Martian ice cloud distribution obtained from SPICAM nadir UV measurements

Abstract

[1] The Spectroscopy for the Investigation of the Characteristics of the Atmosphere of Mars (SPICAM) instrument on board Mars Express has successfully performed one Martian year of measurements. Nadir UV (200-310 nm) measurements allowed it to build maps of ice cloud optical depth distribution for all seasons. The development and decay of the aphelion cloud belt (ACB) and polar hoods were observed. The characteristic values of the cloud optical thickness were 0.1-0.3 at the early stage of the ACB formation in the solar longitude range L s = 20-60°. After L s = 93°, the well-developed ACB showed cloud optical thicknesses varying between 0.3 and 0.8. The ACB quickly decayed after L s = 140°. Both polar hoods were observed during their development and decay stages, showing cloud optical thicknesses of about 0.35. The north polar hood started to develop at L s = 160°and the south one at L s = 330°. Estimates of water content in the ice clouds gave values of 0.35-1.8 gm À2 for ACB and 0.4 gm À2 for the polar hoods. A comparison with water vapor abundance showed that only a small fraction (10-20% for ACB and 30% for the polar hoods) of total water content in the atmosphere was accumulated in clouds. The Martian surface albedo at the wavelength 300 nm appeared very low (0.004-0.018) and exhibited anticorrelation with the visual albedo consistent with optical properties of iron oxides abundant in Martian soils. The investigation of a regional dust storm allowed the estimation of dust optical parameters at the wavelength 300 nm (asymmetry factor g d = 0.8 and single scattering albedo s d = 0.6).

Mars' south polar hood as observed by the Mars Climate Sounder

“…The cloud bridge extends approximately from 20°S to 35°S latitude, but the longitudinal breadth varies depending on the season. The cloud bridge has been observed in previous studies as well [ Mateshvili et al , 2007; Smith , 2004; Liu et al , 2003] (see also section 3.3). The dynamical effects producing this remain obscure, but its behavior is described along with the nature of the other belt clouds during the period of coverage in sections 3.1.1 through 3.1.17.…”

“…The calculated water content values are given in Figure 3, for optical depth values corresponding to those in Figures 1a–1q. A value of τ = 0.10 represents the core of the belt when it is at its seasonally thinnest or the outer edges of the belt when it is thickest, and corresponds to a water content of m = 0.08 pr μ m. A value of τ = 0.25 represents the core of the thickest belt, corresponding to m = 0.20 pr μ m. These values are comparable to those of Mateshvili et al [2007] using SPICAM nadir UV measurements (0.40 pr μ m), taking into consideration different assumed ice particle radii (1.36 μ m versus 2 μ m).…”

Mars' north polar hood as observed by the Mars Climate Sounder

“…This time is shorter than for our observations (L S = 150°–30°), the reason being that their observations are restricted to south of 60°N in the daytime, while we see clouds only from 60°N to the pole in the daytime at L S = 15°–30° and L S = 150°–165°. Mateshvili et al [2007] presented water ice cloud observations from SPICAM nadir UV measurements in MY 27. They see the onset of NPH clouds at L S = 160° and the recession at L S = 30°, which agrees well with our observations.…”

A Reverse Ozone Hole on Mars

“…Martian clouds are particularly frequent over the poles, where they are referred to as polar hoods, and over the high volcanic mountains. [7] The highest mountain is Olympus Mons, about 100 times the size of the largest Earth volcano and nearly three times higher than Mount Everest. On both planets such high mountains are regularly covered in clouds.…”

“…Conversely, during winter and spring the very low temperatures give rise to cloud formation, which additionally reduces the source of radicals by trapping up to 30 % of the water column. [7] Furthermore, the ice clouds provide a surface for heterogeneous HO x loss. [6] These conditions curb the chemical sinks so that the O 3 concentration strongly increases over the poles.…”