Printable Version of Topic

Posted by: dvandorn Mar 30 2006, 11:01 AM

As I recall, the ambiguous results of the Viking landers' life detection experiments were explained at the time by postulating that the surface soils are highly enriched with extremely oxidized clays (I seem to recall descriptions of clays enriched with peroxides).

Nearly identical results were seen at both V1 and V2 sites, so, assuming that the results are explained by what was called "exotic soil chemistry," this chemistry would have to be widespread on Mars.

We now have some very good elemental analyses of the Martian soils, both from orbit and ground-truth from the MER rovers. In these analyses, clays seem not at all widespread, only appearing in very, very old outcrops that were presumably laid down during a very short geological timeframe during which non-acidic water was common on the Martian surface.

Soils in the Viking landing sites would appear, from the more advanced sensors we've flown since Viking, to be basaltic with admixtures of ferrous sulphates. Not exactly the exotic chemistry required to explain the Viking results.

So, the million-dollar question seems to be: if the Viking experiments can only be explained by *either* biotic processes that do not involve what we always considered the pre-requisite organic molecules, *or* by ubiquitous exotic chemistry in the soils that we're simply not seeing with more advanced instruments, which theory are we forced to accept as fact?

If neither, then what theory *does* account for the Viking results?

-the other Doug

Posted by: mcaplinger Mar 30 2006, 03:24 PM

No, we don't. We have some really crude elemental abundances from orbit and some slightly better ones from the ground. But I don't think elemental abundances tell you much if anything about either clays or peroxides, you need instruments that can look at molecular properties, not just elemental abundances. Phoenix is the first mission that will be able to do that, and then MSL.

Posted by: ElkGroveDan Mar 30 2006, 05:02 PM

Good question.



Good answer.

Wouldn't it be true though that firm knowledge of even just the elemental abundances would narrow the infinite possibilities of possible reactions that we were facing at the time of Viking?

Posted by: BruceMoomaw Mar 30 2006, 06:15 PM

Not really -- the main element involved is oxygen, in combination with either hydrogen or a wide variety of metals already known to exist in Martian minerals. (That's not counting Albert Yen's alternate theory, which simply calls for unusually shaped and electrically charged microscopic surfaces on ordinary Martian mineral crystals.) Nailing down the Oxidant Mystery is likely to be the major contribution of Phoenix's wet chemistry experiment.

Posted by: paulanderson Mar 30 2006, 06:47 PM

I would note also that some in the anti-life camp still use the lack of organics found by Viking as an argument, despite the fact that it was later shown that the detector used on Viking was not able to verify the smaller traces of organics in testing done in the Antarctic (I think that's where it was done), which were in the soil and confirmed by better instruments. If similar trace amounts of organics had been in the Martian soil, Viking could or would have easily missed them completely.

I've also seen other later papers which now dispute the idea that the Martian soil is so highly oxidizing, yet that is also still used as a primary anti-life argument and has been widely accepted as fact, but again there is no real firm consensus or agreement on that.

Posted by: BruceMoomaw Mar 30 2006, 07:06 PM

There are certainly SOME major oxidants in the soil -- the fact that Mars' soil retained much of its oxidizing power even after being roasted at several hundred degrees proves that. Any germs capable of withstanding that must hail from the planet Krypton. This, in turn, makes it a suspicious coincidence that the results from the Labeled Release experiment could be due to actual germs living in the same highly oxidizing environment -- and more likely that the LR results are just from another type of nonliving oxidant which does break down under heating.

In this connection, the new LPSC had an abstract connected with the recent studies of the very powerful (and bacteria-destroying) oxidants which have been discovered in the Atacama Desert's soil, produced by regular Earth sunshine: http://www.lpi.usra.edu/meetings/lpsc2006/pdf/1778.pdf . It claims that "hydrogen peroxide together with hematite reproduces the kinetics of the LR experiment", and indeed the reaction does follow the time curve of the Viking results very closely -- but the study didn't test whether this reaction ceases when the soil is heated. (There are a number of LPSC abstracts dealing with Atacama soil, and I haven't yet read all of them -- I'll see whether any of them mentions temperature-dependent effects.)

Posted by: BruceMoomaw Mar 30 2006, 07:22 PM

Well, there are no LPSC abstracts this year dealing with the thermally "labile" (unstable) oxidant suggested by the Viking LR experiment, although there are four other interesting papers dealing with the overall question of the surface destruction of Martian organics: # 2098, 2162, 2262 and 2397.

Posted by: JRehling Mar 31 2006, 12:05 AM

I'll add to Mike's and Bruce's answers by drawing an analogy: Knowing the elemental abundances when you're wondering about the chemistry is a little like knowing how many As, Bs, Cs, etc., are in a paragraph when you're wondering what the paragraph means. The way you put the little parts together matters -- a LOT.

