* 15:54 09 September 2005
* NewScientist.com news service
* Maggie McKee
This image taken by the Huygens probe reveals what appears to be a major river channel (Image:NASA/JPL/ESA/University of Arizona)
Brief but powerful monsoons of liquid methane may lash parts of Saturn's giant moon Titan every few hundred years, suggest new calculations based on observations with the Cassini spacecraft. Astronomers also think they have observed thunderstorms growing and raining down on the moon.
Methane makes up about 5% of the Titan’s thick atmosphere and is thought to rain down to the surface and then evaporate back into clouds in a cycle similar to the water-cycle on Earth.
Indeed, observations with both the Cassini spacecraft in orbit around Saturn and the Huygens probe sent down to Titan in January 2005 seem to bear this out. They show dark, river-like channels carved into higher, lighter terrain and a round feature resembling a lake near the south pole.
But so far scientists have not been able to prove that liquid methane flows through these features today. "Everything looks pretty fresh on Titan," says Jonathan Lunine, a planetary scientist at the University of Arizona in Tucson, US. "It's akin to seeing riverbeds that don't have water in them, and knowing there was rain in the last season."
Now, University of Arizona colleague Ralph Lorenz says Titan may well experience short, periodic downpours like those in deserts in the south-west US – though much more intense. Lorenz presented the "methane monsoon" theory – named after an Arthur C Clarke novel in which astronauts get blown away by a methane storm – on Thursday at a planetary sciences meeting in Cambridge, UK.
The evidence comes from imaging radar data from the Cassini spacecraft. These reveal canyon-like features that end in triangular slopes that appear to be littered with cobbles a few centimetres wide.
That may indicate flash flooding, says colleague Jason Barnes. He says most sediment transport in rivers on Earth occurs during the "peak discharge" of such events. "So if you a river jumps to ten times its normal level, like the Colorado River through the Grand Canyon, most sediment is transported during the three days of the flood," he told New Scientist.
The radar also shows sinuous shapes resembling rivers that only branch off in a few places. The curviness, size, and number of branchings resemble channels in Earth's deserts that are carved in heavy bursts of rainfall that quickly seep into the porous soil.
"The shapes of river channels on Titan resemble those we see in the desert southwest," says Lorenz. "Where I live in Tucson, you see river channels and there's water in them for two or three days a year and it's dry the rest of the time."
He says it may be raining somewhere on Titan at any given time, but that centuries may pass between rainstorms in a particular region. Sunlight drives the rain-evaporation cycle, and models of Titan's haze, reflectivity, and temperature suggest the Sun can only produce an average of about a centimetre or two of rain per year over all of Titan.
"That doesn't say whether it's a millimetre every month, a centimetre every year, or several metres every thousand years," says Lorenz.
But if the methane in Titan's atmosphere were concentrated into a single liquid layer, it would cover the entire moon in a blanket 10 metres thick. "The air is holding a lot of methane," says colleague Jason Barnes. So if it rains less often, it dumps more liquid – perhaps as much as a metre - at a time, he told New Scientist.
Whether or not these episodic methane monsoons occur, Cassini scientists believe they have now seen evidence for methane rain on the moon.
Using the spacecraft's visual infrared mapping spectrometer (VIMS), they have studied the evolution in height and thickness of long clouds that tend to cluster around the moon's mid-southern latitudes.
The team observed "vigorous centres" in the clouds as they rose from a height of 23 kilometres to 44 km at speeds of several tens of metres per second. Then, the cloud tops dissipated or fell 10 kilometres over the next 30 minutes. This suggests the clouds "evolve convectively and dissipate through rain," reported Caitlin Griffith of the University of Arizona on Thursday.
All of the recent Titan observations, including the detection of a limited amount of liquid methane just under the surface where Huygens landed, point to "an active hydrological cycle", says Barnes. "It's more similar to Earth than you might think."
The research was presented at a meeting of the American Astronomical Society's Division of Planetary Sciences in Cambridge, UK.