Abstract

The Rosetta spacecraft has investigated comet 67P/Churyumov-Gerasimenko from large heliocentric distances to its perihelion passage and beyond. We trace the seasonal and diurnal evolution of the colors of the 67P nucleus, finding changes driven by sublimation and recondensation of water ice. The whole nucleus became relatively bluer near perihelion, as increasing activity removed the surface dust, implying that water ice is widespread underneath the surface. We identified large (1500 square meters) ice-rich patches appearing and then vanishing in about 10 days, indicating small-scale heterogeneities on the nucleus. Thin frosts sublimating in a few minutes are observed close to receding shadows, and rapid variations in color are seen on extended areas close to the terminator. These cyclic processes are widespread and lead to continuously, slightly varying surface properties.

Highlights

  • Subsequent images covering the Anhur region were acquired on 4 to 5 June, revealing that the water ice had fully sublimated from the surface, leaving a layer spectrally indistinguishable from the average nucleus (Fig. 2)

  • The extended bright patches observed in the Anhur/Bes regions indicate a local enrichment of water ice, pointing to compositional heterogeneities in the uppermost layers of comet 67P

  • OSIRIS was built by a consortium led by the Max-Planck-Institut für Sonnensystemforschung, Göttingen, Germany, in collaboration with CISAS, University of Padova, Italy; the Laboratoire d’Astrophysique de Marseille, France; the Instituto de Astrofísica de Andalucia, CSIC, Granada, Spain; the Scientific Support Office of the European Space Agency (ESA), Noordwijk, The Netherlands; the Instituto Nacional de Técnica Aeroespacial, Madrid, Spain; the Universidad Politéchnica de Madrid, Spain; the Department of Physics and Astronomy of Uppsala University, Sweden; and the Institut für Datentechnik und Kommunikationsnetze der Technischen Universität Braunschweig, Germany

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Summary

Introduction

We report on seasonal and diurnal color variations of the surface of 67P’s nucleus from observations with the narrow-angle-camera (NAC) of the OSIRIS imaging system [11] onboard Rosetta, as caused by the evolution of the dust mantle and exposures of water ice. In addition to the change of color, the amount of phase reddening (the increase of spectral slope with phase angle) decreased by a factor of two when the comet approached perihelion in the 2015 observations compared with those from August 2014 [12, 15], indicating a change in the physical properties of the outermost layer of the nucleus.

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