The Martian North Polar regions exhibits a strong spectral variability, with high level of surface hydration as derived from the 3μm band (Jouglet et al. 2007 ; Milliken et al., 2007 ; Audouard et al., 2014) and the presence of hydrated minerals, mostly interpreted as Ca-sulfates, surrounding the perennial polar cap and highlighted by the 1.9 μm BD (e.g., Langevin et al., 2005 ; Poulet et al., 2008 ; Horgan et al., 2009 ; Massé et al., 2012).
A specific feature in the 3 μm region (the narrow 3 μm band), which is correlated with a 4 μm feature, highlight an open-ring shaped region surrounding the perennial North Polar Car (between ~68°N and ~76°N). While the origin of this signature has not yet been firmly identified, it comes from a recent and potentially still ongoing process, and may involved salts and relate to some sort of modification of the physical (grain size) or chemical (hydration level) properties of the high-latitudes ice-free soils (Stcherbinine et al., 2021).
Maps are in geotiff or fits format, which extent goes from 50°N to 90°N.