Identification of a new spectral signature at 3 $\mu$m over Martian northern high latitudes: implications for surface composition

Abstract

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 $\lambda$ $\ge$ 3.8 $\mu$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 $\mu$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.

Submitted to Icarus

References in corpus (6)

• Recovery of surface reflectance spectra and evaluation of the optical depth of aerosols in the near-IR using a Monte-Carlo approach: Application to the OMEGA observations of high latitude regions of Mars
• Water in the Martian regolith from OMEGA/Mars Express
• Strong variability of Martian water ice clouds during dust storms revealed from ExoMars Trace Gas Orbiter/NOMAD
• Mars Express measurements of surface albedo changes over 2004 - 2010
• Yearly and seasonal variations of low albedo surfaces on Mars in the OMEGA/MEx dataset: Constraints on aerosols properties and dust deposits
• Martian water ice clouds during the 2018 global dust storm as observed by the ACS-MIR channel onboard the Trace Gas Orbiter

Cited by in corpus (1)

• Diurnal and Seasonal Mapping of Martian Ices With EMIRS