Albedo features on Mars

The classical albedo features of Mars are the light and dark features that can be seen on the planet Mars through an Earth-based telescope. Before the age of space probes, several astronomers created maps of Mars on which they gave names to the features they could see. The most popular system of nomenclature was devised by Giovanni Schiaparelli, who used names from classical antiquity. Today, the improved understanding of Mars enabled by space probes has rendered many of the classical names obsolete for the purposes of cartography; however, some of the old names are still used to describe geographical features on the planet.

Early telescopic astronomers, observing Mars from a great distance through primitive instruments (though they were advanced for their day), were limited to studying albedo contrasts on the surface of the planet. These lighter and darker patches rarely correspond to topographic features and in many cases obscure them. The origins of the albedo contrasts were a mystery. The lighter patches at the poles were correctly believed to be a frozen substance, either water or carbon dioxide, but the nature of the dark patches seen against the general reddish tint of Mars was uncertain for centuries. When Giovanni Schiaparelli began observing Mars in 1877, he believed that the darker features were seas, lakes, and swamps and named them accordingly in Latin (mare, lacus, palus etc.). Within a few decades, however, most astronomers came to agree that Mars lacks large bodies of surface water. The dark features were then thought by some to be indications of Martian vegetation, since they changed shape and intensity over the course of the Martian year. They are now known to be areas where the wind has swept away the paler dust, exposing a darker surface, often basaltic rock; so their borders change in response to windstorms on the Martian surface that move the dust around, widening or narrowing the features.

The dust-storms themselves also appear as light patches, can cover vast areas and sometimes last for many weeks; when Mariner 9 arrived in Martian orbit in November 1971 the entire planet was covered by a single enormous dust-storm, with only the peaks of the four or five highest mountains showing above it. This variability may explain many of the differences between telescopic observations over the years.

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