Hematite is an iron oxide very common on Mars; but the coarse-grained form almost certainly must have crystallized in some kind of surface water reservoir on that part of Mars -- either a big lake (the favored theory) or a cluster of volcanic hot springs.

The area corresponds with what MGS' camera shows to be "a smooth, layered, friable unit that is interpreted to be sedimentary in origin."

A series of papers dealt with this "Hematite Patch", which has now been picked as the backup landing site for the next U.S. Mars Surveyor Lander since it's a promising place to search for microfossils.

Ken Edgett and Michael Malin reported that MGS photos showed the surface to be covered with an odd pattern of parallel ridges that "appear to be an erosional form" rather than dunes -- and since it is densely covered with ancient, eroded meteor craters while revealing few fresh ones, C. Kelsey concludes that "the implication is that the area has been exposed, perhaps exhumed, very recently" by erosive processes.

TES chief experimenter Phillip R. Christensen reported that MGS has now located a second, much smaller patch of coarse hematite -- about 60 km square -- filling a crater in the Aram Chaos area which seems to have served as an ancient lake.

The puzzle is why more such areas haven't been found; Kelsey suggests that "the reason that lakebed deposits of ancient evaporites are so rarely found by TES is that [wind-blown] sediments have covered most lakebeds, and we need the lucky circumstance of recent removal to expose an intact layer" -- since, if such a deposit of soft evaporite minerals is exposed for more than a few hundred million years, meteor impacts and Martian winds will erode it away too.

Since this area is smoother than the Isidis Highlands that are the next prime target for a Mars lander, plus the new emphasis on arranging a safe landing after the Mars Polar Lander failure may make its final selection more likely -- especially since the lander is currently scheduled to carry two high-resolution soil microscopes and a "Mossbauer spectrometer" designed to study Martian iron minerals in detail.

This instrument should be able to clearly identify forms of crystallized iron minerals that are deposited only by living bacteria on Earth -- and if any such minerals are mixed in with this deposit, it might find them too.

But the TES instrument has so far completely failed to locate its most important goal: carbonate minerals such as calcite and gypsum. These were expected to exist in large amounts on Mars.

We know from the erosion rates of the oldest Martian craters and other geological features that early Mars had a dense carbon dioxide atmosphere which it later lost -- and while some of it may have escaped into space, the belief is that most of it eventually dissolved in Mars' early supply of surface liquid water or near-surface hot springs, and then reacted with the planet's silicate rocks to form carbonates - the fate of most of Earth's initial CO2 supply.

And, in fact, most of the handful of Martian meteorites found on Earth (including ALH84001) do contain significant amounts of carbonates. But Earth-based telescopic searches for carbonates have been frustratingly ambiguous -- and now MGS' instrument has failed to detect them at all, indicating that they must form a much smaller proportion of the surface than had been thought.

Why?