Cratering and Crater Counting
- Debris continually rains down upon all surfaces
in the Solar System.
- This material is left over from the
formation of the solar system.
- The debris ranges in size from dust grains to
comets and asteroids tens of kilometers in diameter.
- When a meteoroid impacts a planetary surface at speeds of
tens of kilometers per second its energy of motion is
instantly converted to heat. The resulting explosion
excavates a crater.
- Since the energy of motion of the object is coverted into
an explosion the resulting crater tends to be round regardless
of the direction of impact. (Although slow small impactors can excavate elongated craters).
- The impact/explosion flings material out in all directions
creating an "ejecta blanket" of shattered rock around the
- If the impact is energetic enough the planetary surface
can rebound, producing multiple rings and a central mountain peak in the crater. - - - -
- The rain of debris was much heavier in the
past (just after the solar system formed) and has since tapered off
to a steady drizzle.
- Craters gradually accumlate on an old planetary
surface like a sidewalk getting wet in the rain. You can tell
how long it has been raining (at least at first) by how wet
the sidewalk is.
- In a similar fashion you can estimate the
``age" of a planetary surface by the density of craters.
- Many Craters: ``old"
- Few Craters: ``young"
- Note: ``Age" means time since solidification of the surface or the
time since the last resurfacing. All of the planets and
satellites are 4.6 billion years old.
- Resurfacing mechanisms
Revised October 23, 2006