A cyanide cloud formed over Titan's south pole as the strange moon of Saturn entered its seven-year winter in 2009, astronomers reported on Wednesday.

An enigma of the Solar System, Titan is studded with lakes of liquid hydrocarbons and a choking nitrogen-methane atmosphere, according to data sent back by a US-European scout mission.

The satellite and its mother planet are so far from the Sun -- 1.4 billion kilometres (886 million miles) -- that each season on Titan lasts seven Earth years instead of three months.

Reporting in the journal Nature, scientists led by Nick Teany at the University of Bristol in western England, said the US orbiter Cassini first spotted a cloud over Titan's south pole five years ago.

Two years of observation found the cloud comprised icy particles of hydrogen cyanide.

The discovery showed that the onset of winter on Titan is sharp and brutal even in this extremely cold region of space.

"Titan's south pole must be extremely cold to allow hydrogen cyanide to condense," a university press statement said.

"In fact the upper atmosphere must have cooled by over 50 degrees (Celsius or 90 degrees Fahrenheit) in less than a year to reach a blisteringly cold minus 150 C."