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The Planetary Society Blog

By Emily Lakdawalla

LPSC: Wednesday and Thursday: A few Mars Exploration Rover-related talks

Mar. 16, 2006 | 15:10 PST | 23:10 UTC

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Today has been an interesting one for the discussion of the future of outer planets exploration, but I don't want to get ahead of myself; I need to continue catching up with the talks I went to yesterday.

In the afternoon, I was faced with an even tougher choice than I had in the morning: there were concurrent sessions on Cassini's studies of the icy satellites and rings; the rovers; and results from Deep Impact. After last Friday's Enceladus hubbub I thought that Cassini would be the shoo-in, but as I read the abstracts I decided that several of them sounded awfully similar to results I'd seen at the DPS meeting last September. As it developed, I ended up going to a couple of rover talks, then went to Saturn, and then took a much-needed break from sitting in the dark and typing, and finally finished in Deep Impact.

I only went to two rover talks but Steve Squyres and Matt Golombek talk so danged fast that I took almost as many notes as I had for the Titan session. Steve's job was to give an overview of the most recent 150 sols of the Spirit mission, which I think pretty much took people almost but not quite back to DPS (here's the abstract). "Happy sol 781, everybody," he began. "My talk will feature exclusively stuff after Haskin Ridge and heading down into the inner basin." He said he'd talk about four areas: (1) what they call "the Land of Olivine" up to (2) the sand dunes of El Dorado and then (3) Arad, a crazy soil location, and finally, (4) Home Plate.

The Land of Olivine: (I put this in quotes because it's impossible to write down notes from what Steve says without doing it in his voice, but as usual I warn that this is more accurately a paraphrase than a quote:) "As we came down off of Haskin ridge, we went over lots of exposed terraces. We went over a class of material we called Seminole. Seminole, Algonquin, Comanche, all of these rocks, and indeed this whole area very very mafic in composition. High in magnesium, low in iron. If you do normative mineralogy you get up to 50% olivine; up to 70% of the iron in these rocks is in olivine." The "normative mineralogy" bit means that he isn't saying the rocks are made of 50% olivine, it's just that when you count up all the atoms of all the elements they detect with APXS and then start stuffing them into chemical formulae according to the order they'd crystallize if the rock started as a melt, you'd get that much olivine. It's a standard way of comparing the elemental composition of dissimilar rocks to each other -- it's sort of melting them by thought experiment.

Click to enlarge >

El Dorado, Gusev Crater, Mars
This panorama from the Mars Exploration Rover Spirit was captured on sol 708 and 710 while Spirit was exploring a site named El Dorado. El Dorado is a field of dark, rippled sand that is visible from space as a dark spot on the side of the Columbia hills. You can download the full resolution view from the Planetary Photojournal (1.6 MB). Source Credit: NASA / JPL / Cornell

Steve continued: "El Dorado is a spectacular dune field. We didn't know what it was going to be until we got on top of it. We think that, in terms of how this thing got here, because of the configuration of this terrain with respect to the prevailing winds, it may be an aeolian cul-de-sac. We did a Mini-TES raster on the dunes. Mini-TES spectra of El Dorado look like the dark soils exposed by wheels during traverses: pyroxene, plagioclase, olivine of Fo₄₅ or so. What you see in the Microscopic Imager is a sand that is very well sorted, very well rounded, grain sizes a few hundred microns, chemistry very similar to average Gusev soil though somewhat higher in olivine. This is very clean stuff. Mössbauer mineralogy: lots of olivine, pyroxene, essentially unaltered, low Fe³⁺, very clean.

"Arad was a surprise. We were driving along minding our own business and the wheels churn stuff up and there's this white bright-toned soil exposed. Not like Paso Robles. 38% SiO₂, 35% SO₃, 19% FeO, 4% MgO, not a lot of anything else. Remarkably high in ferric sulfates. Very intense salt content; unlike Paso Robles, no significant phosphate.

"Home Plate has occupied most of our energy and attention for the last month or so. It is a spectacular plateau a couple of meters high, 80-90 meters across. It has several distinct units. It has a lower unit, that is very coarse in nature. Poorly sorted, chock-full of coarse granules up to several mm in size. Somewhat rounded, prominent throughout lower reaches. Laterally, you get to one of the most interesting things we have seen, a probable bomb sag in this deposit. A rock has fallen from the sky and has deformed these layers. This is the only one of these we have found anywhere and we have looked hard. The upper unit is quite fine grained, much more well-sorted, very, very finely layered. You can get some hints of low angle cross-stratification. We drove around to the side of Home Plate and we saw some beautiful cross stratification, lovely crossbed sets, some of the most spectacular we've seen in the entire unit and indeed on Mars. Upper unit is distinctly different from lower unit. Here you see extremely well-sorted, extremely well-rounded grains. In searching through our images, the best soil analog we can find anywhere is El Dorado!" Steve got a laugh from the audience when he pointed mentioned that one image from his talk, showing outcrops at Home Plate, was referred to by the team as "Rock Monster" because of you could see the eyes, nose, and mouth…

