6 Planetary Geology Planets Rock!

6 Goals What determines why planets look the way they do. What are the 4 main process that affect a planet’s surface. How does planetary interior affect planetary atmospheres?

6 Why are planets different? Different planets look different. What determines this? To understand the outside, you must understand the insides.

6 Concept Test The cores of the terrestrial worlds are made mostly of metal because ______. a.the terrestrial worlds as a whole are made mostly of metal. b.the core contained lots of radioactive elements that decayed into metals. c.over billions of years, convection gradually brought dense metals downward to the core. d.metals sunk to the centers a long time ago when the interiors were molten throughout. e.None of the above.

6 Internal Heat Why is heat the driver of geological processes? What is heat? Think of temperature on an atomic scale. Where do you expect change?

6 Accretion Planets form out of primordial cloud of gas and dust. Atoms to dust to grains to rocks to asteroids to planetesimals to planets.

6 Differentiation Accreted material is all mixed. Friction generates heat.

6 Radioactive Decay Natural nuclear reactors. Oklo, Gabon. Copyright – Robert D. Loss, WAISRC

6 Heat and Matter Hot things cool. How? 1.Convection – Material moves around. 2.Conduction – Atoms bump in to one another. 3.Radiation – Thermal radiation caries energy.

6 Lithosphere – Radius at which rock is too cool to flow by convection.

6 Concept Test Given this description of planetary interiors, we expect: a.Smaller planets should cool off faster than bigger ones. b.Larger planets should have thinner lithospheres than smaller planets. c.There should be fewer signs of geological activity on the surfaces of smaller planets compared to larger ones. d.All of the above. e.None of the above.

6 Concept Test Which of the following is NOT generally true of all the terrestrial world lithospheres? a.The thickness of the lithosphere depends on interior temperature, with cooler interiors leading to thicker lithospheres. b.The lithosphere is broken into a set of large plates that float upon the softer rock below. c.The lithosphere extends from somewhere in the mantle all the way to the surface. d.Rock in the lithosphere is stronger than rock beneath it. e.None of the above.

6 How do we know? How do we know what is really going on inside planets? 1.Look for signs of surface activity. Has anything changed recently? 2.Look for magnetic fields around planet. 1.Fluid core 2.Convection of fluid 3.Rapid rotation

6 Concept Test Recent evidence suggests that Mars, like Earth, once had a stronger magnetic field. Assuming this is true, which of the following could explain why Mars today lacks a global magnetic field like that of Earth? a.Because Mars's interior is cooler than Earth's, its liquid core layer may not be undergoing convection. b.Mars is too far from the Sun to have a global magnetic field. c.The Martian core is made of rock, while Earth's core is made of metal. d.Mars rotates much slower than the Earth. e.All of the above.

6 Concept Test The choices below describe four hypothetical planets. Which one would you expect to have the hottest interior? (Assume the planets orbit a star just like the Sun and that they are all the same age as the planets in our solar system.) a.Size: same as Mars. Distance from Sun: same as Earth. Rotation rate: once every 18 hours. b.Size: twice as big as Earth. Distance from Sun: same as Mercury. Rotation rate: once every 6 months. c.Size: same as Venus. Distance from Sun: same as Mars. Rotation rate: once every 25 hours. d.Size: same as the Moon. Distance from Sun: same as Mars. Rotation rate: once every 10 days.

6 Comparative Planetology 4 main surface altering processes –Cratering (exogenic) –Volcanism (endogenic) –Tectonics (endogenic) –Erosion (endogenic) In order for endogenic processes to occur, energy must be available. Small planets (and moons, and asteroids) generally have little internal heat, and, therefore, little endogenic activity. Exogenic processes affect everything.

6 Comparative Planetology 2 In terrestrial planets, the main sources of heat are: –Radioactivity –Sunlight –Energy of formation (original heat)

6 Volcanoes

6

6 Tectonics

6

6

6 Plate Tectonics

6 Erosion

6 Dunes

6 Alluvial Fan

6 Craters

6 New Surface vs. Old Impacts happen over time. Can date a surface by the number of craters on its surface. Date since last geologic activity. Pot holes on road: –Many holes = old road. –Few holes = new road Resurfacing = energy –One or more endogenic processes. Another clue to insides.

