That earlier scheme was a wildly over optimistic, underfunded rush to a Mars sample return attempt in 2005, which would have probably collapsed through its sheer implausibility even if the 1998 missions had been totally successful.

The revised program is now well defined through 2005 - consisting of two fairly long-range, sophisticated rovers in 2003, and an orbiter in 2005 equipped with an extremely high-powered telescopic camera (among other instruments) and capable of transmitting data back to Earth 12 times faster that Mars Global Surveyor now in Mars orbit can do. Beyond, 2005, however, the program remains flexible while funding positions are more established.

The Bush Administration's proposed budget provides NASA with a hoped-for infusion of money which would allow it to fly a very sophisticated mission in 2007: a big "smart lander" which would test systems allowing soft landers to land within a few kilometers of their target point, detect and dodge dangerously rough or sloped terrain during their actual descent, and survive a very rough landing.

The lander would then deploy a long-range rover that would travel dozens of km over Mars' surface for up to a year, analyzing the surface with as much as 300 kg of scientific instruments.

It would also serve as the test flight for the actual Mars sample-return lander in 2011, which would use a similar rover to collect samples that would then be launched back into Mars orbit by a small rocket where a small container would be ejected to be scooped up by a joint French-American orbiter before blasting out of Mars orbit for the return trip to Earth with its extremly precious Mars samples.

Howqever, given NASA's continuing budgetary woes it's quite possible that Congress may reverse Bush's proposed boost to Mars funding. Indeed, the U.S. Senate is currently trying to cut $50 million out of this year's funding for the program.

This would not be a catastrophe, though - for, even without Bush's boost, the Smart Lander test flight and the first sample return mission would be delayed two years, until 2009 and 2013.

In any event, though, there will definitely be at least one other 2007 U.S. Mars mission - the first Mars Scout mission, a lower cost $300-million mission - essentially a Mars-specialized version of NASA's current Discovery program for general Solar System exploration.

The Mars Scout missions - to be launched every four years - would be selected by an appraisal board from a set of proposals provided by separate competing teams of scientists and engineers.

The selection process for that first Scout mission has already begun with a May workshop in Pasadena where 39 proposals were submitted by various scientific teams.

These were not official submissions - they were, instead, provided as a kind of "practice run" to see what kind of proposals the scientific community is likely to provide when the official Announcement of Opportunity is released next February, and to "identify achievable science objectives and technology development priorities".

The best ten proposals, however, were chosen for 6-month study contracts costing the government $100,000 to $150,000 each - and they will undeniably be in a relatively favorable position when the actual finalists for the Scout mission are chosen in May 2003 (with the final choice being made in January 2004).

This still doesn't make it even probable that one of these ten favored proposals from the Pasadena workshop will be chosen. After all, a similar workshop was held at the start of the Discovery program in 1992 - and among the eight Discovery missions since chosen for flight, only two have come out of the 13 proposals chosen for extended study after that workshop.

But these 10 proposals do provide us with a glimpse of the kinds of missions that will actually be submitted - and eventually chosen - for the 2007 Mars Scout mission.

The actual structure of the Mars Scout program - the range of missions that will be considered for it - has become more flexible since the revised U.S. Mars program was first announced last year.

At that time, the primary role of the Scouts was seen as that of a flock of little, low-cost reconnaissance vehicles to check out various possible landing areas on Mars, both for their landing safety and for their scientific interest, to see if they would be worthwhile targets for later, bigger and far more expensive landers, including the sample return missions.

These vehicles might be durable little hard landers like the 30-kg Beagle 2 to be carried on Europe's 2003 Mars express mission, and Pathfinder in 1997 - which would not only examine an area after landing, but also take photos of it on the way down.

Or they might be aerial vehicles - balloons, gliders, or even powered airplanes - which would use cameras and other instruments to examine Martian terrain from above, in more detail than orbiters can. But while they would be free to carry other instruments, their primary role really would be that of "scouts" for later, more expensive - and less expendable - landers.

However, at its March meeting, NASA's Space Science Advisory Committee (SSAC) reexamined the Mars Scout program and decided to make it far more flexible in structure:

"The Scout program should permit all missions to Mars space that fit within the cost caps and schedule constraints of the Program. In addition to orbital and landed missions to Mars, missions should be permitted that focus primarily on Phobos and Deimos, the upper atmosphere, or 'network science' [long-lived landers for seismic and weather measurements]. Any of these would complement the missions that are part of the main Mars program."

In short, NASA's science committee advised that the Mars Scouts be turned into a Mars-focused version of the Discovery missions, in which virtually any kind of mission is acceptable if the appraisal board thinks that its scientific return will be worth its cost.

Indeed, several Discovery missions have previously been proposed to study Mars - and one, the "Aladdin" mission to return material from Mars' moons to earth at low cost, has been a finalist twice.

The feeling now seems to be that suitable landing sites for the earliest sample return missions can be selected on the basis of the information we get from the current and future Mars orbiters, without the need for detailed on-the-spot inspection of a large number of possible sites.

Sure enough, the 43 proposals offered up at the May workshop - and the 10 selected for detailed study - are a wildly mixed bag.

Only about four of the ten fit at all into the original definition of the program as scouts for more advanced landers, and only four would carry any instruments capable of detecting any possible evidence of Martian life (although no U.S. Mars probe between the Vikings of 1976 and the 2007 smart lander will do that, either).

In fact, three of them would focus largely or entirely on studying Mars' atmosphere - and four of the missions would never even touch Mars' surface. This most definitely does not mean, however, that they're scientifically inferior - only that the Scout program is now much different from what it used to be.

In describing them, we will begin with the three atmosphere-oriented missions - two of which are Mars orbiters to study the planet's atmosphere and weather from above. One - MACO (Mars Atmospheric Constellation Observatory), is proposed by Robert Kursinski of the University of Arizona.

