Thus those four different example "pathways" identified by MEPAG.

The first -- Understanding Habitability Though Space and Time -- would simply continue the generalized survey of the planet as a whole, following up the survey of Mars from orbit in this decade by sending relatively cheap landers to a large number of different sites to check out their properties with in-situ experiments in order to better understand both the planet's geological history and the potential of various sites as possible locales for ancient fossils.

It would also include, if needed, more orbiters and some larger landers along the Smart Lander line, as a prelude to the choice of sites for sample-return missions.

The other three pathways are more specialized, aiming at exploration focused on particular kinds of regions in response to possible interesting results from the missions through 2005.

One of these pathways, known as Focus on Polar Climatic and Habitat Records, would result from a combination of:

• the recent results from Mars Odyssey's neutron spectrometers indicating the presence of surprisingly large amounts of near-surface ice at high latitudes;
• climatic models indicating that some of this ice can melt during polar summers or those 100,000-year cycles in which Mars' axial tilt dramatically increases to provide a habitat for life; and
• results from the extremely detailed photos and IR composition maps from America's 2005 Mars Reconnaissance Orbiter ("MRO") pinpointing places where erosion has exposed layers of water ice and carbon dioxide ice mixed with sediment that serve as a record of the varying climate of Mars over the past few tens of millions of years (with such ice perhaps also containing near-intact frozen remains of old Martian microbes).

Such findings might very well lead to the 2009 MGE being aimed for such a layered site to study it in detail, along with later landers of various sizes -- and, later, sample-return missions -- also being aimed at Mars' polar regions as the most promising area of the planet to study its past habitability and look for fossils.

Another pathway ("Focus on Ancient Geologic and Habitat Records") would respond to results from Mars Odyssey, the 2003 MER and Beagle 2 landers, and the MRO confirming firmly that there are places on Mars where layers of sedimentary rock have been formed from sediments laid down at the bottom of ancient Martian lakes or even seas, or where ancient hot springs have deposited minerals.

Any such areas would be a clearly logical hunting ground for microbial fossils, and once again the 2009 rover, later landers, and finally sample-return missions might concentrate on them.

Finally, a "Focus on Subsurface Exploration" might result from several different kinds of findings: evidence of a few currently active geothermally heated regions by Mars Odyssey's high-resolution thermal maps, or of near-surface deposits of ice or even pockets of liquid water by the radar sounders on the 2003 Mars Express orbiter and the MRO.

Alternatively, digging may be needed to get below the surface layer of destructive oxidants to look for organic chemicals or get access to water-altered rocks and minerals on Mars buried beneath wind-blown dust and sand.

The 2009 MGE will probably drill a meter or so into the ground, and later landers could sound the subsurface in more detail using long-wavelength radar or seismic techniques -- after which more sophisticated and bigger landers would drill deeply into the surface to obtain samples for in-situ analysis, and later for sample return. This Pathway would obviously require the development of another new technology not needed for the others -- reliable and fairly low-powered deep drilling, at least to several dozen meters.

MEPAG's Astrobiology Subgroup also says that the proposed "SHARAD" instrument on the 2005 MRO -- which would make a sensitive search for layers of ice and liquid-water pockets in the upper few hundred meters of Mars' surface -- is very important and should be moved from its current second-priority ranking on that mission to be one of the top-priority instruments.

Italy, which was supposed to provide SHARAD, is now vacillating on whether it will do so -- although it still seems to be leaning towards it, with the decision scheduled within a few weeks. But the MEPAG subgroup says that if Italy does pull out, NASA should "take steps to ensure" that such a device is flown on MRO, even if the U.S. has to fund SHARAD itself.

All likely future exploration Pathways do have some traits in common. In particular, the National Academy of Sciences' "Decadal Survey" laying out an optimal program of Solar System exploration this summer urged that an inexpensive "Mars Scout" mission -- picked out from competitive proposals by different scientific teams -- should not only be flown in 2007, but if possible at every second Mars launch opportunity afterwards. NASA has indeed decided that a second Mars Scout in 2011 is now a near-certainty, and that others are likely to follow.

Also, all the Pathways do call for sample-return missions in the not-distant future - although the first Pathway, involving a continued general survey of Mars, would naturally call for them a bit later than the more focused Pathways.

All such plans, however, leave the other serious questions NASA now faces in regard to Mars sample return missions. Where do they really fit into such exploration sequences? Is there any way to lower their exorbitant cost? Given their high cost and our uncertainty as to where to look for those faint traces of ancient life or prebiotic evolution, does it really make sense to fly ANY sample return mission until we have finished very extensive in-situ exploration of Mars to locate the best conceivable sampling sites?

The answer of quite a few scientists (and this reporter) was, until recently, "no" -- they felt that even the very first sample return mission to Mars should be delayed until after a long series of much cheaper reconnaissance missions to Mars.

But that philosophy is now changing. The reason for the change is that -- as stated in a National Academy of Sciences report last year, and reaffirmed by the "Minimal Sample return" science subgroup of MEPAG this summer -- the very first return of samples from Mars, even if it contains absolutely no meaningful evidence for or against life, will in itself serve as one of the most important of those "reconnaissance" missions.

The whole point of returning samples to Earth, rather than analyzing them on the spot with remote-control landed instruments, is that the chemical, physical and microscopic analyses that can be conducted on samples from other worlds using Earth-based instruments are so stupendously superior to those that can be done by remote-control instruments on spacecraft that there is simply no comparison.

To quote J.A. Wood and William Boynton: "Most of what we know about terrestrial and extraterrestrial rocky materials has come from imaging at the microscopic level, and in recent years from the use of microbeam instruments to reveal microchemical structures and isotopic patterns in these materials.

"The machinery required to prepare materials for these studies and perform the analyses is complex and massive; it does not lend itself to miniaturization for spacecraft payloads... Microbeam analysis techniques have advanced to the point where a very large amount can be learned -- in terrestrial laboratories -- from a single 10-micron mineral grain."

And such huge ground-based analysis instruments can also detect and distinguish elements, isotopes, and different minerals in extremely tiny trace amounts that no small spacecraft instrument weighing only a few kilograms could possibly sense. Also, once samples are back on Earth, an endless series of new analytical techniques can be used on them using new instruments -- many of which may not even have been invented at the time the samples were first collected.

Many of the super-detailed microscopic-scale studies of Mars meteorite ALH84001 which are being used in the current debate as to whether it shows traces of fossil life could not have been done at all when the meteorite was first collected in 1984. It's a safe bet that in coming years more and more powerful new techniques will be applied to this meteorite alone, and additional experiments will be designed to test as-yet undeveloped hypotheses about it.

Such super-sensitive, super-detailed and super-flexible chemical and physical analytical techniques aren't just important in looking for faint traces of fossil life; they're invaluable in understanding the geological and chemical history of any rock or soil sample -- which means they're crucial in correctly understanding the geological and climate conditions to which the samples have been exposed throughout their history.

To quote the MEPAG subgroup: "Detailed and precise understanding of crustal evolution with time, determining unambiguously the existence and nature of minute amounts of prebiotic or even biotic compounds, determining the timing and nature of any wholesale planetary differentiation, understanding the nature and formation of any regolith, determining the nature and abundance of volatiles, and deciphering the evolution of any atmosphere, are only possible through the laboratory analysis of samples."