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Last Updated: Sunday, 2 January, 2005, 15:26 GMT
Cassini passes 'two-faced' moon
The Cassini orbiter's images of Iapetus show a world of sharp contrasts

The Cassini spacecraft has made a close pass of Saturn's moon Iapetus, a striking world of two halves.

One side of Iapetus' surface is as bright as snow, while the other is coated in a material as dark as tar.

At 0130GMT on 1 January, Cassini flew by the frigid moon at a distance of 123,400km on its closest approach.

Some scientists think the dark material on Iapetus' surface came from space, while others believe it could have spewed out from the moon's interior.

It looks like the skin of a stitched baseball
Dr Carolyn Porco, Space Science Institute
The flyby could bring scientists a step closer to knowing which of these processes was responsible for its bizarre appearance.

"[Iapetus] is novel and it's intriguing and may signify some important planetary process that's just begging for further investigation," Dr Carolyn Porco, who leads the mission's imaging team, told the BBC News website.

Cassini got roughly 10 times closer to the moon than the Voyager 2 probe did in 1981. The closest approach occurred over the moon's dark terrain, which has never been seen at close range.

The best images taken of Iapetus by Voyager had a resolution of about 8km per pixel, while on this pass Cassini got down to a resolution of about 1km per pixel.

Cover of darkness

The dark coating is rich in organic (carbon-based) molecules and blankets the side of Iapetus that leads in the direction of orbital motion around Saturn (apart from the moon's poles).

It may have started off as a cloud of material ejected into space by an impact on an outer moon of Saturn. Over time, the orbiting dust could have moved inward towards the ringed planet.

Eventually, Iapetus flew through the cloud, which "painted" it on one side with the organic-rich material.

Iapetus pictured by Voyager 2, Nasa
Voyager 2 caught this glimpse of Iapetus in 1981
The other possibility is that the dark coating originated within Iapetus itself.

"It could have been extruded out on to the surface in a violent event or in some other way; we just don't know," Dr Porco explained.

"Volcanism can mean different things; it could be the fissure type we see in Iceland. But if eruptions are involved, it is going to be icy volcanism."

Scientists also want to know how the dark material came to have its particular shape and the boundary.

"It looks like the skin of a stitched baseball," the Cassini imaging team leader observed.

Mountain peaks

Also visible in Cassini's latest images of Iapetus are a line of mountains that appear as a string of bright dots around the moon's equator.

The mountains were originally detected in Voyager images, and might compete in height with the tallest mountains on Earth, Jupiter's moon Io and possibly those on Mars.

Cassini has also detected a large circular feature in the southern hemisphere which is probably an impact crater. It seems to have a diameter of more than 400km (250 miles).

With a diameter of 1,436km (892 miles), Iapetus is Saturn's third largest moon.

In Greek mythology, Iapetus was a Titan - one of a race of god-like giants who were born from Uranus and Gaia.

The $3.2bn Cassini-Huygens mission is a joint venture between the US space agency (Nasa), the European Space Agency (Esa) and the Italian Space Agency (Asi).

On 25 December, the piggybacked Huygens space probe separated from Cassini to begin its journey to study Saturn's largest moon Titan.

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