It would use the time-honored technique of radio occultation to make vertical profiles of Mars' atmosphere. But instead of the spacecraft transmitting the radio beam to Earth, MACO would consist of four little satellites that would continuously transmit radio beams back and forth between each other, monitoring their strength, frequency and phase as the beams periodically sliced through a cross-section of Mars' atmosphere.

Despite the simplicity of MACO's observation system, it could in many ways do an excellent job at monitoring global weather changes on Mars - especially since the four minisatellites, once they had been aerobraked into low circular orbits around Mars with different inclinations, would have their interlinked radio beams occulted over 200 times per day in positions scattered all around Mars.

Moreover, such occultation cross-sections can provide much better vertical resolution of the many properties of a planet's atmosphere than even the best radiometer-type sensors on a single satellite can do with resolutions as little as 100 meters possible. The GPS network of navigation satellites is currently being employed using this same technique to provide data on Earth's weather.

The MACO satellites would profile temperature, pressure, and humidity, while providing significant data on wind speeds and even some measurement of the trace levels of deuterium present in Mars' water vapor - which we already know is somewhat more concentrated than on Earth, confirming that part of Mars' initial atmosphere has been swept away from the planet by the solar wind.

The MACO birds would also each carry a lightweight IR sensor to measure atmospheric dust - which is one of the biggest influences on the temperature of Mars' thin air - and use a microwave receiver to measure water vapor.

The Mars Environmental Observer - proposed by Michael Janssen of the Jet Propulsion Laboratory - is a more conventional Martian meteorological satellite, but a very capable one.

It would be a modified version of the Stardust spacecraft - which, to set up its flyby of the comet Wild 2, has to carry out one deep-space maneuver so big that Stardust's fuel tanks are quite adequate to brake a copy of the craft into Mars orbit. MEO would then aerobrake into a 400-km orbit around Mars, and set about its atmospheric observations, using four different instruments.

Two of them would be radiometers measuring very short wavelength microwaves, which are now starting to be used for space studies in a major way.

One, a French instrument with the unlikely name of BOSCO, would measure waves with a wavelength of a few millimeters, while the American, "MIMAS"-- based on the American "MIRO" microwave spectrometer which will fly on Europe's Rosetta comet probe in 2003 - would measure shorter, sub-millimeter waves.

There's some feeling, though, that the two are redundant - and the French are seriously considering deciding instead to fly BOSCO on their 2007 test flight of their big Mars sample return orbiter.

Either or both of them, peering at the Martian horizon, would provide a very sensitive profile of air temperature and humidity throughout the thickness of the Martian atmosphere - and they could also measure winds at various altitudes down to only two or three meters per second, a capability that the PMIRR instrument doesn't have.

Another copy of the miniaturized PMIRR from the 2005 U.S. Mars orbiter would, however, also be carried, and by comparing its data to the microwave data, the amount of dust at different altitudes could be very sensitively measured.

Finally, MEO would carry a miniature version of the "ATMOS" instrument flown on the Space Shuttle: an extremely sensitive IR Fourier spectrometer capable of taking such detailed spectra that it could measure a huge variety of different trace gases in Mars' air down to only a few parts per billion - and, for some gases, a few parts per trillion!

Not only would this provide vast new knowledge of Mars' atmospheric chemistry - and allow more isotopic measurements to study the atmosphere's history - but it could detect tiny near-surface concentrations of such gases as methane and nitrous oxide, which, if found, would provide strong evidence that Mars still has a small subsurface population of living bacteria to manufacture them.

Thus MEO - despite being an atmospheric orbiter - is one of the few Scout proposals that could look for actual evidence of Martian life.

Another Scout proposal - "Pascal", from Robert Haberle of the Ames Research Center - would be a network of 24 tiny hard landers scattered all over the Martian surface as a network of miniature weather stations. (Haberle has proposed Pascal before as a Discovery mission, although it wasn't picked as a finalist.)

These rugged little cylindrical capsules, weighing only 2.1 kg each, would be dropped by simple parachute - being padded enough to survive impacts of 140 km/hour - and their sensors would be evenly distributed over their surfaces so it wouldn't matter which way they ended up lying.

They would be powered by tiny nuclear batteries, allowing them to measure air temperature, pressure, humidity and dust content for fully 10 Martian years (18 Earth years) - thus enabling a portrait of long-term variations in the yearly cycle of Mars' weather, such as the occasional global dust storms, to be formed.

Despite their tiny size and low communications bit rate, they would also each carry a tiny color CCD camera, which would take 10 pictures of Mars' surface on the way down, and then transmit those pictures very slowly over the following months.

Haberle is also considering adding another tiny camera to each of them that could send back a single photo per month. They would be carried to Mars on a copy of the Mars Climate Orbiter/Mars Odyssey spacecraft , which would eject them in various directions a few weeks before reaching Mars to arrange their landings scattered over the planet - and the carrier bus would then brake itself into a low Mars orbit to daily receive their data transmissions and relay them back to Earth. To save money, however, the bus would carry no instruments of its own.

The remaining six Mars Scout concepts concentrate on the surface - although two of them would never touch it. One - "SCIM" (Sample Collection for Investigation of Mars), proposed by Laurie Leshin of Arizona State University - is an actual low-cost sample return mission, but one, which would never touch Mars' surface.

Instead, it would hurtle nonstop over Mars' surface at 40 km altitude and scoop up a sample of both the atmosphere and the very fine dust particles floating in it, and return them to Earth.

This dust wouldn't include any meaningful biological evidence - but it would provide us with much more knowledge of the mineralogy of Mars' surface in general, as opposed to the current Mars meteorites that seem to have come from only a handful of locations on the surface.

