Re-examining the horizontal wavelength spectrum of gravity wave activity in Mars's lower atmosphere

Abstract:

Gravity wave activity in Mars's atmosphere has modulated the escape of Mars's atmosphere over billions of years; gravity wave breaking can enhance turbulent mixing in Mars's equivalent of the mesosphere-lower thermosphere region and increase the concentration of heavier gases at the exobase, while heating/cooling associated with gravity waves propagating in the thermosphere can drive stronger thermal escape. The ongoing Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft has characterized gravity wave activity in Mars's upper atmosphere in unprecedented detail, while on-planet radiance observations from Mars Climate Sounder on board Mars Reconnaissance Orbiter (MRO-MCS) have helped characterize the lower atmospheric (below 30 km) climatology of sub-hourly period, 10-100 km horizontal wavelength gravity waves before and during MAVEN.

Frustratingly, these upper and lower atmospheric climatologies so far disagree. Gravity wave activity is strongest in the upper atmosphere when lower atmospheric gravity wave activity is weakest. One possible explanation for this disagreement is that the upper atmospheric gravity wave activity is primarily sampling 100-500 km horizontal wavelengths, motivating more careful attention to the horizontal wavelength spectrum of gravity wave activity in the lower atmosphere. A previous study of this question by Imamura et al. (2007, J. Atmos. Sci., 64, 1717-1726), however, reported substantial and potentially spurious interannual variability in the horizontal wavelength spectrum derived from observations by the Thermal Emission Spectrometer (TES) on the now-defunct Mars Global Surveyor (MGS) spacecraft, requiring re-examination.

Here, we adapt the methodology previously applied to study gravity wave activity in MRO-MCS on-planet observations to investigate MGS-TES nadir observations during Mars's equivalent of August in two different Mars years to test whether there was interannual variability in lower atmospheric gravity wave activity, evaluate the contribution of Mars's strong thermal tides to the apparent horizontal wavelength spectrum, and determine what horizontal wavelengths should be targeted for a broader survey of lower atmospheric gravity wave activity in MGS-TES observations.

Supported by NASA Mars Data Analysis (80NSSC19K1215)