It is, for instance, unclear whether a second Groundbreaking sample-return mission, for general insight into Mars' geological and climate history rather than for biological evidence, will be needed from a different location. It's also quite possible that -- as suggested in some of the example Pathways listed above -- some sufficiently interesting specialized site may be discovered by the coming near-future missions that even the very first Groundbreaker sample-return mission will be targeted for there.

Nor do we have any idea when the more sophisticated, specialized sample return missions specifically aimed at looking for fossil evidence will start to fly.

However, MEPAG does point out that the Smart Lander platform actually has enough spare capability to land a payload on Mars that it probably won't need to be modified to fly those more advanced sample-return missions. The projected payload for Groundbreaker is actually only about 400-600 kg (with 250-300 kg of it being the Mars Ascent Vehicle).

This is actually considerably less than the likely weight of the big MGE rover scheduled to be Smart Lander's first payload in 2009. A Smart Lander could easily carry both the Groundbreaker equipment and the 200-kg MER-class sample-collecting rover, with a one-kilometer range, originally planned for the Mars sample return mission.

Alternatively, as mentioned, a future sample-return lander could touch down near a big future MGE-type rover that had collected a variety of samples during an expedition across dozens of kilometers, and either the big rover or (if it's no longer functioning) a smaller "retrieval" rover sent out by the sample-return lander could load that collection into the sample-return lander for return to Earth.

Also, this same spacecraft might be used for another high-priority mission proposed by the Solar System Decadal Survey: landing in the great Aitken Basin on the Moon's far side to recover samples of rocks that may have been excavated from as deep as the Moon's mantle by the impact that created that huge basin. NASA is considering trying to fly this mission as early as 2009.

The Smart Lander, sample-retrieval orbiter -- and perhaps also the short-range small sample-collecting rover based on MER -- could be used for it with remarkably few modifications. The main one would be to replace the heat shield and parachute on the lander with a big ejectable retrorocket package to cancel out most of the Moon lander's descent speed, letting the Smart Lander's own throttleable rockets get rid of the final few hundred meters per second.

Finally, MEPAG does list the next big development in Mars exploration technology that should be developed after Groundbreaker: namely, systems capable of drilling really deeply into Mars' subsurface.

Any microbes still surviving on Mars must be buried far below the surface, in the layer of geothermally warmed liquid water thought to still exist within the pores of Mars' rock below the "cryosphere layer" of permafrost thought to occupy the upper kilometer or two of its surface even at the warmer equatorial regions. (They might be a good deal closer to the surface in any small geothermally warmed hot-spring area still existing on the planet, but even there we'd have to dig deep.)

And any mission to search for such extant microbes must be capable of drilling at least hundreds of meters deep into Mars' subsurface rock layers, and maybe kilometers -- using the low power levels available on an acceptably light lander.

The drill head must also be capable of carrying out fairly sophisticated analyses of the local rock using little in-situ instruments built into it, or else the drill must be capable of transporting small samples of rock all the way back up to analytical instruments on the surface lander itself. Obviously, if you actually want to return samples from Mars' deep subsurface to Earth, the latter is necessary.

JPL has been looking into the possibility of doing this with a "Subsurface Explorer" which would literally be a self-hammering nail: a thin pointed probe a meter or two long with a motorized weight that repeatedly slams down and then slides back up inside it, ramming the probe deeper and deeper into the surface while it trails a thin power and communications cable back up to the surface lander.

That cable might also include a thin two-way tube filled with high-pressure liquified carbon dioxide manufactured by the lander out of Mars' air itself, which could be pumped down to the probe and then pumped all the way back up carrying bits of rock sample back up to the lander.

Another possibility is the "inchworm digger" proposed by Honeybee Robotics, which would actually be capable not only of drilling down kilometers into the ground, but then reversing itself and using a second drill on its upper surface, plus retractable hooks capable of grabbing the rock on either side of the digger, to crawl all the way back up to the surface with its sample cache!

Equally obviously, such systems will take a great deal of technological development, given the notorious difficulty of drilling operations and the unexpected problems they routinely face. (So far, JPL's prototype for the Subsurface Explorer has drilled only a couple of meters during its tests.) MEPAG says that the first step, to be carried out between 2015 and 2020, is to launch a Mars lander carrying a one-way probe with in-situ instruments capable of drilling down several hundred meters into the ground.

On the positive side, such subsurface explorers don't even exist for Earth yet, and would obviously have huge potential uses in studying the geology of THIS world -- which means that the mining industry and the geological science community might both be willing to assist NASA in a very big way in developing them.

The main conclusion, however, is that the ever-changing U.S. Mars program is about to change dramatically yet again. We'll know more about this when NASA issues its final report on the new program's design in December.

But it's already clear both that it will be modified toward much greater flexibility to deal with new scientific findings from future Mars probes, and that the first sample-return mission has been considerably simplified -- which will probably strip it of any chance (always rather slim) that it will discover actual biological evidence, but also means that it may provide us with crucial new clues as to Mars' geological and climate development as early as 2016.