Space

Water-Ice Found On Comet 67P's Surface

January 13, 2016 | by Jonathan O'Callaghan

Photo credit: Shown is one of the regions of water-ice. Nature & Filacchione

For the first time, scientists have found water-ice on the surface of comet 67P/Churyumov-Gerasimenko. While we already knew water vapor existed in the coma (cloud) surrounding the comet, this is the first detection that has been made on the ground. 

The nucleus (cores) of comets like 67P are known to have water as a major constituent, but a common theme among all comets is a lack of water-ice on the surface. Only on one other comet – Temple 1 – has such a feature been hinted at before.

Using the orbiting Rosetta spacecraft’s Visual InfraRed Thermal Imaging Spectrometer (VIRTIS-M), a team of scientists led by Gianrico Filacchione from INAF-IAPS in Rome examined an area of the comet called Imhotep. Here, they found hidden water-ice at infrared wavelengths. Their results are published in the journal Nature.

“The presence of water on comets was known in the past, because we see a lot of water vapour in the coma,” Filacchione told IFLScience. “But this is the first time we can see surface deposits of water, and so we have the opportunity to look at the interior of the comet and see the composition of the comet in the nucleus.”

Shown are the two regions labelled BAP1 (white arrow) and BAP2 (yellow arrow). Nature & Filacchione

At the Imhotep region, water-ice was found at an abundance of about 5 percent in the surrounding dark material by VIRTIS-M. In particular, it was found in “walls,” or scarps, extending from the ground, and also at their base, in the form of millimeter-sized pure water-ice grains.

Comet 67P has a 6.44-year orbit around the Sun, and made its closest approach (perihelion) in August 2015, when it was most active. But interestingly, Filacchione said these particular water-ice regions may have been exposed at the previous perihelion, and have remained exposed ever since.

Two regions at Imhotep were found with water-ice, one about 100 meters (330 feet) across and another smaller one 50 meters across (165 feet). The technique used to find them – imaging spectroscopy – relies on the surface being illuminated by the Sun, which is the case here – although parts are in shadow, which is how the water-ice remains stable. But it’s possible there is actually more water-ice on the surface in other darker regions, such as the night portion of the comet.

“We do not exclude that there are deposits on the part of the comet not illuminated by the Sun,” said Filacchione. “But we don’t have the capability to detect them with this kind of technique.”

As mentioned earlier, the importance of finding water-ice is that it allows scientists to examine the interior of the comet. This material is thought to be exposed during periods of increased activity, when dust is blown from the surface. It could allow the structure of the nucleus to be studied, and with the water-ice being so close to the surface, it could also improve modeling of the comet.

“Comets are really dynamic objects,” added Filacchione. “They orbit across the frost line, 2.7 AU [Astronomical Units, 1AU is the Earth-Sun distance], which defines the start of activity of water-ice. It allows us to better understand how the Solar System formed, because they are remnants of the formation of the Solar System.”

Image in text: During perihelion the comet goes through increased activity that can expose sub-surface water-ice. ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

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