The third of NASA's Astrobiology Science Conferences -- held every two years at Ames Research Center in Mountain View, California -- has just ended. Every one of these has drawn a considerably bigger crowd of scientists than the previous one. This might seem peculiar for what one scientist has described as "the most lively scientific field not to have any actual subject material yet".

But another, at the Conference, noted that a sizable number of graduate students are attending these conferences AGAINST the advice of their advisors, thereby proving that the field has genuine emotional drawing power and an ability to ignite people's interest which may allow it to endure for a very long period of time until the first evidence of alien life finally does turn up.

Whereas the first conference in 2000 consisted entirely of sequential talks in a single multi-day session held in an auditorium, the number of speakers has now increased to the point that several hours of each day was devoted to six sessions of talks held simultaneously with each other, making it impossible for this poor reporter to make it to a lot of the talks he would have liked to attend.

The majority of them -- including all the single-session talks in the mornings and evenings -- were held in a literal Big Tent: an enormous plastic affair whose sides snapped and cracked in the Bay Area's winds like the sails of a three-master, leading to many an apprehensive upward glance on the part of the audience. But everything held together for the duration, and so I'm here to give a rundown on some of the conference's more interesting talks.

There were a great many talks -- some of them extremely esoteric -- on the chemical mechanisms that might play a role in the initial appearance and evolution of life, or in allowing so-called "extremophile" microbes to survive under incredibly rugged conditions that no one 20 years ago thought microbes possibly COULD survive.

There were, however, also much more accessible talks on subjects of far more interest to the general public -- and, as one might expect, Mars was the star of the show. As we'll see, there were a much smaller number of interesting talks and posters on Europa and the planets of other stars -- in fact, this time out there were only three actual talks on Europa.

The problem may be that, now that the Galileo spacecraft has gone to its reward, it's likely to be a  long time before we acquire any new science data on that strange distant world -- whereas new data was flooding in from five Mars spacecraft even as the conference went on, with the next one scheduled for launch next year.

The talk with the biggest attendance -- by reporters and the general public, as well as scientists -- was, as one might expect, by Cornell's Steve Squyres on the continuing findings of the two Mars Exploration Rovers. He did indeed have some intriguing tidbits that had not been mentioned up to then in any of the weekly MER press conferences at the Jet Propulsion Laboratory.

He had relatively little to say about the findings of the MER-A rover, "Spirit", on the floor of Gusev Crater. Gusev may well have served as a giant water-filled lake during Mars' earliest "Noachian" era 4 billion years ago. But Spirit -- while functioning beautifully, except for its two-week fit of now-corrected software collywobbles -- has so far mostly confirmed the fears of many scientists that any ancient lake sediments on its floor have since been covered over by a thick layer of non-aqueous material from other sources such as windblown dust, ancient volcanic ash from a big volcano to the crater's north, or rocks thrown into the crater by other giant meteoroid impacts on the surrounding Martian southern highlands.

Scientists had just finished driving it 250 meters to the 200-meter wide "Bonneville" crater in hopes of finding chunks of deeply buried sedimentary rock dug up from under that surface covering by the impact that formed the crater. But Spirit's photos of the crater's insides after its arrival showed a disappointing lack of any signs of exposed bedrock, and so the rover -- after analyzing a series of rocks on the crater's rim -- has now set off to see how close it can get to the low "Columbia Hills" 2.5 km to the south before its systems finally give out in the coming months.

The latest tests of the rovers' condition indicates that -- barring some unexpected breakdown -- they should both work for an absolute minimum of eight months, and perhaps a good deal longer; so there's a real chance that Spirit will make it all the way to the hills. Failing that, it has a good chance of getting close enough to analyze some fragments of their rock that have rolled down their slopes onto the plain over the millennia.

Orbital views indicate that "Columbia Hills" may actually be one of the remnants of an even older layer of terrain that was mostly worn away by erosion before the newer material was deposited onto the crater's floor, and so it's still possible that they and the boulders thrown from them may be made of the hoped-for ancient Gusev lakebed sediment.

And on the day the conference ended, the latest MER press conference revealed that Gusev had finally found a rock on Bonneville's rim which seems not to be made of the unaltered volcanic basalt lava the rover had found in every rock up to then -- and which has apparently been modified, at least on its outer crust, by exposure to some limited amount of liquid water. (More on this later.)

