Comet 67P/C-G: Surface Temperatures as Derived by Rosetta/VIRTIS in the Mapping Phase

Abstract:

We show spatially-resolved temperature maps of comet 67P/Churyumov-Gerasimenko, main target of the ESA Rosetta spacecraft, as derived from infrared hyperspectral images acquired by the VIRTIS imaging spectrometer onboard the Rosetta Orbiter in the Mapping phase carried out in August and September 2014. These data were obtained during the Mapping phase, at variable spatial resolution (from roughly 100 m/px down to ~10 m/px), illumination conditions, and heliocentric distances (spanning the range from 3.6 to 3.4 AU).

VIRTIS infrared spectra in the range longward of ~4 µm are affected by the thermal emission of the comet, hence the measured radiance in that spectral region can be used to determine surface temperatures and spectral emissivities by means of temperature-retrieval algorithms. We use a Bayesian approach that was previously applied to Rosetta/VIRTIS data obtained during the close flybys of asteroids 2678 Steins and 21 Lutetia, as well as to the entire dataset of infrared data acquired by the VIR mapping spectrometer aboard the Dawn spacecraft during its orbital phase at asteroid Vesta in 2011-2012.

The VIRTIS instrument onboard Rosetta is not sensitive to physical temperatures on the nightside of the comet, where the signal is considerably low. Typically, ~170 K is the minimum temperature that allows one to retrieve surface temperatures while preserving high accuracies. On the other hand, for a given local solar time, the maximum temperature depends on the solar incidence angle and on surface properties such as thermal inertia and albedo.

The availability of spatially-resolved, accurate temperature observations, significantly spaced out in local solar time, provides clues to the physical structure of specific surface units, which complements the mineralogical investigation based on imaging spectroscopy data collected at shorter wavelengths.

Acknowledgements

This work is supported by the Italian Space Agency (ASI), ASI-INAF Contract n. I/024/12/0. We acknowledge funding from the French Space Agency (CNES) and the German Aerospace Center (DLR). Support of the Rosetta and VIRTIS Science, Instrument, and Operation Teams is gratefully acknowledged.