Rosetta data shows gas content of comet’s coma

As we count down to Wednesday’s attempted landing on comet 67P/Churyumov-Gerasimenko,  the European Space Agency has released information about the data its Rosetta spacecraft is already collecting. It is already possible to tell quite a lot about the gas in the coma, even before the landing vehicle Philae heads for the surface.

In early October 2014, activity in the region above the “neck” of the comet became high enough for water and carbon dioxide to be detected by the instrument’s high spectral resolution channel, VIRTIS-H. 

The data shows that 67P/C-G is far less carbon dioxide rich than other comets such as 103P/Hartley, also a Jupiter-family comet that was measured by NASA’s EPOXI mission during its brief fly-by on 4 November 2010.

There is relative abundance of 4% carbon dioxide in 67P/C-G compared with Hartley’s 20%.

The ESA says the detection of gases in the comet’s coma in this early phase of the mission is important for understanding the ices inside the comet.

MIRO and ROSINA have already detected water and carbon dioxide, respectively, and VIRTIS can detect the same molecules, adding robustness to the measurements. But because it can see both with the same instrument, it can determine their relative abundance directly.

This is very important because from the ratios of these two molecules that make up the ice in the comet, scientists can gain vital insights into the make-up and structure of that ice. Ultimately, these new measurements will help meet one of the key goals of the Rosetta mission: what is comet 67P/C-G made of?


VIRTIS has also been measuring the temperature of the surface of the comet, which is currently around  –70 °C. That falls to around –183°C a kilometre above the surface due to gases accelerating away from the surface and expanding in the coma.

As the comet moves towards its closest position to the Sun in August 2015, activity will increase. VIRTIS will continuously map the distribution of carbon dioxide and water, as well as that of other minor gases including carbon monoxide (CO), methanol (CH3OH), methane (CH4), formaldehyde (CH2O), and hydrocarbons such as acetylene (C2H2), and ethane (C2H6).

relative abundance of carbon dioxide with respect to water is estimated to be about 4%, showing that comet 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 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 There are exciting times ahead as the icy treasure chest starts to give up its secrets.

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