by Bruce Moomaw for SpaceDaily.com
Mountain View CA - Apr 27, 2004
While Steve Squyres didn't mention it in his March 30 talk before NASA's Third Astrobiology Science Conference at Ames Research Center, the separate MER press conference held at JPL on the last day of the Conference revealed that the Gusev landing site of the first MER rover, "Spirit" -- which had seemed to be a relatively dull area of volcanic basalt rocks unaltered by water -- was finally starting to show some clear evidence of aqueous alteration after all.
Specifically, Spirit had just finished analyzing "Mazatzal", a large rock near the rim of Bonneville Crater that seemed distinctly different in appearance from any of the basalt boulders Spirit had analyzed up till then -- it was very light-colored and seemed to be more wind-eroded than the basalt rocks were. The rover analyzed its outer crust with the instruments on the end of its arm, then used its "RAT" tool to grind a fraction of a centimeter for more analyses, and then ground yet further into it.
What it found was clear evidence of a weathered crust on the rock composed of two separate layers -- an outer light-colored layer with a very dark layer underneath it, followed by the somewhat lighter-colored unweathered rock. The outer pinkish weathered layer seems to be similar in composition to Mars' windblown dust -- rich in salts of sulfur and chlorine. The underlying dark weathered layer was not only also sulfur-rich, but surprised scientists by being even richer in chlorine than Mars' soil. And the rock underlying these two crusts was itself laced with cracks and small cavities filled with whitish minerals that also seem to be the result of exposure to liquid water.
The surface coatings, at least, are not the result of Mazatzal being exposed during part of its lifetime to underground liquid water, as first thought. It had been thought all of the lighter-colored rocks visible at Gusev were just ordinary basalt boulders with a somewhat thicker coating of windblown Martian dust -- but at the next MER press conference on April 8, science team member Ray Arvidson of Washington University announced that quite a few genuinely light-colored rocks similar to Mazatzal have now been recognized in Spirit's pictures and long-range infrared spectra, among the rocks thrown out onto the plain by the meteoroid impact that excavated Bonneville Crater. And, judging from their shapes, these rocks have been substantially eroded by blowing sand BEFORE the light-colored weathered crust formed on them. They must therefore have been coated with at least a very thin film of liquid water for fairly long periods of time while still sitting on the surface of Mars.
Arvidson said that this meshes with the new portrait of Mars' climatic "obliquity cycles", such as I described in my recent piece about the Mars papers delivered at last year's Division of Planetary Sciences Conference. Mars' obliquity (the tilt of its spin axis) slowly rocks back and forth, over cycles of roughly 100,000 years, between maximal and minimal tilts. For the past 3 million years, it has been rocking between tilts of 13 and 35 degrees. Earlier, there was a period of several million years in which computer analyses indicate that its tilt was varying instead between 26 and 46 degrees.
The effect on Mars' weather patterns during its periods of more extreme tilt can be extraordinary. Since the two poles are alternatively titled much more directly toward the Sun than is now the case, during summer all the water ice located on or near the surface at either of Mars' polar regions actually tends to sublimate into vapor and migrate down toward lower latitudes to refreeze. During tilts of 40 degrees, it may migrate all the way down to the equator (and Gusev), leaving Mars with an equatorial "ice belt" rather than two polar ice caps. When it reaches those lower latitudes, it either refreezes as frost on the planet's surface, or actually falls as snow. And -- even given modern Mars' extremely thin air -- during daily and seasonal temperature shifts, there will be periods when the ice melts to form thin and short-lived films of liquid water on the planet's rocks and soil which can nevertheless weather them significantly.
It's also possible that, during the periods of highest obliquity, carbon dioxide that has been frozen near the poles, or adsorbed in large amounts by the cold polar soil, is released into the atmosphere to raise Mars' surface air pressure several fold. This would still make it, at absolute most, about one-twentieth as dense as Earth's surface air -- but that would be enough to greatly increase the ability of Mars' winds to blow dust and sand around. Indeed, a German team has recently concluded, by counting the small craters on top of some of Mars' sand dunes, that most of them were formed several hundred thousand years ago and have been pretty much unmoving since then. It's possible that the rocks at Gusev have been carved by blowing sand only during high-obliquity periods -- and that, as the tilt of Mars' spin axis then diminishes, there's a brief period during which the planet's air pressure and thus the erosive force of its winds greatly drops but there is still a layer of frost or snow covering the rocks. This would explain why the light-colored rocks at Gusev have an aqueous weathered crust that formed on them after their last erosion by wind.
