First Detection of the Nitric Oxide Dayglow on Mars
Résumé
Nitric oxide (NO) is a well-known indicator of solar and auroral activity in the terrestrial upper atmosphere. Direct measurements of NO on Mars can therefore constrain studies of energetic processes controlling the composition and structure of its upper atmosphere (80-200 km). Identifying and quantifying these processes is one of the science objectives of NASA’s Mars Atmosphere and Volatile Evolution (MAVEN) mission currently orbiting Mars. NO can be observed directly by solar resonance fluorescence in the mid-ultraviolet (MUV). Indeed, this approach has routinely been used to measure terrestrial NO for 50 years. On Mars, this “dayglow” emission is very weak relative to other bright MUV features and thus has confounded attempts at its detection there for nearly the same amount of time. Here, we report the first detection of the NO dayglow in the Martian atmosphere using limb observations by the Imaging Ultraviolet Spectrograph (IUVS) on the MAVEN spacecraft. The detection is enabled by the spectral modeling and removal of the carbon monoxide Cameron bands, which dominate the MUV limb spectra. We focus on the spectral region between 213.0-225.5 nm, where three NO gamma bands emit. We will infer NO densities from the dayglow spectra and compare our observations with predictions from a photochemical model. We will discuss the implications, particularly in the context of previous in situ measurements.