Interpretation of COSAC mass spectrometer data acquired during Rosetta's Lutetia fly-by 10 July 2010

On 10 July 2010 the Rosetta spacecraft flew-by asteroid Lutetia. At that time all the mass spectrometers in the payload of the Philae Lander capable of gas analysis were in operation. Among these, the COSAC (Cometary Sampling and Composition) experiment is a gas-chromatograph mass spectrometer which was switched on twice during the fly-by, once 3h prior to Closest Approach for 126s and 1h afterwards for 126s. The instrument performed well and two mass spectra were obtained from which the composition of the gas phase was evaluated. These spectra show no evidence of the presence of an exosphere at Lutetia. The most likely interpretation of the records is that traces of residual gas from Philae and from COSAC itself were recorded, with a slight change in composition between the two measurements. A mean upper pressure limit inside COSAC derived from the two measurements is of the order of 5×10−9mbar. This is the highest limit quoted by the various onboard instruments but is explicable in the context that COSAC was designed to operate in environments characterized by relatively higher pressure.