The near-nucleus gas coma of comet 67P/Churyumov-Gerasimenko prior to the descent of the surface lander PHILAE

Abstract

Context. The European Space Agency (ESA) Rosetta mission was the most comprehensive study of a comet ever performed. In particular, the Rosetta orbiter, which carried many instruments for monitoring the evolution of the dusty gas emitted by the cometary nucleus, returned an enormous volume of observational data collected from the close vicinity of the nucleus of comet 67P/Churyumov-Gerasimenko. Aims. Such data are expected to yield unique information on the physical processes of gas and dust emission, using current physical model fits to the data. We present such a model (the RZC model) and our procedure of adjustment of this model to the data. Methods. The RZC model consists of two components: (1) a numerical three-dimensional time-dependent code solving the Eulerian/Navier-Stokes equations governing the gas outflow, and a direct simulation Monte Carlo (DSMC) gaskinetic code with the same objective; and (2) an iterative procedure to adjust the assumed model parameters to best-fit the observational data at all times. Results. We demonstrate that our model is able to reproduce the overall features of the local neutral number density and composition measurements of Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA) Comet Pressure Sensor (COPS) and Double Focusing Mass Spectrometer (DFMS) instruments in the period August 1–November 30, 2014. The results of numerical simulations show that illumination conditions on the nucleus are the main driver for the gas activity of the comet. We present the distribution of surface inhomogeneity best-fitted to the ROSINA COPS and DFMS in situ measurements.

Dust-to-Gas and Refractory-to-Ice Mass Ratios of Comet 67P/Churyumov-Gerasimenko from Rosetta Observations

“…It must be noted that the coma models used to invert ROSINA DFMS data into activity maps (Fougère et al 2016a) do not have unique solutions (in particular because each measurement is at one specific location within the coma at a time, and coverage can only be partial due to spacecraft trajectories). Zakharov et al (2018) have shown that multiple other solutions to the activity distribution can yield the results obtained by ROSINA, some of these solutions having little to no H 2 O originating from the Hapi region itself. Furthermore, the surface activity maps recently derived from ROSINA/DFMS data show a widely varying distribution of activity, with Hapi and potential fallback materials only partially contributing to the activity in the North.…”

“…Coma models are used to predict the entire outgassing pattern from the nucleus and determine the local density, temperature, and velocity of the gas within the portions of the coma probed along the line of sight for remote sensing instruments (Biver et al 2019), or at the spacecraft for ROSINA (Bieler et al 2015a(Bieler et al , 2015bKramer et al 2017;). These models typically use as basis either the physical interactions between gas molecules, such as the Direct Simulation Monte Carlo approach (Fougère et al 2016a;Zakharov et al 2018) or a simplified collosionless coma as used to invert source location from ROSINA/COPS (Kramer et al 2017) and ROSINA/DFMS data , or the geometric characteristics of the coma as observed (Biver et al , 2019. For all approaches, the 67P coma is found to be extremely asymmetric, with water activity originating primarily from the illuminated portions of the nucleus at any point in time, and additionally with latitudinal variability for some species (Hässig et al 2015).…”

The backscattering ratio of comet 67P/Churyumov-Gerasimenko dust coma as seen by OSIRIS onboard Rosetta

“…The presence of a cloud of dm-sized junks close to the nucleus can be corroborated with complex 3D models of the cometary coma and intensity scattering calculations using the complete ensemble of the dust size distribution. At present several authors of this manuscript are involved in using the recently published RZC coma model (Zakharov et al 2018a) to follow this approach. The results of this investigation will be the subject of separate future papers.…”

The refractory-to-ice mass ratio in comets

“…Fluid-dynamical codes of 67P gas coma, observed inbound at heliocentric distances > 3 au, have provided the Hapi's active area fraction ranging from 1.2% (homogeneous model) to 7.5% (Hapi's-dominated inhomogeneous model) (Marschall et al 2017). Other more general time-dependent fluid-dynamical coma models provide the best fit of the same data assuming inhomogeneous solutions with Hapi being a minor water contributor (Zakharov et al 2018b). All these 3D coma models take into account the local insolation, the radiation reflection from facing surfaces, and gas outflow focussing due to the nucleus shape, and show that any adopted inhomogeneous active area fraction over the nucleus surface is still arbitrary.…”

“…2.2.3 Hapi is a dust deposit, nevertheless inbound outgases a water mass similar to the average northern surface (Zakharov et al 2018b).…”