content long 19-February-2019 01:07:35

Winners - United Kingdom

16 - 18 years old - Iapetus

Author: Elena Rastorgueva

My chosen target is Iapetus, a moon of two halves, a long-standing puzzle and, I feel, the worthiest candidate for further investigation. As tempting as Dione and Saturn are, Iapetus stands out because of its scientific mystery and distinctive appearance.

Giovanni Cassini himself was perplexed by this 'yin and yang' moon. The trailing side of Iapetus, which faces backwards, opposite to the direction of travel, is white. No surprises there: Iapetus is not very dense and so is probably composed mainly of frozen water, as well as some rock. However, the leading side, which faces forwards, is black with a hint of red. It's thought that this dark material is made up of various compounds containing carbon, including cyanides.

No other object in the solar system has such an extreme contrast. Even more intriguing is that the contrast is very distinct: the change from stark white to coal-black can occur in 20 metres.

There have been attempts at explanations.

Maybe the material was transferred from Phoebe, another distant moon, after a rogue object slammed into it and blasted the material into space. Then again, the colours don't quite match up.

Perhaps the material has been spewed out from within Iapetus due to volcanism. But then why haven't we seen these ice volcanoes?

Much uncertainty remains. Yet with newly-acquired evidence, we may get closer to the root of this 300-year-old problem.

Another mystery was only revealed in 2004, when the Cassini probe took its first detailed photos of Iapetus. They show the moon to look very much like a walnut, albeit with a radius of 700km. This similarity is because of a very tall and narrow ridge that stretches almost all the way round the moon's equator. How could the ridge have formed?

It may have been during an early period in the moon's history when it was spinning much faster and caused the ice and rock to form the ridge much like a salad spinner. Another explanation is that Iapetus had a moon of its own: a sub-satellite. Gradually, it spiralled in towards Iapetus and broke into pieces as it was torn apart by Iapetus's gravity. These pieces eventually landed along the equator and formed the ridge seen today.

This may sound odd, but then Iapetus is a very unusual moon in being so very far from its planet: we are yet to observe any moons with sub-satellites because they are too close to their planet. The planet has such a strong gravitational pull that it effectively takes all the moons for itself. It would be very exciting, both among the scientific community and the general public, if it was discovered that the sub-satellite had existed: a moon which orbits a moon, which orbits a planet, which orbits a star.

It is very difficult to resist the intrigue of Iapetus and its unique appearance. By focusing on this particular target in the Cassini probe's next fly-by, we may be able to unravel some more of its mystery

Last Update: 27 February 2014

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