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

美国宇航局的一艘航天飞船发现足够的证据证明水星极地存在水冰。

radar¹-bright patches line up perfectly² with areas of permanent shadow" src="http://www.enread.com/upimg/allimg/120322/1_120322072114_1.jpg" width="624" height="351" />
The radar-bright patches line up perfectly with areas of permanent shadow
Though surface temperatures can soar above 400C, some craters³ at Mercury's poles are permanently⁴ 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 precisely⁵ with the shadowed craters.

Messenger is only the second spacecraft - after Mariner⁶ 10 in the 1970s - to have visited the innermost（最里面的） 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 Observatory⁷ on the latest photos of Mercury's poles taken by the MDIS imaging instrument aboard Messenger.

"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 Applied⁸ 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（绝缘的） debris⁹ 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 interpretation¹⁰ 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 latitudes¹¹."

"The new data from Messenger... is strengthening the evidence that there is some sort of volatile¹² 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 conundrum¹³: "I think this is a question that we can come to a definitive¹⁴ answer on, as opposed to 'we think it may be this'," the MIT researcher explained.