The Cassini spacecraft has spotted the brightest area yet on Saturn's moon, Titan - but how it formed remains a mystery.
"It's the brightest area on Titan in every wavelength we've looked at," says Jason Barnes, at the Lunar and Planetary Laboratory at the University of Arizona, US.
The bright red spot is about 400 kilometres across and lies south-east of another bright area named Xanadu. But it is almost twice as bright as Xanadu. "The question is why," says Barnes.
The reflective area could indicate a layer of methane-rich ground fog or the sheen of methane rainfall. Other observations indicate that Titan may have seas of liquid hydrocarbons and methane clouds.
The Visual and Infrared Mapping Spectrometer aboard Cassini has observed the spot over a period of nine months, and the Keck Observatory in Hawaii has observations going back over 4 years.
The persistence of the spot suggests it may not involve clouds, as these tend to break up within hours or days on Titan. However, a long-lived cloud controlled by the flow of the atmosphere across surface features, such as small mountains, might be possible. On the other hand, the spectrum of the spot does not correlate well with that of clouds.
It may be a fog hovering over a lake, hot springs or volcanic region. "But if it's ground fog, it's really persistent," Barnes told New Scientist.
A bright arc in Titan's surface, known as "The Smile", might also be responsible for the new bright spot. "The Smile" could vent some material that blows northeast and then settles on the bright region. The spot's colour and brightness suggest that it is a relatively recent formation.
The team's early speculation was that the area could be a volcanic hot spot. But Cassini's instruments did not find elevated surface temperatures.
Another possible explanation is that the spot is made of highly reflective frozen carbon dioxide. However, Cassini does not have the capability to confirm this.
NASA's Monika Kress and Chris McKay predicted in 2004 that there should be a lot of carbon dioxide on Titan, given a predicted bombardment by carbon-dioxide-rich comets early in the moon's existence.
Journal reference: Science (vol 310, p 92)
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