Chapter 9 9.1 Connecting Planetary Interiors and Planetary Geology: Surfaces Earth and the Other Terrestrial Worlds • Our goals for learning • What are terrestrial planets like on the inside? • What causes geological activity? • Why do some planetary interiors create magnetic fields? What are terrestrial planets like Seismic Waves on the inside? • Vibrations that travel through Earth’s interior tell us what Earth is like on the inside Earth’s Interior Terrestrial Planet Interiors • Core: Highest density; nickel and iron • Mantle: Moderate density; silicon, oxygen, etc. • Crust: Lowest density; granite, • Applying what we have learned about Earth’s basalt, etc. interior to other planets tells us what their interiors are probably like 1 Differentiation Lithosphere • Gravity pulls • A planet’s outer high-density layer of cool, rigid material to center rock is called the • Lower-density lithosphere material rises to • It “floats” on the surface warmer, softer • Material ends up rock that lies separated by beneath density Strength of Rock Special Topic: How do we know what’s inside a planet? • Rock stretches when pulled slowly but • P waves push breaks when pulled matter back rapidly and forth • The gravity of a large world pulls slowly on its rocky content, shaping the world into a • S waves sphere shake matter side to side Special Topic: What causes geological activity? How do we know what’s inside a planet? • P waves go through Earth’s core but S waves do not • We conclude that Earth’s core must have a liquid outer layer 2 Heating of Interior Cooling of Interior • Accretion and • Convection differentiation transports heat as when planets hot material rises and cool material were young falls • Conduction • Radioactive transfers heat decay is most from hot material important heat to cool material source today • Radiation sends energy into space Role of Size Surface Area to Volume Ratio • Heat content depends on volume • Loss of heat through radiation depends on surface area • Time to cool depends on surface area divided by volume 4πr2 3 surface area to volume ratio = = 4 πr3 r 3 • Smaller worlds cool off faster and harden earlier • Moon and Mercury are now geologically “dead” • Larger objects have smaller ratio and cool more slowly Why do some planetary interiors Sources of Magnetic Fields create magnetic fields? • Motions of charged particles are what create magnetic fields 3 What have we learned? Sources of Magnetic Fields • What are terrestrial planets like on the inside? • A world can have – Core, mantle, crust structure a magnetic field if – Denser material is found deeper inside charged particles • What causes geological activity? are moving inside – Interior heat drives geological activity • 3 requirements: – Radioactive decay is currently main heat source – Molten interior • Why do some planetary interiors create magnetic – Convection fields? – Moderately rapid – Requires motion of charged particles inside planet rotation What processes shape planetary 9.2 Shaping Planetary Surfaces surfaces? • Our goals for learning • What processes shape planetary surfaces? • Why do the terrestrial planets have different geological histories? • How does a planet’s surface reveal its geological age? Processes that Shape Surfaces Impact Cratering • Impact cratering • Most cratering – Impacts by asteroids or comets happened soon after • Volcanism solar system formed – Eruption of molten rock onto surface • Craters are about 10 • Tectonics times wider than object that made them – Disruption of a planet’s surface by internal stresses • Small craters greatly •Erosion outnumber large ones – Surface changes made by wind, water, or ice 4 Impact Craters Impact Craters on Mars Meteor Crater (Arizona) Tycho (Moon) “standard” crater impact into icy ground eroded crater Volcanism Lava and Volcanoes • Volcanism happens when molten rock (magma) finds a path through lithosphere to the surface • Molten rock is called lava after it reaches the Runny lava makes flat Slightly thicker lava Thickest lava makes surface lava plains makes broad shield steep stratovolcanoes volcanoes Outgassing Tectonics • Convection of the mantle creates stresses in the crust called tectonic forces • Volcanism also releases gases from Earth’s interior • Compression forces make mountain ranges into atmosphere • Valley can form where crust is pulled apart 5 Plate Tectonics on Earth Erosion • Earth’s continents • Erosion is a blanket term for weather-driven slide around on processes that break down or transport rock separate plates of • Processes that cause erosion include crust – Glaciers –Rivers –Wind Erosion by Water Erosion by Ice • Colorado River • Glaciers carved continues to the Yosemite carve