Water Ice Clouds in the Martian Atmosphere: a Comparison of Two Methods

Introduction: Over the past decade there has been an increased interest in water-ice clouds in the Martian atmosphere and the role they may play in the water cycle. Water-ice clouds in the Martian atmosphere have been inferred, historically, through observations of " blue " and " white " clouds [1], but the first positive confirmation of water-ice in the atmosphere came during the Mariner 9 mission, through the Infrared Interferometer Spectrometer experiment [2]. Despite these observations of water-ice clouds, they were not thought to be a significant player in the climate or water cycle. Even through the Viking era (mid-1970's), the true extent and ubiquity of water-ice clouds on Mars was not appreciated. More recently, work has suggested that clouds forming in a low-latitude belt during the northern spring/summer time frame (the aphelion cloud belt) may be retaining water in and scavenging water to the northern hemisphere [3,4]. This cloud belt forms as a result of the cross-equatorial Hadley circulation, which has its upwelling branch in the northern hemisphere, during northern spring/summer. At the current epoch, Mars' northern spring/summer timeframe coincides with aphelion (L s =71°), causing the atmospheric temperatures to be lower in this season, allowing water-ice clouds to form more readily than during the perihelion season. Ice particles in the clouds may gravitationally settle, confining water to the northern hemisphere. This settling may also remove dust acting as cloud condensation nuclei, decreasing the radiative heating potential of the atmosphere further. This is a powerful mechanism to explain why Mars' northern hemisphere contains substantially more water than the southern hemisphere. Furthermore, the aphelion cloud belt was not originally recognized as an annual feature in the Martian climate and was originally proposed to be due to a climate change since Viking [3]. However, Viking data did show the aphelion cloud belt over two Martian years [5], as well as widespread and frequent water ice clouds throughout the Martian year. Water ice clouds, and specifically the aphelion cloud belt, have also been seen through the MGS TES data set [6-10]. Since this belt is now seen as most likely an annual feature of the Martian climate, it is desirable to understand its characteristics , such as its spatial and temporal extent, and