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ASTR-1010 Planetary Astronomy Day - 25 Announcements Smartworks Chapter 6: Due Today, March 22. Smartworks Chapter 7: Due Friday, March 26. 1st Quarter Observing Nights: Tues & Thurs, March 23 & 25 -- 7:30pm Lab this week: Earthquakes Lab next week: Rotation Rate of Mercury Chapter 7 Lecture Outline The Terrestrial Planets and Earth’s Moon ClassAction Stuff • SS Splash page questions • Terrestrial Planets • Iceland Volcano Two broad categories of planets: Earthlike and Jupiterlike • All of the planets orbit the Sun in the same direction and in almost the same plane • Most of the planets have nearly circular orbits Density m D V • The average density of any substance depends in part on its composition • An object sinks in a fluid if its average density is greater than that of the fluid, but rises if its average density is less than that of the fluid • The terrestrial (inner) planets are made of rocky materials and have dense iron cores, which gives these planets high average densities • The Jovian (outer) planets are composed primarily of light elements such as hydrogen and helium, which gives these planets low average densities The Terrestrial Planets • The four inner planets are called terrestrial planets – Relatively small (with diameters of 5000 to 13,000 km) – High average densities (4000 to 5500 kg/m3) – Composed primarily of rocky materials Seven large satellites are almost as big as the terrestrial planets • Comparable in size to the planet Mercury • The remaining satellites of the solar system are much smaller Hydrogen and helium are abundant on the Jovian planets, whereas the terrestrial planets are composed mostly of heavy elements Similar but Different • Terrestrial planets: – Mercury – Venus – Earth – Mars – Earth’s Moon (or simply, the Moon) • All are rocky/metallic, dense. • Smallest two have little/no atmosphere. Mass is Key • The differences between the planets are largely driven by mass. • Different processes depend on the mass of the planet. Mass ratio to Earth Moon 0.012 Mercury 0.055 Mars 0.11 Venus 0.82 Earth 1.00 Comparative Planetology • We can learn a lot by comparing the planets. • The same processes operate on each planet: – Tectonism (moving crustal plates) – Volcanism (volcanoes) – Impacts (cratering) – Gradation (smoothing by weathering and erosion) • These processes are stronger or weaker on the different planets. Cratering on planets and satellites is the result of impacts from interplanetary debris • When an asteroid, comet, or meteoroid collides with the surface of a terrestrial planet or satellite, the result is an impact crater • Geologic activity renews the surface and erases craters, so a terrestrial world with extensive cratering has an old surface and little or no geologic activity • Because geologic activity is powered by internal heat, and smaller worlds lose heat more rapidly, as a general rule smaller terrestrial worlds are more extensively cratered Impacts • Craters on the Moon are relics of the last phase of planetary accretion. • All terrestrial planets experienced this. • Venus and Earth have few craters. • Subsequent tectonism and gradation erases the craters. • Some large impacts on the Earth have influenced the evolution of life. Cratered Region on the Moon NASA/JSC 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. 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. Cratering Rate NASA/JPL/Caltech 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. 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 (Fig 4.13) Completely vaporize the oceans Global surface temperature rise 2000 C (3600 F) Last ~4.2-3.8 Gyr ago How Old is the Earth? Age of the Earth – oldest rocks 4.0 Gyr “Zircons” ~4.4 Gyr Suggest the crust separated from interior ~4.5 Gyrs Moon rocks ~4.4 Gyr Therefore, Moon existed by this time. Formation of the Moon • Moon formed in large collision between Earth + Mars-sized protoplanet. • The collision scattered material into Earth orbit; this collected by accretion to form the Moon. • Composition of Moon is like that of Earth’s crust. • Dark areas on Moon (maria) are ancient lava flows from later large impacts. Impact Energies and these are the small ones A Model of the Earth • We model the Earth’s interior by studying earthquakes. • Sound moves at different speeds through different materials. • P (primary) waves travel through solids and liquids. • S (secondary) waves go through solids only. • Earth’s layers are: crust, mantle, liquid outer core, solid inner core The Earth’s Interior