Spectrophotometric variegation of the layering in comet 67P/Churyumov-Gerasimenko as seen by OSIRIS

G. Tognon (1) , Sabrina Ferrari (1) , L. Penasa (1) , F. La Forgia (2) , M. Massironi (1, 3) , G. Naletto (2, 1, 4) , M. Lazzarin (2) , P. Cambianica (1) , A. Lucchetti (5) , M. Pajola (5) , F. Ferri (1) , C. Güttler (6) , B. Davidsson (7) , P. Deshapriya (8) , S. Fornasier (8) , S. Mottola (9) , I. Tóth (10) , H. Sierks (6) , Philippe Lamy (11, 12) , R. Rodrigo (13, 14) , D. Koschny (15) , C. Barbieri (2) , M. Barucci (8) , Jean-Loup Bertaux (12) , I. Bertini (2) , Dennis Bodewits (16) , Gabriele Cremonese (5) , V. da Deppo (17) , S. Debei (18, 19) , M. de Cecco (20) , J. Deller (6) , Michel Franceschi (3) , E. Frattin (2, 5) , M. Fulle (21) , Pedro J. Gutiérrez (6) , W.-H. Ip (22, 23) , H. Keller (9, 24) , Luisa M. Lara (25) , J. J. Lopez-Moreno (25) , F. Marzari (2) , V. Petropoulou (1) , X. Shi (6) , C. Tubiana (6)

1 CISAS - Centro di Ateneo di Studi e Attività Spaziali “Giuseppe Colombo”
2 Dipartimento di Fisica e Astronomia "Galileo Galilei"
3 Dipartimento di Geoscienze [Padova]
4 IFN - CNR Istituto di Fotonica e Nanotecnologie [Padova]
5 OAPD - INAF - Osservatorio Astronomico di Padova
6 MPS - Max-Planck-Institut für Sonnensystemforschung
7 JPL - Jet Propulsion Laboratory
8 LESIA (UMR_8109) - Laboratoire d'études spatiales et d'instrumentation en astrophysique
9 DLR Institut für Planetenforschung
10 Konkoly Observatory
11 LAM - Laboratoire d'Astrophysique de Marseille
12 PLANETO - LATMOS
13 ISSI - International Space Science Institute [Bern]
14 CAB - Centro de Astrobiologia [Madrid]
15 ESTEC - European Space Research and Technology Centre
16 Department of Physics [Auburn]
17 IFN - CNR Institute for Photonics and Nanotechnologies
18 Unipd - Università degli Studi di Padova = University of Padua
19 Department of Industrial Engineering [Padova]
20 University of Trento [Trento]
21 OAT - INAF - Osservatorio Astronomico di Trieste
22 SSI - Space Science Institute [Macau]
23 IANCU - Institute of Astronomy [Taiwan]
24 IGEP - Institut für Geophysik und Extraterrestrische Physik [Braunschweig]
25 IAA - Instituto de Astrofísica de Andalucía

Between August 2014 and September 2016, the ESA space mission Rosetta escorted comet 67P/Churyumov-Gerasimenko (67P) during its perihelion passage. The onboard OSIRIS Narrow Angle Camera (NAC) acquired high-resolution multispectral images of the cometary surface. These datasets allowed a characterization of the spectrophotometric variegation of the layering of the large lobe, correlated with the layer structural elevation. We perform a spectrophotometric characterization of the outcropping stratification of the small lobe of 67P as a function of its structural elevation, and consequently, a spectrophotometric comparison of the layered outcrops of the two lobes. We selected two sequences of post-perihelion OSIRIS NAC images (~2.4 au outbound tothe Sun), from which we built up two multispectral images, framing an extended geological section of the onion-like stratification of the small lobe. Then we classified the consolidated areas of the outcropping and the relative coarse deposits that were identified in the multispectral data with a two-class maximum likelihood method. For this, we defined the classes as a function of the structural elevation of the surface. As a result, we identified a brightness variegation of the surface reflectance that is correlated with the structural elevation. The outer class, which is located at higher elevations, appears darker than the inner class. This fits previously obtained results for the large lobe. The reflectance values of the nucleus of 67P tend to decrease with increasing structural elevation. The observed spectrophotometric variegation can be due to a different texture as well as to a different content of volatiles and refractories. We suggest that the outer outcrops appear darker because they have been exposed longer, and the inner outcrops appear brighter because the surface has been more effectively rejuvenated. We interpret this variegation as the result of an evolutionary process.

Domains: Sciences of the Universe [physics]; Astrophysics [astro-ph]

G. Tognon, Sabrina Ferrari, L. Penasa, F. La Forgia, M. Massironi, et al.. Spectrophotometric variegation of the layering in comet 67P/Churyumov-Gerasimenko as seen by OSIRIS. Astronomy and Astrophysics - A&A, 2019, 630, A16 (12 p.). ⟨10.1051/0004-6361/201834884⟩. ⟨insu-02293636⟩