QUESTION:
Rocks at the Viking and Pathfinder sites all seem to have the general
appearance of vesicular basalt, at least to my "earth-trained" eye, and
things like the Yogi spectrum seem to support this. Discovery of more
evolved igneous rocks, or sedimentary rocks like carbonates, would seem to
make mars a much more "lively" target for geological exploration, and the
silica saturation of Barnacle Bill seems to have been a major coup. If the
rocks around the Sagan Station were, at least in part, deposited by a flood
several hundred million years ago, or even billions, they've had a LONG time
to sit there weathering. Is it conceivable that some aspect of Martian
aeolian weathering could, give enough time, preferentially erode certain
mineral grains in non-basaltic rocks, giving them a "vesiculated"
appearance?  Could some of these "basalts" really be VENTIFACTS, perhaps
with some sort of desert varnish?  Could the APXS "see through" such a rind?

ANSWER from Mike Mellon on August 13, 1997:
Even at the Viking landing sites, the exposed surface rocks have the look of
vesicular basalt. And since those missions, some scientists have suggested
that the observed pockets may be caused by some sort of erosion. The rocks
at the Pathfinder landing site are no surprise in this respect. To resolve
the question of rock type will require more information on rock mineralogy,
which simply is not available from the Pathfinder instruments.

I don't know if the APXS can "see" through a desert varnish-like chemical
coating. I know that there has been some concern regarding "seeing" through
dust coatings observed on some of the rocks.  The upcoming Mars Global
Surveyor mission (and orbiter) has a Thermal Emission Spectrometer onboard.
This instrument is capable of "seeing" through a thin chemical varnish (like
a desert varnish) to determine rock mineralogy.

Mike Mellon