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Derek Lowe The 2002 Model

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Derek Lowe, an Arkansan by birth, got his BA from Hendrix College and his PhD in organic chemistry from Duke before spending time in Germany on a Humboldt Fellowship on his post-doc. He's worked for several major pharmaceutical companies since 1989 on drug discovery projects against schizophrenia, Alzheimer's, diabetes, osteoporosis and other diseases. To contact Derek email him directly: Twitter: Dereklowe

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October 25, 2006

Mass Spec on Mars

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Posted by Derek

There's an interesting analytical chemistry paper in the preprint section of PNAS (open access if you want to read it) that may reopen an old controversy. It's from a large multinational team (Mexico, Spain, France, NASA-Ames) investigating the GC-mass spec instrumentation that was flown to Mars on the Viking landers in 1976. That's a key instrument in the life-on-Mars debate, so an attack on it is significant. First, though, some background - it's a tangled story.

The Viking landers each had three biology experiments to look for possible signs of Martian life, whose results were famously difficult to interpret. They produced both excitement and confusion at the time (scroll down in that NASA history page) and they've been fuel for arguments ever since.

There was the pyrolytic release experiment, which incubated Martian soil with 14C-labled carbon monoxide and carbon dioxide. After several days, the sample was purged, then heated to 650C and analyzed for the release of any labeled carbon compounds that might have been formed by living organisms. A control sample was heated before incubation, to kill off any such life forms. Seven out of the nine runs of this experiment seemed to produce positive results - that is, volatile labeled carbon was produced after pyrolysis.

The gas-exchange experiment used the same sort of apparatus, exposing the soil to either water vapor or nutrient solution under a mixed atmosphere of gases. The headspace was analyzed for changes in the concentrations of the various components, which could be due to biological uptake or release. This one showed a strong release of oxygen and carbon dioxide from the samples once moisture was added, but the amount decreased over time, leading to theories that this was the product of an inorganic reaction rather than a signature of life.

The labeled release experiment put Martian soil into a dilute nutrient broth, with several small organic compounds which were all labeled with 14C. After incubation, the headspace of the experimental cell was analyzed for any released labeled gases and again, a control experiment was done with pre-heated soil. This one produced exciting data, with release of labeled gas in the experimental samples well over those in the controls. One odd result, though, was that the subsequent injection(s) of nutrient solution did not produce a further spike of released gas. The final curves ended up looking neither like what you'd have expected from a classic bacterial positive, nor from a simple chemical reaction. This ambiguity has meant that the LR results have been re-analyzed and re-fought ever since the 1970s, with the experiment's designer, Gilbert Levin, leading the effort to rescue the data as a case for Martian life.

But then there were the GC-MS data, from an experiment considered to be the backstop test in case the biology experiments were difficult to interpret. Since they certainly were that, from beginning to end, this experiment became for many people the most important one on the landers. (It already had been for the people - a not insignificant group - who thought from the start that the biology tests were unlikely to provide a conclusive answer). This one heated soil samples directly and looked for volatile organics. Heating to 200C showed little or nothing in the way of carbon compounds, and very little water besides. By contrast, another sample taken up to 500 degrees released a comparative flood of water, but still showed no evidence of organic molecules.

And that, for most observers, was that. No organic molecules, no life. Explanations after the GC-MS results mainly turned to what sorts of inorganic chemistry might have given the behavior seen in the three other experiments. Martian soil was thus hypothesized to be a sterile mixture of interesting chemicals (iron peroxides? carbon suboxide polymers?) that had fooled the biology test packages, but couldn't fool the GC-MS.

There's always been an underground, though, that has held that the results were indeed the result of life. Gilbert Levin has never given up. In 1981, he pointed out that tests of a Viking-style GC-MS instrument had shown that it was insensitive to organics in a particular Antarctic soil sample, but that this same soil nonetheless gave a positive result in the LR experiment. And he really put his opinions out in the store window in 1997, with a paper that flatly concluded that the 1976 LR experiments had indeed detected Martian life.

