by Bruce Moomaw
Cameron Park - April 11, 2000 - The second main subject of the final Martian LPSC papers was the seemingly never-ending wrangle over whether Mars Meteorite ALH 84001 does or does not contain significant chemical -- and perhaps microscopic -- evidence of fossilized ancient Martian microbes.
The original Johnson Space Center team who reported the possible find in 1996 continued to stand by their conclusions, reporting further evidence that the tiny magnetite crystals found inside the meteorite strongly resemble in shape those produced only by some Earth bacteria.
And B.P. Weiss (in a paper subtitled "Panspermia Lives On!") concluded that the magnetic properties of the minerals inside ALH84001 prove that it was never heated above 40 deg C -- and thus that the tiny carbonate nodules that contain most of the possible evidence were not (as some researchers think) actually formed in water heated to far above the boiling point by volcanic activity or meteor impact, so that no real microbes could have lived in them.
However, there were more pessimists at the Conference. Allen Treiman and D.C. Golden both flatly disagreed with Weiss. And T. Stephan, A. Steele and J.K.W. Toporski all concluded that there simply is no way to tell whether the organic compounds found inside ALH84001 -- or any other meteorite -- were actually the result of Earth contamination.
Toporski found clear microscopic evidence of Earth microbes deep inside the "Nakhla" Mars meteorite -- and, in another paper, the pro-Martian life Johnson Space Center team actually agreed with Steele and Toporski that meteorites become contaminated with germs so quickly on Earth's surface that "apart from some low-weight organic material... all other [organic] compounds [in any meteorite] could be the products of microbial and terrestrial contamination."
In short, the JSC team seems to have completely given up on one of their lines of evidence -- the idea that the "PAH" hydrocarbons found deep inside ALH84001 provide significant evidence of Martian germs.
And since a growing number of researchers have concluded that another of their lines of evidence -- the microscopic cylindrical shapes that they thought might be fossilized bacteria -- can easily be manufactured by nonliving processes, it appears that their only remaining piece of really suggestive evidence lies in those oddly shaped magnetite crystals, which (as I noted) have also been called into question.
It seems virtually certain now that ALH84001, by itself, contains no really strong evidence for Martian life which supports the view of some skeptical observers that NASA trumpeted that "evidence" back in 1996 as "propaganda" for the space program.
This is hardly surprising, though, since ALH is an igneous rock -- a lump of hardened lava -- and no such rocks have been found even on Earth that contain provable microfossils.
A much more promising place to look for Martian life would be in a sedimentary meteorite -- a piece of Martian sandstone or shale -- but none has ever been identified, quite possibly because geologists did look at them but concluded that they were actually "meteorwrongs" (native Earth rocks that look like meteorites in some ways).
Further evidence of this is the fact that, of the 15 identified Martian meteorites, only one -- ALH84001 -- is more than 1.3 billion years old, despite the fact that over half of Mars' surface is known to be much older.
D.M. Schneider conducted tests to find out what sedimentary meteorites might look like, and concluded that they would be coated with white or clear crusts rather than the black one usual on meteorites. In addition, a team that attached rock samples to the heat shield of an unmanned Russian spacecraft reported that Martian carbonate meteorites could also be identified by the effect that entry friction heating would have on their oxygen isotopes.
If we can find even a few such meteorites, we may not need to return samples of Mars to Earth to prove that life once existed there.
There were, of course, a lot of LPSC papers on Mars that dealt with other subjects than these two. William K. Hartmann repeated his earlier claim that MGS' closeup photos show some lava flows on Olympus Mons and the Elysium Plain to be so sparsely cratered that they must be less than 10 million years old -- which would make it virtually certain that Mars still has occasional volcanic eruptions today.
He does add, though, that "Lavas of age 10 to 100 million years are relatively rare on Mars; most lava plains have higher crater densities and older ages, probably of the order of 1 billion years."
John Longhi and G.G. Ori agreed with earlier suggestions by Jeffrey Kargel and M.D. Max that Mars may be a "planetary Fizzie", with a huge supply of subsurface carbon dioxide (and perhaps methane) mixed with its thick permafrost layer as a "clathrate" -- so that even a modest warming by subsurface volcanic activity might cause them to explode back into gaseous form with tremendous pressure, accounting for the huge and mysterious "catastrophic water outflows" that peppered the planet up to about 2 billion years ago; they would have been titanic seltzer fountains!
J.M. Dohm, Victor Baker and Robert Strom proposed a geological mechanism that would cause Mars to undergo occasional strong episodes of volcanic activity at intervals of tens or hundreds of millions of years -- setting off such huge seltzer eruptions on a planetwide scale, and restoring, if only for a few tens of thousands of years, the planet's original dense CO2 atmosphere and liquid water seas, before it refroze.