There is another complicating factor: it's considered increasingly likely that sulfur -- and sulfuric acid in particular -- play a major role in the surface chemistry all over Mars. Theories of planetary formation suggest that Mars may originally have formed out of a part of the original near-solar nebula that was more sulfur-rich than the part of the nebula that formed Earth. In any case, what volcanic vents Mars has left (and it certainly has some active vents even now) emit various sulfurous gases that are virtually certain to react with the 0.15% of free oxygen that exists in Mars' atmosphere right now. (The oxygen is briefly liberated by the Sun's ultraviolet light from the carbon dioxide and water vapor in Mars' air, although it quickly combines chemically again with the atmosphere's other components.)
That oxygen -- when it reacts with Mars' tiny traces of volcanic sulfurous gases -- quickly turns them into sulfur trioxide, which in turn reacts immediately and eagerly with any water it contacts to form sulfuric acid. (It's this same kind of process -- working on the tiny traces of water vapor and sulfur dioxide that still exist in Venus' air -- that forms that planet's sulfuric-acid cloud deck.)
Sulfuric acid, in turn, is extremely "hydrophilic" -- it's powerfully chemically drawn to blend with the rest of Mars' surface water, whether liquid or frozen. And mixtures of sulfuric acid and water can have amazingly low melting points. In fact, as such an acid-water mixture drops to temperature levels enough to refreeze part of the water in it, the remaining liquid portion of the mixture becomes a more and more concentrated liquid solution of sulfuric acid until it finally becomes about 39% H2SO4 -- at which point it will remain liquid down to temperatures as low as -74 deg C (-101 deg F), 10 deg C lower than the lowest average yearly surface temperature on Mars at the current time. Moreover, such an acid solution is so eagerly chemically attracted to more molecules of water vapor in the air that -- even in modern Mars' near-vacuum trace of air -- it will not evaporate; it pulls more water vapor molecules out of the thin air as fast as it releases them into it. In short, it appears increasingly likely that small moist traces of sulfuric acid solution may frequently exist on or near the surface of many parts of Mars right now, even far from any volcanic vents, remaining stubbornly liquid even in the planet's current hostile environment. And during those high-obliquity periods when Mars had significant amounts of water ice alternately refreezing and evaporating on its surface on a seasonal basis, it would be even easier for significant amounts of sulfuric acid solution to exist there, weathering the planet's rocks and soil particles more efficiently than water alone could do.
Such a phenomenon would also very nicely explain the thin surface crust of salts that seems to cement soil particles together all over Mars. This crust seems to be composed largely of magnesium sulfate -- natural Epsom salt -- and this is precisely the substance produced in large amounts when the mineral olivine, found in Martian basalt, is weathered by sulfuric acid. The dark weathered crust of Mazatzal is indeed richer in sulfur and chlorine salts than the unaltered original rock. And at the Lunar and Planetary Science Conference in Houston earlier in March, it was announced that the "OMEGA" near-infrared spectrometer on Europe's orbiting Mars Express, just while mapping the first one percent of Mars' surface, has found particularly dense concentrations of Mg sulfate in many of the light-colored patches seen on the floor of Mars' great Marineris Valley -- suggesting that these regions, like Meridiani, may have been exposed to particularly large amounts of groundwater mixed with sulfuric acid.
(OMEGA, by the way, is far more sensitive to sulfates than that longer wavelength IR spectrometer on our own MGS orbiter that discovered the Meridiani hematite concentration in the first place. This explains why MGS did not detect the very large amounts of sulfates in the light-colored Meridiani Etched rock from orbit.)
