Permafrost surrounds both Martian poles, the Mars Odyssey spacecraft has found.
Initial observations made in March with the spacecraft’s Gamma Ray Spectrometer found hydrogen around the south pole. Scientists believe the most likely chemical form for the hydrogen is water ice.
Now a further month of detailed observations also reveal a similar icy area near the north pole. That bolsters the case for an early wet period in Martian history during which life could have existed, before the planet froze.
The instrument used can only analyse the top metre of the Martian soil, but if the water ice seen there permeated the planet’s entire soil and rock layer, it could contain enough water to cover Mars with an ocean 500 to 1500 metres deep.
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The water spotted so far may “represent only the tip of an iceberg frozen under ground,” says Jim Bell of Cornell University in the journal Science, where the scientific results have now been published.
The ice also offers a useful resource for future human exploration, although NASA is not believed likely to announce plans for such a mission, despite press speculation of a 20-year timetable.
Water map
The Gamma Ray Spectrometer includes a gamma ray detector, a neutron spectrometer and a high-energy neutron detector, which monitor the products of cosmic ray strikes on the upper metre of Martian soil. These reveal the hydrogen signal.
To help map the distribution of water ice, researchers used a simple model of the surface: a top layer a few tenths of a metre thick that is relatively dry above a thicker layer containing more ice.
The top layer is dry in low latitudes, with as little as 0.25 per cent ice. But by 42 degrees south it rises to 1 per cent, and reaches a plateau of 2 per cent at 60 degrees south, estimates a group led by William Boynton of the University of Arizona.
They estimate the lower layer contains the bulk of the ice – about 35 per cent by mass in polar regions and up to 73 per cent by volume. That implies the ice fills the voids in a porous layer of rocks and soil, i.e. permafrost.
In the northern hemisphere, hydrogen-rich deposits start to appear at about 45 degrees north, but are obscured above 60 degrees north by seasonal deposits of carbon dioxide ice.
The discovery of water ice will shape future Martian missions. NASA had earlier targeted the south polar zone with its Mars Polar Lander, which was to land at 76 degrees south with instruments designed to detect subsurface ice. However, a software failure caused the spacecraft to crash on descent on December 3, 1999.
Journal reference: Science (DOI’s:10.1126/science.1073541, 1073722, 1073616)
