Scientists think Mars has a bad case of rust. Martian soil is full of
iron-bearing compounds that, over the eons, have reacted with trace amounts
of oxygen and water vapor in Mars’ atmosphere to form iron oxide — the same
chemical that covers innumerable rusty nails in garages and workshops on

The word “rust” conjures up images of things that are red –like Mars and
old nails– but not all iron oxide is the same color. Here on Earth a
gray-hued variety of iron oxide, a mineral called hematite, can precipitate
in hot springs or in standing pools of water.

Gray hematite is not the sort of rust you might expect to find on a
desert-dry planet like Mars. But perhaps Mars wasn’t always as dry as it is
today. There are many signs of ancient or hidden water on the Red Planet
including flash-flood gullies, sedimentary layers … and hematite.

In 1998, an infrared spectrometer on NASA’s Mars Global Surveyor (MGS)
spacecraft detected a substantial deposit of gray hematite near the Martian
equator, in a 500 km-wide region called Sinus Meridiani. The discovery
raised the tantalizing possibility that hot springs were once active on

“We believe that the gray hematite is very strong evidence that water was
once present in that area,” said Victoria Hamilton, a planetary geologist at
Arizona State University (ASU). “We think the deposit is fairly old. It was
buried, perhaps, for several hundred million years or more and now it’s
being exposed by wind erosion.”

Gray hematite has the same chemical formula (Fe2O3) as its rusty-red cousin,
but a different crystalline structure. Red rust is fine and powdery; typical
grains are hundreds of nanometers to a few microns across. Gray hematite
crystals are larger, like grains of sand.

“Red and gray iron oxides on Mars are really just different forms of the
same mineral,” explained Hamilton. “If you ground up the gray hematite into
a fine powder it would turn red because the smaller grains scatter red

The coarse-grained structure of gray hematite is important, says ASU’s Jack
Farmer, head of the NASA Astrobiology Institute’s Mars Focus Group, because
“to get that kind of coarsening of the crystallinity, you would need to have
a reasonable amount of water available” where the hematite formed.

The link between water and gray hematite makes the so-called “Hematite Site”
(Sinus Meridiani) an alluring target for future Mars landers as well as for
remote sensing instruments on the 2001 Mars Odyssey spacecraft — slated to
launch on April 7th.

Odyssey will carry an infrared imaging camera called THEMIS (short for
Thermal Emission Imaging System) that can identify surface minerals from
orbit by analyzing their spectral “fingerprints.”

“It turns out that all materials vibrate at the atomic scale,” explains
Hamilton. “For minerals, the rate at which the atoms vibrate corresponds to
the thermal infrared part of the electromagnetic spectrum, between about 5
and 50 microns. Those are longer wavelengths than what our eyes can see.”
Every mineral has a unique infrared spectrum that identifies it as surely as
the fingerprints of a human being, she added.

THEMIS is a “next-generation” instrument that can capture sharper images
than TES, the Thermal Emission Spectrometer that is orbiting Mars now aboard
Mars Global Surveyor. THEMIS will be able to discern the mineral content of
geological features only 100 meters across, compared to 3 km for TES.

Of many candidate landing sites for NASA’s 2003 Mars Exploration Rovers, the
Sinus Meridiani region is one of the most intriguing to scientists. THEMIS
data could help planners pinpoint the best places to land, especially if the
maps reveal deposits of other aqueous minerals such as carbonates or

“The interesting thing about carbonates and sulfates,” says Phil
Christensen, principal investigator for THEMIS, “is that these materials can
be better (than hematite) at preserving a fossil record. Some of them, like
carbonates, would also indicate that standing bodies of water were present
on the surface.” Hematite minerals, on the other hand, might have been
formed by hydrothermal water deep underground.

So far, instruments on MGS have found no direct evidence for carbonates or
sulfates anywhere on Mars. The absence of such aqueous minerals is a mystery
if liquid Martian water — in the form of lakes, rivers or oceans — was
indeed abundant in the planet’s geological past.

Christensen cautions that the spatial resolution of TES on Mars Global
Surveyor might not have been good enough to detect small deposits of
carbonates. With its superior resolution, THEMIS has a better chance. For
example, TES would not have detected the carbonate layers in Earth’s Grand
Canyon, but THEMIS would.

Until someone finds signs of carbonates or sulfates on Mars, perhaps in some
future THEMIS image, gray hematite remains the best known mineral signpost
for ancient Martian water.

The hematite makes scientists wonder, was there once a Martian equivalent of
Yellowstone National Park where steaming hot springs formed hematite-laden
pools? And are underground springs still present there today? Human
exploration of the Red Planet could hinge on the answers. And there may be
no better place to find out than Sinus Meridiani, where the lure of hematite
is powerful indeed.