Xanadu's Channels
On the final flyby of Cassini's original four-year tour, its radar mapper captured these unusual channels on Titan at the edge of Xanadu, the widest seen in this area.

Methane on Titan behaves essentially the way water does on Earth, cycling through states as an ice, liquid and gas in Titan’s environment. Evidence of the aftermath of rainfall seems to abound on Titan’s surface, and it has long been presumed that methane rainfall is a key mechanism in perpetuating Titan’s methane cycle, the same way precipitation maintains Earth’s water cycle. But the observation of actual rainfall on Titan has been elusive.

Now, astronomers studying Titan from telescopes in Chile and Hawaii, assisted by Cassini infrared spectrometer measurements of Titan, have found evidence that fits with the existence of methane raindrops, not just vapor, low in the atmosphere over a continent-size feature called Xanadu. Whether these particular droplets actually hit the ground before evaporating is still unknown, but at least the methane precipitation appears to have been identified.

Astronomer Mate Adamkovics of University of California at Berkeley and colleagues found the low atmosphere near Xanadu grew more opaque in the morning and evening in a way that is consistent with methane drizzle.

With complementary observations from Chile’s Very Large Telescope, the Canada-France-Hawaii Telescope on Mauna Kea, Hawaii, and Cassini’s visual and infrared mapping spectrometer (VIMS), the team ruled out questions of whether they had mistaken the reflectivity of surface features for methane droplets in earlier observations.

Combining telescope and Cassini VIMS observations to get at the question of methane rain was particularly useful, says co-author Dr. Jason W. Barnes, an associate member of the VIMS team and an assistant professor of physics at University of Idaho, Moscow. “VIMS and ground-based datasets could be analyzed in the exact same way, at the same time, and both could be brought to bear on the same problem.”

Cassini VIMS offers data collection free from the interference of Earth's atmosphere. The VIMS instrument can target an area of interest from different angels and the data acquired is of high spatial resolution. Telescope observations from Earth can be made more frequently since they are not dependent on flights past Titan, and offer higher spectral resolution. “Combining the two really makes a lot of sense,” says Barnes.

Titan and seasonal changes on its surface are a major target of investigation throughout Cassini's Equinox mission. Barnes is hopeful the geometry of future Titan flybys will allow VIMS observations of Xanadu in the morning, mid-afternoon and evening, and from different angles, to further refine our knowledge of Titan’s rainfall in this region.

Xanadu is about the size of Australia. It has been thought to be a raised topographical feature, though there is recent debate about whether it is in fact high or low. Ádámkovics and others have postulated that Xanadu’s geography may affect atmospheric circulation. Cassini radar images of Titan’s sand dunes have shown that prevailing easterly winds flow toward Xanadu. If Xanadu is really higher than surrounding terrain, winds may blow a moist airmass upslope where it cools, condenses into a mist and drizzles over the landmass.

Scientists are still far from being able to provide routine weather reports for Titan, says Barnes. “But we can for sure tell you where it is cloudy. But clouds don't always produce rain, and when they do, the rain doesn't always reach the ground. The Xanadu rain in question is particularly interesting in that we haven't found corresponding rain clouds. So this kind of mist is still a mystery.”

This Cassini Science League entry is an overview of a science paper authored, or co-authored, by at least one Cassini scientist. The information below was derived from the following publication: “Evidence for condensed-phase methane enhancement over Xanadu on Titan,” Planetary and Space Science (article in press), M. Adamkovics (University of California, Berkeley (UCB)); I. de Pater (UCB); M. Hartung, Gemini Observatory, La Serena, Chile; J.W. Barnes, University of Idaho, Moscow, ID

-- Mary Beth Murrill, Cassini science communication coordinator