Titan’s Upper Atmosphere
Composition from Cassini UVIS
Observations
Joshua Kammer
California Institute of Technology
January 6, 2012
Outline
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Titan’s Atmosphere and Photochemistry
The Cassini Mission and UVIS Instrument
Retrieval Method and Data Processing
Results from Observation and Modeling
Discussion and Implications for Future Work
Titan
• Atmosphere: Mostly
nitrogen (98%), with
methane and various
hydrocarbons
• Surface pressure of
~1.5 atm
• Temperature of
atmosphere ranges
from 80K to 180K
Hidden in Haze
• Thick layers of haze
obscure the surface
• Photochemistry drives
complex suite of
organic reactions
• These organics have
distinct absorption
features in the UV
region of spectrum
Hidden in Haze
• Motivation:
– What chemistry occurs
on Titan, and where?
– How might the products
of this chemistry vary in
space and time?
– Connection to lower
atmosphere/surface?
Enter Cassini
• Launched in 1997
• Arrived in Saturn
system in 2004
• Wide array of
instruments
– Ultraviolet Imaging
Spectrograph (UVIS)
Stargazing with Cassini UVIS
• Cassini UVIS instrument
– EUV Spectrograph used
for solar occultations
– FUV Spectrograph used
for stellar occultations
– Measures integrated UV
photon flux
(from Esposito et al. 2003)
Stargazing with Cassini UVIS
• Cassini UVIS instrument
– EUV Spectrograph used
for solar occultations
– FUV Spectrograph used
for stellar occultations
– Measures integrated UV
photon flux
• Occultations with UVIS
can probe atmospheric
regions that no other
instruments can easily
observe
Photon Counts to Optical Depth
• Process in two steps:
– Calculate Io (λ) spectrum from above atmosphere
– Optical depth is calculated as:
τ (λ,h) = - ln [I (λ,h) / Io (λ)]
where I (λ,h) is the integrated photon flux at each
wavelength λ and occultation ray height h
Optical Depth to Abundance
• Species absorption cross-sections
– From laboratory measurements
• Instrument response function
– For given species abundance (cm-2), can calculate
contribution to optical depth as seen by UVIS
• Rodgers retrieval methodology
– Finds best fit parameters of forward model in
iterative process that minimizes cost function
• J(x) = (x-xa)T Sa-1 (x-xa) + (y-Kx)T Se-1 (y-Kx)
Abundance to Density
• Assume spherically
symmetric atmosphere
• Convert line of sight
abundance to local
density using inverse
Abel transform
Future Work
• Large amounts of UVIS data still unprocessed,
and more continues to be acquired
• Examine possible seasonal and spatial
variability of methane and other hydrocarbons
• Further integration and comparison with Titan
global atmospheric chemical models
• Similar approach for Saturn observations