Posted by: edstrick Mar 31 2006, 09:40 AM

Plausible post-Viking studies interpreted the weathered component of the soils as being dominated by "palagonite", which is a mineraloid -- not a mineral -- derived by hydration and weathering of volcanic glass, without well defined mineral content or well-crystalized phases. This goes with the lack of well defined oxide or oxy-hydroxide mineral spectral signatures for a lot of the reddish iron color in the global dust and reddish soils. Added to these dust coponents in the soils were assumed to be basaltic and related sand minerals, sulfates and other water soluble salts and oxidizing compounds, possibly metal peroxides.

One thing Gil Levin doesn't seem to want to talk about much is the glaring LACK of one finding from his experiment. While he found a sterlization-destroyable release of carbon-bearing gas from his radioactive "soup", the reactions were "one-shot" events that happened immediately upon addition of the soup, and there was no growth signature, no delay and increase in gas output whatever. This behaviour is consistent with a rapidly reacting chemical in the sample and not growing organisms. Abundant metabolizing organisms that progressively die off is consistant with the observations, but I don't know to what extent he modeled the plausible biomass required for the prompt gas evolution rates.

Posted by: djellison Mar 31 2006, 10:11 AM

And to add to this - the APXS's on MER can't detect hydrogen, and if your trying to find peroxides........

To carry on JR's analogy - It's like knowing how many of every letter EXCEPT E there are in a paragraph, and wondering what it means.

Doug

Posted by: chris Mar 31 2006, 10:46 AM

To put Doug's post another way:

T:17 N:17 A:17 R:15 O:15 I:11 D:11 Y:7 S:7 H:7 G:7 P:5 W:4 C:4 X:3 M:3 L:3 F:3 U:2 K:2 V:1 J:1

Chris

Posted by: ljk4-1 Mar 31 2006, 02:01 PM

Silly Question of the Day: When they designed Viking to look for
life on the Martian surface, why didn't they include instruments to
fully analyze the materials to separate and detect the inorganic
elements?

Didn't they want to know what the Martian surface was made of
outside of the life issue? I am still amazed at how much of the
results were left to guesswork.

Posted by: BruceMoomaw Mar 31 2006, 02:45 PM

Actually, the one instrument for inorganic analysis of the surface -- the X-ray spectrometer -- was itself a last-minute addition to the payload, put on only in 1971 after a growing number of scientists raised a stink about the lack of any such instrument whatsoever. Viking was explicitly designed as a biological mission, becuase that was by far the highest-priority scientific question about it -- and it was assumed at the time that more Mars missions would soon follow to do other research. (Which, of course, they would have done had the Shuttle not eaten up all the space-science funding in the 1980s.)

Posted by: The Messenger Mar 31 2006, 07:38 PM

Without the inertial of the shuttle, would there have been any funding for any space programs in the 1980's? The Reagan White House operated with a slash-and-burn mentality.

Posted by: BruceMoomaw Mar 31 2006, 08:02 PM

This brings us back yet again to that interminable debate: if the manned program (in this case the Shuttle) was removed, would the total funding for the unmanned program increase or decrease? I still see absolutely no logical argument that it would have decreased. The Space Pork Congressmen, without a manned program to supply their districts with that pork, would have supported a compensatory bigger unmanned program; and the public seems quite interested in some spects of unmanned space exploration -- what they want to see is something new and interesting, and they don't much care whether it's Buzz Lightyear or Robbie the Robot holding the camera. TOTAL space spending would certainly have decreased (explaining by itself NASA's frantic support for Shuttle and Station by any means necessary, up to and including regularly committing perjury before Congress), but that's an entirely different matter.

As for the Reagan Administration having a "slash and burn" mentality: if they really had had one, they wouldn't have inflated the budget deficit to the size of the Crab Nebula. David Stockman -- the one member of the Administration who seriously favored zapping the hell out of the unmanned space program -- later wrote a bitter book on how Reagan had deceived him by falsely saying that he DID favor big spending cuts.

Posted by: ElkGroveDan Apr 1 2006, 01:01 AM

OK I get it. My point was that one could rule out any discussion of xylophones in that paragraph if there were no X's.

Posted by: BruceMoomaw Apr 1 2006, 01:08 AM

Well, yeah -- but virtually all the theories floating around regarding what the oxidants may be (and there are a lot) already rule out any elements which we know not to exist (or to be likely to exist) in Mars soil or atmosphere.

Posted by: RGClark Apr 1 2006, 04:14 PM

There have been some recent reports about the lack sensitivity of the GCMS for the detection of low amounts of organics on Mars. For example, the recent Science article on using Viking analogue instruments on samples from the Atacama found organics in amounts below that which the Viking GCMS could have detected them.
My guess is that the current view by Mars scientists is that the conclusion of no organics on Mars may have been premature.


Bob Clark

Posted by: paulanderson Apr 1 2006, 04:31 PM

Exactly. That's what I was getting at in my previous post also. But I still see some critics of the life idea conveniently gloss over this little fact... Viking could easily have missed such lower levels of organics, which we know now, but didn't back then. So to use the "no organics found" claim as a main argument against possible positive life results, is false. We need to move past that now.

