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-mNM Skies 14’’

Titan

• 16 day rotation period• 27 degree obliquity• Thick atmosphere with a

surface pressure of 1.5 bar.

• Main atmospheric constituents

• N2 (90-97%)• Methane (2-5%)• Assorted hydrocarbons &

nitriles (C2H2, C2H6, HCN,…)

Phase diagram of water

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

http://www.lsbu.ac.uk/water/phase.html

TE

Phase diagram of methane

T

Gas

Solid

Liquid

Specular reflections on the Nile

http://visibleearth.nasa.gov

Arecibo 14-cm radar observations

after Campbell et al. 2003 Science

Re

lativ

eS

tre

ng

th

West et al., Nature, 2005.

No specular reflections @ 2m

Re

lativ

eS

tre

ng

th

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

Surface maps

90N

0

90S

West Longitude

0180

Latit

ude

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 transmissionAdaptive optics atKeck 10-mGemini 8-m

Surface maps (compare)

90N

0

90SWest Longitude

0180

Keck

Cassini

Clouds at Titan’s south pole

10 Dec. 2001 11 Dec. 2001 28 Feb. 2002

Keck 2 AO/NIRSPEC and NIRC2, K’ filter (1.95-2.29 μm)

11/11/03 11/12/03 11/13/03 11/14/03

K’

2.12

2.17

Titan through different filters

South polar cloud locations

Why are clouds near the south pole?

Mean daily insolation on Titan

Temperature profile (1)

temperature

heig

ht

dry adiabat

surface temperature

Stable

Temperature profile (2)

temperature

heig

ht

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, 14-18 km above surface.

Griffith et al. 1998 Nature

What causes large cloud outbursts?

• Surface heating?

• Increased condensation nucleii?

• Increased methane humidity• Injected somewhere else and brought to the pole?

September 2nd 2004(first seen December 18th 2003)

Roe et al. 2005 ApJLRoe et al. 2005 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)

mid-lat cloud summary

New coupled dynamics-microphysics model

Rannou et al. 2006 Science

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

Seasonal Change?

Mean daily insolation on Titan

Typical Titan images:November 2001- November 2004

Titan Images:December 2004 - 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. 2006 PNAS

Titan General Circulation Model

IRTF spectroscopic monitoring

Keck & Gemini Titan Images:December 2004 - Present

Griffith et al. 2000 Science

Stratosphere

Troposphere

SurfaceSmall variations in brightnessat 2.13-2.18 micronscorrespond to troposphericClouds covering 1% of Titan’s disk

IRTF Spectral Data (April-May 2006, Oct-Nov 2006)

Spectra deviateat <2.13 micronsindicating extremely low <0.15% tropospheric cloud activity

Subtracted spectra

Large cloud event

1% cloud coverageat 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 GeminiOSIRIS and NIFS

Continued Low-res near-IR spectroscopic monitoring with IRTF

Continued 14” photometric monitoring

conclusion

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

conclusion

Nightly 14’’ Telescope Photometry

(Schaller et al. 2005)

Tokano 2005

1995 2002

2005 2010

Keck Image 09/19/2005

Cloud at 58 degrees latitude

Comparison to 1995 Event

(Schaller et al. 2006)

LFC LFC

LFC

CH

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