We could, for instance, understand more clearly what kind of weathering processes have affected Mars' surface minerals - and whether the minerals found in the dust indicate that Mars had large amounts of liquid water on its surface in the past.

And the accompanying air sample would be invaluable in itself - for instance, by analyzing the percentages of trace isotopes in it, we could estimate how much of Mars' original dense atmosphere was actually swept away into outer space by the flow of solar wind past the planet, as opposed to chemically reacting with Mars' surface rocks or being blasted into space by giant meteorite impacts.

SCIM would take off in Sept. 2007 and make its first Mars flyby in late 2008 - but that would be a high-altitude flyby, allowing Mars' gravity to redirect it into a different solar orbit.

Then, in July 2009, SCIM would return to Mars and hurtle over its southern hemisphere at 40 km altitude, where using two small pads of shock-absorbent "aerogel", similar to those employed by the Stardust mission, will collect micron-sized dust samples.

The extremely fluffy aerogel can capture these "dust" samples without causing serious scientific damage to them - even at speeds of 20,000 km/hour. Simultaneously, SCIM would scoop up a separate sample of Martian air.

To survive such a low pass through Mars' atmosphere (unlike the accidental low pass that doommed Mars Climate Orbiter in 1999), the SCIM spacecraft would be mostly wrapped in a heat shield - but, unlike the shield on any earlier spacecraft, it would be long and pointed to minimize the extent to which air friction slowed down the craft during its fiery dive.

Immediately after skimming over Mars' edge and then out again into space, it would fire a rocket motor to compensate for the 2200 km/hr of speed it had lost during the pass, so that it would then be on course again for a return to Earth in May 2010, at which point it would drop off its collected samples in a small Earth-return capsule - again like the one being employed for Stardust. During the entire trip, it would be powered by a single long solar panel trailing behind it, which would be safely located in its nonheated wake during its dive through Mars' air.

The dust - unlike material returned directly from Mars' surface - would have been sterilized by solar UV light during their long period floating in the Martian atmosphere, and so wouldn't need to be quarantined. And - given the extreme sensitivity of modern, big Earth-based analytical instruments - individual dust grains could be inspected and analyzed in detail far beyond anything that in-situ Mars instruments could possibly do, which of course is one the central arguments for sample return missions.

During a recent competition held by the European Space Agency's for the possible use in 2005 of a copy of the Mars Express bus, France proposed its own Mars atmospheric sample-return mission. But it was rejected as too expensive for the time being, and in any case it would have dipped no lower than 120 km altitude into Mars' atmosphere - too low to collect any dust, and with even its air sample being atypical of the planet's lower-atmospheric gases.

Another mission - "Mars Scout Radar", proposed by Bruce Campbell of the Smithsonian Institution - is just what its name indicates: an orbiter (again based on the MCO bus), equipped with an umbrella-like radar antenna fully 5 meters across.

Mars Express and the 2005 U.S. orbiter are scheduled to use very low-frequency radar to probe several kilometers below Mars' surface, looking for subsurface ice and pockets of liquid water. But MSR's radar - its only instrument - would use a much shorter wavelength, for a completely different purpose.

Mars Global Surveyor's high-resolution photos and IR spectra of Mars' surface minerals have confirmed something long suspected: most of Mars' surface is blanketed with a layer of wind-blown dust and sand - sometimes meters thick - which is concealing most of its ancient surface features, and making analyses of local minerals almost impossible, as the dust has been mixed to an even composition all over the planet.

MSR would use three shorter wavelengths - 3, 25 and 75 cm - to construct high-resolution "synthetic-aperture radar" pictures of Mars' surface similar to those that Magellan used to map Venus, which would look like real photographs.

But the two longer wavelengths would punch through as much as 5 meters of dust or sand to reveal the features of the underlying bedrock, making a planetary map with a resolution of half a km.

This would allow revelations of a vast variety of features of the surface of ancient Mars that are still unknown today - just as similar radar on the Space Shuttle has punched through the desert sands of the Sahara to reveal networks of ancient, buried river valleys.

MGS has found that most of Mars' ancient "valley networks" are partly filled with windblown sediments, making it impossible to tell from the cross-section shape of their beds whether they were carved by fluid (presumably water) running along early Mars' surface, or tunneled out by slower underground streams of fluid until the tunnel's roof caved in - in which case ancient Mars' surface might have been below freezing.

MSR could settle that - and it could also pierce sediments along the edge of Mars' great northern lowland plains to find if they were, as some think, the shores of a vast northern ocean.

By looking for such ancient river valleys, lakebeds and ocean shorelines MSR could provide critical new data for choosing good landing sites of later missions. And it could also provide a much better count of craters under sand-shrouded terrain to find out how old it was when it when it was first sediment-covered. Moreover, the longer-wavelength radar could pierce as much as 70 meters through the ice of Mars' polar caps, providing insight into the changes in their deposition and of local weather over the millennia. Meanwhile, the shortest, 3-cm radar could provide an altitude map of Mars' features with a horizontal resolution of only 100 meters - far better than MGS' laser altimeter.

MSR would be able to map Mars completely in only about 6 months. The scientific usefulness of this mission is so clear that NASA's current Mars plan - if the extra money from Bush comes through - tentatively calls for such a radar-mapping orbiter to be launched anyway in 2009, probably in collaboration with Italy. But Campbell's proposal would provide a 2-year jump on this.

There has been, for a long time, a great deal of interest in possible Mars Scout-type missions involving aerial vehicles - balloon, gliders, and even powered airplanes - to map wide swaths of Mars' surface in much sharper detail than orbiters can.

The 43 proposals presented at the Workshop included many designs for airplanes capable of flying in Mars' wispy air (since drones already exist capable of flying in the comparably thin air 35 km above Earth's surface), but surprisingly few balloons.

