QUESTION:
I seem to recall a pair of Viking orbiter photos a few years back that
showed a possible geyser eruption. Was that confirmed or discounted?

ANSWER from Mary Urquhart on September 30, 1997:
I have never heard of any Viking images which were ever believed to show
possible geyser activity.  Hydrothermal systems (which in certain
circumstances can produce geysers) almost assuredly existed on Mars and may
possibly even exist today. However, no direct evidence of hydrothermal
activity except alteration of the martian meteorites has been found.  Mars
Global Surveyor may find evidence of hydrothermal activity.  Seeing a geyser
is unlikely, even if they are present (which they probably aren't), but we
might find minerals on the surface that were produced by hydrothermal
processes.

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