A Numerical Model for the Dynamics and Composition of the Venusian Thermosphere

Abstract

The structure, composition and winds of the mesosphere and thermosphere of Venus are investigated using a nonlinear time-dependent hydrodynamic model. The assumption that all variables depend only on altitude and distance from subsolar point allows a two-dimensional formulation of the problem. Within this framework the model provides an entirely self-consistent treatment of the multi-component fluid. The system solved consists of four time-dependent equations for motion, temperature, and the distributions of O and CO, and two diagnostic equations representing continuity and hydrostatic balance for the total fluid. The model is forced by absorption of solar radiation which provides heating of molecules and dissociation of CO2 into CO and O. A large-scale circulation is calculated, the gross features of which-resemble those derived in an earlier simplified model, consisting of a single cell with rising motion on the dayside, sinking motion on the nightside, and a day-to-night horizontal flow. This circulation in the global mean acts to remove the light gases to balance the photodissociation. Relatively large concentrations of light gases build up on the nightside. The consequent increase of the pressure at a given level acts to block the nightward circulation. Hence little motion occurs in a large region centered around the antisolar point. Instead, most of the downward vertical motion occurs within an internal boundary layer just to the night-side of the terminator. Exospheric temperatures predicted by the model, using an EUV heating efficiency of 0.30, range from greater than 600 K at the subsolar point to less than 300 K at the antisolar point, whereas there are typically 20 40 K horizontal variations of temperature in the mesosphere. Maximum horizontal velocities are order of 300 m s?1 and occur on the dayside near the terminator at the level of the exobase. The model predicts that CO and O will have relative number densities of 4% on the dayside at the level of the F-1 ionospheric peak, provided vertical eddy mixing is negligible.