First Results From The APXS-Analyses On Pathfinder: Mars More Earth-Like Than Previously Assumed

December 04, 1997

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 für 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. Wänke, A. Turkevich, J. Crisp, J. Brückner, 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 posess 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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