[X] My Assistant

[ElkGroveDan]

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.

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.

[Guest_BruceMoomaw_*]

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.

[Guest_RGClark_*]

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.

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

[Guest_paulanderson_*]

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.

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.

[ElkGroveDan]

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.

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

[dvandorn]

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

[mcaplinger]

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?

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.

[JRehling]

why didn't they include instruments to
fully analyze the materials to separate and detect the inorganic
elements?

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.

[Guest_BruceMoomaw_*]

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.