In the Martian north, summer brings a nightly dusting of snow.
This surprising scene comes courtesy of new simulations of flip-flopping layers in the Martian atmosphere, which mix more vigorously than expected and produce stormy weather.
Though still virtual, the snow shower fits quite well with an observation made by a robot placed on Mars in 2008—and it may offer an explanation for how a very different type of snow falls out of the red planet’s polar skies.
If the simulations are correct, the summertime snow on Mars happens in bursts that can last for several hours, scientists report in a study describing the find published today in the journal Nature Geoscience. Flakes of water ice fall from clouds high in the planet’s atmosphere, sometimes failing to reach the ground, but perhaps occasionally leaving a frosty fingerprint that greets the dawn.
“There’s not enough to build a snowman,” says study coauthor Aymeric Spiga, a planetary scientist at the French National Center for Scientific Research. Still, the snow is probably a substantial player in the planet’s water cycle.
“The snowfall, the downbursts, that’s all very novel, very neat,” says John Wilson of NASA’s Ames Research Center. “That probably is what’s taking place in the real Mars atmosphere, and it’s bound to have an impact on how water gets distributed.”
Dry Ice Storms
The case for snow falling on Mars has been building for some time. For starters, we know the red planet has clouds and subsurface water ice.
NASA’s Phoenix lander, which set itself down near the Martian north pole in 2008, spotted wispy structures high overhead that looked a lot like virga streaks on Earth. Here, these streaks are formed when precipitation doesn’t quite reach the ground, so scientists concluded that Phoenix had seen a high-altitude snowstorm of water ice.
Fast forward to 2012, when scientists announced that NASA’s Mars Reconnaissance Orbiter observed what looked like a cloud of carbon dioxide snowflakes over the southern pole.
It’s the only time such dry-ice snow has been spotted falling anywhere in the solar system. Unlike the delicate virga in the north, it not only reaches the ground, but also contributes significantly to the planet’s seasonal carbon dioxide ice caps, says Paul Hayne of NASA’s Jet Propulsion Laboratory.
“The CO2 storms are much more intense,” Hayne says. In terms of its contribution to the caps, “we estimated up to 20 percent of the seasonal accumulation, which is several meters in total.”
To figure out how the planet’s water ice clouds contribute to Martian meteorology, Spiga and his colleagues set up a high-resolution computer model and watched what happened in their patch of digital Martian atmosphere.
Over the Martian night, water-ice clouds radiate infrared light that cools the surrounding atmosphere. This causes blobs of very cold air to settle on top of warmer air. Those cool blobs then sink, producing strong currents and tempestuous winds that weren’t anticipated to occur in Martian clouds.
The icy particles in the clouds get pushed around by the convective currents and ultimately shoved toward the surface in bursts of precipitation.
Hayne says the clouds’ self-promotional snowfall is one of the key findings of the simulation.
“By radiating energy, the clouds cause the surrounding air to cool more rapidly, which leads to more cloud growth and ultimately snowfall,” he says, although he isn’t sure whether the same mechanism might explain the more dramatic dry ice snowstorms he observed.
Still, it’s a nice result that fits in neatly with the Phoenix observations, Wilson says. A former meteorologist at NOAA, Wilson is now working on simulating Martian climate at NASA. He says these are the kinds of observations that could become important ingredients in the complex global models used to understand Mars.
Ultimately, scientists would like to use these models to rewind time and glimpse the planet’s watery past—but such things can’t be done confidently until the models accurately replicate its current climate.
“Ten years ago, the water-ice clouds were thought to have little impact on Mars climate,” Wilson says. “Now, we realize they have a big impact … and so things become richer and more complex and interesting than we suspected.”