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Weathered Rocks from a FIDO Mars Rover Test Site

fido_rover.jpg (244605 bytes) FIDO is the prototype rover for a Mars mission, complete with an Athena-like science payload. The focus of Mars rover missions is to traverse across sites that may have been ancient shallow seas or lakes. The theme is to acquire data and samples to test the hypothesis that Mars once supported warm, wet conditions and that organic materials and perhaps microorganisms developed in such environments.

The Silver Lake area, located north of Baker in the Mojave Desert, CA, is an excellent analog for rover trials because lacustrine, fluvial, and groundwater processes were extensive during wet Pleistocene glacial epochs and occasionally during the Holocene. We have examined a suite of rocks collected at the site to anticipate the problems expected in field analyses on surfaces of similar rocks (as opposed to surfaces that have been prepared in the laboratory for optimal analysis conditions).

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Individual cobbles used in our analyses were collected from surfaces of alluvial fans on the northwestern side of a playa. They typically have three distinct coatings. Tops of cobbles have a dark-gray, glossy, thin coating of desert varnish. Bottoms, which were sitting in fine-grained aeolian accretion mantles, have a red varnish. Both coatings are inferred to be fixed to rock surfaces by bacterial action [4]. Also, some surfaces are coated by thin caliche deposits. mojave_rock6.jpg (64738 bytes) mojave_rock4.jpg (61960 bytes)

 

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Above is a Raman spectrum showing dolomite and calcite peaks from the weathered surface of a carbonate cobble. (left) A cobble of gneiss yielded a spectral peaks of quartz, K-feldspar, and plagioclase, and minor epidote, anatase, biotite, and clinozoisite.

(above) On the same cobble, a dark-coated surface yielded the same major minerals, plus anatase, calcite, and a phyllosilicate (saponite);  the red-coated surface, however, yielded only hematite, quartz, and feldspar of poor (low S/N) quality.

Conclusion: Given the different rock types and surface coatings present on rocks at the Silver Lake site, we can make several generalizations:

(1) Rocks such as the carbonates, which cannot sustain a stable surface and thus do not develop varnish coatings, present no obstacles to the determination of their mineralogy. High fluorescence backgrounds occur on some weathered carbonate surfaces, however, and we are investigating their causes.

(2) Rocks that develop dark gray or black coatings yield a good representation of their mineralogy as long as the coatings are thin. These glossy surfaces have also been abraded by wind blown silt, and the coatings tend to be only a few m m thick except where there is local microrelief in the surface.

(3) Coated surfaces where the coatings are thicker than a few tens of m m present a challenge to determining mineralogy, but spectral patterns of the coatings provide insight into important atmosphere-rock interactions.

(4) The quality of the spectra along a traverse also reflects the nature of the rock surface; fresh surfaces yield excellent spectra (high S/N) whereas oxidized and otherwise coated surfaces typically yield poorer S/N spectra (Fig. 2).

(5) Important additional information is obtained from multiple vs. single-mineral spectra. The minerals that occur together within a given spectrum also provide information about mineral associations, and therefore rock textures. Our method of multi-point traverses is key to exploiting this information.

Reference: Jolliff B. L., Wang A., Kuebler E. K., Haskin A. L, Arvidson R., "Raman Analysis of Weathered Rocks from the FIDO Mars Rover Test Site, Silver Lake, California", Thirty Lunar and Planetary Science Conference, (1999) Huston, Texas, USA.

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