New Scientist Breaking News –

Radar reveals ice deep below Martian surface

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Ice bowl

Johnson and colleagues have now revealed subsurface measurements of two regions in the planet’s northern hemisphere – the mid-latitude lowlands called Chryse Planitia and the northern polar cap.

They believe a 250-kilometre-wide circular structure that lies between 1.5 and 2.5 kilometres below the surface of Chryse Planitia is an impact crater that was buried with volcanic ash or soil several billion years ago. The team sees no radar boundaries in material that fills the bowl of the crater and the radar signals lose little strength when passing through it. That suggests the infill must contain a large proportion of ice, which is nearly transparent to radar.

Substantial amounts of ice in the soil would make sense given the crater’s location in what appears to be a basin where ancient rivers once converged. “If the water could be captured in a basin and preserved for several billion years, it may still be there,” says Plaut.

Intriguingly, the signal reflected from the bottom of the crater is so strong and appears so flat that it may be liquid water. “If you put water there, that’s what the signal might look like,” Johnson told New Scientist. But he cautions the data is based on only one pass over the region and could be caused by another material.

Rare pass

MARSIS also studied the northern polar cap and found nearly pure water ice stretching down 1.8 kilometres below the surface, with an icy layer of sand underneath.

The researchers are encouraged that such interesting features have emerged from only three data-gathering passes. MARSIS has only been able to make this small number of observations because the subsurface results can only be obtained under special circumstances.

It can best study the subsurface when it is closest to Mars – just 26 minutes of each 7-hour orbit – and when it is also on the planet’s “night” side. That is because energetic electrons in the sunlit portions of the planet’s outer atmosphere, or ionosphere, block the radar’s longest, ground-penetrating wavelengths.

For the last several months, these conditions have not existed at all. But, the conditions are now right again and will remain so until May 2006. The next study regions are in the southern hemisphere, including the south pole.

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