Image copyright NASA Image caption The radar-bright patches line up perfectly with areas of permanent shadow

A Nasa spacecraft has found further tantalising evidence for the existence of water ice at Mercury's poles.

Though surface temperatures can soar above 400C, some craters at Mercury's poles are permanently in shadow, turning them into so-called cold traps.

Previous work has revealed patches near Mercury's poles that strongly reflect radar - a characteristic of ice.

Now, the Messenger probe has shown that these "radar-bright" patches line up precisely with the shadowed craters.

Messenger is only the second spacecraft - after Mariner 10 in the 1970s - to have visited the innermost planet. Until Messenger arrived, large swathes of Mercury's surface had never been mapped.

The bright patches were detected by ground-based radio telescopes in the 1990s, but as co-author Dr Nancy Chabot explained, "we've never had the imagery available before to see the surface where these radar-bright features are located."

The researchers superimposed observations of radar bright patches by the Arecibo Observatory on the latest photos of Mercury's poles taken by the MDIS imaging instrument aboard Messenger.

Mercury: The inner-most planet Mercury was visited first by the Mariner 10 probe in the 1970s; and by Messenger currently

The planet's diameter is 4,880km - about one-third the size of Earth

It is the second densest planet in Solar System; 5.3 times that of water

The Caloris Basin is the largest known feature (1,300km in diameter)

Scientists speculate there is water-ice in the planet's permanently shadowed craters

Mercury's huge iron core takes up more than 60% of the planet's mass

It is an extreme place: surface temperatures swing between 425C and -180C

Mercury is the only inner planet besides Earth with a global magnetic field

Messenger is the first spacecraft to go into orbit around the planet

"MDIS images show that all the radar-bright features near Mercury's south pole are located in areas of permanent shadow," said Dr Chabot, from Johns Hopkins University Applied Physics Laboratory (JHUAPL).

"Near Mercury's north pole such deposits are also seen only in shadowed regions, results consistent with the water-ice hypothesis."

However, she cautions, this does not constitute proof, and for many craters, icy deposits would need to be covered by a thin layer (10-20cm) of insulating debris in order to remain stable.

Maria Zuber, from the Massachusetts Institute of Technology (MIT), who is a co-investigator on the Messenger mission, told BBC News: "The most interesting interpretation of [the radar observations] is that they were due to water ice.

"Sulphur had been proposed, there had also been some suggestion it was roughness - though there was no reason craters at the poles should be rougher than those at low latitudes."

"The new data from Messenger... is strengthening the evidence that there is some sort of volatile there, and water-ice seems quite likely."

She said information from several instruments on Messenger was currently being analysed in order to answer the ice conundrum: "I think this is a question that we can come to a definitive answer on, as opposed to 'we think it may be this'," the MIT researcher explained.

On Wednesday, scientists from the Messenger mission published findings that Mercury had been geologically active for a long period in its history.

Data from the probe shows that impact craters on the planet's surface were distorted by some geological process after they formed.

The findings, reported in Science magazine, challenge long-held views about the closest world to the Sun.

Scientists also presented a new model of Mercury's internal structure, which suggests the planet's huge inner core is encased in a shell of iron sulphide - a situation not seen on any other planet.

Messenger was launched in 2004, and entered orbit around its target in March last year. Nasa recently announced that its mission would be extended until 2013.

Paul.Rincon-INTERNET@bbc.co.uk and follow me on Twitter