Click to enlarge >

The "Rock Monster"
Spirit captured this false-color image on sol 764, performing RAT brishings on two rock targets in the upper part of the series of units exposed in the layered outcrop. The image is jokingly referred to as the "Rock Monster" because of the resemblance to a face, complete with eyes, nose, and mouth. Credit: NASA / JPL-Caltech / Cornell

Continuing to speak about Home Plate, Steve said, "Mineralogy and geochemistry are consistent through the whole stack. The lower and upper unit are consistent in mineralogy and major element chemistry. It's an altered basalt with a lot of magnetite in it. What we see is a fairly typical basaltic composition, somewhat elevated in potassium and sodium compared to Adirondack, high phosphorous, titanium, low chromium. Also notably high in chlorine and bromine, by this may be surface coating effect" noting that the toothless RAT can no longer scrape away altered surface rinds. "While Home Plate by itself is a fascinating structure, we do not know how widespread Home Plate-like material may be. We have not seen it before, but there may be outcrops else where, possibly on McCool Hill. Lower unit: poor sorting, coarse granules, and probable bomb sag point strongly toward a pyroclastic or impact origin. Upper unit could also be a cross-stratified base surge deposit. But the pronounced sorting and rounding, also allow for, and may suggest aeolian reworking. Even if so, it's aeolian reworking of the same stuff" as at the base. "Is it impact or volcanic in origin? I'm leaving this one open for now. Home Plate is clearly associated nearby with vesicular basaltic blocks that have compositions very similar to Home Plate.

"We are now in a drive-or-die situation. We are down to about 350 Watt-hours. The vehicle is very dusty and we are getting into winter, we need to get to north facing slopes 120 meters away in order to continue doing science during the winter. We need to drive like hell and get to safe winter haven. Our focus right now is on Foget, Korolev, and Oberth." These are all names for spots on McCool Hill that the drivers have identified as possible winter parking spots for the rover. "Spirit has just completed a drive in the direction of Oberth and Korolev, we hope in a week or so to be on one of those nice toasty slopes that will allow us to survive another winter at Mars."

So that was all one fifteen-minute talk. I should add, in this context, that I asked one of the people I know from the rover mission later about the problems that have been cropping up again in Spirit's balky wheel. He confirmed that there were problems but that they don't have the luxury of stopping for two months and figuring out the problem and an optimal solution; they have to get Spirit to a north facing slope, or it'll die, so it's damn the wheel problem and full speed ahead for the team.

I took nearly as many notes for the following talk, by Matt Golombek, but they are much less coherent (here's the abstract). Really, his talk was intended to unmistakably drive home two points. The first point is that there is no evidence at all in Gusev crater for the paleolake deposits they had hoped to find there. "The cratered plains of Gusev are dominated by aeolian processes. THe rock distributions can be related directly to the impacts that occurred here. Liquid water has not shown its face ANYwhere except in alteration rinds; this has been dry and dessicated since the lava flows formed in the Hesperian. If you go to any place on Mars where the rock crystallization age is equal to the crater age, this is what it will look like."

However, in regards to that crater age, Matt had another point that he wanted to drive home. To paraphrase, the craters that Spirit has seen, at every scale from Bonneville to the tiniest "hollow" only 40 centimeters across, is very shallow, with a depth-to-diameter ratio of less than 1 to 10. Though some are certainly filled with sediment, the freshest look reasonably fresh, with rocky bottoms. The shallowness, Matt argued, strongly suggested that every crater that Spirit has seen to date is a secondary. (That means that they didn't form from an original meteorite hit; they formed when the spray of debris from a single large meteorite hit fanned out and splatted into the ground. The lower speeds at which secondary craters form result in the shallower depths.) This observation "has very important implications for age dating using very small craters," Matt finished. What he's implying is that people may be significantly over-counting craters on Martian surfaces similar to the one that Spirit's sitting on.

At that point I skipped over into the Cassini talks. Since I'm a day behind now though, I think I'll jump forward in time to one more rover related talk I attended this morning, given by L. Paul Knauth, titled "Impact Surge as the Simplest of the Proposed Hypotheses for the Origin of Sediments at the Opportunity Landing Site on Mars." (here's the abstract.) If you didn't catch the implication of his talk title, let's just say he may as well have subtitled it "The MER Team Has Got It All Wrong." So of course I had to go and see what he had to say, which was basically that you can explain the Opportunity landing site rocks with impact-related deposits, and you don't need liquid water sitting around as the rover team has argued.

