When an asteroid or comet impacts a planet, the explosion ejects huge amounts of material, sending it flying in all directions. But there are also plumes of material, long fingers of rock and dust that stream out as well. The boulders and such inside this plume then fall back to the ground, making linear chains of secondary craters. We see lots of these on our Moon, moons in the outer solar system, and Mercury, too.
If these features are long enough, it's inevitable two chains from two different primary craters would cross somewhere. And it turns out this has been seen... but where?
Well, X marks the spot!
This MESSENGER image of Mercury shows exactly that: two crater chains from two separate impacts crossing over each other (and a third, shorter chain is at the bottom, too). They're almost exactly perpendicular to each other, which is cool, and the intersection happens to lie in a big, shallow crater about 120 km (72 miles) across that fills this image. Unfortunately, MESSENGER hasn't been orbiting Mercury long enough to have surveyed the whole planet yet, so I wasn't able to find the source craters of these two chains.
Interestingly, both chains have elongated craters at their ends, one on the upper left and the other at the top. That indicates a very low-angle impact; anything hitting the ground from an angle above about 10° tends to make a circular crater. However, the one on the left appears to be right on the big crater's rim, so the elongation may be due to the ground angle changing. The other may be coincidence; both are far too small to have been the source craters for the chains.
I'm not sure there's any real scientific value in knowing these crater chains intersect or examining the intersection in detail. Still. They're fun to look at, fun to explore, and they're just seriously nifty.
Image credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington
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