Adaptive optics image from Gemini Observatory of Titan passing in front of much larger Saturn. Titan, as usual, looks quite bland, with no hint of clouds.
Credit: Gemini Observatory/AURA/Henry Roe, Lowell Observatory
SYDNEY: Large clouds seen over the equator of Saturn's moon Titan may be tropical storms with hydrocarbon rain, say scientists.
The clouds - perhaps created by periodic volcanic eruptions of methane - are the first evidence of rain near the moon's equator. Detailed recently in the journal Nature, the findings may also shed light on dramatic carved features such as valleys and channels visible on Titan's surface.
"[The clouds] help to explain what is going on on Titan - how there could be big stream valleys and flood plains in regions which appear so dry," said lead researcher Mike Brown, an astronomer from the California Institute of Technology in Pasadena, USA.
Cold and wet
Titan is the largest of Saturn's moons and the only heavenly body other than Earth known to have permanent surface liquid. It's not water though, as the average temperature on Titan is -180 °C. At this temperature hydrocarbons that are gaseous here on Earth, such as methane, are liquids.
Sporadic clouds have been previously recorded near the south pole, occurring during Titan's summer solstice, when the surface heats up causing convection clouds to condense. Months can go by between clouds, however, and no clouds have ever been seen closer to the equator. Furthermore, Titan's dense atmosphere distorts visible light, making telescopic observations difficult.
To get around these problems, Brown and co-workers used two types of telescopes to search for clouds. Firstly, nightly monitoring images from NASA's infrared telescope on Mauna Kea, Hawaii, were used to detect small variations in Titan's brightness.
Bright conditions
When the conditions were brighter than usual (hinting at the presence of cloud cover) a second telescope, fitted with powerful optics that correct for atmospheric distortion, was quickly trained onto Titan.
After almost 10 years of observation, a large bright spot was seen over Titan's equator. Waves of storm clouds spread out across the moon, travelling all the way to the south pole and lasting for around one month.
"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 travelled around the planet, triggering their own clouds," said Brown's co-worker and graduate student Emily Schaller. "Within days a huge cloud system had covered the south pole."
