A colour-enhanced image from the Phoenix lander shows the Martian surface at night, covered with a thin film of frost or snow. (Courtesy of Science)

On Mars, snow falls in the early morning from wispy, feathery clouds that many Earthlings would recognize as cirrus clouds, a Canadian-led research team has reported.

"We found ice clouds and precipitation that were surprisingly Earth-like – certainly more so than expected," said Jim Whiteway, the professor at Toronto's York University who headed the study published Friday in Science. It was the first time precipitation had been observed falling to the ground on Mars.

Whiteway told CBC News that the Martian clouds are similar to very thin clouds seen in Earth's Arctic in the winter.

"They're called diamond dust. And if you look up at the sky, you can still see the stars, but you see some sparkling ice crystals falling, sparkling in the moonlight."

Whiteway and 22 collaborators used data gathered by NASA's Phoenix Mars Lander using a Canadian-designed light-detection-and-ranging (LIDAR) instrument. The lander spent five months in the Martian Arctic during the Martian summer last year and completed its mission in November.

Prior to the mission, Whiteway said, researchers knew that the polar ice caps advanced as far south as the Phoenix site, located at 62 degrees north, in winter.

"But we didn't know how the water vapour moved from the atmosphere to ice on the ground," Whiteway said.

NASA first reported Phoenix's observations of falling snow in September, but at that time it wasn't clear whether the snow ever made it to the ground. Now that the data has been analyzed, researchers think they have a better understanding of the water cycle on Mars.

During the day, they suggest, the water vapour is lifted by turbulence and convection. At night, as the temperature drops below the frost point, it forms ice crystal clouds and falls back to the surface as snow.

The LIDAR was part of the Phoenix's meteorological station. The instrument fires a laser into the sky, then measures the backscattered light, similar to the way radar emits and detects radio waves or microwaves. Signals detected on Mars can be compared to LIDAR signals of clouds and precipitation on Earth.

Clouds creep out at midnight

The Mars data showed that during the day, there were no clouds in the atmosphere, just dust. However, starting at midnight, clouds were detected near the surface, and at 1 a.m., a second cloud layer started forming about four kilometres above the surface.

(Time was measured in terms of local time at the site of the lander. A day on Mars is roughly equivalent in length to a day on Earth, lasting only 39 minutes longer.)

A few times, around 5 a.m., streaks indicating precipitation appeared in the signals. In one case, the snow fell to the lowest level detectable by the instrument — about 50 metres above the surface.

"It is reasonable to assume that the ice crystals would have continued to descend through the saturated air to reach the surface," the paper said.

'It would look like frost'

However, Whiteway thinks the snow cover on the ground would have been very thin.

"It would look like frost," he said.

A photograph of the Martian surface taken by the lander shows what appears to be a film of frost on the rocks and dust, but Whiteway said there is no way to tell from one image whether some of that was snow, particularly since there also would have been frost anyway.

Twenty-three authors contributed to the study, including researchers at Dalhousie University, the Geological Survey of Canada, the Canadian Space Agency, Vaughan, Ont.-based Optech Inc. and Brampton, Ont.-based MacDonald Dettwiler and Associates, as well as a number of U.S. and Belgian researchers. The Phoenix mission received $37 million US in funding from the Canadian Space Agency.

The mission also discovered a layer of ice water five to 18 centimetres beneath the soil at the Martian north pole, and there is evidence that water has physically modified the soil there in the past. Those findings, also published in Science Friday, suggest the area might once have been habitable by life as we know it.