Diffusion limited escape of hydrogen from Mars

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 ∼100K in the early morning hours to ∼300K 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. •A new, more accurate, formulation of the diffusion-limited flux for an escaping species is provided.•The exospheric temperature on Mars spans the range at which hydrogen escape is kinetically limited to diffusion limited.•Efficient ballistic transport through the exosphere results in a strong day-night flow of hydrogen.•Because of the ballistic flow, the upward transport of H on Mars is diffusion limited on the nightside despite the low temperatures.•The globally integrated H escape rate is equal to the globally integrated diffusion limit.