Two craters

It really is a weird planet -- weird enough for "Weird" Wink Wilkinson himself. Two recent examples:

The "lake" crater in a Themis image that's being studied by students from Saratoga Springs, NY (via Martian Soil) doesn't really have a lake in it -- at least I think it doesn't, and I don't think the Themis team and the students really think so either. In fact I asked Phil Christensen, and they don't. But there's no denying that the dark splot looks weird and intriguing. A dark area in the same crater was visible in Viking images, but it didn’t look as dramatic as it looks to Themis. Has the patch changed since Viking? Is the "lake" really as flat as our eyes want to make it? How is it related to the blurry bit coming down the rim off to one side? Search me.

There seem to be quite a lot of dark patches in the northern parts of craters in those latitudes, but even so this one still seems pretty dramatic. Maybe the process involved is something like what's going on in this MOC image of a crater at a similarly high southern latitude, one of the original pictures that led to the discovery of the gullies. It would be nice to see Themis images of the crater in the infrared, which would show whether the dark stuff was unlike the lighter stuff -- sand as opposed to dust, say. And it would be nice to see MOC images, too. The only MOC image I can find of the "lake" crater is one taken late on in the southern winter with quite poor light; it isn’t particularly gripping.

Not that there's any shortage of gripping MOC images -- with pride of place, at the moment, going to the recent picture of the crater in Xanthe Terra that people at Malin are nicknaming "Mojave". A MOC picture of the crater's interior flanks was first unveiled at the Lunar and Planetary Science Conference a month or so ago, in a session on the last day that took place after quite a lot of people had already left. You can see details from the picture in the pdf of the paper's abstract. (The original MOC image, rather harder to get your eyes into, is here.)

The crater's superposition over two of the great outflow channels that drain northwards into Chryse Planitia means it must be pretty young. The outflow channels come from Mars's "middle age", well after the days of drainage network formation that might speak of a "warm wet" early Mars, and the crater obviously has to be younger than the outflow channels it sits astride. So presumably it formed in the geologically recent past -- not as recent as the past of the gullies and ice ages, but certainly in the second half of martian history, when the Martian climate is thought to have been pretty much the way it is today. Yet inside this crater are features that look labout as water-carved as you could imagine. Specifically, there are erosion fans of the sort you see in the Mojave desert, where they're created by storm drainage. The fans are covered with dense networks of channels. Lots and lots of them. When Dick Kerr wrote the paper up in Science a couple of weeks ago he quoted Jeff Moore of NASA Ames calling the scars on the crater's walls "gullies on steroids".

So -- downpours on Mars? Apparently not in any widespread way. There's no evidence of anything remotely similar in the other craters nearby. Indeed there's nothing similar in any of the other MOC images. The drainage fans cannot be tokens of a bygone global climate. So the MOC team -- in this instance, Rebecca Williams, Ken Edgett and Malin himself -- is leaning towards thinking that the erosion features were created very quickly more or less straight after the impact. They may be the result of processes that take place in quite a lot of craters, their handiwork seen in this crater rather than any others because it's pretty young for its size. Maybe other craters looked like this too, in their youth, and what;s unique about this one is just its youth.

Here's an amateur's picture of how it might happen. After the initial impact (a fairly big one - the "Mojave" crater is 60km across) a pulse of heat spreads out through the surrounding crust. If there's ice in the subsurface, it melts or boils as the heat hits it, depending on the temperature and the local pressure. Cracks in the brecciated crust let the water -- or steam -- flow back into the crater itself; the floor of the crater is low, and it's there that the water reaches the surface. Above the hot lakes in the crater's depths a tower of cloud starts to climb into the cold sky. Even as the cloud is still rising, water and ice condensing start to fall back to the surface below. The rain courses down the crater's warm fresh flanks and pools on its floor; all the time ever more vapour is lofted back into the atmosphere. Winds from every direction are drawn into the thermal updraft, setting it spinning. A cycle of self-sustaining weather settles in above the crater, a microclimate that from a distance looks like a mountain wreathed in lightning, a hurricane anchored to the ground and straining for the sky.

