An article published in the journal “Nature Astronomy” reports a series of measurements of the temperature of comet 67P/Churyumov-Gerasimenko’s nucleus obtained thanks to ESA’s Rosetta space probe’s VIRTIS instrument. A team of researchers led by Federico Tosi of the National Institute of Astrophysics’ Space Astrophysics and Planetology Institute in Rome, Italy, used the infrared images captured by VIRTIS to generate thermal maps from which they obtained the temperatures reconstructing the daily and seasonal variations but also the ones related to its morphological characteristics and the chemical-physical characteristics of the top surface layer.
The Rosetta space probe mission ended on September 30, 2016 with its landing on the surface of comet 67P/Churyumov-Gerasimenko and its destruction due to the fact that it wasn’t designed to land. The maneuver was programmed only because Rosetta had reached the end of its mission. However, the enormous amount of data collected keeps on getting analyzed, in this case the infrared images captured by the VIRTIS (Visible InfraRed and Thermal Imaging Spectrometer) instrument between August and September 2014, about a year before it reached perihelion.
The image (Courtesy Tosi et al. All rights reserved) shows a comparison between the temperature detected on the surface of comet 67P/Churyumov-Gerasimenko on August 22, 2014, in panel a, and the one simulated using a thermophysical model that assumes a top layer of the surface dominated by dust and minimal sublimation projected on a model of 3D digital shape of the nucleus, in panel b. Panel c shows the difference between the measured values and the ones derived from the simulation in which green shows an agreement. In the moment of maximum sunstroke there’s a shadow of the small lobe of the nucleus on the big lobe.
The average temperature detected in the period examined was approximately 213 Kelvin, -60° Celsius, but some areas were warmer, reaching -43° Celsius. This is due to a phenomenon called self-heating which occurs in sinking of the surface where the internal walls reflect heat. That discovery was surprising because comet 67P/Churyumov-Gerasimenko reflects only 6% of sunlight so it approximates a so-called black body. The analysis of the data showed that the anomalies were concentrated in the neck of the nucleus, the area that joins the two lobes. The presence of a few millimeters of dust and a minimal sublimation of volatile materials may be at the basis of self-heating.
Other variations were detected when the two lobes generate shadows at the maximum sunstroke. The resulting thermal stress can fragment the materials on the surface. To better understand the situation, the researchers studied in particular the region of the comet 67P/Churyumov-Gerasimenko called Imhotep, which tends to be smooth and without shadows with the consequence that self-heating is limited.
Data from another instrument, MIRO (Microwave Instrument for the Rosetta Orbiter), were also examined because it could detect temperature at greater depths. The only plausible explanation seems to be that the top centimeter on the surface is composed mainly of non-compact dust.
Eight months later comet 67P/Churyumov-Gerasimenko was much closer to the Sun but the temperatures detected by the VIRTIS instrument didn’t increase as predicted in the presence of a surface layer composed mainly of dust. This was explained by the changes that occurred following the approach to the Sun, which brought more and more volatile materials to the surface.
Angelo Zinzi of the Italian Space Agency’s SSDC (Space Science Data Center), one of the authors of this research, pointed out that he and his colleagues had to develop a system to visualize data directly on comet 67P/Churyumov-Gerasimenko’s nucleus shape because its shape made it difficult to analyze them with a classic 2D projection.
The excellent quality of the data collected by the VIRTIS instrument allowed to obtain these accurate temperature measurements on the surface of comet 67P/Churyumov-Gerasimenko. They help to understand the processes taking place on a comet in its period of activity and among other things imply that under the upper layer of its surface it is still more or less intact.