In a case of persistent interplanetary detective work using powerful ground-based telescopes, a team of astronomers located and tracked the first bright but transient clouds over tropical latitudes on Saturn's moon Titan. The astronomers used the Gemini North telescope and NASA's Infrared Telescope Facility (IRTF) in an almost-nightly observing program providing new insights into the nature of Titan's tropospheric (low-elevation) clouds. The team's paper, Storms in the tropics of Titan, will be published in the August 13, 2009 edition of the journal Nature. The astronomers observed a convective pulse at mid-latitudes that generated a wave in Titan's atmosphere. This wave went on to trigger cloud formation over both the equatorial and south polar regions. These new observations of this type of equatorial cloud may help explain the formation of liquid methane-carved channels and rivers located in the vicinity of the Huygens probe landing site.

"These types of dramatic global weather events on Titan are rare and only last a few weeks," said Henry Roe, Lowell Observatory astronomer and team member. "The best way to observe them and understand how they compare with more normal weather on Titan is through a coordinated effort obtaining frequent observations on several telescopes." The observing team consists of Lowell astronomer Henry Roe; Emily Schaller, a Hubble Fellow now working at the University of Arizona's Lunar and Planetary Laboratory; and Caltech's Tapio Schneider and Michael Brown.

Using the 3.0-meter NASA Infrared Telescope Facility (IRTF), operated by the Institute for Astronomy, University of Hawaii, and The Gemini North 8.0-meter telescope, the team made many observations over a longer than two-year period. During this period, which covered the mid-to-late northern spring season on Titan, they observed remarkably few clouds anywhere on Titan. On April 13, 2008 observations with the IRTF showed a dramatic increase in cloud activity on Titan. The next night, and for many nights thereafter, they acquired quick imaging snapshots of Titan using the larger Gemini North telescope. These images revealed that the initial cloud system was at 30 deg S latitude, but within days additional clouds appeared over Titan's tropics and south polar region. "It was as though we observed a giant storm covering South Africa that days later caused clouds to form over Antarctica and Indonesia," said team member Schaller.

Titan, Saturn's largest moon, is larger than Mercury and Pluto and is the only moon in our solar system to be surrounded by an atmosphere. While weather on Earth is based on water, on Titan conditions are too cold for liquid water to exist. However, conditions on Titan are just right for weather to be methane based. In January 2005 NASA's Cassini mission deployed the Huygens probe into Titan's atmosphere. The probe landed near Titan's equator and returned images of channels and dry streambeds. This posed a conundrum as weather models predicted that Titan's equatorial regions were a desert and should never be cloud covered or receive rainfall. These same models predicted the south polar clouds the team had earlier observed during early southern summer on Titan, but the models indicated that these south polar clouds should have disappeared by mid-to-late summer.

By observing clouds in the equatorial region, where clouds were never expected to form, and in the south polar region, where clouds were not expected to form in the current season, this new research has shown that clouds can form in times and places not predicted by the current generation of computer weather models of Titan's atmosphere. On Titan strong storm activity in one area can trigger clouds and storms elsewhere on the moon. "These observations show that the channels and streambeds in Titan's tropical desert can be explained by infrequent but strong downpours, much like many of the landforms here in the Southwestern United States," said Roe.

"Titan's year is 30 Earth years long and so far we've only been observing Titan with this type of precision and frequency for less than one Titanian season," said Roe. "Imagine trying to understand Earth's weather having only seen what happens in January, February, and part of March. We have our work cut out for us to continue watching the weather on Titan for many more years."

Funding for this research was provided by NASA's Planetary Astronomy Program and from an NSF Planetary Astronomy Grant. Support was also made through the Hubble Postdoctoral Fellowship. The IRTF is operated by the University of Hawaii under a cooperative agreement with the Planetary Astronomy Program of the NASA Science Mission Directorate. Gemini Observatory is operated by the Association of Universities for Research in Astronomy, Inc., under a cooperative agreement with the National Science Foundation on behalf of the International Gemini partnership.