Unfinished Earth History and modern continuation of planetary accretion

Impacts and Popular Culture “Armegeddon” “Deep Impact” Dinosaur Extinction – 65 Ma

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

Craters on the Moon – Hadean and later bombardment

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

Location of Craters on Earth

Craters in North America

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

Shoemaker-Levy 9

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 Monday: Questions 36-52