Seasonal variability of the hydrogen exosphere of Mars

The Mars Atmosphere and Volatile EvolutioN (MAVEN) mission measures both the upstream solar wind and collisional products from energetic neutral hydrogen atoms that precipitate into the upper atmosphere after their initial formation by charge exchange with exospheric hydrogen. By computing the ratio between the densities of these populations, we derive a robust measurement of the column density of exospheric hydrogen upstream of the Martian bow shock. By comparing with Chamberlain-type model exospheres, we place new constraints on the structure and escape rates of exospheric hydrogen, derived from observations sensitive to a different and potentially complementary column from most scattered sunlight observations. Our observations provide quantitative estimates of the hydrogen exosphere with nearly complete temporal coverage, revealing order of magnitude seasonal changes in column density and a peak slightly after perihelion, approximately at southern summer solstice. The timing of this peak suggests either a lag in the response of the Martian atmosphere to solar inputs or a seasonal effect driven by lower atmosphere dynamics. The high degree of seasonal variability implied by our observations suggests that the Martian atmosphere and the thermal escape of light elements depend sensitively on solar inputs. Plain Language Summary We utilize a new technique to measure the density of hydrogen escaping into space from Mars, allowing us to observe the variability of hydrogen around Mars over a full Martian year. We find that the current escape of hydrogen from Mars varies dramatically over the Martian year, suggesting a sensitive dependence of the atmosphere and the escape of hydrogen on the input of energy from the Sun, as well as a possible influence of seasonally varying weather patterns. Understanding the present-day and historical hydrogen escape from Mars is crucial to solving the problem of how Mars was transformed from an early warm wet state to the cold dry planet we see today. Our results provide a quantitative data set with good temporal coverage which is complementary to other measurements of Martian hydrogen and which provides important new constraints on models of the Martian atmosphere.