In the last few years, others have joined the battle. Steven Benner at Florida, whose work I wrote about here, published a PNAS paper in 2000 which maintained that organic molecules on Mars would likely be retained as higher molecular weight carboxylates, which would not have been volatile enough for the Viking GC/MS instrument to detect. And now this latest group has weighed in.

They've also analyzed various Antarctic and temperate desert samples, and found that all of them contain organic matter that cannot be detected by thermal GC-MS analysis. And the ones that contain iron, including the NASA reference simulated Mars soil (a weathered basalt sample from near Mauna Kea), tend to oxidize their organics quickly under heating. The conclusion is that while much of the water and carbon dioxide produced in the Viking experiment from heating the Martian soil was surely inorganic, some of it could have been from the oxidation of organic material. The paper concludes that the Viking GC-MS results are. . .inconclusive, and should not be taken as evidence either way for the presence of organic molecules or life. The question, they feel, is still completely open.

The good news is that future missions are relying on other technologies. In addition to good ol' thermal volatilization/GC-MS, there are also plans for solvent extractions, laser desorption mass spec, short-path sublimation, and other nifty ideas. If these various US and European missions get off the ground (and on the Martian ground), we're going to have some very interesting data to look at. And argue about.

Comments (6) + TrackBacks (0) | Category: Analytical Chemistry | Life As We (Don't) Know It


1. Daniel Newby on October 26, 2006 1:14 AM writes...

All I want to see are traces from an HPLC-polarimeter with two or three different columns, using water as the solvent. Organic life virtually has to be chiral, and for simple molecules that means lots of optical isomers. If it sends back traces with sugar and amino acid peaks, we can justify sending a whole army of instruments.

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2. Eric Johnson on October 26, 2006 10:18 AM writes...

> Organic life virtually has to be chiral

Organic life virtually has to be able to make chiral discriminations - but organisms built from racemic monomers could still make chiral discriminations. A polypeptide comprising both dextro and levo amino acids could still recognize ligand A and ignore the enantiomer of A.

> sugar and amino acid peaks

There's no reason these particular molecules have to be involved.

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3. GATC on October 26, 2006 11:17 AM writes...

"The 2006 Astrobiology Follies: Return of the Phantom Martian Microbes" Gest, H. 2006.

Oh to be an emeriti! Now, on to global warming!

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4. Daniel Newby on October 26, 2006 4:52 PM writes...

"... but organisms built from racemic monomers could still make chiral discriminations."

Certainly that is possible. However you can make reasonable arguments that once organisms start synthesizing their own monomers, natural selection will rapidly settle on single enantiomers. Catalysts and their substrates can get a much better fit, and therefore faster reactions at lower energy cost, if they are highly enantiopure.

Re. sugars and amino acids, certainly they are not necessary. However they are sufficient, the set of sensible small-molecule compounds is finite, and we know that the primordial tar included at least some of them.

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5. Anonymous on October 26, 2006 6:21 PM writes...

This post reminds me of the time we consulted with the late Henry Rapaport. One of my colleagues was telling him how he got an unusual product from a reaction. Rapaport asked him how he knen that was the product produced. My colleague told him he saw it by mass spec. Rapoport was unimpressed and shot him down with "They found life on Mars with a mass spec".

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6. Monte Davis on October 27, 2006 9:56 AM writes...

Henry S. F. Cooper's The Search for Life On Mars is still well worth reading after 25 years. One reflection it inspires is that for obvious reasons, we know medically relevant biochemistry in more depth than biochemistry in general, and biochemistry in more depth than chemistry in general -- so when anomalous results like Viking's turn up, there's a scramble to figure out what "exotic" peroxides and superoxides might be doing.

IOW, there are lots of curious corners of inorganic chemistry we've simply not been motivated to explore -- just waiting to challenge parochial ideas of "things that only life does"...

GATC: thanks for the link to the Gest paper -- very sharp!

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