This report models the water cycle on Mars at high obliquity and concludes the GRS high hydrogen readings seen in some equatorial regions, including the Gusev and Meridiani MER rover landing sites, could be due to near surface ice deposits:

Explaining the Mid-Latitude Ice Deposits with a General Circulation Model.
M. A. Mischna1,2, M.I. Richardson2, R.J. Wilson3 and A. Zent4, 1Department of Earth and Space Sciences, University of California, Los
Angeles, CA 90095-1567 (mischna@****.edu) 2California Institute of Technology, Pasadena, CA 91125 (mir@****.edu) 3Geophysical Fluid Dynamics Laboratory, Box 308, Princeton, NJ 08542 (rjw@gfdl.noaa.gov) 4Ames Research Center, MS 245-3, Moffett Field, CA 94035 (aaron.p.zent@****.gov)
"Conclusions: We have examined the influence of
a regolith on the water cycle with a focus on high
obliquity periods on Mars. Our findings show that
while the regolith will almost certainly interact with
the atmosphere initially, it is only a transient effect,
and ice will form on the surface once the regolith is
effectively isolated from the atmosphere. These low
latitude deposits could conceivably be ice deposits
formed at high obliquity and are certainly presently out
of thermal equilibrium, but remain due to the insulating
effect of a dust lag."
http://www.lpi.usra.edu/meetings/lpsc2004/pdf/1861.pdf

Bob Clark