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

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

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,…)

Phase diagram of water T E

Phase diagram of methane T Gas Solid Liquid

Specular reflections on the Nile

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

West et al., Nature, No specular 2  m Relative Strength

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

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

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

Titan’s spectrum McKay et al., 2001

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

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

Clouds at Titan’s south pole 10 Dec Dec Feb Keck 2 AO/NIRSPEC and NIRC2, K’ filter ( μm)

11/11/0311/12/03 11/13/0311/14/03 K’ Titan through different filters

South polar cloud locations

Why are clouds near the south pole?

Mean daily insolation on Titan

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

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

June 2005 Cassini Image

Tokano 2005 (Icarus)

Mean daily insolation on Titan

Large cloud events

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

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, km above surface. Griffith et al Nature

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

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

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

mid-lat histogram

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

New coupled dynamics-microphysics model Rannou et al Science

Types of Clouds Small scale south polar ~1% coverage of Titan’s disk Consistently present from 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

Seasonal Change?

Mean daily insolation on Titan

Typical Titan images: November November 2004

Titan Images: December Present

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

Mitchell et al PNAS Titan General Circulation Model

IRTF spectroscopic monitoring

Keck & Gemini Titan Images: December Present

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

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

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

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!

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.

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

conclusion

Nightly 14’’ Telescope Photometry

(Schaller et al. 2005)

Tokano

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

Comparison to 1995 Event (Schaller et al. 2006)

LFC CH 4