This image shows early morning fog in the Noctis Labyrinthis,at
the westernmost end of Valles Marineris. This fog, which isprobably
composed of water ice, is confined primarily to the low-lyingtroughs,
but occasionally extends over the adjacent plateau. Theregion
shown is about 300 kilometers (186 miles) across. (CourtesyNASA/LPI)
Although not as pronounced as on Earth, clouds are common featureson
Mars. The Martian atmosphere has only a trace of water vapor;however,
the temperature and pressure is such that the atmosphereis usually
close to saturation and produces clouds. Even fromEarth based
telescopes, clouds have been observed by transientbrightening
on the surface of Mars. Numerous cloud patterns havebeen seen
from the Marineer and Viking spacecraft and have beenclassified
into various categories (Carr, 1981; French et al.1981):
1.Lee waves. These clouds form in the lee of large obstacles suchas
mountains, ridges, craters and volcanoes. The air in theseregions
undergoes wavelike oscillations.
2.Wave clouds. These clouds appear as rows of linear clouds.They are common at the edge of the polar caps.
3.Cloud streets. These clouds exhibit a double periodicity.They appear as linear rows of cumulus-like, bubble-shaped clouds.
4.Streaky clouds. These clouds have a direction withoutperiodicity.
5.Fog or ground hazes. Fog usually occurs in low areas suchas valleys, canyons and craters. It forms during the coolest timesof the day such as dawn and dusk. Sometimes ground haze is causedby dust in the atmosphere; however, if the atmosphere is clearground fog can be easily identified.
6.Plumes. These are elongated clouds. They appear to havea
source of rising material and in many case are composed of dustparticles.
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