Rosetta: Comet’s South Heats Up

The southern side of Rosetta’s comet will change dramatically in the next months. Under the influence of the Sun it may lose a surface layer of several meters.

The northern and southern hemisphere of Rosetta’s comet 67P/Churyumov-Gerasimenko experience sun-driven erosion to a very different extent. This is the result of a recent analysis performed by Rosetta’s OSIRIS team. Based on data acquired by the scientific imaging system OSIRIS, the scientists used a thermal model to estimate how much material both hemispheres lose during one orbit as ice sublimates from the comet’s surface carrying grains of dust with it. While the southern hemisphere may be widely reshaped shedding a layer of several meters, the northern hemisphere will be much less affected. Since Rosetta’s arrival, the comet’s southern hemisphere has been facing away from the Sun. Starting in May it will be illuminated again. The scientists expect to see dramatic changes then.

Much like Earth, Rosetta’s comet undergoes different seasons as it travels around the Sun: while some regions are subject to strong solar irradiation, others at the same time receive much less sunlight. Some areas close to the poles even go through phases of complete darkness or non-stop illumination. The reason for this is the inclination of the body’s rotation axis with respect to its orbital plane. With an angle of 52 degrees, this inclination is much larger in the case of comet 67P/Churyumov-Gerasimenko than in the case of Earth. Together with its complex shape and strongly elliptical orbit this leads to a very uneven distribution of summer and winter months among both hemispheres. While the comet’s northern hemisphere experiences a long summer lasting for 5.6 years when the comet is far away from the Sun, the southern hemisphere has a short, but intense hot season of about ten months.

Currently, the southern side of the comet still faces away from the Sun and is trapped in polar night. However, in May it will start to see sunlight again. “We then expect the erosion there to pick up significantly”, says OSIRIS Principal Investigator Holger Sierks from the Max Planck Institute for Solar System Research in Germany.

In their model calculation, the OSIRIS team estimated the maximum loss of material for both regions during one orbit. Their model assumes that the subsurface water ice is covered by a very thin and extremely porous dust layer of only 50 micrometers. “There is, of course, still some uncertainty in this value. It does, however, reproduce the erosion rates we presently see very well. We therefore believe  it to be  a good starting point for our model”, says OSIRIS scientist and emeritus Principal Investigator Horst Uwe Keller from the Institute for Planetary Research of the German Aerospace Center (DLR), who led the analysis. The erosion is proportional to the sublimation of water ice, which the scientists determined as a function of insolation.

The calculations are based on a three-dimensional shape model of the comet derived from OSIRIS data. Morphological details were neglected and the entire surface modeled by approximately one hundred thousand small triangles.

“Assuming that four times more dust is emitted than ice, our model leads to very different scenarios for the northern and southern hemisphere”, says OSIRIS scientist Stefano Mottola from the Institute for Planetary Research of the German Aerospace Center (DLR). “While during its short but intense summer the southern hemisphere may lose a surface layer measuring up to 20 meters in thickness, this value should be much smaller for the northern hemisphere. According to our estimations, only very few prominent peaks and cliffs may erode by more than ten meters over the course of one orbit.”

The scientists therefore expect the southern side to change dramatically as 67P approaches perihelion in August. “Quite possibly, after perihelion 67P will no longer be the comet we have grown used to in the past months”, says Sierks. “Witnessing these changes from up close will be an unsurpassed adventure”, he adds.

The neck area between the comet’s two lobes is particularly weakly insolated. At the same time, it has displayed the strongest and earliest dust activity in the past months. The scientists therefore believe that possibly this region has a different composition than the rest of the comet.

Rosetta is an ESA mission with contributions from its member states and NASA. Rosetta's Philae lander is provided by a consortium led by DLR, MPS, CNES and ASI. Rosetta is the first mission in history to rendezvous with a comet, escort it as it orbits the Sun, and deploy a lander to its surface.

The scientific imaging system OSIRIS was built by a consortium led by the Max Planck Institute for Solar System Research (Germany) in collaboration with CISAS, University of Padova (Italy), the Laboratoire d'Astrophysique de Marseille (France), the Instituto de Astrofísica de Andalucia, CSIC (Spain), the Scientific Support Office of the European Space Agency (The Netherlands), the Instituto Nacional de Técnica Aeroespacial (Spain), the Universidad Politéchnica de Madrid (Spain), the Department of Physics and Astronomy of Uppsala University (Sweden), and the Institute of Computer and Network Engineering of the TU Braunschweig (Germany). OSIRIS was financially supported by the national funding agencies of Germany (DLR), France (CNES), Italy (ASI), Spain (MEC), and Sweden (SNSB) and the ESA Technical Directorate.