Ground-based observations of Saturn's giant moon Titan have revealed tantalizing evidence that its parched desert-like surface can support large-scale storm clouds that rain liquid methane.

Published today in the journal Nature, astronomers using the Gemini North telescope and the Infrared Telescope Facility (IRFT) on Hawaii announced the discovery of a cloud formation spanning three million square kilometres – equivalent to the area of India – over the moon's tropical zone near its equator.

Leading up to the discovery cloud cover over the moon was well below one percent, and no significant clouds over the tropical regions were spotted during the previous eight years of observations. Then, over a three week period in April 2008, the cloud cover increased dramatically, first forming at a latitude of 30 degrees south then later closer to the equator.

"The first cloud was seen near the tropics and was caused by a still-mysterious process, but it behaved almost like an explosion in the atmosphere, setting off waves that traveled around the planet, triggering their own clouds," says Mike Brown of the California Institute of Technology. "Within days a huge cloud system had covered the south pole, and sporadic clouds were seen all the way up to the equator."

Although cloud formations at Titan's mid latitudes and polar regions had been observed for many years, prior to this discovery it was not known whether significant cloud formation was possible over equatorial regions.

"In April 2008 we observed what was a global event that shows how storm activity in one region can trigger clouds, and probably rainfall, over arid regions, such as the tropics where Huygens landed," says team member Henry Roe from Lowell Observatory. "Of course these rain showers are not liquid water like here on Earth, but are instead made of liquid methane. Just like the streambeds and channels that are carved by liquid water on Earth, we see features on Titan that have been created by flowing liquid methane."

Small scale channels imaged by the Huygens probe that descended through the moon's thick atmosphere in January 2005 seemed to contradict atmospheric models predicting dry desert-like conditions near the equator; until now these had been explained by liquid methane possibly seeping out of the ground. Now it appears the speculation is over. "No one considered how storms in one location can trigger them in many other locations," says Brown. The same phenomenon occurs in the Earth's atmosphere.

Currently the Cassini spacecraft flies by Titan once every six weeks or so, so the continuous ground-based monitoring from the Earth plays an important role in studying features like these rapidly forming storms. But Titan's year is 30 Earth years long so there is a long way to go before fully understanding seasonal variations in the weather. "Imagine trying to understand Earth's weather having only seen what happens in January, February, and part of March," says Roe. "We have our work cut out for us to continue watching the weather on Titan for many more years."

Our latest 132-page Astronomy Now special edition is an extravaganza of astronomy for the year ahead, with a complete 30-page guide to observing the planets, moon, meteor showers, two solar eclipses, and the deep sky in 2010.
 U.K. STORE
 E.U. STORE
 U.S. & WORLDWIDE STORE

Hubble Reborn
Hubble Reborn takes the reader on a journey through the Universe with spectacular full-colour pictures of galaxies, nebulae, planets and stars as seen through Hubble's eyes, along the way telling the dramatic story of the space telescope, including interviews with key scientists and astronauts.
 U.K. STORE
 E.U. STORE
 U.S. & WORLDWIDE STORE

3D Universe
Witness the most awesome sights of the Universe as they were meant to be seen in this 100-page extravaganza of planets, galaxies and star-scapes, all in 3D!
 U.K. STORE
 E.U. STORE
 U.S. & WORLDWIDE STORE

This special publication features the photography of British astro-imager Nik Szymanek and covers a range of photographic methods from basic to advanced. Beautiful pictures of the night sky can be obtained with a simple camera and tripod before tackling more difficult projects, such as guided astrophotography through the telescope and CCD imaging.
 U.K. STORE
 E.U. STORE
 U.S. & WORLDWIDE STORE

Explore the Universe with these new versions of the award-winning Starry Night Software. Available now from the Astronomy Now Store.
 U.K. STORE
 E.U. STORE
 U.S. & WORLDWIDE STORE

Astronomy Now is pleased to announce the publication of Exploring Mars. The very best images of Mars taken by orbiting spacecraft and NASA's Spirit and Opportunity rovers fill up the 98 glossy pages of this special edition!
 U.K. STORE
 E.U. STORE
 U.S. & WORLDWIDE STORE

This new poster features some of the best pictures from NASA's amazing Mars Exploration Rovers Spirit and Opportunity.
 U.K. STORE
 E.U. STORE
 U.S. & WORLDWIDE STORE