In the end, only one of the 10 finalists was an aerial vehicle - "Kitty Hawk", from Wendy Calvin of the University of Nevada, who had earlier proposed it as a Discovery mission.

Kitty Hawk consists of four unpowered little gliders, dropped separately to fly over appropriate locations around the great network of valleys that lead into Mars' gigantic 4000-km long Valles Marineris (Marineris Valley). Previously, Dan Goldin had ordered initiation of a project to fly one little actively powered airplane over this region in 2003 to celebrate the first century of powered flight, but this project was quickly dropped due to cost.

Designing a powered airplane for Mars is a lot harder than designing a glider - especially since an engine powerful enough to drive a propeller-driven craft through such thin air must take up a great deal of the plane's total weight, so that only about 10 percent of its mass could consist of scientific instruments.

By contrast, half the total weight of a glider could consist of instruments - and so several of them would weigh no more than a single powered plane with the same payload of instruments, allowing them to cover a total flight distance across the surface that compares pretty well with the flight distance from a single long-range powered plane.

Each of the four Kitty Hawk gliders would travel about 120 km over a period of 10 minutes before crashing. They would be dropped one at a time from an orbiting bus, which would record the high-speed data transmissions from each one and play it back to Earth later.

The Marineris Valley and its tributaries are especially suitable for such aerial reconnaissance because of the very dramatic layering that can be seen in their walls, revealed particularly clearly by MGS' high-resolution photos - both thick layers apparently laid down by successive lava flows in Mars' early days and running the complete height of the canyon's' walls (up to 8 km high), and narrower layers visible on the slopes' lower flanks and running across the flatter canyon floors, piling up again into isolated mesas scattered all over the canyons and towering up to 5 km high themselves.

These latter layers seem to be made of softer, more easily erodable rock, which is probably sedimentary - but how was it laid down? Was it made out of wind-blown dust or perhaps deposited by various episodes of volcanic eruption, before being slowly cemented together by Mars' slow weathering processes?

Or could these be layers of water-borne sediment deposited on the floors of the canyons during Mars' more clement days, either by intermittent torrents of water gushing at high speed through the canyons, or gradually during periods when the canyons may actually have been deep, water-filled lakes?

Clearly, if they're water-deposited, these layers will be especially promising hunting grounds for ancient Martian fossils.

But to understand them and the general geological history of the region, we need both higher-resolution photography of the layering (some of it only a meter or less thick, judging from MGS) and the small channels carved through it in places, and high-resolution IR spectra of the minerals in the individual layers.

Each of the Kitty Hawk gliders would be dropped at a separate location outside one of the canyons, inside a small heat shield, with its wings and tail folded over its main body. At about 15 km, it would unfold - a process already tested on Earth - into a glider with a 2-meter wingspan.

Then navigating by gyros, the glider would angle down to about 5 km, cruise over the canyon's lip, and glide down at an angle following the slope of the canyon wall (which is never more than about 30 degrees), and perhaps making occasional pre-programmed turns to view especially interesting areas.

By the time it reached the canyon's floor, it might be only 2 km up, and would then simply glide across the floor and lower mesas, sending back images to its orbiter until it crashed.

Its instruments would consist of wide and narrow-angle multispectral cameras - with the latter's photos showing details of the canyon layering as small as 10 cm - and a near-IR spectrometer mapping the layers' minerals with a resolution of only 3 meters.

It would have time to transmit only 20 photos, but this would be enough to provide vastly improved understanding of the geology of these great formations - and to judge their value as landing sites for future missions.

One of them would probably be dropped over part of the Candor Canyon where MGS' own IR spectrometer has located what seems to be a smaller deposit of the same coarse-grained hematite which covers a great part of the Simus Meridiani plain - and which may very well have crystallized at the bottom of a lake of standing water.

Two more proposed Mars Scout missions would be specialized landers - each one studying only one location, not to judge its suitability for later bigger landers, but to carry out specific scientific studies of Mars that can be done in no other way.

The first - proposed by Frank Carsey of JPL - would be the CryoScout. It would land on Mars' northern "residual" polar cap - the relatively small permanent cap of water ice, thousands of meters thick and covered only during Mars' winters with an additional 2-meter thick layer of frozen carbon dioxide - and then release a "Cryobot", a torpedo-like device that would slowly melt its way down through the ice over several months to a depth of at least 200 meters, analyzing the ice and sending its data back to its surface mother lander through a thin cable.

If this sounds familiar it is, because Carsey is also head of JPL's attempt to design a more ambitious Cryobot that would slowly melt all the way through Europa's kilometer-thick shell of ice to reach the liquid-water ocean thought to exist beneath it, analyzing both the ice and the liquid water for evidence of microbial life. It's obviously a huge engineering endeavor - which may take decades before being attempted on Europa - and the Mars polar CryoScout would serve as a test run.

Like the planned Europa version, it would be about a meter long and 10-12 cm wide, weighing about 20 kg. Its nose would be heated - although it hasn't been decided yet whether this would be by electric heaters powered by the lander through the connecting cable, or by an actual small block of plutonium-238 heating elements.

NASA is now starting to seriously reconsider its current reluctance to put plutonium-powered electrical generators on its space probes - and one part of this is that they are specifically allowed in the current designs for Mars Scouts.

But Dr. Carsey tolds SpaceDaily that the decision as to whether to use plutonium heaters or electric ones in the CryoScout is the biggest remaining question about its design. If it did use electric heaters, then - in order to supply enough electricity - the surface lander might very well have to carry an RTG nuclear battery itself, given the low sunlight level at Mars' poles.