At the time of Squyres' talk, however, that news had not yet been released -- and so his discussion was mostly of the genuinely fascinating and very solid evidence of ancient surface or near-surface liquid water discovered at the landing site of the second rover, "Opportunity", on the Meridiani Plain.

This, of course, is the plain revealed by infrared spectral maps by the MGS orbiter to be uniquely covered with the coarse-grained gray version of the iron oxide hematite -- which (unlike the extremely fine-grained red "nanophase" hematite dust which gives Mars its red color) was thought likely to be an indicator that the region's rocks had been exposed to liquid water at some point in the planet's ancient past.

Meridiani has now firmly confirmed that fact. After its lander rolled into a 20-meter crater, it discovered a 40-cm thick stratum on the crater wall ("Opportunity Ledge") apparently made out of the same light-colored "etched" rock which underlies the hematite-rich surface layer all over Meridiani -- and which turned out, on analysis, to be unlike anything seen on Mars before.

This rock is formed out of separate layers only 1-2 mm thick, revealed by the rover's three spectrometers to consist of soft "amorphous silicates" mixed with an astonishing 30-40% of sulfate salts, including the hydrated iron sulfate jarosite -- a fact which, by itself, virtually confirms that the light outcrop rock has been exposed at some point to large amounts of liquid water. (Sulfates are fairly common in Mars' windblown soil, but not in amounts anything like this.)

This rock is riddled with birdshot-sized round "blueberries" (actually gray), which the rover has confirmed to consist of at least several tens of percent hematite. They seem to comprise most of the gray hematite seen from orbit. The evidence is that -- as some team members guessed before landing -- the light-colored etched rock, after forming during Mars' more temperate Noachian days, was then covered over by windblown material or volcanic ash for several billion years before being uncovered again by the planet's winds only a few tens of millions of years ago. (Evidence for includes the fact that the craters on this part of the Meridiani plain are an odd mix of extremely old, eroded ones and a sprinkling of small, very fresh ones that can only have formed on its surface recently.)

Since that exhumation, dry basalt sand has been gradually blowing in from other waterless parts of Mars and slowly eroding and powderising the soft light-colored matrix rock of the etched layer from the top down, leaving behind the harder gray hematite berries to mix in a concentrated way with the invading basalt sand to form a dark upper layer perhaps less than a meter thick on top of the remaining layer of light soft etched rock.

The remaining layer of Etched rock is still dozens -- and perhaps hundreds -- of meters thick, judging from orbital photos. The fact that only a very narrow band of it was exposed on the slopes of "Eagle Crater" where Opportunity landed is due just to the fact that the basalt sand has sifted into the crater and covered up all the Etched outcrop in the crater's lower parts. (The Opportunity rover -- having finally left the crater and ventured out onto the flat dark plain around it -- has already discovered little hollows elsewhere on the plain where more small patches of the bright Etched rock have been exposed by the wind.)

The formation of the "blueberries" themselves isn't fully understood. But the rover's very detailed "Microscopic Imager" photos of Opportunity Ledge indicate that they actually formed inside the soft thin layers of sedimentary Etched rock as "concretions" -- iron minerals dissolved out of the matrix rock by water, which later recrystallized as little round balls of hardened hematite encasing some of the individual mineral grains in the matrix rock. Such round hematite concretions are found in several different places in Earth rock strata, and can be formed in several different ways -- but they're all water-related.

The blueberries show no sign of bending the thin horizontal matrix rock strata around them -- which indicates that they crystallized on the spot, rather than forming elsewhere as hardened drops of molten lava or pellets of congealed volcanic ash and then falling into the sediments out of which the matrix rock later hardened. And a few of the blueberries, when cut in half by Opportunity's rock grinding "RAT" tool, show signs of the same thin sediment layers neatly preserved within their crystallized interiors.

The same close up "Microscopic" rover images also show little hollow slots a centimeter or so wide and just one or two mm thick, scattered through the light-colored outcrop. Slots like these -- called "vugs" -- are commonly found in sedimentary strata on Earth, where some salts dissolved out of the sediment grains by liquid water recrystallized again as flat tabular crystals, which in turn later re-dissolved or were eroded away by the wind to leave the slots. Gypsum -- the soft and very soluble mineral, calcium sulfate -- is especially famous for doing this.

