Planetary Impacts Planetary Impacts Impacts between planets & asteroid-sized bodies have played an important role in determining the planets properties. In the case of Mercury, a large head-on impact is invoked to explain its unusually large iron core. An impact may explain the formation of the Moon An impact may explain the slow spin of Venus Impacts are an essential element of the history of the Solar System. Crater Schematic Crater Structure Depends on Energy Small craters can be simple bowls (Moltke, 7km, Moon) Bessel (16km, 2km deep,moon) In bigger craters, the walls slump and the center rebounds into a peak (Euler, 28km, 2.5km deep, Moon) Bigger still you, you get a peak ring inside the crater (Tycho 85km, King 77km, peak only) Bigger still you, you get a ring inside the crater, now called an impact basin (Schrodinger, 320km, Moon) The largest show multiple rings in the impact basin (200km double ringed basin on Mercury) 1
Approximate energetics (do not study in detail) An object of density and diameter D has mass M=(density)(volume)=10 12 (D/1km) 3 kg for density of =2500kg/m 3 (rock) A typical impact velocity (for Earth) is v=20km/s, so the kinetic energy is E=Mv 2 /2=3 10 20 (D/1km) 3 Joules = 60,000 (D/1km) 3 Megatons of TNT Hiroshima = 13 kilotons, Bikini 15 megatons typical fusion bomb 250 kilotons Will it reach the ground? The atmosphere protects us against small meteorites basically if the mass of the air run into by the meteorite equals the mass of the meteorite it will slow down before hitting the ground. This is true for meteorites with radii less than about 1m If you have no atmosphere (e.g. the Moon) everything hits, while if you have a much thicker atmosphere (e.g. Venus) the meteorites have to be still larger. Or, since it takes 2500 kilojoules to heat and vaporize 1kg of water, a 20 km/s impact can vaporize 80 times it s weight in water. And Rates Diameter Event Energy (TNT) Eathqua ke (M) Crater (km) 1m Fire ball 60 tons 6 days 3m Fire ball 2 kilotons 3 months 10m airburst 45 kilotons 4 years 30m airburst 2 megatons 58 years 100m impact 60 megatons 6.8 1.3 0.2 1000 years 300m impact 2000 megatons 7.7 3.8 0.4 14000 years 1km impact 60000 megatons 8.8 12.5 0.6 260,000 years 3km impact 2 million megatons 9.8 37.1 1.2 3.6 million years 10km impact 60 million megatons 10.8 122 1.2 66 million years 30km impact 2 billion megatons 11.8 327 1.7 1 billion years rate Mike November 1952 10 Megatons 2 km crater You begin to have global effects for objects >1km or so. Wolf Crater, Australia 0.85km, about 300,000 years old Mistastin Lake, Newfoundland 28km, about 38 million years old 2
Tunguska, Siberia 1908 Air burst, probably ~60m, 10-20 Megatons flattened 2000 sq. km of forest Between 1972 and 2000, IR satellites watching for missile launches detected 518 events of roughly 1 kiloton and larger about 30 per year. Peekskill, NY:1992 Moving at about 15 km/s, 50km up at start. Probably 0.5-1.0m in size (2-10 tons) before fragmenting. The streak is about as bright as the full moon. The piece that hit the car was 12.4kg. Chelyabinsk meteor: 2013 https://www.youtube.com/watch?v=90omh7_i8vi Chicxulub The End of the Dinosours? ~10 km, 65 million years ago, off Yucatan Peninsula Crater is 140-180km across 20 meters diameter, 15 million kg, 500 kilotons Comet Shoemaker-Levy 9 (SL9) In March 93, a new comet was found, analysis showing it was bound to Jupiter Spreading Out As it began to break up into fragments, it was realized that some time in 1992 it had passed too close to Jupiter and been torn apart by Jupiter s tides About 22 fragments, the largest ~1km, were on an orbit which would take them crashing into Jupiter in July 94 at about 60km/sec (130,000 miles/hour!) with the biggest fragments Releasing the equivalent of 600 million megatons of TNT Everyone tuned in for the crash of the century 3
Impacts Fading impacts Fragment A (SAAO) After many impacts Fragment C (MSSSO) Crater Chains Again? Similar scar noticed by an amateur in 2009 Ganymede Callisto Lunar Impacts The Moon 4
Key Ideas: Surface of the Moon: Young, dark Maria Old, heavily cratered highlands Thick regolith of pulverized rock Interior of the Moon Crust and Mantle, but no Core (?) No magnetic field Origin of the Moon The Neighbor Moon After the Earth, the Moon is the best known body in the Solar System: Closest companion in space We ve visited it and returned samples of rock for study. Planted instruments and seismographs on the surface to probe the interior Study it up-close with robotic probes, for US Lunar Reconnaissance Orbiter and LCROSS most recently. Two Faces of the Moon The Moon is Black (albedo=0.11) Maria Impact Basin Near Side Cratered Highlands Far Side 16% of surface fewer craters (younger surface) older craters filled in by lava flows higher iron content in the rocks (basalts) Maria ( seas ) Highlands Light Colored Heavily Cratered Overlapping craters High Mountains Deep Valleys 5
