Snow on Mars Is Great for Skiing, Terrible for Snowball Fights

Here in the United States, spring's thaw is right around the corner. In Mars' northern hemisphere, however, winter is just getting started, and that means it's time for snow. Although Mars is millions of miles away and still poses plenty of scientific riddles, thanks to discoveries over the last year, researchers think they now have a pretty decent idea of what Martian snow is like.

In short: Mars has two kinds of snow, and one isn't like Earthly snow at all.

Most of the snow that reaches the ground and accumulates on Mars is not made of frozen water, but of frozen carbon dioxide. It's a purer version of the "dry ice" that you might use to fog up a punch bowl at a Halloween party. And when it comes to frozen CO2, you can forget the beautifully varied, six-sided, H20 crystal snowflakes that fall from our skies. Instead, the carbon dioxide solidifying in Mars' thin atmosphere likely forms cubic pellets with the corners lopped off and replaced by triangles, called cuboctahedrons.

Furthermore, the Red Planet's white stuff is probably small compared to our everyday terrestrial snowflakes. Carbon-dioxide snow is microscopic, and the second variety, the rare water-ice snow on arid Mars, is similar to the "diamond dust" in our polar regions.

In northern Martian climes carbon-dioxide snow is a mere 8 to 22 microns in diameter. Molecules falling on the southern highlands, meanwhile, range from 4 to 13 microns. It's microscopic in both cases, and about the breadth of a human red blood cell. "The particles are so small they wouldn't be like flurries," says Kerri Cahoy, an assistant professor of aeronautics and astronautics at MIT and co-author of a recent paper on Mars snow. "The snow would really be like a very thin fog that you'd see."

Alas, for future colonist children, snowball fights on Mars don't look like a realistic option. Superfine, frozen carbon dioxide powder mixed with a bit of water-ice packs poorly. "It would be next to impossible to make a snowball," says Daniel Cziczo, an atmospheric chemist at MIT. "Just like on Earth when the temperature gets too cold?below zero degrees Fahrenheit?you don't get the sticky snow that has some liquid water in it that you need to make snowballs. They'd be so powdery they'd fall right apart."

For the same reasons, however, sledding, skiing, or snowboarding on Mars could be X Games?level gnarly. "I think it would be quite good for sledding on Mars when there's snow on the surface," says Jim Whiteway, a professor of space engineering at York University. "It would be quite slippery compared to what we have on Earth."

So how do scientists know all this?

You might be surprised to learn that the first definitive detection of carbon-dioxide snow falling on Mars came in a study published last September. A spectrometer aboard the Mars Reconnaissance Orbiter made infrared light observations in 2006 and 2007 that revealed carbon-dioxide ice crystal clouds extending to the ground over Mars' south pole in the frigid dark of winter. "We needed an infrared camera, like night vision, to see the clouds," says lead author Paul Hayne, a planetary scientist at NASA's Jet Propulsion Laboratory. "In the snowiest parts of the polar regions, it snows pretty much all winter."

On Mars?which has four seasons like Earth because of a similar tilt of its planetary axis?winter lasts for about six Earth months because the Martian year is nearly twice as long. Yet the snow accumulation over this time is not as much as you might expect?perhaps a foot or two.

Hayne says that snowfall contributes about one-fifth of the material to the seasonal ice cap of carbon dioxide in the south. In the north as well as the south, these seasonal caps grow to more than six feet in height on top of the permanent, year-round caps composed of water-ice. (To give you a sense of scale, the north pole's permanent ice cap spans some 600 miles across, about three times as big as the south's cap.)

This carbon dioxide frost, along with its snowy precipitation, can occur when local temperatures plunge below minus 193 degrees F. Amazingly, about a quarter of the entire Red Planet's atmosphere gets alternately dumped on the poles each cold season. "That's a thing that's so strange about all this?we're not talking about a minor component of the atmosphere," Hayne says. "It's almost the equivalent of [atmospheric] nitrogen on Earth freezing out."

As with the CO2 snow, the first detection of more-familiar water-ice snow on Mars happened recently. In 2008, near the north pole, the Phoenix lander's LIDAR instrument shot a green laser beam up into the atmosphere to measure the backscattering of light from airborne particles. The instrument sensed the gentle falling of water-ice crystals from cirrus-like clouds several miles up that form at night as winter approaches.

Phoenix did not record snow alighting on the ground in daylight. Whiteway says that after the sun rose, any deposited water-ice snow would not last long, sublimating within hours back into water vapor. -Before vanishing, though, it might look rather nice. "There would've been a frosting on the ground," says Whiteway, lead author of a Science paper describing the findings. "You would notice the odd crystal and sparkle here and there, like in the early autumn."

The Martian winter's chill killed the Phoenix lander before it ever got to see carbon-dioxide snow or frost forming. Our insights into the shape of carbon-dioxide snow relate back?albeit indirectly?to the landers Viking 1 and 2 that touched down on Mars in the summer of 1976. Paul Doherty, now a senior staff scientist at the Exploratorium science museum in San Francisco, wondered if the Viking cameras would detect halos around the sun in the Martian sky like those on Earth?called sun dogs or parhelia?caused by floating ice crystals. To gauge where and how Martian sun dogs might form, Doherty went to look up Martian snow crystal shapes in the literature?except there wasn't any. "I couldn't find any information on that, so decided I'd better do what a scientist does and run an experiment," Doherty says.

With an engineer colleague named Clarence Bennett, Doherty built a "Mars chamber" about the size of a breadbox. Inside it, he simulated Mars' low atmospheric pressure?less than 1 percent of Earth's?and winter temperature conditions. Gauzy fibers around a nitrogen-cooled pipe served as the nucleating agents for carbon dioxide to glom on to and grow into crystals. The resulting snowflakes that precipitated to the chamber floor, when photographed under magnification, turned out to have a cuboctahedral shape. Based on this, Doherty calculated where Martian sun dogs should appear, though a search of Viking as well as subsequent lander and rovers' photos of the sky hasn't yielded halos?yet. "On Mars, we've looked a lot already and haven't seen any halos, so they must be rarer [than on Earth]," Doherty says.

Beyond preparing a slope report for would-be interplanetary skiers, there is a point to all of the science work regarding Martian snow. For example, determining the impact of snow on Mars' energy budget?how cloud and surface particles reflect and absorb energy from the sun?will be an integral part of better modeling Mars' climate. "Understanding that whole process helps us understand how Mars works," says Cahoy, "and what it might have looked like earlier in its history and in the future."

Source: http://www.popularmechanics.com/science/space/moon-mars/what-snowstorms-are-like-on-mars-15207302?src=rss

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