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Phil. Trans. R. Soc. A (2009) 367, 697–711
doi:10.1098/rsta.2008.0164
Published online 20 November 2008
Dynamical implications of seasonal and spatial
variations in Titan’s stratospheric composition
B Y N ICHOLAS A. T EANBY 1, * , P ATRICK G. J. I RWIN 1 , R EMCO
2
AND C ONOR A. N IXON
DE
K OK 1
1
Atmospheric, Oceanic & Planetary Physics, Clarendon Laboratory,
Department of Physics, University of Oxford, Parks Road, Oxford OX1 3PU, UK
2
Department of Astronomy, University of Maryland, College Park,
MD 20742, USA
Titan’s diverse inventory of photochemically produced gases can be used as tracers to
probe atmospheric circulation. Since the arrival of the Cassini–Huygens mission in July
2004 it has been possible to map the seasonal and spatial variations of these compounds
in great detail. Here, we use 3.5 years of data measured by the Cassini Composite
InfraRed Spectrometer instrument to determine spatial and seasonal composition trends,
thus providing clues to underlying atmospheric motions. Titan’s North Pole (currently in
winter) displays enrichment of trace species, implying subsidence is occurring there. This
is consistent with the descending branch of a single south-to-north stratospheric
circulation cell and a polar vortex. Lack of enrichment in the south over most of
the observed time period argues against the presence of any secondary circulation
cell in the Southern Polar stratosphere. However, a residual cap of enriched gas was
observed over the South Pole early in the mission, which has since completely dissipated.
This cap was most probably due to residual build-up from southern winter. These
observations provide new and important constraints for models of atmospheric
photochemistry and circulation.
Keywords: Titan; atmosphere; Cassini
1. Introduction
Saturn’s largest moon, Titan, has the most substantial atmosphere of any
planetary satellite in our Solar System. Titan’s atmosphere has a surface pressure
of approximately 1.5 bar and is composed mainly of molecular nitrogen (95–99%)
and methane (1–5%). The atmospheric temperature structure is similar to the
Earth’s, although much colder, with a well-defined tropopause and stratopause
(figure 1). Temperatures range from 70 K at the tropopause up to approximately
220 K in the winter stratosphere. Titan’s low gravity and high surface pressure
mean that its atmosphere is highly extended vertically.
*Author for correspondence ([email protected]).
One contribution of 14 to a Discussion Meeting Issue ‘Progress in understanding Titan’s
atmosphere and space environment’.
697
This journal is q 2008 The Royal Society
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698
200
1
10
10 2
10 3
300
stratosphere
100
tropopause
troposphere
0
80 120 160 200 240
temperature (K)
(c) 10 –3
10 –2
400
10 –2
10 –1
300
1
200
10
100
10 2
500
400
300
10 –1
200
1
100
10
altitude (km)
stratopause
400
500
pressure (mbar)
10 –1
mesosphere
10 –3
altitude (km)
pressure (mbar)
10 –2
(b)
500
pressure (mbar)
10 –3
altitude (km)
(a)
N. A. Teanby et al.
10 2
10 3
0
80 120 160 200 240
temperature (K)
10 3
0
80 120 160 200 240
temperature (K)
Figure 1. Example temperature profiles at representative latitudes for the Cassini epoch based on
Cassini CIRS spectra (stratosphere and mesosphere) and additionally the Voyager radio
occultation profile from Lindal et al. (1983) for the lower atmosphere. Most trace species condense
at altitudes close to the cold (70 K) tropopause. Note the hot stratopause near the North (winter)
Pole. (a) 708 S, (b) 08 N and (c) 708 N.
Titan’s orbit lies in Saturn’s equatorial plane, so it has an axial tilt (obliquity)
of 26.7 8 to the ecliptic and a year equal to 29.46 Earth years, the same as Saturn.
Therefore, Titan experiences seasons and numerical models predict different
atmospheric circulation regimes throughout the Titan year as the point of
maximum solar heating shifts in latitude (Hourdin et al. 1995; Tokano et al.
1999; Luz et al. 2003; Hourdin et al. 2004; Rannou et al. 2005; Richardson et al.