Other heaters on its sides would prevent the thin layer of liquid water from refreezing there - and the water would actually be sucked up at the cryobot's rear and recycled by pumps to come out its nose as hot-water jets.

This technique should wash away the fine dust which is mixed in large quantities with Mars' polar ice, and which would otherwise cake into thick layers in front of the cryobot's nose - a technique which may also be necessary on Europa, given the large amounts of salts that are thought to be mixed with the ice there. The hot water jet technique would also double CryoScout's descent speed beyond that of a simple heated-nose probe.

Preliminary tests indicate that this design can not only plow through the 80-20 mixture of ice and dust that is thought likely in the residual ice cap, but - if it should accidentally land on the darker "layered terrain" beyond the cap's edge - it could probably melt through that as well, despite the fact that it is thought to consist of a 50-50 mixture of ice and dust.

One cost-cutting technique planned for this mission would be that the CryoScout surface lander would be the legged soft lander built for the 2001 Mars Surveyor Lander mission - which was based on the Mars Polar Lander's design, and thus cancelled and boxed away after that craft crashed.

There's a feeling among many scientists that not using it is a serious waste, and that we understand the likely cause of the Polar Lander crash well enough now that a repeat could probably be avoided.

The lander itself would carry descent and post-landing cameras and weather sensors. It would lower the Cryobot onto the surface inside a "silo" whose purpose would be to keep the liquid water melted by the Cryobot during the first meter of its descent from sublimating immediately into vapor in the thin Martian air, which would keep its descent from being properly lubricated. After it was completely buried below the surface of the ice, this problem would no longer exist.

Carsey's team for about a month won't decide the actual miniature scientific instruments that the Cryobot would carry, but there are plenty of candidates. To quote last year's International Conference on Mars Polar Science: "The Martian polar deposits are believed to preserve a record of geological and climatic history that extends back at least 100,000 to 100 million years."

And the 200 meters distance covered by CryoScout would examine at least several million years of Martian climate history, neatly recorded in deposited layers.

Mars' lack of a large stabilizing moon causes its axial tilt to rock back and forth between 0 degrees and 45 degrees over cycles of about a hundred thousand years, and every few million years it may keel all the way over to a 60-degree tilt - all of which has startling effects on its weather during these periods.

During these high tilt periods on a year-round average, Mars' poles are actually warmer than its equator, and the planet as a whole warms up somewhat - ejecting enough CO2 from its cold soil layers to increase its air pressure several fold, and also causing the water in its polar ice to sublimate and then refreeze in a belt around its equator, until the axial tilt decreases again and the process reverses.

As the air pressure increases, Mars' air carries far denser dust storms, so this cycle is thought to cause the alternate light-dark layering of the dust-ice mixture laid down a fraction of a millimeter per year in the layered deposits and the polar ice itself.

This means that, by using a side-looking camera and optical sensors to profile the fine layering of the ice through which it descends, CryoScout could give us an actual record of Mars' climate cycles over a million years or more.

Moreover, if a microscope capable of viewing extremely thin dark dusty layers was carried, it might even be able to identify individual planet-wide dust storms like the one currently blanketing much of the planet.

Moreover, CryoScout would carry some instruments to analyze the chemical makeup of the dust and ice - as well as looking for organic compounds frozen into the ice.

The polar ice may be one of the best places to look for such well-preserved frozen evidence of past or even present life, since water ice is thought to neutralize the powerful oxidants that apparently destroy organic compounds in the soil on the rest of Mars' surface.

It would likely carry Raman and UV fluorescence spectrometers, which would examine the dust and ice around the Cryobot to both identify a wide variety of minerals and provide a sensitive test for many kinds of organic compounds. (The Raman sensor could also determine whether or not much of Mars' "water ice" is really a "clathrate", a frozen mixture of water and CO2.)

Meanwhile some of the meltwater the probe produced would be sucked up in its rear and analyzed using electrochemical sensors and perhaps a tiny liquid chromatograph, further analyzing both dissolved soil salts in the ice and possible organics.

CryoScout would clearly be a specialized lander, designed to carry out one very specialized kind of scientific study of Mars' surface, rather than searching for promising landing sites for future missions.

Another such highly specialized one-place lander would be "Urey", proposed by Jeffrey Plescia of Arizona State University, whose single purpose would be accurately dating the age of rocks in one place on Mars - that is, the time since they originally hardened out of lava.

This would allow us, for the first time, to determine how accurate our current techniques are for estimating the age of different places on Mars by counting the number of meteor craters accumulated on their surfaces.

Up to now, all our estimates of the age of the surface of Mars in different places - that is, the time since it was it was originally laid down either as lava flows or as sediment deposits (or since it was exposed by a period of erosion in Mars' past) - have been based on such counts of accumulated craters both large and small, with the cratering rate of the Moon's history as a guide.

This is crucial if we are to understand both the history of Mars' internal geological activity over time, and the history of its climate changes - including the real time in its early history when it apparently lost its initial thick CO2 atmosphere, which may have made it warm enough for liquid water and thus life to exist on its surface. We can't judge how accurate our theories of Martian history may be until we have such reasonably good dating of its surface features.

Mars' geologic history has been divided into three main "ages". The oldest, the "Noachian", is the early period during which Mars was still undergoing very heavy bombardment by big objects - and during which (perhaps not coincidentally), it seems to have had a much denser atmosphere than today, perhaps even one capable of warming its surface temperature to the point that liquid water and life could exist there.

Judging from the counts of big craters, only a relatively small part of Mars' current surface still dates from that time; but that part contains most of Mars' "valley networks" which may be the beds of ancient rivers.

Most of Mars' southern highlands have few very large craters, but still have a large number of smaller ones - indicating that they date from the "Hesperian" age, during which the rate of bombardment of the planets slacked off dramatically, and during which most of the planet's atmosphere also disappeared.

