Impacts on Earth Barringer (Meteor) Crater, Arizona 50,000 years old

Impact Frequency of meteors and comets Size Effect of atmosphere 50,000 /yr > 0.1 kg > 1 kg 13,000 /yr Significantly slowed > 10 kg 2000 /yr broken up (10 m) Few /1000 yrs unaffected (100 m) 1 /5000 yrs (1 km) 1 /300,000 yrs (10 km) 1 /100,000,000 yrs

Because there are more small asteroids than big asteroids, many more small craters form than big ones.

55 impact features in North America Over 200 recognized across Earth 50 larger than 100 km

Vredefort basin, South Africa Oldest (2 By) and largest (300 km) impact found on Earth

Chicxulub 65 My 180 km across - 10 km asteroid or comet Yucatan Peninsula

Cretaceous-Tertiary (K-T) Boundary Event Mass extinction at 65 My 70% of all species, 90% of all marine species, 100% of all land species larger than 10kg Possible culprit: 10 km asteroid or comet hits Earth causing global firestorm intense volcanism massive tsunamis loss of ozone layer 6 months of darkness from dust Long-term climate change Severe acid rain

Chesapeake Bay Impact ~35 Million years ago Probably originally ~40 km wide

Tunguska Event Remote Siberian forest, June 30, 1908 2000 km2 of scorched then flattened trees; no crater, no meteorites 30 m asteroid that exploded on hitting the atmosphere? Frequency: every 1000 years?

Near-Earth Objects (NEO’s) ~ 500 known asteroids that cross or come close to Earth’s orbit Probably millions, with ~2000 larger than 1 km Source: asteroid belt Collisions in asteroid belt, Some pieces are perturbed by Jupiter into elliptical orbits Usually they collide with Sun or planet or ejected from solar system ~ typically 10-100 My (exposure age of the meteorite surfaces)

Computer Simulations 1 km comet striking at 60 km/s