The water ice on the surface of the comet 67P/Churyumov-Gerasimenko offers clues about its evolution

Photos of the Imhotep region on the comet 67P/Churyumov-Gerasimenko where water ice on the surface was confirmed (Image ESA/Rosetta/NavCam – CC BY-SA IGO 3.0)
Photos of the Imhotep region on the comet 67P/Churyumov-Gerasimenko where water ice on the surface was confirmed (Image ESA/Rosetta/NavCam – CC BY-SA IGO 3.0)

An article published in the journal “Nature” describes research that led to the confirmation of the presence of water ice exposed on the surface of the comet 67P/Churyumov-Gerasimenko. The VIRTIS instrument on ESA’s Rosetta space probe recorded the data that were analyzed to determine the composition of the surface top layer, which is mainly a dark material, dried and rich in organic substances containing a small part of water ice. On the comet there’s a lot of ice but it’s interesting to examine that on the surface because it allows us to understand better some of its creation processes, even underground.

This research focused on scans performed between September and November 2014, shortly after the Rosetta space probe reached the comet 67P/Churyumov-Gerasimenko. In June 2015 a study of the presence of ice was published in the journal “Astronomy & Astrophysics” and now the one published in “Nature” provides further evidence that water ice may exist on the surface and new information on the size of its grains.

Two areas over 50 meters across in the region of the comet 67P/Churyumov-Gerasimenko called Imhotep show water ice on the surface. They’re associated with vertical walls and accumulation zones beneath them where the measured temperature is about -120° Celsius (-184° Fahrenheit). The ice makes up about 5% of the areas examined while the rest is made up by the dark material of mixed composition.

One of the news from this study consists of the new information discovered on the ice grains. Previous observations that allowed to discovere a water ice cycle again made with the VIRTIS (Visible and Infrared Thermal Imaging Spectrometer) instrument detected grains of a few micrometers in diameter. Instead, this time they found grains of two types: one of several micrometers in diameter and another even around 2 millimeters in diameter.

The fact that there are such different types of grains implies different formation mechanisms that take place at different timescales. In the region called Hapi examined in the previous research the grains are very small and are associated with a thin ice layer that is formed as part of the daily ice cycle. This is the result of a rapid condensation in that region during each rotation of the comet that takes a little more than 12 hours.

The history of the grains found in the Imhotep region seems more complex. Probably there the grains were formed more slowly over time and only occasionally are exposed through erosion. Ice crystals can grow up to the size of a few millimeters, for example by sintering. Another possible cause is the sublimation because part of the water vapor emitted that comes out from the comet’s nucleus may condense again forming the various layers.

These studies show the evolution that takes place on the surface of the comet 67P/Churyumov-Gerasimenko. Figuring out where changes occurred and what other parts of the comet remained unchanged after its formation is complex but essential. The Rosetta mission ais exactly to find out how comets evolve to understand their role in the history of the solar system.



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