The Cassini spacecraft has discovered evidence of a vast ethane cloud on Saturn's moon Titan.
The clouds are seen in red
Scientists believe flakes of ethane "snow" or drops of ethane "rain" may be falling from the cloud into lakes of liquid methane.
Before the mission, researchers expected to find the moon awash with oceans of liquid ethane.
But so far, the evidence for them has been scarce, suggesting the ethane may be tied up as ice at Titan's poles.
"We think that ethane is raining or, if temperatures are cool enough, snowing on the north pole right now," said planetary scientist Dr Caitlin Griffith, from the University of Arizona in Tucson, US.
"When the seasons switch, we expect ethane to condense at the south pole during its winter.
"If polar conditions are as cool as predictions say, ethane could accumulate as polar ice."
The cloud was detected by Cassini's visual and infrared mapping spectrometer as a bright band at heights of between 29km (18 miles) and 60km (37 miles) on the edge of Titan's arctic circle.
In the coming months, Cassini's cameras will search for direct evidence of polar caps of ethane ice on Titan.
Currently, the moon's north pole is in winter darkness, making it difficult to observe.
"We're going to start making more polar passes in the coming months," said Dr Griffith.
"By the end of next year, Cassini will have recorded the first polar temperature profile of Titan, which will tell us how cold conditions are at the pole."
The $3.2bn Cassini-Huygens project is a joint venture between the US space agency (Nasa), the European Space Agency (Esa) and the Italian Space Agency (Asi).
The spacecraft entered orbit around Saturn on 1 July 2004. It is the first to explore the Saturn system of rings and moons from orbit.
The ethane observations were reported in the journal Science.