NASA's Jet Propulsion Laboratory has announced that the Mars Curiosity rover has found the most direct evidence that surface water once flowed freely on the Martian surface, likely billions of years ago.
Evidence consists of water borne gravel
According to JPL, Mars Curiosity has imaged rocks with water borne gravel embedded in them. By noting the sizes and shapes of the stones, scientists have ascertained that the ground in Gale Crater where Mars Curiosity is traveling was once a stream that moved three feet per second and had a depth between ankle and hip deep. While channels thought to have once been streams or rivers have long been noted on the Martian surface, this is the first time that water borne gravel has been found and examined. The gravel ranges in size from a grain of sand to a golf ball.
Ancient stream flowed south from north rim of Gale Crater
The stream entered Gale Crater from the northern rim and fanned out, according to images taken by orbiting spacecraft, according to JPL. The area examined by Mars Curiosity is midway between the northern rim and the foot of Mount Sharp, the central mountain peak of Gale Crater. The gravel likely washed down from above the rim where the Peace Vallis feeds in.
Channels first discovered by Mariner 9
Mariner 9, which orbited Mars in 1971 and 1972, provided the first detailed image of the Valles Marineris, the huge canyon that runs along the Martian equator at five times the length of the Grand Canyon in Arizona. It also discovered flow features on the Martian surface that suggested the presence of surface water at some point in Mars's ancient history. In the mid 1970s, the Viking orbiters discovered more evidence of channels and other features that were likely created by the flow of surface water. Subsequent Mars missions have uncovered similar evidence.
Water on Mars currently
Mars Odyssey discovered the presence of ice below the Martian surface as well as at the poles, usually hidden by layers of dry ice (frozen carbon dioxide.) Scientists are puzzled as to where all of the surface water went,according to an article by Space.com. The amount of ice that has been detected does not account for the amount of water thought to have been extant in Mars' youth. Besides ice at the poles and as part of subsurface permafrost, some of the water may have become part of Martian clay beds near where rivers and streams first ran billions of years ago. Surface water is very unlikely because the Martian atmosphere is too thin to sustain it.
Mark R. Whittington is the author of Children of Apollo and The Last Moonwalker. He has written on space subjects for a variety of periodicals, including The Houston Chronicle, The Washington Post, USA Today, the L.A. Times, and The Weekly Standard.
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