Astronomy: Moisture on Mars

To early astronomers, it seemed obvious that there was water on Mars. What else could form the white polar caps that shrank so noticeably every spring and began to grow again in the fall? And what could possibly produce the springtime darkening of the Martian surface other than rapid vegetation growth stimulated by water released from the melting caps?

Contemporary scientists have been far less convinced that Mars has water. They believe that the caps consist not of snow or ice but largely of frozen carbon dioxide, the principal constituent of the atmosphere of Mars. Some have suggested that the color...

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