The polar ice caps of Mars are instantly recognizable. But take a close look at them, and the familiar gives way to some pretty strange landscapes.
Consider, for example, the "Swiss cheese" terrain that marks the icy reaches of the southern cap. These are places where carbon dioxide ice sublimates (turns from a solid directly to a gas) leaving behind pits a few meters deep. The pits form weird figures and patterns that change shape over time.
As if that weren't enough, in one of these regions something left a four-kilometer-wide scar in the shape of a nearly perfectly circular pit. Exactly what happened? It's hard to say for sure. According to the team poring over high-resolution images obtained by the Mars Reconnaissance Orbiter, the pit could be an impact crater, but where is the material that was blasted out of it? It could be a collapse pit, but where are the fractures typically seen around such features? It doesn't help that the whole thing is covered in dry ice Swiss cheese several meters thick.
Here's another look, zooming in directly on the center of the pit. More detail emerges, but is it enough to settle the question?
It's just another example of how much exploration is left for those who will come after us: maybe robots, maybe human travelers. We can only make educated guesses about exactly what they'll find in the ice pits of Mars.