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Watery Tales From Texas

the salty tears of mars
photo by NASA/JPL/MSSS
by Bruce Moomaw
Cameron Park - May 4, 2001
Some theorists at this year's Lunar and Planetary Sciece Conference again reiterated their belief that there is evidence that substantial amounts of liquid water flowed on at least some parts of Mars not just in its earliest times, but occasionally in much more recent geological times -- perhaps less than a hundred million years ago.

Victor Baker cited an accumulation of evidence to that effect, including Jeffrey Kargel's aforementioned claim of evidence for very recent Martian glaciers around the Argyre Basin area; Natalie Cabrol's survey of apparent lakebeds filling Martian impact craters, some of which may have been water-filled within the past few hundred million years; areas in the higher latitudes covered with uncratered and fresh large patterns of cracked ground resembling patterns left on Earth by episodes of liquid water above permafrost; and totally uncratered dunes at the summits of Mars' towering Tharsis volcanoes, which Baker thinks could only have been wind-created at those great altitudes if Mars' air pressure had very recently been much higher than it is now.

William K. Hartmann, D.C. Berman and Jennifer Grier reiterated their claim that there is very strong evidence that some areas of Mars -- especially in the Cerberus Plains and on the slopes of the Tharsis volcanoes -- do show virtually conclusive signs of large volcanic lava flows erupted as recently as a few tens of millions of years ago, for the simple reason that these hard-rock surfaces are virtually uncratered.

This meshes well with the age-dating of several of the Martian meteorites on Earth, which seem to have hardened out of lava as little as 170 million years ago.

Hartmann also proposed an explanation for the puzzling fact that, so far, all the Martian meteorites recognized on Earth are made of lava only a few hundred million years old -- except for the famous "ALH84001" supposedly carrying evidence of Martian fossils, which is fully 4.5 billion years old.

Where are the large number meteorites that should have come from the intermediate-age materials that cover most of Mars' surface? His theory is that Mars' ancient surface, in most areas, was pulverized long ago by the rain of small meteors onto it, and that this pulverized surface served as a perfect medium to contain Mars' near-surface ground ice, so that it consists of "ice-rich soils, loosely conglomerated sand and gravels, sedimentary layers, conglomerates, and sandstones" -- all of which are either crumbly enough that they cannot survive being blasted into space by the giant impacts that launch Mars meteorites to Earth, or else are "sedimentary rocks that would not be recognized by meteorite collecting parties".

Thus the only Mars meteorites we have so far are either from fresh, still-hard lava flows, or (in the case of ALH84001) from a few patches of underlying ancient Martian bedrock that have been recently exposed by erosion.

If there have been occasional, fairly large volcanic eruptions on Mars into recent times, these could provide enough heat to melt ground ice and release huge amounts of carbon dioxide stored in the cold Martian subsurface, thus providing Baker's theorized brief "clement interludes" of air pressures and temperatures high enough to allow large amounts of liquid water to exist once again on the surface.

Indeed, Berman, Hartmann, and James Head all see signs that the Cerberus Plains are marked not only by very young lava flows, but by signs of large amounts of water flowing across them less than 100 million years ago -- which presumably was produced when the volcanic eruptions thawed local ground ice.

Baker's theory of recent large-scale liquid-water episodes on Mars, however, is still very much open to question.

For instance, William Banerdt's analysis of MGS' laser topography maps of Mars suggest that "Mars was not subjected to significant erosion by flowing water over any extended period of time since the end of [Noachian] heavy bombardment" -- for he finds few signs of mature tributary drainage channels leading out of Mars' great lowland basins, and also that the sheer number of such isolated basins argues that there has been very little "erosional breaching and filling of basins" on Mars over the past few billion years (although this is a very efficient process on Earth).

There are also some puzzling indications in Mars' surface mineralogy -- to the extent we yet know it, since our data on the subject is still very limited.

(Perhaps the most unfortunate part of the Mars Polar Lander failure is the fact that its "TEGA" instrument would have given us our first detailed mineralogical analysis of Mars' soil.) J.J. Hagerty suggested the heating of Mars' surface rocks by the rain of meteor impacts onto them would have caused them to react with Mars' groundwater to produce great amounts of powdery clays which now comprise Mars' soil.

