Spectrophotometric properties of the nucleus of comet 67P/Churyumov-Gerasimenko from the OSIRIS instrument onboard the ROSETTA spacecraft

Abstract

Context. The European Space Agency Rosetta mission is orbiting the comet 67P/Churyumov-Gerasimenko since August 2014, and it is now in the escort phase of the comet. A large complement of scientific experiments designed to complete the most detailed study of a comet ever attempted are on board Rosetta. Aims. In this work we present the results on the 67P nucleus photometric and spectrophotometric properties derived from the OSIRIS imaging system, constituted of a Wide Angle Camera (WAC) and a Narrow Angle Camera (NAC). The observations presented here were performed in the July- beginning of August 2014 timeframe, during the approach phase, when OSIRIS was mapping the surface of the comet with several filters at different phase angles (1.3°–54°), and with a resolution up to 2.1 m/px. Methods. The OSIRIS images were processed with the OSIRIS standard pipeline, then converted in I/F radiance factor and corrected for the illumination conditions at each pixel using the Lommel-Seeliger’s disk law. Colour cubes of the surface have been hence produced by stacking registered and illumination corrected images. Furthermore photometric analysis was performed both on integrated photometry in several filters, and on disk resolved images acquired with the NAC orange filter, centred at 649 nm, using Hapke modelling. Results. The disk-averaged phase function of the 67P nucleus shows a strong opposition surge with a G parameter value of - 0.13±0.01, in the HG system formalism, and an absolute magnitude H^_{v}(1, 1, 0) = 15.74±0.02 mag. The integrated spectrophotometry in 20 filters covering the 250-1000 nm wavelength range shows a red spectral behaviour, without clear absorption bands except for a potential absorption centred at ∼ 290 nm, possibly due to SO^_{2} ice. The nucleus shows an important phase reddening, with disk- averaged spectral slopes increasing from 11%/(100 nm) to 16%/(100 nm) in the 1.3°–54° phase angle range. The geometric albedo of the comet is 6.5±0.2% at 649 nm, with local variations up to ∼ 16% in the Hapi region. From the disk resolved images the spectral slope has been computed, together with local spectrophotometry, and three distinct groups of regions (blue, moderately red, and red) have been identified. The Hapi region is the brightest, the bluest in term of spectral slope, and the most active surface on the comet. Local spectrophotometry puts in evidence an enhancement of the flux in the 700-750 nm associated to coma emissions.