"Titan's Dynamic Atmosphere"

Planetary atmospheres are complex systems. Studies of extraterrestrial atmospheres have drawn on the much more extensive data and analysis of Earth, but its atmosphere is only one realization in the range of possibilities.  Efforts to understand other atmospheres and predict their behavior must be closely tied to observations.  The atmospheres in the solar system are large-scale natural laboratories with different “external” drivers (e.g., planetary rotation, solar forcing, internal heat fluxes).  Examining their response to these different conditions is an important step toward understanding the physical processes that govern them.

This talk focuses on one such laboratory, Saturn’s giant moon Titan, which has been explored through Cassini-Huygens and earlier Voyager spacecraft measurements, as well as from ground-based observations.  Titan’s atmosphere is an intriguing blend in the planetary line-up.  In several respects, it resembles Earth’s.  Its atmosphere is primarily N2, and its surface pressure is about 50% larger.  The second most abundant constituent is not O2 but CH4, and there is evidence for a hydrological cycle involving CH4.  Titan’s winter stratosphere has different chemistry than on Earth, but it has an analog to the ozone hole, with strong circumpolar winds, condensate ices, and anomalous concentrations of organic gases near the pole.  However, Titan is a much slower rotator than Earth: its “day” is 15.95 Earth days.  In this regard, it is more like Venus, another slow rotator, and it provides the second example in the solar system of an atmosphere with global super-rotation, i.e., its winds whip around the body in much less time than a Titan day.  Because of Saturn’s large axial tilt relative to its orbital plane, Titan is a Venus with seasons.  The origin and maintenance of the strong winds on Venus and Titan are poorly understood, and the seasonal changes in Titan’s winds and temperatures—observations from the Cassini spacecraft are already showing them—provide clues for understanding these systems.