The nucleus became increasingly blue as the comet approached the Sun, showing that the surface composition is enrighed with water ice.
These changes are due to the effect of condensation of water vapour and by the intense activity as the comet approached perihelion, leading to a gradual thinning of the dust layer which covers the comet, exposing the underlying layers enriched with ice.
As the cometary activity intensified as the comet approached perihelion and as the dust llayer thinned, the comet underwent spectacular daily colour changes over large areas of its surface and revealed thin layers of frost in regions which go rapidly from night to day
In these regions, over short periods of time (between 30 and 40 minutes), the authors saw that the regions emerging from night have a bluer spectrum than the neighbouring surfaces already illuminated by the Sun, in as well as the formation of thin layers of frost close to the edges of the shadows, which sublime in a few minutes following illmination by the Sun.
These observations can be understood in terms of the daily water cycle : transformed rapidly into vapour by the light of the Sun during local day, the water recondenses again forming a thin layer of frost and ice as the temperature falls following the setting of the Sun locally, and then sublimes again the following day.
Observation of two large patches of ice
This work also reports the first observation of two large and bright patches of ice (each 1500 m2) in the southern hemisphere in the regions called Anhur and Bes, This observation highlights shows This observation highlights the direct exosure of water ice mixed with cometary dust in the Cette observation met en évidence l’exposition directe de glace d’eau mélangés aux poussières cométaires.in the cometary nucleus.
These formations, which are four to six times brighter than the surrounding terrain, appeared at the end of April 2015, and disappeard about ten days later.
The authors Les auteurs thus think that this is a mixture containing between 20% and 30% of water ice, i.e. a quantity of ice 6 times higher than that in other bright patches observed earlier on the surface of the comet.
Although large, the thickness of these bright patches is probably equivalent to less than 30 cm of pure water ice.
Their detection shows clearly that there is a variation in the water abundance on the surface layers of the cometary nucleus, and indicates that the composition of the comet is locally heterogeneous on scales of a few tens of metres.
These patches enriched with water ice are presumably formed by the re-condensation of volatiles during earlier perihelion passes of the comet.
Conclusion
Long term observations of comet 67P/CG give us information, in particular, about the composition of the upper surface layers of the nucleus.
They show that water ice is abundant on the nucleus, and appears on the surface as the comet approaches perihelion. However most of the time it is covered by dust.
During the observations, OSIRIS has also shown that mixtures rich in ice and water (up to 30%) are sometimes present on the nucleus, and moreover that the lifetime of the ice exposed to the Sun can vary from several minutes to several days.
Most of the bright parts can only be seen with very high resolution and under rather special lighting conditions.
It is most likely that there are similar phenomena on other comets, which helps to explain why cometary nuclei are so dark even though they contain abundant quantities of ice and water.
Reference :
"Rosetta’s comet 67P/Churyumov-Gerasimenko sheds its dusty mantle to reveal its icy nature", Fornasier et al.
Paper published on line in Science le november 17th 2016 : 10.1126/science.aag2671 (2016)