Gazing upon the cratered surfaces of solar system objects like the Moon, Mercury, and Mars, it's hard not to think of them as having existed forever. After all, surely the forces that could produce such tortured terrain must be long gone. In a sense, it's true - many of these surfaces are billions of years old. And, certainly, the sorts of events which produced the continent-sized impacts still visible to this day are a distant memory. But that doesn't mean the surfaces of the planets, moons, and asteroids in the solar system are no longer changed!
One could hardly ask for a more visceral reminder of this than the crater recently discovered on Mars by NASA's Mars Reconnaissance Orbiter (MRO). MRO's HiRISE imager, one of the best instruments ever deployed to another planet, has returned hundreds of thousands of stunning pictures of the surface of Mars. Significantly, it also observes the same regions multiple times over the course of MRO's eight years in orbit. This allows it to record changing surface processes. On November 19th of last year, acting on an observation from 2012, it snapped the image seen above: a brand new 30-meter crater on the surface of Mars!
Now, a new crater on the surface of Mars isn't too surprising - after all, hundred of objects strike Mars (and the Earth!) every year. But this one is unusually large, making it easy to pick out in HiRISE imagery. So when did the impact occur? Sometime between July 2010 and May 2012 - the interval between HiRISE images of this region.
Discovering a young crater like this is quite valuable. Planetary scientists and geologists can now watch how the Martian weather affects it in near real-time. And this isn't just interesting for its own sake: the age of every surface in the solar system is determined by the craters embedded in it. In order to find the precise age of a surface, we need to pick up some of its rocks, take them to a laboratory, and measure the ratios of certain elements within. We've done this for only one body - the Moon, using lunar rocks returned by the Apollo astronauts. For every other surface (Mercury, Mars, moons, asteroids, etc) we make a simple assumption: that craters build up on surfaces around the solar system at about the same rate everywhere. Thus, an area with about as many craters as a similar region on the Moon likely has a commensurate age. So, understanding how craters degrade could have a very real impact (no pun intended!) on the ages of objects everywhere!