As always, many of the papers (especially those dealing with the detailed geochemical analysis of meteorites and returned Moon samples) were so specialized and dry in their subject matter that they hold appeal only for fellow scientific specialists, and/or masochists -- but as always, many were of great interest to anyone with a reasonable degree of interest in the exploration of other worlds.

As to be expected a major theme of this year's LPSC was the ongoing debate as to just how much liquid water Mars had on or near its surface during its earliest days, and how much it has now.

The relevance of this to the question of whether ancient Mars had microbial life -- and even whether Mars may still have some, buried deep beneath its savagely hostile present-day surface -- is obvious.

And the debate is still as furious as ever.

Back in 1996, when the MGS spacecraft first entered orbit around Mars and began the first really detailed close-up scientific survey of the planet since the Viking missions, the single most popular model of the planet's history -- what might be called the "Modern Classic" view -- ran as follows.

During the first billion or so years after its creation -- the so-called "Noachian" period -- Mars had a carbon dioxide atmosphere that was belched from out of its early volcanoes, to provide a far denser atmosphere than its faint wisp today.

Indeed, its surface air pressure may have been as much as present-day Earth's, or perhaps even several times greater.

One major piece of evidence for this is the fact that craters dating back to that epoch are much more eroded than all the craters existing on areas of Martian land which (judging by their sparser total crater count) were volcanically resurfaced after the Noachian era -- indeed, these oldest craters are so much more eroded as to indicate that only wind erosion in a genuinely dense atmosphere could have done it.

And one consequence of that dense CO2 atmosphere would have been a powerful greenhouse effect -- strong enough to warm much of Mars' surface above the freezing point of water.

The best evidence for this is the scattering of ancient "valley networks" across the planet -- which look very much like branching dry riverbeds, and were almost surely formed by the flow of a moderate amount of some fluid across the surface over long periods of time.

Some researchers, however, think they see subtler signs that Noachian Mars had a lot of liquid water on its surface -- everything from grooves in some of Mars' southern highlands that may have been gouged by glaciers created by accumulated snowfall on its mountains, to features around the edge of the great lowland depression taking up most of Mars' northern hemisphere which just might be the shorelines of an ancient ocean that once filled that lowland (which is also floored with plains of material so extremely flat that they may be seafloor sediment).

And this is just the sort of environment in which microbial life might very well have evolved on ancient Mars at about the same time it was first evolving on Earth.

However, Mars -- unlike Earth -- then gradually lost that early dense atmosphere.

Some of it -- because Mars' gravity is so much weaker than Earth's -- may have been splashed into space by the huge asteroid impacts which were still common in those early days of the Solar System.

The current understanding of the interior of Mars suggests that it can be modeled with a thin crust, similar to Earth's, a mantle and a core. Using four parameters, the Martian core size and mass can be determined. However, only three out of the four are known and include the total mass, size of Mars, and the moment of inertia. Mass and size was determined accurately from early missions. The moment of inertia was determined from Viking lander and Pathfinder Doppler data, by measuring the precession rate of Mars. The fourth parameter, needed to complete the interior model, will be obtained from future spacecraft missions. With the three known parameters, the model is significantly constrained. If the Martian core is dense (composed of iron) similar to Earth's or SNC meteorites thought to originate from Mars, then the minimum core radius would be about 1300 kilometers. If the core is made out of less-dense material such as a mixture of sulfur and iron, the maximum radius would probably be less than 2000 kilometers. This image is Copyright � 1998 by Calvin J. Hamilton. See more of Hamilton's award winning Planet Scapes

Some of the rest may have been lost when Mars' initial core of liquid iron cooled down and solidified around 4 billion years ago, shutting off the planet's magnetic field and thus allowing the "solar wind" of charged particles racing past the planet to gradually skim away gases from Mars' thin upper atmosphere.

But the favored view, at least until MGS arrived, was that most of Mars' early CO2 gradually dissolved into its surface liquid water and then reacted with Mars' silicate crustal rocks to form solid carbonate minerals.

On Earth, the process of "crustal tectonics" (or, as it's often called, "continental drift") eventually -- after periods of up to 100 million years -- drags these surface carbonates down into Earth's hot interior, where they're broken down by the heat and their CO2 is then belched back up to the surface by Earth's volcanoes to begin the cycle all over again.

But Mars, because it's smaller than Earth and so has more area relative to its interior volume, could never store up nearly as much trapped subsurface heat from the traces of radioisotopes in its rocks.

And since it's that excess internal heat that drives crustal tectonics, Mars had none -- and so, after most of its CO2 air had been turned into surface layers of carbonate minerals, it remained in that form permanently.

Its air was gone for good.

And without the greenhouse warming from that dense blanket of CO2, all the planet's surface water froze solid, forming a layer of permafrost (the so-called "cryosphere") several kilometers thick.

This process was, of course, gradual.

Indeed, up to a billion years ago, Mars had occasional titanic "catastrophic outflows" of subsurface liquid water, caused because the remaining liquid water in the rock pores of its warmer interior achieved immense pressure in some lowland areas where there was a linked but higher-altitude liquid water table in nearby highlands.

When a volcanic spasm or an occasional giant meteor impact cracked the thick surface shell of permafrost, this water would gush out in immense floods for hundreds of kilometers, dwarfing any floods on Earth -- but lasting only a few days before the subsurface water reservoir ran out, and the surface water froze solid, leaving only the channels formed by the immense brief flood.

Eventually, though -- as both Mars' interior volcanism and its surface continued to cool -- the cryosphere grew to such a thickness that such ruptures ceased to occur.

Indeed, by about 3 billion years ago Mars had lost virtually all of its air -- since, even after all of Mars' surface liquid water froze, some of its air was still being blasted into space by meteor impacts, stripped away by the solar wind or turned into subsurface carbonate deposits by the planet's few remaining volcanic hot springs.

And so Mars' frozen, virtually airless surface became hopelessly inhospitable to life -- but if life ever did evolve on Mars, there may still be beds of microscopic fossils and ancient organic material (much better preserved than Earth's oldest fossils, thanks to Mars' lack of crustal tectonics and water erosion).

And there may even be Martian microbes surviving today in the remaining liquid "water table" kilometers below its surface, and perhaps a lot closer to the surface around any of the planet's remaining geothermally heated areas for its volcanism has not completely died away even today.