The lunar landscape is ancient, pocked with craters and mountains that are billions of years older than anything on Earth. Because the Moon has no atmosphere, there is no erosion to wear down geographical features. Because the Moon is (geologically speaking) dead, with no churning magma beneath the crust, there’s no volcanic or tectonic activity to lift up new mountains. Almost every major feature we see on the Moon has been there for almost 4 billion years (since the Late Heavy Bombardment phase of the Solar System’s development ended).
But even the Moon isn’t completely static — there is still plenty debris moving around in our region of space, and some of it continues to rain down on the lunar surface. The image to the left is a comparison of two photographs of a region of the Moon taken 38 years apart. The red circle highlights a bright new crater – at some point between these images being taken by the Apollo 15 mission in 1971 and the Lunar Reconnaissance Orbiter in 2009, a sizeable chunk of rock collided with the moon at a speed of tens of thousands of kilometers per hour.
While we’ve always known that such impacts happen continuously (in fact we see them regularly burning up in Earth’s protective atmosphere), this is the first time we’ve seen direct evidence of an impact on the moon after the fact. Which is rather cool!
For more information, have a look at the formal announcement from the LRO team here: http://lroc.sese.asu.edu/news/index.php?/archives/260-New-Impact-Crater-on-the-Moon!.html