D. Shemansky
6/25/2006
Titan mesosphere/stratosphere structure:
A photometric analysis of the altitude range below 1000 km has been carried
out at the highest possible ray-height resolution (3-5 km) for the lsco and
avir TB occultations.
FUV Spectral ranges accumulated:
px
174-367
649-752
828-899
944-1008
wavelength (A)
1250.-1400.
1620.-1700.6
1759.9-1815.1
1850.3-1900.2
designation
_CH4
_C2H4
_C6H6
_C6N2
_CH4: Dominated by CH4 but contains a mix of higher order hydrocarbons.
_C2H4: Dominated by C2H4 but also mixed with other higher order hydrocarbon and
cyano species.
_C6H6: Peak region of the C6H6 cross section. Mixed primarily with C2H4,
but dominated by aerosol scattering at lower altitudes.
_C6N2: Peak region of C6N2 cross section, dominated by aerosol scattering.
Figures 1 & 2 show ln(I/I0) against altitude for the lsco photometric regions on two
different scales. The _C6H6 and _C6N2 components show significantly different shapes
below 900 km. The subtraction of a constant 0.12 fraction of the optical depth of the
_C2H4 component from _C6H6 gives the points plotted as the dark gray curve with
solid diamonds (C6H6_lnII0_cC2H4). The result is identical to the _C6N2 component
within data noise over the entire altitude range. Both curves show no structure in altitude
down to the sudden change in slope at 590 km. There is an indication of an extinction
layer at 516.6 km. The conclusion is that the _C6N2 photometric component contains
only extinction by the aerosol component referred to earlier as tholin, and the _C6H6
component contains extinction by tholin and C2H4. This is consistent with the spectral
analysis in which upper limits were obtained for C6H6 and C6N2. The results from
Figures 1 & 2 indicate further that there is no measured evidence for C6H6 or C6N2 in
the lsco occ data.
Figures 3 & 4 are the figures for the avir occultation corresponding to 1 & 2 for lsco.
Extraction of the C2H4 contribution to the _C6H6 signal in this case required a fraction
0.15 of the _C2H4 optical depth, in order to match the _C6N2 optical depth curve. In this
case, however, there are two locations where deviations go beyond the noise level, one at
580.5 km, and the other in the region 400-450 km, as shown in Figures 3 & 4. The
deviation at 400-450 km has an inconclusive origin at this time. The apparent extinction
layer at 580.5 km can be interpreted in two possible ways; The layer may be C6H6, or it
may represent a strong change in the partitioning of the higher order species below the
level of C6H6. This can only be resolved by spectral analysis. At 596.3 km the _C6H6
optical depth corrected for the _C2H4 component (_C6H6_lnII0_cC2H4) is nearly
identical to the optical depth of the _C6N2 component (Fig. 3, 4). Figure 5 shows the avir
optical depth spectrum at 596.3 km compared to the lsco spectrum at 537.3 km. Although
the lsco spectrum is about one scale height lower than the avir spectrum, the spectra
contain nearly equal abundances of the higher order hydrocarbons. Figure 6, however,
shows the avir spectrum at 576.5 km which covers the region between 576.5 and 596.3
km. Here the spectrum shows a large increase in the abundance of C2H4 relative to the
other species. The lsco spectrum at 514.4 km shown in Figure 6 shows much less C2H4
and therefore does not have the same property in magnitude of variation in abundance
within a scale height in altitude. The conclusion is that the extinction peak at 580.5 km in
the _C6H6_lnII0_cC2H4 plot in Figures 3 & 4 is caused by an increased partitioning in
C2H4 in absolute amount and relative to other species.
Figure 7 shows the _C2H4 component before and after removal of an estimate of the
tholin contribution to the optical depth, in the lsco occultation. Three extinction peaks are
identified in the figure. The result is consistent with the earlier conclusion that the higher
order hydrocarbon and cyano species are confined in altitude range and the observations
infer that these species are not detectable below about 500 km in this experiment (ie the
observed absorption spectrum at lower altitudes arises in the higher altitude regions along
the line of sight). The primary component responsible for extinction below 600 km is
aerosol. Figure 8 shows the _C2H4 component for both avir and lsco after correction for
the tholin contribution. The evident much deeper extinction in the avir occultation below
600 km apparently is caused by the sharp increase in C2H4 abundance that does not have
an equivalent in the lsco data. Figure 9 gives a comparison of the tholin components in
avir and lsco where strong differences in distribution are evident below 500 km.
Figure 10 shows the spectral analysis for lsco at 546.5 km, providing extracted
abundances for the species. The values for C6H6 and C6N2 are upper limits. The
magnitude of the tholin component assumes the scatterer is the same composition as the
tholin solid in the Khare et al. 1984 paper. The shape of the extinction cross section
derived from the Khare etal. refractive index provides a satisfactory fit to the observed
spectra, but inferred absolute abundance will depend on modeling.
Comparison to Voyager results:
Figure 11 compares the _C2H4 component for avir and lsco with the occultation result
for Voyager (Smith et al, 1982) in the appropriate wavelength range. The altitude scale of
the Voyager data is apparently accurate based on internal consistency of the data. The
Voyager occultations were close to equatorial. The main differences to note are that the
apparent strong extinction by aerosols takes effect about 100 km higher for Voyager and
significantly more absorption is evident for Voyager in the 700 km – 1000 km region.
The Voayger data show a broad extinction maximum near 770 km.
