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[139] THE MARTIAN ATMOSPHERE is persistently hazy. The haziness is due to the scattering of light by suspended dust and condensate particles. This haze causes the Martian sky to be gray to yellow instead of blue as on Earth; the blueness of Earth's sky is due to the scattering of light by air molecules. Superimposed on the Martian haze are various types of local condensate clouds and fogs. At times, dust storms raise great yellowish clouds that stand out against the haze and ultimately contribute to it.


Because the axis of Mars is tilted with respect to its orbit plane, the Martian atmosphere undergoes seasonal changes analogous to those on Earth. Viking spacecraft arrived just before northern summer solstice. Approach images show a relatively dense haze covering the northern hemisphere and a much clearer atmosphere in the south. With the beginning of southern spring, an even denser haze blanket formed over the southern hemisphere, largely obscuring the surface even from vertical view. Later this southern haze thinned but, as southern summer approached, dust storms again obscured far, e areas.


Northern latitudes were obscured by condensate clouds and hazes during fall and winter in that hemisphere. North of about 60° latitude, this "polar hood" was diffuse and featureless and, because of the very low atmospheric temperatures s in these regions, is believed to be at least partly carbon dioxide ice particles. The zone between 40° and 60° N was swept by fronts that moved south out of the polar regions; cloudiness was associated with these weather systems.


Images of the Martian limb regularly show a high, layered haze structure extending to more than 35 km above the surface, with individual layers typically extending over large areas. The vertical distribution of light-scattering particles is not directly proportional to the brightness profile in the limb image. This condition is because lower layers are seen along paths of varying length through upper layers. The true distribution of scatterers was calculated, and results revealed the existence of clear layers between the cloudy ones.


The diffuse haze blanket itself is not without structure. In some regions its features include broad longitudinal streaks, cellular lumpiness, and wave trains. Cells, which range in size from about 1 to 10 km, indicate convection within the haze blanket. Wave trains up to several hundred kilometers long are visible in a large percentage of high-altitude frames near the morning terminator. These waves are visible because of the alternate condensation and evaporation of ice crystals in the troughs and crests of a pressure wave traveling through an atmosphere of high static stability.


One prominent type of condensate cloud on Mars forms around the giant volcanic mountains of Tharsis and Olympus. These clouds, evidently formed [140] by orographic uplift, form in late morning and obscure the flanks of the volcanoes up to an elevation of about 20 km, leaving the summits unobscured. In Earth-based observations, these clouds have bet n known for decades as the "W clouds" because of their repeating configuration. Other types of condensate clouds occur over less than 1 percent of the Martian surface at any particular time. These include convective-like-formations, cirrus-like wisps, and low-lying canyon clouds.


Observers using telescopes have known for many years that global-scale dust storms arc common when Mars is closest to the Sun in its relatively elliptical orbit. Such a storm enveloped the planet when Mariner 9 arrived at Mars. Two smaller global dust storms were observed by Viking orbiters during the e extended mission. The first occurred early in the southern spring, and the other shortly after southern summer solstice. Both storms probably started in the Thaumasia- Solis Planum region, and rapidly engulfed most of the planet. They greatly affected meteorology at the landing sites, and each prevented the acquisition of clear images of the Martian surface for 2-3 months. Several dozen localized dust storms were also observed by the Viking spacecraft. Most of these occurred near the retreating south polar cap or in the region to the south of the canyons on the southeastern slopes of Tharsis.



Water-Ice Cloud on Flanks of Ascraeus Mons. This southern view of the dawn side of Mars was taken during August 1976 by Viking Orbiter 2 as it approach' d the planet. Because it was winter in the southern hemisphere at that time, the south pole is in the dark. Part of the adjacent seasonal frost cap is visible at tile bottom center. The great equatorial canyon system, Valles Marineris, is faintly visible at center right; but hazy atmosphere obscures surface features north of that e except for the protruding summit of the giant volcano, Ascraeus Mons. The white feature on its western flank is thought to be a type of water-ice cloud frequently observed in that region. [P-19009]






Early Morning Clouds in the Tharsis Montes and Valles Marineris Region. Ascraeus Mons and Pavonis Mons are prominently displayed in this mosaic, and dense cloud blankets cling to their northern slopes. High cirrus clouds lie to the west of Tharsis, and waves are visible in the clouds surrounding the peaks. Bands of clouds appearing to have a cellular structure extend north from the canyon, and the areas within and immediately surrounding the chasm exhibit water-ice fogs. [211-5049; 5°S, 105°W]




