by Jim Nintzel
Phil Plait at Slate's Bad Astronomy blog highlights a recent photo captured by the UA's HiRISE camera aboard the Mars Reconnaissance Orbiter:
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.