Over time the heat dissipates. Water flows more slowly, more intermittently, into the crater from the surrounding bedrock. Rain starts to drain back into the crust. The winds that rushed in to the updraft above the crater calm themselves. The hurricane abates. The surface drys, and freezes. Below the surface the water pools, the rock still warm enough to keep it liquid, for a while. Maybe a long while.

Is that how it was? Perhaps. But I'd lay money it was a fair bit weirder.

Update: At Oded Aharonson's suggestion I looked at "Aeolian processes in Proctor Crater on Mars: Sedimentary history as analyzed from multiple data sets" by Lori K. Fenton, Joshua L. Bandfield and A. Wesley Ward (JGR, Vol 108, NO. E12, 5129, doi:10.1029/2002JE002015, 2003 -- available as a 3.3mb pdf from Lori Fenton's webpages here). There's a dark patch in Proctor crater that is clearly a field of sand dunes, and after looking at that it's easier for me, at least, to see the "lake" in the Themis image as sand, too; looked at that way I'd guess the blur to the side is sand blown up the rim by the wind.

Comments

[Rupert]

I know a way to find out. We find a convenient lump of rock in the neighbourhood - perhaps chip off a chunk of Phobos, the whole thing might be a bit excessive - and deorbit it while taking notes. Probably get some money from the DoD's petty cash fund to offset expenses.

[Oliver Morton]

You hit on a hobby horse. I keep meaning to write an article or even a paper on how a controlled asteroid impact on Mars could be the first great astrobiological/planetary engineering experiment. We'd understand impacts a lot bettter, I suspect, if we could watch a few -- and the hot wet transient environment in a fresh impact crater would be a perfect place to look for life. If you believe it's worth learning to deflect the orbits of asteroids anyway, as a sensible precaution for the inevitable (though probably far off) time when we find one headed for the earth, then deflecting one or two to hit Mars offers a trial run with lots of added benefits. It's really not as Strangelovely as it sounds...

[Lori Fenton]

On the "lake": Yep, it's a dune field. Oded was right. It's hard to tell that it is a dune field, because the dunes are not well defined. I've found that dunes near the south pole tend to look somewhat rounded, which may mean that they are inactive and somewhat eroded.

Many craters in this region have dark dune fields on their floors. The sand probably got blown in from the surrounding plains, but then it got stuck in the craters by topographically-enhanced winds.

If you look carefully, you can see that there are hundreds of thin filamentary streaks that run generally northwest-southeast. They are parallel to the large dark streak coming off the dune field that heads up the northwest crater rim. The thin streaks are thought to be dust devil tracks, where bright dust is scoured away by dust devils are they are slowly blown downwind. The dark streak coming off the dune field is probably dark sand being blown off the dunes themselves. It's likely that the same wind (blowing during the same time of day and the same season) made both types of features, since they are oriented in the same direction.

Hope this helps.

[Neil Saunders]

It is a fascinating image. I think the initial reaction is due in part to image scaling; if you zoom to 100% you begin to see features that make the "lake" appear much less black and flat.
Disclaimer: I am in no way an expert in either image processing or planetary photogeology. However, I do enjoy messing with images using The Gimp. I played around with this image by cropping to the "lake" area, then enhancing the contrast and levels. You can see the result at http://psychro.bioinformatics.unsw.edu.au/neil/tmp/mars/. It seems to me that the "lake" is actually raised, with little ridge features around the edges and I'd agree with others that the image is consistent with wind moving in a SE-NW direction.
I think it's great that those kids are involved and that this stuff is out on the net for anyone to play with.

[Zac W]

I was actually in the group that took the image. I am from Saratoga Springs in New York. There actually is an infared image taken of this crater and you can find it at http://themis.asu.edu . THanks for everyones comments

[Ken Edgett]

It really IS a lake. Hydrocarbon lake.
Just kidding. The south high latitudes have lots of intracrater dune fields that have been indurated and then planed-off by the wind to almost flat forms. The "lake" in that initial gully crater in AB1-07707 also turns out to be dune material.