Impact Craters and Age - Review

Finding ages of objects Absolute ages are assessed through radioactive decay (1/2 lives) – Carbon 14, half-life of 5700 years – Uranium 238, half-life of 700 million years! Requires lab analysis of physical samples

How it works Each half life reduces the amount of the radioactive element by 1/2 – 2 half lives = 1/4 the amount left – 3 half lives = 1/8 the amount left If a half life is 700 million years, and there is 1/8 the original amount left… – 1/8 amount = 3 half-lives x 700 million years = ??? – 2100 million (2.1 billion) years.

No lab samples, no problem! Craters can provide a RELATIVE age – Formation of craters (and other features) will not always happen in a fresh area. – They overlap each other!

Crosscutting and Superposition Older craters/features are broken up and overlaid by newer features – Superposition: craters overlapping others – Crosscutting: features sliced through by channels Try it again:

Try again A B C

Crater Classification Erosion of a crater can happen *anywhere* – (it is just faster on some astronomical bodies that others…) – The appearance of a crater can reveal its (relative) age – 3 main classes Preserved (A) Modified (B) Destroyed (C)

Describe the main feature - HYPOTHESIZE!

Which is the older surface? How do you know?

A second method Crater Density Method! It’s pretty simple: – More craters = older! This assumes relatively stable rates of impacts over time. Also, assumes relatively even distribution.

Famous impacts Many on earth, but hard to see sometimes… Not always though!

Barringer Crater Arizona, 50,000 years ago 1.2 km diameter, 0.2 km deep Created by 300,000 ton iron meteor, 50 m 20 km/s

Tunguska Event 1908, Siberia – Blue column of light – Bright as the sun! Object airburst – Broke apart before hitting the ground Blast = 1000x hiroshima – Knocked down 80 million trees over 2,150 km 2 – Blew out windows several hundred km away

Peekskill Meteorite 1992, Eastern US 12 kilo stony meteor First major meteor captured on video from multiple angles/areas Recovered and sold for $70,000

Chicxulub! Dino Killer! PDX sized object! 200 km crater! Global effects!

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Shoemaker-Levy fragments Each left an impact scar the size of earth.