Photometric behaviour of 67P/Churyumov-Gerasimenko and analysis of its pre-perihelion diurnal variations

Abstract

Spectral parameters measured by the Rosetta/VIRTIS imaging spectrometer on the nucleus of comet 67P/Churyumov-Gerasimenko were studied as a function of incidence, emission and phase angles. A statistical analysis was undertaken for a number of spectral parameters: reflectance at seven wavelengths (0.55, 0.75, 1.2, 1.7, 2.0, 2.8 and 4.0 μm), the depth and centre of the 3.2 μm organics band, and the visible and infrared slopes. Different photometric behaviours were assessed over the entire comet surface, on the four different comet macro-regions (head, neck, body and bottom) and for different local solar times. Phase functions are on average both spatially and temporally constant, indicating an overall homogeneity of optical properties. Comparison between phase functions of 67P/Churyumov-Gerasimenko and of other comets explored by space missions indicates a variation of phase curve steepness among comets, larger than expected for objects having similar albedo. This can be ascribed to different surface roughness, in turn due to different levels of surface erosion. Band depth appears to be independent of phase angle, which may help in constraining the surface composition. However, its band centre slightly moves to longer wavelengths with increasing incidence angle, possibly due to exposure of low amounts of water ice due to a weak cometary activity at increasing temperatures. Water ice abundance is also the main driver of the photometric behaviour of the visible and infrared slope, since larger water ice abundance causes a slope decrease and a phase reddening increase in the neck region and during the nighttime.