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First Results From APXS-Analyses on Mars Pathfinder

Max Planck Institute of Chemistry
Mainz, Germany

Contact: Heinrich Wanke
Phone: +49-6131-305-231
Fax: +49-6131-371-290

4-12-97

First results from the APXS-analyses on Pathfinder: Mars more
Earth-like than previously assumed

A German-American research group present in this week's Science
magazine first results from the Alpha Proton X-ray Spectrometer
(APXS) on Pathfinder. The APXS, designed and built by researchers
at the Max Planck Institute of Chemistry in Mainz/Germany, in
cooperation with scientists of the US and Russia, provided new
insights into the chemical composition of Martian soil and rocks.

The Viking soil analyses and the information from the 12
meteorites assumed to be of Martian origin pointed to a rather
primitive surface composition of Mars. The first in-situ analyses
of rocks during the Pathfinder mission showed that the Martian
crust is probably as highly differentiated as the Earth's crust.

To obtain rock samples for chemical analyses from the surface of a
remote planet requires tools, which are very difficult to
accommodate on and to operate from light-weight landers. For this
reason, the cosmochemistry group at the Max-Planck-Institut fur
Chemie in Mainz has concentrated its efforts from the very
beginning on the design of a sensor with the capability to analyse
almost all major and minor elements in any sample with which it is
brought into contact. An instrument employing such a sensor -- the
Alpha Proton X-ray Spectrometer (APXS) -- was originally designed
and built by the MPI Mainz in cooperation with an international
group of researchers (including colleagues from the USA and
Russia) for a Russian mission to Mars (Mars-92). This mission was
postponed twice (Mars-94, Mars-96) and then unfortunately lost
soon after launch. Evidently, the group at the MPI Mainz was very
pleased when asked by the Pathfinder project to also provide such
an APX-Spectrometer for the NASA Mars Pathfinder mission to be
mounted on the Sojourner microrover.

APXS results will for the first time be published in an article in
SCIENCE, entitled "The Chemical Composition of Martian Soil and
Rocks Returned by the Mobile Alpha Proton X-ray Spectrometer:
Preliminary Results from the X-ray Mode" by R. Rieder, T.
Economou, H. Wanke, A. Turkevich, J. Crisp, J. Bruckner, G.
Dreibus and H.Y. McSween, Jr.

Soil analyses at the Pathfinder landing site gave almost identical
values for each of the five spots visited. These compositions are
also very similar to the ones measured at the two Viking landing
sites. The soil is obviously -- at least up to mean latitudes --
thoroughly homogenized by dust storms on a planet-wide scale. It
is rich in Mg and Fe, indicating mafic rocks as the dominant
surface component, diminuated by impacts and weathering, and by
reactions of the diminuated rock material with volcanic gases like
SO2 and HCl, respectively.

On top of this, Pathfinder yielded the first chemical analyses of
Martian rocks: The first rock analysed was called "Barnacle Bill".
Its composition was a big surprise to all Mars scientists, as it
turned out not to be mafic at all. The concentration of Mg was in
fact quite low, but Si and Al were high, indicating a felsic
composition as opposed to the general believe of a rather
primitive mafic composition of Mars' surface material. Much to the
contrary, this rock appears to be highly fractionated, similar to
typical highly evolved terrestrial crustal rocks, and this also
with respect to its high abundance of K, hinting at high
abundances of all incompatible elements.

Comparing the composition of the rocks at the Pathfinder landing
site -- which again appear quite similar to one another -- with the
composition of the soil into which they are embedded and which is
distinctly different from that of the rocks, it is obvious that
this soil cannot be made from rocks typical for this site:
Material from mafic geological provinces, compositionally similar
to rocks that we can hold in our hands (the Martian meteorites,
alias SNC meteorites), needs to be added after diminuation.

The researchers conclude that Mars must possess a variety of
geological provinces with very different compositions, and that
Mars is a planet, much more evolved than previously assumed. Thus,
mobility is a must for all future landing missions to Mars, if
Science wish to unravel the geological and chemical evolution of
our neighbour planet, which appears to be almost as complex as
that of our home planet -- the Earth.
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