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- The Martian atmosphere, like that of Venus, is primarily
- carbon dioxide. Nitrogen and oxygen are present only in small
- percentages. Martian air contains only about 1/1,000 as much
- water as our air, but even this small amount can condense out,
- forming clouds that ride high in the atmosphere or swirl around
- the slopes of towering volcanoes. Local patches of early morning
- fog can form in valleys.
-
- There is evidence that in the past a denser martian
- atmosphere may have allowed water to flow on the planet. Physical
- features closely resembling shorelines, gorges, riverbeds and
- islands suggest that great rivers once marked the planet.
-