Tuesday, December 8, 1998 Published at 18:56 GMT
Mars' north pole revealed in stunning 3D
The cap is cut by canyons and troughs
By BBC News Online Science Editor Dr David Whitehouse
A space-borne laser has produced the first three-dimensional view of Mars' North Pole. This has enabled scientists to estimate the volume of water in the ice cap and speculate that much of the planet's original water is either hidden below the surface or missing.
The elevation measurements were collected by the Mars Orbiter Laser Altimeter (MOLA), an instrument aboard NASA's Mars Global Surveyor spacecraft, during the spring and summer of 1998. The spacecraft is still orbiting Mars but at that time it was in an interim elliptical orbit.
MOLA sends laser pulses toward the planet and measures the time taken for the signals to be reflected. Mars was bombarded with approximately 2.6 million of these pulses. The topographic map reveals that the ice cap is about 1,200 kilometres across, with a maximum thickness of three kilometres. The cap is cut by canyons and troughs that plunge as deep as one kilometre beneath the surface.
"Similar features do not occur on any glacial or polar terrain on Earth," said Dr. Maria Zuber of the Massachusetts Institute of Technology and NASA's Goddard Space Flight Center, "They appear to be carved by wind and evaporation of ice." The MOLA data also show that large areas of the ice cap are extremely smooth, with elevations that vary by only a few feet over many miles.
In some areas, the main ice cap is surrounded by large mounds of ice, tens of miles across and up to half a mile in height. "These structures appear to be remnants of the cap from a time when it was larger than at present," Zuber said. Impact craters surrounding the cap appear to be filled with ice and dust that was either deposited by wind or condensation, or perhaps further remains from an earlier larger ice cap.
The shape of the polar cap indicates that it is composed primarily of water ice, with a volume of 1.2 million cubic kilometres. The cap has an average thickness of one kilometre and covers an area 1.5 times the size of Texas. But the volume of the Martian north polar cap is less than half that of the Greenland ice cap, and about four percent of the Antarctic ice sheet.
The estimated volume of the north ice cap is about 10 times less than the minimum volume of an ancient ocean that some scientists believe once existed on Mars. If a large body of water once existed on the red planet, the remainder of the water must now either be stored below the surface and in the much smaller south polar cap, or have been lost to space. But such a large amount of unaccounted-for water is not easily explained by current models of Martian evolution.