The Nucleus of comet 67P/Churyumov–Gerasimenko – Part I: The global view – nucleus mass, mass-loss, porosity, and implications

The radio science experiment RSI on-board Rosetta determined the mass of the nucleus of comet 67P/Churyumov-Gerasimenko at the start of the prime mission from 2014 August to November (GM = 666.2 +/- 0.2 m(3) s(-2) or 9982 +/- 3 x 10(12) kg) and shortly before the end of the mission from 2016 July to September (GM = 665.5 +/- 0.1 m(3) s(-2) or 9971.5 +/- 1.5 x 10(12) kg). The mass-loss is Delta M = 10.5 +/- 3.4.10(9) kg, about 0.1 per cent of the nucleus mass. Almost 50 per cent of the mass-loss occurred during the 32 d before and 62 d after perihelion. The nucleus mass combined with the new very precise nucleus volume of 18.56 +/- 0.02 km(3) yields a bulk density of 537.8 +/- 0.6 kg m(-3). This low bulk density suggests that the nucleus is highly porous. The porosity is constrained by the observed bulk density, the density of ices, mostly water ice, and the density of compacted nucleus dust material. For a range of compacted dust material density from 2000 to 3500 kg m(-3), the porosity varies between 65-79 per cent when the dust-to-ice mass ratio F-nucleus for the nucleus body lies in the range 3 <= F-nucleus <= 7. The nucleus is thus a highly porous very dusty body with very little ice. The total mass-loss Delta M puts hard constraints on the models of interpretation of the observations from other instruments on Rosetta. The loss from gas, based on ROSINA and MIRO observations, suggests that F-coma, the dust-to-gas mass ratio of cometary matter in the coma beyond the Hill sphere of the nucleus is of the order of F-coma <= 0.5 and therefore at least six times, and perhaps as much as 14 times, smaller than F-nucleus. It seems that the lost gas mass was overestimated by the instruments. The lost gas mass shall be 5.25 x 10(9) kg for F-coma = 1. For any other gas mass values > 5.25 x 10(9) kg F-coma is < 1. Most of the lifted dust mass will not leave the cometary Hill sphere but falls back to the nucleus. For 3 <= F-nucleus <= 7 and F-coma = 0.5, the fallback mass is between 1.8.Delta M and 4.3.Delta M.