MARSIS uses two 20-meter long antennas to transmit radar signals at four extremely long wavelengths (62-167 meters), capable -- preliminary tests indicate -- of piercing as much as 4 to 5 km of Martian rock and returning detectable echoes from the top of a subsurface layer of permafrost, or from the border between a permafrost layer and a layer of rock containing liquid water.

It will make such vertical profiles of most of the crust of Mars with an accuracy of 50-100 meters. Its main intention is to look for deeply buried Martian water -- but it would obviously have no trouble locating areas where it is shallowly buried.

Indeed, it may not be the only radar sounding instrument to arrive at Mars in Dec. 2003. "Nozomi" (formerly called "Planet-B") -- the small Mars probe launched by Japan in 1998 -- ran into the same curse that has afflicted most recent Mars probes: while firing its onboard engine to propel it from an elongated Earth orbit onto its path to Mars, it developed a leaky engine valve, narrowly depriving it of the fuel it would need to brake into Mars orbit if it had continued there on a direct path.

However, its ground controllers hastily devised an alternate route -- requiring it to stay in solar orbit for 4 extra years (during which it will make one gravity-assist flyby of Earth) -- after which it can approach Mars in 2003 when the planet is somewhat closer to the Sun, allowing Nozomi to enter its elongated Mars orbit with a smaller burn than was originally necessary.

Nozomi was not designed to work this long; indeed, its S-band radio transmitter failed last year. But its backup X-band transmitter is still working -- and if nothing else fails, it will still be able to carry out its planned mission.

Nozomi's experiments are designed to study Mars' upper atmosphere and ionosphere -- but one of its instruments, a plasma wave detector, can double as a radar sounder capable of piercing 100-200 meters of Martian rock.

Its total area coverage would be much less than that of Mars Express -- but if briny liquid water really is as close to the surface in some spots as the MGS photos indicate, it too could detect it.

And it is also possible that if the U.S. picks another Mars orbiter rather than a long-range surface rover as its 2003 Mars mission (a decision due in a few weeks), it too might carry a MARSIS-type sounder. Such an instrument was not mentioned in the original list of possible experiments suggested for it -- but that was before the MGS revelations.

Finally, NASA is beginning to pay serious attention to a recent proposal by MARSIS' experimenters for a follow-up Mars radar orbiter -- "MEEM" (Mars Environment and Evolution Mission), a lightweight Delta-launched polar Mars orbiter carrying both an improved deep Mars subsurface sounder and a higher-frequency synthetic-aperture radar system similar to that flown to Venus on the Magellan spacecraft, capable of constructing actual 2-D "photos" of Mars' surface with a resolution of only 50 meters.

Tests indicate that such shorter-wavelength radar could pierce only 5-10 meters into the Martian surface. But this would be enough to solve one of the most annoying problems in orbital Mars exploration: the fact (solidly confirmed by MGS) that over the eons, Mars' wind storms have covered over, with meters of fine windblown dust and sand, most of its surface features left over from its earliest days -- when it had a dense atmosphere and very possibly large amounts of surface liquid water.

Similar SAR photos of Earth's deserts taken from orbit have easily pierced 5 meters of sand to reveal ancient dried-up river beds buried for thousands of years, whose existence had been totally unexpected -- and similar maps of ancient Martian watercourses could provide us with invaluable information both on Mars' overall history and on promising places to look for fossil (or even still-existing) microbial life.

NASA thinks highly enough of MEEM that it was recently officially mentioned in NASA's newly revised overall "Roadmap" map plan for near-future Solar System exploration. The earliest it could have been launched to Mars under NASA's earlier plan was 2007 -- but it would be quite possible to accelerate this to 2005, if NASA does decide on a program of extensive Mars aerial reconnaissance before focusing on more landers.

Finally, high-resolution IR spectroscopy may also have a role to play. Mars Express (and the proposed 2003 U.S. orbiter) will both carry near-IR mapping spectrometers capable of mapping a somewhat different set of water-created surface minerals with a resolution of only about 0.5 km -- and future THEMIS-type instruments, using longer-wavelength IR, could look for small spots on the surface where there are unusual concentrations of water vapor (or traces of other gases -- such as hydrogen and methane -- which are usually found only where there are living subsurface bacteria).

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MARS WATER SPECIAL