Diffusion limited escape of hydrogen from Mars

Abstract

Hydrogen escapes from Mars primarily by the Jeans mechanism but the rate is variable and the controlling factors complicated. One of the complications is that the temperature at the Martian exobase varies from ∼100 K in the early morning hours to ∼300 K in the afternoon. At the cold temperatures on the nightside of Mars, H escape rate is limited by Jeans escape, but on the warm dayside H escape is limited by the diffusion rate through the thermosphere. Nevertheless, the hot and cold regions are coupled by efficient ballistic transport through the exosphere. Because of this, H diffuses upward at the diffusion-limited rate even on the nightside and, once H reaches the exosphere, it is transported rapidly by ballistic flow to the warm dayside, where it escapes. As a result, escape is not at all limited by the cold regions of the exobase. The globally integrated escape flux is equal to the globally integrated diffusive limit. Because of this it is important to precisely calculate the diffusion-limited flux and we present a new formulation that is more accurate than the classical formula.