Seasonal surface frost at low latitudes on Mars

During the past 4 Mars years, Mars Orbiter Camera imaging capabilities have been used to document occurrence of seasonal patches of frost at latitudes as low as 33° S, and even 24° S. Monitoring reveals bright patches on pole-facing slopes; these appear in early southern winter and disappear in mid winter. The frost forms annually. Thermal Emission Spectrometer and daytime Thermal Emission Imaging System observations show surface temperatures on and near pole facing slopes reach the condensation temperature of CO2, indicating the patches consist of carbon dioxide rather than water frost. For several months, temperatures on pole-facing crater walls are so low that even carbon dioxide condenses on them, although the slopes are illuminated by the Sun every day. Thermal model calculations show slopes accumulate a several centimeter thick layer of CO2 frost. The frost becomes visible only months after it has begun to form, and has an orientational preference which is due to illumination bias at the time of observation. H2O condenses at higher temperatures and water frost must therefore also be present. Potential opportunities to observe seasonal water frost at low latitudes are also described.