And the Microscopic images also show little ripple structures, just a few centimeters long, in the thin matrix strata -- like those which geologists find in Earth strata which have hardened out of sediment layers that actually had water flowing over their upper surfaces while the sediment was being laid down.

This "cross-bedding" suggests to geologists that the sediments out of which the Meridiani light rocks hardened were not just laid down originally as a dry layer of windblown dust or volcanic ash and were later chemically modified by having large amounts of buried groundwater run through them.

Instead, they seem to have actually had a layer of liquid water running over their tops, as though they were sediments originally laid down on the bottom of some ancient Martian lake or pond, which later gradually hardened while undergoing the further aqueous processes that formed the hematite concretions within them.

This much had been revealed in the MER press conferences -- but Squyres, as I say, provided a few fresh details. For one thing, there's the precise nature of the sulfate salts found by Opportunity in the Etched layer. It had already been announced that the rover's Mossbauer spectrometer -- which analyzes iron minerals by bombarding a rock with gamma rays and noting the frequency of the gamma rays that the rock emits in return -- had found large amounts of jarosite, a hydrated sulfate of iron and potassium.

But Squyres added that, after the first tentative detection of this mineral, the team had decided to confirm it by tuning the spectrometer to the specific frequency band used to measure jarosite and then leaving it running for fully 24 hours. In the process they not only solidly confirmed the jarosite, but found that the etched rock contains not a trace of pyroxene and olivine -- two minerals found in huge amounts in the basalt lava rock which covers most of Mars. So, if the etched rock started out as basalt or volcanic ash, it has been very massively altered indeed by water.

Meanwhile, other spectra of the rock from the rover's "Mini-TES" thermal infrared spectrometer -- a surface version of the orbiting spectrometer on MGS which detected gray hematite at Meridiani in the first place -- indicate that jarosite is actually not the commonest sulfate in the Etched rock, and that sulfates of magnesium and calcium are probably even commoner. This mixture -- along with iron oxides and amorphous silica, both of which also exist in the Etched rock -- is precisely what one gets when a solution of sulfuric acid runs through a bed of basalt.

Such acid can be made when sulfurous volcanic gas dissolves in liquid water, or it can be made when ordinary water with oxygen dissolved in it runs through a bed of pyrite (iron sulfide) -- the famous "fools' gold" -- which has been laid down by earlier volcanic or hydrothermal processes. In the case of the latter, the pyrite is turned into a mixture of sulfuric acid and dissolved iron.

This mix is exactly what we find in the Rio Tinto river of southern Spain, a strange place which is now under study by Spanish and American scientists as a very promising terrestrial analog of Meridiani. It was the subject of a whole series of talks at the Conference by David Fernandez-Remolar, Wendy Calvin and Carol Stoker.

The river runs through the world's largest surface deposit of pyrite, in the process turning into a weird blend of sulfuric acid solution -- with a pH of fully 1 to 3, as strong as battery acid -- and dissolved iron (in the form of "ferric" ions), which turns it as red as the region's "Tinto" wine. After it later emerges from the deposit and starts mixing with less acid water from other streams, the dissolved iron precipitates out as huge deposits of goethite -- iron oxyhydroxide -- which, as it later dries out, slowly breaks down into hematite.

Meanwhile, the acid river reacts with the minerals on its shoreline, thus adding large amounts of dissolved sulfate salts to its mix -- which, on the river's banks, dry out into big banks and bluffs of crystallized yellow jarosite and sulfates of magnesium and calcium.

It seems highly likely that the Etched layers at Meridiani were formed at some point in Mars' very early history by a similar layer of sulfuric acid. But such acid could not have formed on Mars by the same process as in Spain, since the Tinto process requires a large amount of oxygen dissolved in water to react with the pyrite, and no such large amounts of oxygen could have existed in early Mars' air. Instead, the sulfuric acid at Meridiani would have been expelled directly from the geochemistry of volcanic vents.

