J/A+A/666/A140 Titan's atmosphere analysis (Rannou+, 2022)
Analysis of four solar occultations by Titan's atmosphere with the infrared
channel of the VIMS instrument: Haze, CH4, CH3D, and CO vertical profiles.
Rannou P., Coutelier M., Rey M., Vinatier S.
<Astron. Astrophys. 666, A140 (2022)>
=2022A&A...666A.140R 2022A&A...666A.140R (SIMBAD/NED BibCode)
ADC_Keywords: Solar system ; Planets
Keywords: planets and satellites: atmospheres -
planets and satellites: composition
Abstract:
Titan, the largest satellite of Saturn, has a dense atmosphere mainly
composed of nitrogen, methane at a percent level, and minor species.
It is also covered by a thick and global photochemical organic haze.
In the last two decades, the observations made by the Cassini orbiter
and the Huygens probe have greatly improved our knowledge of Titan's
system. The surface, haze, clouds, and chemical species can be studied
and characterised with several instruments simultaneously. On the
other hand, some compounds of its climatic cycle remain poorly known.
This is clearly the case of the methane cycle, which is, however, a
critical component of Titan's climate and of its evolution.We
reanalysed four solar occultations by Titan's atmosphere observed
with the infrared part of the Visual Infrared Mapping Spectrometer
(VIMS) instrument. These observations were already analysed, but here
we used significantly improved methane spectroscopic data. We
retrieved the haze properties (not treated previously) and the mixing
ratios of methane, deuterated methane, and CO in the stratosphere and
in the low mesosphere. The methane mixing ratio in the stratosphere is
much lower (about 1.1%) than expected from Huygens measurements (about
1.4 to 1.5%). This is consistent with previous results obtained with
other instruments. However, features in the methane vertical profiles
clearly demonstrate that there are interactions between the methane
distribution and the atmosphere circulation. We also retrieved the
haze extinction profiles and the haze spectral behaviour. We find that
aerosols are aggregates with a fractal dimension of Df≃2.3±0.1,
rather than Df≃2 as previously thought. Our analysis also reveals
noticeable changes in their size distribution and their morphology
with altitude and time. These changes are also clearly connected to
the atmosphere circulation and concerns the whole stratosphere and the
transition between the main and the detached haze layers. We finally
display the vertical profiles of CH3D and CO for the four
observations. Although the latter retrievals have large error bars due
to noisy data, we could derive values in agreement with other works.
Description:
Haze extinction at three wavelengths, haze spectral slopes and methane
mixing ratio retrieved with occultations.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
et10e.dat 124 23 Haze extinction at three wavelengths retrieved
with occultation T10E (Figure 9 and 18)
(kext_T10E.dat)
et10elw.dat 46 12 Haze extinction at wavelength 4898nm retrieved
with occultation T10E (Figure 18)
(kextT10ELW.dat)
et53e.dat 124 46 Haze extinction at three wavelengths retrieved
with occultation T53E (Figure 9 and 18)
(kext_T53E.dat)
et53elw.dat 46 32 Haze extinction at wavelength 4898nm retrieved
with occultation T53E (Figure 18)
(kextT53ELW.dat)
et78e.dat 124 30 Haze extinction at three wavelengths retrieved
with occultation T78E (Figure 9 and 18)
(kext_T78E.dat)
et78elw.dat 46 13 Haze extinction at wavelength 4898nm retrieved
with occultation T78E (Figure 18)
(kextT78ELW.dat)
et78i.dat 124 30 Haze extinction at three wavelengths retrieved
with occultation T78I (Figure 9 and 18)
(kext_T78I.dat)
et78ilw.dat 46 14 Haze extinction at wavelength 4898nm retrieved
with occultation T78I (Figure 18)
(kextT78ILW.dat)
eslt10e.dat 46 23 Haze spectral slope dLn(kext)/dLn(wavelength)
relative to alpha_0 = -2.18 retrieved with
occultation T10E (Figure 10)
(kextslopeT10E.dat)
eslt53e.dat 46 46 Haze spectral slope dLn(kext)/dLn(wavelength)
relative to alpha_0 = -2.18 retrieved with
occultation T53E (Figure 10)
(kextslopeT53E.dat)
eslt78e.dat 46 30 Haze spectral slope dLn(kext)/dLn(wavelength)
relative to alpha_0 = -2.18 retrieved with
occultation T78E (Figure 10)
(kextslopeT78E.dat)
eslt78i.dat 46 30 Haze spectral slope dLn(kext)/dLn(wavelength)
relative to alpha_0 = -2.18 retrieved with
occultation T78I (Figure 10)
(kextslopeT78I.dat)
xch4t10e.dat 46 23 Methane mixing ratio retrieved with occultation
T10E (Figure 11 - top) (xch4_T10E.dat)
xch4t53e.dat 46 46 Methane mixing ratio retrieved with occultation
T53E (Figure 11 - top) (xch4_T53E.dat)
xch4t78e.dat 46 30 Methane mixing ratio retrieved with occultation
T78E (Figure 11 - top) (xch4_T78E.dat)
xch4t78i.dat 46 30 Methane mixing ratio retrieved with occultation
T78I (Figure 11 - top) (xch4_T78I.dat)
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Byte-by-byte Description of file (#): et???.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 7 F7.3 km Alt Altitude
10- 20 E11.6 m-1 Ext884 Extinction at 884nm
22- 34 E13.6 m-1 e_Ext884 Lower value of extinction at 884nm error
35- 46 E12.6 m-1 E_Ext884 Upper value of extinction at 884nm error
48- 59 E12.6 m-1 Ext1540 Extinction at 1540nm
61- 72 E12.6 m-1 e_Ext1540 Lower value of extinction at 1540nm error
74- 85 E12.6 m-1 E_Ext1540 Upper value of extinction at 1540nm error
87- 98 E12.6 m-1 Ext2199 Extinction at 2199nm
100-111 E12.6 m-1 e_Ext2199 Lower value of extinction at 2199nm error
113-124 E12.6 m-1 E_Ext2199 Upper value of extinction at 2199nm error
--------------------------------------------------------------------------------
Byte-by-byte Description of file (#): et???lw.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 7 F7.3 km Alt Altitude
10- 20 E11.6 m-1 Ext4989 Extinction at 4989nm
23- 33 E11.6 m-1 e_Ext4989 Lower value of extinction at 4989nm error
36- 46 E11.6 m-1 E_Ext4989 Upper value of extinction at 4989nm error
--------------------------------------------------------------------------------
Byte-by-byte Description of file (#): eslt*.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 7 F7.3 km Alt Altitude (km)
9- 20 E12.6 --- Extsl Extinction slope - 2.18
22- 33 E12.6 --- e_Extsl [] Lower value of extinction slope - 2.18 error
35- 46 E12.6 --- E_Extsl [] Upper value of extinction slope - 2.18 error
--------------------------------------------------------------------------------
Byte-by-byte Description of file (#): xch4*.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 7 F7.3 km Alt Altitude
10- 20 E11.6 --- CH4 CH4 mole faction
23- 33 E11.6 --- e_CH4 Lower value of CH4 sigma mole faction error
36- 46 E11.6 --- E_CH4 Lower value of CH4 sigma mole faction error
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Acknowledgements:
Pascal Rannou, pascal.rannou(at)univ-reims.fr
(End) Patricia Vannier [CDS] 11-Oct-2022