Rosetta Mission: Ptolemy sniffs next piece of the comet puzzle

The Ptolemy team have been surprised by the results as, based on the findings of the ROSINA instrument on board the Rosetta orbiter, they were expecting to see larger concentrations of CO on the surface. ROSINA, like PTOLEMY, is a mass spectrometer and at the time of landing was analysing the gases rising from the surface some 30 km above Comet 67P. Results by ROSINA, acquired shortly before landing (published in January 2015), found that the concentration of CO, although variable, was up to four times that of CO₂, whereas the Ptolemy measurements found that CO was about ten times less than CO₂.

According to Dr Andrew Morse, lead author on the paper, these findings could suggest that either the coma gas composition changes through various chemical reactions as it moves away from the comet, or that the gas vaporised from the comet varies by location, making it a heterogeneous comet. He says: “Though it is a possibility that carbon monoxide is produced in the coma as it moves away from the comet, a more probable account of such a large change would be that the gases released from the comet’s surface differs according to location.”

One hypothesis is that a heterogeneous comet is the result of it being accumulated from diverse building blocks during its formation in the solar system. Alternatively it is the result of uneven heating in its journey into the inner solar system. The ROSINA instrument could help to answer this by making further measurements of the water coming off the comet’s surface. Dr Morse adds: “Our results, from Comet 67P’s surface, has both surprised us as well as opened up a variety of new questions about how comets form and how they work. We’re eagerly awaiting new results which should help us to clarify whether Comet 67P is indeed heterogeneous in nature or if there is another explanation. Either way, these results offer up an important piece of the complex, yet fascinating puzzle of how comets are formed.”

Co-author Dr Geraint Morgan says: “The questions raised by Ptolemy show the value of landers, even high risk ones, in establishing a ‘ground truth’ measurement on the surface to compare with on-going measurements from orbiting spacecraft.  Despite Philae’s eventful journey the data produced could be the key to help the Rosetta mission unlock the secrets of Solar System.”

The paper ‘Low CO/CO₂ ratios of comet 67P measured at the Abydos landing site by the Ptolemy mass spectrometer’ was published in the academic journal Astronomy & Astrophysics. It was authored by the OU Ptolemy team which consists of Dr’s Andrew Morse, Simon Sheridan, Geraint Morgan, Dan Andrews, Simeon Barber and Professor Ian Wright, and Olivier Mousis from Laboratoire d'Astrophysique de Marseille.