Impact Craters and Age

Impact Craters and Age - Review

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

How it worksEach half life reduces the amount of the radioactive element by 1/22 half lives = 1/4 the amount left3 half lives = 1/8 the amount left

If a half life is 700 million years, and there is 1/8 the original amount left1/8 amount = 3 half-lives x 700 million years = ???2100 million (2.1 billion) years.No lab samples, no problem!Craters can provide a RELATIVE ageFormation of craters (and other features) will not always happen in a fresh area.

They overlap each other!

A B CCrosscutting and SuperpositionOlder craters/features are broken up and overlaid by newer featuresSuperposition: craters overlapping othersCrosscutting: features sliced through by channels

Try it again:Try again

ABC

Crater ClassificationErosion 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) age3 main classesPreserved (A)Modified (B)Destroyed (C)

Describe the main feature - HYPOTHESIZE!Which is the older surface?How do you know?

A second methodCrater Density Method!

Its pretty simple:More craters = older!

This assumes relatively stable rates of impacts over time.

Also, assumes relatively even distribution.Famous impactsMany on earth, but hard to see sometimes

Not always though!

Barringer CraterArizona, 50,000years ago

1.2 km diameter, 0.2 km deep

Created by 300,000 ton iron meteor, 50 m across @ 20 km/s

Tunguska Event1908, SiberiaBlue column of lightBright as the sun!

Object airburstBroke apart beforehitting the ground

Blast = 1000x hiroshimaKnocked down 80 million trees over 2,150 km2Blew 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!

k-t

Shoemaker-Levy 920+ fragmentsEach left an impact scar the size of earth.