Snow on Mars seems to have a much more exciting night life than researchers previously thought. Credit: NASA/JPL-Caltech/MSSS

When night falls on Mars, plunging temperatures can lead to snowstorms with whipping winds that could rattle a rover as it tries to land, according to a new study.

The analysis upends researchers’ previous assumptions that Martian snow falls slowly and gently from the sky. Getting a handle on the planet’s weather is important for future exploratory missions. But it could also help to explain how Mars lost a lot of its water, and what might happen to the water that remains. The study, published on 21 August in Nature Geoscience1, moves researchers closer to an answer by providing the first detailed peek into what happens to the water in the Red Planet’s clouds.

Scientists combined powerful computer models that enabled them to run global climate measurements, calculate air turbulence and even make localized weather predictions for Mars. If a researcher uses only a global climate model, they can predict where clouds might be, but they won’t have any idea about the dynamics within a cloud, says Aymeric Spiga, a planetary scientist at Pierre and Marie Curie University in Paris and lead study author.

“That’s the reason why these snowstorms were not discovered before,” he says.

“It’s a bit like a Russian doll, with each successively higher-resolution model fitting inside the other,” says Paul Hayne, a planetary scientist at NASA’s Jet Propulsion Laboratory in Pasadena, California, who was not involved with the study. “Very clever.”

Falling fast

During the day, icy clouds about 10–20 kilometres above Mars’s surface absorb sunlight and help to keep the atmosphere warm and stable. But Spiga and his colleagues found that when the sun sets, the temperature inside the clouds cools rapidly, dipping 4 degrees Kelvin per hour.

Cold, descending air masses mix with hot air rising from Mars’s surface. By about 2 a.m. local time, powerful columns of rushing wind reach speeds of 10 metres per second. The descending winds bring snow-like ice particles with them.

These miniscule particles — only one-thousandth the size of a raindrop — take 5–10 minutes to freefall 1–2 kilometres, which is much faster than the 4 hours that scientists had previously estimated. This means that clouds must be within a kilometre or two of the planet’s surface for snow to accumulate on the ground.

The researchers’ simulations not only demonstrated how Mars’ atmosphere transports water closer to the surface, it also revealed some upward release of water vapour in the clouds near the planet’s poles — where NASA’s Phoenix Lander first spotted falling snow in 2008.

The authors note that they can’t conclude whether atmospheric mixing patterns favour sending water higher into Mars’ atmosphere — where it could potentially be lost to space — or down towards the surface.

Sticking the landing

If this relatively low-level atmospheric mixing pushes water higher up into the atmosphere, that could explain how the Red Planet lost a lot of its water, says Mike Chaffin, a planetary scientist at the University of Colorado Boulder who studies Mars’ upper atmosphere.

“More and more, we’re understanding that the Mars atmosphere is connected vertically in ways we didn’t expect,” Chaffin says.

Understanding the dynamics of the planet’s atmosphere will help to eliminate surprises for future missions to Mars. Alhough the wind speeds in these snowstorms would be considered moderate on Earth — they wouldn’t be strong enough to ground a commercial flight — Mars’s thinner atmosphere would amplify wind turbulence, Spiga says.

“Any time NASA sends a new lander or rover to Mars, the landing of these spacecraft is always very risky process,” says Spiga. Every little bit of information on what a rover — or a crewed mission — would encounter in the planet’s atmosphere will help to set the craft down on the surface in one piece.