9 Planetary Geology Planets Rock!

9 Goals What determines why planets look the way they do. What are the 4 main process that affect a planet’s surface.

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

9 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?

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

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

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

9 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.

9 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.

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

9 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.

9 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.

9 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

9 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.

9 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.

9 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.

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

9 Volcanoes

9 Tectonics

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9 Plate Tectonics

9 Hot spots

9 Erosion

9 Dunes

9 Alluvial Fan

9 Craters

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

9 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.

9 Homework #11 Due Friday 3-Oct: Nothing