The Horizontal Wavelength Spectrum of Gravity Wave Activity in Mars’s Lower Atmosphere: The Perspective from MGS–TES Nadir Observations

Abstract

Connecting upper-atmospheric gravity waves (GW) to their lower- and middle-atmospheric (0–30 and 30–100 km altitude) origins can improve understanding of the evolution of Mars’s atmosphere as well as its present thermal structure and general circulation. A recent study using observations from the Mars Climate Sounder (MCS) on board the Mars Reconnaissance Orbiter (MRO) characterized the climatology of lower-atmospheric GW with 10–100 km horizontal wavelengths, but GW observed in the upper atmosphere have horizontal wavelengths of up to 500 km, motivating more careful attention to the horizontal wavelength spectrum of lower-atmospheric GW. A previous study of observations by the Thermal Emission Spectrometer (TES) on Mars Global Surveyor (MGS) reported substantial interannual variability in the horizontal wavelength spectrum of wave activity defined broadly, including GW. Here, we derive the horizontal wavelength spectrum of wave activity from the brightness temperature variance spectrum of MGS–TES observations resampled to the MRO–MCS spectral channels during L s = 120°–150°, Mars Years 24 and 25. Lower-atmospheric wave activity is strongest at wavelengths <200 km, a population which resembles that observed by MRO–MCS and is likely GW activity. This short-wavelength population is distinct from a wave population with wavelengths >1000 km, which appears to be a mixture of tides and planetary waves. We find that interannual variability in GW activity previously identified using MGS–TES data largely arises from a change in the instrumental noise characteristics of MGS–TES during the first quarter of MY 25.

Martian Gravity Waves Observed by the Thermal Emission Imaging System (THEMIS) During Northern Summer

“…Many THEMIS swaths exhibit GWs, and the incredibly long duration of the THEMIS data set allows for the assessment of interannual variability and direct comparison to MCS GW assessments to test the coverage and sensitivity of THEMIS to many GW lengths and orientations. We provide several insights on Martian GWs: The largest GW activity occurs poleward of 60°S, with magnitudes up to $${\mathrm{log}}_{10}\left({\hat{{\Omega }}}_{GW}\right)$$ = −4 K 2 K −2 , potentially due to reduced filtering of waves from the surface from a strong winter‐time westerly jet (Heavens, Pankine, Battalio, & Wright, 2022; Heavens, Pankine, Battalio, Wright, Kass, et al., 2022). Reduced GW activity occurs along the equator and in the northern hemisphere. Inter‐annual variability of GWs is substantial but is mainly correlated within zonal bands.…”

“…The unique measurement platform provided by THEMIS enables an extensive analysis of Mars's GW activity at 25 km altitude. While Band 10 of THEMIS broadly observes in a single vertical layer, horizontal information in the troposphere is expansive compared to observations provided by TES or MCS (e.g., Heavens, Pankine, Battalio, & Wright, 2022; Heavens, Pankine, Battalio, Wright, Kass, et al., 2022). Each THEMIS swath contains thousands of individual baselines (Ξ λ , θ ), each of which is a measure of GW activity ($${\hat{{\Omega }}}_{GW}$$) in a specific orientation and length, but the resultant spectrum contains binned baselines ( ξ λ , θ ).…”

“…The largest GW activity occurs poleward of 60°S, with magnitudes up to $${\mathrm{log}}_{10}\left({\hat{{\Omega }}}_{GW}\right)$$ = −4 K 2 K −2 , potentially due to reduced filtering of waves from the surface from a strong winter‐time westerly jet (Heavens, Pankine, Battalio, & Wright, 2022; Heavens, Pankine, Battalio, Wright, Kass, et al., 2022). Reduced GW activity occurs along the equator and in the northern hemisphere.…”