  Mars northern polar latitudes are known to harbor an enhanced 3 ${\mu}$m
spectral signature when observed from orbit. This may indicate a greater amount
of surface adsorbed or bound water, although it has not yet been possible to
easily reconcile orbital observations with ground measurements by Phoenix. Here
we re-analyzed OMEGA/Mars Express observations acquired during the Northern
summer to further characterize this 3 ${\mu}$m absorption band increase. We
identify the presence of a new specific spectral signature composed of an
additional narrow absorption feature centered at 3.03 ${\mu}$m coupled with an
absorption at ${\lambda}$ ${\geq}$ 3.8 ${\mu}$m. This signature is
homogeneously distributed over a high-albedo open ring surrounding the
circumpolar low-albedo terrains between ~ 68{\deg}N and 76{\deg}N and ~
0{\deg}E and 270{\deg}E. This location includes the Phoenix landing site. This
feature shows no time variability and can be confidently attributed to a
seasonally stable surface component. All together, the stability, spectral
shape and absence of significant correlation with other signatures in the 1 $-$
2.5 ${\mu}$m range discard interpretations relying on water ice or easily
exchangeable adsorbed water. Sulfates, notably anhydrite, provide interesting
comparisons to several sections of the spectrum. Analogies with Earth samples
also show that the spectral signature could result from a latitudinal
modification of the hydration state and/or grains size of salts contaminants.
While the exact full spectral shape cannot be easily reproduced, plausible
explanations to this observation seem to involve geologically recent water
alteration at high northern latitudes.
