Current Cratering Rate

The above figure shows the cratering rate for the Moon in units of number of craters with diameter > 1 km formed per year. The cratering rate for the land masses on the Earth is roughly 10 times higher (can you think of reasons why?). As a first guess at the cratering rate on the Earth, in Homework 5 you count impact craters on the Earth to estimate the cratering rate.

• Based on the current impact rate, events like the one that supposedly got rid of the dinosaurs, the impact of the object which formed the Chicxulub crater in the Yucatan 64.98 million years ago, diameter ~170 km, occur 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 hundred-thousand years or so on the Moon which means that such an impact roughly occurs every 10,000 (or so) years on the Earth.

Lunar Chronology (and the history of the Cratering Rate)

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