Seasonal Change in Titan’s Cloud Activity Observed with IRTF/SpeX Emily Schaller (Caltech) Henry Roe (Lowell Observatory) Michael Brown (Caltech) Image Credit: NASA/JPL/Space Science Institute

Titan’s Methane Weather Cycle Image credit: NASA/JPL/ESA Titan provides us with a unique laboratory in which to study a hydrological cycle on a planet other than Earth with a different condensable species (methane on Titan, water on Earth). Understanding how Titan’s hydrological cycle changes with season over Titan’s 29-year year is vital for determining formation mechanisms of surface features observed by Cassini/Huygens.

Titan’s Methane Clouds Rapidly varying tropospheric methane clouds were regularly observed near Titan’s south pole in Keck and Gemini adaptive optics images from 2001-2005. During this time, Titan’s south pole was in continuous sunlight Our hypothesis is that the locations of Titan’s clouds are controlled in a complex way by the seasonally varying insolation and changes in Titan’s global circulation. As Titan moves toward southern autumnal equinox in 2009, the latitudes of Titan’s clouds should move north. While infrequent observations from Cassini are useful for studying the morphologies of clouds, only a nightly monitoring program can provide the type of dataset necessary to determine the frequency, duration and altitudes of large and small cloud systems and the change with season.

IRTF Titan Spectroscopic Monitoring Disk integrated spectra of Titan from 0.8-2.4 microns Data taken by Telescope Operators nearly every night SpeX is on the telescope (76 nights so far over two semesters). Total time to take data each night is less than 20 minutes. Disk integrated spectra allow us to determine Titan’s total fractional cloud coverage and cloud altitudes. Data are rapidly reduced and increases in cloud activity allow us to trigger target-of-opportunity observations with the adaptive optics systems on either Keck or Palomar to determine cloud latitudes (one such TOO was triggered on November 28, 2006).

Ten individual nights of Titan spectra divided by a G2V star Ten individual nights of Titan spectra divided by a G2V star. The wavelengths at which the spectra deviate from each other and the magnitude of the deviation allow us to determine the altitude and brightnesses of any clouds present. Surface Troposphere Stratosphere The spectrum of 2/23/06 is shown subtracted from that of 2/24/06. We can constrain the difference in total cloud coverage to be less than 0.15%. From Gemini images we took several days earlier along with images from the T11 Cassini flyby (Feb 27), we know that these two nights correspond to a baseline of extremely low cloud activity. Wavelength (microns)

Discussion Titan observations over the past two semesters with IRTF have shown a dramatic decrease in cloud activity. On all but two nights Titan’s total cloud coverage was less than 0.15%. The near lack of cloud activity in IRTF observations contrasts sharply with similar observations of Griffith et al. (1998 & 2000) near equinox and indicates a striking seasonal change in cloud frequency and magnitude. Observations of the latitudes, magnitudes, altitudes, and frequencies of Titan’s clouds as Titan moves toward southern autumnal equinox in 2009 will help elucidate when and how Titan’s methane hydrological cycle changes with season. IRTF is poised to make a major contribution toward understanding the long and short-term evolution of cloud systems and the hydrological cycle on Titan.