At his talk, Squyres also elaborated on one other piece of evidence for this scenario found by the Opportunity rover. It had already been stated that the rover's "APX" spectrometer -- which analyzes the percentages of various elements -- had found surprisingly high (though still trace) amounts of bromine in the outcrop and that this was further evidence that it was crammed with minerals that had crystallized out on earlier liquid solution; but this line of reasoning was not explained at the time.

Squyres now explained it. The ratio of bromine to its close cousin chlorine is usually very stable and reliable in Earth rocks and meteorites, as well as in seawater -- but bromine becomes several times more concentrated in the upper layers of saltbeds made from evaporating bodies of briny water on Earth. The reason is that chlorides are a good deal less soluble than bromides -- and so, as such a body of water starts to dry up, the chloride salts tend to precipitate out of solution first and form a layer on the bottom of the saltbed.

The bromides, however, tend to stay in solution until the (literally) bitter end -- crystallizing out of the small remaining amount of brine only when it is almost completely dried up, so that they are thus concentrated in the salt bed's upper layers. Whether the water soaking the Meridiani etched sediments actually did form a surface lake or was merely a layer of buried groundwater soaking the sediments underground, this same layered structure of salts would have formed in it as it dried up.

There are still a swarm of major mysteries about this spectacular discovery, of course. For one thing, while Squyres is absolutely confident that the mineralogy of the Etched layer proves that it must have been soaked in liquid water for a protracted period of time, he emphasized that he personally is still not entirely certain that the water actually formed a surface layer, with the Etched sediments sitting on its bottom (or actually crystallizing out of the briny water as it dried up).

The only evidence so far that it was not, instead, just an underground water table -- soaking a buried layer of sediments that had been deposited earlier as repeated layers of windblown dust or volcanic ash from local eruptions -- is those ripples preserved in the hardened thin layers of deposited sediment, whose small size suggests that they were originally formed by flowing water.

Such ripples are much larger when they're formed by winds in sediment layers on Earth -- but if Noachian Mars had a thinner atmosphere than Earth, it's possible that purely windblown ripples in dust and sand on its surface may have been smaller.

However, Squyres also pointed out that all five successful Mars landers have taken closeup pictures of countless windblown dust and sand ripples on the present-day surface of Mars, where the air is a lot thinner than it must have been during Mars' Noachian days -- and we still have yet to see any such "aeolian" ripples as small as those preserved in the layers of the Opportunity outcrop.

Then there's the question of exactly why the hematite formed in those strange little beadlike concretions -- and, for that matter, whether all the Meridiani hematite DOES exist in that form. Now that Opportunity has finally emerged from the small "Eagle" crater in which it landed onto the main Meridiani plain, it is seeing an awesome landscape of seemingly endless wind-rippled dark basalt sand mixed with hematite beads -- a wind-eroded layer which, however, may be no more than a meter or so thick on top of the remaining layer of light Etched rock, which is already visible in many patches at the bottoms of small hollows and troughs in the dark upper sand.

But its Microscopic Imager's pictures are also showing a large number of more irregular and sometimes porous bits of rock intermixed with the round gray beads -- and, as the University of Nevada's Wendy Calvin noted, there are indications from Mini-TES spectra and multispectral color pictures of them by the rover's "Pancams" that some of them are also made out of hematite.

The rover has also inspected and analyzed virtually the only big rock visible for hundreds of meters in any direction: a dark chunk 30-40 cm wide on the edge of Eagle Crater and christened "Bounce Rock" -- because, by an unlikely chance, the airbag-encased lander managed to roll and bounce across during landing. Squyres, during his talk, said that the rover's Mini-TES suggests that it, like the gray "Blueberries", was rich in gray hematite -- but this turned out to be a false alarm.

The rover's more detailed analyses of the rock since then -- including a reappraisal of the Mini-TES spectra of it -- have revealed that it contains virtually no hematite, and indeed seems to be a chunk of unweathered basalt that may have been thrown dozens of kilometers from one of the large ancient meteorite craters bordering the Meridiani landing region. So the question of whether there are other forms of hematite at Meridiani besides the Blueberries remains unsettled.

The Blueberry concretions remain puzzling in themselves. Such concretions, when they slowly form in layers of water-soaked sedimentary rock on Earth (like the dark round "Moqui Marbles" in the great sandstone layers of Utah), form and grow around some central nucleus that chemically triggers their appearance -- but it's not clear yet what these nuclei were in the etched Meridiani rock.

