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The Planetary Society Blog
By Emily Lakdawalla
Cool crater on Mars from Mars Express
Jan. 5, 2006 | 06:19 PST | 14:19 UTC
Here's a nifty image that was recently released by the Mars Express imaging team (though it was actually captured more than a year ago): Elongated crater, Hesperia Planum, Mars Mars Express captured this image of an elongated crater on May 5, 2004. The crater is about 25 by 11 kilometers (15 by 7 miles) in size. Such an elongated crater forms when the impactor comes in from a very low angle, less than 10 degrees. The image also contains several examples of "rampart craters," craters that are surrounded by an ejecta blanket with an abrupt edge, as well as many curving "wrinkle ridges," features that form when tectonic forces squeeze the ground and cause it to buckle. SourceCredit: ESA / DLR / FU Berlin (G. Neukum) |
Elongated or elliptical craters such as these are found on the Moon and Mercury and other places, but seeing one makes you wonder: why aren't more craters elliptical? If impactors can come from pretty much any direction, then most impacts should happen with the impactor coming in at some angle, that is, they should be "oblique" impacts. Instinct tells you that if an impactor comes in at a shallow angle -- say, 40 or 30 degrees from horizontal -- then it should blast a crater that's elongated in the direction that the impactor was traveling. But it just doesn't happen that way. Impact experiments have shown that because of the way the huge kinetic energy of the impactor gets transferred to the surface that it hits, you wind up with a circular crater pretty much regardless of the angle that the impactor arrives at. It's only when the impactor comes in from the very lowest of angles, the most glancing of blows, that you get an elongated crater like this one. Some experiments with super high speed photography have caught one of the mechanisms by which you can get an elongated crater in action. When an impactor hits, the bottom of the impactor starts blasting a crater, but the top of the impactor tries to keep moving and shears off, impacting the surface down-range. The experimenters call this "decapitation" of the impactor.
And here's a thought: one good way to get an impactor coming in at a tremendously shallow angle is to de-orbit a moon. One of Mars' moons, Phobos, orbits close enough to Mars that its orbit is decaying over time, getting closer and closer to the surface. One of these days, its orbit is going to try to take it below the surface, and there will be a new elliptical crater -- and one less moon -- at Mars.
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