Dark side of comet 67P/Churyumov-Gerasimenko in Aug.–Oct. 2014

MIRO/Rosetta continuum observations of polar night in the southern regions

¹ Jet Propulsion Laboratory/California Institute of Technology, 4800 Oak Grove Dr., Pasadena, California 91109, USA
e-mail: mathieu.choukroun@jpl.nasa.gov
² University of Massachusetts, 619 Lederle Graduate Research Tower, Amherst, MA 01003, USA
³ LESIA, Observatoire de Paris, CNRS, UPMC, Université Paris-Diderot, 5 place Jules Janssen, 92195 Meudon, France
⁴ LERMA, Observatoire de Paris, PSL Research University, UPMC, CNRS, 61 avenue de l’Observatoire, 75014 Paris, France
⁵ National Central University, Jhongli, 32001 Taoyuan City, Taiwan
⁶ Max Planck Institut für Sonnensystemforschung, Justus von Liebig Weg 3, 37077 Göttingen, Germany
⁷ Planetary Science Institute, 1700 East Fort Lowell, Suite 106, Tucson, AZ 85719, USA
⁸ Laboratoire d’Astrophysique de Marseille, BP 8, 13376 Marseille Cedex 12, France
⁹ Institute for Geophysics and Extraterrestrial Physics, 38106 TU Braunschweig, Germany

Received: 25 March 2015
Accepted: 24 September 2015

Abstract

The high obliquity (~50°) of comet 67P/Churyumov-Gerasimenko (67P) 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 during the period August-October 2014. Before these observations, the southern polar regions had been in darkness for approximately five years. Subsurface temperatures in the range 25−50 K are measured. Thermal model calculations of the nucleus near-surface temperatures carried out over the orbit of 67P, 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, loose, regolith-like surface. These values are similar to those derived elsewhere on the nucleus. A large difference in the brightness temperatures measured by the two MIRO continuum channels is tentatively attributed to dielectric properties that differ significantly from the sunlit side, within the first few tens of centimeters. This is suggestive of the presence of ice(s) within the MIRO depths of investigation in the southern polar regions. These regions started to receive sunlight in May of 2015, and refinements of the shape model in these regions, as well as continuing MIRO observations of 67P, will allow refining these results and reveal the thermal properties and potential ice content of the southern regions in more detail.