Current Cratering Rate

The above figure is for the cratering rate of the Moon. The cratering rate for the land masses on the Earth is roughly 10 times higher (can you think of reasons why?).

• Based on the current impact rate, the event which got rid of the dinosarus (crater diameter ~100 km) occurs once every billion years or so on the Moon. For the Earth, this means that such an impact occurs roughly every 100 million years.
• The impact which formed the Barringer Crater in AZ (diameter ~1.2 km) occurs roughly every few thousand years on the Moon which means that such an impact roughly occurs every 10,000 (or so) years on the Earth.

History of the Cratering Rate (and the Lunar Chronology)

Event                                 Billions of Years Ago

Lunar Formation                              4.6

   surface soft

   rate is unknown because of
   cratering saturation--craters
   overlap destroying evidence of
   earlier impacts

Crust Formation                              4.4

   Heavily cratered highlands regions
   are formed 

Start of Heavy Bombardment                   3.9

   Basin Formation 

   final heavy bombardment
    (final surge due to an unique
     event--debris from disruption 
     of an asteroid?)

Maria Formation (filling in of basins with
  lava flows                               
                                           3.9-3.2

Slow constant bombardment                    3.8

Today                                         0