 |
||||
|
||||
|
|
The lure of hematite: what makes the Red Planet red 04.11.01: 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 Earth. 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 Mars. "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 light." 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. Odyssey (a Martian spacecraft to be launched on April 7, 2001) 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. 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 sulfates. "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, no direct evidence for carbonates or sulfates anywhere on Mars has been found. 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.
|
||
|
||||