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Last Updated: Tuesday, 4 January, 2005, 14:27 GMT
Probe passes 'moon of two halves'
Flyby images may help solve the mystery of Iapetus' bizarre appearance

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.

The flyby went spectacularly well. It has been very scientifically fruitful
Dr Carolyn Porco, Cassini imaging leader

"The flyby went spectacularly well. It has been very scientifically fruitful," Dr Carolyn Porco, who leads the mission's imaging team, told the BBC News website.

"While we don't have a final, definitive answer on why Iapetus looks the way it does, we have taken a giant leap forward in getting closer to an answer."

A Nasa tracking station in Goldstone, US, began to receive the signal and science data collected by Cassini at 0700GMT on Sunday.

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.

Organic "paint"

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.

Nasa/JPL/Space Science Institute
Dark material "painted" the moon's surface some time in the past (Image: Nasa/JPL/SSI)
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.

Impact site

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 400 km (250 miles).

Iapetus, Nasa/JPL/Space Science Institute
The moon has been left pock-marked by past bombardments (Image: Nasa/JPL/SSI)
Friday's flyby was the first close encounter of Iapetus during the four-year Cassini tour. The second and final close flyby of Iapetus is scheduled for 2007

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

Cassini's next task will be to provide the Huygens probe with communications support during its descent through the atmosphere of Saturn's largest moon Titan.

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

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).




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