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Unformatted text preview: Terrestrial planet formation In the inner Solar System, think dust (no ices) grew in size via collisions + sticking Initial phases don't involve gravity - too weak for dust particles, cm and meter-sized bodies Form planetesimals with sizes of km and after that, gravity dominates the further growth Rapid formation (< 1 million years) of large bodies - sometimes called planetary embryos (Moon to Mercury size) Slower assembly of 50-100 embryos into final terrestrial planets. 10 - 100 million years. Extraterrestrial Life: Spring 2008 Extraterrestrial Life: Spring 2008 Formation of the giant planets Most popular theory for giant planet formation: core accretion. Cores of the giant planets (M ~ 10-20 Earth masses) form just as the terrestrial planets, but faster because of the additional surface density of icy materials beyond the snowline. Cores then capture envelopes of gas from the disk before the gas is lost (within ~10 million years): several hundred Earth masses for Jupiter smaller envelopes for Saturn, Uranus and Neptune One of the last giant impacts is thought to have formed the Moon (SwRI / Robin Canup simulation)
Extraterrestrial Life: Spring 2008 Extraterrestrial Life: Spring 2008 Water on the Earth Temperature in the protoplanetary disk at the location of the Earth was too high for water to condense Meteorites from asteroids inside about 2.7 AU are very water-poor (0.1% by mass or less) Possible delivery of water from: comets asteroids in the Main Belt Constraint: composition of the Earth's oceans 2 isotopes of hydrogen: ordinary hydrogen (1 proton in atomic nucleus) deuterium (`heavy hydrogen') - 1 proton plus 1 neutron in the nucleus The ratio of deuterium to hydrogen in the Earth is measured at ~150 parts per million (ppm) Extraterrestrial Life: Spring 2008 Extraterrestrial Life: Spring 2008 Movies: Derek Richardson 1 Total mass of water on the Earth is estimated at about 0.05% (5 x 10-4 Earth masses). If this was delivered by asteroids that are 10% water by mass, need: M asteroids = 10 " 5 "10#4 M Earth = 5 "10#3 M Earth
Carbonaceous chondrites: meteorites thought to come from outer asteroid belt D/H: 120-180 ppm ...about 10 times the current mass of asteroids in the asteroid belt. Comets: D/H ratio of about 300ppm !
Suggestion: initial asteroid belt was much more massive, formation of Jupiter destabilized orbits and led to asteroids impacting the Earth Seems most likely that the Earth's water came from the asteroid belt region rather than from comets
Extraterrestrial Life: Spring 2008 Extraterrestrial Life: Spring 2008 Giant planets also play a role in the outer Solar System: Jupiter ejects comets into the Oort cloud (protects Earth) Evolution of the outer planet orbits may have captured Pluto into its unusual, Neptune crossing orbit, and scattered debris into the inner Solar System We can study all this in detail in the Solar System: how important are similar processes in other planetary systems? See the influence of Jupiter on the asteroids in the number of asteroids at different distances from the Sun One way in which giant planets may be important for life
Extraterrestrial Life: Spring 2008 Extraterrestrial Life: Spring 2008 2 ...
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- Spring '08