Download Mitchell et al. 2006 PNAS Models of Titan Cloud Activity with season

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
T
E
http://www.lsbu.ac.uk/water/phase.html
Phase diagram of methane
Liquid
Solid
T
Gas
Credit: H. Roe
Adaptive optics at
Keck 10-m
Gemini 8-m
Typical Titan images:
2001- 2005
Schaller et al. 2006
Mean daily insolation on Titan
Models of Titan Cloud Activity with season
Present
Mitchell et al. 2006 PNAS
Present
Rannou et al. 2006 Science
Latitude
Cassini ISS Surface Map
0
x
West Longitude
Credit: NASA/JPL/Space Science Institute
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
Relative Flux
surface
troposphere
stratosphere
15%
2.03 micron flux
IRTF
0
-15%
ISS Surface Map
Schaller et al. 2010 submitted
Relative Flux
Flux
IRTF Spectral Data
Spectra deviate
Spectra deviate
at <2.12 microns
at <2.12 microns
indicating extremely
indicating extremely
low <0.15% tropospheric
low <0.15% tropospheric
cloud activity in 90% of
cloud activity in 95% of
all nights
all nights
Typical Titan images:
2001- 2005
Schaller et al. 2006
Titan Images:
2005-2008
IRTF Spectrum
Schaller et al. Nature 2009
Cassini Titan flybys (March 31, May 20) completely missed event
Schaller et al. Nature 2009
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 groundbased observations will provide the key
for interpreting the origin and evolution of
the fluvial surface features.