires motion of charged particles inside planet Summary • What processes shape planetary surfaces? – Cratering, volcanism, tectonics, erosion – Amount of cratering tells us how long ago a surface formed • Why do the terrestrial planets have different geological histories? – Differences arise because of planetary size, distance from Sun, and rotation rate 11 Summary of Geology of Planets • How do we know that Earth’s surface is in motion? – Measurements of plate motion confirm idea of continental drift • How is Earth’s surface shaped by plate tectonics? – Plate tectonics responsible for subduction, seafloor spreading, mountains, rifts, and earthquakes – Reason for plate tectonics still a mystery Summary • How did Earth’s atmosphere end up so different? – Temperatures just right for oceans of water – Oceans keep most CO2 out of atmosphere – Nitrogen remains in atmosphere – Life releases some oxygen into atmosphere • Why does Earth’s climate stay relatively stable? – Carbon dioxide cycle acts as a thermostat • How might human activity change our planet? – Destruction of ozone – High rate of extinction – Global warming from production of greenhouse gases 12 Summary of Geology of Planets • What geological processes shaped our Moon? – Early cratering still present – Maria resulted from volcanism • What geological processes shaped Mercury? – Cratering and volcanism similar to Moon – Tectonic features indicate early shrinkage Summary of Geology of Planets • What are the major geological features of Mars? – Differences in cratering across surface – Giant shield volcanoes – Evidence of tectonic activity • What geological evidence tells us that water once flowed on Mars? – Features that look like dry riverbeds – Some craters appear to be eroded – Rovers have found rocks that appear to have formed in water – Gullies in crater walls may indicate recent water flows 13 Summary • What is Mars like today? – Mars is cold, dry, and frozen – Strong seasonal changes cause CO2 to move from pole to pole, leading to dust storms • Why did Mars change? – Its atmosphere must have once been much thicker for its greenhouse effect to allow liquid water on the surface – Somehow Mars lost most of its atmosphere, perhaps because of declining magnetic field Summary of Geology of Planets • What are the major geological features of Venus? – Venus has cratering, volcanism, and tectonics but not much erosion • Does Venus have plate tectonics? – The lack of plate tectonics on Venus is a mystery 14 Summary • What is Venus like today? – Venus has an extremely thick CO2 atmosphere – Slow rotation means little weather • How did Venus get so hot? – Runaway greenhouse effect made Venus too hot for liquid oceans – All carbon dioxide remains in atmosphere, leading to a huge greenhouse effect 15.