Scientists have identified an unexpected, high-altitude methane ice cloud on Saturn's moon Titan that is similar to the exotic clouds found far above the Earth's poles.

The cloud, imaged by NASA's Cassini spacecraft, was part of the winter cap of condensation over Titan's north pole.

Researchers have determined that it contains methane ice that produces a much denser cloud than ethane ice previously identified there.

"The idea that methane clouds could form this high on Titan is completely new. Nobody considered that possible before," said Carrie Anderson, scientist at the NASA's Goddard Space Flight Center in Maryland in the US.

Methane clouds were already known to exist in Titan's troposphere, the lowest layer of the atmosphere.

Like rain and snow clouds on Earth, these clouds form through a cycle of evaporation and condensation, with vapour rising from the surface, encountering cooler and cooler temperatures and falling back down as precipitation.

On Titan, however, the vapour at work is methane instead of water.

"Titan continues to amaze with natural processes similar to those on the Earth, yet involving materials different from our familiar water," said Scott Edgington, Cassini deputy project scientist at NASA's Jet Propulsion Laboratory in Pasadena, California.

"As we approach southern winter solstice on Titan, we will further explore how these cloud formation processes might vary with season," he concluded.

The results appeared in the journal Icarus.