1. Seasonal Change in Titan’s Cloud Activity (A Titan Weather report) Emily Schaller (Caltech) Mike Brown (Caltech), Henry Roe (Lowell Observatory)

2. Current observing scheme Keck 10-m Gemini North 8-m NASA IRTF 3-m NM Skies 14’’

3. Titan 16 day rotation period 27 degree obliquity Thick atmosphere with a surface pressure of 1.5 bar. Main atmospheric constituents N 2 (90-97%) Methane (2-5%) Assorted hydrocarbons & nitriles (C 2 H 2, C 2 H 6, HCN,…)

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

5. Phase diagram of methane T Gas Solid Liquid

6. Specular reflections on the Nile http://visibleearth.nasa.gov

7. Arecibo 14-cm radar observations after Campbell et al. 2003 Science Relative Strength

8. West et al., Nature, 2005. No specular reflections @ 2  m Relative Strength

9. On the whiteboard in the coffee room in the Planetary Science department at Caltech…..

11. Surface maps 90N 0 90S West Longitude 0180 Latitude Credit: NASA/JPL/Space Science Institute x

12. How long ago did it rain at the Huygens landing site? Or: How long ago was it cloudy?

13. Titan’s spectrum McKay et al., 2001

14. Narrowband imaging Methane transmission Adaptive optics at Keck 10-m Gemini 8-m

15. Surface maps (compare) 90N 0 90S West Longitude 0180 Keck Cassini

16. Clouds at Titan’s south pole 10 Dec. 2001 11 Dec. 200128 Feb. 2002 Keck 2 AO/NIRSPEC and NIRC2, K’ filter (1.95-2.29 μm)

17. 11/11/0311/12/03 11/13/0311/14/03 K’ 2.12 2.17 Titan through different filters

18. South polar cloud locations

19. Why are clouds near the south pole?

20. Mean daily insolation on Titan

21. Temperature profile (1) temperature height dry adiabat surface temperature Stable

22. Temperature profile (2) temperature height dry adiabat surface temperature convection condensation buoyancy cloud tops wet adiabat

23. June 2005 Cassini Image

24. Tokano 2005 (Icarus)

25. Mean daily insolation on Titan

26. Large cloud events

27. Large Cloud Outbursts (Schaller et al. 2006a Icarus)

28. Spectroscopic evidence for large, infrequent clouds On two nights out of 14, Titan brightened by up to 200% in atmospheric windows. Clouds covered 7% of disk, 14-18 km above surface. Griffith et al. 1998 Nature

29. What causes large cloud outbursts? Surface heating? Increased condensation nucleii? Increased methane humidity Injected somewhere else and brought to the pole?

30. September 2 nd 2004 (first seen December 18 th 2003) Roe et al. 2005 ApJL Roe et al. 2005 Science Discovery of mid-latitude clouds

31. Mid-latitude cloud locations Map courtesy NASA/JPL/Space Science Institute

32. mid-lat histogram

33. Characteristics of temperate-latitude clouds: -localized (but move) -short lived (~1 earth day) -streaky -sausage link morphology -tight convective cores (Cassini/VIMS) Conclusions: -not orographic -convection into dry troposphere -surface source[s] + (inconsistent?) wind mid-lat cloud summary

34. New coupled dynamics-microphysics model Rannou et al. 2006 Science

35. Types of Clouds Small scale south polar ~1% coverage of Titan’s disk Consistently present from 2001-2004 Large cloud outbursts Clouds increase in brightness by ~15 times over typical levels Last for ~1 month Observed in two different seasons Midlatitude (40S) clouds Streaky, short lived Not evidence for seasonal change Likely tied to the surface

36. Seasonal Change?

37. Mean daily insolation on Titan

38. Typical Titan images: November 2001- November 2004

39. Titan Images: December 2004 - Present

40. South polar cloud latitude vs. time Schaller et al. 2006b (Icarus) Titan Southern Summer Solstice South Pole ceased to be area of maximum solar insolation

41. Mitchell et al. 2006 PNAS Titan General Circulation Model

42. IRTF spectroscopic monitoring

43. Keck & Gemini Titan Images: December 2004 - Present

44. Griffith et al. 2000 Science Stratosphere Troposphere Surface Small variations in brightness at 2.13-2.18 microns correspond to tropospheric Clouds covering 1% of Titan’s disk

45. IRTF Spectral Data (April-May 2006, Oct-Nov 2006) Spectra deviate at <2.13 microns indicating extremely low <0.15% tropospheric cloud activity

46. Subtracted spectra Large cloud event 1% cloud coverage at 25 km altitude

47. IRTF data from yesterday showed evidence for a small ~0.5% cloud! Triggered Keck interrupt program this morning and NIRC2-AO images were taken 4 hours ago!

48. Conclusions: Seasonally varying insolation and uplift from the general circulation appears to control the location of clouds on Titan - clouds may be over Huygens ~2010 Large cloud events occur in different seasons of Titan’s year and may be caused by increased methane humidity or CCN. The dissipation in Titan’s south polar clouds is the first indication of seasonal change in Titan’s weather.

49. What’s next? Observations over the coming year will determine how seasonal change will progress. Integral field spectroscopy at Keck and Gemini OSIRIS and NIFS Continued Low-res near-IR spectroscopic monitoring with IRTF Continued 14” photometric monitoring

50. conclusion

52. Nightly 14’’ Telescope Photometry

54. (Schaller et al. 2005)

56. Tokano 2005 19952002 20052010

57. Keck Image 09/19/2005 Cloud at 58 degrees latitude

58. Comparison to 1995 Event (Schaller et al. 2006)

59. LFC CH 4