QUESTION:
Some of the explanations for images of "two toned rocks" such as Yogi
suggest that the Martian wind-blown dust may be responsible. However, it
looks as though these rocks may have been buried up to the two tone
demarcation at one time, and then fell over due to some geological process.
The darker half of the boulder may have been the part that was buried. Is
this a plausible explanation?

ANSWER from Mary Urquhart on October 22, 1997:
Dust contamination may well be responsible for the "two toned rocks" such as
Yogi.  The evidence isn't just from color, but also from Pathfinder's APXS
measurement of Yogi's surface.  Preliminary results show that Yogi appears
to be contaminated by dust adhering to its surface.  Other effects may be
affecting the appearance of Yogi and other rocks as well such as direct vs.
diffuse lighting conditions.  Right now all results are *very* preliminary.

It is possible that another process is producing the apparent "two toned"
effect on some rocks at the Pathfinder landing site.  If a geologic process
has moved the rocks as you suggest, then more evidence other than the color
of the rocks will be needed.  The data from Pathfinder will take months, and
in many cases years, to properly analyize.  At this point it is too early
for anything more than preliminary results and working hypotheses.

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