But this was also an era during which the number of huge, short-lived "catastrophic outflow" floods from underground increased - and during which most of the great "Tharsis bulge" and the associated Marineris Valley formed on one side of Mars.

And virtually all of Mars' northern lowlands contain very low crater populations, and are thus said to present surfaces from the most recent, "Amazonian" era - during which the cratering rate dropped still lower, after the north had been resurfaced by great lava flows and/or by massive deposits of water- or gas-borne sediment.

The four great shield volcanoes on the summit of the Tharsis bulge also have flanks covered with lava flows that are uncratered, and thus must have occurred during this period.

But there are tremendous uncertainties in our current estimates of the periods these ages actually cover, because we are similarly uncertain of the rate at which cratering bombardment did slack off on Mars.

We have excellent knowledge of the ages of different terrain on the Moon, because we have returned samples that can be radioactively dated for comparison with the number of craters on their origin areas. But since Mars is so much closer to the Asteroid Belt, we are very uncertain how fast cratering slacked off for it.

Estimates of the end of the Noachian Age, vary from 4.3 to 3.85 billion years ago - and since cratering slacked off very dramatically for the Moon about 3.8 billion years ago but we don't know whether this also happened on Mars, estimates of the end of the Hesperian and the start of the Amazonian vary wildly from 3.8 billion years ago to a mere 1.3 billion years ago! And our estimates of the ages of individual areas within those terrains are of course comparably murky.

Dating the ages of rocks involves very complex analyses of elements existing in only traces within them, and it's usually been regarded as requiring the return of Mars samples to Earth labs.

But Spudis and his co-experimenters are convinced that even getting the ages of Martian surface areas down to an uncertainty of 15 percent would be a huge increase in our knowledge of the planet, and that we now have miniature instruments which could be carried on Mars landers to do just that.

Thus Urey, which would soft-land and age-date the rocks in a single place on Mars - so that, by comparing that age with the local crater count, we could finally get a reliable estimate of how the overall cratering rate on Mars has compared with that of the Moon, providing a "benchmark" from which we could estimate the ages of Mars' other surface formations with far greater confidence.

To do this, Urey will try to land in an area where virtually any rock it analyzes is likely to be the same age - but most of Mars' surface is covered with rocks and soil that have been carried there from other places by wind, water, or the ejecta from distant giant craters, making any estimate of the region's age from just a few analyzed samples far less reliable.

So it will aim for the Cerberus Highlands, a region covered with some of the most recent lava flows on Mars - which judging from their sparse cratering, are not only recent but all about the same age.

The lava flows there may be very young indeed; William H. Hartmann estimates from his crater count that they may be as little as 10 million years old, but Spudis thinks they're more likely about 200 million years old, and that some of the youngest rocks found among the Mars meteorites may come from here.

This also means that dating the rocks here could only give us a measurement of Mars' cratering rate in recent times, and there would be uncertainties in how it applied to earlier ages - but Spudis still thinks that, given the uniformity of age in any rock samples analyzed here, this is the best spot on Mars for the purpose, especially given the fact that they seem to have very little wind-blown dust covering them.

Urey's rover would use a drill originally developed for the Mars sample return mission to collect rock cores about two centimeters long, and then date them with two different techniques for comparison. In both, the rock would be struck with a small but very high-powered laser, hot enough to boil some of the more volatile elements in them into gas whose isotopes would be analyzed by mass spectrometers.

In one, the laser system would be used to measure the radioactive potassium-40 in the rock - after which the rock core would be ground into powder and roasted in a tiny oven to release argon-40 produced by the potassium's decay and then trapped inside the rock.

Since any argon produced by this process before the rock solidified out of lava would have escaped, by measuring the ratio of the two elements, we could estimate how long the rock has been in a solid form from which the argon couldn't escape. The tiny British Beagle 2 lander will try to use a cruder version of this technique when it lands on a sediment-covered plain in 2003.

In the other technique, the very narrow laser beam would be fired at several different spots on the rock core, thus analyzing grains containing different types of minerals with different amounts of the element rubidum.

Rubidium-87 is mildly radioactive and slowly decays into strontium-87 - so, by measuring the amounts of rubidium in the various grains and also measuring the differing ratios of strontium-87 to strontium-86 in the same grains, we can judge how long the grains have been solid and thus trapped different amounts of strontium-87 within themselves.

The original plan was for Urey to be a copy of the two 180-kg Athena rovers that NASA plans to land on Mars in 2004 using the Pathfinder airbag system, so that it could then crawl as much as a kilometer across the landscape to collect its samples.

But Spudis now has some doubts that this rover design could carry those larger age-dating instruments without major modifications - and since all the rocks in Cerberus are likely to be the same age, he is considering a cheaper alternative design which (like CryoScout) would use the unflown Mars Surveyor 2001 lander, and collect all its rock samples from just a few dozen meters away using a much smaller rover that would collect the samples with its core drill and then return them to the dating instruments on the main lander. The rover might even be tethered to the lander by a signal cable, allowing its control computer to stay on the main lander.

Urey would also carry other instruments. Depending on the design, either the big Athena rover or the stationary Surveyor lander would carry a stereo camera and a mineral-mapping near-IR spectrometer on a mast - and either the big rover or the small one would carry an arm with two other instruments to analyze rocks on the spot.

One would combine a magnifying camera, an X-ray spectrometer to measure rock elements, and an X-ray diffractometer to analyze specific minerals and which, unlike most diffractometers, wouldn't need to grind up the rock first.

The other would use UV fluorescence to look for tiny traces of organic compounds in the rocks - and while it couldn't tell whether such organics came from ancient microbial fossils or from nonliving sources, just knowing whether some traces of organic compounds can survive on Mars' surface despite the destructive chemical processes discovered by the Vikings would be important.

