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Control/Tracking Number: 07-RC-204-AAS-DPS
Activity: Research Contributed
Current Date/Time: 7/27/2007 8:59:42 AM
Titan Atmospheric Structure From 2000 Km To 300 Km: Models Compared To Cassini Uvis
Observations
Author Block: D. E. Shemansky1, M. Liang2, Y. L. Yung3
1Space Environment Technologies, 2Research Center for Environmental Changes/Academia Sinica,
Taiwan, 3California Institute of Technology.
Abstract:
Cassini UVIS occultations at Titan have provided measurements of atmospheric structure in the altitude
range 2000 km - 300 km at several latitudes and longitudes. The molecular nitrogen abundance has been
measured to 350 km above the exobase with apparent temperatures close to 200 K. Vertical profiles of
hydrocarbons and nitriles show distinct dependence on latitude. Extinction by aerosols have been
measured for the first time over the altitude range 100 km - 300 km, indicating abundance latitude
dependence below 500 km, and large scale heights above 500 km. Derived vertical abundances of the
higher order species shows significantly declining mixing ratios below altitudes ranging from 750 km to
550 km. C2H2 follows the atmospheric scale of CH4 over the range 800 km to 1000 km. C2H4/CH4
abundance mixing ratios show latitudinal abundance variation by as much as a factor of 4. At latitude 35o the abundance of CH4 is depleted in the sunlit atmosphere above 880 km relative to the darkside,
and higher order species by factors of 2 to 4. Below 880 km CH4 shows no longitude dependence, but
higher order species remain depleted in the subsolar atmosphere. Aerosol extinction is unchanged at
latitude -35o, independent of longitude and the 2 year span between observations. Model atmospheres
have been calculated. Physical chemistry models including atmospheric dynamics have been calculated
using the observed latitudinally dependent distributions as constraints. Limits on the processes of
formation and lifetimes of aerosols based on the observed profiles will be discussed.
This work is supported by the Cassini and the NASA PATM Programs
:
Category (Complete): 31. Titan: Atmosphere (Chemistry, Clouds, and Dynamics)
Facility Keywords (Complete): {\it Facilities:} \facility{Cassini ()}
Presentation Preference (Complete): 1 - Oral Preferred
Additional Information (Complete):
I am willing to serve as a Chair: : Yes
(1) Area of Expertise: : 15. Gaint planets: Aurorae and Magnetospheres
(2) Area of Expertise: : 23. Mercury and Moon
https://www.abstractsonline.com/submit/SubmitPrinterFriendlyVersion.asp?ControlKey=...
7/27/2007
Titan atmospheric structure from
2000 km to 300 km: Models
compared to Cassini UVIS
observations
D. Shemansky, M. Liang, Y. Yung
10/11/07
Observed Properties
• Vertical hydrocarbon partitioning is variable with
apparent differences at different latitudes and
longitudes with some possible temporal variation.
• Extinction by aerosols is measured from 300 km
to 1000 km. Significant variability in aerosol
abundance is evident at different latitudes only
below 500 km.
Species investigated
•
•
•
•
•
•
N2
CH4 methane
C2H2 acetylene
C2H4 ethylene
C2H6 ethane
C4H2 diacetylene
•
•
•
•
•
C6H6 benzine
C6N2 dicyanodiacetylene
C2N2 cyanogen
HCN hydrogen cyanide
HC3N cyanoacetylene
UVIS occultations
Event
Latitude (deg)
Longitude (deg)
TB λSco
-36
318
TB αVir
58 -- 45
310 -- 330
T10 solar
-62 -- -51
0
T26 solar
-76
0
T21 αEri
-34
116
T32 solar
-56 -- -44
0
Summary: Longitude dependent
vertical structure
• At latitude ~ -35o the sunlit atmosphere above 880 km is
depleted relative to the dark atmosphere in CH4 and higher
order species by factors of 2 to 4.
• Below 880 km CH4 abundance (based on a single pair of
occultations) has no longitude dependence, but higher
order species remain depleted in the subsolar atmosphere.
• Aerosol extinction is identical from 300 km to 900 km at
latitude ~ -35o , independent of longitude and the 2 year
interval between occultations.
Summary: Latitude dependent
vertical structure
• Darkside abundance of CH4 1000 km – 700 km is
unchanged in magnitude and vertical distribution between
latitude 58o – 45o and –36o.
• Higher order hydrocarbons such as acetylene are depleted
above 700 km by factors as large as 2 in the darkside
occultations between latitude 58o – 45o and –36o.
• North/south differences in aerosol extinction below 500
km are significant in structure and magnitude, with
extinction below 425 km substantially smaller in the north
latitude region.
Summary: Vertical structure
• The higher order hydrocarbons show rising
mixing ratios relative to CH4 with increasing
altitude above 650 km.
• Peak densities of the higher order hydrocarbons
are indicated in the range 550 km to 700 km.
