Traces of ice on 67P

From the pictures of the Rosetta’s OSIRIS camera, evolves the hypothesis that the region Hapi, located on the neck of the comet, may have traces of ice.
Traces of ice are to be found on Hapi, a flat area situated on the neck of the comet 67P / Churyumov-Gerasimenko. And as suggested by the latest images of the OSIRIS camera, onboard the ESA Rosetta probe, this strange region of the comet portrays imperceptibly the bluest of the rest of the surface.
What we may find surprising at first sight, is that Rosetta has accustomed in recent months to offer us beautiful landscapes, all strictly in black and white.
Indeed, the high-resolution images spread so far reflect the real and the surface that is unquestionably gray when viewed by human eyes. But the tools of a space mission gather much more information than the human eyes would meet.


Credit: ESA/Rosetta/MPS

The OSIRIS camera, thanks to its filters, it is able to detect slight variations in the amount of light reflected by a body. The mechanism is simple: taking pictures at the same surface in various colors and comparing between them, the team of the camera has been able to assess that the area in question is brighter in reflecting the blue thus a greater amount of light is found in this length wave. Among the most credible hypothesis stands the fact that this bluish color depends on the presence of ice water to the mixed powder that forms the surface layers.
“Although these color variations of the surface are minimal, they can provide important clues,” says Holger Sierks of the Max Planck Institute for Solar System Research (MPS) in Germany, PI of OSIRIS.
First, the data underscore the uniqueness of Hapi, a very different region from the rest of the surface, which on the whole is characterized by a slightly flushed spectrum, typical of cometary nuclei and other bodies considered primitive. “We know that the properties of reflectivity are closely related to the morphology of the surface,” says Sonia Fornasier of the Paris Observatory, scientist of the OSIRIS team. Scientists connect this particular bluish reflectivity to a greater abundance of ice water or just below the surface due to the comparison with the results of previous missions to Hartley 2 and Tempel 1 comets.


Credit: NASA/JPL-Caltech/UMD

Why just in this area, you can find plenty of ice, is explained by the team with the special lighting conditions of the region. “At perihelion -point orbit of closest approach to the Sun-67P when it warms up for its proximity to the Sun, Hapi is in the shadow. On the far side of the orbit, when Hapi receive back the sunlight, “says Fornasier,” the comet is at a great distance from the Sun and the temperature is very low. So Hapi could be a region where the ice has survived during the previous orbit of the comet around the sun. One area that today could conceal enough “fuel” to create the activities that we have observed in the past months.”
Comforting these assumptions based on the data of the OSIRIS camera, who can study only a limited number of wavelengths, the VIRTIS spectrometer made by a French-German-Italian consortium, was provided by the Italian Space Agency, with the scientific responsibility of INAF.
“You have to think that VIRTIS has the ability to identify the presence of specific molecules on the surface observed,” explains Fabrizio Capaccioni, INAF-IAPS and PI of VIRTIS “and provide a quantitative assessment of their abundances. As for the region of Hapi, even our spectra collected in the neck region and published in a recent article in Science (Capaccioni et al, Science 347, 2015) show a variation of the spectrum in that region. This characteristic may be associated with the presence of ice, although other explanations of the phenomenon could be possible. VIRTIS is also investigating another region of the surface, where it seems to see very clearly the presence of ice. The data are being analyzed, and in the very near future, we expect significant results.”

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