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Tiger stripes and ice volcanoes

27 Jan, 2012

In the latest of our shortlisted entries to the 2011 Wellcome Trust Science Writing Prize, Kelly Oakes tells us about a distant moon.

Its surface is white as snow and covered in ice. Large expanses are smooth and unblemished, belying a history of constant meteorite bombardment. In one site near this moon’s south pole there are cracks tens of miles long. Ice and water, from vast underground oceans, are constantly spewed out of these cracks into the blackness of space.

Sounds like something from Doctor Who, right? Well, this moon actually exists, and it’s closer than you might think.

Enceladus is the second smallest of Saturn’s major moons. Until the Voyager 2 satellite passed in 1981, we knew little about it. Now, with the Cassini-Huygens satellite performing having performed several close fly-bys, the story of Enceladus is beginning to become clear.

Enceladus has one of the most reflective surfaces of any object in the solar system. It is also one of the most geologically active solar system bodies. This may seem counterintuitive at first, but it is no surprise to the scientists that study Enceladus — geological activity is what makes the moon so pristine and reflective in the first place. Ice volcanoes are able to constantly replenish the surface, covering up the scars of meteorite bombardment.

As it orbits Saturn, the pull of the planet contorts Enceladus. Saturn inflicts so much force on the tiny moon that ‘hotspots’ are created near its south pole, creating four giant cracks. Each is around 80 miles long, a mile wide and 500 metres deep. These cracks, affectionately known as ‘tiger stripes’, are 100°C hotter than the rest of the moon.

Cassini took the first clear pictures of the tiger stripes in July 2005. Their signature however, had been seen a few months earlier. In February 2005, small changes in the magnetic-field data revealed Enceladus had an atmosphere containing water vapour. There was something a little out of the ordinary about it, as the atmosphere was concentrated around the moon’s south pole, right above the tiger stripes. Ultraviolet images later confirmed what scientists had suspected: the stripes were the source of the giant plumes of water vapour and ice being ejected into space.

In the 1980s, scientists had some idea that these plumes might exist. During its time around Saturn, Voyager 2 searched for them but found no conclusive evidence of their existence. Scientists working on the Cassini mission decided that they could not take any chances — a fly-by had its path altered, bring it closer to the surface of Enceladus. It flew through one of the gas clouds created by the plumes and confirmed the presence of water, dust and molecules containing carbon and hydrogen.

One of the few things that we knew about Enceladus before Voyager 2 and Cassini was that it was icy. Since the discovery of the tiger stripes and plumes, scientists have speculated that there is also liquid water beneath the surface.

Although the forces exerted by Saturn on Enceladus are insufficient to explain the heating occurring around the tiger stripes, combing their effect with that of a vast ocean beneath the surface, able to transport heat more efficiently, might explain the phenomenon.

Evidence of a hidden ocean was suggested in December 2008 when scientists noticed that the tiger stripes had moved. They think that something similar to the movement of Earth’s tectonic plates is occurring on Enceladus, with the surface of the moon splitting and material coming up from underneath to fill the gap. On Earth, the material that fills the gap is molten rock. On Enceladus, it could be water.

Yet more evidence came a few months later. Unusually high levels of salt were discovered in water from the plumes. Salt tends to come from large bodies of water; oceans are big enough to have significant amounts dissolved within them, while ponds are not.

Liquid water, combined with heat and the organic molecules seen in the plumes, increases the chance that life might develop on Enceladus. If water does exist on Enceladus, it will join Mars and Europa, a moon of Jupiter, at the top of the list of places to look for life in the solar system.

We are discovering more about this moon all the time. Earlier this year, for example, it was revealed that the ice volcanoes on Enceladus create an electrical circuit between the moon and its planet. This produces aurora, better known as the northern lights here on Earth, on Saturn.

If these recent discoveries are anything to go by, Enceladus is only going to get more interesting. The Cassini mission was originally meant to run for four years, until 2008. It has now been extended twice, and will keep exploring Saturn and its moons until at least 2017. Watch this space.

Kelly Oakes

This is an edited version of Kelly’s original essay. Views expressed are the author’s own.

Find out more about the Wellcome Trust Science Writing Prize in association with the Guardian and the Observer and read our ‘How I write about science‘ series of tips for aspiring science writers.

Over the coming months, we’re publishing the shortlisted essays in this year’s inaugural competition.

Image Credit: kokogiak on Flickr

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