Comparison between Navier–Stokes and Direct Monte–Carlo Simulations of the Circumnuclear Coma: I. Homogeneous, Spherical Source

The structure of the near-nucleus H2O atmosphere formed by sublimation under solar heating of a hypothetical large dusty ice sphere is computed (1) by the direct simulation Monte–Carlo method (DSMC) and (2) by solving Navier–Stokes equations (NSE) combined with a locally plane-parallel solution of the collisional Boltzmann equation for the nonequilibrium near-surface Knudsen layer. For Hale–Bopp-like comets, perfect agreement is obtained between the two methods on the day and night sides. This excellent agreement is maintained on the near-nucleus dayside for less productive comets, even down to production rates as low as those expected for Comet P/Wirtanen near 3 AU. It provides a direct validation of the gasdynamic simulations performed in support of the ESA Rosetta mission lander descent optimization studies (Crifo et al. 2001) and also confirms the similarity between the dayside coma and underexpanded axially symmetric free jets pointed out in Crifo (1986). On the nightside, moderate to high discrepancies appear between the two solutions as the production rate decreases, revealing the limits of the NSE method. The limits of the present study are delineated, and directions for future investigations are indicated.