2007). This results in a stratospheric circulation that switches from two
symmetrical Hadley cells at the spring and autumn equinoxes, to a circulation
dominated by one large hemisphere-to-hemisphere Hadley cell during interequinox periods. The stratosphere appears to have zonally symmetric
temperature (Flasar et al. 1981, 2005) and composition (Teanby et al. 2006,
2008) structures—with very little longitudinal variation. This suggests that nonsymmetric atmospheric features such as planetary waves, which are important
for creating non-zonal structures on the Earth, are less significant on Titan.
Titan has a very photochemically active atmosphere. Solar UV photons and
magnetospheric electrons break apart nitrogen and methane molecules to form
radicals such as CH3 and N (Vuitton et al. 2007). These radicals combine to form
more complicated photochemical daughter products, which in turn are broken up
again to form even more complex species. The result is a rich atmospheric
chemistry, with a vast array of trace gases. Titan’s haze, which gives it an orange
appearance at visible wavelengths, is also a product of this photochemistry. Most
trace species are created in the upper atmosphere above 500 km (Wilson &
Atreya 2004) and infiltrate lower altitudes by eddy diffusion. Eventually, most
compounds condense near the tropopause cold trap and are removed from the
stratosphere. This source–sink arrangement should result in stratospheric
abundance profiles of trace species and haze that increase with increasing
altitude, towards the production zone.
Most of Titan’s trace gases have spectral features in the mid-infrared.
Therefore, infrared remote sensing, either from spacecraft or ground-based
telescopes, provides an excellent way to determine the global distribution of
these gases. It is also possible to measure the composition directly using mass
spectrometers, such as Cassini’s INMS instrument (Waite et al. 2005) and the
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Titan’s stratospheric composition
radiance (nW cm–2 sr –1/cm–1)
800
(a)
C2H2
600
400
200
0
600
100
80
60
40
20
0
HC3N
C4H2
CO2
C3H4
650
HCN
700
C2H6
C3H8
750
(b)
800
850
CH4
C2H4
CH3D
900
950
1000 1050 1100 1150 1200 1250 1300 1350 1400
wavenumber (cm–1)
Figure 2. (a,b) Synthetic spectrum of Titan in the mid-IR at CIRS maximum spectral resolution
(0.5 cmK1) showing the vast array of emission features from trace species in the stratosphere.
(Note on units: x cmK1Z10 000/x mm.)
Huygens probe GCMS (Niemann et al. 2005). These direct samplings provide
important constraints on atmospheric composition, but do not provide global
coverage—this requires infrared remote sensing. Figure 2 shows an example of
Titan’s mid-IR spectrum and shows the vast array of compounds present in the
atmosphere. Emission features in this region of the spectrum are produced in
the middle and upper stratosphere in the 10–0.1 mbar pressure range and can
thus be used to determine stratospheric temperature and composition.
The rich photochemistry of Titan is extremely interesting in its own right, as it
has important implications for the Earth’s early atmosphere, which is believed to
have had a similar composition. However, in this paper we concentrate on exploiting
variations in composition across Titan’s disc to probe its equally fascinating
atmospheric circulation. Because trace gases have a vertical gradient, they can be
used as tracers of vertical motion in the atmosphere, as illustrated in figure 3.
Upward motion of the atmosphere advects tracer-poor air from the lower
atmosphere, and as a consequence of the vertical profile leads to a decrease in
abundance at each atmospheric level. Conversely, subsidence advects tracer-rich air
from the upper atmosphere to lower levels and leads to an increase in abundance. In
this paper, we use this simple consequence of Titan’s photochemistry to probe
atmospheric circulation. Owing to the zonal symmetry of Titan’s atmosphere,
variations in composition with latitude are the most significant.
The first observations of latitudinal composition variations were from the
Voyager I flyby of Titan in 1980 (Coustenis & Bézard 1995). These observations
showed an enrichment in most species near the North Pole, which at that time was
experiencing early spring. The next observations of composition variations were by
Roe et al. (2004) using the Keck I telescope, which showed an enrichment of C2H4
over the South Pole from data taken in 1999–2002 (late northern autumn). A vast
increase in our understanding of Titan’s atmosphere was made possible by the
arrival of the Cassini–Huygens spacecraft at the Saturnian system in July 2004.