It's possible that -- as in the lower stretches of the Rio Tinto -- the briny acid solution in the Meridiani etched layer was later mixed with less acid liquid water, which would have caused its dissolved iron to crystallize out as the iron oxydroxide, goethite. (Goethite might thus have crystallized out around tiny relatively alkaline specks in the rock.)

Squyres says that the Mossbauer spectrometer -- which can distinguish very well between goethite and the non-hydrated iron oxide hematite -- has found no significant trace of goethite. But goethite, exposed for 3.5 billion years or more to the extremely thin and dry atmosphere that has covered Mars since its Noachian days, might very well have decomposed completely into hematite by now. Even in the humid atmosphere of southern Spain, the goethite deposits precipitated out of the Rio Tinto turn into coarse-grained hematite within only two million years.

Finally, there's the billion-dollar question: are the ancient water-soaked sediment layers of Meridiani the sort of place that might possibly have supported ancient Martian microbes? As Angeles Aguilera pointed out in a poster, even the battery-acid brine of the Rio Tinto is highly populated not just with bacteria but with one-celled algae, protozoans and rotifers unperturbed by its high acidity and its bizarre chemical conditions. Indeed, some species of bacteria in its upper reaches actually digest pyrite, greatly accelerating the rate at which it reacts with the oxygen in the river's water to turn into sulfuric acid and dissolved iron.

Such bacteria still breathe oxygen, reacting it with sulfides to derive their life energy. This makes it impractical for them to have existed in the oxygen-scarce air of early Mars. But there are also many species of "anaerobic" bacteria on Earth which substitute a wide variety of other oxidizing substances for oxygen -- including carbon dioxide, sulfur, sulfates or ions of ferric iron. Most of these microbes which still exist on modern-day Earth derive their metabolic energy by reacting these oxidizers with the complex organic compounds produced by the other parts of Earth's vast ecosystem.

But there are several species that "rough it" to an even greater extent. Some, first discovered just a few years ago, live in the pores of Earth's basalt layers up to three kilometers down, utilizing traces of hydrogen produced when water weathers such basalt, and reacting that hydrogen with carbon dioxide to derive both their energy and the carbon compounds that they need to build and repair themselves.

And the waters of the Rio Tinto are very rich in one bacterium, Acidithiobacillus ferrooxidans, which plays a major role in using oxygen to digest the iron and sulfur in the region's pyrite -- but which, in 2002, was shown by lab tests to be astonishingly versatile. In the total absence of oxygen, it can also make a living by completely substituting ferric iron for oxygen and reacting it with either sulfur or hydrogen -- or, if iron is also absent by reacting sulfur with hydrogen!

It's possible that this bacterium -- or similar anaerobic ones -- grows in the oxygen-free water table hundreds of meters below the surface of the Rio Tinto region, deriving its energy in this strange way (while utilizing that energy to digest carbon dioxide and turn it into its own cells' organic compounds). Bacteria of the same type may very well have been among the first living things to evolve on early Earth, before photosynthetic bacteria and plants had evolved to pump oxygen into its air -- and, by the same token, they might very well have been able to develop and survive in the soil and water of early Mars.

In one of the Conference's talks, the Ames Research Center's Carol Stoker talked about the "MARTE" project at Rio Tinto: drilling hundreds of meters down into the ground at the site to look for just such anaerobic bacteria, This requires not only hauling up samples of sediment and rock from those depths, but immediately stuffing them into oxygen-free tents for analysis, since oxygen often poisons such anaerobic bacteria.

The results are extremely tentative, but some kind of bacteria have indeed been identified living in such deep ground layers there -- although it's not yet certain whether they have just this type of anaerobic metabolism. That part of the study is still going on.

(MARTE's procedures also serve as a technique to develop ways of extracting and analyzing such samples without contaminating them -- thus serving as a useful procedure to rehearse similar future biological drilling operations on Mars. In fact, next year another project will actually set up a simulated Mars lander at the site to drill automatically deep into the ground and analyze the samples it extracts without direct intervention by human engineers or scientists.)

