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  1. Science & Tech
25 June 2014updated 01 Jul 2021 11:42am

An island appeared in a lake on a moon around Saturn, then disappeared

A mysterious island has materialised in a methane lake on Saturn’s largest moon – only to vanish just weeks later.

By Ajit Niranjan

Saturn is famous for its rings, but it should be just as famous for its moons. Dozens of natural satellites orbit the great gas giant, ranging from miniscule moonlets smaller than a running track, to vast celestial bodies the size of planets – and even now it appears that more moons are being created from the dust of Saturn’s rings.

One moon in particular, however, has been the subject of fascination since its discovery in 1655. Titan, a rock mass larger in volume than both the Earth’s Moon and the planet Mercury, has geographical features strikingly similar to Earth’s. Undulating rivers meander through mountains and volcanoes, cutting deep crevices into the otherwise smooth surface. Channels, seas, dunes and shorelines are all features of Titan’s surface, beneath the only dense, sustained gas atmosphere on any moon in the Solar System.

There’s a key difference here though. Unlike on Earth, Titan’s surface-atmosphere cycle isn’t composed of water. Brian Cox, in Wonders of the Solar System, explains:

On Titan, methane plays exactly the same role as water does here on Earth. So where we have clouds of water, Titan has clouds of methane with methane rain. Whereas we have lakes and oceans of water, Titan has lakes of liquid methane.”

Such vast quantities of highly flammable fuel might appear unstable, but there is little chance of it burning: Titan’s atmosphere is composed almost purely of nitrogen, an inert gas. As a result, Titan is the only object that we know of beyond Earth to have both an atmosphere and stable bodies of liquid on its surface. It’s in one of these vast methane-ethane lakes – Ligeia Mare – that scientists have discovered an unidentified floating object.

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In an article published in Nature Geoscience, the research team analysed photos from Nasa’s Cassini probe to find a new geographical feature unseen in previous years. The ‘island’ in question (a small grainy splodge in the middle of the photo) measures about twelve by six miles – less than half the size of the Isle of Wight. Bafflingly, photos taken just weeks later of Ligeia Mare failed to show any sign of the mysterious island, prompting researchers to investigate the nature of the speck.

Speaking to the BBC, planetary scientist Jason Hofgartner said: “‘Magic Island’ is a colloquial term that we use within the team to refer to this. But we don’t actually think it’s an island.”

The researchers have proposed four potential explanations for the moving mass (or “transient features”, as the authors call it). According to Hofgartner, lead researcher of the international team, “these bright features are best explained by the occurrence of ephemeral phenomena such as surface waves, rising bubbles, and suspended or floating solids”.

There’s nothing exciting about “ephemeral phenomena” and “transient features” – that just means they’re short-lived. These could be in the form of ‘methane-bergs’ floating in the water, or deep-sea activity creating bubbles or waves. This last explanation seems to be popular, but may run counter to previous scientific thought: a study earlier this year found the surface of Ligeia Mare to be “extremely smooth, likely to be mostly methane in composition, and exhibit no surface wave activity”. The surface ripples were less than a millimetre high. Lakes and ponds on Titan, compared to Earth, would seem completely still.

This doesn’t mean Titan lacks waves though. Scientists have cautioned against drawing conclusions as the data was taken over a pretty short time frame – and considering seasons on this moon each last seven years, we can only speculate about the moon’s climate over time. Hofgartner’s team suggest that as summer on Titan draws closer, hot air brings stronger winds making greater waves. These could be enough to register as a fleeting object on the images.

The explanations are plausible, but astronomers are agreed that further research into Titan’s geography is needed. Speaking to the Guardian, space scientist John Zarnecki spoke of potential future explorations.

Is it floating solids or erupting gas bubbles from below or wave action? We just don’t know. The one thing we can say with certainty is that we just have to go back to Titan – but this time with a sea floater so that we can see close up just what is happening in the seas of this incredible place.”

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