changes on Mars
William Herschel's drawings of the Red Planet in the 1780s provided early evidence of changes on the planet, both in the shape and coloration of surface features. These changes were studied in greater detail by the map-makers Beer and Mädler who noted that, between 1837 an 1839, the dark region surrounding the north polar cap appeared to shrink and fade. This could be explained, they suggested, if the dark region was marshy soil moistened by meltwater from the retreating snow. By the 1860s, the idea that Mars had both oceans and continents was commonplace. Liais, however, attributed the shifting appearance of the dark areas to the seasonal growth and decline of vegetation, a suggestion later elaborated in spectacular style by Lowell. The vegetation hypothesis remained viable throughout the first half of the 20th century. Yet sadly, Mars's coloration variations are now known to be due not to the presence of life but to great clouds of wind-borne dust which occasionally engulf the planet and leave behind dramatic evidence of their power, including vast dune fields, wind streaks, and wind-carved features.