New analysis of data collected by ESA's Rosetta orbiter has revealed significant quantities of water ice on the comet 67P/Churyumov–Gerasimenko (67P). While the presence of water had previously been observed on 67P both in the comet's coma, and as frost on the surface, this discovery represents the first time that a surface deposit of water ice has ever been definitively confirmed on the comet.

The discovery was made using data from Rosetta's Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) collected between September and November 2014. The instrument focused on two bright deposits present on the comet's Imhotep region.


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Vast quantities of water vapor had previously been detected in the 67P's coma, with scientists theorizing the source of the water to be located beneath the dust shrouded crust of the comet. Therefore, the newly confirmed surface deposit may be the result of some form of erosion process.

Image of 67P highlighting the positions of the two ice deposits (Credit: ESA/Rosetta/NavCam – CC BY-SA IGO 3.0)

Pixel sampling of the bright deposits via the VIRTIS instrument revealed a yield of roughly 5 percent pure water ice. Rosetta scientists were also able to define two separate sizes of grains present in the samples – one tens of micrometers in diameter, and a second much larger population around 2 mm in diameter.

There are two theories as to how the larger grains may have come to form. The first involves a process known as sintering, by which many smaller particles are compacted together to form secondary ice crystals. The second possibility is that the grains could have formed as a result of sublimation.

Sublimation occurs when heat emanating from the Sun warms the comet, evaporating the water ice deposits buried beneath its crust. It is possible that, in very cold conditions such as those prevailing within 67P, the sublimation process could be aided by extra energy created as amorphous ice deposits transform into crystalline ice on a molecular level.

ESA graphic detailing the presence of water ice present in the Imhotep region of 67P (Credit: ESA/Rosetta/NavCam–CC BY–SA IGO 3.0; VIRTIS images and data: ESA/Rosetta/VIRTIS/INAF-IAPS, Rome/OBS DE PARIS-LESIA/DLR; G. Filacchione et al (2016))

However, the majority of the water vapor created via the process fails to escape the surface. ESA has carried out laboratory tests simulating the sublimation process believed to be taking place on 67P. The results indicated that only a relatively small percentage would escape the surface, roughly 80 percent of the vapor could remain near the surface, potentially forming an ice layer several meters thick.

Looking forward, Rosetta's science team intend to analyze data collected mid-way through 2015 as the comet made its approach to perihelion in order to determine the extent to which the exposed deposits of water ice were affected by close proximity to the Sun.

A paper on the study has been published online in the journal Nature.

Source: ESA

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