"Water on Mars!" Headlines like this almost write themselves. The media jump at the opportunity to trumpet such reports — and what the implications may be for life on other worlds. It happens with Mars (frequently), Jupiter's moon Europa (often), and even Saturn's moon Enceladus (the latest entry in the water/life sweepstakes).
So, media accounts came as no surprise when planetary scientists announced in early December the discovery of fresh deposits in a couple of martian gullies. The light-colored material appears to be debris carried downhill by flowing water, which can exist for a short time before freezing in the frigid conditions. Michael Malin of Malin Space Sciences System and his colleagues found the fresh deposits by comparing Mars Global Surveyor (MGS) images from early in the spacecraft's mission with more recent ones.
But there's more to this story. The water-on-Mars headlines won out over an equally important aspect of the same research: the current rate at which craters form on Mars. Even the title of the Malin team's report — "Present-Day Impact Cratering Rate and Contemporary Gully Activity on Mars" (Science, December 8, 2006) — gives precedence to the formation of craters.
The researchers found 20 fresh impact sites with craters ranging from approximately 7 feet (2 meters) to 490 feet (150m) in diameter. Seven of the 20 impact events produced multiple craters. Most of the new impact craters appear dark, suggesting the impacts removed light-colored dust. However, a few show light-toned ejecta, implying the impact excavated lighter material from beneath the surface.
The observed cratering rate on Mars appears consistent with that of our Moon. This means areas devoid of craters must be young (in a geologic sense), and some as-yet-unrecognized process must remove older craters. It also means that, if you lived on Mars, chances are a meteoroid would hit close enough that you'd hear an impact every 10 to 20 years.
You didn't hear as much about the cratering rate as you did about possible water because — let's face it — water on another world is far more exciting than a crater. For most people, a crater isn't worth noticing unless it lands in your backyard. But science is often about the seemingly mundane, and what it can tell us about how the universe works.