Manuscript Prepared on December 7, 2006
Photolytically generated aerosols in the mesosphere and
thermosphere of Titan
Mao-Chang Liang1,2 , Yuk L. Yung2 , and Donald E. Shemansky3
∗
E-mail:
mcl@gps.caltech.edu
ABSTRACT
Analysis of the Cassini Ultraviolet Imaging Spectrometer (UVIS) stellar and
solar occultations at Titan to date include 12 species: N2 (nitrogen), CH4
(methane), C2 H2 (acetylene), C2 H4 (ethylene), C2 H6 (ethane), C4 H2 (diacetylene), C6 H6 (benzene), C6 N2 (dicyanodiacetylene), C2 N2 (cyanogen), HCN (hydrogen cyanide), HC3 N (cyanoacetylene), and aerosols distinguished by a structureless continuum extinction of photons in the EUV. Upper limits have been
placed on benzene, dicyanodiacetylene, cyanogen, and cyanoacetylene, although
the limits on the first two in this list are not considered significant. The aerosol
component shows extinction properties compatible with those of laboratory generated solid deposition defined as tholin, from N2 /CH4 discharges The introduction of aerosol particles in the gas phase, assumed to retain the same refractive
index properties as tholin, with radius ∼25 Å using Mie theory provides a satisfactory fit to the spectra. The derived vertical profile of aerosol density shows
distinct structure, implying a reactive generation process reaching altitudes more
than 1000 km above the surface. A photochemical model presented here provides
a reference basis for examining the physical processes leading to the distinctive
atmospheric opacity at Titan, although a detailed comparison over altitude has
not been established. We find that dicyanodiacetylene is condensable at ∼650
km, where the atmospheric temperature minimum is located. This species is by
far the simplest molecule identified to be condensable. Observations are needed
to confirm the existence and production rates of dicyanodiacetylene.
1
Research Center for Environmental Changes, Academia Sinica, Taipei 115, Taiwan
2
Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125,
USA
3
Planetary and Space Science Division, Space Environment Technologies, Pasadena, CA 91107, USA
Aerosols at Titan
Liang, Yung, Shemansky
0
-1
-2
ln(I/I0)
-3
-4
λ Sco synth
λ Sco obs
Model C
aerosol + CH4
-5
-6
-7
-8
1300
1400
1500
1600
1700
1800
λ (A)
Latitude -36 deg. Altitude 517 km -- 537 km.
Data compared to synthesized spectrum, Liang et al.
model C. Synthesized aerosol component is included.
1900
1000
[CH4] X 10-7
900
tholin
CH4 X 10-7
h (km)
800
700
600
500
400
300
9
10
11
12
13
LOG([N] )
14
15
16
T (K)
120
1100
1000
130
140
[CH4] X 10-7
150
160
170
180
[aerosol]
T (K)
[CH4] X 10-7
800
700
600
500
400
300
1.0
200
Aerosol density distribution
900
h (km)
190
2.0
3.0
4.0
LOG([N])
5.0
6.0
7.0
1000/h
3.517261
3.253935
3.0273
2.830187
2.657179
2.504112
2.367724
2.245429
2.135153
2.035205
1.944199
1.860987
1.784608
1.714255
1.649241
1.588981
1.532971
1.480777
1.432023
1.386378
1.343555
1.3033
1.265389
1.229622
1.195823
1.163834
1.133512
1.104732
1.077378
1.051347
1.026545
1875A
tau
13.81551
11.346957
4.8264664
3.1416577
2.2323192
1.6034666
1.054202
0.710529
0.450298
0.495661
0.529182
0.455162
0.371294
0.250838
0.225375
0.241223
0.216100
0.154147
0.173138
0.107753
0.118872
0.134755
0.049293
0.080669
0.062693
0.089840
0.098269
0.056389
0.020578
0.028494