Knauth puts together a good presentation, and he opened with a slide showing a nuclear bomb explosion, which I have to say I wasn't mentally prepared to see first thing in the morning. But his point was "Here's a nuclear explosion that shows a surge deposit that flows out from the base of the stem. These things produce sedimentary rocks. All impacts produce some kind of basal surge." He showed lots of extremely lovely slides of Earth rocks formed by impact or volcanic surges, with selected inset images from Opportunity that did look strikingly similar.

To address the subject of the hematite blueberries at the Opportunity landing site, he pointed out similarities to small, spherical structures in surge deposits called accretionary lapilli. "Accretionary lapilli form like hailstones in the cloud as it goes along, and they rain out. They are made of target material particles. So the question is, could we get a lot of iron oxide in the lapilli? On Mars, there are very high-iron lava flows. In magma chambers, sulfides separate as immiscible drops, and you can get cumulates at the bottom, and disseminated particles in the magma as it cools. This happens to some extent in any magma. If, at Meridiani, you had some of these, you could have had an impact onto it -- it could have hit a huge cumulate. You could have had multiple impacts with many surges, or one enormous one. It could also be that Meridiani was hit by a large iron meteorite. The important thing is that basalt grains, accretionary lapilli, salts, ice, sulfides, and brine are mechanically emplaced in base surges. You don't need an acid lake or an acid aquifer to make jarosite, all you need is sulfides and water vapor."

He went on to consider the festoon structures observed by the MER team. At this point, he let contempt creep into his statements, and he began to lose my interest; when scientists waste argumentative effort being contemptuous, I always begin to feel less inclined to believe their stories, no matter how logical they seemed to that point. He argued that the festoons identified by John Grotzinger are "nothing but topography" and anyway it "turns out it doesn't matter because you do see festoons in base surges." He went on to say that "there are also some problems with the MER team in geochemistry. You can't maintain an acid aquifer in the presence of basaltic minerals."

As you might expect, as soon as he finished talking about six people jumped up and beelined for the microphones. John Grotzinger got there first, and unleashed a little contempt of his own, saying "We appreciate what you're trying to do but while you were trying to download slides yesterday you obviously missed my talk." He said that the scales of Knauth's examples did not match the scales of things on Mars. (Personally I'm a little bit leery of arguments based only on scale, because the different force of gravity and other stuff on Mars tends to make some Earth processes scale strangely.) Grotzinger continued, "The most amazing thing about stratified rocks is that they're stratified. Details matter. All the data suggests that the evaporite playa is the best model."

Knauth responded: "You're using a eutectic brine," meaning that Grotzinger's models relied on a water that was utterly and completely saturated with salt. "Do you know what the viscosity of brine is? That flow regime diagram you're using doesn't apply…." Grotzinger replied that the diagrams are nondimensional and that changes in viscosity have a negligible effect. Knauth asserted viscosity is not negligible, and he'd do the experiment to test it.

After this exchange there was no time for further questions, so the other five questioners were told to sit down. One of them, Ray Arvidson, was sitting next to me, and I asked him what he had wanted to say. He said that "the regional geology doesn't work" in Knauth's scenario. "Most of the sediments are sitting on the craters," meaning that they could not have formed as a result of the impact craters themselves.

I haven't read the rover team's papers closely enough, or indeed Knauth's for that matter, for me to be able to independently evaluate the relative veracity of the two very opposing viewpoints. I can tell you, however, from having been inside MER mission operations during the first couple of months that (a) the MER team is a very large one with a very diverse group of scientists and (b) that one of the central themes driving their choices of observations to make with the rovers' finite time on Mars is to form multiple hypotheses and come up with observations to test those hypotheses. I know that when they first saw those blueberries they had a lot of hypotheses, which included concretions, accretionary lapilli, and other possibilities, and that they designed tests for all those hypotheses, and therefore their results are the results of not only observations but careful tests.

So I'm generally inclined to believe the MER team's story unless there are a lot more than one guy objecting. However, people like Knauth are very, very important to the advancement of our understanding of the solar system. By raising objections, they force other scientists to be methodical and meticulous, and to develop tests and counter-arguments to every one of the objector's arguments. It improves the quality of the science overall, by suggesting new tests and new experiments to perform, and forcing scientists to address weak points in their theories. And, every once in a while, they are right, and the new line of investigation yields a whole new viewpoint. This give-and-take is central to the scientific process.

By the way, here is the latest news we have on our site from the rovers, and I know that A. J. S. Rayl is working on the next update, which I'm guessing you'll see on this site in the next week or two.

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