Observations of the northern seasonal polar cap on Mars II: HiRISE photometric analysis of evolution of northern polar dunes in spring

We present an overview of our analyses of HiRISE observations of spring evolution of selected dune areas of the north polar erg. The north polar erg is covered annually by seasonal volatile ice layer, a mixture of CO2 and H2O with mineral dust contamination. In spring, this layer sublimes creating visually enigmatic phenomena, e.g. dark and bright fan-shaped deposits, dark–bright–dark bandings, dark down-slope streaks, and seasonal polygonal cracks. Similar phenomena in southern polar areas are believed to be related to the specific process of solid-state greenhouse effect. In the north, it is currently unclear if the solid-state greenhouse effect is able to explain all the observed phenomena especially because the increased influence of H2O on the time scales of this process has not yet been quantified. HiRISE observations of our selected locations show that the ground exhibits a temporal behaviour similar to the one observed in the southern polar areas: a brightening phase starting close to the spring equinox with a subsequent darkening towards summer solstice. The resolution of HiRISE enabled us to study dunes and substrate individually and even distinguish between different developments on windward and slip face sides of single dunes. Differences in the seasonal evolution between steep slip faces and flatter substrate and windward sides of dunes have been identified and compared to CRISM data of CO2 and H2O distributions on dunes. We also observe small scale dark blotches that appear in early observations and tend to sustain a low reflectivity throughout the spring. These blotches can be regarded as the analogue of dark fan deposits in southern polar areas, leading us to the conclusion that both martian polar areas follow similar spring evolutions.

Spectroscopy and detectability of liquid brines on mars

Recent geomorphological observations as well as chemical and thermodynamic studies demonstrate that liquid water should be stable today on the Martian surface at some times of the day. In Martian conditions, brines would be particularly more stable than pure water because salts can depress the freezing point and lower the evaporation rate of water. Despite this evidence, no clear spectral signature of liquid has been observed so far by the hyperspectral imaging spectrometers OMEGA and CRISM. However, past spectral analysis lacks a good characterization of brines׳ spectral signatures. This study thus aims to determine how liquid brines can be detected on Mars by spectroscopy. In this way, laboratory experiments were performed for reproducing hydration and dehydration cycles of various brines while measuring their spectral signatures. The resulting spectra first reveal a very similar spectral evolution for the various brine types and pure water, with the main difference observed at the end of the dehydration with the crystallization of various hydrated minerals from brines. The main characteristic of this spectral behavior is an important decoupling between the evolution of albedo and hydration bands depths. During most of the wetting/drying processes, spectra usually display a low albedo associated with shallow water absorption band depths. Strong water absorption band depth and high albedo are respectively only observed when the surface is very wet and when the surface is very dry. These experiments can thus explain why the currently active Martian features attributed to the action of a liquid are only associated with low albedo and very weak spectral signatures. Hydration experiments also reveal that deliquescence occurs easily even at low temperature and moderate soil water vapor pressure and could thus cause seasonal darkening on Mars. These experiments demonstrate that the absence of water absorptions in CRISM in the middle afternoon does not rule out water activity and suggest future spectral investigations to identify water on the Martian surface.