But most of the remaining LPSC papers on Mars were provided by three people: Ken Edgett, Michael Malin, and James W. Head. Edgett and Malin released a whole series of papers on their conclusions from studying the super-detailed photos of Mars from MGS's telephoto camera -- and revealed both spectacular surprises and mysteries.
Their announcement that the puzzling networks of dark streaks across much of Mars' surface are drawn by giant dust devils has already drawn much public attention -- and they revealed two photos clearly showing dust devils about 200 meters across in the very act of drawing such streaks., along with other photos showing occasional avalanches of wind-deposited dust down slopes that have occurred during the two Earth years that MGS has been surveying Mars.
Other photos revealed that, if Mars Polar Lander had successfully touched down on the "layered terrain" near Mars' south pole, it would have found layers less than 10 meters thick, laid down by relatively mild shifts in Mars' climate as its axial tilt increases and decreases over cycles of hundreds of thousands of years -- and they also revealed that the nature of this layering is distinctly different for the north and south poles, confirming P.C. Thomas' highly publicized photos showing clear differences in the makeup of the northern and southern polar caps themselves (although we still don't understand these differences).
Other photos revealed seasonal changes in the frosting and defrosting of polar dunes that suggests that they contain large amounts of water ice or frozen CO2; unexpected rock layering exposed in ancient Martian crust all over the planet which indicates that Mars has had "a substantially more complicated geologic history than previously thought" although we still aren't sure whether the layering is sedimentary or lava flows. Moreover details of the strange paradoxical "rough-smooth" nature of most of Mars' surface, wherein areas that look rugged in the large-scale Viking photos turn out to look surprisingly smooth in the finer-scale MGS photos, Meanwhile plains that looked smooth to Viking usually turn out in the MGS photos to be covered by still-mysterious ridges and grooves that make them "extremely rough at meter scales".
Edgett and Malin conclude: "Most of the surface is unlike what might be expected on the basis of photos from previous spacecraft. Many meter-scale surface features defy explanation on the basis of terrestrial analogs and field experience."
Meanwhile, James W. Head provided a whole series of papers further supporting his belief that early Mars not only had liquid water; it had enough of it to fill the northern lowlands as an ocean and to cover much of the south with huge glaciers.
As evidence of the former, he and H. Hiesinger cited a possible ancient shoreline (termed "Contact 2"), 800 km of which was examined by MGS' laser altimeter and revealed to be all at the same altitude, with six giant runoff channels in the Chryse Plain all suddenly ending as soon as they reach that altitude.
Then in the lowlands north of it, the surface is astonishingly smooth (as though made out of ancient sea-bottom sediment plains), and riddled with so-called "polygonal ground" and "lobate impact craters" thought certain to indicate large local supplies of groundwater.
As evidence of his polar icecap theories, Hiesinger cites more photos and laser altimetry data revealing channels and long, sinuous sediment ridges -- apparently carved by meltwater flowing under a layer of ice -- extending for hundreds of kilometers north of the current south polar cap, suggesting that it may have originally extended that far and then shrunk during Mars' "Hesperian Age" 3 to 4 billion years ago (at about the same time that the Chryse drainage channels were carved).
He also claims evidence, from the way one large crater has been partially covered by polar layered terrain, that there is some evidence that the southern icecap later partially grew back before shrinking again.
However, Edgett and Malin repeated their claims both that there is no photographic evidence of an ancient northern ocean shoreline, and that there are "no obviously marine or lacustrine [lake-caused] landforms on the northern plains."
What everybody agrees is that we need more data; Ralph Lorenz suggested that we look for great salt flats on the northern plains (and agreed with R.A. Beyer that there is some evidence for such flats on the floor of the Marineris Valley). A.I. Malyshev suggested that Mars' catastrophic outflows may have been great eruptions of mud rather than water -- which might mesh with K.L. Tanaka's earlier suggestion that Mars' ancient "Northern Ocean" may actually have been an ocean of mud, which could explain the lack of clear shoreline features noted by Edgett and Malin.
Indeed, as should be clear by now, "We need more data!" is still the war cry in virtually any area of Martian studies.
MGS's findings -- and, to a lesser extent, those of Mars Pathfinder -- have opened up whole areas of new and fascinating questions about Mars, and the revelation that it is both different from, more complex than, and more mysterious than we could possibly have anticipated before those missions.
But -- thanks largely to the failure of six of the last eight American and Russian Mars probes -- we still mostly have new questions rather than new answers.
MGS is scheduled to go on busily mapping Mars for at least another year, and hopefully by the time of the next LPSC conference we may have some more answers.
But it is clear that, to really make any major progress in cracking the complex secrets of Mars, we need some more successful Mars probes -- which makes it even more imperative that the U.S. should clear up the problems currently bedeviling its Mars program.
As during the first half of the LPSC conference, of course, there were other subjects than Mars -- and in my final report on the Conference, we will examine those.
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