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(a)
(b)
altitude
altitude
N. A. Teanby et al.
volume mixing ratio
volume mixing ratio
Figure 3. Consider a planet with a hypothetical atmospheric circulation. If, as on Titan, trace
species abundance (or volume mixing ratio) increases with altitude then advection by vertical
atmospheric circulation leads to: (a) depletion for upwelling motion and (b) enrichment for
subsiding motion at a particular atmospheric level. Dashed lines show the initial gas profile and
solid lines show the advected profile due to a circulation in the direction of the arrow. The variation
of trace species with latitude then gives a diagnostic for the circulation.
The Composite InfraRed Spectrometer (CIRS) instrument on-board the Cassini
orbiter has so far provided millions of far- and mid-IR spectra that can be used to
determine the detailed composition of Titan’s atmosphere. So far, latitudinal
variations in composition using CIRS data have been determined by Flasar et al.
(2005), Teanby et al. (2006), Coustenis et al. (2007), de Kok et al. (2007a) and
Teanby et al. (2008). De Kok et al. (2007b) discussed variations in Titan’s
photochemical hazes. These studies all found enrichment of trace species or haze
near the northern winter pole. Wind velocities derived from CIRS spectra by Flasar
et al. (2005) and Achterberg et al. (2008) have indicated the presence of a North
Polar vortex. This is corroborated by the composition mapping from Teanby et al.
(2007). Therefore, current Cassini observations are consistent with subsidence
occurring inside a North Polar vortex, leading to enrichment of trace species.
Vertical profiles of many of Titan’s trace species and hazes were determined by
de Kok et al. (2007a,b), Teanby et al. (2007) and Vinatier et al. (2007). These
studies confirm that around the equator, the abundance of trace gases and haze
increases with altitude. This corroborates disc-averaged ground-based studies in
the sub-mm regime (e.g. Hidayat et al. 1997; Marten et al. 2002; Gurwell 2004;
Livengood et al. 2006), which also inferred abundance profiles that increased with
altitude. Near the North Pole the profiles become more complicated (Teanby
et al. 2007; Vinatier et al. 2007).
Seasonal changes in composition were first noted at mid-northern latitudes by
Roe et al. (2004) who compared ground-based C2H4 determinations with those
from Voyager I. This was the first confirmation that Titan’s atmospheric
composition is indeed seasonally dependent. Comparison of Cassini and Voyager
observations shows that some species were more enriched at the Voyager epoch
(early northern spring) than at the Cassini epoch (northern winter). This also
suggests seasonal variations. However, direct comparison of the results from
different instruments requires us to assume that all instrument-related offsets
have been accounted for. The first determination of seasonal variations using a
single instrument (Cassini CIRS) was presented by Teanby et al. (2008), which
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Titan’s stratospheric composition
701
showed a trend for decreasing trace species abundance in the south. This study
has the advantage that it is based on continuous observations from one
instrument, but has the disadvantage that it only covers 2.5 years or one-twelfth
of a Titan year. In this paper, we extend the coverage to 3.5 years (almost the
entire prime mission) and include variations of additional gases, CO2, C2H4 and
C2H6. The results are used to investigate seasonal and latitude variations in
composition and probe general features of Titan’s atmospheric circulation.
2. Cassini CIRS observations and analysis
The CIRS instrument is described in detail by Kunde et al. (1996) and Flasar et al.
(2004). Briefly, CIRS has three focal planes covering three different spectral
regions: FP1 (10–600 cmK1); FP3 (600–1100 cmK1); and FP4 (1100–1400 cmK1).
The spectral resolution is adjustable to have an apodized full-width half-maximum
of 0.5–15 cmK1, but is generally used at 0.5, 2.5 and 15 cmK1 resolutions.
In this paper, we use 2.5 cmK1 resolution mapping and 0.5 cmK1 resolution singleswath nadir (downward looking) observations, which have sufficient spectral
resolution to resolve individual gas peaks. Only data from FP3 and FP4 are used
as these have better spatial resolution than far-IR FP1 data. Examples of
latitude/longitude coverage for both observation types are shown in fig. 1 of Teanby
et al. (2008). Typically, observations have a spatial resolution of 2–68 latitude.
