Mars Hot Oxygen Density and Effective Temperature Derived From the MAVEN IUVS Observations

The escape of hot oxygen atoms from Mars is important for the evolution of the planet's H2O and CO2 inventories. Owing to the lack of data constraints prior to the Mars Atmosphere and Volatile Evolution (MAVEN) mission, previous understanding of this key loss channel of the Mars atmosphere relied mostly on physics-based models. Using the optical observations from the MAVEN Imaging Ultraviolet Spectrograph (IUVS) during 2014-2018, we perform the first inversion analysis of the coronal scans at 130.4 nm to quantify the density and effective temperature of the Mars hot oxygen corona. Our results indicate that the exobase densities of the hot oxygen atoms are similar to 6.3-8.0 x 103 and similar to 1.2 x 104 cm-3 during low and moderate solar activities, respectively, which agree well with recent empirical estimates using MAVEN's in situ measurements and, however, are about a factor of two higher than previous model predictions. The effective temperature varies only slightly between similar to 4100-4500 K. The temperature and density variations with solar, seasonal, and dust conditions are consistent with previous models. Comparison with inversion results from IUVS limb scans indicates that the cold and the hot oxygen populations dominate below similar to 200 and above similar to 600 km, respectively, between which the kinetic distribution depends on both populations. Our results demonstrate for the first time the feasibility of extracting information about the Mars hot oxygen corona directly from optical observations, opening up a powerful means for monitoring the Mars oxygen escape on a global scale in future missions. The Mars hot oxygen corona is a tenuous but important layer of energetic oxygen atoms in the Martian exosphere. These energetic atoms can escape from Mars to space, forming one of the key loss channels of the planet's once abundant surface water. To date, direct measurement of the Mars oxygen escape rate is not instrumentally achievable. Since 2014, the Mars Atmosphere and Volatile Evolution spacecraft has been routinely observing the 130.4 nm emission from the Mars hot oxygen corona, which can be used to infer the escape rate indirectly. However, interpretation of these optical observations is a challenging task due to their low signal-to-noise ratios and the complex multiple scattering effects. Here, by binning and averaging a large number of individual observations and using a novel two-population radiative transfer model, we demonstrate for the first time the feasibility of extracting information about the Mars hot oxygen density and effective temperature directly from the optical observations. Our inversion results are consistent with previous model predictions and recent empirical estimates under typical conditions. With the development of more sensitive instruments, our method can provide a powerful means to measure both long-term and short-term variations of the Mars oxygen escape on a global scale in future missions. We demonstrate for the first time the feasibility of deriving Mars hot oxygen density and effective temperature from optical observationsThe derived density and temperature under various solar, seasonal, and dust conditions are consistent with previous model predictionsOur method opens up a powerful means for long-term and global-scale monitoring of Mars hot oxygen density and temperature in future missions