0.024991
ln(tau)
2.625792
2.42895
1.574115
1.144751
0.803041
0.472168
0.052784
-0.341745
-0.797845
-0.701863
-0.636423
-0.787102
-0.990762
-1.382947
-1.489988
-1.422036
-1.532016
-1.869846
-1.753668
-2.227915
-2.129704
-2.004301
-3.009972
-2.517401
-2.769508
-2.409722
-2.320044
-2.875476
-3.883524
-3.558075
-3.689247
1875A
1875A
C0
ln(tau_fit) tau_fit
h
model abnd Tau_actual C1
2.693832 14.78824 284.3121
C2
2.20191 9.042269 307.3202
1.718473 5.576006 330.3274
1.40E+17
4.62
1.252834
3.50025 353.3336
9.50E+16
3.135
0.809509 2.246804 376.3389
6.10E+16
2.013
0.390282 1.477398 399.3432
3.90E+16
1.287
-0.004614 0.995397 422.3466
2.50E+16
0.825
-0.375812 0.686731 445.3491
1.58E+16
0.5214
-0.724406 0.484612 468.3506
1.03E+16
0.3399
-0.701863 0.495661 491.3511
1.03E+16
0.3399
-0.636423 0.529182 514.3507
1.12E+16
0.3696
-0.877514 0.415815 537.3493
7.80E+15
0.2574
-1.007108 0.365274
560.347
6.95E+15
0.22935
-1.145975 0.317914 583.3437
5.40E+15
0.1782
-1.290924 0.275017 606.3394
4.75E+15
0.15675
-1.439532 0.237039 629.3342
4.40E+15
0.1452
-1.589958 0.203934 652.3281
3.50E+15
0.1155
-1.7408
0.17538
675.321
2.05E+15
0.06765
-1.890998 0.150921 698.3129
1.95E+15
0.06435
-2.039752 0.130061 721.3038
1.30E+15
0.0429
-2.186466 0.112313 744.2938
-2.330699 0.097228 767.2829
6.60E+14
0.02178
-2.472131 0.084405 790.2709
5.80E+14
0.01914
-2.61054 0.073495 813.2581
-2.745774 0.064199 836.2442
-2.877743 0.056262 859.2294
-3.006398
0.04947 882.2137
5.30E+14
0.01749
-3.131727 0.043642
905.197
-3.253744 0.038629 928.1793
-3.372485 0.034304 951.1607
-3.488 0.030562 974.1411
3.60E+14
0.01188
– 14 –
Fig. 4.— Modeled (model C) vertical profiles for CH4 (thick solid), HC3 N (dashed), HCN
(dash-dotted), C6 N2 (triple-dot-dashed), and C6 H6 (long-dashed). Thin solid line represents
modeled CH4 by model A. The saturation density of C6 N2 extrapolated from high temperature measurements (295-369 K, Saggiomo 1957) is shown by dotted line. The resulting H2
(3×10−3 ) and CH4 (2.3%) mixing ratios at 1174 km and H2 escape flux (7×109 molecules
cm−2 s−1 ) at the top are in good agreement with the observations (4±1×10−3 , 2.7±0.1%,
and 1.2±0.2×1010 molecules cm−2 s−1 , respectively) (Yelle et al. 2006).
– 15 –
Table 1. Summary of Model Results
Molecule
N2 (×1021 )
CH4 (×1019 )
C2 H2 (×1017 )
C2 H4 (×1016 )
C2 H6 (×1016 )
HCN (×1017 )
C4 H2 (×1015 )
C6 N2 (×1014 )
C6 H6 (×1014 )
HC3 N (×1015 )
C2 N2 (×1015 )
Tholin (×1013 )
Cassini
5.8
6.0
2.1
4.0
7.0
1.0
4.5
<1.0
<1.4
<3.9
<4.0
5.8
Model A
5.8
15
15
9.3
200
5.6
59
15
18
27
0.84
···
Model B Model C Model D WA04
5.8
9.4
9.1
5.7
110
3.7
37
16
13
25
1.2
···
5.8
9.5
1.9
4.0
17
0.69
12
5.7
12
6.9
0.34
···
5.8
9.7
1.7
2.0
9.2
0.53
2.1
8.0
0.041
7.8
0.42
···
···
15
1.7
3.5
22
0.017
45
···
0.23
···
···
···
Note. — Values are line-of-sight column integrated abundances, in molecules
cm−2 , reported by matching the observed N2 abundance. Model A: hydrostatic
atmosphere. Model B: non-hydrostatic atmosphere, an ad hoc downward wind
and extinction due to the derived tholins are assumed (see text). Model C: same
as Model B but also with additional sinks for the tabulated nine photochemical species (see text). Model D: same as Model C but with the hydrocarbon
chemistry from Moses et al. (2005). WA04: model results from Wilson & Atreya
(2004).