Weather Report for Titan
Storm Brews over Titan's Tropical Dessert
The research, published in the August 13, 2009 issue of the journal Nature, announces the discovery of significant cloud formation (about three million square kilometers) within the moon's tropical zone near its equator. Prior to this event (in April 2008) it was not known whether significant cloud formation was possible in Titan's tropical regions. This activity in Titan's tropics and mid-latitudes also seems to have triggered subsequent cloud development at the moon's south pole where it was considered improbable due to the sun's seasonal angle relative to Titan.
The evidence comes from a team of US astronomers using the Gemini North telescope and NASA's Infrared Telescope Facility (IRTF) both on Hawaii's Mauna Kea. "We obtain frequent observations with IRTF giving us a 'weather report' of sorts for Titan. When the IRTF observations indicate that cloud activity has increased, we are able to trigger the next night on the Gemini telescope to determine where on Titan the clouds are located," said team member Emily Schaller who was at the University of Hawai‘i Institute for Astronomy when this work was done. In fact, in the past eight years of monitoring at Gemini no significant clouds over Titan's tropical regions were spotted.
"In April 2008 we observed what was a global event that shows how storm activity in one region can trigger clouds, and probably rainfall, over arid regions, such as the tropics where Huygens landed," said team member Henry Roe from Lowell Observatory. Roe adds, "Of course these rain showers are not liquid water like here on Earth, but are instead made of liquid methane. Just like the streambeds and channels that are carved by liquid water on Earth, we see features on Titan that have been created by flowing liquid methane."
Unlike the Earth, on Titan, where the temperature is some -178 degrees Celsius, methane (or natural gas) is a liquid and it the dominant driver of the moon's weather and surface erosion. Any water on Titan is frozen on or below the moon's surface and resemble rocks or boulders on Titan's surface.
During this three-week episode clouds forming at about 30 degrees south latitude were observed, followed several days later by clouds closer to the equator and at the moon's south pole. The apparent connection between the cloud formations leads to the possibility that cloud formation in one area of the moon can instigate clouds in other areas by a process known as atmospheric teleconnections. This same phenomena occurs in the Earth's atmosphere and is caused by what are called planetary Rossby waves which are well understood.
The high-resolution Gemini images of Titan were all obtained with adaptive optics technology which uses a deformable mirror to remove distortions to light caused by the Earth’s atmosphere and produce images showing remarkable detail in the tiny disk of the moon. "Without this technology this discovery would be impossible from the surface of the Earth," said Schaller.
Currently the Cassini spacecraft is orbiting Saturn but only flies by Titan once every 6 weeks or so. This makes continuous ground-based monitoring important for studying features like these with shorter periods on the order of 3-weeks like this storm.