Wave and Dust Clouds in Arcadia Planitia. This mosaic of Viking Orbiter 2 frames shows an area north of Olympus Mons. Surface detail north of 45° is obscured by the polar hood. Well-developed wave clouds, seen at the upper right, are produced by strong westerly winds perturbed by the large crater, Milankovic (55 °N, 147° W). The wavelength (distance between crests) of these clouds is about 60 km; their persistence through more than 500 km implies stability in the atmosphere which prevents the dissipation of the waves by turbulence. The dust clouds at the lower left are probably associated with passage of a cold front moving out of the polar hood region. [211- 5373; 43° N, 124° W]



Condensate Clouds over the Viking Lander 1 Site. During the summer, the northern hemisphere of Mars is generally quite hazy-as shown in the Orbiter views taken in red, green, and violet light (left to right) from a distance of 32 000 km. Because all colors show some obscuration, the haziness is probably caused by both dust and condensates. The large diffuse cloud near the top center, however, is brighter in violet light than in red, suggesting that it is largely composed of condensates. It appeared over the Viking Lander 1 site in the (Chryse basin just a few days before landing. [211-5143; 25°N, 45°W]

Changes in Atmospheric Clarity. These two views in violet light illustrate the dramatic change in the clarity of the atmosphere in the region east and northeast of the Argyre basin during winter in the southern hemisphere. (a) Most of the snow-covered Argyre basin is shown. This was taken just after the winter solstice when solar heating was minimal. (b) This view was taken in late winter when the area had started to warm. The cold southern regions may trap water vapor from the much warmer northern hemisphere to form these clouds, or water vapor may be released from the seasonal polar cap as it retreats. [(a) 34A13, (b) 81B21; 47°S, 22°W]


Wave Clouds. Wave clouds form in a stable stratified atmosphere when winds pass over topographic features such as crater rims. The distance between crests (wavelengths) depends on the dimensions of the perturbing feature and on the speed and vertical profile of the wind. (a) These 20-km-wavelength clouds seem to be formed by westerly winds perturbed by the small ridge to the west of the clouds. (b) This complex pattern of waves has wavelengths between 2 km and 15 km, and may be connected with the south polar crater field seen through the haze or perhaps with instability induced by wind shear. The air is quite dusty in the picture, which was taken in red light soon after the onset of the second global dust storm. (c) This view shows waves to the west of Argyre which are associated with a weather system which also produced the Argyre dust storm. [(a) 40A21; 30°S, 88°W, (b) 287B43; 60°S, 154°W, (c) 131B64; 55°S, 65°W]





Cirro-Cumulus and Strato-Cumulus Clouds. Clouds with cellular structure resembling terrestrial cirro- cumulus and strato-cumulus clouds are quite common on Mars, especially in the polar-hood region. Small convective cells, created when the base of the cloud layer is heated b! ground radiation, are responsible for the structure. (a) Cellular cloud layers are seen at the edge of the polar hood, viewed from a distance of 15 000 km. Note the lee waves produced by the crater. (b) View, taken from a distance of 1400 km, of cellular clouds in the north polar hood, showing the alignment of the cells into "streets." These features can be produced by vertical wind shear. [(a) 470A07; 40° N, 210° W, (b) 138B53; 73°N, 318°W]


Limb Pictures. Limb pictures (those that include the edge of the planet's disk) show that condensates, and perhaps dust, exist in layers in the atmosphere up to 40 km above the planet's surface. The limb structure in the southern hemisphere is shown in (a) during the early winter and in (b) during the late winter. View (c) depicts the north polar limb and (d) the south polar limb. Both polar views were obtained during the late summer for each hemisphere. [(a) 53A65; 40°S, 40°W, (b) 79B06; 48°S, 253°W, (c) 78B71; 80°N, 346°W, (d) 393B01; 78°S, 84°W]






Clouds Surrounding Olympus Mons. In this mosaic, Olympus Mons, wreathed in clouds at midmorning, was viewed obliquely (at an angle of 70° from vertical) from a range of 8000 km through a violet filter. The season is early summer when Olympus Mons receives close to its maximum solar flux. The top of the cloud blanket is about 19 km above the mean ground level and 8 km below the summit. Water-ice, which condenses as upslope air currents cool, is thought to form these clouds. Parts of the cloud cover have a cellular appearance, indicating convection within the clouds. A well-developed wave cloud several hundred kilometers long is visible toward the limb. [P-17444; 18° N, 133° W]

Clouds around Pavonis Mons. Early morning views, taken 3 weeks apart, show Pavonis Mons, the central volcano of the Tharsis Mons receives close to its maximum solar flux. The top of the cloud blanket is about 19 km above the mean ground level and 8 km below the summit. Water- ice, which condenses as upslope air currents cool, is thought to form these clouds. Parts of the cloud cover have a cellular appearance, indicating convection within the clouds. A well- developed wave cloud several hundred kilometers long is visible toward the limb. (a) 40A95, (b) 62A18: 0°N, 113° W]