Conclusions:
The high radial resolution photometric data for the TB occultations show results in
general agreement with the earlier spectral analysis. The main conclusions brought
forward by this data reduction are:
1) There are apparent significant differences in partitioning and abundance for the higher
order hydrocarbons between the north latitude (avir) (60o – 47o) and south latitude (-36o)
(lsco) below 850 km. Production of the higher order hydrocarbons appears to stop below
500 km in lsco and 450 km in avir. There is roughly a factor of 3 more higher order
hydrocarbon in avir compared to lsco at 575 km. The north latitude higher order
hydrocarbons appear in a single broad layer, while the south latitude data show three
possible narrow layers extending from 754. to 528. km. The Voyager equatorial results
appear to show a broad layer at 770 km, and an aerosol cut-off near 450 km, ~ 100 km
higher than the Cassini results.
2) Aerosol extinction is dominant above 1840 A at all altitudes. This extinction is
detectable at about 800 km in the transmission data, and dominates all absorbers at all
wavelengths in the fuv except for CH4, below 450 km – 400 km.
3) C6N2, the end point in the chain to formation of aerosols is not detectable in the TB
data. C6H6 is also not detectable. Upper limits have been set.
4) The tholin cross section shape (Khare et al) allows a satisfactorily fit to the observed
spectrum when included with the hydrocarbon and cyano species.
f1159_ww0_lnII0_t
Figure 1
0.1
-0.0
-0.1
_C6N2_lnII0_t
_C6H6_lnII0_t
_C2H4_lnII0_t
_C6H6_lnII0_cC2H4
_CH4_lnII0_t
-0.2
516.7 km
ln(I/I0)
-0.3
-0.4
-0.5
-0.6
-0.7
-0.8
-0.9
-1.0
400
500
600
700
800
h (km)
900
1000
1100
1200
f1159_ww0_lnII0_t
Figure 2
0
ln(I/I0)
-1
_C6N2_lnII0_t
_C6H6_lnII0_t
_C2H4_lnII0_t
_C6H6_lnII0_cC2H4
_CH4_lnII0_t
-2
-3
-4
400
500
600
700
800
h (km)
900
1000
1100
1200
f1140_avir_ww0_lnII0_t
Figure 3
0.1
-0.0
-0.1
_C6N2_lnII0_flt_t
_C6H6_lnII0_flt_t
_C2H4_lnII0_flt_t
_C6H6_lnII0_cC2H4
_Ch4_lnII0_flt_t
-0.2
580.5 km
ln(I/I0)
-0.3
-0.4
-0.5
-0.6
-0.7
-0.8
-0.9
-1.0
300
400
500
600
700
h (km)
800
900
1000
1100
f1140_avir_ww0_lnII0_t
Figure 4
0
ln(I/I0)
-1
-2
_C6N2_lnII0_flt_t
_C6H6_lnII0_flt_t
_C2H4_lnII0_flt_t
_C6H6_lnII0_cC2H4
_Ch4_lnII0_flt_t
-3
-4
300
400
500
600
700
h (km)
800
900
1000
1100
ftb_avir_348_11_40_11_r5_lnII0a_037
Figure 5
0
_037_avir
_027_lsco
-1
ln(I/I0)
-2
037_avir: 596.3 km
027_lsco: 537.3 km
-3
-4
-5
-6
-7
1100
1200
1300
1400
1500
λ (A)
1600
1700
1800
1900
ftb_avir_348_11_40_11_r5_lnII0a_036
Figure 6
0
_036_avir
_026_lsco
-1
ln(I/I0)
-2
036_avir: 576.5 km
026_lsco: 514.4 km
-3
-4
-5
-6
-7
1100
1200
1300
1400
1500
λ (A)
1600
1700
1800
1900
f1150_ww0_lnII0_C2H4_tholin
Figure 7
0
753.5 km
_C2H4_lnII0_cC6N2
_C2H4_lnII0_t
528.2 km
-1
ln(I/I0)
578.7 km
-2
-3
400
500
600
700
800
h (km)
900
1000
1100
1200
avir_lsco_ww0_lnII0_C2H4_tholin
Figure 8
0.0
690.2 km
ln(I/I0)
-1.0
_C2H4_avir_knII0_cC6N2
_C2H4_lsco_lnII0_cC6N2
574.6 km
-2.0
753.5 km
578.7 km
-3.0
528.2 km
-4.0
400
500
600
700
800
h (km)
900
1000
1100
1200
avir_lsco_ww0_lnII0_tholin
Figure 9
0
_C6N2_lsco_lnII0_t
_C6N2_lavir_nII0_t
ln(I/I0)
-1
-2
-3
-4
200
300
400
500
600
h (km)
700
800
900
Figure 10
ftb_lsco_348_11_59_59_R5_lnII0s_027d
Fit to lsco transmission spectrum at h= 546 km
0
-1
-2
h= 546.5 km
N2 =3.8 x 1021
CH4 = 4.20 x 1019
C2H2= 2.50x1017
ln(I/I0)
C2H4=4.20x1016
-3
-4
C2H6=3.50x1016
HCN=1.70x1017
_027x
_042
_c2h2_005
_c2h6_003
_c2h6_013
_hcn_009
_c2h4_010
_ln_027x
_027c_05_lnII0
ln_027d_04_00
C4H2=4.5x1015
-5
-6
C6N2=1.0x1015
C6H6=6.5x1014
Tholin=4.0x1015
-7
-8
1150
1250
1350
1450
1550
λ (A)
1650
1750
1850
1950
avir_lsco_voy_C2H4
Figure 11
0
-1
C2H4_lsco
voy_lnII0_1632
C2H4_avir
ln(I/I0)
-2
-3
-4
-5
-6
300
400
500
600
700
h (km)
800
900
1000
1100