If someone woke me out of a sound sleep and forced me at gunpoint to say which is my favorite camera in the solar system, they'd probably have to shoot me. But I think that HiRISE onboard the Mars Reconnaissance Orbiter would be in the top three. And it's pictures like this one that put it there:
[Click to get to greatly embiggened pictures.] That is not a closeup of my chin before I shave. It's Mars, a dune field in the far north; at latitude 83.5° to be precise, less than 400 km (240 miles) from the north pole. The eternal Martian wind blows the heavy sand into dunes, and you can see the hummocks and ripples from this across the image. The sand on Mars is from basalt, which is a darkish gray color. The red comes from much smaller dust particles which settle everywhere. But what are those weird tendril thingies? In the Martian winter, carbon dioxide freezes out of the air (and you thought it was cold where you are). In the summer, that CO2 sublimates; that is, turns directly from a solid to a gas. When that happens the sand gets disturbed, and falls down the slopes in little channels, which spreads out when it hits the bottom. But this disturbs the red dust, too, which flows with the sand. When it's all done, you get those feathery tendrils. Note that at the tendril tips, you see blotches of red; that's probably from the lighter dust billowing a bit before settling down.
Now, you might think I'm making this all up. How do we know this stuff is flowing downhill like that? Ah, because in this picture we've caught it in the act! In this image, a closeup of a region just to the left of center of the big image, you can actually see the cloud of dust from an avalanche as it occurs. Oh, baby. The cloud is only a few dozen meters across, and can't be more than a few seconds old. I love stuff like this. I tend to think of Mars as a stiff, still, unchanging place, but then HiRISE goes and slaps me in the face with something like this. Mind you, this is an avalanche. On another planet. Caught as it happened. Awe. Some. We've seen this before on Mars, but it's still shocking and amazing. I can imagine some future settlers on the Red Planet, dealing with the lack of air, bitter cold, dust in all the machinery, radiation hazards from the Sun. And, apparently, they'll have to dodge landslides too. It'll be a tough life for sure... but then, I look at pictures like this and think it would be worth it, just to stand on the surface of another world and be able to simply look around. If we can see this kind of thing from space, with robotic probes, what will humans see when they go there and can kick over some rocks?