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

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 18-35• Monday: Questions 36-52