The Phoenix Mars lander casts a shadow on a trench on the surface of Mars. In soil samples from such a trench, mission scientists discovered two minerals that suggest interaction with water in the past.

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The UA-led Phoenix Mars Mission has discovered a substance nearly as alien to Tucson as the red planet itself: snow.

A series of weather experiments made public on Monday shows that clouds recently formed above the spacecraft have been releasing snow into the Martian atmosphere, though Phoenix has yet to see any of it hit the ground.

And while the latest discovery twists scientists' understanding of weather on Mars, another series of experiments has mission leaders believing that water frozen beneath the arctic soil once existed as a liquid on the surface of the region.

The snow discovery was made after scientists used a laser to pierce clouds circling over Phoenix, said Jim Whiteway, the lead scientist for the Meteorological Station aboard Phoenix.

By analyzing how the laser is reflected by a cloud, the instrument developed by the Canadian Space Agency generates an image that shows what's inside.

In this case, scientists saw snow as it dropped from the clouds and was carried away by wind, Whiteway said during a NASA press conference held in Washington, D.C.

"Nothing like this view has ever been seen on Mars," he said. "We'll be looking for signs that the snow may even reach the ground."

But snow wasn't the only water discovery Phoenix scientists unveiled on Monday.

Using two different experiments, scientists have found a mineral and clays on the surface of Mars that are telltale signs of past liquid water.

Results from an ovenlike instrument known as the Thermal and Evolved Gas Analyzer, or TEGA, along with the spacecraft's chemistry analyzer, known as MECA, have found a noticeable amount of calcium carbonate, a salt formed by liquid water, along with clays that hold water within their molecules.

Calcium carbonate is formed by the interaction of water and carbon dioxide, meaning that the water frozen in the Martian arctic was liquid at some point in the planet's past, said Michael Hecht, a lead scientist for MECA.

The discovery, coming on top of past revelations into the nature of the soil around Phoenix, is changing how scientists view Mars, Hecht said.

"We're really rewriting the book on Martian geochemistry," he said.

Unlike other areas of Mars, where the evidence of erosion points to a liquid-water past, the arctic doesn't contain similar features on its surface, said William V. Boynton, a lead scientist for TEGA and a professor at the University of Arizona.

"What's new here is that we've seen evidence of water in the northern plains where there isn't visible evidence of flowing water," he said.

It's unclear when or how the now-frozen water may have existed in liquid form, though Peter Smith, the UA's lead scientist for the mission, ventured a guess.

The tilt of Mars' axis varies widely, meaning that at some point in the past, the planet's north pole was pointed almost directly at the sun, which could have increased the region's temperature.

But much work and analysis remain before mission scientists can pin down answers to those questions as well as whether life could have existed in the liquid water, Smith said.

"Is this a habitable zone?" he asked. "We're approaching that hypothesis, though it's too soon to be sure of this."

Phoenix landed on Mars over Memorial Day weekend, travelling more than 420 million miles through space before touching down on the red planet's northern arctic plains.

The spacecraft's science operations have been led by the UA, which became the first public university to lead a Mars mission.