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Iapetus: The Two-Faced Ridged Moon Of Saturn - Yellow Magpie

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Published on October 25th, 2011 | by Yellow Magpie


Iapetus: The Two-Faced Ridged Moon Of Saturn


Iapetus is a dazzling sight. Possessing some of the largest mountains in the Solar System it is full of strange features. One of the most recognisable is its two-coloured appearance as well as an enormous ridge running along its equator. It also has a huge 580 kilometre (360 miles) wide crater called Turgis. 

Not A Sphere

A sphere with a ridge, Iapetus is 1,470 kilometres in width (913 miles). It takes 79 days for the moon to complete a trip around Saturn, orbiting at a distance of 3.5 million kilometres (2.1 million miles).

Iapetus is an ice world with an average temperature of minus 190 degrees Celsius (minus 310 degrees Fahrenheit) and the satellite is mainly made up of water-ice with a substantial portion comprised of rocky materials.

Although on first viewing the moon appears to be spherical this is not quite the case. The two poles are compressed and along the equator a large ridge is present.

Iapetus's Casini Regio

Light And Dark

Iapetus is a two-coloured satellite. The poles and the trailing hemisphere (the side facing away from Saturn) are brightly coloured wheras the leading hemisphere (the side facing the ringed-planet) is a dark red colour. The same red colour is also found on Iapetus’s sister moon, Hyperion. The dark side of the planet is named the Cassini Regio. Together these two colours create one of the most striking features of the Solar System.

Scientists believe that the difference in temperature between the distinct regions caused the formation of the bright and dark sides.

They maintain that the slow, 79-day rotation of the satellite means that much of the surface ice would have been sublimated leaving a dark residue in its stead.

However, it is important to emphase that this was not a fast process by any means and took hundreds of millions of years. According to scientists at NASA, it would have taken over one billion years for 20 metres of ice to have been sublimated in the dark region. While over the same period of time only ten centimetres would have been lost in the bright region.

Iapetus Saturn's Moon's Bright Region

The Ridge

One of the most impressive features of Iapetus is its equatorial ridge which is a huge structure at 1,300 kilometres (800 miles) in length, 20 kilometres (12 miles) in width and 13 kilometres (eight miles) in height. The ridges have peakes that are more than 20 kilometres in height (12 miles).

Although there are several ideas that have been put forward by theorists none explain why this ridge is confined to Cassini Regio so at best the thoughts are incomplete.

Iapetus's Equatorial Ridge

Breaking The Orbital Pattern

As one of the Saturn’s biggest moons (the third largest), Iapetus should have an orbit much closer to the ringed-planet. However, it doesn’t. The satellite’s orbit is very far out. As of yet, no one knows why this is the case.

Like all of the objects in the Solar System, mysteries still remain about Iapetus. We still do not know how one of the most striking features, its equatorial ridge was formed. Nor do we know why it orbits so far away from Saturn. Perhaps, like all great mysteries the answers will come in good time.

Highly Recommended Reading

Check out Yellow Magpie’s The Planet Saturn: The Ringed Wonder Of The Solar System for more insight into the largest planet in the Solar System.

Cosmos is a highly recommended book. It contains large, full-page pictures of the asteroids and writing on the subject by the highly competent author, Giles Sparrow.

For people living in Ireland or the United Kingdom, you can access: Cosmos from here.

For those who live in Canada, you can obtain: Cosmos here.

For Germany: Cosmos.

For France: Cosmos.

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