Rosetta was the first space mission to closely escort a comet on its journey around the Sun. Among the many discoveries made on comet 67P/Churyumov-Gerasimenko, Rosetta also achieved continued measurements of the surface temperature of a cometary nucleus with an unprecedented spatial resolution. The Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) onboard the Rosetta orbiter captured infrared images of the comet’s nucleus, which were then turned into thermal maps. We were thus able to study changes in the nucleus temperature over almost two months in August and September 2014, about one year before the perihelion passage.
Our data analysis overall revealed that surface temperatures are the result of the chemical and physical state of the material closest to the surface, a few centimetres thick at most. Daytime and / or seasonal variations in temperature have a very limited influence on layers deeper than one meter. Despite the loss of gas and dust at each pass near the Sun, the inner part of the nucleus is therefore essentially primitive.
How did we get to this result?
First we measured the average temperature of the comet’s nucleus on its dayside. While the average surface temperature in this period was 213 K or -60°C, we came across specific points as warm as 230 K or -43°C, which corresponded to a pit, namely a sinking of the surface where the inner walls, reflecting the heat, give rise to a phenomenon called “self-heating”.
Comet 67P is very dark and reflects only 6% of sunlight. In the considered period, the comet was still far away from the Sun and the activity was relatively small, with no ice outcrops. Hence, we expected the temperature to approach a black-body law. Interestingly, analyzing the variability of temperature in several morphological regions of the comet’s nucleus, we realized that self-heating affects a particular macro-area, that is, the narrowing that connects the two lobes of the nucleus. Here the measured temperature values did not follow a black-body law, but were higher than expected. Assuming a surface dominated by dust in the uppermost surface layer as thick as a few millimetres, and a minimal sublimation of volatile materials, self-heating is due to surface roughness, which can be imagined as a random distribution of mini-concavities, such as dimples on a golf ball, scattered almost everywhere on the nucleus’ surface. In the narrowing, self-heating is enhanced by the prominent concave shape.
Comparison between surface temperature values as retrieved by VIRTIS on 22 August 2014 during a rotation of the nucleus of comet 67P (panel a), and surface temperature values simulated by a thermophysical model that assumes an uppermost surface layer dominated by dust, with minimal sublimation, projected onto a 3D digital shape model of the comet's nucleus (panel b). On the right, panel c displays the difference between the measured and predicted temperature values, with the green colour representing a substantial agreement. Note the prominent shadow casted by the small lobe on the big lobe of the nucleus during maximum insolation.
Another unprecedented measure of the comet 67P nucleus involved thermal stress due to sudden shadowing produced alternately by the two lobes on each other during the maximum daytime insolation. Such temperature variations are strong enough to induce very effective fragmentation of the surface material. Indeed, in this region the temperature differences measured over a few minutes exceed by tens of times those that usually occur on the dayside of the nucleus.
Finally, to better investigate seasonal temperature effects on the nucleus, we focused on a region called Imhotep, which is smooth on average and unaffected by substantial self-heating. Here we compared the observations of VIRTIS with those of the Microwave Instrument for the Rosetta Orbiter (MIRO) also carried onboard the Rosetta orbiter, and able of measuring temperature at greater depths. Imhotep’s observations obtained by VIRTIS and MIRO in August 2014 can be explained assuming a surface layer dominated by loose dust in the uppermost cm-thick layer. Imhotep was also observed eight months later, when the comet was much closer to the Sun. VIRTIS-derived temperatures were clearly higher, but less than expected in the case of a surface layer made of loose dust only. The explanation is that the composition in the uppermost cm-thick layer must have evolved through time, with an increasing amount of volatiles inducing an increasing degree of sublimation that in turn can decrease surface temperatures, compared to the case of a dust-only layer. These results are in excellent agreement with previously published papers.