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Weathered Rocks from a FIDO Mars
Rover Test Site
||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).
|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 . Also, some surfaces are coated by thin caliche
|Above is a Raman spectrum showing dolomite and
calcite peaks from the weathered surface of a carbonate
||(left) A cobble
of gneiss yielded a spectral peaks of quartz, K-feldspar, and plagioclase,
and minor epidote, anatase, biotite, and clinozoisite.
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.
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
(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
(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.
(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.
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,
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