While scientists and the public alike have been astounded by the unexpected shape of Comet 67P/ Churyumov-Gerasimenko’s nucleus, what lies beneath the surface is just as important scientifically.
Comet interiors preserve a unique record from the formation of the Solar System 4.6 billion years ago. Reaching that information was one of the key tasks of Rosetta and Philae. There were two principal experiments designed to 'see' inside the comet's nucleus: the COmet Nucleus Sounding Experiment by Radio-wave Transmission (CONSERT) instrument and the Radio Science Investigation (RSI).
Image taken with the navigation camera (NavCam), 28 January 2016. Credit: ESA/Rosetta/NAVCAM, CC BY-SA IGO 3.0
Comets are known to be mixtures of dust and ice. Yet puzzlingly, measurements show that 67P/C-G's density is much lower than ice. This means the comet has a high porosity, and could be an indicator that there are huge empty caverns inside it. However, neither CONSERT nor RSI find any evidence to support this.
Instead, the investigations have shown that the porosity comes about because the comet is a more or less uniform mixture of ice and 'fluffy' dust grains, rather than a honeycomb structure featuring large voids.
CONSERT works by 'sounding' the comet's nucleus. This involves beaming ...