Some other anaerobic Earth bacteria gain their energy by combining dissolved sulfate ions with hydrogen -- yet another possible way for Martian germs to survive in the sulfate-saturated waters of early Meridiani, since hydrogen was also produced in fairly large amounts by the planet's early volcanoes.

The real question, however, is just how long the wet episode at Meridiani lasted. Was it just for a few thousand years -- quite long enough to carry out the types of mineral reactions seen at the site, but ridiculously short for life to have any chance of evolving at the site? Or was it millions -- or even hundreds of millions -- of years, in which case the evolution of life on or under the Meridiani plains might be plausible after all? Alternatively, were there other wet places on or just under the surface of Noachian Mars -- their surface traces now long since destroyed -- that DID last long enough for life to evolve on Mars?

Unfortunately, the very tentative sprinkling of data the Opportunity rover has derived from looking at that little 40-cm high stack of exposed sediment cannot tell us a thing about this. To gain some idea of how long the Meridiani wet episode lasted, either the rover or its later successors must look at the far thicker layers of Etched sediment still mostly buried beneath the region's thin surface covering of blown-in basalt sand and liberated hematite berries.

The remaining, uneroded part of that layer could be very thick indeed -- MER team member Ray Arvidson estimates that the Meridiani hematite-rich region is covered with as much as 300 meters of sediments that were stacked on top of the original ancient cratered plain during the Noachian.

"Opportunity" is now on its way to Endurance Crater, a 160-meter crater about 720 meters from its landing site. It's regarded as having an excellent chance of getting there, since it recently drove 100 meters across the flat, hazardless plain in a single day. Orbital pictures reveal that Endurance Crater -- like Eagle Crater -- is largely filled with inblown dark sand, but still has a band of exposed light Etched rock around its upper rim -- but in this case, the accessible segment of Etched rock strata seems to be a dozen meters tall instead of only 40 cm.

Views of the strata in this vastly thicker visible layer should give us a much better understanding of the general timing and rhythm of the deposition of the Etched material's layers. (Squyres, at the Conference, expressed his belief that the Etched layer is a "sabkha" -- a salt flat that has gone through repeated cycles of being drenched with water and then drying out.) It's even possible that the slopes of Endurance Crater lower down may reveal earlier, darker strata that were laid down at Meridiani before the light-colored, water-soaked Etched sediment.

For these reasons, it's possible that the rover will be ordered to descend down into the crater to examine and analyze this big cross-section close-up, even it there is a risk that it will be unable to climb back out. Otherwise, after examining Endurance crater in detail from its rim it will turn south and start driving toward a much bigger region of interlaced bands of surface Etched rock and dark sand about 2 km to the southeast, inspecting various exposed features on the plain in the process.

Even such greater stratigraphic studies may not give us anything close to a complete understanding of the overall history and duration of the Meridiani plain's "wet" episode, and whether it lasted long enough to serve as a possible site for the evolution of life -- but the rover will at least enable us to learn a lot more about it.

Even if the wet epriod here did not last long enough for life to actually evolve here, it's entirely possible that life might have evolved elsewhere on Noachian Mars -- in some region where the evidence of ancient liquid water is now less clear or even completely erased -- and that the Martian microbes which originated there then spread across Mars to Meridiani.

If ancient Martian microbes DID exist at Meridiani, the chances that recognizable fossil evidence has been preserved there are excellent. As Fernandez-Remolar and others have pointed out, they are extremely well preserved in the mineral deposits that have crystallized out of the Rio Tinto -- often encased in solid iron oxide or in the amorphous silica mineral chert, which can survive for very long periods.

In one poster, the Johnson Space Center's Carlton C. Allen pointed out that some fossils produced by microbes in such an environment might even consist of structures big enough to be seen by Opportunity's cameras! The fossil record contains abundant examples of macroscopic biofilms that are observable at the resolution of the Microscopic imager.

Many hot springs precipitate minerals that fossilize and preserve colored microbial mats, as well as masses of organisms many cm across... Bacterial remains are preserved in iron- and manganese oxide deposits within hot spring travertines in Morocco. These microfossils were found in cm-scale black 'shrubs'. Filamentous and coccoidal bacteria were observed densely packed within the shrubs, but no microfossils were found in the enclosing aragonite and calcite laminae."

