Planetary Geology Planets Rock! Goals • What determines why planets look the way they • What are the main process that affect a planet’s surface Why are planets different? • Different planets look different • What determines this? • To understand the outside, you must understand the insides Internal Heat • Why is heat the driver of geological processes? • What is heat? • Think of temperature on an atomic scale • Where you expect change? • Planets form out of primordial cloud of gas and dust • Atoms to dust to grains to rocks to asteroids to planetesimals to planets Accretion Differentiation • Accreted material is all mixed • Friction generates heat Radioactive Decay Natural nuclear reactors Oklo, Gabon Copyright – Robert D Loss, WAISRC 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 Heat and Matter • Hot things cool How? Convection – Material moves around Conduction – Atoms bump in to one another Radiation – Thermal radiation caries energy Lithosphere – Radius at which rock is too cool to flow by convection 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 b c d Size: same as Mars Distance from Sun: same as Earth Rotation rate: once every 18 hours Size: twice as big as Earth Distance from Sun: same as Mercury Rotation rate: once every months Size: same as Venus Distance from Sun: same as Mars Rotation rate: once every 25 hours Size: same as the Moon Distance from Sun: same as Mars Rotation rate: once every 10 days Comparative Planetology • 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 Volcanoes Tectonics 9 Plate Tectonics Hot spots Erosion Dunes Alluvial Fan Craters 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 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 b c d e Its composition Its size Its distance from the Sun Whether or not it has liquid water None of the above Homework #11 • Due Friday 3-Oct: Nothing