Assessing Radiative Transfer in Water-Ice Clouds above the Phoenix Mars Landing Site in Multiple Wavelengths

Characterizing the climate and dynamics of the Martian atmosphere depends on a thorough understanding of the radiative properties of water-ice clouds. These clouds alter atmospheric dynamics through scattering, absorption, and emission of radiation, such that Mars Global Climate Models need radiatively active water-ice clouds to recreate near-surface air temperatures. The opacity of the clouds is a key indicator for determining their radiative influence.

The Phoenix Mars lander, situated in the northern polar region of Mars, observed water-ice clouds throughout its 151-sol mission using the lidar and Surface Stereo Imager (SSI). Here, we analyze the water-ice clouds above the Phoenix landing site by examining the radiative transfer in thermal and visible wavelengths. First, a complete record of water-ice clouds is built by modelling the surface energy balance at the Phoenix site, isolating the thermal contribution from the clouds above the lander. This is achieved by modelling the MET temperature data acquired nearly continuously throughout the mission. We observe that clouds emit up to 30 W m^-2 of longwave radiation toward the surface throughout the night, increasing the near-surface air temperature. Next, we use images taken with the SSI in combination with a doubling-and-adding radiative transfer model to derive the water-ice content of the atmosphere. This allows us to make a direct comparison between the visible opacity of the cloud and the amount of thermal radiation emitted by the cloud to better constrain their effect on the atmosphere.