Mars has nighttime snow storms

Nighttime may be snow-time on Mars.

Low evening temperatures could cool clouds and trigger turbulent winds that fuel fast-falling snowstorms on the Red Planet, new simulations suggest. This process, reported August 21 in Nature Geoscience, may account for surprising observations of snowfall by one of NASA’s Martian landers.

“Clouds and snowfall have emerged in recent years as central players in the water cycle and climate of Mars,” says Paul Hayne, a geophysicist at NASA’s Jet Propulsion Laboratory in Pasadena, Calif., who wasn’t involved in the research. The new study was able to create the rapid snowfall by simulating climate on a much finer scale than previous simulations. That shows the need for local-scale weather data to understand the most fundamental aspects of Mars’ climate, Hayne says.

Satellite observations show that snow made of both water and carbon dioxide cover the Martian poles, and, in 2008, NASA’s Phoenix lander detected streaks of snow falling below a nighttime water-ice cloud (SN Online: 6/20/08). This was the first time scientists had seen evidence of snowfall on Mars.

At the time, scientists thought that snow on the Red Planet formed similarly to the main way it forms on Earth — ice particles in clouds collide, stick together and then eventually become heavy enough to fall to the ground. But Mars has less water vapor in its clouds and less gravity than Earth. That means that snow there possibly falls too slowly to explain the Phoenix lander observations. Aymeric Spiga of Pierre and Marie Curie University in Paris and colleagues wondered what would happen if simulations took into account regional or local wind disturbances or strong changes of temperature, which often accompany snowstorms on Earth.

By looking at temperature differences in the layers of the Martian atmosphere, the team found that cold Martian nights could trigger the development of rapidly descending plumes of snow. These plumes resemble the streaky cloud structures and precipitation seen by the Phoenix lander. Instead of water-ice particles slowly growing into snow and then falling, low temperatures cool water-ice clouds, creating strong winds that push water-ice crystals toward the ground, creating fast-falling snow, the simulation shows. Similar squalls form on Earth; it just wasn’t clear that they could happen on Mars, too.

Spiga says the simulations match variations in clouds and snowfall from the Phoenix lander observations down to the hour and offer a straightforward explanation for how the snow forms. To reach the ground on Mars, snow would need to fall from low clouds, only a kilometer or two above the Martian surface, Spiga notes. Or, mountains and other features would have to tower high in the sky. Otherwise, the snow would sublimate, transforming from flakes back into water vapor before hitting the ground.

“This is an important discovery because it allows us to understand the conditions under which snowfall can deposit water ice on the surface,” Hayne says.

Spiga says the result may require researchers to rethink the role water-ice clouds play in stirring up the lower atmosphere of Mars. Scientists thought the clouds were fairly static. But the results show that the mixing of water at these lower altitudes is “not like we thought,” he says. Adding this movement of water to Mars climate models could give clues to how water moves around the planet now and how it dried out over time.