Through a telescope, Mars is an intriguing
sight. There are darkish areas, reddish-ochre tracts, and white caps covering the
planet's poles. Early observers believed that the dark areas were seas and the red
regions were land, but for almost a century now, it has been realized that there can be no
oceans on Mars. In fact, water cannot exist as a liquid on Mars, excpet at the very
lowest areas of the surface, such as the great bowl of Hellas, where the atmospheric
pressure is great enough for liquid water to form. A little water vapor has been
detected in the atmosphere, and the existence of extensive permafrost has been suggested,
although there is considerable doubt as to the amount of water locked up at the poles in
the form of snow or ice. The visible surface of the white caps is certainly carbon
dioxide snow, but under this, water ice exists.
It is tempting to suggest that the ochre tracts are deserts, and recent researh indicates that this may be a good name for them. Information from orbiting Soviet spacecraft Mars 3 suggests that they are siliceous - that is they contain, or consist ofm silica. However, they are cold by terestrial standards. On Mars, at noon on the equator in midsummer, the temperature may rise to over 12 ' C, but at night a thermometer would show more than 70 ' C below zero.
When Mars is viewed through a telescope, it looks like a red and orange disk. An observer can easily see white ice caps at the north and south poles of Mars. These caps grow and shrink throughout the Martian year, just as thepolar caps of Earth do. The darker areas of Mars’s surface may look greenish to the observer, but this is an optical illusion caused by the contrast in color between the dark patches andbrighter areas. Scientists believe that the dark areas are regions of relatively unweathered bare rock, while the bright areas are regions with deposits of weathered material, especially fine dust.
At certain times of the year, usually the southern Martian spring and summer when Mars is closest to the sun, great dust storms appear as yellow clouds. The largest of these storms can cover the globe of Mars and last for months. At other times white clouds of water vapor are visible. Scientists now believe that the "canals" people observed on Mars during the 19th century are actually another optical illusion, caused by the mind’s tendency to draw connections between irregular patches in a fuzzy image.
The Hubble Space Telescope (HST) provides the clearest Earth-based views of Mars, and astronomers use it to study the weather on the planet. The HST has provided images of dust storms with such detail that scientists can pinpoint the areas on the planet in which the storms started. The telescope also makes general studies of the atmosphere possible. Using HST images, astronomers have determined that the atmosphere of Mars is cooler, clearer, and drier than it was in the mid-1970s, the last time scientists were able to monitor the atmosphere closely.