Moon Rocks Our most detailed knowledge of the Moon comes from ~382 kg of samples returned by 9 space missions: United States: 6 Apollo Landings (Apollo 11-17) 1969-1972 12 astronauts visited maria and highlands Soviet Union: 3 Luna robotic missions (1970, 1972, 1976) returned samples by capsule Apollo and Luna Landing Sites A12 A14 A15 A16 A17 A11 L24 L20 L16 Basalt rapidly cooled lava Breccia impact cemented rocks Anorthosite slowly cooled lava Regolith Layer of dust and fragmented rock. Produced by meteor impacts Covers the uppermost surfaces of moons, planets and asteroids Lunar Regolith: Single-mineral grains & rock fragments Impact breccias: heat-fused grains & rocks 2-8 m thick in Maria >15 m thick in the Highlands Apollo 11 Footprint Lunar Regolith Apollo 17 (Jack Schmidt) 6
Maria Apollo 17: Taurus-Littrow Valley Composed mostly of dark volcanic basalts rich in iron and magnesium. No water or hydrated minerals, unlike Earth basaltic lavas. Titanium content is 10x higher than Earth. Maria are flows of magma from deep fractures in the crust caused by the impacts of asteroid-sized bodies (many km) Ages of 3.1-3.8 Gyr: time of last major impacts Apollo 17 by Lunar Reconnaissance Orbiter Surface Composition Measured by taking ratios of light reflected at different wavelengths by Clementine orbiter (mainly) rover 7
Highlands Highlands are thoroughly pulverized by impacts. Older than the Maria as expected (3.8-4.0 Gyr) Age marks the end of an intense period of bombardment that started 4.6 Gyr ago Unusual mineral content of highland breccias: Suggests that the moon was almost completely molten 4.35 Gyr ago! Some idea of age from cratering Young surface Old surface Cratering History Mare Orientale (1000km) Bombardment was strongest 3.8-4.5 billion years ago The big impact basins formed 3.8-4.0 billion years ago (e.g. Mare Orientale) Volcanic flooding of the basins 3.1-3.8 billion years ago Topography Apollo 16 - Descartes Highlands 8
Summary of Results The Lunar Topography was entirely shaped by impacts and their effects: Maria are younger (3.1-3.8Gyr), forming after the end of the last epoch of heavy impacts Highlands are older (3.8-4.0 Gyr) and heavily cratered. Highlands may have solidified ~4.3 Gyr ago. Before that the Moon was mostly molten. Lunar Interior Crust: ~70 km thick on average, >100 in highlands ~10 km thick in the maria Thicker on far-side than near-side Mantle: Solid, makes up 90% of lunar volume Moonquakes, triggered by tidal stresses from the Earth, occur in the lower mantle Core: None? Seismography This is also known from seismography using instruments place by the Apollo astronauts The Moon has small earthquakes (magnitude 1-2) driven by the varying tidal stresses from the Earth over the elliptical orbit of the Moon Crust Mantle These occur much deeper (800-1000 km) than on Earth (1-10km) surface earthquakes are rare Also detected ~1700 impacts on the moon with estimated masses from 0.5-5000 kg Core? Is there a Lunar Core? The center of mass is offset from the center of the figure by 2km -- the crust is thicker on the far side than on the near side. This is currently unknown: Moon has no global magnetic field now, so no molten core like Earth. Samples of very old moon rocks show residual fossil magnetism they cooled in a magnetic field. Did the moon have a molten core and dynamo magnetic field like Earth 3.6-3.8 Gyr ago? Current data suggests it is unlikely at most about 3% of the Moon s mass, as compared to about 30% for the Earth 9
Origin of the Moon Any theory of Moon formation must explain these facts: Moon has much less iron than the Earth Moon lacks water and other volatiles Moon rocks most resemble Earth s mantle Identical proportions of 3 oxygen isotopes in Earth and Moon rocks (very different proportions are found in meteorites). Theories of Moon Formation Co-Formation: Earth & Moon formed as a pair Does not explain iron differences, or lack of volatiles. Capture: Earth gravitationally captured the moon Cannot explain iron differences, identical oxygen isotope ratios, or lack of volatiles Also is hard to do (but not a theory killer) Theories of Moon Formation Fission: Moon split off from a fast-spinning proto-earth Composition issues OK except the volatiles. Also hard to do. Giant Impact: (favored theory) Proto-Earth smacked off-center by a Mars-sized body Only iron-poor mantle stuff knocked off Moon formed from aggregation of the debris. The impact theory is favored. The impact theory has many strong points: Impactor s iron would have been in the core, and that core would have sunk into the Earth A molten post-impact moon would have boiled off all its volatiles. Moon formed of mostly Earth s mantle debris, explaining compositional similarities. Still questions, but it does the best so far. 10
Water? There are craters at the poles that are permanently shadowed from the Sun (white regions in the radar map) Lunar Prospector detected excess hydrogen in the polar regions Not certain it is water, but it is possible Try Again -- LCROSS Designed to do the crash experiment impactor satellite followed by a chase satellite (which also crashes) Oct. 2009 Spectra of ejected material confirmed presence of significant quantities of water Radar map Crashed Lunar Prospector into a likely region hoping to detect water spectroscopically in kicked up debris, but found nothing Smaller craters in the Aitken basin IR (Heat) emission from impact seen by chase satellite Recent News 11