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 lake bottom are all possibilities, the scientists say.
The study by an international team has been published in the journal Nature Geoscience.
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 up many of the roles played by water on Earth.
Indeed, the seas and lakes that pepper the moon's north polar region are filled not with water, but with methane and ethane. These exist in a gaseous state on Earth, but at typical Titan temperatures of -180C, they are liquids.
Titan - 'Looking-glass Earth'
- 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 mysterious bright feature was spotted in pictures from a Cassini flyby of Titan on 10 July 2013. The 'island' appears to be absent in imagery of Ligeia Mare taken on three previous flybys of the moon.
By the time of the next pass of Titan, on 26 July, the feature had vanished, and it could not be seen in two subsequent Cassini flybys over the area.
''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 island. So the team were left with a handful of potential explanations.
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 over the course of time, causing waves to pick up on the surface of lakes and seas. Waves are one potential explanation for the 'magic island' and researchers have already seen possible evidence for small waves on another body of liquid known as Punga Mare.
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 mean that methane-ethane ice would sink in the winter and float in the summer.
It looks like something is going on in Ligeia Mare. Titan surprises us at every turn'
End Quote Prof John Zarnecki Open University
But the moon's surface is also thought to be covered in various organic (carbon-based) compounds, some of which could have sufficiently low densities to allow them to float. One such compound, known as polyacetylene, could be suspended below the surface of the 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.
'We do have evidence for volcanism on Titan in the past, or even going on now. But we haven't found the smoking gun,' said Mr Hofgartner.
John Zarnecki, an emeritus professor at the Open University in Milton Keynes, was chief scientist for the Surface Science Package - an instrument carried to Titan aboard the European-built Huygens probe. He was also a co-author of the first paper to predict wave heights on Titan, but 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 the England football team's exploits in 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. When I first embarked on this project, over 20 years ago, my nightmare was that Titan, after all the hype, would turn out to be dull and boring but nothing could be further from the truth.
'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 transient feature is seen to move its location on successive passes by Cassini, it could be more 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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