The VIRTIS team have produced maps of the surface temperature of comet 67P/C-G showing how the temperature varies across regions and with local time. Some examples of these were shown today during the Rosetta special session at the European Planetary Science Congress (EPSC).
Over the past two months, the VIRTIS imaging spectrometer has recorded about three million spectra of the surface of 67P/C-G as Rosetta closed in from about 14000 km to less than 100 km from the comet’s nucleus. One thing that can be extracted from these spectra is the surface temperature, which is derived from measurements of the radiation emitted in the spectral range 4.5 to 5.1 microns.
Back in mid-July, the first temperature measurements – made when the comet occupied just a few pixels in the VIRTIS field of view – yielded an average surface temperature of 205 K, already at that time ruling out a surface covered exclusively in ice.
By early August, as the comet filled more of the VIRTIS field of view, variations in the temperature could be seen, modulated by the comet’s rotation period.
Following Rosetta’s arrival at the comet on 6 August, the VIRTIS team have been able to map variations in the surface temperature across the nucleus. These data have been an essential input to the on-going landing site selection process since one of the criteria for choosing a site is that it be neither too hot nor too cold for the Philae lander.
At the Rosetta special session today at the EPSC, the VIRTIS team described how they have made temperature maps of the entire sunlit surface of the comet, noting that temperatures as high as 230 K have been recorded. They have also been able to map how the surface temperature varies with local time – as can be seen in the figure above.
Measurements of the surface temperature can provide clues to the composition and physical properties of a comet. These new VIRTIS measurements have allowed the team to rule out some models of the comet surface and to favour a comet surface composed of a porous and highly thermally insulating dusty crust that is depleted of water ice. As they reported today, this is also consistent with the VIRTIS global measurements of thermal inertia – a measure of a body’s resistance to changes in temperature – that is compatible with the value for high porosity dusty materials.
The relatively high spatial resolution of the VIRTIS measurements have also allowed the team to investigate thermal shocks that happen when a region enters or exits from shadow. This is of interest because thermal shocks can give rise to stresses in the surface, which can lead to micro-cracks and eventually result in fractures in the surface.
The team have also been poring over the spectra to search for hints about the chemical makeup of the surface of comet 67P/C-G. Among the preliminary results reported today was no evidence of water ice on a global scale, confirming that the outer surface is generally dehydrated.
On the other hand, they see some strong hints of carbon-bearing compounds, with some spectral features that are compatible with the complex macromolecular carbonaceous materials found in the most primitive carbonaceous meteorites. These materials are often referred to as “organics”, even if their origin is unrelated to life.
The picture of comet 67P/C-G that is beginning to emerge from these early VIRTIS measurements is of a dark, dry, and dusty comet surface with a rich and complex chemistry.