clouds in the tropics of titan emily schaller lunar and planetary laboratory, university of arizona...
DESCRIPTION
Phase diagram of water T ETRANSCRIPT
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Clouds in the Tropics of Titan
Emily Schaller
Lunar and Planetary Laboratory, University of Arizona
2010 Hubble Fellows Symposium
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Titan
• Thick atmosphere with surface pressure ~1.5 bar.
• Major gases in atmosphere: N2,(~98%) CH4 (~2%)
• 27 degree obliquity
• 16 day rotation period
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Phase diagram of water
http://www.lsbu.ac.uk/water/phase.html
T E
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Phase diagram of methane
T
Gas
Solid
Liquid
Credit: H. Roe
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Narrowband imaging
Adaptive optics atKeck 10-mGemini 8-m
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Typical Titan images:2001- 2005
Schaller et al. 2006
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Mean daily insolation on Titan
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Mitchell et al. 2006 PNAS
Models of Titan Cloud Activity with seasonPresent Present
Rannou et al. 2006 Science
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Surface maps
0
West Longitude
Latit
ude
Credit: NASA/JPL/Space Science Institute
x
Cassini ISS Surface Map
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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,
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But… small-scale surface features seen by Huygens probe near equator show evidence for rainfall
http://www.esa.int/SPECIALS/Cassini-Huygens/index.html
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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
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IRTF Spectral Data surface troposphere stratosphere
Rel
ativ
e Fl
ux
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2.03 micron flux
0
15%
IRTF
ISS Surface Map
-15%
Schaller et al. 2010 submitted
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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
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Typical Titan images:2001- 2005
Schaller et al. 2006
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Titan Images:2005-2008
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Schaller et al. Nature 2009
IRTF Spectrum
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Schaller et al. Nature 2009
Cassini Titan flybys (March 31, May 20) completely missed event
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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
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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
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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.