[It seems likely that in the 350 years since the telescope was invented more time has been devoted to viewing and photographing Mars than any other planet. The reason is that Mars is the only planet (apart from the earth) on which it is possible to perceive permanent surface markings. The spacecraft Mariner IV made 22 photographs of Mars that represented an improvement in optical resolution over earlier photographs.
At the 15 to 17-year intervals when Mars makes a particularly close approach to the earth (a distance of some 35 million miles), its disk is about a 70th the diameter of the moon as it is seen from the earth, or about half the size of a typical lunar crater. Within the compass of this tiny area three centuries of astronomers have given specific names to dozens of surface features. The most prominent of all, first shown in a drawing made by Christian Huygens in 1659, is Syrtis Major, which in shape and location somewhat resembles the terrestrial subcontinent of India. Syrtis Major actually projects into the northern hemisphere of Mars, but according to astronomical custom photographs of Mars are usually printed with the Martian south pole at the top; hence Syrtis Major appears to point down.] (1)
The most widely discussed features of Mars are of course the "canals"—those straight-line markings that are firmly vouched for by many leading observers of the planet, and just as firmly doubted by others. [Âĺńŕuse fine detail on Mars is continuously shifted in and out of focus by thermal inhomogeneties in the earth's atmosphere, the canals have been particularly difficult to capture on photographic plates. Nevertheless, photographs do provide sonic evidence for their existence.]
Concerning other features of Mars there is no dispute. Photographs show clearly that something resembling an ice cap forms first on one pole, then on the other, [as the inclination of the planet's axis to the plane of its orbit around the sun produces summer and winter seasons. The polar cap slowly disappears with the coming of the Martian spring. Because the atmosphere of Mars is exceedingly thin (as has been verified by the occultation experiment performed by Mariner IV), it is somewhat difficult to believe that it contains enough water vapor to give rise to a polar cap with such whiteness and such a slow rate of retreat. It has been suggested, however, that the polar caps may consist not of frozen water but of frozen carbon dioxide, an alternative that seems much more in keeping with what is known about the composition of the Martian atmosphere.]
With the changing seasons there are also apparent changes in the coloration of dark regions [such as Syrtis Major). The observed color is reported to range from yellowish brown to blue-green. Infrared photography and other tests demonstrate conclusively that the blue-green color is not due to the presence of chlorophyll. [The color change may represent a purely inorganic phenomenon, for example a change associated with alterations in the degree of hydration of certain minerals.] (3) Finally, in addition to the “canal”, polar caps and polar changes, there are clearly discernible disturbances in the Martian atmosphere that appear to be clouds and dust storms. [After such disturbances there arc often pronounced changes in the visibility of the planet's surface features, and îver the years certain features change in shape and color.