This mosaic, by the Mariner 10 spacecraft, is from images taken while it retreated from the planet on March 29, 1974. (NASA)

Among the inner planets of our solar system, Mars has attracted the most interest from space agencies around the world, mostly because scientists believe the planet may have once held, and might still hold, liquid water, a key ingredient for supporting life.

Venus has also attracted attention with the recent arrival of the European Space Agency's Venus Express satellite. But what about Mercury, the planet closest to our sun?

Like the fleet-of-foot Roman god of the same name, the planet Mercury has proven elusive for astronomers, both amateur and professional alike. Its size and proximity to the bright sun make it difficult for Earth-bound observers to view the planet without a telescope. Likewise, astronomers avoid turning the sensors of orbital telescopes in Mercury's direction for fear of damaging the sensitive instruments looking so near the sun.

Over the last 30 years, most of the information about Mercury has been gleaned from ground-based telescopes or from observations of Mariner-10, the satellite that last flew by the planet in 1975.

That lack of knowledge has started to change, however, with the Jan. 14 flyby of the Messenger probe, which is scheduled to make two more passes of the planet before settling into an orbit in 2011. Messenger took more than 1,000 images during its flyby and took the first readings of the planet's chemical composition, both on its surface and in its atmosphere.

The findings, first revealed on Jan. 30 and later published in a series of studies in the July 4 issue of the journal Science, show that beneath Mercury's rocky exterior is a liquid iron-rich outer core powering its magnetosphere and giving shape to its most prominent tectonic feature — the huge cliffs that mark the top of crustal faults, a sign that as the core cools, the planet has shrunk by about 0.05 to 0.1 per cent.

Mercury 101

Mercury is the closest planet to the sun, travelling in an elliptical orbit at an average distance of about 58 million kilometres. (Compared to 150 million kilometres for Earth.) Because it is so close to the sun, Mercury also travels faster than any other planet, at a speed of about 48 kilometres per second, completing a circuit of the sun once every 88 days.

The planet's rotation on its axis is slow, taking about 59 Earth days to complete. The combination of the planet's quick pace around the sun and slow rotation means the time between one sunrise and the next on the planet lasts 176 Earth days.

The unusually long day, combined with the planet's thin atmosphere and close proximity to the sun, means Mercury experiences enormous swings in temperatures: with temperatures getting as hot as 467 C and as cold as –170 C on the dark side of the planet.

The harsh conditions mean Mercury is not likely to have ever supported any form of life.

Since Pluto was demoted from its status as a planet, Mercury is also the smallest planet in our solar system. Mercury has a diameter of about 4,879 kilometres, or about two-fifths that of Earth.

On its barren surface, pock-marked with craters from meteors and small comets, Mercury resembles Earth's moon.

But Messenger's findings point to its active past. In addition to the cliffs — called lobate scarfs — that create "wrinkles" on Mercury's surface, Messenger has also found evidence of volcanic vents along the margins of the Caloris basin, a prominent remnant of an impact with a meteor. These suggest that Mercury had a more active geological past and may help explain the planet's smooth plains, since such plains could have been formed in the aftermath of lava eruptions on the planet's surface.

Analysis of Mercury's chemical composition revealed that though Mercury is more than 60 per cent iron by weight, iron is relatively scarce in Mercury's surface minerals — and thus probably also in its crust and mantle. That, scientists studying the planet say, is unusual compared to the other planets of the inner solar system. Further study of the chemical composition of Mercury will have to wait until Messenger is in orbit around the planet in 2011, they said.

Messenger also detected a number of charged particles in the planet's thin atmosphere, including ionized sodium, magnesium, oxygen and even traces of water. Earth-based radar observations of impact craters on the planet's poles suggest they might contain water ice shielded from sunlight by the depth of the craters, an observation Messenger will check once in orbit.

Missions to Mercury

Prior to Messenger's arrival, Mariner 10 was the lone space probe to visit Mercury, passing by the planet three times in 1974 and 1975 after previously flying past Venus. It flew to within 740 kilometres in the closest of its passes to the planet but managed to only photograph 45 per cent of Mercury's surface. It also detected the planet's magnetic field.

NASA astronomers have high hopes Messenger, launched in 2004 and run at a cost of $446 million US, will continue to provide a more complete picture. It's already mapped 21 per cent more of the planet's surface than the previous probe during its first flyby.

Part of the reason there have been so few missions to Mercury is time and cost. By the time Messenger arrives in orbit around Mercury, the space probe will have travelled 7.9-billion kilometres and flown past Earth once, Venus twice and Mercury three times.

The mission, first conceived in 1996, was a long time coming also because material science advances were needed to create a probe capable of withstanding radiation from the sun, says Messenger's principal investigator, Sean Solomon, of the Carnegie Institution of Washington.

But another reason is that — compared to the search for water on Mars — Mercury's environment offers little future potential for exploration.

Why that is the case is a question astronomers hope the latest mission will answer.

"Mercury ended up as a small, dense planet with a dynamic molten core and little atmosphere," said Solomon. "We want to know how it ended up with this extreme outcome."

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