Mars H Escape Rates Derived From MAVEN/IUVS Lyman Alpha Brightness Measurements and Their Dependence on Model Assumptions

Abstract

AbstractMars has lost a large fraction of its water to space, with the H component of this loss thought to occur mainly as a result of thermal (Jeans) escape from the upper atmosphere. Constraints on H loss have historically been made using hydrogen Lyman alpha (121.6 nm) light scattered in the planet's extended upper atmosphere or corona. Here we employ observations from the Mars Atmosphere and Volatile Evolution (MAVEN) mission's Imaging Ultraviolet Spectrograph (IUVS) to constrain H escape in December 2014 and August 2016, when MAVEN observed the dayside corona at low latitude. To obtain adequate fits and address systematic sources of uncertainty including instrument calibration, we fit in exobase number density and escape rate instead of density and temperature, employing Markov Chain Monte Carlo techniques. This produces better model fits to data than most previous analyses. When we assume a single population of H atoms, we obtain H temperatures inconsistent with expected trends and a shape mismatch between observed and modeled profiles, similar to previous studies. Introducing either a second population of H (at a distinct temperature and density) or adding deuterium to the corona allows for essentially perfect fits. Despite this model ambiguity, derived loss rates for both periods are within a factor of four, 3.3–8.8×108cm−2/s in December 2014 (Ls∼250) and 0.6–2.3×108cm−2/s in August 2016 (Ls∼200). These rates are similar to those found in prior studies and confirm the known seasonal trend—doing so while incorporating the substantial uncertainty in absolute calibration insufficiently explored by previous studies.