Wind‐Enhanced Hydrogen Escape on Mars.

Hydrogen escape is a metric of water loss on Mars, which controls the long‐term evolution of the planet. While the Jeans formulism has been traditionally used to evaluate hydrogen escape on Mars, here we propose that this escape could be significantly enhanced by horizontal winds near the exobase. The enhancement reaches 20% for H and 100% for H2, and presents solar cycle, seasonal, and diurnal variations well correlated with the variations of the horizontal wind velocity. Physically, the enhancement is driven by the departure of the particle velocity distribution from ideal Maxwellian and is critically controlled by the Mach number as the ratio of the horizontal wind velocity to the thermal velocity. A larger Mach number (due to a larger wind velocity, a lower temperature, a larger particle mass, or any combination of the three) implies a stronger degree of departure and consequently more enhanced escape over the Jeans case. Plain Language Summary: Hydrogen escape is a metric of water loss on Mars which plays a crucial role in the long‐term climate evolution of the planet. The traditional Jeans formulism, proposed one century ago, has been widely used to evaluate hydrogen escape on Mars, but previous experiences with atmospheric escape of light species on both the Earth and Titan imply that the Jeans escape flux may be seriously underestimated due to the neglect of horizontal winds in the upper atmosphere. In this study, we perform simple kinetic calculations that confirm the scenario of significantly enhanced hydrogen escape on Mars as driven by the presence of horizontal winds. The degree of such an enhancement is surprisingly large, which could reach 20% for H escape and even 100% for H2 escape, urging reassessment of water loss from Mars both at the current epoch and over the history. Physically, the enhancement of hydrogen escape is linked to a significant departure of the particle velocity distribution from ideal Maxwellian. Meanwhile, the horizontal wind field in the Martian upper atmosphere is known to demonstrate interesting solar cycle, seasonal, and diurnal variations, which are well inherited by the predicted variations of the wind‐driven enhancement of hydrogen escape. Key Points: Horizontal winds near the Martian exobase can enhance the H and H2 escape fluxes by up to 20% and 100%Wind‐driven enhancement of hydrogen escape is more prominent for fast horizontal winds and low temperatures near the exobaseWind‐enhanced hydrogen escape presents significant solar cycle, seasonal, and diurnal variations [ABSTRACT FROM AUTHOR]