MIRO Observations of the Southern Regions of 67P/Churyumov-Gerasimenko: From Deep Polar Night, to Summer at Perihelion.

Abstract:

The high obliquity (50°) of 67P/Churyumov-Gerasimenko is responsible for a long-lasting winter polar night in the Southern regions of the nucleus. We report observations made with the submillimeter and millimeter continuum channels of the Microwave Instrument onboard the Rosetta Orbiter (MIRO) of the thermal emission from these regions since rendez-vous with the comet in August 2014. Prior to these observations, the Southern Polar Regions had been in darkness for approximately 5 years. Subsurface temperatures in the range 25-50 K were measured early in the mission. Thermal model calculations of the nucleus near-surface temperatures carried out over the 67P/C-G orbit, coupled with radiative transfer calculations of the MIRO channels brightness temperatures, suggest that these regions have a thermal inertia within the range 10-60 J.m^-2.K^-1.s^-0.5. Such low thermal inertia values are consistent with a highly porous, dusty, regolith-like surface. These values are comparable to those derived elsewhere on the nucleus. Differences between the two channels are tentatively attributed to dielectric properties that differ significantly from the sunlit side, within the first few tens of centimeters. This was suggestive of the presence of ice(s) within the MIRO depths of investigation in the Southern Polar Regions early in the mission. As Rosetta pursues its observations of 67P/Churyumov-Gerasimenko through and after perihelion, we are revisiting analysis of MIRO thermal emission data of these regions, and their interpretation.