The presence of large amounts of sulfuric acid on the surface and near-surface of Mars could also explain one of the planet's biggest puzzles: its so-called "carbonate paradox". It's known with reasonably certainty that Noachian Mars had a fairly dense carbon dioxide atmosphere -- maybe considerably denser than Earth's total present-day air. But if the planet was also warm enough during these days for large amounts of liquid water to exist on or near its surface (thanks to the greenhouse effect from all this CO2), then that same liquid water should have caused most of the CO2 to react with Mars' silicate rocks and form large permanent beds of carbonate minerals. In effect, the early habitable Mars would have self-destructed -- something that doesn't happen on Earth only because our crustal tectonic cycle (which Mars is too small and thus too internally cool to have) constantly drags Earth's accumulated beds of carbonates back down into its hot interior and break them back down into CO2, which its volcanoes then belch back up into the atmosphere.
However, repeated searches from Earth-based telescopes and the orbiting MGS and Mars Odyssey spacecraft have failed to reveal any large carbonate deposits -- which has led many scientists to conclude that even Noachian Mars' dense CO2 blanket wasn't powerful enough to warm the planet's surface above freezing, so that there were never more than small and temporary patches of liquid water on or near its surface -- maybe always concealed, when they did exist, beneath a layer of ice. This, of course, would have made ancient Mars vastly less promising as a possible spot for the evolution of life. (It would also mean that Mars instead lost most of its early CO2 when it was splashed into space by the giant meteroid impacts of the period, or swept into space by the solar wind that has blown directly over Mars' upper atmosphere ever since its very early initial magnetic field shut down.
However -- as Jeffrey Moore points out in the April 15 "Nature" -- if Noachian Mars' surface water bodies actually consisted of even a very weak 0.1% solution of sulfuric acid, that acid would prevent any CO2 dissolved in the water from reacting with Mars' silicate rocks to form carbonates. It's thus possible that Noachian Mars really might have had large bodies of surface liquid water without leaving any carbonate evidence of this behind today. Indeed, since such an adulteration by sulfuric acid would considerably lower the freezing point of water, it would also make the existence of such bodies more likely.
And since such a body of weak sulfuric acid solution, as it did finally start to freeze from the top down, would leave a more and more concentrated layer of liquid acid solution underneath the thickening top layer of pure water ice, Steve Squyres has pointed out that this gives us another possible mechanism by which the sediment layer at Meridiani might have been exposed to a layer of H2SO4 concentrated enough to break down basalt sand or volcanic ash into the mess of amorphous silicates, sulfate salts, and iron oxides that the Opportunity rover has found in the Etched rock layer there today. There would still have had to be some currents running through the liquid layer beneath the ice to produce the "cross-bedded" ripples found by the rover in the sediment layers, but he thinks this possible.
Another talk at the Conference concerned yet another complication in this story. Phil Christensen of Arizona State University -- the chief investigator for the "TES" thermal IR spectrometers on both the orbiting MGS spacecraft and the two MER rovers -- has recently announced his belief that his instruments have finally detected evidence for small amounts of carbonates (probably magnesium carbonate) lacing the soil of Mars planet wide. These apparently make up only 2 to 3 percent of Mars' soil globally, although the "Mini-TES" on the Spirit rover has found somewhat larger amounts of the substance in the soil at Gusev. But even this small percentage -- if it's also spread through the material of Mars' rocks down to a depth of one to three kilometers -- would be enough after all to have absorbed one to three bars (that is, Earth atmospheric pressures' worth) of atmospheric CO2.
If this is so, then -- to quote Jeffrey Moore in "Nature" -- "It could be that Mars sustained a thick CO2 atmosphere and supported liquid water, so long as the CO2 was precluded from forming carbonates by [small amounts of ] sulfur dioxide in the air and water...Once the abundance of SO2 [from Mars' early volcanism] dropped below the critical level to suppress carbonate formation, the atmosphere would have rapidly collapsed to near its present size, leaving carbonates very little time to form as layered marine deposits" of the sort that have been unsuccessfully looked for on Mars.
However, at the Conference Janice Bishop -- speaking for the "BioMars" scientific team at the U. of California at Berkeley -- expressed their belief that the two IR spectral features which Christensen thinks indicate soil carbonates -- plus another one which he thinks indicates some as-yet unidentified mineral chemically combined with small amounts of soil water -- may actually all instead be due to several percent of hydrated iron sulfates in the soil: jarosite or its cousins. Their ground-based spectra of such minerals match up quite well with all three spectral features -- and Bishop announced that soils containing a few percent of them also match up very well with Spirit's Mossbauer gamma-ray spectra of the minerals in Gusev's soil. So the question of whether any of Mars' early water is locked up today in carbonate minerals on its surface remains uncertain.
