Impact

The Solar System

• Nine Eight Planets• Over 170,000 catalogued asteroids• ???? Centaurs• ???? KBO’s (including one disgruntled ex-

planet)• Millions of Comets

Impact Mechanics• Energy of motion turns into heat• 10-m rock at 20 km/sec = 150 kt = 10

Hiroshima atomic bombs• Energy released in seconds• Impacting object destroyed• Crater 20-30 times diameter of impacting

object• Crater is round regardless of approach path

The Impact Process

• Atmospheric Entry• Contact-Compression • Penetration – Transient Crater• Excavation• Collapse

Impacts on Earth

Small Impacts Make Simple Pit Craters

A Simple Lunar Crater - Bessel

Recent Impact – Meteor Crater

Medium Impacts Create Central-Peak Craters

How Central Peaks Form

Tycho (93 km) Moon

Upheaval Dome, Utah

Peak Ring Craters

Schrodinger: A Peak-Ring Crater

Clearwater Lakes, Quebec

Large Impacts Result in Multiple-Ring Basins

Mare Orientale,

Moon

Comets and Planetary History

• Superposition• Crater Saturation• Crater Degradation

Superposition

Crater Saturation

Crater Degradation

Recognizing Craters on Earth• Craters rapidly destroyed by erosion• Circular geologic structures• Impact melts• Impact breccias• Shock metamorphism• Shatter cones

Manicouagan, Quebec

Sudbury, An Ancient Impact Site

A Shatter Cone

Fallback Breccia

Mega-Impacts:

As Usual, Gary Larson Gets There First

Computer Simulations by H.J. Melosh (University of Arizona)