Rocks From Mars
Cameron Park - June 1, 2001
Finally, the debate continued to rage on at the Conference as to whether the famous Martian meteorite ALH84001 does or does not contain meaningful fossil evidence of ancient Martian microbes.
Indeed, this rock is starting to look as Sphinx-like to scientists as the entire planet currently does.
Kathie Thomas-Keprta and her team at the Johnson Space Center -- who set off a sensation in 1996 by announcing the supposed evidence in the first place -- reiterated their belief that some of the microscopic magnetite crystals within the tiny carbonate granules found within the rock are shaped such unusual "elongated hexa-octahedral" forms, and so uniform in size, that there is almost certainly no nonliving process that could have crated them -- and that they therefore must have been created by ancient Martian "magnetotactic" bacteria that evolved to take advantage of Mars' now-dead magnetic field by using such crystals inside their cells to point themselves like compass needles in the right direction to take maximum advantage of some kinds of nutrients in underwater sediment, as some Earth bacteria have done.
Allan H. Treiman countered by saying that "each characteristic of [these crystals] is consistent with an abiotic origin", and that they could easily have been formed by the rapid heating of iron carbonate in the carbonate granules by volcanic heat, or even by the heat of the giant meteor impact that launched the meteorite off Mars' surface and onto its long voyage to Earth.
Imre Friedmann, however, countered Treiman by repeating his argument -- recently detailed in "Science" -- that some of these unusual magnetite crystals are laid out in chain-like formations that simply could not have been created by any such nonliving crystallization process -- indeed, that they look as though they were originally separated at even distances from each other by now-vanished organic material.
While no paper at the LPSC countered this argument, though, it has been reported elsewhere that the visual microscopic evidence of such chains is actually far more ambiguous than Friedmann and his supporters make it out to be.
And one thing that has become clear is that these magnetite crystal chains are pretty much the "last stand" where possible fossil evidence of life in ALH84001 is concerned -- every other apparent piece of evidence can now be convincingly explained as having perhaps been made in other ways by nonliving processes.
Robert Folk, who has been playing the scientific maverick for some years by claiming widespread microscopic evidence of super-tiny "nanobacteria", said that his new studies of this and another Martian meteorite showed that tiny nanometer-scale mineral ovoids similar to the possible "bacterial fossils" reported inside AL:H84001 also exist in very large numbers on the outside surfaces of both meteorites -- indicating that these ovoids, whether they're produced by nonliving processes or living "nanobacteria", were very likely deposited on and inside the meteorites AFTER their arrival on Earth and therefore do not constitute evidence of Martian microbes.
And J.P. Greenwood and J. Farquhar reported that, while peculiar ratios of sulfur trace isotopes often serve as a sign in Earthly environments that living microbes have been metabolizing nutrients there in the past -- there are no clear signs of such isotopic anomalies within the minerals of ALH84001 and two other Mars meteorites studied.
Farquhar reiterated his recent report in "Nature" that there's a good chance that simple solar UV light striking Martian minerals can also produce such sulfur isotoic "fractionation" effects, making it doubtful that sulfur isotopes can ever be used by themselves as any meaningful evidence of Martian life.
Finally, two researchers suggested longer-shot evidence that the surface of present-day Mars -- despite its seemingly savage environment -- may even contain large amounts of alive and thriving microbes.
D.M. Warmflash suggested that the discovery of traces of organic compounds within the Mars meteorites may mean that the Viking landers were wrong in indicating a total absence of organic compounds in Mars' surface soil -- and, therefore, that the puzzling results from the landers' "Labeled Release" tests for living microbes, which are still rather hard to explain by means of the hypothesized organic-destroying "soil oxidants", may really have detected living and growing germs., as its designer Gilbert Levin still insists.
And Hungarian scientist A. Horvath made an even bolder proposal regarding the remarkable dark spots that MGS has discovered appearing in great numbers on the tops of Martian polar-region dunes in spring and expanding as the season progresses.
Michael Malin and Ken Edgett think this is simply due to the fact that sunlight hitting the tops of the dunes thaws the frosts of CO2 and/or water off them first, and then that this darker frost-free soil is heated by the sunlight and in turn heats the soil around it to rapidly expand the thawed area.
Horvath, however, thinks the dark spots may actually be large films of living Martian algae taking advantage of water frost melting on the dunes in spring to rapidly grow and reproduce before retreating back into dormant spore form after the liquid film of water disappears.
His main piece of evidence for this is that the dark spots tend to elongate and form long streaks running down the surfaces of dunes, indicating that they are associated with the actual presence of liquid water on the dune surfaces.
But there are possible alternative explanations for this -- for instance, E.F. Albin reported similar dark streaks running down frost-free Martian slopes elsewhere, which may be due to local releases of subsurface carbon dioxide which have formed miniature versions of Nick Hoffmann's "cryoclastic" eruptions and swept material downslope to blow away lighter fine surface dust.
It may be that Malin and Edgett's theory just needs to be mildly modified -- that frozen CO2 or water vaporizing out of the upper centimeters of dune material in the dark spots tends to flow downslope and either heat up and evaporate the frost down there, or sweep away lighter-colored fine dust on the dune surfaces to expose the darker coarse dust underneath.
(It is certainly known that frozen CO2 thawing out of the soil around the south polar cap in spring kicks enough dust into the air to absorb sunlight, further warm the surrounding air, and so create Mars' self-amplifying dust storms.) And MGS has also found the tops of the polar dunes are also the very first spots to refrost in fall, further indicating that all of this is just a nonliving weather process.
In short, the major conclusion one is forced to from this year's LPSC papers on Mars is that -- while we are teetering on the very brink of a general understanding of the planet's geological and climate history -- we still haven't crossed that brink.
Indeed, the new "White Mars" theory that most of what we thought was caused by liquid water on Mars' surface is really due to carbon dioxide has actually further complicated the picture in the past two years.
We still need more data to solve the problem.
MGS will continue to photograph more and more small spots on the planet with 2-meter resolution; and the Mars Odyssey craft now on its way to orbit Mars should certainly provide us with more useful evidence with its THEMIS infrared camera (which can try to map carbonates and other surface minerals with a resolution of only 100 meters, search for local spots of still-active geothermal warmth, and photograph much of the planet with 30-meter resolution) and its gamma-ray spectrometer (which should give us our first overall global map of the distribution of important elements in Mars' surface rocks, and can also detect both water ice and frozen CO2 at depths down to 30 cm below the soil surface) -- but it seems unlikely that these two instruments by themselves can crack the puzzle.
We are much more likely to make that final breakthrough in understanding the nonliving processes on Mars from the spacecraft set to travel to Mars in 2003 -- the two American "MER" rovers, Europe's "Mars Express" orbiter with its large set of new and important instruments for surface mapping (especially its subsurface radar sounder), and the little Beagle 2 lander that it will carry.
And only after we have that understanding of nonliving Mars will we be in a really good position to estimate the chances that early Mars did evolve life -- and even that present-day Mars may conceivably still bear it in preserved shelters under the planet's surface.