But -- while we won't have really good near-IR spectra of Mars' surface until the European and American orbiters of 2003 and 2005 -- the Earth-based spectra we do have suggest that there may actually not be much clay on Mars.

And K.L. Mitchell suggested that Mars' soil may actually be made up mostly of volcanic ash, which he thinks could have been explosively spewed out of Mars' early volcanoes in great amounts even if their lava didn't come in contact with groundwater that would have formed steam.

A.S. Yen -- who has believed for some time that Mars' mineralogy suggests that there has been little surface water on its surface -- goes further, saying that anywhere from 2 to 30% of Mars' soil may actually be the remnants of the micrometeorites that have hit the planet, and that the iron oxide in the soil may have been created by the heat of the meteorites' impact rather than by any exposure to water.

It's also an embarrassing fact that -- as I've said -- MGS has so far failed to turn up any sign of the carbonate deposits that were expected to exist in large amounts on Mars' surface if early Mars had a dense CO2 atmosphere and any substantial amount of liquid water (although the just-launched Mars Odyssey orbiter will make a more thorough search).

It may be that atmospheric processes have now broken all the carbonates on Mars' surface back down into CO2, leaving large subsurface beds of it.

But it's also a fact -- as C.D. Cooper and John Mustard pointed out -- that MGS has also found no really concentrated deposits of sulfates, which should also have been left behind by surface bodies of water on Mars.

It has found fairly strong evidence of sulfates in the soil, but these don't seem to be located in lowland basins where water would most likely have accumulated, or near volcanoes -- instead, they seem to exist mostly in the "duracrust" that earlier Mars landers have found widely distributed on the planet, which seems to have been slowly formed over the millennia by the reactions of the soil with tiny, short-lived traces of melted water frost.

This leaves the one mineral MGS DOES seem to have found large deposits of on Mars that is usually associated with liquid water: coarse-grained hematite, which apparently exists in one very large patch in the Terra Meridiani or "Meridiani Highlands" (which is now the highest-priority landing site for one of the 2003 U.S. Mars rovers), and in several smaller patches in the Aram Chaos regions and several of the canyons branching off from the great Marineris Valley.

Most theories indicate that this stuff had to be formed by Martian iron minerals dissolved in large amounts of liquid water -- either in surface lakes or near-surface groundwater.

Indeed, D.C. Catling suggested at the Conference that such hematite could have been formed by only two processes: either a large amount of oxygen dissolved in the water, in which case early Mars' air must have included a large supply of oxygen that could only have produced if the planet had a large population of one-celled green plants -- or else UV sunlight causing the dissolved iron minerals to react with the water itself, in which case a great deal of hydrogen would have been pumped into early Mars' atmosphere as a side effect, which would be a situation "metabolically very favorable for simple life should it have appeared on early Mars", since bacteria find it very easy to use hydrogen plus iron as a source of their necessary energy.

Even here, though, there's a catch: one other mineral, which MGS does now seem to have discovered in significant amounts on Mars, is olivine -- a greenish iron mineral which breaks down quickly into clays when exposed to liquid water.

And T.M. Hoefen reported that there seems to be a large concentration of olivine near the Terra Meridiani, quite close to the supposed Hematite Formation -- which seems to imply both that Mars as a whole may always have had a very dry surface, and that the "hematite" indicated by MGS' thermal-IR spectra may perhaps be some other mineral.

The analyses of rocks carried out by Mars Pathfinder's little rover --measuring the amounts of different elements in them -- are also producing conflicting results.

These analyses had surprised everyone by indicating that the rocks in Mars' northern lowlands contain much more silica than expected, a mineral which indicates that the magma out of which the rocks were formed encountered a great deal of underground water.

But there were problems properly calibrating the results from the rover's "APX" spectrometer, and two new reports on the now-improved data point in opposite directions.

C.N. Foley reported that the spectrometer's alpha-particle data shows that the rocks contain much more oxygen than expected -- suggesting that they may contain a good deal of chemically bound water, and perhaps may even be sedimentary rocks rather than volcanic ones as had been thought -- but J. Bruckner said that his recalibrations of the X-ray data show that the rocks contain less silica than originally thought.

  • Click For Part Four

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