Seasonal temperature effects on comet 67P/Churyumov-Gerasimenko as inferred from Rosetta/VIRTIS

Abstract

The Visible InfraRed Thermal Imaging Spectrometer (VIRTIS) on board the orbiter of the ESA Rosetta mission operates in the spectral range 0.25-5.1 μm. VIRTIS is continually used to map and investigate the surface composition of the comet nucleus in its uppermost layer within a depth of a few tens of microns. Moreover, the temperature of these surface layers can be mapped utilizing VIRTIS’ long wavelength spectral measurements. To do this, the VIRTIS team uses a Bayesian approach to nonlinear inversion, which was already adopted in the past for other small bodies. The superficial layer sampled by VIRTIS is significantly different from the much thicker layers sampled by the Microwave Instrument for the Rosetta Orbiter (MIRO). Furthermore, VIRTIS is not sensitive to physical temperatures on the nightside of the comet, where the signal is considerably low. Typically, 170 K is the minimum surface temperature that can be measured while preserving small formal errors (<1 K on retrieved temperatures). Temperatures below ~170 K have increasingly lower accuracy, and should be carefully interpreted.

In previous papers, we have provided a thorough description of surface temperature maps and thermal properties of comet 67P/Churyumov-Gerasimenko as derived from infrared data acquired by Rosetta/VIRTIS in the early Mapping phase carried out in August and September 2014. We divided those maps into intervals of true local solar time and we evaluated, for the available coverage, the average daily temperatures. In addition, we studied correlations between temperature and albedo at different wavelengths, and derived diurnal temperature profiles for several local sites, representative of as many macro-regions of the comet. In this new work, we focus on effects determined by the season and the heliocentric distance. Due to the low thermal inertia of the nucleus surface material, the surface temperature is essentially dominated by the instantaneous value of the solar incidence angle. Small values of this angle result in high surface temperatures, but, due to comet 67P’s obliquity, for each location the smallest achievable value of insolation angle depends on the season. During 2014, VIRTIS’ visible and infrared measurements covered only the northern regions of the cometary surface and the equatorial belt became gradually unveiled, while the southern region is going to be revealed from 2015 onwards.