Discrete Clouds on Volcano Slopes. Discrete clouds are frequently seen above the slopes of the large volcanoes. (a) The unusual plume cloud was repeatedly seen over Ascraeus Mons in the early morning during the summer. (b) The cloud shown is located over the northwest slopes of Ascraeus Mons; the picture was taken when the local season was early autumn and the time about 2:00 p.m. Picture (c) shows an unusual combination of cirrus-like clouds, thin wave clouds, and a prominent discrete cloud (which may be a turbulent rotor) over Arsia Mons. [(a) 58A12;11° N, 105°W, (b) 225A05;12°N, 104°W, (c) 344B88; 9°S, 120°W]




Cirrus Clouds. Clouds resembling terrestrial cirrus clouds are often seen in the Martian atmosphere. That these clouds are condensate phenomena is well illustrated by the greater contrast in (a), taken through a violet filter, than in (b), taken through a red filter at the same time. Without shadows to determine altitudes and a knowledge of temperatures at the proper heights, it is difficult to distinguish water and carbon dioxide ices. It is not improbable that both types of cirrus clouds exist. The group of cirrus clouds in view (c) occurred to the north of the Valles Marineris canyon system; the varying orientations the clouds may indicate differences in wind direction at the altitudes at which particular clouds occur. Picture (d) shows a bright winter cloud as it appeared over the Electris region. It was observed to recur at the same place on several days during that season. Bjerknes Crater is at the lower left. [(a) 101A10; 6°N, 244°W, (b) 101A07; 6°N, 244°W, (c) 58A02; 6°S, 76°W, (d) 88A03; 42°S, 192°W]







Cloud Shadows. Shadows of clouds may be used to determine the altitudes of clouds. This information, coupled with height profiles of temperature and pressure, can lead to a determination of the composition of a cloud. In mosaic (a), high-altitude clouds are seen over ancient, cratered terrain to the east of the Hellas basin. Arrows connect three small condensate clouds to their shadows, which appear to be about 200 km away. Using simple geometric relationships involving the cloud, its shadow, and the sun elevation angle, one finds the clouds to be at approximately 50 km altitude. View (b) shows a larger (100 km long) cloud south of Valles Marineris, about 50 km above the surface. At this altitude, where temperatures and pressures are low, carbon dioxide is the probable composition. [(a) 97A75, 77, 79, 81; 50° S, 246° W, (b) 318A24; 20° S, 44° W]



Enigmatic Clouds. These four frames show Martian atmospheric phenomena that do not fit into any of the preceding categories. Picture (a) is an unusual polar hood cloud formation associated with the large crater, Mie. Superposition of lee waves from parts of the terrain around Mie could produce such a formation under appropriate atmospheric conditions. In (b), a cloud in the southern hemisphere can be seen; to catch the Sun's rays the cloud must be high in the atmosphere. Linear, optically thin streaks are seen in the Thaumasia region in (c). Streaky, condensate hazes that have developed near the dawn terminator during the onset of autumn in the southern hemisphere are seen in (d). [(a) 470A05: 18° N, 220° W, (b) 211B60; 55° S, 234° W, (c) 67A06; 39° S, 85° W, (d) 431B03: 26°S, 280°W]

Early Morning Surface Fog. The presence of morning fogs in some crater and channel bottoms is a Viking discovery with possible implications for the future biological exploration of Mars. These early morning views of the Memnonia region were taken one-half hour apart using a violet filter to enhance the contrast of the condensates. The areas marked by arrows are noticeably brighter in the later picture. The fogs indicate specific spots where water is exchanged, probably on a daily cycle, between the surface and the atmosphere. The surface and lower air layers in this region become unusually cold at night because of the thermal properties of the surface. When the surface warms in the morning, it seems that a small amount of water vapor-estimated to be about one-millionth of a meter thick if liquefied is driven off; this vapor recondenses in the atmosphere, which warms more slowly, to form a ground fog of ice particles. [P17487; 13°S,147° W]





Early Morning Clouds in Noctis Labyrinthus. Condensate clouds are seen here in early morning in the canyons of Labyrinthus Noctis, which lies at the western end of the equatorial Valles Marineris system. This picture, which covers about 90 000 km2, was made by combining three frames of the same field taken through violet, green, and red filters. Although these clouds lie mainly down inside the canyons, they evidently extend above the walls and spill over some of the surrounding plateau. Like most condensate clouds in the Martian troposphere, they are believed to be composed of water-ice crystals. [P18114, 9°S, 95°W]

