Phoenix detects Red Planet snow

The laser pulses a green light into the sky 100 times a second

The Phoenix spacecraft on Mars has detected snow above its landing site.

The US robot used its lidar instrument to probe the structure of clouds and saw large water ice-crystals falling through the Martian "air".

The instrument, which works by scattering pulses of laser light off particles in the sky, did not follow the snow to the ground.

The data suggests the snow vaporised before it reached the surface - but Phoenix is monitoring the situation.

"We're going to be watching very closely over the next month for evidence that the snow is actually landing on the surface," said Jim Whiteway, of York University, Toronto, lead scientist for the Canadian-supplied Meteorological Station on Phoenix.

"This is a very important factor in the hydrological cycle on Mars with the exchange of water between the surface and the atmosphere."

Changing seasons

Phoenix touched down successfully on Mars' northern plains on 25 May this year.

The static lander carries a number of instruments to study the geochemistry and environment of the Martian "Arctic".

The ice crystals fell through the clouds from 4km up down to about 2.5km

Its weather station continuously monitors the temperature, pressure and wind around the robot.

The station has detailed the rise in temperatures as the summer season has taken hold; and seen the temperatures begin to fall again as the Sun dips below the horizon for increasing periods of time with the onset of winter.

"Over the first two months of the mission, the humidity of the atmosphere was increasing as water [ice] sublimated from the ground and the polar ice cap; and over the second half of the mission we've started to see frost, ground fog and clouds. And this is now occurring every night," explained Dr Whiteway.

Standing water?

Other key results released this week by the US space agency (Nasa) include the identification in the soil around Phoenix of calcium carbonate, which on Earth is a chief component of limestone rock.

Phoenix has also detected sheet-like particles which are probably clays of some kind.

The significance of both minerals is that they form only in the presence of liquid water.

The Sun is spending an increasing amount of time below the horizon

Orbiters have identified such minerals in other parts of Mars, but only in areas where there is clear evidence of flowing water. The difference with the Phoenix landing site is that it is an open plain with no obvious geological features that have been cut by running water.

"Assuming we really do need liquid water to form these carbonates - which appears to be the case - then what this says is that we might have had standing water at some point in the past," speculated Bill Boynton of the University of Arizona and the lead scientist on the Tega instrument.

"It's possible that the ice that is there [just below the surface] has melted in-situ and stayed right in that spot and the reaction happened there."

Cold death

Phoenix's original mission was set to last just 90 Martian days, but this has now been extended indefinitely - not that engineers expect the spacecraft to live for much longer.

The Arctic Sun now spends four hours a day below the horizon, and the lander is getting less power into its batteries and is having to expend more energy on heating its systems.

The nights will gradually get longer until, in April next year, the Sun will stay below the horizon for three months.

The ability of Phoenix to gather sunlight is also expected to be severely compromised by the settling of carbon dioxide frost on its solar panels.

On current projections, the robot will probably go silent at the end of November or in early December.

"We're trying to... make hay as the Sun shines and get the most out of the instruments in these last few days before the end of the mission," said Barry Goldstein, the Phoenix project manager at Nasa's Jet Propulsion Laboratory.

"As the ice builds up on the solar arrays, we may see the solar arrays crack, if not fall off. We're going to go through a glassification temperature where the material around the circuit boards gets so brittle that it turns into a glass-like material and actually cracks."

The temperature at winter solstice will be at most -120C (-184F) and perhaps even colder.

Phoenix carries seven science instruments