Mars is weird. Right? I mean, it's a whole other planet. So you expect it to be weird.
But then I see pictures like this one from the Mars Reconnaissance Orbiter's HiRISE camera, and I am reminded just how weird it is:
[Click to chicxulubenate.]
Most craters you see are pretty simple: something impacts the ground at high speed, BOOM!, and you get a crater like a dish tossed into soft sand. But this one has two rings, one inside the other. That can happen with huge impacts producing craters hundreds of kilometers across, but this one is small, only 230 meters from side to side - an American football stadium would just fit inside this crater.
The most likely explanation for the double ring is that the Martian landscape here is layered. There's rock and sand on the surface, but underneath that is a layer of ice. The big rim is from the displaced rock, and the inner, smaller ring is from the impactor plowing through the ice. Each layer has a different strength - rock is harder than ice - so it's as if two craters were formed, one inside the other. Radar observations of Mars from orbit have indicated there's ice under the surface in this region, so that fits.
Similar double-ringed craters have been seen on Mars - though the structure and history is by no means well understood! - and some have been found on the Earth's Moon as well. Those tend to be big, as I mentioned, though they don't have to be.
By the way, the image above is color enhanced to show details. The blue may be from carbon dioxide frost, which can be seen in similar color-enhanced HiRISE images. The ripples in the center are sand dunes, sculpted into parallel waves by the ceaseless Martian wind.
Craters this small on Earth are extremely unlikely to form; the impactor would be maybe 20 meters or so across, and objects that size tend to break up when they ram through our thick atmosphere at high speed. Mars has much thinner air, so rocks that size can hit intact. Studying craters on Mars is a chance to see what these hypervelocity impacts are like under very different conditions, which helps us understand them. The physics of extremely high-speed collisions is hard to study experimentally - accelerating large objects to that kind of speed is both difficult and more than slightly dangerous - so it's nice to have a lab like Mars where we can observe these effects.
Tip o' the lens cap to HiRISE on Twitter. Image credit: NASA/JPL/University of Arizona.