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Mars northern polar latitudes are known to harbor an enhanced 3 μ 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 μ 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 μ m coupled with an absorption at λ ≥ 3. 8 μ m. This signature is homogeneously distributed over a high-albedo open ring surrounding the circumpolar low-albedo terrains between ∼ 68°N and 76°N and ∼ 0°E and 270°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 μ 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. • Using OMEGA data, we have identified a new spectral signature composed of a narrow 3 μ m band associated with a 4 μ m absorption. • This signature is observed exclusively over an open-ring area surrounding the North polar dunes between 68°N and 76°N. • This observation may indicate a specific physical or chemical alteration potentially involving sulfates in a polar environment. [ABSTRACT FROM AUTHOR]

In 2020, hydrogen chloride (HCl) in the gas phase was discovered in the atmosphere of Mars with the Atmospheric Chemistry Suite (ACS) onboard the Trace Gas Orbiter (TGO) mission (Korablev et al., 2021). Its volume mixing ratio (VMR) shows a seasonal increase of up to 5 ppbv during the perihelion season, followed by a sudden drop to undetectable levels, contradicting modelling estimates of the HCl lifetime of several months. In the Earth's stratosphere, heterogeneous uptake of HCl onto water ice is known to be a major sink for this species. This reaction is now also considered when modelling HCl abundances in the Martian atmosphere. In this work, we use simultaneous measurements of water ice and HCl obtained by the ACS instrument to find particular structures in the vertical profiles as detached gas layers at ice-free altitudes ("ice-holes"). From these particular examples we conclude that the heterogeneous uptake of HCl onto water ice operates on Mars and is a fast mechanism regulating the HCl abundance in the atmosphere of Mars. • Simultaneous vertical profiles of water ice and HCl analysed over 2.5 Martian years. • HCl gas layers at ice-free altitudes reveal local anticorrelation of HCl vs water ice. • Heterogeneous uptake of HCl onto water ice rapidly regulates HCl abundance. [ABSTRACT FROM AUTHOR]