Posted by: ElkGroveDan Apr 1 2006, 04:36 PM

Thank you. That's the answer I was looking for when I first posed the question. Just defending my line of reasoning.

Posted by: dvandorn Apr 1 2006, 05:12 PM

OK -- I think I understand what y'all are saying. I guess the absolute certainty with which the MER team, for example, comes out with statements like "these soils are ground-up basaltic dust with a small admixture of sulphate binding materials" made me think that they had identified the chemical composition of the soils pretty definitively -- and with absolutely no mention of the peroxides, etc., necessary for the old explanations of the Viking results.

So, the truth really is that the MER team states definitively what they know to be in the rocks and soils, but leaves out any references to what *else* might be in the rocks and soils? And they're making somewhat confident statements about the origins of the rocks and soils, when a whole suite of constituents -- those necessary to recreate the Viking results on chemical reactions alone -- aren't detected by their instruments and, so, aren't even discussed?

Is that what y'all are saying?

-the other Doug

Posted by: mcaplinger Apr 1 2006, 06:43 PM

First off, they don't have any way of telling the rocks from the soils; the APXS and the Mossbauer just see whatever is in front of them. They can, maybe, distinguish the soil from the rock by making a measurement and then brushing the rock and making it again, but that's not foolproof. And second, there is always some ambiguity in these analyses, so don't get the impression that they're so "definitive".

Neither of the MER instruments measures all elemental abundances. The Mossbauer detects iron oxidation states, and the APXS looks primarily for "rock-forming" elements. From the Cornell tech briefing: "The x-ray mode is sensitive to major elements, such as Mg, Al, Si, K, Ca, and Fe, and to minor elements, including Na, P, S, Cl, Ti, Cr, and Mn. The alpha mode is sensitive to lighter elements, particularly C and O." I don't know what the carbon detection thresholds are for alpha mode, but I doubt it's that great. The MER instruments are for geology, not organic chemistry. Note that they can't even detect hydrogen or nitrogen.

Rocks typically have silicon, oxygen, and metals in them. That's what MER was designed to look at. They simply can't see peroxides at all, and they can't see organics to any significant degree.

Posted by: JRehling Apr 1 2006, 07:51 PM

I don't think any such instrument exists. There are so many nifty instruments out there that you might forget that we're just exploiting a few loopholes in nature, which has no intrinsic desire to make our analysis easy.

Mass spectrometry of gases works because you can measure the mass of molecules as they ping off of a detector, but that wouldn't work with a solid. It is ambiguous when two compounds share molecular mass.

Vis/IR absorption/emission spectroscopy works because electron shells have different energy levels in ways that sometimes fingerprint a compound uniquely. That works terribly if you're trying to find trace components, or something that lacks any distinct absorption lines, or if multiple, endless possible constituents happen to have similar patterns and are highly ambiguous.

Proton/alpha/X-ray spectroscopy pings the nuclei of a sample to give you a census on those. You have to design the experiment according to the element (alpha particles would smash hydrogen nuclei out of the way), and in the best case, it only tells you the elements, not how they are combined. It's also ambiguous if two isotopes of different compounds have the same number of baryons.

The "what the heck is this solid mineral" made of experiment doesn't exist. A battery of experiments could get down to the heart of the matter, but not in an 8-kg package. That's why sample return is worthwhile; otherwise, we'd just put the magic experiment on a rover and be done with the questions.

Posted by: BruceMoomaw Apr 1 2006, 08:59 PM

David Des Marais' discussion in the Jan. 2005 "Geochemical News" that I mentioned on another thread ( http://gs.wustl.edu/archives/gn/gn122.pdf, pg. 9-16) contains quite a detailed discussion of the instruments which he regards as crucial to a remote analysis of Martian material by a rover. One is a good detector and analyzer of trace organics -- specifically, a GCMS -- and the other is a "definitive mineralogy analyzer", for which he regards an X-ray diffractometer as being best. MSL '09 will carry both -- but both require a quite sophisticated system for ingesting, grinding up and distributing hard samples like rocks, which the MERs were simply too small to carry. (The proposed follow-up "Viking 3", interestingly, probably would have carried all of these; but even if it was mobile, its range would have been far more limited than that of MSL, or for that matter of the MERs.)

And neither of these instruments would do a good job of analyzing Martian oxidants, which are very unstable and will require specially designed analysis techniques to nail them down precisely. Wet chemistry tests -- of the sort that Phoenix will do -- will probably be best for that purpose; I don't know how good MSL will be at analyzing them, although its GCMS may provide some data. In fact, oxidants probably cannot even be returned successfully in Mars sample-return missions -- they're too unstable and would break down en route, without constant reapplication of the Martian environmental processes that created them in the first place. Thus they will likely require in-situ analysis.

About a decade ago, NASA published a very authoritative guide to the sorts of analyses that different instruments on planetary landers can do -- if I can track the damn thing down. At the moment I can't find it on Google, but I know I have a copy of it somewhere.