Rosetta’s ROSINA instrument, the Rosetta Orbiter Sensor for Ion and Neutral Analysis, has detected its first cometary volatile molecules. The results were presented at the European Planetary Science Congress, EPSC, held in Portugal this week.

ROSINA's reflectron time of flight mass spectrometer (RTOF).

ROSINA’s reflectron time of flight mass spectrometer (RTOF).

The detections were made early August when Rosetta was within 200 km of comet 67P/Churyumov-Gerasimenko, and over 500 million kilometres from the Sun – the first time that a comet’s coma has been analysed in situ this far from the Sun.

Since then, ROSINA has been almost continuously measuring the density and the composition of the comet’s coma. It has already acquired more than 40,000 high- and low-resolution spectra with its two mass spectrometers (DFMS and RTOF).

Overall, the density of the coma is relatively low at this early stage, far from the Sun, but should increase as activity picks up, as the comet moves closer to the Sun over the next year. The density is seen to vary during the comet ‘day’, as it rotates over a 12.4 hour period.

As expected, the main species in the comet’s coma are found to be water, carbon monoxide, and carbon dioxide, which are being released from below the surface layer of the nucleus, which VIRTIS has shown to be dark, porous, and probably dry.

Rosina's double focusing mass spectrometer (DFMS). Credit: ESA/Rosetta/ROSINA/UBern/BIRA/LATMOS/LMM/IRAP/MPS/SwRI/TUB/UMich

Rosina’s double focusing mass spectrometer (DFMS).

However, ROSINA has made the surprising observation that the ratio between these species varies quite significantly, depending on where in the coma Rosetta is. Sometimes carbon monoxide is almost as abundant as water; sometimes it’s only around 10%. In addition, ROSINA has not only detected these main species already, but many of the expected minor ones, such as ammonia, methane, and methanol.

As Rosetta gets closer to the comet and as comet activity increases, it will soon be possible to measure the ratio of hydrogen to deuterium – an isotope of hydrogen with an added neutron – in the cometary water. This ratio is constant in Earth’s ocean water and thus can be used as a way of tracing the still unknown origin of that water: for example, was it incorporated into the Earth at the time of formation, or was it delivered from space at some later date?

In particular, 67P/C-G is a Kuiper belt comet, and the hydrogen to deuterium ratio measured for its water will help constrain how much of Earth’s water could have come from a population of impacting Kuiper belt comets, soon after the birth of the Solar System.