The south polar region of Mars from 24 Viking Orbiter images taken in 1977

Bit by bit, feature by feature, Mars is slowly revealing itself to us. A century ago, it was thought of as a planet crisscrossed by a network of canals — surely the great irrigation channels of a highly advanced race. Then we flew by for a first close look in the early 1960s and saw nothing but a cratered wasteland, little better than our own moon, save for the rusty color. But closer study has revealed much more. Mars was indeed once wet, as the dry seabeds and riverways that score its surface attest. Indeed, it still has some water, as new sightings of seasonal streaking down mountain faces caused by springtime ice melt show.

Now comes word from NASA’s Mars Reconnaissance Orbiter (MRO) that it’s snowing on the Red Planet — a lot. A massive, 500-km-diameter cloud persists all winter long over the planet’s south polar cap, dumping snow to blizzard-level depths. The catch: this snow isn’t made from water crystals, but carbon dioxide, and that’s just a little bit of what makes the new findings — and the way they were uncovered — so intriguing.

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As anyone near a tailpipe knows, atmospheric CO 2 readily absorbs and radiates heat, making it synonymous with planetary warming. But sometimes even carbon dioxide meets its match, and that’s what happens at the south pole on Mars, where brutal winter temperatures of –125ºC make the notorious greenhouse gas flutter down from the Martian atmosphere as a dry, coarse snow similar to corn starch.

Researchers went looking for south-pole snow using the Mars Climate Sounder, a spectrometer aboard the MRO that measures the visible and infrared light emitted from cloud particles in the atmosphere. In this case the infrared readings were crucial since Mars’ polar regions, like Earth’s, remain dark all winter. That means there isn’t enough visible light to study the area when the snow is falling.

Infrared, on the other hand, offers scientists something of a night-vision peek at the atmospheric particles above the south pole. On Mars, those particles do the same thing they do on Earth: scatter, absorb and radiate light. Conveniently, they do so at distinct wavelengths depending on the composition of the particle. Scientists can therefore compare those different wavelengths to identify the chemical makeup of any particular particle. This also allows them to map the cloud formations producing the snow — and those formations are impressive. The main cloud extends from an altitude of 32 km, down almost to the planet’s surface. Latitudes farther from the pole have smaller 9.7-km-by-9.7-km snow clouds that appear sporadically over a seasonal ice cap.

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“The smaller ones flash on and off for a day or two at a time, like little snowstorms,” says Paul Hayne, a geophysicist at NASA’s Jet Propulsion Laboratory and the report’s lead author. “If you had a thick enough space suit you could go skiing.” Hayne and his colleagues aren’t certain how deep the snow is, but they say it’s at least several feet, with radar evidence suggesting that some deposits are well more than a mile deep.

Don’t expect captivating snowflake patterns on Mars, however. Though the scientists aren’t certain what shape the falling snow takes, Earth-based experiments suggest they are solid particles — and they’re nasty ones too. Catch one of these on your tongue, and you’ve got instant frostbite.

This isn’t the first snowfall found on Mars; NASA’s Phoenix Lander mission detected falling water-ice at the planet’s northern regions in 2008. But until now, researchers had only indirect evidence of falling CO 2 snow: a bed of CO 2 ice on the surface, temperatures cold enough for CO 2 to freeze and satellite-based scans suggesting the presence of dense CO 2 clouds. “We had all those different lines of evidence,” Hayne says, “but this is the first direct three-dimensional observation of the cloud and the composition information.”

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The MRO discovery does more than reveal the increasing richness of Mars. It also helps explain more about the planet’s atmospheric engine. A portion of the annual CO 2 snowfall sublimates back into the Martian atmosphere — a key element of Mars’ climate cycle since 30% of the planet’s atmospheric mass is exchanged between the polar caps and the atmosphere every year. “That’s a huge climate driver, and we don’t really understand how that process works on Mars,” says Hayne.

That could have implications for, yes, life. If more research reveals that additional amounts of CO 2 are released from polar ice caps in similar cycles, that would mean periods of higher atmospheric pressure, making water more stable on the surface. That, in turn, would make polar investigations a NASA priority. Someday we may very well see a rover tricked out in snow tires, making its way across the plains on an improbable world where a greenhouse gas offers only chills.

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