‘Butterfly’ impact crater in Hesperia Planum

http://www.esa.int/SPECIALS/Mars_Express/SEMZLM8A9HE_0.html

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contact trinary?

It MIGHT have been from a meteoroid that broke up in the atmosphere immediately before impact; but ESA states that it's probably due to an object that came in at a very shallow angle (<10 degrees).

Mars has a 10x or so excess of oblique impact craters compared with the moon and mercury and ganymede/callisto. Back in the 80's, Pete Schultz noticed that they vary in orientation of the long axis of the craters, with the older, more degraded having a greater tilt relative to the equator than the younger ones. He proposed that the majority of them are the results of tidal decay of a pieces of one or more disrupted moons that were close to Mars than Phobos, and that their orientations reflect the equatorial plane of Mars *AT THE TIME* of the tidal decay and impacts. He argued (others have suggested, also) that the formation of the Tharsis bulge caused a shift in the moments of inertia of Mars, and caused true polar wander from some 4 billion years ago to the present, as Tharsis grew.

I don't think there were any really strong arguements against his hypothesis, but it was not generally accepted and there was little or no followup research by the early 90's on the idea. I don't know if there's been any more since.

Most of the papers I've seen recently on polar wander on Mars seem to be making arguments from magnetic and gravity anomalies, though there was one interesting one by Jafar Arkani-Hamed recently based on the locations of impact basins:
Giant impact basins trace the ancient equator of Mars

There are a lot of people doing work with Mars Geographic Information Systems (GIS) these days. It seems like this would be an interesting problem for a grad student to follow up Pete Schultz's ideas using GIS.

--Emily

Oddly enough, way back when I asked a similar question in this forum regarding the orientation of impact craters but never got much response - I was, however, thinking very much of the big southern basins. The matter of crater-chains, butterfly craters, clusters etc had escaped me, however. I'd think that with the current level of Mars mapping, it's down to global statistics and that the resolution of what should be a series of smooth distributions into peaks and troughs shouldn't be too hard a task. Naturally, there will be a range of perturbing factors to take into account, such as the polar wandering of Mars, but out of zillions of craters some patterns ought to emerge.

I *like* crater counting - so long as somebody else does it (though, in all honesty, this is just the job for somebody with the GIS software and a clever search program, so there's no need to feel too sorry for the poor devil who actually does the work!).

Bob Shaw