6 Concept Test Based on all we know about the terrestrial worlds, what single factor appears to play the most important role in a terrestrial planet's geological destiny? a.Its composition. b.Its size. c.Its distance from the Sun. d.Whether or not it has liquid water. e.None of the above.

6 Why is Earth so Different? Why do we have oceans of water? Why doe we have an atmosphere with: –so little carbon dioxide? –so much O 2 and N 2 ?

6 Atmosphere Origins Outgassing Volcanoes. CO 2 and H 2 O main gases. Mt. St. Helens

6 Concept Test For which terrestrial planets is the main atmospheric gas one of the two main gases produced by outgassing? a.Only Moon & Mercury b.All five planets c.Only Venus, Earth and Mars d.Only Earth e.Only Venus and Mars

6 The Terrestrial Planets World Atmos. Compsn. Surface Pressure Ave Surface Temp. Winds, Weather Clouds, Hazes Mercury He, Na, O barDay: 800F Night: -280F none Venus 96% CO2 3.5% N2 90 bars880FSlow winds, acid rain Sulfuric acid clouds Earth 77% N2 21% O2 H2O 1 bar60FWinds, hurricanes, rain, snow H2O clouds, pollution Moon He, Na, Ar barDay:260F Night: -280F none Mars 95% CO2 2.7% N bar-60FWinds, dust storm H2O and CO2 clouds, dust

6 Concept Test The Earth retained so much outgassed water relative to Venus and Mars because: a.The Earth is closer to the Sun than Mars and so was kept warmer, whereas Venus was too hot. b.The Earth is farther from the Sun than Venus and so didn’t lose water due to thermal escape, but Mars was too cold and it all froze. c.Both Venus and Mars lost their water due to evaporation and then UV-dissociation (while much of Mars’ water also froze). d.The Earth simply outgassed much more water than the other two. e.None of the above.

6 I like Rain 1.Temperature just right. 2.Water vapor could rain. 3.Liquid oceans didn’t completely evaporate. 4.CO 2 could dissolve in oceans. –60x CO 2 dissolved in oceans as atmosphere. –170,000x CO 2 in carbonate rocks as atmosphere.

6 Goldilocks Planet Delicate balance: Earth is just right. –No oceans without greenhouse gasses. –Without oceans too many greenhouse gasses. –Without oceans no life. –Without life, no ozone. –No ozone, no life. How does Earth maintain this balance?

6 Carbon Cycle Tectonics are crucial!

6 Nature’s Thermostat Carbon Cycle has negative feedback. Increasing temperatures lead to feedback that decreases temperatures, etc. Positive and negative feedbacks have nothing to do with desirability of outcome.

6 Too Cool Less Rain Less CO 2 dissolves in oceans. More CO 2 builds up in atmosphere Increased greenhouse Heats Up.

6 Too Warm More Rain More CO 2 dissolves in oceans. Less CO 2 builds up in atmosphere Decreased greenhouse Cool Down.

6 Concept Test Under what circumstances is thermal escape likely to be important? a.Light gases on big planets far from the Sun b.Heavy gases on big planets far from the Sun c.Light gases on small planets near the Sun d.Light gases on small planets far from the Sun e.Equally well on any kind of gas from any kind of planet

6 Cycles

6 Human Input Affecting CO 2 cycle. Moving C from ground to air at increased rate. No increase in rate of C from air to ground.

6 Mars Today

6 Positive Feedback Fossil Fuels – Increase CO 2 to atmosphere. Deforestation – decrease CO 2 from atmosphere Increase CO 2 – Increase greenhouse effect Increase temp – Melting glaciers, ice sheets, polar caps Less ice – lower planetary albedo Lower albedo – Earth absorbs more light More absorption – Increasing surface temperatures.

6 Homework #6 Due Monday January 30: Read Bennett Chapter 6. Do: Chapter 6 Quiz