Is (Was?) There Life on Mars?

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On July 4, 1997, the spacecraft Pathfinder bounced to a landing on the surface of Mars. It had been 21 years since humans last placed an object on Mars. That was when Unlike Pathfinder, both Viking missions carried equipment designed to look for evidence of life.

There were five different types of instruments:

1. Television cameras.

No images suggesting the presence of life were ever seen.

2. A gas chromatograph combined with a mass spectrometer.

This apparatus examined the martian soil for the presence of organic molecules. Even though sensitive to concentrations in the parts per billion (ppb) range, no organic matter was detected (except for traces of the solvents that had been used on earth to clean the equipment). Even if organic molecules could be formed on Mars, the intensity of the ultraviolet light at the surface would soon destroy them.

3. The Labeled-Release (LR) experiment

Metabolism is a universal property of life on earth. The LR experiment was designed to look for evidence of catabolism by any microorganisms that might have been present in the Martian soil. In this experiment, a soil sample was incubated with a dilute soup of organic molecules (such as the amino acid glycine) which had been synthesized with the radioactive isotope 14C. Over a period of 10 days, the atmosphere above the sample was monitored for the appearance of radioactive gases such as carbon dioxide (CO2). The results: Thus most scientists concluded that the gas was produced by nonliving chemistry (brought about by oxidizing agents in the soil). This conclusion was strengthened by similar results using soils from a dry desert in northern Chile. (See Navarro-Gonzalez, R., et al, Science, 7 November 2003)

4. The Pyrolytic-Release (PR) experiment

The PR experiment was designed to look for evidence of anabolism.; specifically whether there were any microorganisms in the martian soil that could synthesize complex organic molecules from carbon dioxide (CO2) and carbon monoxide (CO). In this experiment, a mixture of radioactive CO2 and CO was introduced into a vessel containing a soil sample.

Because the incubation mixture was illuminated with a bright arc lamp.

After 5 days, The result: organic matter was detected in 7 of 9 runs. However, some positive results were achieved even on runs where the soil had first been heated to such a high temperature that any microorganisms present would have been killed (at least here on earth).

5. The Gas-Exchange (GEX) experiment

In this experiment, a known mixture of gases was placed in the chamber along with the soil sample and then analyzed periodically to see if any gases (e.g. CO2) had disappeared from — or been added to — the mixture.

So what can we conclude from these data?

The LR, PR, and GEX experiments all produced some positive results.

However: So the Viking studies probably did not reveal the presence of life on Mars. But this is not the same as saying that life does not now nor ever did exist on Mars!

Perhaps:

The Evidence from Martian Meteorites

Some meteorites are thought — because of their peculiar chemistry — to have reached earth from Mars. One of these ALH84001 (found in the Allan Hills of Antarctica in 1984) has been subjected to intensive analysis for ingredients suggestive of life processes.
In it have been found:
  1. polycyclic aromatic hydrocarbons (PAHs). But
  2. minerals within the meteorite (e.g. carbonates, magnetite) that are formed by living organisms here on earth and appear to have been deposited in the rock of the meteorite at some later time in its history;
  3. objects that under the scanning electron microscope look like fossils of tiny microorganisms. However, even the largest of these "nanofossils" have diameters of only 100 nanometers (nm) (0.1 µm, about the size of a ribosome). This is smaller than the smallest microorganisms here on earth (the mycoplasmas, with diameters of about 300 nm) and is smaller than the estimates of the minimum diameter (200 nm) needed to provide the volume necessary to build a living cell.

ALH84001 is thought to have landed in Antarctica some 13,000 years ago. But in July 2011, another Martian meteorite landed in the Moroccan desert. With much less time for terrestrial contamination to occur, it may help settle some of the controversy over the significance of the features found in ALH84001.

What would answer our question?

Organic matter is, despite its name, not the exclusive product of life. Many other meteorites contain organic matter and organic molecules can, of course, be synthesized in the laboratory from inorganic precursors. What does distinguish the organic molecules produced by life is the restriction to one enantiomer or the other. For example, all proteins synthesized by living things here on earth use L-amino acids exclusively. Synthesis of amino acids in the chemistry laboratory produces a 50:50 mixture (called a racemic mixture) of the L- and the D- forms. There is nothing to suggest that life couldn't work just as well with D-amino acids. What is unlikely is the ability of proteins (e.g. enzymes) to be able to function if they are made from a mixture of L- and D- enantiomers. So if martian soil should reveal the presence of all-L (or all-D) enantiomers, this would be powerful evidence that life had produced them.

However,

The case for life on Mars — whether today or in the past — is neither proven nor disproven. Stay tuned!

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31 May 2012