The mean temperature of the nucleus of comet 67P/Churyumov-Gerasimenko is -70°C (205K +/- 5K). VIRTIS obtained this result after making measurements between the 13th and the 21st of July, from a distance of 10 000 kilomètres.
From this distance, the comet covers several pixels in the VIRTIS field of view. It was thus not possible to find the global temperature of the body, taking into account the local variations in temperature. To do this, the light emitted by the body must be taken as a whole.
These preliminary measurements seem to support the notion that most of the nuclear surface is too hot (even though -70°C might seem to the man in the street to be very cold) to be made up of pure ice. It is presumably covered by some kind of dark, dust-like and pourous material,.
This result is extremely interesting, since it furnishes for the first time clues about the composition and the physical properties of a cometary surface.
These temperature measurements have been compared to temperatures of a theoretical model of a comet consisting mainly of water ice : the temperature measured on comet 67P/CG turns out to be 20 to 30 K higher than the temperature expected for an icy body.
While the very low reflectivity of the comet allows us to exclude the possibility that the surface is made up entirely of pure ice, there might still be occasional regions covered by relatively pure ice.
In the very near future…
Once Rosetta has come close to the comet, VIRTIS will be able to create real temperature maps of its nucleus, thereby revealing regions made up of pure ice.
In addition, VIRTIS will study the daily temperaturer variations of various regions of the comet in order to see how quickly they react to changes in solar radiation. This behaviour is related, in particular, to the conductivity of the surface material which is there (to a depth of 10 cm), as well as to its porosity and density. This information is of first rate importance to the rest of the mission, when the best landing sites for the Philae module will have to be selected.
VIRTIS will also measure the change in temperature as the comet approaches the Sun.
By combining observations from the various instruments on Rosetta, VIRTIS will provide an accurate description of the physical properties of the cometary surface as well as of the gases in the coma.
Virtis, in brief…
VIRTIS is the visible and infra-red thermal spectro-imager of the Rosetta probe of the European Space Agency (ESA). VIRTIS was built by a consortium under the scientific leadership of the Institut for space astrophysics and planetology of the l’INAF (Italy), which also heads the science operations. The consortium includes the Laboratoire d’Etudes Spatiales et d’Instrumentation en Astrophysique (laboratory for space studies and instrumentation of the Paris Observatory, and the Institute fur Planetenkundung of the DLR (Germany). The development of the VIRTIS instrument was financed and led by theAgenzia Spaziale Italiana (ASI – the Italian Space Agency), le Centre national d’études spatiales (the National Centre for Space Studies, France), and the Deutsches Zentrum fur Luft - und Raumfahrt (DLR)(Germany).
VIRTIS will furnish quantitative information about the solid material making up the nucleus, and map their surface distribution ; it will also provide information about the gases and molecules which are in the coma.