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

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 clis, 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.