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

¹ Physikalisches Institut, University of Bern, Sidlerstrasse 5, 3012 Bern, Switzerland
e-mail: antoine.pommerol@space.unibe.ch
² Centro di Ateneo di Studi ed Attivitá Spaziali, Giuseppe Colombo (CISAS), University of Padova, 35131 Padova, Italy
³ Aix-Marseille Université, CNRS, LAM, UMR 7326, 38 rue Frédéric Joliot-Curie, 13388 Marseille, France
⁴ Max-Planck-Institut für Sonnensystemforschung, Justus-von-Liebig-Weg, 3, 37077 Göttingen, Germany
⁵ LESIA, Obs. de Paris, CNRS, Univ. Paris 06, Univ. Paris-Diderot, 5 place J. Janssen, 92195 Meudon, France
⁶ Department of Physics and Astronomy, Uppsala University, 75120 Uppsala, Sweden
⁷ LATMOS, CNRS/UVSQ/IPSL, 11 boulevard d’Alembert, 78280 Guyancourt, France
⁸ Department of Physics and Astronomy, University of Padova, Vicolo dell’Osservatorio 3, 35122 Padova, Italy
⁹ Deutsches Zentrum für Luft- und Raumfahrt (DLR), Institut für Planetenforschung, Rutherfordstraße 2, 12489 Berlin, Germany
¹⁰ Institut für Geophysik und extraterrestrische Physik (IGEP), Technische Universität Braunschweig, Mendelssohnstr. 3, 38106 Braunschweig, Germany
¹¹ The University of Kent, School of Physical Sciences, Canterbury, Kent, CT2 7NZ, UK
¹² Department of Information Engineering, University of Padova, via Gradenigo 6/B, 35131 Padova, Italy
¹³ CNR-IFN UOS Padova LUXOR, via Trasea, 7, 35131 Padova , Italy
¹⁴ International Space Science Institute, Hallerstraße 6, 3012 Bern, Switzerland
¹⁵ Centro de Astrobiología, CSIC-INTA, 28850 Torrejón de Ardoz, Madrid, Spain
¹⁶ Scientific Support Office, European Space Agency, 2201 Noordwijk, The Netherlands
¹⁷ PAS Space Research Center, Bartycka 18A, 00716 Warszawa, Poland
¹⁸ Department of Astronomy, University of Maryland, College Park, MD 20742-2421, USA
¹⁹ INAF–Osservatorio Astronomico, vicolo dell’Osservatorio 5, 35122 Padova, Italy
²⁰ University of Trento, via Mesiano, 77, 38100 Trento, Italy
²¹ Department of Industrial Engineering – University of Padova, via Venezia 1, 35131 Padova, Italy
²² INAF–Osservatorio Astronomico, via Tiepolo 11, 34014 Trieste, Italy
²³ Instituto de Astrofísica de Andalucía (CSIC), c/ Glorieta de la Astronomía s/n, 18008 Granada, Spain
²⁴ National Central University, Graduate Institute of Astronomy, 300 Chung-Da Rd, 32054 Chung-Li, Taiwan
²⁵ Operations Department, European Space Astronomy Centre/ESA, 28691Villanueva de la Canada, Madrid, Spain
²⁶ Institut für Datentechnik und Kommunikationsnetze der TU Braunschweig, Hans-Sommer-Str. 66, 38106 Braunschweig, Germany

Received: 26 February 2015
Accepted: 15 May 2015

Abstract

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.