The landing sites of one of the MER rovers is at Gusev crater at about -15 lat. and 185 W long. As can be seen in the first map on this page:

Water on Mars
http://www.lanl.gov/worldview/news/photos/mars.shtml

this lies within the area of high hydrogen content near the equator.

This is also discussed in this report of Natalie Cabrol et.al. on the MER mission:

Synthesis of the Testable Hypotheses for Gusev.
http://marsoweb.nas.nasa.gov/mer2003/doc/arcadia_03/talks/Cabrol/cabrol.pdf

This landing site also lies within a near equatorial area that I suggest shows *seasonal* differences in the hydrogen amounts which would suggests this is free water, not chemically bound water, which could become liquid for some part of the day:

Microsymposium 36, MS063, 2002
SEASONAL VARIATIONS OF SUBSURFACE HYDROGEN AS SEEN BY HIGH ENERGY NEUTRON DETECTOR, MARS ODISSEY.
M. L. Litvak, I. G. Mitrofanov, A.S. Kozyrev, A.B. Sanin, W. Boynton, C. Shinohara, D. Hamara, R. S. Saunders.
http://www.planetary.brown.edu/planetary/documents/Micro_36/Abstracts/063_Litvak_etal.pdf

Bob Clark