The Surface Composition and Thermal Properties of the Organic-Rich Surface of Comet 67P/Churyumov-Gerasimenko : VIRTIS/Rosetta Results (Invited)

VIRTIS is a dual channel spectrometer; VIRTIS-M (M for Mapper) is a hyper spectral imager covering a wide spectral range from 0.25 through 5μm. VIRTIS-M uses a slit and a scan mirror to generate images with spatial resolution of 250 μrad over a FOV of 3.7°. The second channel is VIRTIS-H (H for High-resolution), a point spectrometer with high spectral resolution (λ/∆λ=3000 @3μm) in the range 2-5 μm. The nucleus observations are performed in a wide range of conditions with spatial resolution varying from the initial 500m down to 2.5m. The surface temperature has been determined since the first distant observations when the nucleus filled one single VIRTIS-M pixel. On the mid of July 2014 from a distance of 15000km the mean surface temperature has been measured as 205±5K. This pointed to a surface structure largely covered by a porous crust, mainly devoid of water ice. Maximum temperature determined so far are as high as 230K on the subsolar point. The VIRTIS composition analysis has showed evidence of carbon-bearing compounds on the nucleus of the comet 67P/Churyumov-Gerasimenko. The very low reflectance of the nucleus (normal albedo of 0.060 ± 0.003 at 0.55 μm), the spectral slopes in VIS and IR ranges (5-25 and 1.5-5 % kÅ-1) and the broad absorption feature in the 2.9-3.6 µm range present across the entire illuminated surface, are compatible with opaque minerals associated with nonvolatile organic macromolecular materials: a complex mixture of various types of C-H and/or O-H chemical groups, with little contribution of N-H. Authors acknowledge the support from national funding agencies.