Temperature and reflectance derivation from VIRTIS-H observations of 67P

A specific thermal emission model is applied to observations of 67P/Churyumov-Gerasimenko surface by the high-resolution channel of VIRTIS. Signal inversion provides both an effective surface temperature (averaged inside the FoV) and a reflectance spectrum corrected from thermal emission. Details of the organic material band at 3.2 μm [1] and longer wavelengths can then be studied at resolution R ~ 1500-3000 with increased contrast and accuracy

Fresh emplacement of hydrated sodium chloride on Ceres from ascending salty fluids

The surface and internal structure of Ceres show evidence of a global process of aqueous alteration, indicating the existence of an ocean in the past. However, it is not clear whether part of this ocean is still present and whether residual fluids are still circulating in the dwarf planet. These fluids may be exposed in a geologically young surface, and the most promising site to verify the occurrence of present fluids on Ceres is Cerealia Facula dome, in Occator crater. This very young facula exhibits minerals that are relatively rare in our Solar System, the formation of which requires the presence of liquid water in combination with hydrothermal activity. Here we report the discovery of hydrated sodium chloride on Cerealia Facula. These newly identified chloride salts are concentrated on the top of the dome, close to a system of radial fractures. The spatial distribution of the hydrated phase suggests that chloride salts are the solid residue of deep brines that reached the surface only recently, or are still ascending. These salts are very efficient in maintaining Ceres's warm internal temperature and lowering the eutectic temperature of the brines, in which case ascending salty fluids may exist in Ceres today

Mapping of thermal properties of comet 67P/C-G and temporal variations

The long-term evolution of the surfaces of comets depends mainly on the erosion rate that is driven by the thermal properties of the regolith and the sub-surface material. Following the diurnal and the seasonal thermal cycles, dust and gas are released progressively, increasing the erosion process. The amount of dust released depends on the surface and subsurface temperatures and thus on thermal inertia and bulk composition.The ESA's Rosetta spacecraft has followed the comet 67P/Churyumov-Gerasimenko over several months from 4 AU to 1.28 AU heliocentric distance, and the VIRTIS/Rosetta imaging infrared spectrometer was capable of detecting the thermal emission of the surface longward of 3 microns.The surface temperature was mapped over a large fraction of the nucleus and was previously used to derive thermal inertia of the main geomorphological units.In this presentation, we now focus on two different aspects: (1) We aim to present a complete detailed map of the thermal inertia by combining measurements of similar areas obtained at different viewing angles ; and (2) we track the evolution of the local thermal properties derived over months when the comet was moving towards perihelion. We then discuss and compare our results with the textural features observed at the surface

Laboratory simulations of the Vis-NIR spectra of comet 67P using sub-µm sized cosmochemical analogues

International audienceLaboratory spectral measurements of relevant analogue materials were performed in the framework of the Rosetta mission in order to explain the surface spectral properties of comet 67P. Fine powders of coal, iron sulphides, silicates and their mixtures were prepared and their spectra measured in the Vis-IR range. These spectra are compared to a reference spectrum of 67P nucleus obtained with the VIRTIS/Rosetta instrument up to 2.7 μm, excluding the organics band centred at 3.2 μm. The species used are known to be chemical analogues for cometary materials which could be present at the surface of 67P. Grain sizes of the powders range from tens of nanometres to hundreds of micrometres. Some of the mixtures studied here actually reach the very low reflectance level observed by VIRTIS on 67P. The best match is provided by a mixture of sub-micron coal, pyrrhotite, and silicates. Grain sizes are in agreement with the sizes of the dust particles detected by the GIADA, MIDAS and COSIMA instruments on board Rosetta. The coal used in the experiment is responsible for the spectral slope in the visible and infrared ranges. Pyrrhotite, which is strongly absorbing, is responsible for the low albedo observed in the NIR. The darkest components dominate the spectra, especially within intimate mixtures. Depending on sample preparation, pyrrhotite can coat the coal and silicate aggregates. Such coating effects can affect the spectra as much as particle size. In contrast, silicates seem to play a minor role