Grand Valley Canyon Erosion by Wind Erosional Debris • Wind wears • Erosion can away rock and create new builds up sand features by dunes depositing debris 6 Why do the terrestrial planets have Role of Planetary Size different geological histories? • Smaller worlds cool off faster and harden earlier • Larger worlds remain warm inside, promoting volcanism and tectonics • Larger worlds also have more erosion because their gravity retains an atmosphere Role of Distance from Sun Role of Rotation • Planets close to Sun are too hot for rain, snow, ice and so have less erosion • Planets with slower rotation have less weather and • More difficult for hot planet to retain atmosphere less erosion and a weak magnetic field • Planets far from Sun are too cold for rain, limiting • Planets with faster rotation have more weather and erosion more erosion and a stronger magnetic field • Planets with liquid water have most erosion How does a planet’s surface reveal History of Cratering its geological age? • Most cratering happened in first billion years • A surface with many craters has not changed much in 3 billion years 7 Cratering of Moon Cratering of Moon • Some areas of Moon are more heavily cratered than others • Younger regions were flooded by lava after most cratering Cratering map of Moon’s entire surface What have we learned? 9.3 Geology of the Moon and Mercury • What processes shape planetary surfaces? – Cratering, volcanism, tectonics, erosion • Our goals for learning • Why do the terrestrial planets have different geological histories? • What geological processes shaped our – Differences arise because of planetary size, Moon? distance from Sun, and rotation rate • What geological processes shaped • How does a planet’s surface reveal its Mercury? geological age? – Amount of cratering tells us how long ago a surface formed What geological processes shaped Lunar Maria our Moon? • Smooth, dark lunar maria are less heavily cratered than lunar highlands • Maria were made by flood of runny lava 8 Formation of Lunar Maria Tectonic Features • Wrinkles arise from cooling and contraction of lava flood Early surface Large impact Heat build- Cooled lava covered with crater up allows is smoother craters weakens lava to well and darker crust up to surface than surroundings What geological processes shaped Geologically Dead Mercury? • Moon is considered geologically “dead” because geological processes have virtually stopped Cratering of Mercury Cratering of Mercury Caloris basin is Region opposite • A mixture of heavily cratered and smooth regions largest impact crater Caloris Basin is like the Moon on Mercury jumbled from • Smooth regions are likely ancient lava flows seismic energy of impact 9 What have we learned? Tectonics on Mercury • 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 • Long cliffs indicate that Mercury shrank early in its – Tectonic features indicate early shrinkage history How did Martians invade popular 9.4 Geology of Mars culture? • Our goals for learning • How did Martians invade popular culture? • What are the major geological features of Mars? • What geological evidence tells us that water once flowed on Mars? What are the major geological “Canals” on Mars features of Mars? • Percival Lowell misinterpreted surface features seen in telescopic images of Mars 10 Cratering on Mars Volcanism on Mars • Mars has many large shield volcanoes • Olympus Mons is largest volcano in solar system • Amount of cratering differs greatly across surface • Many early craters have been erased What geological evidence tells us Tectonics on Mars that water once flowed on Mars? • System of valleys known as Valles Marineris thought to originate from tectonics Dry Riverbeds? Erosion of Craters • Close-up • Details of some photos of Mars craters suggest show what they were once appear to be filled with dried-up water riverbeds 11 Martian Rocks Martian Rocks • Mars rovers have found rocks that appear to have • Exploration of impact craters has revealed that formed in water Mars’ deeper layers were affected by water Hydrogen Content Crater Walls • Gullies on crater walls suggest occasional liquid water flows have happened less than a million years ago • Map of hydrogen content (blue) shows that low- lying areas contain more water ice What have we learned? What have we learned? • How did Martians invade popular culture? • What geological evidence tells us that – Surface features of Mars in early telescopic water once flowed on Mars? photos were misinterpreted as “canals” – Features that look like dry riverbeds • What are the major geological features of – Some craters appear to be eroded Mars? – Rovers have found rocks that appear to have formed in water – Differences