Photometric properties of comet 67P/Churyumov-Gerasimenko from VIRTIS-M aboard Rosetta

Abstract

After a cruise phase of more than ten years the Rosetta spacecraft entered orbit around comet 67P/Churyumov-Gerasimenko (CG) on 6 August 2014. From this date the spacecraft began to escort the comet and will follow it till the end of the nominal mission (December 2015). This allowed very accurate nucleus imaging making 67P/CG the sixth comet to be directly observed from a spacecraft after 1P/Halley , 9P/Temple 1, 19P/Borrelly, 81P/Wild 2 and 103P/Hartley 2. The Rosetta orbiter carries eleven instruments and among these, VIRTIS, the Visible and Infrared Thermal Imaging Spectrometer (Coradini et al. 2007). This experiment is composed of two sensor heads, -M (Mapper) and -H (High resolution). The -H sensor is a high resolution punctual spectrometer mainly devoted to the study of the coma properties in the 1.88-5.03 micrometers, while -M produces hyperspectral images of the target in the 0.2-5.1 micrometers range with an angular resolution of 250 micr-rad x 250 micro-rad. Data from VIRTIS-M are crucial to assess nucleus spectrophotometric properties: on global scale with disk-integrated observations and, thanks to its imaging capabilities, at larger spatial resolutions with disk-resolved measurements. The latter case is of paramount importance in order to map compositional variability on the surface and possibly link it to sources of activity. This work focuses on VIRTIS-M data acquired from July 2014 up to February 37 2015. The analysis we performed is divided into two main subjects: full-disk photometry (sec. 2) and disk-resolved photometry (sec. 3). In sec. 2, we discuss rotational curves (sec. 2.1), full-disk phase curves (sec. 2.2), color ratios (sec. 2.3) and the derivation of the geometric albedo (sec. 2.4). In sec. 3, taking advantage of the large dataset acquired by VIRTIS-M we calculate a photometric reduction by means of a Hapke simplified model thus assessing the spectrophotometric properties of the surface. Also a zonal photometric reduction has been derived for four macro-regions, in order to better investigate compositional and morphological differences on the nucleus (sec. 3.5). In sec. 4 the full-disk and the disk-resolved dataset are used together to derive a complete phase curve of the comet. Finally a comparison to photometric properties of other comets is shown in sec. 5 and in sec. 6 a summary of the main findings is reported as well as a discussion of future developments of this work with new observations in the coming months.