The four little "Naiades" hard landers proposed by Robert Grimm of Blackhawk Geometrics of Colorado would have a completely different goal: making a very intensive search for any layer of geothermally warmed liquid water that may still exist deep below Mars' surface.

Current models of Mars predict that it has a thick layer of permafrost - the so-called "cryosphere" - starting at most a few hundred meters below its surface; but that after you get below about 2.5 km deep at the equator or 6 km deep under the poles, Mars' remaining trapped geothermal warmth is high enough to melt it into liquid water trapped in the rock pores, which could conceivably serve as a last redoubt for any surviving Martian microbes.

The big question is whether there are still places where Mars retains enough volcanic activity that deposits of liquid water may be located much closer to the surface, making it much easier for space probes to drill down deep enough to reach them.

The Naiades landers, each about the size of Beagle 2, would be simultaneously released from their bus just before it flew by Mars, and would land in a square pattern only about 16 km from each other in the Dao Valley - an intriguing-looking gash on the flanks of the old shield volcano Hadriaca Patera, which looks very much as if it was produced when the volcano's heat during its early days gradually melted a reservoir of underground ice that then trickled away downslope, finally causing the ground above to cave in.

Hadriaca has been extinct since Mars' early days, but it's one of the most promising places on Mars to have retained enough underground heat that there may still be pockets of liquid water relatively close to the surface.

Europe's Mars Express orbiter will use a low-frequency radar sounder to survey the entire planet for underground water in 2003; the 2005 U.S. orbiter will probably carry another radar sounder to remap the planet at shallower depths but with more accuracy in its depth measurements, and the four little "Netlander" hard landers that France plans to scatter over the planet in 2007 each carry their own small radar to sound the local landing site.

But such radar sounders have limitations. It's hoped that they can penetrate as much as 5 km below the surface, but - since Mars' surface seems to be rich in iron minerals - they may be limited to only about 1.5 km.

And in any case it's quite hard to interpret such radar graphs; different rock layers will reflect back the radar waves in a way that may look very much like layers of either ice or liquid water. The feeling is that these soundings will take a lot of analysis before scientists can estimate how likely it is that some of them may truly have found subsurface water layers.

So the Naiades landers would use a different technique: "magnetotellurics". If far lower-frequency radio waves are beamed into the ground - from a few thousand cycles down to only one cycle per second, as opposed to several million cycles for the radar sounders - they are not reflected back from underground layers of different substances, but instead have their energy absorbed by any layers of electrically conductive stuff they encounter.

And when this happens - since such any single electromagnetic wave consists of a side-to-side oscillation of the local magnetic field - the energy of the wave produces an oscillating electric current in the conductive layer that flows at right angles to the magnetic fluctuation.

Such very low-frequency radio waves are routinely produced in large amounts by natural sources, such as ionospheric fluctuations, that are bound to occur on Mars - and it may even conceivably have radio bursts produced by occasional lightning discharges in its great dust storms.

Each Naiades lander will carry a magnetometer to detect the side-to-side magnetic fluctuations marking such waves - and, after landing, it will also unreel four antennas several meters long from its sides which can measure faint electric currents flowing on the surface of the ground, since a tiny fraction of the oscillating current flow from those buried conductive layers will diffuse upwards all the way to the surface.

Thus it will measure both magnetic oscillations at different frequencies, and look for simultaneous ground electric currents oscillating at the same frequency but at right angles to the magnetic field - a dead giveaway for subsurface conductive layers.

Many ground minerals do conduct electric currents, but liquid water is far better at it - especially if it's briny, as may well be the case on Mars.

And by comparing the relative strengths of the magnetic and electric oscillations for waves of different frequencies, we can get a graph which can be analyzed in various complex ways to get a good estimate of both the depth at which such a conductive water table is buried, and also its thickness. Ground ice can also be detected this way, though with far less sensitivity.

This technique can't be used from orbit, but it is far more sensitive to buried water than radar sounding and can thus locate it at much greater depths - down to tens of kilometers, in fact.

It can also locate water layers with far less ambiguity than radar-sounding graphs, and can measure their thickness (while radar sounding will be able to detect only the top surface of any water layer).

Moreover, the Naiades landers will also sometimes engage in active sounding - transmitting brief pulses of such very low-frequency radio downwards themselves, and precisely timing the tiny period before buried conductive layers absorb their energy and retransmit an electric fluctuation back up to the surface.

This variation on ground radar sounding uses transmitters with far less power than the natural radio sources on Mars, and so would be limited to probing only about a kilometer down - but it's also far more accurate in its depth measurements of any pockets of liquid water that close to the surface, and by listening to each other's transmissions the four landers can also build up a better horizontal map of any such aquifers.

And it's just such possible near-surface water layers - perhaps maintained both by local geothermal heat and by the fact that dissolved salts can greatly lower the melting point of water - that may be responsible for the surprising gullies, apparently carved recently from some fluid oozing from rock strata only 100 meters or so down, that have turned up in MGS photos of a few Martian regions (including slopes in the Dao Valley).

The Naiades landers would probably operate only a few weeks to get adequate sounding data of their landing area, and the only other instruments they'd carry would be rather low-quality surface cameras - but if they could confirm that pockets of liquid water still exist moderately near the surface in this region, it would immediately become one of the most promising sites for future landers engaged in the search for ancient fossils.

Finally, there is "Artemis", the proposal by David Paige of UCLA - which, of all the 10 superior proposals from the Workshop, is the one that bears by far the closest resemblance to the original concept of Mars Scout as a collection of small cheap landers to be scattered widely across Mars' surface. It is perhaps also the concept that would require the least new technological development.