However, we now have other evidence -- separate from the whole question of "missing carbonates" on Mars -- that it would be a serious mistake to overstate the degree to which Mars' surface has been weathered by water or H2SO4, and that the weathering process (at least over the last few billion years) must have been very weak and sporadic compared to weathering processes on Earth's rocks. The mineral olivine, found in large amounts in fresh basalt rock, is completely destroyed by only a few thousand years of exposure to regular water, let alone sulfuric acid -- and yet MGS has found outcrops of it scattered all over Mars' surface. The MER rovers have also now proven that it is present in the basalt rocks and sand of Gusev, and also in the dark basaltic sand that is blowing across the surface of Meridiani and grinding away the soft Etched rock there from the top down to leave the hematite "blueberries" there behind.
Mars' cold temperatures have greatly slowed down all such chemical weathering processes -- but olivine weathers very fast even in cold liquid water (let alone sulfuric acid solution!). This by itself indicates pretty firmly that -- at least since the end of the Noachian Age roughly 3.7 billion years ago -- the olivine laid down in Martian volcanic lava flows has been mostly unmolested by weathering despite its great vulnerability, and that therefore since then liquid water must have existed on Mars' surface only in small amounts and for relatively short periods.
It's still possible that most of the olivine we're detecting now was deposited as lava after the Noachian Age; but the amount of olivine that seems to be present even in the older parts of Mars' surface provides definite evidence that even during the Noachian the amount of unfrozen water on Mars' surface may have been pretty seriously limited. Certainly the "atmospheric" aqueous weathering crusts that we are now finding on the surfaces of Martian rocks and soil, due to the occasional formation of thin films of water and/or acid solution during Mars' highest-obliquity periods, can't ever have involved the existence of more than very small traces of liquid moisture spread over billions of years, or the olivine would have completely vanished from Mars' surface and there would also be much larger amounts of other water-weathered minerals such as clay silicates.
This, however, leads us to yet another interesting point stemming from the discoveries of the Spirit rover at Gusev. While the basalt boulders that cover the plain at Gusev (and most of the rest of Mars) are relatively resistant to such weathering, it's possible that Mazatzal and the other light-colored rocks at the site may have been made from the beginning of some other type of rock more susceptible to such water and/or acid weathering. They seem to be consistently more wind-eroded than the regular basalt boulders at Gusev -- and, as another surprise, Spirit's analyses of the element percentages inside Mazatzal once it had completely ground through the weathering crusts into the unweathered underlying rock indicate that the latter apparently had a surprisingly high amount of bromine in it. This may have to do with those whitish cracks and pits inside Mazatzal that are filled with some light-colored material -- it's possible that this rock really was also exposed to liquid water and/or sulfuric acid while still underground, which spread through the cracks in it and weathered it there to create bromides and other salts. This, in turn, could have chemically modified it and made it more porous, thus making it more susceptible to later acid or aqueous weathering on its outer surface as well.
Such repeated exposure to small amounts of liquid water could happen to rocks while they were still buried underground. After all, the gullies discovered by MGS on slopes scattered over a wide part of Mars are now thought to have probably been carved by liquid water -- and while some theories of their origin involve the melting of snow deposited on the Martian surface during its obliquity changes, other theories involve those same obliquity shifts (or perhaps mild local geothermal heating) generating significant amounts of liquid water underneath Mars' surface, which bursts out occasionally onto the surface.
Moreover, in the case of Gusev, such traces of subsurface melted water may sometimes appear very close to the surface indeed. America's Mars Odyssey orbiter maps the amount of hydrogen in the upper meter of the surface -- and thus the amount of water ice or chemically hydrated minerals. It thus discovered very high concentrations of near-surface water ice within only a few centimeters of the surface in Mars' near-polar regions at latitudes of over 60 degrees. But it also discovered two very large and puzzling low-latitude regions -- centered over the equator -- which also contain really surprising amounts of hydrogen, too high to be explained as anything but significant amounts of near-surface water there too (perhaps 10% of the local soil by weight). Gusev is located in the middle of one of these regions.
