Sen—One thing all solid bodies in the Solar System share in common is craters. Some worlds, like Mercury or the Moon, are covered in them, having no atmosphere to erode them away. Earth has relatively few; our dynamic atmosphere and water circulation wipes them out after a few millennia. And some icy bodies like Saturn’s moon Enceladus or Jupiter’s Europa only have a few because their surfaces are also constantly changing… on a geologic timescale.
But that doesn’t mean all craters are alike. Some are simple bowls, some have central mountains, some flat floors. And then there’s the pair shown above (Arima on the right; an unnamed one on the left), twin 50-km-wide impact craters on Mars.
What is going on with them?
They both have large pits in their exact centers, as if after the initial impact they were each hit a second time, dead center. Obviously, the odds of that are ridiculously low even for one crater. But for two, right next to each other? I’m willing to call that impossible.
The first time I saw a crater like this it was also on Mars. I speculated that maybe there had been a second impact, but there was another double crater located nearby it as well. At this point, you might start to suspect it wasn't the asteroid that hit Mars behind this, but Mars itself that’s the key to this issue.
And you’d be right. The surface of Mars is diverse, but a lot of it is basaltic rock, exuded from volcanoes eons ago. In some places, though, under that layer of rock is a subsurface layer of water ice, hidden and protected by the rock above. We know this is true because we’ve seen it; the Mars Reconnaissance Orbiter has seen fresh craters on Mars—literally, ones just weeks or months old—that show ice splattered out from under the surface by the impact.
The physics of hypervelocity impacts is weird. We don’t fully understand just how materials behave under these conditions, and they’re incredibly difficult to reproduce under lab conditions. But what’s suspected to have happened here is that long ago, twice, an asteroid slammed into the rocky surface of Mars. Perhaps they occurred millions of years apart, or it was a binary asteroid, creating both craters at the same time. Either way, both hit the surface of Mars hard.
What happened next was roughly the same for both craters. Under the surface was that layer of ice. The rock at the surface vaporized violently, turned into plasma by the unbelievable pressures and energies of the impact. This material expanded rapidly, creating an explosion of debris.
In the fraction of a second it took for the asteroid to vaporize as well, its own momentum and energy was focused downward. This punched through to the next layer of ice, causing it to vaporize and explode outward as well. The primary crater formed first, then the release of the lower layer created the crater in the center, where the impact energy was focused.
This may explain why both craters have flat floors instead of being bowl-shaped; the softer ice underneath flowed better, filling in the depression of the primary crater, but not quite enough to cover the central pit. Note that the two craters have different-looking pits; perhaps the ice underneath had different thicknesses, creating different structures.
Also interesting to me is the location of these craters. They’re at a latitude of about -17°, not far from the equator of Mars. The smaller craters showing icy ejected material tend to be at mid-latitudes, farther from the Equator. If it was an ice layer under these craters, then that indicates there may be more water locked up on—well, under—Mars than we previously thought.
The more water there is on Mars, even in the form of ice, the more hospitable it looks for future exploration by humans. The history of water is in many ways the history of Mars itself, shaping the planet we see today. Now we know that water may be its future, and ours, as well.