"The APXS contains radioactive sources and detectors for the measurement of x-rays and alpha particles emitted by the sample," says Rudi Rieder, of the Max Planck Institute for Chemistry, Germany and APXS Payload Element Lead.

"Basically [the three spectrometers] will all support each other: minerals are easier to reconstruct if you know what ingredients are available, what atomic environment persists at the site of iron atoms, and what infrared characteristics have been found. And knowing the mineral composition of a sample will help to reconstruct its origin and history," Rieder says.

All of this analysis is aimed at detecting chemical changes wrought by water, probably long in the past. "Unfortunately, we have no way of directly determining the current water content and our capabilities of detecting carbonates - characteristic of [water-borne] sediments - are limited to comparatively high concentrations," Rieder says.

How Does It Work?
Pressed against a rock or a small patch of soil by the Instrument Deployment Device (IDD), the rover's robotic arm, the APXS bombards the sample (about 38 mm, or 1.5 inches, in diameter) with radiation. Atoms in the sample absorb the radiation, then emit x-rays and alpha particles. The APXS reads both kinds of reemitted radiation, which emerge with energy levels characteristic of different elements. "This is similar to the emission of visible light with characteristic colors when elements are heated, for example in a flame or in fireworks." Rieder says.

Alpha particles work better for exciting relatively light elements, such as sodium, magnesium, aluminum, and silicon. X-rays are more effective in exciting heavier elements, such as iron, cobalt and nickel. The relative effectiveness of the two kinds of radiation crosses over at chromium, which responds as well to either kind of radiation. The combination of the two types of radiation makes the APXS a very sensitive instrument, Rieder says. While the APXS on the MERs conceptually resembles similar instruments on the Mars Pathfinder, it includes a number of improvements: improved sensitivity to x-rays, and stronger resistance to electromagnetic interference and to noise caused by carbon dioxide in the atmosphere.

The brains of the APSX reside in the rover's Warm Electronics Box, where data are assembled into spectra.

"From these spectra, data on the abundance of individual elements in the respective sample (a rock or a spot of soil) can be deduced," Rieder says. "In the case of the x-ray spectra this is comparatively straightforward, as the information about individual elements is contained in well-defined areas of the spectrum in the form of well-resolved, almost line-shaped 'peaks.' Interpretation of the alpha spectra is a little bit more elaborate, as these spectra consist of a superposition of more box-shaped distributions."

The APXS has two small, protective doors, and the researchers can conduct simple functional tests by reading the radiation reflected from the special coating on the inside of the doors. The APXS can also read the magnetite-rich rock sample flown on the rovers for further calibration. This is the same calibration sample that the M�ssbauer spectrometer uses. The APXS team conducted pre-flight calibration on a wide variety of test samples.

"Signal strength in the alpha mode is orders of magnitude lower than in the x-ray mode and therefore much longer measuring periods (at least 10 hours) are required, whereas in the x-ray mode, data with good statistical precision can be obtained within 15 to 30 minutes," Rieder says. Once the spectra are collected, all data analysis will take place back on Earth.

What's Next?
Next stop: Mars, Rieder says. "I hope we get a chance to spend a week with one rock only: first analysis as-is, then with dust removed, and then with the RAT grinding away several consecutive layers. I believe that in this way we may learn a lot about the current weathering processes that ultimately lead to the formation of soil, and about the original constitution of the source material: bulk rock."

"Digging into soil and taking analysis at different depths is another item on my list. Things may be hidden under the surface that 'bleach out when exposed to sunshine.' And a subtle change in chemistry may give us some indirect hunch about current water.

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