Early Activity of Churyumov-Gerasimenko: ROSINA/RTOF Results

International audienceThe European Space Agency’s Rosetta spacecraft is now close to the comet 67P/Churyumov-Gerasimenko (67P/C-G). On board is the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA) instrument suite. ROSINA consists of two mass spectrometers, the Double Focusing Mass Spectrometer (DFMS) and the Reflectron-type Time-Of-Flight (RTOF), as well as the COmet Pressure Sensor (COPS).The first signal with ROSINA/RTOF of the gaseous environment of the comet was a significant increase in water density observed on DOY 218.1 of 2014 (at 3.5 AU) by RTOF above the gaseous envelope of the Rosetta spacecraft. A similar density increase is observed by COPS at the same time. A preliminary analysis shows that the water density is nH2O ≈ 1012 m–3 at 100 km distance from the comet (located at 3.5 AU from the Sun). This gives a density at the surface of nH2O ≈ 6.4·1015 m–3 and a vertical column density of water of NCH2O ≈ 6.5·1018 m–2. Assuming an active area of 4% we arrive at a production rate of QH2O ≈ 5.8·1024 mole s–1. These values are preliminary and will be refined by forthcoming observations. Other than water, no signal related to cometary activity could be observed above the molecular background from the spacecraft at present, e.g. cometary CO and CO2 are not observed in the RTOF data so far. This hints at a possible deficiency of carbon bearing compounds in the comet