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SYFY WIRE Bad Astronomy

Kablam! The Newest Crater on Mars.

By Phil Plait

Meet the newest (as far as we know) impact crater on Mars.

That shot, taken by the HiRISE camera on board the Mars Reconnaissance Orbiter, shows a fresh crater about 10 meters across on the surface of the Red Planet. You can tell it’s new by the crispness of the ejected darker material around it; in older craters that gets eroded by wind, erased slowly over the centuries.

Also, it helps that we have a camera that takes pictures of the surface all the time, so we can literally get before-and-after shots:

That picture was taken by the Mars Context Camera on board MRO; it sees much wider swaths of the planet, providing context for the much narrower HiRISE shots. It also looks for changes in the landscape, generally due to weather and other natural forces … like, say, small asteroid impacts. The before picture is from February 2012, and the after from June 2014. That means when the second image was taken, the crater was at most 28 months old. Fresh indeed!

Update, Jan. 12, 2015, at 15:30 UTC: To be perfectly clear, this is very likely to be the youngest crater of decent size on Mars. There are other craters on Mars that are also quite new; one appeared in an image from May 2012, so it's likely to be older, and another apparently was created by an impact on March 27, 2012, so again it's likely to be older. 

On Earth, you don’t usually get craters this size; our atmosphere slows and stops smaller rocks from space. The air on Mars is less than 1 percent the thickness of ours at the surface, so smaller rocks get through.

The impactor apparently came in from the west (left). It dug a hole big enough to excavate material under the surface, which appears much darker. You can see the characteristic splatter pattern of chunks thrown out, dripping debris down in long plumes, too. This is pretty amazing; we don’t usually see this so cleanly in new craters!

The streak is interesting, too. [Rampant speculation follows.] I wonder if that’s from the shock wave as the rock slammed through the air. The rock would’ve been moving hypersonically, far faster than the speed of sound, so it would generate a shock wave, compressing the air in front of it violently. The rock hit first, then the shock wave thundered down. If I’m seeing this correctly, material blown up by the impact would have blown upward, and then the air moving above and to the left of the impact site would’ve been like a wall, blocking that debris from falling to the left. The material moving to the left would have fallen around the column of air, leaving that streak.

This may seem like just gee-whiz stuff, data like this is important. For one, impacts at interplanetary speeds are really hard to replicate in the lab, and the physics of hypersonic impacts is incredibly complicated. This is a nice in situ experiment for us to study.

For another, we’ve seen a lot of new craters on Mars, and this gives us a rate for impacts, something, oddly enough, astronomers and others who wish to prevent similar asteroid impacts on Earth are interested in.

I suspect it also piques folks who are trying to figure out the ages of surface features on Mars; by watching this new crater over time, we’ll get an idea of how long it takes the various bits of it to fade.

For me personally, this is a reminder. When I was a little kid, we didn’t know much about Mars. Then we started sending probes there, and they returned pictures of a dry, dead planet, frozen in time. But that’s not true at all; Mars is a dynamic world, and that only becomes apparent when we go there and stay there. Change takes time, so we must spend the time to see it.

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