Image copyright NASA Image caption The bright feature was spotted in images from July last year, but a few days later it had vanished

Scientists have outlined their best explanations for a mysterious feature dubbed the "magic island", which has been spotted on Saturn's moon Titan.

The Cassini spacecraft captured the "island" during a flyby, but it had vanished by the time of the next pass.

The bright splodge is seen in Ligeia Mare, one of the seas of methane and ethane found at Titan's north pole.

Icebergs, waves and gas bubbling up from the sea bed are all possibilities, the scientists say.

The study by an international team has been published in the journal Nature Geoscience.

Image copyright NASA Image caption Ligeia Mare is the second largest body of liquid on the saturnian moon

Saturn's largest moon shares much in common with Earth, such as a substantial atmosphere and a seasonal cycle. Wind and rain shape the surface to form river channels, seas, dunes and shorelines.

Titan's mountains and dune fields are made of ice, rather than rock or sand, and liquid hydrocarbons take many of the roles played by water on Earth.

The seas and lakes peppering the moon's north polar region are filled with methane and ethane. These are gases on Earth, but at typical Titan temperatures of -180C, they exist in a liquid state.

Titan - 'Looking-glass Earth'

Image copyright NASA Image caption Titan, seen here with Tethys in the background, is shrouded in an orange haze of organic chemicals

Titan is Saturn's largest moon and the second biggest in the Solar System

It is the only moon in the Solar System with clouds and a substantial atmosphere

Wind and rain create similar features to those found on Earth, such as dunes, lakes and rivers

But on Titan it rains liquid methane, filling the rivers, lakes and seas with hydrocarbons

The bright feature was spotted in pictures from a Cassini flyby of Titan on 10 July 2013. The "island" is absent in imagery of Ligeia Mare taken on three previous flybys.

By the time of the next pass of Titan, on 26 July, the feature had vanished, and was not visible in two subsequent flybys.

"'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," co-author Jason Hofgartner told BBC News.

The feature appears and disappears too quickly to be a volcanic islet. So the team were left with a handful of potential explanations.

Image copyright NASA/JPL-Caltech/USGS Image caption Titan's lakes and seas are thought to be filled with a mixture of liquid methane and ethane

Mr Hofgartner, who is based at Cornell University in New York, explained: "We have four different hypotheses that are all equally preferred. In no particular order they are: waves, rising bubbles, floating solids and suspended solids."

Titan operates on a 30-year seasonal cycle, and the moon's northern region is expected to become a more dynamic place as Titan approaches its summer solstice in May 2017.

"Right now, Titan is basically half way between the vernal equinox (August 2009) - at the beginning of spring - and the summer solstice, the start of summer. It's roughly equivalent to what we would consider the beginning of May," said Mr Hofgartner.

"As Titan approaches its summer, more of the Sun's energy is being deposited in the northern hemisphere."

Winds will get stronger, causing an increase in waves, which are one potential explanation for the "magic island". Researchers have already seen possible evidence for small waves on another Titan sea.

Iceberg ahoy?

Another intriguing possibility is that of floating or suspended solids, including icebergs. However, any 'bergs couldn't be made of water-ice - which, because of its relatively high density, would sink in a liquid hydrocarbon sea. Instead, icebergs on Titan would have to be made from a frozen mixture of methane and ethane.

A previous study by Mr Hofgartner and Prof Jonathan Lunine, also of Cornell, suggested conditions on Titan might cause methane-ethane ice to sink in the winter and float in summer.

It looks like something is going on in Ligeia Mare. Titan surprises us at every turn Prof John Zarnecki, Open University

But the moon's surface is also thought to be covered in various organic (carbon-based) compounds, including one, polyacetylene, with a density low enough to allow it to float. It could be suspended below the surface of a sea much like silt in a terrestrial river delta.

The final possibility is that Cassini captured gas bubbling up to the surface from a subsea volcanic vent. The abundant methane in Titan's atmosphere must be continually replenished, because the molecule can only exist for a short time before it is destroyed by UV rays.

Although researchers haven't yet found the "smoking gun" of volcanic activity on Titan, it remains one of the best candidates for the source of the moon's methane.

John Zarnecki, an emeritus professor at the Open University in Milton Keynes, co-authored the first paper to predict wave heights on Titan. But he said there was little evidence from more recent observations that winds on the moon could raise waves big enough to detect.

Speaking to the BBC from Rio de Janeiro, where he has been following England at the World Cup, Prof Zarnecki referred to the waves he could see crashing on to Copacabana Beach, and said: "I'd love to think that this paper represents the first positive indication of a similar phenomenon on Titan."

He added: "These are clearly observations that are close to the limit of detectability - and therefore very difficult to interpret. But it looks like something is going on in Ligeia Mare. Titan surprises us at every turn.

Image copyright Esa Image caption The Huygens spacecraft touched down on Titan in January 2005

"Is this feature showing us floating solids or gases erupting at the surface - or a phenomenon that we haven't thought of? After all, we tend to think in terms of Earth-like phenomena. But based on this so far sparse data, any suggestion is likely to be little more than speculation until we get some more supporting information."

The authors of the latest study hope that future observations by Cassini might yield evidence of similar phenomena. And if they do, there might be a chance of distinguishing between different possible causes.

For example, if a similar feature is seen to move its location on successive passes by Cassini, it could be suggestive of an iceberg being moved by currents.

Prof Zarnecki commented: "This is just further evidence, if we need it, that we just must go back to Titan with a dedicated mission, ideally to land in one of Titan's seas - a Titan Sea Probe. And then we can understand what is happening on the seas of this incredible place."

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