And Harvard's Andrew Knoll -- part of the American-Spanish team stuudying Rio Tinto -- has recently told Astrobiology Magazine: "There are two features of biology that get preserved in the eyeball-level textures of these [Rio Tinto] rocks. One is that you sometimes get little bubbles forming because of gas emanation fro [microbial] metabolism. And some of those will actually roof over with iron minerals and can be preserved through diagenesis [the chemical hardening of sediments into sedimentary rock]... You can get preserved gas spaces and those spaces are invariably associated with biological production of gases.

"The other thing, which I feel even more strongly about, is that many times, where there are microbial populations, they form these beautiful groups of filaments that just string out across the surface. They almost look like the mane of a horse...When minerals are deposited in these environments, they actually nucleate on these strings of filaments, and you get beautiful sedimentary textures that, again, look like the mane of a horse. You can see them in Yellowstone Park, in both siliceous and carbonate-precipitating strings...

"In Rio Tinto, you can see iron depositing on these filaments; and in the 2-million-year-old [hematite] terraces, you can see these filamentous iron textures. And there, again, I know of no process other than biology that could form those. So that's truly something to keep your eyes out for whenever you're looking at a precipitated rock on Mars...If you took a Pancam [like the rover's main cameras] to Rio Tinto or Yellowstone Partk, they would jump out at you. Absolutely."

However, Allen and Knoll point out that the odds are vastly poorer for finding Martian fossils big enough to be seen with the naked eye (or MER's cameras) than they are for finding microfossils so small that they must be looked for with a genuine microscope: "Mineralized microfossils...have typical dimensions of microns to tens of microns...Most fossil microorganisms in terrestrial rocks are only detectable by optical and electron microscopy. The association of carbon with putative microfossils can only be confirmed using microbeam techniques."

Moreover -- while the goethite deposits at Rio Tinto have (to quote Knoll) "a high capacity for preserving cellular details of cells and tissues caught in Rio Tinto sediments", he also points out that as this goethite dries out and turns into coarser-grained gray hematite (the process which seems likely to have also happened at Meridiani), this "commonly obliterates preserved fossils." Gray hematite, after all, consists of grains 5 to 10 microns wide -- larger than many bacteria.

And nonliving chemical and mineralogical processes have actually produced many of the structures in these older hematite beds that look like possible fossils or microbial remains. This is always a huge problem when trying to detect microbial fossils on Earth. Fernandez-Remolar stated in 2002, "Although the fine-scale lamination found in older terrace deposits [of hematite] resembles microbial mat laminae, this tecture appears to have originated mostly if not entirely during diagenesis. In general, hematite formation decreases the quality of paleobiological preservation."

In his own talk at the Astrobiology Conference, David Fernandez-Remolar (of Spain's Centro de Astrobiologia) emphasized that there are some structures at Rio Tinto that look strikingly like stromatolites -- layered mineral deposits created by large colonies of bacteria, and sometimes as much as a meter tall. But he then stated that -- like a great many supposed ancient stromatolites on Earth -- these may well be due instead entirely to nonliving chemical processes.

It's possible that the best place to look for fossilized microbes in the Meridiani rocks may be, not in the hematite Blueberries, but in the crystallized sulfate salts of iron, magnesium and calcium filling the softer matrix rock around the Blueberries.

Such "evaporite" salts rarely last for any long period on Earth because they are both soft and water-soluble -- but the surface of Mars has been almost entirely bone-dry for billions of years since its more habitable earliest years ended, and it's entirely possible that large numbers of fossilized microbes might still be extremely well-preserved in sulfate salt deposits there. (Some bacteria found in ancient salt crystals on Earth have, in fact, been so well-preserved that they have actually been brought back to life when they were finally rehydrated after 40 million years!)

MER's instruments are also incapable of detecting organic compounds trapped in the crystallized minerals -- another remnant of ancient Martian life that could be very well preserved here. For example, in his talk Squyres mentioned that on Earth concretions very often grow chemically around an original nucleus consisting of some bit of organic material.

Fossils big enough to be seen with the naked eye are often found inside concretions when they are cut open. And even when there are no such visible remnants (there certainly have not been any such yet in any of the "blueberries" that the rover has cut in two with its RAT tool), the crystallized minerals in concretions trap ancient germs and organic compounds very well.

