There is much interest in determining the likelihood of future asteroid and meteorite impacts on Earth. It is very difficult to estimate the probabilities of such events but we can learn something by looking to one of our solar system neighbors – Mars. Because the surface of Mars changes so slowly new impacts there can be readily identified. This allows the number of impacts over a few years and the general size the objects making those impacts knowable. With another decade of data collection we should get a rough idea of the typical number of collisions that a planet the size of Mars makes with rocks flying through space. Although, most of these impacts would not concern us here on Earth because our atmosphere protects us from the many small impacts that we see on Mars, it should still allow us to have some idea of what the frequency of impacts could be for objects large enough to penetrate our atmosphere.
Yesterday I noticed the release of recent photo made available from the HiRISE satellite orbiting Mars. That photo was taken in July and clearly shows a fresh impact crater. How fresh? I am not sure when the last pass was made over this location but this is a newly found impact crater and the entire surface of Mars has been mapped within the past 5 years so this impact occurred within that time span.
Below is a cropped closeup of the actually impact site showing a very small crater I estimate from the original image to be 30 to 40 meters in diameter. The impact must have been nearly vertical judging from the nearly radial debris field surrounding the crater. Had this object struck the earth, it may have reached the surface but it would not have had the same impact because the atmosphere would have taken the brunt of the damage. The Chelyabinsk meteor that broke up in the atmosphere over Russia a few years ago was estimated to have been about 20 meters in length. Had it struck the ground without breaking up the impact might have looked something like this crater on Mars.
It is difficult to see but this impact crater lies on the crater rim of a larger (100 to 150 meter diameter) crater. Over the last decade detailed mapping of the surface and identification of new impact crater has allowed for estimates of current rates of change to the surface of Mars to be obtained by watching how quickly the dark debris lightens over time.
You can see from the larger image that there surface of Mars has many impacts craters but most of these are highly eroded. Without water erosion and a very thin atmosphere, the time required to erode these craters to their present states must be enormous. On Mars we can see an amazing series of craters ranging from barely recognizable highly eroded craters to ones that have formed in the past year. This tells us that Mars has been bombarded by objects over a very long period of time.
I have written several times about craters on Mars and their significance with respect to the question of the age of the Solar System:
Naturalis Historia 