Paige was the principal investigator for "MVACS", which was the package of instruments that made up most of Mars Polar Lander's payload. For Artemis, he decided on a set of small hard landers - probably based on the design of the ESA's little 30-kg "Beagle 2", if that design works out well in its ground tests - and constructing a science team whose members consisted mostly of the members of the Polar Lander team, plus the team responsible for the tiny MUSES-CN "nanorover" that NASA was planning to have Japan carry to a small near-Earth asteroid on its 2002 MUSES-C sample return mission.

Development of the nanorover for MUSES-C was halted last year, due to both price overruns and weight increases - but most of the design work and a lot of the testing for it had already been completed, and it had been intended from the start for use on other worlds as well, including Mars.

Artemis' orbiting carrier bus would land three or four such landers (depending on their weight) on various interesting Martian targets - with one of them assigned to land on Mars' northern or southern polar layered terrain, just beyond the edge of the polar cap, to recover the science lost on Polar Lander.

Each lander's functions would in many ways resemble those of Beagle 2, even if a different engineering design is chosen for the landers. Each lander will carry a robotic arm that will dig up soil samples and also carries a drill to extract cores from nearby rocks which will then be ground up and fed into the lander's main experiment package, where they will be roasted at high temperature and the resultant gases analyzed to look for (among other things) organic compounds.

But the instrument used for this won't be a mass spectrometer as on Beagle. It would be MOD (the Mars Organic Detector), an instrument that would have been piggybacked on NASA cancelled 2003 sample return lander, which was developed by UC-San Diego's Jeffrey Bada to detect amino acids and "PAH" organic compounds with tremendous sensitivity by looking for their UV fluorescence. And MOD's functions will be combined with those of the "TEGA" instrument lost on Polar Lander.

Each oven would gradually heat a small sample of rock or soil powder to 900 deg C, recording the changing rate at which the sample's temperature rose as heat was applied, and the gases given off would be sent to sensors for oxygen, CO2 and water vapor.

Not only would this allow a measurement of the amount of ground ice or adsorbed water in the soil, but the procedure allows a surprising number of minerals to be identified, including many that could indicate whether Mars' surface was exposed to liquid water in the ancient past - especially carbonates. And since its soil is evenly mixed over the planet, this indication would be planet-wide.

Moreover, the CO2 and water sensors would use tunable-diode lasers sensitive enough to measure the trace isotopes of carbon and oxygen, and similar sensors would measure these isotopes in Mars' air - providing another indication of how much of Mars' original atmosphere has been swept into space by the solar wind, and how much of its original "reservoir" of CO2 and water stored in the ground remains.

These are all TEGA's functions - but the gases from the roasted sample would also pass over a cryogenically cooled "finger" on whose surface any traces of organic compounds would tend to condense.

Any "PAH" organics on its surface would naturally fluoresce under UV light, and a chemical that coats part of the finger would react with any amino acids to make them fluoresce too.

This technique could detect such organics in traces of less than one part per trillion - tremendously more sensitive than the instruments that the Vikings used to look unsuccessfully for any organic compounds in Mars' soil, and far more sensitive even than Beagle - as well as the UV organics detectors proposed for CryoScout and Urey.

Such organics might either be the fossil remains of ancient microbes or come from non-living sources as is the case for some meteorites - but their presence or absence would itself be of great importance in judging Mars' biological interest.

It's also possible that an add-on feature would allow some organic samples to be run through a miniature liquid chromatograph setup, which would allow MOD to distinguish "left-handed" and "right-handed" versions of some of the amino acids.

If a strong imbalance in favor of one "handedness" was found in the acids, this would be a very strong indication that they actually were the remnants of ancient Martian life.

Meanwhile, the tiny MUSES nanorover - a mere 1.2 kg and 14 centimeters long - would creep slowly away from the lander, supported on four wheels fastened to the rover's body by motorized hinged struts.

Despite its minute size, it would carry instruments much more capable than those on Pathfinder's "Sojourner" rover: a camera capable of magnified viewing, a near-IR spectrometer to analyze minerals, and an alpha-X ray spectrometer on the rover's rear to analyze rock and soil elements.

The swivelable struts would allow the rover to tilt its front or rear up to change its camera viewfield or plant its element spectrometer against vertical rock faces - and even to continue functioning perfectly well if it accidentally turned upside down. Also despite its minuteness, its designers think it could do a good job of crawling 10 meters or more from the lander, picking its way between rocks.

Finally, each of the four Artemis landers would also carry a copy of weather sensors like Polar Lander's, and a pair of small cameras on the end of the sampling arm which could be used both for panoramas of the landing site and for microscopic closeups of soil and rock - including a search for fine soil layering in the walls of a trench, such as Polar Lander had intended to do in a search for shorter-term shifts in climate of the sort that are thought to have laid down the large-scale layering of dust-ice mixture which makes up the polar layered terrain.

Of all these ten Mars Scout concepts, Artemis may have the best chance of being accepted. It uses virtually no components or instruments that haven't already been developed for other missions, sharply limiting its development costs; but at the same time it allows a really sweeping survey of different kinds of Martian terrain, with a particular focus on questions having to do with life (as well as recovering Polar Lander's lost science).

Like the other concepts, though, it will face stiff competition from the additional torrent of new Scout concepts that are bound to flood in when the actual official proposal for ideas goes out next year.

And, of course, only one Scout mission concept will be picked for 2007, with only one more every four years.

However, it's already clear that NASA's extension of the competitive Discovery concept to Mars exploration is likely to produce missions as fruitful for that purpose as the Discovery missions have already proven to be for exploring the rest of the inner Solar System - and it seems increasingly likely that the same competitive, flexible concept will soon be applied to the selection of the more difficult and expensive missions to explore the giant planets and the outer Solar System.