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The UA-run Phoenix Mars lander scraped the surface of our planetary neighbor to verify the presence of frozen water on Mars, and now cameras orbiting the red planet have found a meter-thick layer of nearly pure ice in recently formed craters.

A subterranean ice sheet may extend from the planet's north pole to its middle latitudes, researchers said Thursday.

"Every indication is that this is forming a broad, continuous sheet beneath the surface, said Shane Byrne of the University of Arizona's Lunar and Planetary Lab.

That would "come close to covering half the planet in a million cubic kilometers of ice," Byrne said.

Byrne is a member of the UA's HiRISE camera team and lead author of a paper in Thursday's journal Science that revealed the discovery.

The purity of the ice and the location of the craters — midway between the north pole and the equator — are the two big surprises in the study, said Byrne.

"It looks like this ice is a relic from a previously wetter climate when Mars' atmosphere was a lot more humid than today," Byrne said.

It took an array of cameras on NASA's Mars Reconnaissance Orbiter to make the discovery of visible and detectable ice in five recently formed craters.

First, the wide-angle CONTEXT camera searched for new craters on the dust-covered surface of Mars — looking for brown scars where objects striking the surface had blown away the dust layer that exists across the middle latitudes.

It found five new craters for HiRISE to photograph at much higher resolution. They were small and visible to CONTEXT only because the rock-sized meteorites that caused them threw up a lot of dust.

HiRISE's first look was a revelation. The crater bottom glowed like a glacial lake.

"We expected another routine, small crater, and we saw this bright, blue stuff," said Byrne.

The HiRISE team began monitoring the craters, snapping eight images over the course of the Martian summer. "We were able to see this ice fade away," said Byrne.

The four other craters, thousands of kilometers apart, revealed the same thing, said Byrne — a meter-thick shelf of ice.

The orbiter's spectrographic instrument, known as CRISM, provided confirmation of HiRise's optical discovery.

"Just because it's bright and white and disappears quickly doesn't mean it's water ice," said Selby Cull of Washington University in St. Louis, a member of the CRISM team.

"Only one of the craters was wide enough to measure spectrographically," Cull said in a NASA teleconference Thursday.

She called her instrument "the ultimate color film" because it measures 544 wavelengths of light and can detect the "spectral signature" of elements. It confirmed the discovery of ice.

"It was unexpected to find ice there," said Byrne. "The atmosphere is so dry at the moment, ice sublimates into water vapor."

Once uncovered by the small asteroids that formed the craters, the ice sublimated quickly, a process captured by the HiRISE camera over a period of about 200 days last year. In a series of flyovers, the camera images show the craters' color diminishing from bright blue or white to the brown color of the soil.

Analysis of the sublimation observed by HiRISE found the ice to be up to 99 percent pure, said Byrne.

The ice is thought to be a "relic" from a much warmer and wetter time on Mars, said Byrne.

Peter Smith, who led NASA's Phoenix mission to Mars earlier this year, said the discovery "really is thrilling, and I'm so proud of Shane and the team."

Smith said it took quick action and coordination to document the quickly fading ice fields.

Smith, whose UA-based Phoenix team was the first to chemically analyze ice from the Martian soil, said he agreed with the scientists leading this study when they surmised that pure ice, like that found in the craters, is the rule and not the exception on Mars. He had previously thought a "dirty" ice mixture of soil and water predominated.

Smith also noted that this ice was found in an area visited by the Viking 2 lander 30 years ago. If Viking had been able to dig a couple of inches deeper, it would have uncovered ice three decades before his team did, Smith said.