Observations of Gravity Waves in the Middle Atmosphere of Mars

Abstract:

Introduction: Internal gravity waves are a common atmospheric feature created by adiabatic displacement of air parcels that are restored by buoyancy [1]. Vertically propagating gravity waves transport momentum and energy upward from the lower atmosphere, influencing local temperatures where they break and deposit energy [2]. Gravity waves are characterized by much shorter horizontal and vertical wavelengths than other atmospheric waves [3], so detecting them requires high resolution vertical profiles. Gravity waves have been detected in the lower atmosphere of Mars by radio occultations [4, 5, 6] and in the upper atmosphere by in situ measurements [7] but detections in the mesosphere at sufficient resolution are more difficult. Earth-based stellar occultations, which utilize the line-of-sight alignment of a star with a solar system body, are capable of sampling the Martian mesosphere with sufficient resolution to detect gravity waves.

Aims: The occultation of ε Geminorum by Mars in 1976 was observed with high resolution but had to be analyzed at much lower resolution due to computational limitations [8]. Without those limitations today and owing to a much-improved stellar occultation analysis (“inversion”), we re-analyze the 1976 occultation to search for gravity waves.

Methods: Inversion solves an Abel transform for each value of stellar flux observed to determine pressure, temperature, and number density of the Martian mesosphere [9]. We use the full resolution, at 25 times greater than the original work. We search for wave activity by measuring consistent departures of number density from the background atmosphere, and compare to models of gravity wave behavior.

Results: We find perturbations of 5 km vertical wavelength at a significance of 2-3σ, undetected in the original analysis. We compare the high-resolution atmospheric profiles to other remote sensing and model results. We consider amplitude variation and horizontal wavelength, among other factors, in finding that these are likely caused by internal gravity waves in the Martian mesosphere [10].

[1] Fritts & Alexander 2003

[2] Mueller-Wodarg 2008

[3] England et al. 2017

[4] Creasey et al. 2006

[5] Tellmann et al. 2013

[6] Patzold et al. 2016

[7] Siddle et al. 2019

[8] Elliot et al. 1977

[9] Elliot, Person, & Qu 2003

[10] Saunders, Person, & Withers, in review