There are several possible techniques by which the sample canister may end up being returned to Earth: if ISPP proves practical and the lander does not have to carry a heavy load of fuel when it is first launched from earth, the entire small Earth- return spacecraft might be launched from Mars' surface directly back to Earth. Otherwise, the ascent rocket could just launch the little sample-return container into Mars orbit for retrieval by another Mars orbiter and return to Earth.

At the Houston conference, R.T. Gamber of Lockheed Martin proposed a third idea: launching the sample-return container into solar orbit, and then having a sample-retrieval spacecraft (based on the Stardust comet probe) rendezvous with and retrieve it in solar orbit rather than Mars orbit, after which it would return to Earth.

Gamber says such a rendezvous and docking in the vast reaches of solar orbit would actually be much easier technically than a rendezvous in orbit around Mars. And since the sample-retrieval spacecraft wouldn't have to brake itself into orbit around Mars and later blast out of that orbit back to earth, it could be much lighter and cheaper, more than making up for the fact that the Mars ascent rocket would have to be somewhat more powerful to launch the tiny sample-return container to Mars escape velocity rather than just into orbit around Mars. There is, however, plenty of time to make such technical decisions.

McKay and Briggs do agree that the first sample-return mission should be to an area where there is evidence that water flowed or pooled on the surface of ancient Mars, and that the second one should be to an area where there is evidence that liquid water existed at some point in the past under Mars' surface.

The overall strategy of U.S. Mars exploration -- "Follow the Water" -- remains valid: only areas where liquid water did or does exist on Mars are ever likely to hold evidence of past life, and in the process of sampling such areas for evidence of fossils, we will automatically acquire a great amount of knowledge about Mars' non-biological geology whereas missions just aimed at returning geologically useful samples are unlikely to return any useful biological evidence. (Editor's note. If you disagree with this statement then please write a rebuttal for publishing on SpaceDaily.)

This, however, takes us to the third thread of the Briggs and McKay Mars exploration plan -- for those sample-return missions are by no means the end point of their plan.

First, there's another very important element of Mars exploration: finding out how to drill deep into Mars for deeply buried subsurface samples -- down into the thick subsurface permafrost layer (the "cryosphere"), and ultimately all the way down to the hypothesized layers where liquid water still exists in the pores of the rock, perhaps kilometers down. If "extant" (still-living) microbes still exist on Mars -- or at least frozen and well-preserved ones -- these are where they will be found.

But such drilling, as noted before, will obviously be extremely difficult -- definitely the hardest thing we've tried to do on Mars yet. Even the Apollo missions had trouble doing very shallow drilling on the Moon. This technology must be developed one step at a time.

The McKay and Briggs' plan calls first for the development of the ability to drill ten meters into the soil and return samples to the surface. [As an aside S. Rafeek of Honeybee Robotics proposed a drill based on Honeybee's subsurface drill for the cancelled Deep Space 4 comet mission that would be capable of doing just that, and this is yet another technology that might perhaps be tested on a 2005 Mars lander.]

The next stage about four years later, would be a lander capable of drilling fully 200 meters deep, perhaps something like Brian Wilcox's "Subsurface Explorer" -- a "self-hammering nail" about 2 meters long that uses an internal piledriver-type sliding hammer to pound its way down into the ground as much as 200 meters.

The main problem here, though, is that having the Explorer just analyze the rock it encounters with in-situ sensors is not enough -- we'll want to return samples from so deep underground to the surface in order to give them any kind of decent in-situ analysis for biological evidence - not to mention returning them to Earth.

Two techniques being considered are,

1. mixing small amounts of ground-up Martian rock with compressed liquid CO2 or xenon that would be pumped from the surface lander down all the way to the mole through a very thin tube and then pumped back up through another tube; and
2. actually having the mole construct its own permanent open shaft to the surface -- by pumping epoxy fluids down to it which it would then mix to extrude a solid hollow shaft -- after which sample plugs could be pneumatically shot back up the shaft to the surface (which might be possible with only a few dozen kg of epoxy). Clearly, however -- as several Conference speakers pointed out -- this will be a major technical undertaking., and it is unlikely that it will be attempted before 2009-11.

Obviously this is about the ultimate conceivable in unmanned Mars exploration -- if it's possible without a manned drill crew at all -- and it is indeed the end point of the Briggs-McKay plan; they don't see any attempt to do it before 2015 at the absolute earliest and more likely 2020-25 would be more realistic - if it can be done at all.

Before such an attempt is made, several earlier Mars landers would have to carry out detailed mapping of the subsurface strata at two or more candidate drilling sites, each using a rover which would lay out a 3-km line of active seismic sensors and explosive charges to map the places at which the subsurface liquid-water layer is closest to the surface and whre the consistency of the rock layers will make drilling easiest.

However, a new factor has recently intervened and led Briggs and McKay to modify their plan -- MGS' discovery of possible recent eruptions of liquid groundwater from only a few dozen or a few hundred meters below the surface.

It remains to be seen whether this really is liquid water and not, say, subsurface carbon dioxide -- but if it is water (perhaps kept liquid by high salinity), obviously the search for extant Martian microbes surviving in subsurface liquid water will be made tremendously easier.

Accordingly, Briggs and McKay have inserted a new type of near-term Mars mission into their plan: a rover capable of landing within a fairly short distance of such a site - even in rugged terrain, driving to it, and then either planting an anchor and rappelling itself down the rather steep slopes on which the eruptions have occurred to reach the surface runoff, or drilling several dozen meters down to reach the possible water layer.

In either case, it would then analyze the material it collected, using in-situ life-detection and organic material sensors, to look for signs of either living or dormant microbes. Needless to say, if it found such evidence, the site for the first Mars sample-return mission would be obvious.

This mission would certainly require some technological steps we have yet to make -- pinpoint landings, landing hazard avoidance, moderately long-range rovers, and rappelling operations and/or moderately deep sample drilling -- but it's also obviously easier than the much deeper drilling operations that Briggs and McKay had originally thought would be necessary, and they envision such a mission as being possible as early as 2007 or 2009.

One mission proposed at the Conference by JPL's C.J. Budney would involve a similar rover -- launched by a Delta as soon as 2007 -- which would drive to the edge of the great Valles Marineris and then rappel as much as 2 km down one of its slopes to examine the geological strata of Mars in this region.

Moreover, Mars Express and that possible follow up U.S. Mars orbiter mentioned above could make preliminary examinations of these sites from orbit to see if they are indeed areas where liquid water is close to the surface.

Besides subsurface radar sounding, one ingenious but fairly simple "oasis detector" proposed at the Conference by P.H. Smith would use the orbiter's near-IR mineral mapping spectrometer to constantly scan Mars' surface for any signs of recent water vapor release from subsurface vents, and then automatically and immediately photograph and mineral-map that spot in detail without waiting for commands from Earth.

Any Mars rovers will have to be carefully sterilized to avoid any risk whatsoever of contaminating the site, and Mars' water table, with Earth germs -- a problem which will become more and more serious as Mars exploration (especially subsurface exploration) progresses.

Nevertheless, it could be one of the most logical next steps for us to take after those preliminary orbiters and technology-testing soft landers have flown.

MGS' spectacular discovery, however, simply proves once again that Mars still unquestionably has plenty of surprises left to throw at us - and that planning out any exploration program for mars too far in advance is a recipe for trouble.

However, as I've said, some advance planning of the overall form of a properly advancing Mars program is necessary -- and Briggs' and McKay's plan received enough positive notice at the Conference that it seems very likely that something pretty similar to it will be adopted by NASA when its official new plan for Mars exploration is released in October or November.

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