Class Survey Results Web access: yes=46 no=1 Core=30 Elective=15 Citation=2 Prior astro: none=27 A100=8 A101=9 other=3 Class 4 : The physics of collisions & crater formation Today, we’ll finish talking about cratering in the solar system… Then, we’ll talk about: The factors that determine the nature and severity of a collision. The processes that shape impact craters. III : Volcanism in the solar system Io – the most active volcanic region in the Solar System Examples… Io Very smooth surface – no craters or rilles. Thought to be a very young surface – lava has filled all craters and valleys. Strong stretching/pulling by Jupiter heats interior and causes volcanic activity. Mars Possesses some giant dead volcanoes (e.g. Olympus Mons… 27km high, 600km wide). Example of a crater not caused by impacts. IV : The outer moons – ice geology at work. Europa Surface ice sheet occasionally seems to break and float around on an underlying liquid water ocean. Now – the details of how craters are made… I : Craters are circular… satellite image of Meteor crater The Moon – all craters are circular! Far side of The Moon Mercury – they’re still all circular!! II : Energy and Momentum A moving object possesses momentum and (kinetic) energy. Suppose the object has mass m and speed v. Then Momentum = mv Kinetic Energy = ½ mv2 Typically both energy and momentum are conserved (i.e., cannot be created or destroyed). Energy & Momentum in Impacts Consider an impact on the Earth. The momentum of the impactor… Excavates material from the crater. Gets soaked up by the Earth (moves it in its orbit by a very, very small amount). The energy of the impactor… Heats the impactor and surrounding rock. Excavates material from the crater. Gets turned into sound and seismic waves. Hypervelocity Impacts Hypervelocity (very fast!) impacts… Large momentum but enormous energy. Impactor completely vaporizes on impact. Circular crater blown out by the force of the resulting explosion (not the gouging action of the impactor!). Also melt or vaporize some of the preimpact surface rocks. III : Simple and Complex Craters Simple craters Simple bowl-like craters. Small (<2-4 km on Earth, <15-20 km on Moon). Have diameter-to-depth ratio of 5:1 or so. Complex craters… Above a certain size, the crater walls collapse. Get terraced walls and central peak Relatively shallower (diameter-to-depth ratios of 10:1 or so). Impact basins – very large craters IV: Other signatures of impact The high temperatures and pressures caused by an impact leave other signs… examples are: Shatter cones Shocked quartz Geological feature characteristic of impacts. Sign of very high pressure & temperature. Impact breccia Rock that’s been shattered, mixed, & melted! Shatter Cone Shocked Quartz Impact Breccia Iridium A rare silvery-white metal (close to platinum & gold on the Periodic Table). Much more abundant in asteroids and comets than on the Earth’s surface. Later, we’ll learn about the “iridium anomaly”… a geological signature coincident with the death of the dinosaurs!