Clouds in the Tropics of Titan

Emily Schaller

Lunar and Planetary Laboratory, University of Arizona

2010 Hubble Fellows Symposium

Titan

• Thick atmosphere with surface pressure ~1.5 bar.

• Major gases in atmosphere: N2,(~98%) CH4 (~2%)

• 27 degree obliquity

• 16 day rotation period

Phase diagram of water

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

T E

Phase diagram of methane

T

Gas

Solid

Liquid

Credit: H. Roe

Narrowband imaging

Adaptive optics atKeck 10-mGemini 8-m

Typical Titan images:2001- 2005

Schaller et al. 2006

Mean daily insolation on Titan

Mitchell et al. 2006 PNAS

Models of Titan Cloud Activity with seasonPresent Present

Rannou et al. 2006 Science

Surface maps

0

West Longitude

Latit

ude

Credit: NASA/JPL/Space Science Institute

x

Cassini ISS Surface Map

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,

But… small-scale surface features seen by Huygens probe near equator show evidence for rainfall

http://www.esa.int/SPECIALS/Cassini-Huygens/index.html

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

IRTF Spectral Data surface troposphere stratosphere

Rel

ativ

e Fl

ux

2.03 micron flux

0

15%

IRTF

ISS Surface Map

-15%

Schaller et al. 2010 submitted

IRTF Spectral Data

Spectra deviateat <2.12 micronsindicating extremely low <0.15% tropospheric cloud activity in 90% of all nights

Rel

ativ

e Fl

uxR

elat

ive

Flux

Spectra deviateat <2.12 micronsindicating extremely low <0.15% tropospheric cloud activity in 95% of all nights

Typical Titan images:2001- 2005

Schaller et al. 2006

Titan Images:2005-2008

Schaller et al. Nature 2009

IRTF Spectrum

Schaller et al. Nature 2009

Cassini Titan flybys (March 31, May 20) completely missed event

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

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

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.