Seasonal variability of the chemical composition
of the Titanʼs upper atmosphere
Alberto Adriani
Institute for Space Astrophysics and Planetology
INAF, Roma!
The team Personnel"
Institute"
Role"
Alberto Adriani"
INAF-IAPS, Rome,
Italy"
Principal Investigator"
Emiliano DʼAversa"
INAF-IAPS, Rome,
Italy"
Co-Investigator"
Maria Luisa Moriconi"
CNR-ISAC, Rome,
Italy"
Co-Principal
Investigator"
Bianca Maria Dinelli"
CNR-ISAC, Bologna,
Italy"
Co-Investigator"
CSIC-IAA, #
Manuel López-Puertas"
Co-Investigator"
Granada, Spain"
Maya García-Comas"
CSIC-IAACSIC-IAA,
Granada, Spain"
Co-Investigator"
Bernd Funke"
CSIC-IAACSIC-IAA,
Granada, Spain"
Co-Investigator"
Miguel Á. López-Valverde"
CSIC-IAACSIC-IAA,
Granada, Spain"
Collaborator"
Competence"
Scientific coordination;
atmospheric remote sensing#
Aerosol retrieval in planetary
atmospheres; image processing;
database management"
Radiative transfer; image
processing; database management#
Infrared spectroscopy; radiative
transfer forward and inverse
modeling at nadir and limb"
Non-LTE modeling; radiative
transfer; planetary atmospheres
energy balance; atmospheric
remote sensing"
Thermal, chemical and dynamical
structures in planetary
atmospheres; retrieval of satellite
data#
Dynamical coupling processes
between middle and upper
atmospheres; non-LTE modeling#
Atmospheric theoretical modeling;
atmospheric infrared emissions;
atmospheric thermodynamics#
Background Informa3on •  Seasonality effects in the Titanʼs lower atmosphere have been
detected both from satellite and ground-based instruments and
are currently under extensive study (Roe et al., 2002; Griffith et
al., 1998; Griffith et al., 2005; Schaller et al., 2006; Brown et al.,
2009)."
•  The thermosphere composition changes because of the
photodissociation induced by solar EUV and because of the
magnetospheric forcing, both connected in different ways to the
seasonal period (Sittler et al., 2010; Arridge et al., 2008)."
•  Chemical processes and compositions in the upper atmosphere
can also depend on season, via its coupling with the
thermosphere at the top and with the stratosphere at the bottom. "
Objec3ve and Working Plan •  We aim to determine the vertical distribution both of CH4 and of some
minor constituents up to 1100 km over different locations above Titan's
surface and for a long period of time to verify if latitudinal and seasonal
variations in molecule distributions in the upper atmosphere can be
detected. "
Working Plan:"
1.  To acquire all VIMS limb observations, to process the data and to
extract vertical profiles of spectral radiances, geo-located and georeferred (limb sequences) as already did in Adriani et al. (2011) and
Garcia-Comas et al. (2011)."
2.  To analyze the limb sequences acquired by VIMS covering the entire
nominal and the first extended period of the Cassini mission
(2004-2012). The main effort of the proposal will be focused on the
development of new analysis tools."
3.  To determine appropriate kinetic temperatures and compute
population ratios of the vibrational levels originating the radiances
measured by VIMS for the simulation of the non-LTE emissions of
some minor constituents including C2H2, CO, CH3D and unknown
aromatics."
4.  To upgrade the retrieval system already used for the analysis of
Titan's limb emission."
5.  Correlate VIMS observations with UVIS and INMS data."
Database Management •  We plan to select those VIMS limb observations with phase
angles lower than 100° and with long integration times (≥500
ms) covering both the nominal and the extended mission
period."
•  A database of geo-located and time resolved vertical
distributions of the gases in lower thermosphere will be
developed. "
•  We point to obtain a statistics of the data as large as possible
to permit a climatology for Titan's lower thermosphere."
Ini3al Selec3on VIMS daytime co-added spectra in
the region of 3.2 µm at tangent
heights from 275 to 1062 km. The
data are taken from the Cassini/Titan
encounters studied by Adriani et al.
(2011)."
Radiance [x10-7W/(m2 sr nm)]
12
275 km
379 km
472 km
527 km
623 km
716 km
816 km
872 km
967 km
1062 km
10
CH4
8
6
4
HCN / C2H2 2
0
2.8
3.0
3.2
Wavelegth (µm)
3.4
3.6
CH3D CO VIMS daytime co-added spectra in
the region of 4.6 µm at tangent
heights from 200 to 650 km. The data
are taken from the Cassini/Titan
encounters studied by García-Comas
et al. (2011)."
"
Development of non-­‐LTE Modeling •  We plan to develop a non-­‐LTE model for C2H2 infrared emission near 3.0 µm to perform a joint retrieval with the HCN emission and to derive a more accurate HCN concentra3on. •  We point to iden3fy the specific polymer fiPng the anomalous emission (PAH) revealed in the analysis of the VIMS 3.3 µm CH4 spectra (Dinelli et al., 2009) and to derive the number density profiles of these polymers from the VIMS radiance profiles. •  We would like to pursue the retrieval of CO from its 4.7 µm emission (Baines et al., 2006), also by the analysis of VIMS limb day3me CO emission. In day3me spectra the R-­‐ and P-­‐branches of the CO vibra3onal bands are clearly visible with the no3ceable emission peak around 4550 nm due to CH3D, as showed in the next slide. C2H2(v3=1) vib. temp. SZA variation
1000
Altitude [km]
Vibra3onal temperatures for C2H2 as func3on of the Solar Zenit Angle 1200
800
Tk
600
400
00_
20_
40_
50_
60_
70_
80_
85_
90_
95_
100
120
150
200
CO VTs. SZA var.. CO 1
150
200
250
300
Vibrational temperature [K]
-30 -20 -10 0 5
VT difference [K]
800
700
Tk
Altitude [km]
600
500
400
300
200
100
140
160
180
200
Vibrational temperature [K]
00
20
30
40
50
60
70
80
85
90
95
100
110
120
130
140
-40 -30 -20 -10 0
VT difference [K]
Vibra3onal temperatures for CO as func3on of the Solar Zenit Angle Forward and Inverse Model •  The retrieval algorithm used for the analysis of Titan's limb atmospheric emission, reported in Adriani et al. (2011) and Garcia-­‐Comas et al. (2011), will be used as a star3ng point. •  The algorithm is based on a forward model updated to perform simula3ons over broad frequency intervals and to compute non-­‐LTE radiances (called from now on GBB-­‐TIT), and has been included in a retrieval code that exploits the op3mal es3ma3on technique [Rodgers, 2000]. •  To perform rou3ne analyses we plan to upgrade the GBB-­‐TIT FM to compute analy3cally some of the required deriva3ves, reducing significantly the compu3ng 3me . Expected Results"
•  New non-LTE models for the vibrational levels of C2H2
and CH3D."
•  A collection of volume mixing ratios for CH4, HCN, C2H2,
CO, CH3D, and PAH in order to build a homogeneous
database of geo-located and time resolved vertical
distributions of the examined gases."
•  Correlate VIMS-UVIS-INMS observations"
•  Decoupling the latitudinal variability from the seasonal
variability of the gases abundances ."
"
Help"
To extract data from an instrument requires good know of the
instruments, the measurement technique, the database and the
tools developed for the analysis."
It requires an effort to develop the database for VIMS that is an
instrument that we know very well. It would require a a strong
effort to repeat the work for UVIS or INMS."
We have started to look at the UVIS data and learn how to deal
with them but it would be much efficient and useful is we could
count on a solid collaboration with somebody of the UVIS team."