Speaking of weird impact craters on Mars... Mars Express is a European Space Agency orbiter that's been snapping away at the Red Planet since late 2003. In August 2010 it took this picture of a bizarre feature on Mars:
[Click to impactenate.] I would've thought this was a canyon of some sort, but in fact it's an elongated crater! Most likely some large object broke up as it entered the atmosphere of Mars, striking the surface at a low angle and creating a series of craters that merged to form this strange thing. Unlike the triple crater I mentioned last time, this one is pretty frakkin' big: it's 78 kilometers (almost 50 miles!) long, 10 km (6 miles) wide at one end and 25 km (15.5 miles) wide at the other. Whatever hit here was pretty big, certainly over a kilometer across before it broke up. Probably several. In the high-resolution image you can clearly see a blanket of material surrounding the site, created as material ejected from the impact settled down. The narrow end of the crater seems to be perched right on the edge of a small cliff, and that's too much of a coincidence; it must have formed by the impact itself. Mars Express has a stereo camera as well, allowing 3D elevation images to be made. Check this out:
Wow, that really gives you a sense of what happened here. The impact must have been tremendous -- something like that happening on Earth now would be a near-extinction level event! At the top right, just inside the crater wall, you can see material that has slumped back down in a landslide kilometers across. The flow of ejected material can be seen as well. One thing I can't quite figure out: was the impact left-to-right or right-to-left? Looking at the edges and the ejecta blanket, I think I could argue convincingly either way. And interestingly there's another elongated crater to the northwest of this one, too. They're almost perfectly aligned, implying strongly they're associated with each other.
[UPDATE: Regular BABloggee Ivan Simic points out to me that in the full 3D image there are small impact marks to the right of the big one. These look like secondary impacts -- probably from material ejected from the first impact -- and they have their narrow ends pointing toward the big impact, as you'd expect from something hitting at an extremely shallow angle. I am now leaning toward the impact direction going from the narrow to the wider end of the scar; that is, right-to-left in the top picture (reversed in the 3D one). Come to think of it, the narrow end has a sharp rim, and the wider end is less well-defined, again consistent with a narrow-to-wider impact direction.]
The more I look at Mars, the cooler it gets. All kinds of weird stuff happened there, and we have literally only begun to scratch at the surface.