It happens all the time. Happens all the time for Earth also. But Mars has many more impacts because it is closer to the asteroid belt. Also in addition to that, has many more of the smaller craters because on Earth many of the smaller asteroids burn up completely or almost completely in the atmosphere and don't make a crater while on Mars then the air is so thin that even meteorites too small to make a crater will often make it intact all the way to the ground. They've spotted 400 new craters on Mars so far. New as in they have before and after images, without and then with the new crater, so they actually "spotted" the crater form though often not able to pin it down to the exact day it happened.
This is the biggest so far, 48.5 meters wide, and they also can tell when it happened too, rather precisely with before and after images, between 27th and 28th March 2012.
Large, Fresh Crater Surrounded by Smaller Craters - Mars Reconnaissance Orbiter
NASA Mars Weathercam Helps Find Big New Crater - Mars Reconnaissance Orbiter
As for estimates for crater rate, here is one example, this is a theoretical curve:
from mars | MrReid.org
There are lots of technical papers in the literature attempting to figure out the exact impact rates of different sizes of crater on Mars. For one thing, it makes a difference to estimates of ages of surfaces which is usually done by counting the craters in them.
This paper from 2013 shows the location of 248 dated impact sites on page 512
http://www.lpl.arizona.edu/~shan...
So since it's now up to nearly 500, that's like 100 new impact craters a year that they are detecting, very approximately, large enough to spot from orbit
This is the biggest so far, 48.5 meters wide, and they also can tell when it happened too, rather precisely with before and after images, between 27th and 28th March 2012.
NASA Mars Weathercam Helps Find Big New Crater - Mars Reconnaissance Orbiter
As for estimates for crater rate, here is one example, this is a theoretical curve:
There are lots of technical papers in the literature attempting to figure out the exact impact rates of different sizes of crater on Mars. For one thing, it makes a difference to estimates of ages of surfaces which is usually done by counting the craters in them.
This paper from 2013 shows the location of 248 dated impact sites on page 512
http://www.lpl.arizona.edu/~shan...
So since it's now up to nearly 500, that's like 100 new impact craters a year that they are detecting, very approximately, large enough to spot from orbit
About the Author
Robert Walker
Writer of articles on Mars and Space issues - Software Developer of Tune Smithy, Bounce Metronome etc.Studied at Wolfson College, Oxford
Lives in Isle of Mull
4.8m answer views110.3k this month
Top Writer2017, 2016, and 2015
Published WriterHuffPost, Slate, and 4 moreHuffPost, Slate, Forbes, Newsweek, Business Insider, and Science 2.0