• Spectral absorption shapes in methane, acetylene,
ethylene, and diacetylene show reduced
atmospheric temperatures below 800 km and
north/south differences.
spic_tran_lsco_018e_lst_01
1100
αEri Lat -34o Long 116o
αEri
λSco Lat -36o Long 318o
CH4X 10-1 CH X 10-1
4
1000
900
λSco
CH4_λSco
C2H6_λSco
HCN_λSco
HC3N_λSco
[CH4] X 10-7
h (km)
800
700
600
500
400
300
11
C2H2_λSco
C2H4_λSco
C4H2_λSco
aerosol_λSco
CH4_λSco_model
12
13
14
15
Log([X] cm-2)
16
17
18
spic_tran_lsco_018e_lst_02
1100
αEri Lat -34o Long 116o
αEri
λSco Lat -36o Long 318o
CH4X 10-1 CH X 10-1
4
1000
900
[CH4] X 10-7
CH4_λSco X 10-1
CH4_αEri X 10-1
C2H2_αEri
C2H4_αEri
C4H2_αEri
800
h (km)
λSco
700
600
500
400
300
11
C2H2_λSco
C2H4_λSco
C4H2_λSco
aerosol_λSco
CH4_λSco_model
12
13
14
15
Log([X] cm-2)
16
17
18
spic_tran_lsco_018e_lst_03
1100
αVir Lat 58o -- 45o Long 310o -- 330o
αVir
anti-solar
λSco Lat -36o Long 318o
CH4X 10-1
λSco
1000
900
[CH4] X 10
CH4_λSco X 10
CH4_αVir X 10-1
C2H2_αVir
C2H4_αVir
C4H2_αVir
-7
800
h (km)
CH4X 10-1
-1
700
600
500
400
300
11
C2H2_λSco
C2H4_λSco
C4H2_λSco
aerosol_λSco
CH4_λSco_model
12
13
14
15
Log([X] cm-2)
16
17
18
The T10 and T26 solar
occultations
• T10: 2006 015, Lat –62 -- -51o
• T26: 2007 069, Lat –76o
• Photometric analysis shows moderate differences
in N2 and hydrocarbon extinction at high altitude.
Mixed differences appear at lower altitudes. The
T26 N2 vertical profile shows a lower apparent
temperature relative to T10 at the top of the
atmosphere.
N2_t10_t26_rv1_2_abnd_model_vs_data
UVIS solar occultations at T10 and T26
2100
T10 N2 data
T26 N2 data
N2 model T∞ =195.7 K
N2 model T∞ = 148 K
T26 CH4 data
T10 CH4 data
CH4 model T∞ = 195.7 K
CH4 model T∞ = 188.6 K
N2 model T∞ = 188.6 K
1900
h (km)
1700
1500
T10: T = 195.7 K Lat = -61o -- -51o
T26: T ~ 188 K Lat = -76o
1300
1100
900
700
13
14
15
16
17
Log([N] (cm-2))
18
19
20
21
CH4_tau_1
abundance mixing ratios to CH4.
αVir
λSco
Lat.(deg)
48.7
-36
tau_1 (km) (1200A)
982.0
1023.2
tau_CH4. (1216A)
0.83
0.71
CH4.
1
1
C2H2.
2.2E-02
4.8E-02
C2H4.
8.2E-03
3.4E-02
C2H6.
8.0E-03
1.8E-02
HCN
3.6E-02
1.0E-01
C4H2.
2.6E-03
5.7E-03
HC3N
4.2E-03
1.0E-02
C2N2
3.0E-03
4.5E-03
avir_lsco_tholin_abnd
Tholin abundances from λSco and αVir occultations
1100
αVir_Tholin
λSco_Tholin
CH4_model
1000
900
800
h (km)
700
600
CH4 X 10-7
500
400
300
200
100
9
10
11
12
13
14
Log([th] (cm-2))
15
16
17
18
CH4_vs_C2H2_dens
UVIS Titan αVir occultation derived densities C2H2, CH4
C2H2
1000
C2H2 αVir_58_45N
CH4 X 10-7 αVir
Tholin X 104
C2H2_CIRS_15S
C2H2_CIRS_80N
900
CH4 X 10-7
h (km)
800
CIRS Vinatier et al 2007
700
600
500
Tholin X 104
400
300
200
100
3
4
5
6
7
8
Log([n] cm-3)
9
10
11
12
Impact on modeling
• The Liang et al (2007) model in general shows higher
abundances of high order hydrocarbons, and in particular
too much dicyanodiacetylene and benzene, relative to
observation. Higher rates of aerosol production and
convective and other losses are indicated.
• The observed aerosol and high order hydrocarbons show
increasing mixing ratios with increased altitude, and this is
not a property of the present model. The UVIS
observations are compatible with the Waite et al (2007)
argument that aerosols develop out of ionospheric
chemistry.
• We presently have no sensible constraints on composition
of the aerosol vertical profile.