On Mars, three instruments will work together
to perform remote analysis of the rock and soil that the Mars rovers encounter.
The Mössbauer spectrometer specializes in detecting
iron compounds. The miniTES reads infrared radiation. And the
Alpha-Particle-X-Ray Spectrometer, or APXS
measures a range of chemical elements in a sample.
|Mini-TES from the side, with the main
electronics board in place.
"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
|A view of the APXS sensor
head. Its detectors can be seen in the center of the
chamber. To the left and the right of the detectors
are door flaps that open during use and close to keep
out martian dust.
Credit: Max Planck Institute
How Does It Work?
| In this APXS
graphic, the red spectrum shows excitation with alpha
particles (PIXE) and x rays (XRF). Click image for larger version. Excitation with
only x rays is seen in the blue spectrum. In this case,
a sleeve was mounted around the curium source holder
that contained thin Al foils to absorb alpha particles.
The suppression of the alpha particles reduced the
signal of low Z elements drastically (fraction of x-ray
excitation in percent is given together with element
label). Higher Z elements beyond Fe are only excited
by plutonium x-ray lines.
Credit: Rudi Rieder, Max Planck Institute
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
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
"From these spectra, data on the abundance of individual
elements in the respective sample (a rock or a spot of soil)
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
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
sample that the Mössbauer spectrometer uses. The APXS team
conducted pre-flight calibration on a wide variety of test
|The rock abrasion tool,
Credit: NASA/ JPL/ Cornell/Honeybee Robotics
"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.
Next stop: Mars, Rieder says. "I hope we
get a chance to spend a week with one rock only: first analysis
dust removed, and then with the RAT grinding away several consecutive
layers. I believe that in this way we may learn a lot about
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
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