A comet is largely composed of water ice and water vapour predominates in its ‘atmosphere’ – the coma that forms as it nears the Sun. However, very few examples of water ice have previously been observed on the surface of a comet. Now, scientists using the Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) instrument carried by the Rosetta spacecraft have detected water ice on two areas of the surface of Comet 67P/Churyumov-Gerasimenko. “From spectrometer data acquired over the Imhotep region of 67P between September and November 2014, we were able to determine that two bright areas – each several tens of metres across – consisted of water ice,” explains Gabriele Arnold from the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR) Institute of Planetary Research in Berlin. Arnold leads the German scientific contribution to the VIRTIS instrument and the international team has published their results in the scientific journal Nature.
This is an important discovery. “Although water vapour is the main gas emitted by the comet during its active phase around perihelion, and the interior of the comet is likely to be rich in water ice, its surface does not show much of this,” explains Arnold. Ice sublimates relatively quickly once it is exposed to the space environment and leaves a cometary crust that is low in water and dark coloured – consisting mainly of complex carbon compounds and minerals. “This is what has been observed in the images previously acquired by Rosetta’s Optical, Spectroscopic, and Infrared Remote Imaging System (OSIRIS) and its navigation camera system – but there has been no ice.”
Ice is visible at infrared wavelengths
Observing in the infrared region of the spectrum makes it possible to study the composition of the comet’s surface. Rosetta’s VIRTIS instrument, which operates at visible and near-infrared wavelengths, makes this possible. The VIRTIS data used for the study was acquired when the comet was still approximately 450 million kilometres from the Sun, when it was just becoming active and Rosetta could still orbit close to 67P. The measurements reveal that the two bright areas in the Imhotep region, which had already been observed due to their contrast with the dark regions surrounding them when viewed in the visible spectrum, are seen to be composed of water ice.
The ice is associated with cliff walls and debris falls, which exposed it on the surface. The temperature at the time of the investigations was around minus 120 degrees Celsius. Pure ice occupies only about four percent of the surface imaged by VIRTIS; the remainder consists of dark material.
Varying grain sizes in the ice indicate different development processes
The size of the ice particles can be determined using VIRTIS data. “We have made a very interesting discovery. The ice has two very different granule sizes,” says Arnold. The researchers discovered ice particles only a few tens of micrometres across, and a second class of particles with a size of about two millimetres – 100 times larger. “This indicates different creation mechanisms and different chronological sequences of origin.” The larger particles at Imhotep behave quite differently to the micrometre-sized particles that were discovered in the Hapi region of 67P. These are thought to be frost or rime, which arises during rapid condensation during the 12-hour day/night cycle on the comet.
In contrast, the ice particles in the Imhotep region have a more complex history. They were probably formed slowly and were only exposed by cometary activity and subsequent erosion processes. Smaller ice particles emerged first, which combined to form larger secondary particles.
One possibility for such processes is a kind of sintering, or ‘caking’ – an increasing consolidation of the original structure. Due to the loss of volatiles during the sublimation of water ice and the subsequent refreezing of the resulting water vapour, cavities and channels between the ice particles are gradually closed and the ice compacted. Solar radiation penetrates the comet’s surface and triggers the sublimation of the near-surface ice, which becomes part of the water vapour coma and recondenses elsewhere as ice. Such sintering processes in the uppermost layers of the comet have also occurred where Rosetta’s Philae lander found, on 12 November 2014, that its Multi-Purpose Sensors for Surface and Subsurface Science (MUPUS) experiment could not penetrate beyond the loose dust covering the surface of the comet.
Revisiting the internal structure of 67P/Churyumov-Gerasimenko
In addition to sunlight, the transformation of amorphous to crystalline water might be another source of energy for the sublimation of ice from beneath the surface. The growth of ice particles could occur layer by layer and thus have an impact on the overall structure of the comet. Thin layers of ice that become exposed could then be the result of cometary change.
Since Rosetta’s arrival at comet 67P, intensive discussions have taken place among the scientists, as the evolution of the comet has occurred. The new results could show that a layered structure was not necessarily present during the early history of the comet. A better understanding of which structures emerged during the development of the comet and which are remnants of its early history will give a new insight into the formation of these bodies. The VIRTIS scientists are now investigating if and how the ice deposits on the comet’s surface have changed during the approach to the Sun in 2015.