Chapter 9 9.1 Connecting Planetary Interiors and Planetary Geology: - PDF document

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 lithosphere • Lower-density material rises to • It “floats” on the surface warmer, softer rock that lies • Material ends up beneath separated by density Special Topic: Strength of Rock How do we know what’s inside a planet? • Rock stretches when • P waves push pulled slowly but matter back breaks when pulled rapidly and forth • The gravity of a large world pulls slowly on its rocky content, • S waves shaping the world into a shake matter sphere 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 • Convection • Accretion and transports heat as differentiation hot material rises when planets and cool material were young falls • Conduction • Radioactive transfers heat from hot material decay is most to cool material important heat • Radiation sends source today 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 surface area to volume ratio = 4 π r 2 3 π r 3 = 3 4 r • Smaller worlds cool off faster and harden earlier • Larger objects have smaller ratio and cool more • Moon and Mercury are now geologically “dead” 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 happened soon after – Impacts by asteroids or comets solar system formed • Volcanism • Craters are about 10 – Eruption of molten rock onto surface times wider than object • Tectonics that made them – Disruption of a planet’s surface by internal • Small craters greatly stresses outnumber large ones • Erosion – Surface changes made by wind, water, or ice 4

Impact Craters Impact Craters on Mars Meteor Crater (Arizona) Meteor Crater (Arizona) Tycho Tycho (Moon) (Moon) “standard” crater standard” crater impact into icy ground eroded 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 Runny lava makes flat Slightly thicker lava Slightly thicker lava Thickest lava makes Thickest lava makes surface lava plains makes broad shield shield steep stratovolcanoes stratovolcanoes lava plains makes broad steep volcanoes 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 • Erosion is a blanket term for weather-driven • Earth’s continents processes that break down or transport rock slide around on 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 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 • What geological processes shaped our different geological histories? Moon? – Differences arise because of planetary size, 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 Early surface Large impact Heat build- - Cooled lava Heat build Cooled lava covered with crater crater covered with up allows up allows is smoother is smoother craters craters weakens weakens lava to well lava to well and darker and darker crust crust up to surface up to surface than than surroundings 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 basin is Region opposite Caloris Region opposite • A mixture of heavily cratered and smooth regions largest impact crater Caloris Basin is Caloris Basin is largest impact crater like the Moon on Mercury on Mercury jumbled from jumbled from seismic energy of seismic energy of • Smooth regions are likely ancient lava flows impact 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 – Tectonic features indicate early shrinkage • Long cliffs indicate that Mercury shrank early in its 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