Mercury poles give up hints of water ice
24 Mar,BBC
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."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.
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