But the same calculations which predict that water ice will be stable right now within a few cm of the surface in Mars' high-latitude regions also predict flatly that water ice cannot currently exist within less than 2 meters of the surface at Mars' equator -- the summer noontime temperatures there, low though they are, are still high enough closer to the surface to have made all the ice closer to the surface sublimate directly into vapor in Mars' wispy air. So why is there apparently ground ice closer to the surface in these two equatorial regions?
In another talk at the Astrobiology Conference -- following one he made at last September's Division of Planetary Sciences conference -- the University of Colorado's Bruce Jakosky described a possible explanation. Both of Mars' polar caps are covered during their winters with a thin layer of carbon dioxide frozen directly out of the air, which evaporates in summer, leaving smaller permanent polar caps made of frozen water ice. But for reasons we don't yet understand, Mars' permanent southern water-ice polar cap -- unlike the northern one -- is also covered throughout the year with a second permanent layer of frozen carbon dioxide up to 8 meters thick, which remains in place throughout the summer and is then covered with the thin additional CO2 snow layer in winter.
If that "permanent" frozen CO2 layer were to be stripped away from the top of the southern water-ice cap, the amount of water vapor evaporated into Mars' atmosphere from the underlying water-ice cap during its summers would be vastly increased. Indeed, the average yearly amount of water vapor floating over each spot on its equator might be increased from the equivalent of a 10-micron layer of liquid water to ten times that. And this higher humidity, even in Mars' current wispy air, would be enough to considerably retard the tendency of water ice to sublimate into the air at the planet's equator -- the temperature at which water ice would remain stable on the surface of Mars would rise from -63 to -53 deg C. This, in turn, would allow water to remain frozen a good deal closer to the surface at the equator than is now the case.
And -- for reasons we once again do not clearly understand, but which may be connected with gradual changes in the pattern of Martian dust storms -- the two American spacecraft currently orbiting Mars have found that the "permanent" frozen CO2 ice layer on Mars' southern polar cap is indeed currently and dramatically shrinking. It's riddled with "Swiss-cheese"-type holes through which the underlying layer of southern water ice is now exposed, and these are now widening from year to year at a rate which, if it keeps up, will make the permanent southern CO2 ice layer vanish completely within only a few centuries!
We don't know whether such a total disappearance will really happen in the next few centuries, or whether the process will reverse -- but if there really are very frequent periods on Mars during which most or all of the "permanent" southern CO2 ice layer vanishes and Mars' atmospheric humidity is thus higher, then the ice deposited in Mars' equatorial regions during its last really high-obliquity episodes over 5 million years ago may have had more "staying power" since then than we previously thought. It may have sublimated back into the air, and refrozen again in Mars' near-polar regions, at a much lower rate than we thought -- and so a considerable amount of it, covered by newly wind-deposited dust and sand, may indeed be left very near the surface in those two big near-equatorial regions, including Gusev.
(The same may be true if -- as some think -- a lot of Mars' near-surface equatorial water is currently stored in hydrated sulfate salts in the soil, rather than as near-surface ground ice. These can themselves store a really surprising amount of water -- sometimes their own weight in it -- while still appearing dry. And such equatorial salts would likewise be able to resist drying up and losing their water far better if Mars' atmosphere frequently has episodes of considerably higher humidity due to the disappearance of its "permanent" southern CO2 ice layer.)
If there really still is an excess of equatorial soil ice (and/or water combined as a hydrate with soil salts) left over from Mars' last really high-obliquity episode millions of years ago, then it will be slowly vaporizing into the local air right now -- and a sensitive map of local concentrations of atmospheric water vapor should be able to detect it. Sure enough, Kara Krelove of Arizona State University reported in another Conference talk that the very preliminary early results of a humidity map by the TES spectrometer on the MGS orbiter indicate several patches on Mars' surface that may be emitting extra amounts of water vapor (especially in summer) -- and one of them is just north of Gusev. (Another is around the four great shield volcanoes at the top of the Tharsis highlands -- a region which, according to calculations, was especially likely to accumulate large amounts of ice and snow during Mars' high-obliquity periods, and which shows some photographic evidence of past glaciers on the volcanoes' slopes.)
