I mentioned yesterday that besides water, the other interesting news from Mars Global Surveyor (MGS) is that they spotted lots of new craters, too (a crater in a new image was not in an older one). This allowed the scientists to determine the rate at which new craters are made, and they found that craters are created at a rate of about one per month. These aren't dinky ones either-- one they showed was the size of a football stadium!
Here's a before-and-after shot of a new crater in Arabia Terra in Mars' northern hemisphere:
It was created some time between January 1, 2004 and February 22, 2006. You can see the impact debris (what we astronomers in the know call "ejecta") spread out over an area a kilometer across.
The MGS team also mentioned that if you lived on Mars for about 20 years, on average you'd be close enough to one impact to actually hear it. Given that NASA plans on sending humans to Mars, this is a matter of real concern! It's a tough problem-- these are rocks that are maybe a few meters across, and so there is almost way to detect them. I have no idea how you could reliably find a large enough number of these potential impactors to do anything about them, and you really don't want one touching down near a settlement.
Anyway for more detail and some cool images, go to Emily's Planetary Society blog. She's an actual planetary scientist and has more intelligent things to say about this than I do (you can also see the original images on the Malin Space Science Systems site. Emily also has a statement by the Planetary Society about what all this means for us as humans. It's a good read.