The Mars rover Curiosity recently recorded a temperature of 132 degrees below zero, an intensity of cold that is difficult to comprehend. Yet on Mars, incredibly, that's too warm for it to snow. For that, it needs to reach minus-193 degrees, the temperature required for carbon dioxide to freeze and dry ice to form.
Fortunately for Martian snow lovers, it can get as cold as about 250 degrees below zero at the planet's south pole.
New NASA research provides convincing evidence that there are snowflakes in the clouds on Mars and that they find their way to the ground.
"We firmly establish the clouds are composed of carbon dioxide -- flakes of Martian air -- and they are thick enough to result in snowfall accumulation at the surface," says Paul Hayne, lead author of a new study on Martian snow being published in the Journal of Geophysical Research.
NASA says Mars is the only planet in the solar system with this kind of snow. But it's not exactly known how the snow gets from clouds to the ground.
"It is unclear whether the deposition occurs as snow or by freezing out at ground level as frost," NASA reports.
But views into the clouds and particle data offered by the Mars Climate Sounder aboard the Mars Reconnaissance Orbiter suggest snowfall, the authors say. (The orbiter has been circling the planet since 2005.)
"One line of evidence for snow is that the carbon-dioxide ice particles in the clouds are large enough to fall to the ground during the life span of the clouds," said NASA's David Kass, the study's co-author. "Another comes from observations when the instrument is pointed toward the horizon, instead of down at the surface. The infrared spectra signature of the clouds viewed from this angle is clearly carbon-dioxide ice particles, and they extend to the surface. By observing this way, the Mars Climate Sounder is able to distinguish the particles in the atmosphere from the dry ice on the surface."
NASA says the most "vigorous" snowfall occurs on Mars' south polar residual ice cap, and it sticks year-round due to the frigid temperatures there.