Clouds in the Polar Night of Mars: Modeling and Observations with the Mars Climate Sounder

Introduction: Polar “cold spots” – regions of unusually low brightness temperature – affect the Martian climate by reducing the outgoing heat flux, which is balanced by the latent heat of CO2 sublimation. Since the Viking era, their cause has remained uncertain. Using radiometric observations from the Mars Climate Sounder (MCS) during southern winter, we previously reported that transient cold spots appearing during polar night are statistically correlated with the presence of CO2 ice clouds [1]. In the present work, we successfully reproduce the MCS observations by employing a radiative transfer model including scattering in a layered, cloudy, dusty atmosphere. Surface emissivity, as well as cloud optical thickness, composition, and altitude are constrained by comparison of the MCS data with the model output. A CO2 cloud of optical depth ~0.1 – 1.0, near 25 km altitude, is most consistent with the available data. Theories of Cold Spot Origins: Cold spots with Tb < 148 K are not expected if the CO2 ice cap is in equilibrium with the gas phase. Nonetheless, they are observed throughout the polar night in both hemispheres (Fig. 1). Three primary explanations have been explored previously [2]: (1) Depletion of atmospheric CO2 [3] (2) Non-unit emissivity of surface CO2 [4] (3) Carbon dioxide clouds [5, 6]. Hess [7] demonstrated that (1) is feasible only under special circumstances due to dynamical instability. The spectral properties of CO2 frost and clouds are similar such that it has been difficult to discriminate between (2) and (3) as viable hypotheses. Rapid variability (time scales of days) observed in some cold spots, as well as morphology and distribution, point toward atmospheric phenomena [11]. In the present investigation, we use radiance measurements of both the atmospheric limb and nadir, along with radiative transfer modeling, to show that (3) explains most, if not all cold spots. Observations of Cold Spots: The Mars Climate Sounder [8] onboard the Mars Reconnaissance Orbiter recorded a series of nadir and limb radiance measurements in nine spectral channels, from ~1–40 μm wavelength. The observations used in this study spanned a range of solar longitude, Ls = 111 to 148, corresponding to southern winter. Radiances are converted to equivalent brightness temperatures using the radiometric response function of each filter and the Planck function. Observations are binned by latitude and longitude (1–5° bins) and Ls, then nadir brightness temperatures, Tb, are compared to limb profiles, Tb.