I have a Martian mystery for you today, and one that is writ quite large and dramatically. It seems weird at first, then simple next, but when you dig deeper -- literally -- things get very weird indeed. It all starts with an out-of-control awesome picture that honestly made me reel back and say "Wow!" I present to you out-of-control awesome:
Wow! Click the pic to embiggen. This unnamed crater is about 700 meters (roughly half a mile) across, and sits in the northern mid-latitudes region of Mars. It's interesting, isn't it? The multiple concentric bowls of the crater are trying to tell us something, but what? My first thought, also mentioned on the HiRISE blog, is that this is a coincidental double impact: the big terraced crater was the original impact, then a later, second object impacted almost exactly in the center of the older one, hitting the bulls-eye like William Tell splitting an arrow. The topography seems to support that; the inner crater has a raised rim, as you might expect from a second impact, and that would be hard to explain in a single impact. The terracing -- shelf-like structures sortof like an upside-down wedding cake layering -- is seen sometimes when an impactor smacks into layered ground. Imagine a layer of dirt on top of ice on top of rocks: each layer reacts differently to the impact, leaving the circular, concentric shelves in the crater bowl. Note too that the central crater doesn't look exactly centered, supporting a second impact. Case closed... but wait, Your Honor! We have a surprise witness! This picture is actually part of a much larger region which provides some context:
You can see the extensive ejecta blanket (excavated material laid down from the impact ) around the crater now, which is nifty. But note the smaller crater to the lower right (indicated by the arrow): it looks a lot like the bigger crater! There's a shallow bowl with a deeper crater almost but not quite in the center. There's no terracing, but it's a smaller impact and wouldn't have dug so deeply into the surface.
So what gives? If all we had here was the big crater, I might believe the coincidence of a nearly perfect second impact bullseye inside it. But two of them? Right next to each other? It seems unlikely, to say the least. And I thought I had an explanation for it... which I'll give you. But note: I chatted for a few minutes with Alfred McEwen, the Principle Investigator of the HiRISE camera (which took the image), and he told me things still aren't quite as they seem. Keep that in mind while I describe my thought... My idea is/was this: both of those craters were single impact events. The terrain itself must explain the weird structures; there must be several layers of material with different solidity. In the lower right crater, the softer surface material deformed and splashed back, forming a shallow bowl. Underneath it is a stronger material, forming the raised rim central crater that's slightly off-center. The fact that's it's not centered may be due to sloping in the surface, or that the surface layer isn't constant in thickness across the surface. Perhaps there is stronger material to the left which resisted the impact pressure, leaving the inner crater off-center once the event was over. This explains the big crater too. The outer bowl is shallow. Inside that is a raised rim, as you'd expect from a stronger material. The impactor was big enough to dig below even that layer to a third, deeper and even more resilient layer, leaving a beautiful raised rim. It's not centered either, again perhaps due to the layers being irregular in thickness or to different material strengths in the layers themselves. Finally, in the context image, you can see lots of shallow smaller craters. Again, I think this shows the top layer is something soft like ice, which leaves those barely visible bowls behind after smaller impacts. Tadaa! Done. But wait! Not so done. As Alfred pointed out to me, note that the second crater is actually sitting on the ejecta blanket from the first one (which is how we know that the smaller crater impact occurred after the bigger one). Since it's on top of that material, the ground underneath the impact would've been different than the ground into which the original impactor hit. The other shallow craters are all sitting in that material as well. So we can't simply state that the terrain was similar to the original event because the original impact changed the surface structure. Also, the detail of the structure is difficult to interpret. Turns out that at this latitude glaciation is common, and that tends to screw up details, changing the way things look. Interestingly, the rim of the innermost crater in the big crater looks pretty fresh, too, like it happened after the original event, supporting the William Tell idea that a second asteroid impact hit right in the middle of the previously excavated crater. Finally, in the top image, look at the floor just outside that innermost crater. See the two crescent-shaped lobes at 1 and 2 o'clock? Those may be slumped material from the walls of the crater. If a second impact happened in the center of a pre-existing crater, you'd get some disturbance of the material, including debris flowing down from the walls. So what do we conclude? This place is a mess. That's what I conclude. Alfred said my idea that layered terrain explains most everything has some merit, but so does the idea that a second impactor did the deed. We simply can't tell. If you think I'm having fun figuring this out, then dingdingding! I am. Because it is fun. This is good old-fashioned sleuthing, detective work on the scale of a city block. When we look at pictures like these we get evidence of a crime scene, perhaps millions of years old -- talk about a cold case! -- but still fresh enough that we can puzzle out what happened. The big crater is the main clue, drawing our attention, but the second, smaller crater may be a smoking gun, the surprise evidence that just might make everything else make sense. It's CSI Mars. But in this case it's not some procedural drama on TV. It's real, it's huge, and it's sitting there on another world for everyone to see. All you need to do is go there and look.