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

Sonia Fornasier (1, 2) , P. H. Hasselmann (2, 3) , M.A. Barucci (2) , C. Feller (1, 2) , S. Besse (4) , C. Leyrat (2) , Luisa M. Lara (5) , Pedro J. Gutiérrez (5) , Nilda Oklay (6) , Cécilia Tubiana (6) , F. Scholten (7) , Holger Sierks (6) , Cesare Barbieri (8) , Philippe L. Lamy (9) , Rafael Rodrigo (10, 11) , Detlef Koschny (4) , Hans Rickman (12, 13) , Horst U. Keller (14) , Jessica Agarwal (6) , Michael F. A'Hearn (15) , Jean-Loup Bertaux (16) , Ivano Bertini (17) , Gabriele Cremonese (18) , Vania da Deppo (19) , Björn Davidsson (13) , Stefano Debei (20) , Mariolino de Cecco (21) , Marco Fulle (22) , Olivier Groussin (9) , Carsten Güttler (6) , Stubbe F. Hviid (7) , W.-H. Ip (23) , Laurent Jorda (9) , Jörg Knollenberg (7) , G. Kovacs (6) , J.-R. Kramm (6) , Ekkehard Kührt (7) , Michael Küppers (24) , Florangela La Forgia (8) , M. Lazzarin (8) , Jose J. Lopez Moreno (5) , Francesco Marzari (8) , K.-D. Matz (7) , Harald Michalik (25) , F. Moreno (5) , Stefano Mottola (7) , Giampiero Naletto (26, 17, 19) , M. Pajola (17) , A. Pommerol (27) , F. Preusker (7) , X. Shi (6) , Colin Snodgrass (6, 28) , Nicholas Thomas (27) , Jean-Baptiste Vincent (6)

1 UPD7 - Université Paris Diderot - Paris 7
2 LESIA - Laboratoire d'études spatiales et d'instrumentation en astrophysique
3 Observatorio Nacional [Rio de Janeiro]
4 RSSD - Research and Scientific Support Department, ESTEC
5 IAA - Instituto de Astrofísica de Andalucía
6 MPS - Max-Planck-Institut für Sonnensystemforschung
7 DLR Institute of Planetary Research
8 Dipartimento di Fisica e Astronomia "Galileo Galilei"
9 LAM - Laboratoire d'Astrophysique de Marseille
10 ISSI - International Space Science Institute [Bern]
11 CAB - Centro de Astrobiologia [Madrid]
12 CBK - Space Research Centre of Polish Academy of Sciences
13 Department of Physics and Astronomy [Uppsala]
14 IGEP - Institut für Geophysik und Extraterrestrische Physik [Braunschweig]
15 Department of Astronomy [College Park]
16 PLANETO - LATMOS
17 CISAS - Centro di Ateneo di Studi e Attività Spaziali “Giuseppe Colombo”
18 OAPD - INAF - Osservatorio Astronomico di Padova
19 IFN - CNR Institute for Photonics and Nanotechnologies
20 Department of Industrial Engineering [Padova]
21 University of Trento [Trento]
22 OAT - INAF - Osservatorio Astronomico di Trieste
23 Institute of Space Science [Taiwan]
24 Operations Department (ESAC)
25 Institut für Datentechnik und Kommunikationsnetze
26 DEI - Department of Information Engineering [Padova]
27 Physikalisches Institut [Bern]
28 School of Physical Sciences [Milton Keynes]

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 Hv (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 SO2 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.

Keywords: 67P/Churyumov-Gerasimenko; Data analysis; Photometry; comets

Domains: Earth and Planetary Astrophysics [astro-ph.EP]

Sonia Fornasier, P. H. Hasselmann, M.A. Barucci, C. Feller, S. Besse, et al.. Spectrophotometric properties of the nucleus of comet 67P/Churyumov-Gerasimenko from the OSIRIS instrument onboard the Rosetta spacecraft. Astronomy and Astrophysics - A&A, 2015, Rosetta mission results pre-perihelion, 583, pp.A30. ⟨10.1051/0004-6361/201525901⟩. ⟨insu-01205664⟩