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

Abstract

Context. The Rosetta mission of the European Space Agency has been orbiting the comet 67P/Churyumov-Gerasimenko (67P) since August 2014 and is now in its escort phase. A large complement of scientific experiments designed to complete the most detailed study of a comet ever attempted are onboard Rosetta. Aims. We present results for the photometric and spectrophotometric properties of the nucleus of 67P derived from the OSIRIS imaging system, which consists of a Wide Angle Camera (WAC) and a Narrow Angle Camera (NAC). The observations presented here were performed during July and the beginning of August 2014, during the approach phase, when OSIRIS was mapping the surface of the comet with several filters at different phase angles (1.3• -54• ). The resolution reached up to 2.1 m/px. Methods. The OSIRIS images were processed with the OSIRIS standard pipeline, then converted into I/F radiance factors and corrected for the illumination conditions at each pixel using the Lommel-Seeliger disk law. Color cubes of the surface were produced by stacking registered and illumination-corrected images. Furthermore, photometric analysis was performed both on disk-averaged photometry in several filters and on disk-resolved images acquired with the NAC orange filter, centered at 649 nm, using Hapke modeling. Results. The disk-averaged phase function of the nucleus of 67P 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 behavior, without clear absorption bands except for a potential absorption centered at ∼290 nm that is possibly due to SO 2 ice. The nucleus shows strong 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 of up to ∼16% in the Hapi region. From the disk-resolved images we computed the spectral slope together with local spectrophotometry and identified three distinct groups of regions (blue, moderately red, and red). The Hapi region is the brightest, the bluest in term of spectral slope, and the most active surface on the comet. Local spectrophotometry shows an enhancement of the flux in the 700−750 nm that is associated with coma emissions.

The morphological, elastic, and electric properties of dust aggregates in comets: A close look at COSIMA/Rosetta’s data on dust in comet 67P/Churyumov-Gerasimenko

“…The porosity of aggregate particles decreases with their size if D < 3, while the porosity of BPCA particles with D ≈ 3 is approximately constant at po ≈ 0.85, if their constituent grains hit and stick on contact (Mukai et al 1992;Kimura et al 2016). This is in accordance with po = 0.87 derived for the outermost dust mantle of comet 67P/Churyumov-Gerasimenko from the so-called Hapke's modeling of a spectral variation in the reflectance of the nucleus (Fornasier et al 2015).…”

Surface evolution of the Anhur region on comet 67P/Churyumov-Gerasimenko from high-resolution OSIRIS images

“…We used radiance factor images produced by the OSIRIS standard pipeline up to level 3B, following the reduction steps described in Tubiana et al (2015). Images were corrected for bias, flat field, geometric distortion, calibrated to absolute spectral radiance (in Wm −2 nm −1 sr −1 ), and finally converted into radiance factor (I/F, where I is the observed spectral radiance, and F is the incoming solar spectral irradiance at the heliocentric distance of the comet, divided by π), as described in Fornasier et al (2015). 2, 7, 8, and 10.…”

Evidence for geologic processes on comets

“…In particular, there were no discernible compositional differences between the two ends of Hartley 2 or among any morphologic regions on Tempel 1. Although Rosetta has much higher spatial resolution and longer orbital coverage, the rendezvous with C‐G yielded very similar compositional results: small variations in overall red spectral slopes [ Capaccioni et al ., ], discrete ice deposits seen both in color images [ Pommerol et al ., ; Fornasier et al ., ] and visible near‐infrared (0.2–5 µm) spectroscopy [ De Sanctis et al ., ], and a more or less uniform broad (2.9–3.6 µm) feature potentially related to organics [ Capaccioni et al ., ]. Like the other comets, C‐G does not appear to have significant compositional variability across its surface, despite the presence of substantial morphologic diversity [ Thomas et al ., ] and two distinct lobes.…”