OSIRIS observations of meter-size exposures of H2O ice at the surface of 67P/Churyumov-Gerasimenko and interpretation using laboratory experiments

Antoine Pommerol (1) , Nicholas Thomas (1) , Mohamed R. El-Maarry (1) , Maurizio Pajola (2) , Olivier Groussin (3) , Anne-Thérèse Auger (3) , Nilda Oklay (4) , Sonia Fornasier (5) , C. Feller (5) , Björn Davidsson (6) , A. Gracia-Berna (1) , B. Jost (1) , R. Marschall (1) , O. Poch (1) , M. A. Barucci (5) , Jean-Loup Bertaux (7) , F. La Forgia (8) , H. U. Keller (9, 10) , Ekkehard Kührt (10) , S. C. Lowry (11) , S. Mottola (10) , G. Naletto (12, 13, 2) , H. Sierks (4) , Cesare Barbieri (8) , Philippe L. Lamy (3) , R. Rodrigo (14, 15) , Detlef Koschny (16) , Hans Rickman (6, 17) , Jessica Agarwal (4) , Michael F. A'Hearn (18) , Ivano Bertini (2) , S. Boudreault (4) , Gabriele Cremonese (19) , Vania da Deppo (13) , Mariolino de Cecco (20) , Stefano Debei (12) , Carsten Güttler (4) , M. Fulle (21) , Pedro J. Gutiérrez (22) , Stubbe F. Hviid (10) , W. -H. Ip (23) , Laurent Jorda (3) , J. Knollenberg (10) , G. Kovacs (4) , J.-R. Kramm (4) , Michael Küppers (24) , Luisa M. Lara (22) , Monica Lazzarin (8) , Jose L. Lopez Moreno (22) , Francesco Marzari (8) , Harald Michalik (25) , Frank Preusker (10) , Frank Scholten (10) , Cécilia Tubiana (4) , Jean-Baptiste Vincent (4)

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

Since OSIRIS started acquiring high-resolution observations of the surface of the nucleus of comet 67P/Churyumov-Gerasimenko, over one hundred meter-sized bright spots have been identified in numerous types of geomorphologic regions, but mostly located in areas receiving low insolation. The bright spots are either clustered, in debris fields close to decameter-high cliffs, or isolated without structural relation to the surrounding terrain. They can be up to ten times brighter than the average surface of the comet at visible wavelengths and display a significantly bluer spectrum. They do not exhibit significant changes over a period of a few weeks. All these observations are consistent with exposure of water ice at the surface of boulders produced by dislocation of the weakly consolidated layers that cover large areas of the nucleus. Laboratory experiments show that under simulated comet surface conditions, analog samples acquire a vertical stratification with an uppermost porous mantle of refractory dust overlaying a layer of hard ice formed by recondensation or sintering under the insulating dust mantle. The evolution of the visible spectrophotometric properties of samples during sublimation is consistent with the contrasts of brightness and color seen at the surface of the nucleus. Clustered bright spots are formed by the collapse of overhangs that is triggered by mass wasting of deeper layers. Isolated spots might be the result of the emission of boulders at low velocity that are redepositioned in other regions.

Keywords: Image processing; comets

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

Antoine Pommerol, Nicholas Thomas, Mohamed R. El-Maarry, Maurizio Pajola, Olivier Groussin, et al.. OSIRIS observations of meter-size exposures of H2O ice at the surface of 67P/Churyumov-Gerasimenko and interpretation using laboratory experiments. Astronomy and Astrophysics - A&A, 2015, 583, A25 (16 p.). ⟨10.1051/0004-6361/201525977⟩. ⟨insu-01182702⟩