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 two weeks 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.