1 October 2008
A cool paper just came out in Icarus this week claiming that a crater in the northern plains of Mars may be the result of the impact of a small moonlet of Mars, possibly just smaller than Deimos. Here’s the crater:
Because the crater is so elliptical, and because of the “blowouts” on the eastern edges of the craters (where ejecta actually blew back through the crater rim), it’s likely that these craters were formed by 2 impactors entering the atmosphere at a very shallow angle, traveling to the west. The impactor probably fractured into 2 pieces as it entered the upper atmosphere. Usually, these kind of impacts are attributed to comets or to asteroids that enter the atmosphere at an unusually shallow angle, but the small separation between the craters implies that the impactor wasn’t going fast enough to be an asteroid when it hit the atmosphere.
So, that means that the most likely impactor that formed these craters was a moonlet. One idea that’s been floated is that Mars used to have a good size population of captured asteroids in orbit as moonlets, but that Deimos and Phobos are the only ones that haven’t succumbed to tidal decay. At least, not yet – Phobos will probably impact the surface sometime in the next 50 million years, and might even leave behind it’s own set of footprints!
Correction: The blowouts are actually due to the impactor and debris travelling forward from the impact, which means that the impactor was travelling west to east. This is the same direction that Mars rotates, which would put the moonlet in a prograde orbit, like Phobos and Deimos. (Thanks JEC!)