An Empirical Predictive Model for Atmospheric H Lyman‐α Emission Brightness at Mars.

Characterizing the abundance of atmospheric hydrogen (H) at Mars is critical for determining the current and, subsequently, the primordial water content on the planet. At present, the atmospheric abundance of Martian H is not directly measured but is simulated using proprietary models that are constrained with observations of H Lyman‐α emission brightness, as well as with observations of other atmospheric parameters, such as temperature and Solar UV irradiance. Publicly available brightness measurements require further processing to have scientific utility. To make the data needed to model H abundances and escape rates more accessible to the community, we use H Lyman‐α emissions made with the Mars Atmosphere and Volatile Evolution (MAVEN) mission. The near decade‐spanning data set is reduced to obtain disk‐pointed averages of the H brightness in the upper atmosphere of Mars and then analyzed for statistical trends across multiple variables. The H Lyman‐α emission brightness is found to be dependent on Solar illumination, Solar cycle, and season. The resulting data trends are used to derive empirical fits to build a predictive framework for future observations or an extrapolative tool for estimates of water content at previous epochs. Data that was intentionally not included in the empirical derivations are used to validate the predictions successfully to within 18% accuracy, on average. This first‐of‐its kind predictive model for H brightness is presented to the community and can be used with atmospheric models to further derive and interpret the abundances and escape rate of H atoms at Mars. Plain Language Summary: The upper atmosphere of Mars contains Hydrogen atoms that can escape into outer space. Since these atoms originate from water, understanding the abundance and variation of these atoms is important to understanding water escape from Mars. There are no direct measurements of H abundances in the upper atmosphere of Mars. However, H atoms emit UV photons, and these photons can be measured and used with models to derive H atom abundances. This work investigates the UV photon emissions of H atoms from the upper atmosphere of Mars using a 9‐year data set by converting the data into scientifically useable form and studying the trends with time. A predictive model is developed to successfully replicate the trends in the UV emissions of H atoms according to illumination, seasonal, and Solar activity conditions. This predictive model can be used with other tools and observed quantities to derive H abundances to infer the present and evolving water content at Mars. Key Points: Nine years of H Ly‐α brightness measurements are found to depend on Solar cycle, illumination, and Martian seasonStatistical fits for H Ly‐α brightness across independent variables are empirically derivedA predictive tool for atmospheric H brightness at Mars is developed and shared with the community [ABSTRACT FROM AUTHOR]