Carbon dioxide snow clouds on Mars: South polar winter observations by the Mars Climate Sounder

Abstract

[1] We present south polar winter infrared observations from the Mars Climate Sounder (MCS) and test three hypotheses concerning the origins of "cold spots": regions of anomalously low infrared brightness temperatures, which could be due to enrichment in non-condensable gases, low-emissivity surface frost, or optically thick CO 2 clouds. Clouds and surface frosts have been historically difficult to distinguish, but the unique limb sounding capability of MCS reveals extensive tropospheric CO 2 clouds over the cold spots. We find that both clouds and surface deposits play a significant role in lowering the infrared emissivity of the seasonal ice cap, and the granular surface deposits are likely emplaced by snowfall. Surface temperatures indicate the polar winter atmosphere is enriched by a factor $5-7 in non-condensable gases relative to the annual average, consistent with earlier gamma ray spectrometer observations, but not enough to account for the low brightness temperatures. A large $500-km diameter cloud with visible optical depth $0.1-1.0 persists throughout winter over the south polar residual cap (SPRC). At latitudes 70-80 S, clouds and low emission regions are smaller and shorter-lived, probably corresponding to large-grained "channel 1" clouds observed by the Mars Orbiter Laser Altimeter. Snowfall over the SPRC imparts the lowest emissivity in the south polar region, which paradoxically tends to reduce net accumulation of seasonal CO 2 by backscattering infrared radiation. This could be compensated by the observed anomalously high summertime albedo of the SPRC, which may be related to small grains preserved in a rapidly formed snow deposit.

Carbon dioxide ice clouds, snowfalls, and baroclinic waves in the northern winter polar atmosphere of Mars

“…We used r eff = 1.5 m following the definition of dust properties in the retrieval calculations of opacities [Kleinböhl et al, 2009]. Q e is set to 0.0027, based on the properties of CO 2 ice clouds [Hayne et al, 2012].…”

Seasonal and diurnal variability of detached dust layers in the tropical Martian atmosphere

“…These two profiles make a pair. The 140 km criterion is derived from the Pythagorean distance between two points separated by half of the cross‐track detector array resolution of MRO‐MCS, and half of the line‐of‐sight horizontal range of tangent points sampled at the vertical resolution of the retrieval [ Kleinböhl et al ., ; Hayne et al ., ]. These two distances are 105 km and 260 km respectively.…”

Titan’s surface composition and atmospheric transmission with solar occultation measurements by Cassini VIMS

“…To estimate the error, we compared results with a more accurate delta-Eddington model, which includes the effects of multiple scattering in an optically thick medium (Hayne et al, 2012). It was found that the relative errors in the simple model for the 2.7-lm surface reflectivity were generally <20%, for example for Ds = 0.6 at a phase angle of 60°and other parameters as specified previously the model overestimates surface reflectance by 12%.…”