Thermal Forcing of Near-Surface Temperatures by Martian Water Ice Clouds

Abstract:

We explore the potential role of water ice clouds in moderating the nighttime surface temperature within Gale crater, Mars, as observed by the Rover Environmental Monitoring Station (REMS) instrument suite on board the Curiosity rover. Leading up to aphelion (late northern spring), nighttime minimum atmospheric temperatures should, to first order, decrease steadily, given Mars’ increasing distance from the Sun. However, just prior to aphelion, a pause in this trend is observed, and nighttime minimum temperatures within Gale crater remain steady, or rise slightly, over a period of a few weeks, before resuming their downward trend. This ‘inflection’ feature has been detected in all Mars years observed by Curiosity to date. Given the season, and supported by direct and indirect observations by the Curiosity payload and other Mars spacecraft, we contend that this behavior is due to the formation of twilight and nighttime clouds, which re-radiate outgoing longwave radiation, and reduce overnight atmospheric and surface cooling.

Twilight clouds are consistent with the brief warming of the atmosphere of ~3-5 K after sunset in this season, depending on the sol; surface temperatures experience a similar level of warming. Such clouds are identifiable in REMS Ultraviolet Sensor signals, as well as, in some circumstances, visibly, in the form of post-sunset, high-altitude noctilucent clouds. Overnight, IR measurements by the Mars Climate Sounder show the presence of similar high-altitude ice clouds over Gale crater. Calculated local energy balance over the course of the night shows an increase in IR reflectance and decrease in IR emissivity, suggesting a thickening of ice clouds as nighttime temperature cool. As we are unable to visibly observe nighttime clouds at night, this behavior could be an alternate means of inferring the presence of nighttime clouds on Mars.