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Earth Impacts

Why study impact structures on Earth?
Approximately 160 impact structures have been identified on Earth.
Major role in formation and early history of the Solar System and the objects in it.
Mass extinctions have been influenced by impact events.
Poses a threat to life today

Accretion Process

Accretion theory of planetary formation - planets form as planetesimals collide - grow into larger planetesimals - protoplanets form as collisions continue
This process formed the planets and is continuing today but at a much slower rate.
There are still thousands to millions of small and large planetesimals in space.

Small Impacts

At this moment, collisions of extra-terrestrial objects with Earth are occurring by the billions.
Current rate? (tons per day)
Almost all this material enters Earth's atmosphere as small particles

Meteors

Meteoroid:
Meteor:
Meteorite:
Micrometeorite:

Small Impacts

Meteoroids a few centimeters to a few meters can survive the passage through the atmosphere to become meteorites.
May produce penetration craters or penetration funnels.

Large Impacts

Rare events on Earth
Large meteoroids, asteroids or comets - a few meters to 1000's of meters in diameter.
Large planetesimals are able to pass through the atmosphere without losing much mass or velocity.
Create hypervelocity impact crater upon collision with the surface.

Close Encounters

1908:
1947:
1972:

Risk Factors

Problems of prediction:
1) Low Frequency:
2) High Velocities:
Typical velocities:
3) High Energy Release:
Energy Released Equation:
What is the energy release of a 21 ton object moving at 20 kps?

What's The Risk?

Crater Diameter	Object Diameter	Energy Release	Impact Interval
450 m	23 m	1 MT	370 yrs
1 km	50 m	11 MT	1,600 yrs
5 km	250 m	1,400 MT	28,500 yrs
10 km	500 m	11,000 MT	100,000 yrs
20 km	1 km	87,000 MT	350,000 yrs
50 km	3 km	1,300,000 MT	4,500,000 yrs
100 km	5 km	11,000,000 MT	26,000,000 yrs
200 km	10 km	87,000,000 MT	150,000,000 yrs

Damage Potential

Large impacts are rare events on Earth
Huge energy release from a single object 23 m or more in diameter could completely destroy:
Larger objects could completely destroy:
The largest objects could devastate:
The largest objects could devastate:

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