We examine the evolution of the water production of comet 67P/Churyumov-Gerasimenko during the Rosetta mission (2014 June-2016 May) based on in situ and remote sensing measurements made by Rosetta instruments, Earth-based telescopes and through the development of an empirical coma model. The derivation of the empirical model is described and the model is then applied to detrend spacecraft position effects from the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA) data. The inter-comparison of the instrument data sets shows a high level of consistency and provides insights into the water and dust production. We examine different phases of the orbit, including the early mission (beyond 3.5 au) where the ROSINA water production does not show the expected increase with decreasing heliocentric distance. A second important phase is the period around the inbound equinox, where the peak water production makes a dramatic transition from northern to southern latitudes. During this transition, the water distribution is complex, but is driven by rotation and active areas in the north and south. Finally, we consider the perihelion period, where there may be evidence of time dependence in the water production rate. The peak water production, as measured by ROSINA, occurs 18-22 d after perihelion at 3.5 ± 0.5 × 10 28 water molecules s −1. We show that the water production is highly correlated with ground-based dust measurements, possibly indicating that several dust parameters are constant during the observed period. Using estimates of the dust/gas ratio, we use our measured water production rate to calculate a uniform surface loss of 2-4 m during the current perihelion passage.

The processes that led to the formation of the planetary bodies in the Solar System are still not fully understood. Using the results obtained with the comprehensive suite of instruments on-board ESA's Rosetta mission, we present evidence that comet 67P/Churyumov-Gerasimenko likely formed through the gentle gravitational collapse of a bound clump of mm-sized dust aggregates ("pebbles"), intermixed with microscopic ice particles. This formation scenario leads to a cometary make-up that is simultaneously compatible with the global porosity, homogeneity, tensile strength, thermal inertia, vertical temperature profiles, sizes and porosities of emitted dust, and the steep increase in water-vapour production rate with decreasing heliocentric distance, measured by the instruments on-board the Rosetta spacecraft and the Philae lander. Our findings suggest that the pebbles observed to be abundant in protoplanetary discs around young stars provide the building material for comets and other minor bodies.

We analyse the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA)-the Double Focusing Mass Spectrometer data between 2014 August and 2016 February to examine the effect of seasonal variations on the four major species within the coma of 67P/Churyumov-Gerasimenko (H 2 O, CO 2 , CO, and O 2), resulting from the tilt in the orientation of the comet's spin axis. Using a numerical data inversion, we derive the non-uniform activity distribution at the surface of the nucleus for these species, suggesting that the activity distribution at the surface of the nucleus has not significantly been changed and that the differences observed in the coma are solely due to the variations in illumination conditions. A three-dimensional Direct Simulation Monte Carlo model is applied where the boundary conditions are computed with a coupling of the surface activity distributions and the local illumination. The model is able to reproduce the evolution of the densities observed by ROSINA including the changes happening at equinox. While O 2 stays correlated with H 2 O as it was before equinox, CO 2 and CO, which had a poor correlation with respect to H 2 O pre-equinox, also became well correlated with H 2 O post-equinox. The integration of the densities from the model along the line of sight results in column densities directly comparable to the VIRTIS-H observations. Also, the evolution of the volatiles' production rates is derived from the coma model showing a steepening in the production rate curves after equinox. The model/data comparison suggests that the seasonal effects result in the Northern hemisphere of 67P's nucleus being more processed with a layered structure while the Southern hemisphere constantly exposes new material.

ABSTRACT67P/Churyumov-Gerasimenko (67P) is a Jupiter-family comet and the object of investigation of the European Space Agency mission Rosetta. This report presents the first full 3D simulation results of 67P's neutral gas coma. In this study we include results from a direct simulation Monte Carlo method, a hydrodynamic code, and a purely geometric calculation which computes the total illuminated surface area on the nucleus. All models include the triangulated 3D shape model of 67P as well as realistic illumination and shadowing conditions. The basic concept is the assumption that these illumination conditions on the nucleus are the main driver for the gas activity of the comet. As a consequence, the total production rate of 67P varies as a function of solar insolation. The best agreement between the model and the data is achieved when gas fluxes on the night side are in the range of 7% to 10% of the maximum flux, accounting for contributions from the most volatile components. To validate the output of our numerical simulations we compare the results of all three models to in situ gas number density measurements from the ROSINA COPS instrument. We are able to reproduce the overall features of these local neutral number density measurements of ROSINA COPS for the time period between early August 2014 and January 1 2015 with all three models. Some details in the measurements are not reproduced and warrant further investigation and refinement of the models. However, the overall assumption that illumination conditions on the nucleus are at least an important driver of the gas activity is validated by the models. According to our simulation results we find the total production rate of 67P to be constant between August and November 2014 with a value of about 1 × 10 26 molecules s −1 .

Context. Since its rendezvous with comet 67P/Churyumov-Gerasimenko (67P), the Rosetta spacecraft has provided invaluable information contributing to our understanding of the cometary environment. On board, the VIRTIS and ROSINA instruments can both measure gas parameters in the rarefied cometary atmosphere, the so-called coma, and provide complementary results with remote sensing and in situ measurement techniques, respectively. The data from both ROSINA and VIRTIS instruments suggest that the source regions of H 2 O and CO 2 are not uniformly distributed over the surface of the nucleus even after accounting for the changing solar illumination of the irregularly shaped rotating nucleus. The source regions of H 2 O and CO 2 are also relatively different from one another. Aims. The use of a combination of a formal numerical data inversion method with a fully kinetic coma model is a way to correlate and interpret the information provided by these two instruments to fully understand the volatile environment and activity of comet 67P. Methods. In this work, the nonuniformity of the outgassing activity at the surface of the nucleus is described by spherical harmonics and constrained by ROSINA-DFMS data. This activity distribution is coupled with the local illumination to describe the inner boundary conditions of a 3D direct simulation Monte-Carlo (DSMC) approach using the Adaptive Mesh Particle Simulator (AMPS) code applied to the H 2 O and CO 2 coma of comet 67P. Results. We obtain activity distribution of H 2 O and CO 2 showing a dominant source of H 2 O in the Hapi region, while more CO 2 is produced in the southern hemisphere. The resulting model outputs are analyzed and compared with VIRTIS-M/-H and ROSINA-DFMS measurements, showing much better agreement between model and data than a simpler model assuming a uniform surface activity. The evolution of the H 2 O and CO 2 production rates with heliocentric distance are derived accurately from the coma model showing agreement between the observations from the different instruments and ground-based observations. Conclusions. We derive the activity distributions for H 2 O and CO 2 at the surface of the nucleus described in spherical harmonics, which we couple to the local solar illumination to constitute the boundary conditions of our coma model. The model presented reproduces the coma observations made by the ROSINA and VIRTIS instruments on board the Rosetta spacecraft showing our understanding of the physics of 67P's coma. This model can be used for further data analyses, such as dust modeling, in a future work.