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

Martian gravity waves (GW) greatly impact the atmospheric circulation and formation of clouds, but many GW observations of the lower atmosphere are confined to specific orientations and wavelengths, leaving many gaps in the continuum of waves. To overcome the issue, we analyze eight Mars years of data during the season of L-s = 120 degrees-150 degrees; from Band 10 (14.9 mu m) of the Thermal Emission Imaging System (THEMIS), sensitive to GWs at similar to 25 km altitude. All horizontal orientations at wavelengths below similar to 40 km are detectable, with the potential to detect north-south oriented GWs up to similar to 1,000 km in length. Most THEMIS observations have brightness temperature variances compatible with GW disturbances. Intense GW activity concentrates poleward of 60 degrees S, with normalized magnitudes up to 10(-4) K-2 K-2; activity decreases toward the equator and remains low throughout the northern hemisphere. The interannual intensity of GWs varies by latitude within three regimes of GW length: short (< 20 km), medium (20-100 km), and long (> 100 km). Gravity wave orientations that are detectable in all directions do not favor a single direction; that is, Martian GWs at 25 km altitude are isotropic. Finally, interannual variability seems to emerge from interactions with dust storms, orography, and planetary waves. Plain Language Summary Small-scale disturbances in the atmosphere-called gravity waves (GW)-transport momentum and energy throughout the atmosphere, so understanding their strength and direction is important for understanding the circulation of Mars's atmosphere. We calculate wave occurrence within a data set previously unused in the documentation of GWs. The Thermal Emission Imaging System allows for calculation of waves in all horizontal directions 25 km above the surface. We find that GWs are strongest during the middle of northern hemisphere summer in the southern hemisphere and do not propagate in any preferred direction. Three regimes of waves that vary in latitude and/or year are found with short, medium, and long wavelengths. Finally, waves can be formed by winds blowing over and around mountains and craters, larger types of waves, or dust events.