The phase of water ice which forms in cold clouds in the mesospheres of Mars, Venus and Earth

Abstract

Water ice clouds can form in the coldest regions of rarefied planetary upper atmospheres, where particles grow through deposition from the vapor phase. Despite the very low partial pressures of water in the upper atmospheres of Venus, Earth, and Mars, the temperatures can be low enough (∼<150 K, depending on the planet) to lead to large supersaturations and the formation of ice particles. At these temperatures multiple metastable forms of ice can exist, but the phases that can form remain poorly defined. In this study, we focus on understanding the phase of ice that forms in this class of very cold clouds which exist in the upper atmospheres of three of the terrestrial planets in the solar system. In Earth's upper mesosphere (80-90 km), nanoparticles composed primarily of water ice form between 100 and 150 K, producing clouds known as Polar Mesospheric Clouds (PMCs) or noctilucent clouds (NLCs) (Hervig et al., 2001;Rapp & Thomas, 2006;Thomas, 1991). In the Martian atmosphere, water ice clouds have also been observed planet wide at altitudes up to 90 km where temperatures can drop to <120 K (Fedorova et al., 2020;Forget et al., 2009;Vincendon et al., 2011). In addition, water ice particles may serve as seeds for CO 2 ice particles in the Martian mesosphere (Plane et al., 2018). On Venus, the possibility of water