Modeling of comet thermal emissions and cometary gas spectral lines for MIRO observations of Comet 67P

Realistic modeling of observations of comet thermal emissions and cometary gas spectral lines is critical in preparation for observations of Comet 67P/Churyumov-Gerasimenko from the ESA Rosetta spacecraft. The Microwave Instrument for the Rosetta Orbiter (MIRO), on board the Rosetta spacecraft, will observe the continuum thermal emissions of the comet nucleus at millimeter and submillimeter wavelengths, and spectral lines of water, carbon monoxide, ammonia, and methanol at submillimeter wavelengths. A thermophysical nucleus model is used to calculate temperature profiles in a subsurface layer for assumed thermal properties of the subsurface. The calculated thermal profiles are used to simulate the continuum thermal emission signal that can be measured by MIRO. These subsurface temperatures are also the input to a molecular diffusion model, used to study the escape process of volatile species from an icy interior through a dust layer in the comet nucleus. The diffusion model then provides the boundary conditions (such as the gas velocity and escape rate) for a Direct-Simulation Monte Carlo (DSMC) model to study the gas kinetics in the cometary coma. Note that over most of its extent, the coma is not in thermodynamic equilibrium. For a given coma profile modeled by DSMC, cometary gas spectral lines that can be measured by MIRO are simulated with a radiative transfer model. By using these models in combination, we study the effect of the physical conditions of the comet nucleus and coma on the observations by MIRO at various observational conditions such as different heliocentric distances, local solar phases, observation distances to the comet nucleus, and viewing angles (e.g. nadir or limb). Figure 1 illustrates a modeled thermal seasonal cycle of subsurface layers of Comet 67P, modeled with a 20 cm thick and 70% porous dust layer on top of an icy interior. The temperature peak (or minimum) at deeper locations in the subsurface is time delayed compared to the peak at the surface due to a high thermal inertia in the icy interior.