Twitter found me an article on Mars. It was from NPR and called “Are Those Spidery Black Things on Mars Dangerous?” by Robert Krulwich.
So on Mars there are formations like sand dunes. Mars has seasons, and in spring/summer these “spidery black things” regularly appear. They come out in the same places usually, and then in winter they disappear again.
They have been known a while, but nothing so far has gotten close enough to see what they actually are. The pictures the article provides (which are beautiful) are from very high up. The most accepted theory is that these are actually geysers of extremely hot carbon dioxide that burst up because it gets so hot so fast.
A less accepted theory is that they are clusters of microorganisms that came up to get some sun.
I kind of like the second theory better, but it would be neat to see those geysers too.