A tool left in out in the weather for a winter gives a first hint: iron will rust in the presence of water and oxygen. A rusted tool in the driest desert shows evidence of past contact with water. Iron, in fact, turns out to provide a gold mine of information about the past environment of dust, soil and rock, and the analysis of iron compounds can reveal much about the past environment of soil and rock.

"The process of iron oxide formation is very different depending on the environmental conditions when it formed," says G�star Klingelh�fer, of Johannes Gutenberg-University, in Mainz, Germany. "We know the pathways for the formation of different iron oxides so we can conclude what kind of weathering happened, whether there was fluid water and the amount of water."

Klingelh�fer and his colleagues have developed a miniaturized version of a complex instrument called a M�ssbauer spectrometer, designed do perform the needed chemical analysis. Rudolf L. M�ssbauer discovered the process behind the instrument in 1958 at the Max Planck Institute in Heidelberg.

If we shine a white light on a plant leaf, the light is absorbed and reemitted (we call that process reflection), but the chemicals in the leaf change the light. Our eyes can detect the change: a preponderance of green light coming from the illuminated leaf.

The M�ssbauer spectrometer works in a similar way, but with radiation of wavelengths we can't detect with our eyes. "It works like an x-ray machine at the dentist," Klingelh�fer says.

A typical, down-to-earth M�ssbauer spectrometer measures radiation transmitted through specimens instead of radiation reflected from a surface. But transmission M�ssbauer spectrometry only works on extremely thin samples, such as dust films and metal foil, which makes it impractical for use on Mars.

The M�ssbauer spectrometer on the Mars rovers instead analyzes reflected radiation. It contains a radioactive source that emits x-rays and gamma rays. It also contains a detector capable of sensing x-rays, gamma rays and electrons reemitted (reflected) by the surface of the sample.

The spectrometer also detects the angles at which the radiation leaves the sample. The detailed nature of the radiation or electrons emitted by the sample give researchers clues as to exactly which iron compounds the sample contains, and different iron compounds in turn provide clues about the environment under that existed in the distant past, when the minerals in the sample formed.

On Mars, the rover's robotic arm will place the instrument directly against a rock or the soil, both for physical stability and to place the radiation source and detector at a consistent distance of about one centimeter from the source.

The M�ssbauer spectrometer irradiates a sample about a centimeter and a half (about two-thirds of an inch) in diameter and collects data the radiation emitted from the sample for several hours. By analyzing these data, Klingelh�fer says, "we can detect iron oxides and iron-containing minerals and determine their properties. It's like a fingerprint measurement."

The instrument includes an independent microprocessor (located in the rover's Warm Electronics Box) for data acquisition and storage. The raw data require computer analysis on Earth, and the net result is a plot showing spikes for each particular type of mineral, particularly different oxides of iron.

"Identification of the mineral hematite without the presence of ilmenite is possible evidence for aqueous processes," Klingelh�fer says. "Identification of ilmenite by itself or with hematite is possible evidence for igneous (non-aqueous) processes." In addition, "minerals which exhibit magnetic properties show up with a very special pattern in the spectrum, so we can identify magnetic minerals very easily."

Once the M�ssbauer spectrometers arrive on the surface, they'll require calibration. "There are actually two calibration samples," Klingelh�fer says. "A reference sample inside the instrument itself allows internal calibration of the setup. And we do have on the spacecraft, in addition, a magnetite-rich rock for further calibration." One of the MER landing sites, Sinus Meridiani, contains the iron-rich gray hematite, according to data from the Mars Global Surveyor.

The European Space Agency's Beagle 2 lander, scheduled to arrive on the surface of Mars near the same time as the two Mars Exploration Rovers, also carries a M�ssbauer spectrometer made by the same researchers. The three spectrometers are the first ever flown to Mars, Klingelh�fer says. "We'll get information on the weathering products at the same time in three different locations," Klingelh�fer says. Sampling three sites will reduce the chances that local conditions at one site wind-blown dust covering samples, for example will hide the true nature of the soil or rock.

Klingelh�fer doesn't know when the M�ssbauer spectrometer will first be deployed, but he and his colleagues expect to begin analyzing the first data from the spectrometers on the rovers and Beagle 2 in January of 2004.

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