The scattering phase function of comet 67P/Churyumov-Gerasimenko coma as seen from the Rosetta/OSIRIS instrument

Ivano Bertini (1) , Florangela La Forgia (1) , Cécilia Tubiana (2) , Carsten Güttler (2) , Marco Fulle (3) , Fernando Moreno (4) , Elisa Frattin (5, 1) , Gabor Kovacs (2) , Maurizio Pajola (6, 7) , Holger Sierks (2) , Cesare Barbieri (1, 7) , Philippe L. Lamy (8) , Rafael Rodrigo (9, 10) , Detlef Koschny (11) , Hans Rickman (12, 13) , Horst Uwe Keller (14, 15) , Jessica Agarwal (2) , Michael F. A'Hearn (2, 16) , Maria Antonella Barucci (17) , Jean-Loup Bertaux (18) , Dennis Bodewits (16) , Gabriele Cremonese (5) , Vania da Deppo (19) , Björn Davidsson (20) , Stefano Debei (21) , Mariolino de Cecco (22) , Esther Drolshagen (23) , Sabrina Ferrari (7) , Francesca Ferri (7) , Sonia Fornasier (17) , Adeline Gicquel (2, 20) , Olivier Groussin (8) , Pedro J. Gutiérrez (4) , Pedro H. Hasselmann (17) , Stubbe F. Hviid (15) , Wing-Huen Ip (24, 25) , Laurent Jorda (8) , Jörg Knollenberg (15) , J.-Rainer Kramm (2) , Ekkehard Kührt (15) , Michael Küppers (26) , Luisa Lara (4) , Monica Lazzarin (1) , Zhong-Yi Lin (25) , José J. Lopez Moreno (4) , Alice Lucchetti (5) , Francesco Marzari (1) , Matteo Massironi (7, 27) , Stefano Mottola (15) , Giampiero Naletto (7, 19, 28) , Nilda Oklay (15) , Theresa Ott (29) , Luca Penasa (7) , Nicolas Thomas (30) , Jean-Baptiste Vincent (2)

1 Dipartimento di Fisica e Astronomia "Galileo Galilei"
2 MPS - Max-Planck-Institut für Sonnensystemforschung
3 OAT - INAF - Osservatorio Astronomico di Trieste
4 IAA - Instituto de Astrofísica de Andalucía
5 OAPD - INAF - Osservatorio Astronomico di Padova
6 ARC - NASA Ames Research Center
7 CISAS - Centro di Ateneo di Studi e Attività Spaziali “Giuseppe Colombo”
8 LAM - Laboratoire d'Astrophysique de Marseille
9 ISSI - International Space Science Institute [Bern]
10 CAB - Centro de Astrobiologia [Madrid]
11 ESTEC - European Space Research and Technology Centre
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 DLR Institut für Planetenforschung
16 Department of Astronomy [College Park]
17 LESIA - Laboratoire d'études spatiales et d'instrumentation en astrophysique
18 PLANETO - LATMOS
19 IFN - CNR Institute for Photonics and Nanotechnologies
20 JPL - Jet Propulsion Laboratory
21 Department of Industrial Engineering [Padova]
22 University of Trento [Trento]
23 OFFIS - Carl Von Ossietzky Universität Oldenburg = Carl von Ossietzky University of Oldenburg
24 SSI - Space Science Institute [Macau]
25 Institute of Space Science [Taiwan]
26 ESAC - European Space Astronomy Centre
27 Dipartimento di Geoscienze [Padova]
28 DEI - Department of Information Engineering [Padova]
29 University of Oldenburg
30 Physikalisches Institut [Bern]

The study of dust, the most abundant material in cometary nuclei, is pivotal in understanding the original materials forming the solar system. Measuring the coma phase function provides a tool to investigate the nature of cometary dust. Rosetta/OSIRIS sampled the coma phase function of comet 67P/Churyumov-Gerasimenko, covering a large phase angle range in a small amount of time. Twelve series were acquired in March 2015 - February 2016 for this scientific purpose. This data allowed, after straylight removal, measuring the phase function shape, its reddening, and phase reddening while varying heliocentric and nucleocentric distances. Despite small dissimilarities within different series, we found a constant overall shape. The reflectance has a u-shape with minimum at intermediate phase angles, reaching similar values at the smallest and largest phase angle sampled. The comparison with cometary phase functions in literature indicates OSIRIS curves being consistent with the ones found in many other single comets. The dust has a negligible phase reddening at α < 90°, indicating a coma dominated by single scattering. We measured a reddening of [11-14]%/100 nm between 376 nm and 744 nm. No trend with heliocentric or nucleocentric distance was found, indicating the coma doesn’t change its spectrum with time. These results are consistent with single coma grains and close-nucleus coma photometric results. Comparison with nucleus photometry indicates a different backscattering phase function shape and similar reddening values only at α < 30°. At larger phase angles the nucleus becomes significantly redder than the coma.

Keywords: Scattering; 67P/Churyumov-Gerasimenko; Data analysis; Photometry; Image processing; Comets

Domains: Earth and Planetary Astrophysics [astro-ph.EP]; Solar and Stellar Astrophysics [astro-ph.SR]; Physics [physics]; Astrophysics [astro-ph]

Ivano Bertini, Florangela La Forgia, Cécilia Tubiana, Carsten Güttler, Marco Fulle, et al.. The scattering phase function of comet 67P/Churyumov-Gerasimenko coma as seen from the Rosetta/OSIRIS instrument. Monthly Notices of the Royal Astronomical Society, 2017, 469 (Suppl_2), pp.S404-S415. ⟨10.1093/mnras/stx1850⟩. ⟨insu-01574406⟩