Seasonal Change in Titan

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Seasonal Change in Titans Cloud Activity
Observed with IRTF/SpeX

• Emily Schaller (Caltech)
• Henry Roe (Lowell Observatory)
• Michael Brown (Caltech)

Image Credit NASA/JPL/Space Science Institute
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Titans Methane Weather Cycle
Image credit NASA/JPL/ESA
Titan provides us with a unique laboratory in
which to study a hydrological cycle on a planet
other than Earth with a different condensable
species (methane on Titan, water on Earth).
Understanding how Titans hydrological cycle
changes with season over Titans 29-year year is
vital for determining formation mechanisms of
surface features observed by Cassini/Huygens.
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Titans Methane Clouds

• Rapidly varying tropospheric methane clouds were
  regularly observed near Titans south pole in
  Keck and Gemini adaptive optics images from
  2001-2005. During this time, Titans south pole
  was in continuous sunlight
• Our hypothesis is that the locations of Titans
  clouds are controlled in a complex way by the
  seasonally varying insolation and changes in
  Titans global circulation. As Titan moves
  toward southern autumnal equinox in 2009, the
  latitudes of Titans clouds should move north.
• While infrequent observations from Cassini are
  useful for studying the morphologies of clouds,
  only a nightly monitoring program can provide the
  type of dataset necessary to determine the
  frequency, duration and altitudes of large and
  small cloud systems and the change with season.

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IRTF Titan Spectroscopic Monitoring

• Disk integrated spectra of Titan from 0.8-2.4
  microns
• Data taken by Telescope Operators nearly every
  night SpeX is on the telescope (76 nights so far
  over two semesters).
• Total time to take data each night is less than
  20 minutes.
• Disk integrated spectra allow us to determine
  Titans total fractional cloud coverage and cloud
  altitudes.
• Data are rapidly reduced and increases in cloud
  activity allow us to trigger target-of-opportunity
  observations with the adaptive optics systems on
  either Keck or Palomar to determine cloud
  latitudes (one such TOO was triggered on November
  28, 2006).

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Ten individual nights of Titan spectra divided by
a G2V star. The wavelengths at which the spectra
deviate from each other and the magnitude of the
deviation allow us to determine the altitude and
brightnesses of any clouds present.
Surface
Troposphere
Stratosphere
The spectrum of 2/23/06 is shown subtracted from
that of 2/24/06. We can constrain the difference
in total cloud coverage to be less than 0.15.
From Gemini images we took several days earlier
along with images from the T11 Cassini flyby (Feb
27), we know that these two nights correspond to
a baseline of extremely low cloud activity.
Wavelength (microns)
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Discussion

• Titan observations over the past two semesters
  with IRTF have shown a dramatic decrease in cloud
  activity. On all but two nights Titans total
  cloud coverage was less than 0.15.
• The near lack of cloud activity in IRTF
  observations contrasts sharply with similar
  observations of Griffith et al. (1998 2000)
  near equinox and indicates a striking seasonal
  change in cloud frequency and magnitude.
• Observations of the latitudes, magnitudes,
  altitudes, and frequencies of Titans clouds as
  Titan moves toward southern autumnal equinox in
  2009 will help elucidate when and how Titans
  methane hydrological cycle changes with season.
• IRTF is poised to make a major contribution
  toward understanding the long and short-term
  evolution of cloud systems and the hydrological
  cycle on Titan.