Astronomy 1010-H Planetary Astronomy Fall_2015 Day-27 Course Announcements • How is the sunset/sunrise observing going? • SW-chapter 7 posted: due Fri. Oct. 30 SW-chapter 8 posted: due Wed. Nov. 4 • • • Exam-3 Wed. Nov. 4: Ch. 6, 7, 8 I will collect the L-T books on Monday, Nov. 23 Take more astronomy! Registration for the Spring semester starts soon so think about taking more astronomy. ASTR-1010/1011: Planetary Astro & lab (Tell your friends) ASTR-1020/1021: Stellar Astronomy & lab (Reg. + Honors) ASTR-2020: Problems in Stellar Astronomy ASTR-3010: History of Astronomy ASTR-3040: Intro. To AstroBiology PHYS-2468: Intro. To Physics Research ASTR-3030/3031: Instrumentation & Techniques Earth’s Moon and the terrestrial planets (Mercury, Venus, Earth, and Mars) have similar and dissimilar properties. Must be able to explain the differences. Comparative planetology: studying planets by comparing them to one another. The Earth’s Interior Layers: Crust: continents (low density silicates) and basins (basalt: higher iron content). Mantle. Core (iron, nickel and other dense materials). Produced by differentiation in the early Earth: dense materials sink; low-density materials rise. Geology & Habitatbility Earth's Interior – Seismic waves Crust – lowest density, Al, Si, Ca Mantle – rocky “plastic” Outer Core – molten Inner Core – solid, Fe, Ni Earth's Interior – Differentiation Melting and sedimentation of heavy material to core Al26 ==> Mg26 contributed a lot of early heat Four processes have shaped Earth: Impact cratering. Tectonism • modifications of the crust. Volcanism • igneous activity • magma/lava. Erosion. Material falling from space onto a planet’s surface create impact craters. Secondary craters can be caused by falling ejecta from the impact. The Moon and all terrestrial planets experienced this. Large impacts can melt and vaporize rock. Venus and Earth have relatively few craters. Craters on Mars suggest it was once wetter. Mercury and the Moon are covered with craters. Hadean Earth, Dawn of Life Late Heavy Bombardment – ~3.9 Gyr ago Relatively quiet between formation and LHB Since then, protected by Jupiter Sterilizing Impacts 350-400 km in diameter Completely vaporize the oceans Global surface temperature rise 2000 C (3600 F) Last ~4.2-3.8 Gyr ago The number of craters indicates the surface’s age. More craters means an older surface and minimal geologic activity. Tectonism and erosion can erase craters. Rocks returned from Moon missions (1969– 1976) give ages through radioactive dating. Almost all cratering happened in the first billion years of the Solar System. On the Moon Rocks returned in the Apollo missions (1969-1972) give ages. Rocks from different places show rate of accretion in the early Solar System. Accretion rate fell sharply after a billion years. Older surfaces have more craters because they were formed when the cratering rate was higher. Concept Quiz The Moon Long Ago Imagine taking a picture of the Moon about 2 billion years ago. What would you expect to see? A. It would have many fewer craters. B. It would have many more craters. C. It would have about as many craters as it does now. Radioactive Dating Some elements can decay from one to another (e.g., uranium to thorium). These changes take place at known rates. Parent element declines, daughter element accumulates. Ratio of parent to daughter abundance gives the age of the rock. Age = time since rock was last molten.