QUESTION:
Reference the  caption for  file marssoil.gif  under Current Science
Data.  The  chart in this  file compares Viking  soil with Sojourner
soil.  The caption concludes  that "Mars has a globally  homogeneous
soil  layer  probably  the  result  of  many  years of mixing by the
winds".

In Pathfinder  photos I've  noticed that  in those  areas where  the
surface dust has  been disturbed or removed, the underlying  soil is
darker and the the  layer of surface dust  appears to be very  thin.
Wouldn't this suggest that the soil is not necessarily  homogeneous?
Or is the  surface dust simply  a lighter (less  heavy) component of
the surface soil?
 
ANSWER from Mary Urquhart on September 13, 1997:
I  haven't  found  the  caption  you  refer  to (it is no longer under
Current Science  Data).   The difficulty  may be  that the homogeneous
soil layer mentioned in the caption  you quote refers to the drift  or
windblown  material  and  not   the  Pathfinder  site  "native"   soil
materials.  Regardless, please  remember that analysis that  you quote
(as all analysis to date has been) was preliminary.

Some of the  soil materials at  the Pathfinder site  are certainly not
identical to the  materials at the  Viking landing site.   The caption
for a comparison of Pathfinder APXS soil analysis results with results
from Viking (hap_2.jpg) states:

"Although the first APXS soil analysis (A-2) was reported to be almost
identical  to  Viking  soils,  subsequent  analyses  demonstrate  some
variability and  a few  significant differences  from Viking analyses.
Specifically,  soils  at  the  Pathfinder  site  generally have higher
aluminum and magnesium, and lower iron, chlorine, and sulfur."

The  soil   at  the   Pathfinder  landing   site  may   be  vertically
inhomogeneous (layered) even if the windblown materials have been well
mixed.   The dust  probably does  represent a  lighter (or uncemented)
layer of soil that can be mixed by wind.

Mary Urquhart
Laboratory for Atmospheric and Space Physics
University of Colorado at Boulder