Loss of Thermal Contact Between Dust and Gas in the Mars Atmosphere

Collisions between gas molecules and dust grains in the lower Martian atmosphere keep them thermally equilibrated at nearly the same temperature. But as Goldensen et al. (2008; GRL, 35) show this equilibrium breaks down as low as 40 km above the surface where gas and dust temperatures begin to diverge. Given that dust has been observed at altitudes of 70-90 km above the surface of Mars today (Heavens et al., 2019; JGR Planets, 124) and is present in significant quantities at those altitudes in land planet simulations of Mars-like planets orbiting close to the sun (Hartwick et al., 2021; in preparation), the difference between dust and gas temperatures may become important for heating rate calculations in general circulation models. We have developed a 1-D radiative-convective model that includes the coupling physics of dust and gas, and report preliminary results here. The model apportions the available radiative energy streams into dust and gas heating while conserving total energy. As in Goldensen et al., we find that dust-gas temperature differences increase above 40 km depending on particle size, time of day, dust optical properties, total dust burden, and vertical distribution. Steady-state solutions for a fixed sun angle show that in the region where thermal contact is lost the gas cools as the dust warms. This is because emission from dust increases as collisional losses are reduced and the grains heat up. Consequently, the gas cools to maintain energy balance. For typical conditions where dust is mostly confined to altitudes below 40 km there is very little effect on gas temperatures. But for situations where dust reaches the 70-90 km level, gas temperatures can cool by as much as 10 K or more. We have also explored the behavior of dust and gas temperatures over a full diurnal cycle. In this case dust grains above 40 km are still generally warmer than the gas during the day but they are much closer to gas temperatures at night. This asymmetry in the behavior of the dust-gas temperature difference is due to stronger daytime heating by solar radiation than nighttime infrared heating from a relatively cold gas. We conclude that there are some conditions on Mars today, e.g., global dust storms, where loss of thermal contact between dust and gas at high altitudes could have an impact on local heating rates.