Exploring Martian Day-Night Mesosphere and Thermosphere by Stellar Occultations

The MAVEN (Mars Atmosphere and Volatile EvolutioN) mission to Mars is currently in its ninth year orbiting around the red planet, providing a vast wealth of observations to study its upper atmosphere and ionosphere, as well as its atmospheric evolution. The Imaging Ultraviolet Spectrograph (IUVS) onboard the MAVEN spacecraft is one of the most powerful ultraviolet (UV) spectrographs sent to another planet, with distinctive pointing and scanning capabilities. Stellar occultation is one of the observation modes of the IUVS and the first dataset to consistently measure both the dayside and nightside Martian mesosphere, with an extensive geophysical coverage. This atmospheric region is closely coupled to the layers above and below and is critical in understanding the coupling processes, global circulation patterns, energy balance, and atmospheric loss.

As the stellar signal of a UV bright star traverses the Martian atmosphere, it gets attenuated due to absorption by atmospheric constituents. From the transmission spectra due to such absorption features, stellar occultations measure column abundances of CO2, O2, O3, and the aerosol optical depth, from which are retrieved the local densities and profiles of temperature and pressure in the 20 – 160 km altitude range. The absorption spectra may at times be contaminated by dark current, cosmic rays, stray light, and atmospheric emissions. Of all these factors, contamination by stray light during dayside or bright terminator observations is most prevalent and affects ~60% of stellar occultation events so far. These dayside contaminated spectra are not processed by the data retrieval pipeline, resulting in a huge data gap and significant loss of important information and science. We have recently succeeded in rescuing these observations by applying an improved stray light removal algorithm. I will talk on the science results from this expanded and enhanced dataset. 

 We have not only been able to establish for the first time how the Martian upper mesosphere/lower thermosphere (80 – 160 km) varies on a diurnal scale, but also found that the model predictions are inconsistent with the observations. For example, our study on the day-night thermal structure reveals that this region is predominantly driven by diurnally varying solar heating as the semidiurnal migrating tides do not seem to propagate to the upper mesosphere as efficiently as predicted by the Mars Planetary Climate Model (PCM). Another study on the day-night variation of O2 shows a local time dependence of the O2 mole fraction, with a nightside enhancement that is explained in terms of the relative role of solar driven rapid horizontal winds and slower vertical diffusion. The PCM predicts a similar diurnal trend but underestimates the O2 mixing ratio. These data-model discrepancies imply that our current understanding of the dynamics and structure of this important region is inadequate, and this dataset can provide unprecedented constraints on the models.