Seen through the cameras of Mariner 9, the parched surface of Mars presents a scientific puzzle. Twisting gullies cut across the stark landscape like the traces of a mighty river system; yet scientists agree that there is now no free-flowing water on the Red Planet. How were the "riverbeds" made?
The answer may lie in a theory suggested by Astronomer Bradford A. Smith of New Mexico State University and others long before Mariner 9 took off. Smith says that water may be stored as ice in the planet's northern polar cap under a thin layer of frozen carbon dioxide, or dry ice....
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