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Space
Forecast: Methane drizzle over Xanadu
The forecast for Saturn’s moon Titan is cold and rainy, according to new research. Scientists have long speculated that the an Earth-like precipitation cycle might be concealed under Titan’s opaque orange atmosphere, and now a team of astronomers has observed what may indeed be a near-daily methane drizzle over the moon’s Xanadu region. If this is a widespread process, it could explain the mystery of how methane cycles from the atmosphere to the surface of the moon.
To get a broad picture of Titan’s surface and atmosphere, a team led by Máté Ádámkovics of the University of California at Berkeley studied infrared images of the moon taken through the Very Large Telescope in Chile and the Keck Telescope in Hawaii. The size and sophistication of the instruments allowed the researchers to see light spectra across Titan, which revealed a dark spot over the surface of Xanadu that appeared when the moon’s surface rotated into the sunlight. By modeling different combinations of aerosols, methane ice and liquid methane against a model of Titan’s surface, the researchers found that a liquid methane drizzle best explained the dark spot they saw in the images.
The study, which appeared Nov. 9 in Science, follows on the heels of data from the European Space Agency’s Huygens probe that landed on Titan in 2004, which also hinted there might be rain on Titan. “There’s no sort of real smoking gun [in] regards [to] precipitation at the Huygens landing site,” says Ralph Lorenz, co-author of Lifting Titan’s Veil and an investigator for future NASA missions to Titan, “but it’s not ridiculous to suggest there was some type of drizzle or rain.” Some researchers interpreted humidity measurements taken during Huygens’ descent to Titan’s surface as evidence of rain clouds, Lorenz says, and others thought dark spots photographed by the probe at its landing sight were indicative of raindrops. “You’d think it would be something easy to be sure about, but it’s not,” he says.
Whereas the Huygens probe data may indicate rain at one specific site on Titan’s surface, Ádámkovics’ study depicts it as being “widespread and persistent,” Lorenz says. This opens up further questions about how enough methane could evaporate on a moon that receives so little sunlight to support an ongoing rain cycle over a large area, he adds.
Most of the drizzle seen in the images, Ádámkovics and colleagues wrote, occurred in the Titan morning, which they speculated could either be caused by cooler nights giving way to warmer days much the way early morning mists or fogs settle in on Earth and then evaporate as the day warms, or by topography and winds. Winds would bring about the drizzle through large-scale “advection,” the team reported, as they would drive moist clouds up Titan’s slope where the clouds cool, condense and then rain, similar to a process seen in coastal areas on Earth, Ádámkovics says. Either one is similar to processes on Earth, he says, except on a much bigger scale due to the amount of methane circulating on the moon.
The research demonstrates that “the whole of Titan is connected,” Lorenz says, “which is why it is so important to look at it with different approaches. We have a lot to learn about weather and hydrology on Titan.”
