  We have analyzed observations of the Acidalia hemisphere of Mars taken by the
Hubble Space Telescope's Near-Infrared Camera Multi-Object Spectrograph
(HST/NICMOS) during July of 1997 (Ls = 152 degrees, northern Martian summer).
The data consist of images at ~60 km/pixel resolution, using both narrow- and
medium-band filters specifically selected to allow us to study the hydration
state of the Martian surface. Calibration was performed by comparison to
Phobos-2 ISM observations of overlapping regions, and atmospheric gas
correction was performed by modeling the atmosphere for each pixel using a
line-by-line radiative transfer code coupled with the MOLA altimetry data. Our
results indicate the presence of at least three spectrally different
large-scale (>1000 km diameter) terrains corresponding to the dark regions of
northern Acidalia, the southern hemisphere classical dark terrain, and the
classical intermediate terrain adjacent to southern Acidalia. We also
identified two other spectrally unique terrains, corresponding to the northern
polar ice cap, and to the southern winter polar hood. Comparisons with mineral
spectra indicate the possibility of different H2O- or OH-bearing (i.e.,
hydroxides and/or hydrates) minerals existing both in northern Acidalia and in
the nearby intermediate albedo terrain. Hydrated minerals do not appear to be
spectrally important components of the southern hemisphere dark terrains imaged
by HST in 1997.
