MAVEN/IUVS Observations of OH Prompt Emission: Daytime Water Vapor in the Thermosphere of Mars

Abstract

AbstractWe report the highest altitude detection of water vapor on Mars to date. The daytime limb observations by the Imaging Ultraviolet Spectrograph (IUVS) on the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft are of hydroxyl (OH) prompt emission near 308 nm, which is excited directly from the photodissociation of water vapor by the solar Lyman‐α flux. Average IUVS daytime water vapor densities near 130 km are 3 × 107 cm−3 around perihelion. The water vapor densities diurnally vary with a peak near midday and no detection at sunrise and sunset. To evaluate the large daytime water vapor densities for self‐consistency, we also report the simultaneous observation of OH solar fluorescence emission near 308 nm in the thermosphere, which enables the retrieval of OH densities. Using a one‐dimensional photochemical model initialized with the daytime IUVS water vapor densities, modeled peak OH densities are in good agreement with the observed IUVS peak OH densities. Because the observed thermospheric temperatures are controlled by solar insolation and cross the water frost point during the day, we suggest that the IUVS observed water vapor is created by the daily sublimation of water ice particles supplied from below. We discuss the implications of the IUVS observations on the present day loss of water vapor from Mars in the form of atomic hydrogen.