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Images of Mercury's surface continue to arrive from NASA's MESSENGER probe (MErcury Surface, Space ENvironment, GEochemistry, and Ranging), after reaching stable orbit about the inner-most planet on 17th March.
Of all those that have so far arrived, one of the most interesting is shown above, taken on 24th April, showing an unnamed crater crossed by three lines of small craters. These small craters, all of very similar dimensions, could not possibly be a chance alignment of random impacts, the chances of this happening are far too low.
Lines of craters such as these are also present on other large bodies of the solar system, for example
the Moon and Jupiter's satellite Ganymede, and are thought to be due to the impact of disintegrated comets whose fragments remain close to eachother on the same orbit (exactly what happened to comet Shoemaker-Levy 9 before impacting on Jupiter).
When the flock of material impacts on a planet or moon the pieces crash one after another, in slightly different places, like a volley of bullets from a machine gun. The result is what is seen in the photo.
It is certainly very strange to see two such crater chains overlapping eachother, and with a third crossing one of them. All within a larger crater!
This incredible situation, and the seemingly very small chances of such a super-position, might suggest that such cometary fragment "trains" are actually more common than thought near the Sun (just think of all the comets identified by SOHO), and that the surface of Mercury could have suffered many more impacts than more distant bodies.
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