QUESTION:
This is a quote from recent findings: "Some partially buried rocks, he said,
are a whitish color and may actually be layers of a hard material called
caliche, a calcium carbonate rock material common in Arizona." I have a
problem with why the APXS hasn't been calibrated for carbon. CaCO3 should
have been high on your list for searching. Why just guess at caliche instead
of actually detecting it?

ANSWER from Wendy Calvin on July 14, 1997:
While in prinicple the APXS can measure the abundance of lighter elements
such as carbon, in practice this is a very difficult measurement to make. 
Then energy of alpha particles scattered by carbon and lighter elements is
very low and if the background signal is too high it will be difficult to
pull out the signal from the background.  The APXS will have to sit on a
rock ("integrate the signal") for a very long time.  I'm sure the team plans
to do this at some point.

ANSWER from Wendy Calvin on August 4, 1997:
Indeed measurements of lighter elements such as carbon are a high priority,
as one of the primary questions in Mars geology is that we expect a large
amount of carbonates and have not yet found any evidence for them.  One
problem with the APXS is trying to measure the abundance of carbon in an
atmosphere that is almost exclusively carbon dioxide.  This requires careful
control measurements and does not just "fall out" of the measurement of any
given rock.  Also, light elements (such as C, or Na) have a weaker signal in
the APXS measurements so that noise is an increased problem in determining
their abundances.