On the beach

So today's announcement at NASA HQ was pretty much what I (and a lot of other people) expected, given the specialities of the people on the panel doing the announcing. There's evidence of cross bedding in the thinly layered sediments that Opportunity has been studying, and the size and shape of the cross bedding makes it look pretty conclusive that the sediments were laid down in water. They're seeing ripples from the floor of some sort of shallow pond or lake or sea. How shallow is not clear: "up to your neck or up to your ankles? I don't think we have a good answer to that right now." But it was shallowish. How large an area the water covered is not clear, either, but if all the haematite seen from orbit is in the form of "blueberry" beads that have been eroded out of this sort of rock then there was once a great deal of it. (That said, it's not a done deal that all the haematite comes from rocks like these.)

Again, the take-home is that that there was a habitable environment on Mars a few billion years ago. In theory, there could be microfossils -- fossilised bacteria -- in the very rocks that Opportunity is focusing on, evidence that the environment was not just habitable, but inhabited. Unfortunately, as Steve Squyres explained, the MER instruments are not sharp eyed enough to discern such microfossils. So though Ed Weiler, NASA's science chief, enthused that "If you have an interest in searching for fossils on Mars this is where you want to go", you can't do that hunting with this rover. You should be able to do it with the next generation rover that is slated to be launched in 2009, though, and as Weiler went on to say, Meridiani now looks like the prime landing site for that mission. (How the science community will feel about such a strong steer from the boss on what could be a hardfought decision has yet to be seen.)

The question of whether the water was actually a blue-water sea is still open. It could have been more marshy or pondy, at least some of the time (the earth analogue showed by Dave Rubin of the USGS, which I think was from the Trimed region of Tibet, was dried out ripples covered by a salt crust a very long way from any sea). And it could have been frozen. Steve Squyres outlined the intriguing possibility that ice, rather than evaporation, might account for the high salinity. Imagine an ice covered lake getting colder and colder. As the ice cover gets thicker, the residual water at the bottom gets more and more briney. So ice could make the rocks salty - they don't have to have sat around in the sun having their water evaporated out. The fact that Steve mentioned this makes me think he believes there could have been enough water movement under the ice to account for the rippling seen in the cross beds.

The next step is to take Opportunity to the lip of nearby Endurance crater, where it should be able to see outcrops of the same rock ten or a hundred times thicker, thus putting together a better picture of what was going on and how it changed over time. (According to some estimates, based on looking at outcrops all over Meridiani, the layer of yellowish rock Opportunity has been looking at is in fact about 800 metres deep; the stuff looked at so far is near the top of this column.) Various people on the panel also mentioned that this sort of sulphurous sedimentary rock would be easily spotted from orbit by the CRISM spectrometer on NASA's 2005 Mars Reconnaissance Orbiter. They didn't mention that much the same science, at a slightly less sophisticated level of resolution, can be done by the OMEGA instrument on ESA's Mars Express -- indeed the OMEGA team is probably doing it right now.

No time for more right now -- I have to go and cook. But there is one last thought. Steve Squyres was adamant that the rovers couldn't see microfossils. But those aren't the only fossils that microscopic organisms leave. In the shallow water of Shark Bay, in Australia, there are decidedly macroscopic bacterial colonies called stromatolites, fossils of which are found through much of the earth's early history. On the bottoms of permanently ice-covered antarctic lakes -- lakes I'm pretty sure Steve has swum in -- there are extensive mats built by microbes. Fossils of things like that, I'd have thought, the MER cameras would be able to see. Whether they'd be able to recognise them, of course, is a different matter. It might be a hard call, one you wouldn't want to even raise the possibility of until you had to.
NB, haematite is also known as hematite

Comments

[billg]

As Homer says, Woohoo! We're getting to the point that explaining an absence of life, past or present, on Mars is going to be a challenge.

Do we understand enough about those little ripples to use them as a baseline for an order of magnitude guess about the lifetime of the body of water that formed them? E.g., do you get ripples in 5 minutes, 5 years, 5 millenia?

Or, is it the case that ripples form rapidly all the time and that the rover found the ripples that happened to be left behind when the last bit of water disappeared?

[Keith Cowing]

At the press conference today I was about to bring up stromatolites and other mega- manifestations of microorganisms as a follow-up to my follow-up question - but I'd already pushed the point - and Steve was not going to go there ....

Have my write up online at

http://www.spaceref.com/news/viewnews.html?id=940

"Beachcombing On the Shores of Barsoom"

Cheers

k

[Oliver Morton]

I should have added to this post a reiteration of something I said about the first announcement: a fundamental discovery being made here is that by using all the instruments on the rover together, and its mobility too, it's possible to make complex, multifaceted geological interpretations. This was clearly assumed, but that doesn't mean it's not extremely exciting to see it prove true.

[Carl Zimmer]

From what I understand about stromatolites, there are a lot of ways they can be formed without microbes, and robot-eyeballing probably won't be able to distinguish between biotic and abiotic ones. But it can certainly find some good candidates for sample return! Amazing stuff.

[Charles Schmidt]

Though I'm no geologist, it had occured to me that the msss MGS MOC image here, at the center of the landing ellipse - some kilometers west of Eagle crater - is reminiscent on earth of springtime salt, when the salt which has been used for deicing leaves a white tracery of salt crystals over dark asphalt. The units on Mars bear a striking visual resemblance.

http://marsprogram.jpl.nasa.gov/mgs/msss/camera/images/2004/01/24/R1104134.gif

Surely, someone in the know had predicted that the white areas on Meridiani Planum would be found to be a salt flat, with salty outcroppings...
The image also underscores how lucky we were to find an outcrop in the western part of the landing ellipse, for here in the center of the ellipse the outcroppings (the high albedo areas, I presume) appear to be pervasive and large, while easterly by Spirit the outcroppings are precious few, but for moderate craters like Endurance, or the impossibly propitious Eagle Crater.

Hey, when did Terra Meridiani become Meridiani Planum? Does the new name supplant the old, or do other places on Mars bear dual proper nouns? I still like the old Terra Meridiani.

The discussion by M. Morton on methane (4/25)is well, awesome, like everything else here, wow thank you so much Oliver.

[Fred Mills]

Speaking of NASA and possible cover-ups does anyone remember the perfect epsilon noted on the surface of a rock being among the very first pictures received from the martian surface? Even before the face. Reference to it seems to have disappeared in the intervening years.

Either someone from NASA or elsewhere had a good sense of humor or "somebody" from Mars did. Just curious.