But, once again, we are talking about very small trace amounts. To quote Allen, "Confirmation of ancient Martian life by direct fossil evidence will require the return of samples to terrestrial laboratories."

To sum up, we may already have found the first example of the Holy Grail that NASA is looking for on Mars -- sedimentary rocks which not only indicate an ancient environment capable of supporting life, but which could preserve fossil proof of this very easily.

It's hardly surprising that NASA officials have already announced that Meridiani is now the "prime landing site" for the big, complex 2009 Mars Surface Laboratory rover, and/or for the first mission to return Mars samples to Earth for analyses tremendously more detailed than any that could be conducted by remote control.

However, it would be seriously premature to say that a return visit to Meridiani has definitely been selected for these missions -- and especially that it will be the target for both of them. There is such a thing as putting too many of your eggs in one basket.

As Knoll says, "If water is present on [a site on] the Martian surface for 100 years every 10 million years, that's not very interesting for biology. If it's present for 10 million years [straight], that's very interesting." As yet, we simply have no idea which of these scenarios the Meridiani site more closely resembles -- and the Opportunity rover, by itself, may very well never be able to tell us.

The MSL will be equipped with a very wide variety of analytical instruments -- probably including high-powered microscopes and instruments capable of detecting traces of organic compounds and isotopes that could serve as possible "chemical fossils" of ancient life.

While its chances of confirming Martian life beyond serious doubt with such small in-situ instruments is small, it can definitely provide additional evidence, and provide us with still more information on the best places from which to actually return Martian samples to Earth for real biological study.

But if MSL is landed on the vast Meridiani plain, then -- even given its great range -- it might spend a large part of its mission never getting a chance to examine other varied types of Martian terrain and geological features, which would be a serious scientific loss if Meridiani does turn out to be devoid of any evidence of life.

Landing the first Mars sample return mission at Meridiani might be an even riskier gamble. That long-delayed mission -- now very tentatively set for launch in 2013 and a return to Earth in 2016 -- has recently been downsized to save money.

It will now probably take the form of a "Groundbreaker" mission in which a stationary lander will just use a mechanical arm to scoop up soil and pebbles from its immediate vicinity, rather than dispatching a rover to collect samples from much greater distances and return them to the lander.

Such a lander, at Meridiani, might very well be unable even to collect samples of the light Etched rock which has proven so fascinating -- it might be limited to just scooping up a sample of the loose dark top material covering the Etched layer, consisting of hematite blueberries mixed with the dark, dry basalt sand which has been blowing into Meridiani from another part of Mars.

And if the blueberries turned out to be devoid of any evidence of life, this very expensive mission might actually end up revealing much less scientifically about Mars as a whole than a mission aimed at one of the more typical parts of Mars like those revealed by the previous four landings -- where it could collect larger rock fragments identifiable as coming from that particular place.

One possible solution to the problem presents itself. Up to now, there have been no plans to have the first "Groundbreaker" sample-return mission land so close to the Mars Surface laboratory that the big rover could actually trundle over to the sample-return lander and deliver a set of samples which it had taken from dozens of kilometers across the surface of Mars.

But the latest plan for MSL calls for it to be nuclear rather than solar-powered, extending its planned minimal lifetime to fully two years and its range to several dozen kilometers. Since -- with any luck -- MSL will actually work much longer and travel much farther that that, it's quite possible that MSL would still be working when the Groundbreaker sample-return lander touches down four years later.

A scenario can be visualized in which MSL -- having already thoroughly explored other, different types of Martian terrain -- makes an optional trip into Meridiani to try to find the sample-return lander and deliver a set of samples which it's collected during its exploring. If the rover failed to reach the sample-return lander, the lander could still collect samples from Meridiani itself and return them to Earth.

All this is speculation, however, is wildly premature. As NASA says, even given the electrifying revelations at Meridiani, it's entirely possible that NASA's further explorations -- including the continuing though orbital mapping of the planet by Europe's Mars Express orbiter and the very sophisticated, high-resolution 2005 Mars Reconnaissance Orbiter -- will end up discovering some spot on the surface even more enticing for the first mobile MSL and the first sample-return mission.

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