Dust Storm in Argyre Basin. A Iocal dust storm in the Argyre basin near the end of winter in the southern hemisphere is seen from a relatively high altitude point in the elliptical orbit of Viking 2. Winds appear to he coming from the west. The turbulent brown dust cloud near the polar cap boundary is roughly 300 km across. This cloud did not develop into a global dust storm of the type that tends to occur a little later in the Martian year where Mars is nearer to the Sun. Part of the receding seasonal frost cap covers the lower half of this picture. It appears yellowed by dust in the Argyre basin, but whiter in the mountains (at bottom of picture) at the southern rim of the basin. [P18598B; 50° S, 40° W]







South Pole Dust Storm. This picture of the periphery of the retreating ice cap was taken the day after perihelion (Mars closest approach to the Sun). The cap had shrunk considerably since the time of the Argyre storm observation. The dust storm at the edge of the frost-covered area, which is just visible in the corner of the picture, is about 201) km across. Plumes of dust can be seen outside the boundaries of the main storm. This picture shows the first global storm in its last phase. Such storms are probably related to winds induced by great surface temperature contrasts. [248B57; 70°S, 60°W]

Local Dust Storms near Noctis Labyrinthus. The region southeast of the Noctis Labyrinthus complex on the slopes of the Tharsis bulge seems to be particularly conducive to the formation of local dust clouds. These frames were taken in the middle of spring (a) and in late spring (b). Both local dust storms occurred in the period between the two global dust storms. The area in which the local storms occurred slopes upward toward Arsia Mons. infrared Thermal Mapper instrument data have shown that because of local differences in surface thermal properties, large temperature contrasts occur in this region. Downslope winds caused by these temperature gradients may be strong enough to create such clouds. [(a) 275B05-10, (b) 211B24; 14°S, 90°W]








Dust Storm over the Chryse Basin. These two pictures of a dust storm over the Viking lander site in the Chryse basin were taken 170 seconds apart. Motion of the clouds can be detected if the pictures are viewed through a stereo viewer. Analysis of the two pictures indicates that portions of the cloud were moving from west to east with speeds ranging from 40 to 60 meters per second. This is consistent with westerly winds at the surface with the unusually high speed of 22 meters per second as recorded by the lander. The lander observation is, however, possibly in error because its wind sensor was damaged. [467A69, 467A31; 22°N, 48°W]

Global Dust Storm. The early stages in the development of the first global dust storm were observed by Viking Orbiter 2 in the Thaumasia region. These images were taken 2 days apart. In (a), a single frame, imaged in red light from a very high altitude, includes the entire weather disturbance; the rest of the southern hemisphere was rather clear at this time. In (b), a mosaic, the frames were also taken through the red filter. They show an area several thousand kilometers wide seething with turbulent clouds of dust. This storm spread rapidly to higher altitudes, and suspended dust obscured much of the planet for a period of 50 days. Increased solar heating as Mars nears perihelion is thought to provide the energy that creates these large-scale disturbances. [(a) 176B02; (b) 211- 5379; 42°S, 108°W]









Low Pressure Cell near North Pole. This Martian storm was observed by Viking Orbiter at about 65° N latitude. The local season corresponds to late July on Earth. The storm is located near Mars' polar front, a strong thermal boundary that separates cold air over the pole from the more temperate air to the south. Shadows indicate that the clouds are relatively low in the atmosphere. Because temperatures in this region are well above the condensation temperature of carbon dioxide, water ice is the probable constituent of the clouds. Water vapor concentrations are high (by Martian standards) during this season in the north polar region.

This system strongly resembles satellite pictures of extratropical cyclones near the polar front on Earth. The counterclockwise circulation is consistent with the winds normal low pressure situation.

The frost-filled crater Korolev (approximately 92 km in diameter) is located to the northeast of the storm. The white patches in the top center of the picture are outliers of the north polar remnant cap. [783A42; 70° N, 200° W]




Cold Front. Viking Orbiter 2 photographed this cold front in the Arctic region when the season on Mars was equivalent to mid-May on Earth. During the 2 days between the upper and lower mosaics, the front moved about 950 km, at an average speed of 20 km/hr. The movement may he seen by comparing the two mosaics: lines connect identical features in the two sets of pictures. Weather systems like this appear to be common in Mars' northern hemisphere. Viking Lander 2 has detected the passage of similar fronts many times. Warm, comparatively moist air is lifted over a wedge of colder, denser air as it pushed south. Moisture in the warm air condenses into ice crystals, forming clouds. Dust, seen frequently in Martian storms, could also be present in these clouds. Some water must be present, scientists say, because wave clouds seen in both mosaics result from condensation and evaporation of ice crystals in the troughs and crests of pressure waves propagating in the atmosphere. [211-5764; 65° N, 135° W]