Unfinished Earth History and modern continuation of planetary accretion.
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Transcript of Unfinished Earth History and modern continuation of planetary accretion.
Earth’s Early History
• Hadean Eon – Earth’s first 500 Ma• Intense bombardment by bolides (meteors, comets, etc.)• No permanent crust – destroyed by impacts• Little direct evidence on Earth• Excellent evidence on Moon, Mars and other terrestrial
planets
Earth – the water planet
• Presently, Oceans cover 71% of Earth’s surface
• Odds of impact on land are 29%
• Water and oxygen weather rocks and destroy evidence of impacts
Crater Preservation
• Arid climate is more likely to preserve craters than humid climate
• Less weathering (chemical and physical breakdown) of rock and impact structure
• Less erosion – removal of structure by running water and/or wind
Crater in Australian “Outback”
Craters in Quebec
• Greater age – probably over 2 billion years old
• Much more humid climate
• Extensively eroded by glaciers in past 2 million years
Chicxulub – 65 Ma Impact
• Timing is close to disappearance of dinosaurs and other creatures – land & sea
• Was this the cause?• Other hypotheses
– Climate change– Extensive volcanism
• Straw that broke the dinosaurs’ back?
Gene and Carolyn Shoemaker
• Gene Shoemaker, geologist• Pioneer in study of impact
structures worldwide• Studied Meteor Crater, AZ• Well-respected in geology,
but not in astronomy• Until Shoemaker-Levy 9!
Meteor Crater, Arizona
• Young impact structure• Approximately 40,000
years old• Arid climate – excellent
preservation• Remnants of meteorite
found in crater and in ejecta
Significance of Shoemaker-Levy 9
• Convinced astronomers that such events were still possible
• Focused attention on near-Earth objects• Showed how little we know of such objects• Showed how completely unprepared we are• Is anyone doing anything?
In-class Activity: Jigsaw Presentations I
• Research assigned question; You may work in teams
• It is permissible to trade questions within your group, but you are responsible for one question
• Prepare a 2-3 minute presentation on your question (visual aids may be helpful)
• Wednesday: Questions 1-17• Friday: Questions 18-35• Monday: Questions 36-52