Infrared analyses by Rosetta confirm the presence of water-ice in two distinct locations in the Imhotep region of the comet, and point to processes responsible for creating the comet’s internal icy layers.
Although water vapour is the main gas observed flowing away from Comet 67P/Churyumov-Gerasimenko, very few exposures of water-ice have been observed on the comet’s surface.
Analysing the comet at infrared wavelengths with the VIRTIS instrument, however, reveals the composition of the comet’s material, which is primarily found to be coated in a dark, dry and organic-rich material.
In the latest study, which focuses on data collected by VIRTIS between September and November 2014, the team confirms that two metre-sized areas in the Imhotep region of the comet that appear as bright patches in visible images are indeed composed of water-ice.
The ice is associated with cliff walls and debris falls and has an average temperature of about –120ºC. Pure water-ice was found to occupy up to about 4% of each pixel sampling area, with the rest comprising the dark material.
Furthermore, the data revealed two different populations of grains in terms of their size: those several tens of micrometers and those that are comparatively larger, around 2 mm in diameter. This contrasts with the very small – just a few micrometer diameter – grains found in the Hapi region, on the ‘neck’ of the comet, observed by VIRTIS in a different study.
“The various populations of icy grains on the surface of the comet imply different formation mechanisms, and different time scales for their formation,” says Gianrico Filacchione, lead author of the study published in the journal Nature.
At Hapi, the very small grains are associated with a thin layer of ‘frost’ that forms as part of the daily water-ice cycle, a result of fast condensation in this region over each comet rotation, which is just over 12 hours.
“By contrast, we think that layers of the larger millimeter-sized grains we see in Imhotep have been subject to a more complex history; they likely formed slowly over time, and are only occasionally exposed through erosion,” says Gianrico.
Assuming a typical grain size of tens of microns for ice grains present on the surface – as inferred on other comets as well as Comet 67P/C-G – then observations of millimeter-sized grains can be explained by the growth of secondary ice crystals.
One way this can occur is via the process of sintering, whereby grains of ice are compacted together. Another method is through sublimation, whereby heat from the Sun penetrates the surface, triggering the sublimation of buried ice. While some of the resulting water vapour may escape from the nucleus, a significant proportion recondenses in layers.
This idea is supported by laboratory experiments that simulate the sublimation behaviour of ice buried under dust, showing that more than 80% of the sublimating ice is not released through the dust mantle but is redeposited below the surface.
Additional energy for sublimation could also be provided by a transformation in structure of the ice at a molecular level, that is, through the crystallization of amorphous ice to crystalline ice, which occurs at the low temperatures observed on comets.
“Ice grain growth can cover layers several metres thick, and can therefore affect the large-scale structure, porosity and thermal properties of the nucleus,” says ??Fabrizio Capaccioni, VIRTIS principal investigator.
“If the thin ice-rich layers that we see exposed close to the surface are the result of the comet’s activity, then they represent its evolution and it does not necessarily require global layering to have occurred early in the comet’s formation history.”
“Understanding which features on the comet are left-over from its formation and which have been created during its evolution is somewhat challenging, but this is why we are studying a comet up close: to try to discover what processes are important at different stages of a comet’s lifetime,” adds Matt Taylor, ESA’s Rosetta project scientist.