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

2. 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

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

4. 55 impact features
in North America
Over 200 recognized across Earth
50 larger than 100 km

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

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

8. 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

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

10. 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?

11. 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 My (exposure age of the meteorite surfaces)

12. Computer Simulations
1 km comet striking at 60 km/s