VIRTIS detects water and carbon dioxide in comet’s coma

In previous blog posts we’ve heard how VIRTIS is able to map the temperature of the comet’s surface. Now, as Dominique Bockelee-Morvan and Stephane Erard (Observatoire de Paris) report, the VIRTIS science team has started to map gas in the coma. One of the scientific objectives of Rosetta’s Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) is to map the emission of different gases from comet 67P/Churyumov-Gerasimenko, and to study how these change with comet activity. In early October 2014, activity in the region above the ‘neck’ of the comet became high enough for water (H2O) and carbon dioxide (CO2) to be detected by the instrument’s high spectral resolution channel, VIRTIS-H. (The surface mapping is done with an imaging spectrometer, called VIRTIS-M.) From these spectra, it is already possible to tell quite a lot about the gas in the coma. The spectra show infrared molecular bands whose shape depends on the temperature in the coma, whereas the intensity is a function of the number of molecules along the line of sight of VIRTIS. From these measurements, the relative abundance of carbon dioxide with respect to water is estimated to be about 4%, showing that comet 67P/C-G is not as rich in carbon dioxide as comet 103P/Hartley, also a Jupiter-family comet, for which a relative abundance of about 20% was measured by NASA’s EPOXI mission during its brief fly-by on 4 November 2010. Ever since July, VIRTIS has been measuring the average temperature of the comet’s surface, finding it to be around –70 °C at the moment. These measurements of the gas in the coma now allow the science team to say something also about the temperature at some distance from the surface. The current measurements correspond to a height of one kilometre above the surface, where the temperature falls by more […]



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