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

Abstract

Since OSIRIS started acquiring high-resolution observations of the surface of comet 67P/Churyumov-Gerasimenko’s nucleus, over one hundred metre-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 decametre-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 re-condensation/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 colour seen at the surface of the nucleus. Clustered bright spots are formed by the collapse of overhangs triggered by mass wasting of deeper layers. Isolated spots are possibly the result of the emission of boulders at low velocity and re-deposition in other regions.