So it's quite possible that there is still a significant (though slowly decreasing) amount of permafrost frozen into the soil right now within only a fraction of a meter of the seemingly dry surface of Gusev, just below the rover's ability to dig -- and that it is periodically thawed into thin films of underground water during the region's summers.
All this still leaves us, though, with the puzzle of why only some of the rocks that Spirit has found are thus water-weathered, while most of them remain dry, unweathered, smooth volcanic basalt. It may be that the impact that gouged out Bonneville crater was itself what melted a lot of the near-surface ground ice at that spot, and that large amounts of it stayed liquid for quite some time afterwards. So only the rocks nearest the impact point were cracked and had large amounts of newly thawed water (mixed with freshly dissolved brines and acids from the surrounding soil) pushed into their cracks, and some of these rocks were also flung out onto the plains around Bonneville by the impact to mix with the unshocked and unmoistened regular basalt rocks that cover all of the plain at Gusev. And these rocks are also more vulnerable later on to water and acid weathering of their outer crusts, because of the fact that they are already riddled with thin salt-filled cracks and pits. This would certainly explain why the water-weathered rocks seem to be commoner near Bonneville crater.
Alternatively, it may be that the salt-filled cracks in these rocks were formed a very long time ago -- perhaps hundreds of millions of years, long before the small impact that formed Bonneville Crater -- and that the Bonneville impact only dug them up from their undergound resting place. Right now -- as is so often the case with Mars -- we just don't know yet.
In any case, the discovery of these salt-filled cracks in some of the Gusev rocks by Spirit nicely meshes with the existence of thin cracks in some of the "SNC" Mars meteorites that have been identified on Earth -- cracks which are also lined with small amounts of various substances produced when igneous rock is exposed to water, such as sulfate and chloride salts, clays, carbonates and iron oxides. There's no doubt that the water that deposited these minerals came from Mars rather than earth -- its content of deuterium (heavy hydrogen) is much higher than that of Earth water -- but there has been much debate over whether the water may have been melted out of Martian ice only by the shock of the giant meteoroid impacts that launched the rocks off Mars into solar orbit.
However, the fact that Spirit has now identified very similar salt-filled cracks in undisturbed Mars rocks seems to prove the belief of other scientists that the minerals in the cracks of the SNC meteorites probably have were mostly deposited by relatively cool liquid water while the rocks were still on Mars itself -- and calculations of the nature of the impact shock wave that launched them off the planet indicates that most of them were probably buried within a hundred meters or less of the surface when the launch happened.
These rocks consist mostly of unaltered, unweathered volcanic basalt -- so they cannot have been exposed to much liquid water at any point in their long histories. But they do seem to have been exposed to small amounts of it, and even such small amounts might be adequate to serve as a means by which small numbers of native Martian bacteria might be able to survive underground for a very long time. The evidence now seems strong that Noachian Mars was never "warm and wet", to quote the view of it held at one time by some optimistic scientists. But we now know beyond doubt-- thanks largely to the MER rovers -- that it was "cool and damp", with some patches where its large supply of surface ice was melted into liquid water for at least moderate lengths of time. What we still don't know -- and probably won't know for some time -- is whether these liquid episodes and places were long enough and extensive enough to serve as an environment where native Martian life could have evolved.
And the water-weathered cracks in the SNC meteorites lead us on to another major subject of the Astrobiology Conference: the continuing debate over whether the famous "ALH84001" meteorite -- the only sample we have so far of Martian lava laid down during Mars' ancient and more habitable days -- actually does contain some fossil evidence that its own water-filled cracks and pits may have served as a home for Martian bacteria. Since the initial hoopla in 1996 over such possible evidence, the level of belief that it does contain such evidence has diminished, and the impression most science reporters have gotten in recent years is that the idea has pretty much been quietly dropped. But -- judging from the Conference -- at least one piece of biological evidence in this meteorite is still promising enough to be the subject of a genuine and furious continuing debate, which I'll describe in my next chapter.
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