Gravity waves perturb the Martian atmosphere’s thermal structure

Atmospheric gravity waves arise when mountains and other features cause a wind’s otherwise smooth horizontal flow to oscillate vertically. On Mars, where the topography is rough and high and where wind speeds can reach 400 km/s, gravity waves carry momentum fluxes that far exceed those of their terrestrial counterparts. A new computational study demonstrates that gravity waves not only perturb the Martian atmosphere’s dynamics, as one would expect, but also perturb its thermal structure. Aymeric Spiga of the Pierre and Marie Curie University in Paris and his collaborators sought to explain a puzzling observation made in 1997 by Mars Pathfinder. As the lander parachuted toward the Martian surface, its sensors detected pockets of air whose anomalously low temperature (around 100 K) was cold enough for carbon dioxide, the atmosphere’s main constituent, to condense. Ice clouds of CO₂ were indeed detected by later missions, but at altitudes (60–80 km) at which climate models predicted the atmosphere would be too warm. Two phenomena were proposed to account for the anomaly: gravity waves and thermal tides. Spiga and his colleagues resolved kilometer-scale gravity waves by means of a circulation model whose grid spacing was 10 times finer than the hundreds-of-kilometers spacing typical of global models. The team’s simulations showed that at around 80 km, fluctuations induced by gravity waves alone cause the temperature to dip below 100 K. How the dry-ice droplets nucleate in the thin Martian atmosphere remains a mystery. Solving it will require building a computational bridge across another gap in length scale: to the microphysics of cloud behavior. (A. Spiga et al., Geophys. Res. Lett. 39, L02201, 2012 .)—Charles Day