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

We 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-alpha flux. Average IUVS daytime water vapor densities near 130 km are 3 x 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. The loss of water from Mars is a compelling topic of study that helps trace its transformation from a warm and wet planet to the cold and dry planet that exists today. Water loss is greatly facilitated by water vapor transport to altitudes above 100 km during the dust storm season on Mars. We report the highest altitude detection of water vapor on Mars to date. Daytime observations between 110 and 150 km altitude indicate a strong diurnal variation of water vapor there, with a peak near midday. The observations are made by the Imaging Ultraviolet Spectrograph (IUVS) on NASA's Mars Atmosphere and Volatile Evolution spacecraft currently orbiting Mars. Simultaneous IUVS observations of hydroxyl, a product of water vapor destruction by solar ultraviolet radiation, reveal a self-consistent picture of daytime water vapor variability. We suggest that the water is lofted to these high altitudes from below as ice particles before they reach altitudes where the temperatures are high enough to release the water in the vapor phase. The IUVS daytime water vapor densities in the upper atmosphere indicate that its escape to space is more efficient than previously thought. OH prompt emission observations provide the highest altitude detection of water vapor on Mars to dateThermospheric water vapor concentrations are diurnally dependent, with a peak near midday at perihelionDiurnal temperature variations are consistent with nighttime sequestration of water as ice, followed by its daytime sublimation to vapor