Clouds and dust storms

Hell Really Exists

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Ground-based observers noted that albedo markings on Mars are often obscured and correctly attributed these changes to clouds within the martian atmosphere. Martian clouds are divided into yellow clouds, white clouds, and polar hoods (discussed in Section 5.3.7). Hazes, which are optically thin while clouds are optically thick, are often seen along the terminator, particularly along the sunrise limb, and have been observed from the surface landers (Figure 6.3). These result from vapor condensation during the low nighttime temperatures. Early morning fog also is seen in low-lying areas such as the Valles Marineris canyon and inside impact craters (Figure 6.4).

Yellow clouds have been observed since 1877 and are now recognized as dust storms. Dust is prevalent across the martian surface because of the lack of liquid water and the range of geologic processes which have eroded surface rocks. Longitudinal differences in atmospheric pressure and temperature can produce strong winds capable of producing dust storms. Dust devil activity, which is more common in the southern hemisphere and extends from late spring to early fall (Fisher et al., 2005; Whelley and Greeley, 2006), helps lift dust into the atmosphere (Basu et al., 2004; Kahre et al., 2006) (Figure 6.5). Martian dust storms can be local, regional, or

Figure 6.3 Early morning clouds and hazes are common sights from the landers. This image shows thin cirrus clouds over the Mars Pathfinder landing site. (NASA/ JPL/Imager for Mars Pathfinder team.)

Image Fog Nasa
Figure 6.4 Fog is often seen in topographic depression in the early morning. This MOC image shows a 36-km-diameter crater, located at 66.4°S 151.4°E, filled with fog. (MOC image R0700964, NASA/JPL/MSSS.)

Figure 6.5 Dust devils help to lift dust from the surface into the martian atmosphere. These dust devils were captured by Spirit's camera as they traveled across the floor of Gusev Crater. (NASA/JPL/Texas A&M University.)

Figure 6.6 Global dust storms can arise quickly on Mars, as demonstrated in these two images taken 2.5 months apart by the Hubble Space Telescope. Albedo features are easily discerned in the image from 26 June 2001, but are completely covered by the dust storm in the image from 4 September 2001. (Image STScI-PRC01-31, NASA/Cornell/SSI/STScI/AURA.)

Figure 6.6 Global dust storms can arise quickly on Mars, as demonstrated in these two images taken 2.5 months apart by the Hubble Space Telescope. Albedo features are easily discerned in the image from 26 June 2001, but are completely covered by the dust storm in the image from 4 September 2001. (Image STScI-PRC01-31, NASA/Cornell/SSI/STScI/AURA.)

Figure 6.7 White clouds can be seen over the Tharsis volcanoes and western Valles Marineris in this MOC regional view. Orographic clouds commonly occur near the tall martian volcanoes. (MOC image MOC2-144, NASA/JPL/MSSS.)

global in extent, depending on the atmospheric conditions. Global dust storms (Figure 6.6) typically occur near perihelion when it is summer in the southern hemisphere. The stronger daytime heating associated with Mars' proximity to the Sun produces the strong winds and dust devil activity which initiate dust storms. Cooling of the surface under the dust storm leads to further temperature gradients and more wind, causing the dust storm to expand. Under the right conditions, this mechanism can continue until the entire planet is engulfed in dust, whereupon the surface temperature variations diminish, the winds decrease, and the dust storm ends.

White clouds and hazes have been observed since the seventeenth century and are primarily composed of H2O, although some CO2 clouds have also been detected (Figure 6.7). The white clouds clearly show on UV and blue filter images of Mars. Many of the white clouds are orographic clouds, produced as the atmosphere is forced by topography to higher altitudes where the lower temperatures allow condensation of the H2O vapor. Ground-based observers in the seventeenth century often reported the presence of a W-shaped cloud in the western hemisphere of Mars. Today we know that the W-shaped cloud is an orographic cloud associated with the Tharsis volcanoes.

Convecting air parcels will rise at the dry adiabatic lapse rate until they reach a level where the temperature equals the condensation temperature of one of the gas components, at which point clouds will form. Air is saturated when the abundance of a condensable gas is at its maximum vapor partial pressure (the amount of atmospheric pressure contributed by the vapor). Evaporation (or sublimation) is balanced by condensation in saturated air. Liquid droplets will condense when more vapor is added to saturated air, resulting in the formation of clouds. The amount of vapor contained in the atmosphere and how close that atmosphere is to saturation is measured by the relative humidity. Relative humidity (RH) compares the partial pressure of the vapor (pv) to that of saturated air (ps):

The value of RH is close to 100% in clouds. The saturation vapor pressure at a particular temperature is given by the Clausius-Clapeyron equation:

where CL is a constant specific to the type of gas condensing and Ls is the latent heat of condensation for saturated air. The release of this latent heat affects the temperature gradient and thus the lapse rate in this region of the atmosphere. The specific heats of the condensing gas become t dw c" = -P 3T - L'dW (633)

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