The comet's icy nucleus
Scientists are particularly keen to investigate the interior of 67P's nucleus and the processes unfolding close to and at the surface.
The mass and volume of the nucleus imply an average density of around 0.5 grams per cubic centimetre, or half the density of frozen water. If one assumes that the dust-to-ice ratio measured in the coma – which is between 6:1 and 2:1 – is representative of the entire comet, the mean porosity is up to 70 percent or more, which means that the comet nucleus largely consists of cavities.
Initially, the plan was to use radar (the CONSERT experiment) to explore the internal structure of the comet’s nucleus and determine the distribution of the pores. The Rosetta orbiter and the Philae landing craft were fitted with radar antennas for this purpose. The landing site was selected because it would have allowed mapping of a substantial portion of the comet's nucleus.
Unfortunately, the final landing site at which Philae ultimately came to rest only permitted the observation of a layer close to the surface measuring a few hundred metres in thickness. At this location, the comet material showed high porosity and homogeneity. But it is certainly conceivable that cavities formed through cometesimal accretion as the comet nucleus grew slowly over time. These may have been invisible to CONSERT. It is possible that these cometesimals exhibit lower porosity.
The diagram above shows the inner structure of the nucleus as it would appear in both of these scenarios – in the lower half we see a homogenous interior with extremely high porosity, and in the upper section we find cavities and lower porosity.
The results of several direct and indirect observations are also visible on the surface: the fact that the landing module rebounded from the surface indicates that layers close to the surface may be hard. These findings were confirmed by measurements using the SESAME seismometer and the failure of the MUPUS probe to penetrate the surface for its own experiment. These hard layers are expected to lie close to the surface beneath a layer of dust several decimetres thick and could cover the entire comet. Exposed ice was observed at only a few locations across the comet. In addition, the diagram shows possible trajectories of boulders that were ejected by the comet's outgassing but failed to surpass its gravitational field.
The mass spectrometers ROSINA and COSAC, as well as other instruments, identified a plethora of substances in the coma that are listed in the table to the left. The substances never before found on a comet are shaded in grey. Thanks to Rosetta, the number of known cometary substances has more than doubled. The large number of complex organic molecules known to be the building blocks for the emergence of life is astonishing. The deposits of highly volatile elementary gases, such as oxygen and nitrogen, are equally remarkable. Their existence demonstrates that the comet nucleus formed at temperatures below -235 degrees Celsius, and that these temperatures may still prevail at its innermost core.
ROSINA was also able to measure the ratio of deuterium to hydrogen. This isotopic fingerprint shows that the water found on Earth cannot come from the type of comet family to which 67P/Churyumov-Gerasimenko belongs, or only does to a negligible extent. It is therefore most likely that water arrived on Earth with other planetesimals from the inner Solar System.