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The Planetary Society Blog
By Emily Lakdawalla
Mars and Mine Dumps
Jan. 5, 2007 | 06:44 EST | 11:44 UTC
NOTE: I've corrected an incorrectly paraphrased comment from Burt in the sixth paragraph below. Burt told me, "Where you stated that 'Burt answered that, indeed, he's saying that nearly all the layered deposits we can see have nothing to do with liquid water', I actually said that they 'need not have anything to do with' liquid water." Sorry for the error. --ESL
During December I received several emails from scientists inviting me to read and discuss their recently published papers in this blog -- for example, the Saturn ring spokes story. Another of these was a message from Donald Burt, a geologist at Arizona State University, pointing to the lead article of the December 5 issue of EOS, the weekly newspaper printed by the American Geophysical Union, titled "Mars and Mine Dumps."
I'm forced to admit here that while my first response to Burt's email was "what a nice message," my second was "uh-oh." Burt and his coauthors, Kenneth Wohletz and L. Paul Knauth, are three scientists whose interpretation of the chemical, textural, and morphological evidence returned from the Mars Exploration Rover mission clashes strongly with that of the Mars Exploration Rover science team, headed by Steve Squyres. I wrote last year about a presentation that Knauth made at the Lunar and Planetary Science Conference that -- while being one of the better presented papers I saw at the conference -- was poorly received by the gathered scientists.
This is an oversimplification of the scientific conflict, but I'll summarize it by saying that where Squyres and coworkers interpret much of the evidence, particularly at Meridiani, as supporting the central role of liquid water in the formation of the textures and chemistry of the rocks, Burt and his coworkers argue that the evidence can be explained by a combination of the effects of impact cratering, volcanism, and the dispersion of the products of these phenomena by wind. Burt's argument has the advantage that it doesn't require conditions on Mars to be very different in the past than they are today, with the exception that both impact cratering and volcanism were, without any question, more common occurrences earlier in Mars' history than they are now. Squyres' argument is the more extraordinary, so they do have the greater burden of proof.
The paper that Burt was asking me to look at draws a comparison between open-pit mines on Earth and impact craters on Mars: the open pits are like craters, and the tailing piles -- the huge dumps of broken-up rock material that is removed from a mine, processed, and discarded -- are like the ejecta deposits from the crater. Burt and coauthors discuss how sulfide-rich mine tailings on Earth frequently weather into acid sulfate minerals without large quantities of liquid water -- moisture from the atmosphere is enough. They conclude that "despite what has been claimed, acid sulfates such as jarosite at Meridiani provide evidence against, not for, liquid water."
Unfortunately, I find myself ill-equipped to criticize the geochemical arguments of either the Burt or Squyres camps. I never took geochemistry and only have one undergraduate class in sedimentology to my credit, so my eyes quickly glaze over whenever I read these papers; I mean, they all sound logical to me, and I don't really know jarosite from smectite from any of these other water-rich minerals, so I certainly can't poke holes in the arguments of either group on my own. I do think it's interesting to consider how impacts and subsequent wind activity on Mars can spread a powdered average-Mars pile of minerals all over the planet, and also to wonder what the regional deposits would look like in the event that a large meteor struck a spot on Mars that was unusually rich in some metal ore.
I certainly agree with Burt insofar as impact cratering and volcanism are way more likely to be the origin of layered rocks and minerals on Mars than they are on Earth, where practically every aspect of geology is dominated by the effects of water in one way or another. I poked at Burt on the second question he addressed in his EOS paper, which was the origin of the layered deposits found everywhere at Meridiani (Opportunity's landing site) and at Home Plate in Gusev Crater (Spirit's landing site). I asked him whether he was suggesting in the paper that the layered sediments found across Mars all have an impact surge origin (that is, they are the distal deposits left behind by fast, hot surface flows of ejecta from craters), and that none (or only a very few) were water-lain.
Burt answered that, indeed, he's saying that nearly all the layered deposits we can see need not have anything to do with liquid water, particularly at the rover landing sites, which are seeing layered deposits right at the surface. "Major contributions from wind and/or volcanism cannot be excluded at this point (although they're certainly not required, and wind mostly just redistibutes impact products). Inasmuch as the layers are intimately interbedded with impact craters at all stages of burial, and seem to involve most of the heavily cratered highlands, they clearly formed at the same time as the impacts, and the simplest explanation would be to blame distal impact surge (and associated atmospheric fallout) for all of them." There would have been water-lain deposits at lower elevations from the outflow channels, he went on, but even these will be covered by a veneer of impact debris, which is what Spirit found at the Gusev Crater landing site -- a place that should certainly be underlain by water-lain deposits, but where they really weren't found at the surface.
The Mars Exploration Rover mission has provided Mars scientists with so huge a data set that it's really difficult to get one's head around it. It should be no surprise that there are differing interpretations -- in fact, it would be surprising if there weren't. It's actually a little surprising to me that I haven't heard more variety of interpretations from within the Mars Exploration Rover team itself, more like the Cassini team, where I talked recently about the two conflicting interpretations of the origins of Enceladus' plumes. (I should note that I can't at all keep up with the volume of publications on the rover mission so I'm sure that there's quite a bit of interpretation variety that I've missed, and I'd be delighted if people more up on the literature than I could point these out.)
Burt and his coauthors are definitely a minority, but science isn't democracy; it's a cyclical process of evidence leading to hypotheses leading to tests leading to the gathering of new evidence and so on. Provided that the scientists participating in the process are open-minded enough to consider objectively the contrary arguments of other scientists, conflict should eventually lead to better-quality (and often more nuanced) explanations.
Mars is getting more difficult, though. The wealth of data presents more than one problem. First of all, there's just so much to take in; no one scientist can be familiar with all the data from Mars anymore. The rovers have provided so much -- and now Mars Reconnaissance Orbiter is exponentially expanding the amount of data to hand. And the more detailed these data become, the more important local geology may become in explaining what we see on Mars. The more places we see, with more detail, the more likely that some of these places will be special, and have origins that don't represent the most likely sequence of events, but rather some special and unique sequence of events. Arguments based upon things that are globally likely may not necessarily apply locally -- but then again, maybe they do.
It's a tough problem. I'll close with the somewhat shameful admission that my response to the problem of the data glut from Mars is to say that Mars is too hard! --Let me study instead a place where we have less data, like, say, Saturn's satellites, or even Neptune's. And hope that someday, in my lifetime, we'll get to a point where we have so much data from them that I'll have to turn my curiosity elsewhere again!
[Want your peer-reviewed planetary science paper to be covered in this space? Send me the paper, and be willing to help me understand it, and I'll give it a shot.]
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