Carbon monoxide fluorescence
from Titan’s atmosphere
M.-A. Lopez-Valverde1, E. Lellouch2, A. Coustenis2, J.-G. Cuby3
1
Instituto de Astrofisica de Andalucia, Granada, Spain
2Observatoire de Paris, F-92195 Meudon, France
3 Observatoire de Marseille, France
OUTLINE:
Introduction
Observations Absorption and Emission Spectra
Analysis
Non-LTE modelling; Sensitivity study
Conclusions
Introduction
Fluorescent emissions not uncommon in planetary atmospheres
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Earth CO2 (15-, 4.3-µm), O3(10-µm), …
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Venus and Mars: CO2, 10-µm (Ground), 4.3-µm (ISO,Mars Express)
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Jupiter and Saturn: CH4 (3.3 µm ), Uranus: CO (4.7 µm), Titan HCN (3 µm)
Often complex to model …
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Radiative transfer in molecular vib-rotational bands (H2O , CO2 , CH4 )
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Complexity of relaxation & thermalisation routes
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Frequent lack on collisional rate coefficients
… but potentially interesting to:
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Detect new compounds (CO on Uranus) and tracer abundances
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Evaluate IR cooling of upper atmosphere (CO2 in Mars/Venus mesosphere)
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Measure local temperatures, especially from rotational distribution
Here, we report on the derivation of CO in Titan stratosphere
from VLT observations of its 4.7 µm fluorescence emission
The atmosphere of Titan
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Nitrogen, Methane, Traces of
Amonia, Argon, Ethane
Surface Pressure 1.5 atm
Surface T 94K, Stratosphere
Profiles of CO & CO2 ?
Infrared Window at 5-µm probes Titan’s
surface and lower atmosphere
(Noll et al, 1993,1996)
Preliminary observations :
VLT/Isaac, R ~ 2120, November 16, 2000,
Range = 4.75-5.10 µm
Detection of emissions at 4.75-4.84 µm,
coinciding with CO(1-0) & (2-1) lines
Evidence for non-thermal mechanism
(Lellouch et al. Icarus 162, 126-143, 2003)
Improved and extended observations
( Lopez-Valverde, et al., Icarus, submitted, 2004 )
November 19-20, 2002
Telescope: VLT – UT1 (Antu)
Instrument: ISAAC (Infrared Spectrometer and Array Camera)
Mode: Long-Wavelength Spectroscopy
Resolution: R = 3000 / D, where D is effective source diameter
Range: 4.51-4.86 µm in two grating positions, R ~ 2570
Calibration on HR2354; reduction from Eclipse and IRAF packages
Detection of MANY (~ 45) emissions :
31 12CO(1-0) lines (R11-R0 and P1-P19
Numerous 12CO(2-1) lines, some (R11-R6 and P3-P9) well separated from the 12CO(1-0) lines
12CO(2-1) and 12CO(1-0) lines have similar intensities
Non-LTE model for CO
T/P profile recommended by Yelle et al (1997)
CO = 32 ppmv constant at all altitudes (Lellouch et al, 2003)
Five excited vibrational
levels of 12CO,
13CO and N
2
Radiative Processes
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Solar excitation in v=1 and v=2 states
For 12CO, full radiative transfer included
For 13CO, only spontaneous emission considered.
CH4 overlapping
Collisional scheme:
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V-T deactivation of CO for collisions with N2, CH4, and H2
V-V exchanges between all CO excited states and N2 (1)
V-V exchanges between CO and 13CO
V-T deactivation for N2 (1)
Non-LTE
Results
Vibrational Temperatures:
• N2(1) and CO(1) out of LTE at all altitudes
but close to LTE at 100-250 km
• CO(2) strongly excited at all altitudes
Contribution Functions:
•1-0 sounds two atmospheric regions at
100-250 and 350-550 km
•2-1 sounds layer 200-300 km
Fit to the Observations
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Produces satisfactory match of 1-0 lines
Underestimates 2-1 lines by factor of ~ 2
SENSITIVITY STUDY :
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CO mixing ratio : From 32 to 60 ppm
Temperature profile: 10K up/down at all levels
Collisional scheme: Tune collision rates within their
uncertainties
Effect of uncertainties (1)
NOMINAL
TEMPERATURE
CO
COLLISIONS
Effect of uncertainties (2)
Best fit models of the CO fluorescent emissions
Conclusions
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Emission from CO(1-0) and (2-1) lines is clearly detected and
corresponds to solar pumping at 4.7- and 2.3 um
While CO absorption lines sound the troposphere, these emission
lines probe stratospheric altitudes
1-0 well explained by nominal model. 2-1 underpredicted by factor 2.
Sensitivity studies show difficulty to match both bands at same time
A combination of modifications could explain both bands BUT …
… an altitude gradient in CO would be necessary !?
Missing unknown non-thermal excitation mechanism ??
Our observations are global …. Variability? Transport by circulation?
Cassini/Huygens will have much higher spatial resolution
Artist’s view of Huygens’ parachuting descent into Titan’s atmosphere
Schedule: Cassini separation 25th Dec 04 Descent 14th Jan 2005