1. Clouds in the Tropics of Titan Emily Schaller Lunar and Planetary Laboratory, University of Arizona 2010 Hubble Fellows Symposium

2. Titan Thick atmosphere with surface pressure ~1.5 bar. Major gases in atmosphere: N 2,(~98%) CH 4 (~2%) 27 degree obliquity 16 day rotation period

3. Phase diagram of water http://www.lsbu.ac.uk/water/phase.html T E

4. Phase diagram of methane T Gas Solid Liquid Credit: H. Roe

5. Narrowband imaging Adaptive optics at Keck 10-m Gemini 8-m

6. Typical Titan images: 2001- 2005 Schaller et al. 2006

7. Mean daily insolation on Titan

8. Mitchell et al. 2006 PNAS Models of Titan Cloud Activity with season Present Rannou et al. 2006 Science

9. Surface maps 0 West Longitude Latitude Credit: NASA/JPL/Space Science Institute x Cassini ISS Surface Map

10. Dry Tropics (dunes) – cloud-free Wet poles (lakes, rivers, other fluvial features) – lots of clouds Images from: saturn.jpl.nasa.gov & Radebaugh et al. 2008,

11. But… small-scale surface features seen by Huygens probe near equator show evidence for rainfall http://www.esa.int/SPECIALS/Cassini-Huygens/index.html

12. IRTF spectroscopic monitoring Disk integrated spectra of Titan covering 0.8-2.4 microns with a resolution of 375 Data taken every night SpeX instrument is on the telescope Disk integrated spectra: –total fractional cloud coverage –cloud altitudes –Interrupt at Gemini to determine latitudes

13. IRTF Spectral Data surfacetropospherestratosphere Relative Flux

14. 2.03 micron flux 0 15% IRTF ISS Surface Map -15% Schaller et al. 2010 submitted

15. IRTF Spectral Data Spectra deviate at <2.12 microns indicating extremely low <0.15% tropospheric cloud activity in 90% of all nights Relative Flux Spectra deviate at <2.12 microns indicating extremely low <0.15% tropospheric cloud activity in 95% of all nights

16. Typical Titan images: 2001- 2005 Schaller et al. 2006

17. Titan Images: 2005-2008

18. Schaller et al. Nature 2009 IRTF Spectrum

19. Schaller et al. Nature 2009 Cassini Titan flybys (March 31, May 20) completely missed event

20. Tropical Clouds Two days after initial large tropical cloud, clouds also appeared near the south pole Simple calculations reveal that a Rossby (planetary) wave would take ~2 earth days to reach the south pole Rossby waves trigger clouds by forming areas of low pressure Schaller et al. Nature 2009

21. Tropical Clouds - tie to the surface? 15S, 250W contains a small cloud in in all images for 20 days. Surface heating/methane injection at this location? Great place for Cassini to look for potential cryovolcanism or surface changes due to methane rainout Schaller et al. Nature 2009

22. Conclusions Locations and intensities of Titan’s clouds vary significantly with season Tropical clouds do occasionally form on Titan Clouds can form via teleconnections mediated by large-scale waves Large cloud events may be caused by increased methane humidity, surface heating, or other factors Observations of Titan’s clouds over the next few years by Cassini and ground- based observations will provide the key for interpreting the origin and evolution of the fluvial surface features.