A Northern Arizona University (NAU) astronomer, and her colleagues, recently discovered daily clouds in the atmosphere of Saturn's largest moon, Titan. This discovery adds to the growing body of evidence that Titan uniquely resembles Earth with clouds, rain and seas.

Artist's concept of Titan. Photo: NASA

The Oct. 20 issue of Science will publish the findings of Caitlin A. Griffith, Joseph L. Hall, and Thomas R. Geballe of Gemini Observatory in Hawaii. Griffith is an associate professor in the NAU Department of Physics and Astronomy. Hall is a research assistant at NAU and recently graduated from the Physics and Astronomy Department.

Titan receives special attention from astronomers because of its Earth-like characteristics. It is mainly nitrogen and has a surface pressure near terrestrial. In addition, Titan's atmosphere, containing large quantities of methane and no free oxygen, is similar to some theoretical models of the Earth's prebiotic atmosphere.

Discovery of the daily occurrence of clouds on Titan follows the finding two years ago, by Griffith, Geballe, Gary A. Miller and Tobias Owen, of a hurricane-sized cloud in Titan's lower atmosphere. The origin of this massive cloud system remains unknown, because it was observed only once. In contrast, Griffith's recent and repeated observations of daily clouds reveal the nature of weather on Titan.

"Titan's clouds are quite bizarre," Griffith said. "They show up at the same altitude everyday, and are extremely sparse, covering less than a percent of Titan's globe. In comparison, on Earth clouds can be seen from the surface to a 16-km altitude, and they typically cover half the globe.

"Titan provides astronomers with an immense laboratory with which to study processes that occur on Earth, such as liquid cycles between the atmosphere and surface. Ultimately we need to understand terrestrial processes in strange environments in order to investigate the evolution and variety of planetary and extra-solar atmospheres.

"While similar to Earth, Titan's environment is quite exotic. The atmosphere is cool (-180 C) and 10 times more massive than Earth's. This renders the atmosphere sluggish. Titan also receives approximately 100 times less power from the Sun, due to its distance from the Sun. This is an important distinction, because on Earth, solar energy drives weather."

Artist's concept of European Huygens probe descending into Titan's atmosphere later this decade. Photo: ESA

Griffith and her colleagues studied the characteristics of Titan's clouds to determine an energy source for its cloud formation.

"We compared the energy sources on Earth with those on Titan, and we found that Titan was more abundant in one: latent heat, the energy released when gas condenses, for example to form clouds," she said. "We then calculated the heights that the clouds would reach if they were driven by latent heat, and found that they would reside right where we found them at approximately a 25-km altitude.

Griffith therefore proposes that unlike on Earth, where solar radiation powers weather, on Titan, latent heat plays a greater role.

An image of Titan's conditions is gradually emerging.

"Standing on the surface of Titan, we would see a very dimly lit world, as bright as Earth under a full moon," Griffith said. "Below the orange sky, the Sun would appear as a diffuse light source through Titan's high smog. At night, we would not see stars through the smog's veil. On the ground, the atmosphere would be clear and the visibility unobscured.

"Temperatures would be uniform and winds quiescent. Every week, sparse clouds would appear below the orange haze but still high in the sky, barely visible. They would quickly produce rain and disappear. Very infrequently, perhaps once a year, clouds would blanket the sky for a day or two. At this point, we are unsure why and how this cloudy day would happen."

This and other basic questions about Titan still challenge astronomers.

"Little is known about Titan's surface and the presence of oceans is still a hypothesis," Griffith said. "Fortunately we have the opportunity to explore Titan. The Cassini Mission is presently en route to Titan and the Saturnian system. This spacecraft will deploy a probe into Titan's atmosphere to explore this methane-rich world."