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Images of a fresh meteorite crater 12 metres across located within Arcadia Planitia on Mars show how ice faded with time. The images were taken in November 2008, left, and January 2009. ((NASA/JPL-Caltech/University of Arizona))

Fresh meteorite impacts on Mars have revealed ice under the surface far closer to the Martian equator than astronomers expected to find it. 

The new craters, found about halfway between the planet's north pole and the equator, revealed bright deposits of ice that may be 99 per cent pure.

Ice is known to exist on Mars. The white polar caps are made largely of ice, and the Phoenix lander directly sampled ice in July 2008 at its landing site in the northern plains.

"We knew there was ice below the surface at high latitudes of Mars, but we find that it extends far closer to the equator than you would think, based on Mars' climate today," said Shane Byrne of the University of Arizona, in a statement.

The purity of the ice seen on Mars was also surprising, said Byrne, whose research with 17 co-authors appears this week in the journal Science.

"The thinking before was that ice accumulates below the surface between soil grains, so there would be a 50-50 mix of dirt and ice. We were able to figure out, given how long it took that ice to fade from view, that the mixture is about one per cent dirt and 99 per cent ice," he said.

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The HiRISE camera on NASA's Mars Reconnaissance Orbiter took this image of a new meteorite impact crater eight metres in diameter in Vastitas Borealis, Mars, on Nov. 1, 2008. ((NASA/JPL-Caltech/University of Arizona))

The scientists used several instruments on the Mars Reconnaissance Orbiter to capture the images of ice at the new craters, found at five different sites on Mars.

In August 2008, researchers examined images from one of the orbiter's cameras to look for dark spots that weren't visible in previous images, spots that would indicate recent meteorite impacts on Mars.

Another team then used a high-resolution camera, called HiRISE, to take detailed pictures of the dark spots, formed by craters from 0.5 metres to 2.5 metres deep.

"We saw something very unusual when we followed up on the first of these impact craters, and that was this bright blue material poking up from the bottom of the crater," Byrne said. "It looked a lot like ice."

Material fading away

Later images showed that the bright material was fading away, just as you would expect ice to fade away. Ice is unstable in the thin Martian atmosphere and sublimates, or turns directly to vapour.

A spectrometer on the orbiter detected the spectral signature of ice in the crater, confirming the discovery.

"All of this had to happen very quickly because 200 days after we first saw the ice, it was gone," said Byrne.

The fact that the ice wasn't dirty with Martian dust, but was nearly pure, led to several theories about how such a pure deposit of ice could form on Mars.

Byrne said the most promising theory was that the ice forms under the ground in "ice lenses," in the same way that they form on Earth. The formation of ice lenses involves very thin films of liquid forming around particles of ice and dirt.

"It would be of great interest if we could discover a process that involved liquid in today's climate, and not just in some of the warmest areas of the planet but in some of the coldest areas of the planet in the high latitude regions," said Byrne.