Chapter 9 Planetary Geology: Agenda Earth and the Other Terrestrial Worlds • Announce: – Part 2 of Projects due – Read Ch. 11 for Tuesday • Ad hoc, ex post facto • Rover Update • Ch. 9—Planetary Geology • Lab: Parallax or Waves on a String
Rover Update Ad Hoc • More than 1000 Martian days
• From Wikipedia: – Ad hoc is a Latin phrase which means "for this [purpose]." It generally signifies a solution that has been designed for a specific problem, is non-generalizable and can not be adapted to other purposes. – Ex post facto : from the Latin for "from something done afterward"
1 9.1 Connecting Planetary Interiors and What are terrestrial planets like Surfaces on the inside? • Our goals for learning • What are terrestrial planets like on the inside? • What causes geological activity? • Why do some planetary interiors create magnetic fields?
Seismic Waves Earth’s Interior
• Vibrations • Core: Highest that travel density; nickel through and iron Earth’s • Mantle: Moderate interior tell us density; silicon, what Earth is oxygen, etc. like on the • Crust: Lowest inside density; granite, basalt, etc.
Terrestrial Planet Interiors Differentiation • Gravity pulls high-density material to center • Lower-density material rises to surface • Material ends up • Applying what we have learned about Earth’s separated by interior to other planets tells us what their interiors density are probably like
2 Lithosphere Strength of Rock • A planet’s outer • Rock stretches when layer of cool, rigid pulled slowly but rock is called the breaks when pulled lithosphere rapidly • It “floats” on the • The gravity of a large warmer, softer world pulls slowly on rock that lies its rocky content, beneath shaping the world into a sphere
Special Topic: Special Topic: How do we know what’s inside a planet? How do we know what’s inside a planet?
• P waves push • P waves go matter back through Earth’s core but S and forth waves do not
• We conclude • S waves that Earth’s core shake matter must have a liquid outer side to side layer
Thought Question Thought Question
What is necessary for differentiation to occur in a What is necessary for differentiation to occur in a planet? planet?
a) It must have metal and rock in it a) It must have metal and rock in it b) It must be a mix of materials of different density b) It must be a mix of materials of different density c) Material inside must be able to flow c) Material inside must be able to flow d) All of the above d) All of the above e) b and c e) b and c
3 What causes geological activity? Heating of Interior • Accretion and differentiation when planets were young
• Radioactive decay is most important heat source today
Cooling of Interior Role of Size
• Convection transports heat as hot material rises and cool material falls • Conduction transfers heat from hot material to cool material • Smaller worlds cool off faster and harden earlier • Radiation sends energy into space • Moon and Mercury are now geologically “dead”
Why do some planetary interiors Surface Area to Volume Ratio create magnetic fields? • 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 • Larger objects have smaller ratio and cool more slowly
4 Sources of Magnetic Fields Sources of Magnetic Fields • A world can have a magnetic field if • Motions of charged particles charged particles are moving inside are what create • 3 requirements: magnetic fields – Molten interior – Convection – Moderately rapid rotation
What have we learned?
• What are terrestrial planets like on the inside? – Core, mantle, crust structure – Denser material is found deeper inside • What causes geological activity? – Interior heat drives geological activity – Radioactive decay is currently main heat source • Why do some planetary interiors create magnetic fields? – Requires motion of charged particles inside planet
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?
5 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
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
6 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
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
7 Erosion by Wind Erosional Debris
• Wind wears • Erosion can away rock and create new builds up sand features by dunes depositing debris
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
8 Thought Question Thought Question
How does the cooling of planets and potatoes vary How does the cooling of planets and potatoes vary with size? with size?
a) Larger makes it harder for heat from inside to a) Larger makes it harder for heat from inside to escape escape b) Larger has a bigger ratio of volume (which needs b) Larger has a bigger ratio of volume (which needs to cool) to surface area (the surface is where cooling to cool) to surface area (the surface is where cooling happens) happens) c) Larger takes longer to cool c) Larger takes longer to cool d) All of the above d) All of the above
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
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
9 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
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
10 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
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
11 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
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
12 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
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
13 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 in cratering across surface – Gullies in crater walls may indicate recent – Giant shield volcanoes water flows – Evidence of tectonic activity
What are the major geological 9.5 Geology of Venus features of Venus?
• Our goals for learning • What are the major geological features of Venus? • Does Venus have plate tectonics?
14 Radar Mapping Cratering on Venus
• Impact craters, but fewer than Moon, Mercury, Mars
• Thick atmosphere forces us to explore Venus’ surface through radar mapping
Volcanoes on Venus Tectonics on Venus
• Many volcanoes, • Fractured and including both shield contorted surface volcanoes and indicates tectonic stratovolcanoes stresses
Erosion on Venus Does Venus have plate tectonics?
• Photos of rocks • Most of Earth’s major geological features taken by lander can be attributed to plate tectonics, which show little gradually remakes Earth’s surface erosion • Venus does not appear to have plate tectonics, but entire surface seems to have been “repaved” 750 million years ago
15 What have we learned? 9.6 The Unique Geology of Earth • Our goals for learning • What are the major geological features of • Our goals for learning Venus? • How do we know Earth’s surface is in – Venus has cratering, volcanism, and tectonics motion? but not much erosion • How is Earth’s surface shaped by plate • Does Venus have plate tectonics? tectonics? – The lack of plate tectonics on Venus is a mystery • Was Earth’s geology destined from birth?
How do we know Earth’s surface Continental Motion is in motion?
• Motion of continents can be measured with GPS
Continental Motion Seafloor Crust • Idea of • Thin seafloor continental drift crust differs from was inspired by thick continental puzzle-like fit of crust continents • Dating of seafloor • Mantle material shows it is erupts where usually quite seafloor spreads young
16 How is Earth’s surface shaped by Seafloor Recycling plate tectonics?
• Seafloor is recycled through a process known as subduction
Surface Features Surface Features • Major geological • Himalayas are features of North forming from a America record collision history of plate between plates tectonics
Surface Features Rifts, Faults, Earthquakes • Red Sea is • San Andreas forming where fault in plates are pulling California is a apart plate boundary
• Motion of plates causes earthquakes
17 Plate Motions Hot Spots • Measurements of plate motions tell us past and future layout of continents
• Hawaiian islands have formed where plate is moving over volcanic hot spot
Was Earth’s geology destined Earth’s Destiny from birth? • Many of Earth’s features determined by size, rotation, and distance from Sun
• Reason for plate tectonics not yet clear
What have we learned? What have we learned?
• How do we know that Earth’s surface is in • Was Earth’s geology destined from birth? motion? – Many of Earth’s features determined by size, distance from Sun, and rotation rate – Measurements of plate motion confirm idea of continental drift – Reason for plate tectonics still a mystery • How is Earth’s surface shaped by plate tectonics? – Plate tectonics responsible for subduction, seafloor spreading, mountains, rifts, and earthquakes
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