The European Space Agency’s (ESA) Rosetta mission observed comet 67P/Churyumov-Gerasimenko (hereafter 67P) and its activity for two years through perihelion. The observations revealed a bi-lobate shape (Sierks+2015) and diverse morphology (Thomas+2015). One of the objectives was to determine the refractory-to-volatile ratio. In a simplified view where any ejected material is lost to space two measurements are sufficient to determine this ratio. First, the total mass loss during one apparition (Pätzold+2019). Second, the total volatile mass loss, indirectly determined by the in-situ measurements (e.g. Fougere+2016) or remote sensing data (e.g. Marshall+2017). But the surface morphology has revealed large dust deposits that indicate that possibly a large fraction of the ejected dust is re-deposited (Thomas+2015). If that is indeed the case, then the two above mentioned quantities cannot constrain the ejected dust mass but rather only the escaping dust mass, thus obscuring the emitted dust-to-gas ratio.
Here we present results that simultaneously constrain the dust size distribution, dust-to-gas ratio, dust re-deposition, and total mass production rates for comet 67P. We use a 3D DSMC gas dynamics code to simulate the inner gas coma and constraine it by ROSINA/COPS data. Further, the inner dust coma is modelled with a 3D dust dynamics code which is used to produce synthetic images that are compared to the OSIRIS data set. Applied to the entirerty of the mission these simulations allow us to constrain the properties of the dust coma and the total gas and dust production rates.
We show how the dust-to-gas ratio, the power-law exponent of the dust size distribution, the fraction of dust fall back, and the scattering properties of the dust are inter-related and constrain each other. Because these parameters are not independent they need to be fit simultaneously. Using the total mass loss of the comet during the 2015 apparition we will show that only a narrow parameter set fits all observations.
We determined a total volatile mass loss of (6.1 ± 1.5) 109 kg during the 2015 apparition. Further, we found that power-laws with q = 3.7+0.57-0.078 are consistent with the data. This results in a total of 5.1+6.0-4.9 109 kg of dust being ejected from the nucleus surface, of which 4.4+4.9-4.2 109 kg escape to space and 6.8+11-6.8 108 kg (or an equivalent of 14+22-14 cm over the smooth regions) is re-deposited on the surface. This leads to a dust-to-gas ratio of 0.73+1.3-0.70 for the escaping material and 0.84+1.6-0.81 for the ejected material. We have further found that the smallest dust size must be strictly smaller than 30 μm.