Relating in situ gas measurements to the surface outgassing properties of cometary nuclei

The sensitivity of the gas flow field to changes in different initial conditions has been studied for the case of a highly simplified cometary nucleus model. The nucleus model simulated a homogeneously outgassing sphere with a more active ring around an axis of symmetry. The varied initial conditions were the number density of the homogeneous region, the surface temperature, and the composition of the flow (varying amounts of H2O and CO2) from the active ring. The sensitivity analysis was performed using the Polynomial Chaos Expansion (PCE) method. Direct Simulation Monte Carlo (DSMC) was used for the flow, thereby allowing strong deviations from local thermal equilibrium. The PCE approach can be used to produce a sensitivity analysis with only four runs per modified input parameter and allows one to study and quantify non-linear responses of measurable parameters to linear changes in the input over a wide range. Hence the PCE allows one to obtain a functional relationship between the flow field properties at every point in the inner coma and the input conditions. It is for example shown that the velocity and the temperature of the background gas are not simply linear functions of the initial number density at the source. As probably expected, the main influence on the resulting flow field parameter is the corresponding initial parameter (i.e. the initial number density determines the background number density, the temperature of the surface determines the flow field temperature, etc.). However, the velocity of the flow field is also influenced by the surface temperature while the number density is not sensitive to the surface temperature at all in our model set-up. Another example is the change in the composition of the flow over the active area. Such changes can be seen in the velocity but again not in the number density. Although this study uses only a simple test case, we suggest that the approach, when applied to a real case in 3D, should assist in identifying the sensitivity of gas parameters measured in situ by, for example, the Rosetta spacecraft to the surface boundary conditions and vice versa

Comparison of DSMC and Euler Equations Solutions for Inhomogeneous Sources on Comets

Cometary activity arises from the sublimation of surface ices into the gas phase. Dust is entrained in the gas and is accelerated by gas drag as the gas escapes into interplanetary space. Previous observations [1, 2] of cometary nuclei have shown remarkable, diverse structures in the near‐nucleus dust distribution. The gas from the comet expands into vacuum and the flow therefore passes through a wide range of densities and pressures and hence though different flow regimes. While a direct simulation Monte Carlo (DSMC) simulation [3] should give, in all cases, accurate results if applied correctly, the calculation becomes slow in denser regions. Solving the Euler equations (EE) is in these cases faster, but they are only an approximation for dilute gas. A direct comparison between DSMC and EE was made by Lukianov et al. [4] for water vapor subliming from an ice sphere into vacuum. He found that the Euler equation with correct boundary conditions gives a good approximation to the velocity and density field in the supersonic region even at global Kn>10⁻³. We have performed further comparisons of the DSMC and EE methods for a case with an ‘active cap’: A region with a half opening angle of 10° has twice the production rate compared with the rest of the sphere. We found that far away from the active cap, the flow is like one from a sphere and our results are in a very good agreement with Lukianov et al.’s [4]. However the Euler equations start to fail close to and at the sides of the jet produced by the active area at lower Knudsen numbers

Comparison of DSMC and Euler Equations Solutions for Inhomogeneous Sources on Comets

Abstract. Cometary activity arises from the sublimation of surface ices into the gas phase. Dust is entrained in the gas and is accelerated by gas drag as the gas escapes into interplanetary space. Previous observations Cometary nuclei are small having a typical diameter of a few kilometers. From Hubble observations [8], a size of 3x5 km is estimated for 67P/Churyumov-Gerasimenko. For ground-based observations, the nucleus is difficult to observe directly because of dust emission. This leads to the need for spacecraft observations in situ as those that will be provided by Rosetta. Our aim is to interpret the appearance of structures in the dust and gas coma and to prepare for the data analysis phase of Rosetta starting in 2014. We particularly wish to understand the relationship between surface activity and the gas and dust distribution in the inner coma. The activity of a comet is produced by the sublimation of water ice and therefore depends strongly on the distance to the sun which provides the energy source to drive the sublimation. Dust is entrained in the gas and is accelerated by gas drag as the gas escapes into interplanetary space. The dust acts to (partially) obscure the surface from view. The Rosetta spacecraft will go into orbit around the comet and accompany it from 4 astronomical units through to perihelion. To plan the observations it is necessary to understand and interpret observations quickly so that the results can be used to plan follow-up sequences. It is also a special situation because the boundary conditions are not well known: E.g. there are contradicting interpretations regarding the thermal conductivity of the surfac