Retrieving the distribution of comet 67P/Churyumov-Gerasimenko surface temperatures for individual Rosetta/VIRTIS-M spectra (pixel) by linear spectral unmixing - Method and first results -

Abstract

Knowledge of surface temperature and its variations as function of illumination conditions is key for understanding the thermodynamical properties, the chemical properties and the physical structure of the regolith (porosity, roughness) of planets and small bodies in the solar system. The surface temperature can be retrieved from near-infrared spectra at wavelengths where thermal emission becomes non-negligible with respect to the reflected components. At 5 micron, the longest wavelength measured by VIRTIS-M on the Rosetta mission which observed comet 67P/Churyumov-Gerasimenko (67P/C-G), the minimum brightness temperature that can be measured is ~150K (instrumental noise equivalent temperature). The usual technique is to fit a Planck function to each spectrum, providing one temperature per pixel. However, the calculation of a distribution of temperatures per pixel is justified by the fact that the local topography changes at all scales resulting in variable illumination conditions (variable incidence angle and shadow casting) within the area covered by each pixel. This causes a distribution of temperatures which turns out in a distribution of thermal emission contributions. Furthermore, the combination of different thermal emission contributions (the linear combination of several Planck curves) is not a Planck function. Consequently, fitting one Planck function to a spectrum results in retrieving a value for the brightness temperature that is not representative of thermophysical properties of the regolith, because it is not the average of all temperatures in the area covered by that pixel.