Using two data resolutions provides complementary information and acts as a
consistency check on the results. The 2.5 cmK1 mapping observations have better
spatial and temporal coverage, whereas the 0.5 cmK1 swath observations contain
more altitude information on the temperature structure and have better signal to
noise because a sit-and-stare viewing geometry allows many more spectra to be
taken at each location.
Composition mapping by Teanby et al. (2006, 2008) shows little variation with
longitude. Therefore, to increase signal to noise, data were averaged into 108 wide
latitude bins with restricted emission angle range following Teanby et al. (2008).
The data cover 39 flybys for almost the full 4 years of Cassini’s prime mission and
are summarized in table 1.
The method used to determine atmospheric temperature and composition is
described in detail in Teanby et al. (2008). In summary, we use a constrained
iterative nonlinear inversion technique (Irwin et al. 2008) based on the correlated-k
approximation (Lacis & Oinas 1991). The inversion is a two-stage process.
First, inverting for a continuous temperature profile using the n4-band of methane
(1240–1360 cmK1). Second, fixing the temperature and inverting for gas
abundances using the 600–1000 cmK1 spectral region and assuming constant
gas volume mixing ratios above the condensation level.
In addition to the gases studied by Teanby et al. (2008) we also invert for C2H4
and C2H6 abundance using 790–1000 cmK1. For this region of the spectrum, we
found that a constant infrared haze cross section with wavenumber gave the best
fit to the data. For other spectral regions, a cross section generated using the
refractive indices of Khare et al. (1984) was used. Spectral data for gases and
haze are as described in Teanby et al. (2008) and de Kok et al. (2007b), except for
C2H6, for which we use updated line parameters from Vander Auwera et al.
(2007). Figure 4 shows example fits to the observed spectra.
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N. A. Teanby et al.
Table 1. Cassini CIRS observations used for this study. (S.S.Lat., sub-spacecraft latitude; N,
number of spectra in each observation; Res., spectral resolution. Latitude range is the usable range
for composition determinations.)
date
orbit
flyby
Res. (cmK1)
S.S.Lat. (8 N)
lat. range (8 N)
N
11
22
17
10
23
24
26
11
14
26
08
24
27
28
14
28
18
04
21
21
Dec 2004
Jun 2005
Mar 2006
Oct 2006
Oct 2006
Nov 2006
Dec 2006
Jan 2007
Jan 2007
Jan 2007
Mar 2007
Mar 2007
Mar 2007
Mar 2007
May 2007
May 2007
Jul 2007
Dec 2007
Dec 2007
Dec 2007
00B
010
022
030
031
033
036
037
037
038
040
041
041
041
044
045
048
053
054
054
TB
T22
T23
T23
T24
T26
T27
T27
T27
T30
T31
T34
T38
T39
T39
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
K9
K38
K0
K26
27
86
37
44
K49
50
K48
K40
36
35
14
12
0
K6
19
19
0
K77
K60
K81
K32
39
K23
K10
K82
K0
K82
K82
15
27
12
58
32
78
65
83
56
1
77
4
21
29
K46
7
K53
K58
46
16
71
68
K33
K42
55
36
5505
3129
3258
895
3185
2537
2344
2385
480
1367
929
564
3679
1362
1744
299
485
1120
550
1470
02
12
01
22
28
27
14
21
02
06
22
22
08
24
11
27
12
12
13
29
30
09
25
11
28
Jul 2004
Dec 2004
Apr 2005
Aug 2005
Oct 2005
Dec 2005
Jan 2006
May 2006
Jul 2006
Sep 2006
Sep 2006
Sep 2006
Oct 2006
Oct 2006
Dec 2006
Dec 2006
Jan 2007
Jan 2007
Jan 2007
Jan 2007
Jan 2007
Mar 2007
Mar 2007
Apr 2007
Apr 2007
000
00B
005
013
017
019
020
024
025
028
029
029
030
031
035
036
037
037
037
038
038
040
041
042
043
T0
TB
T4
T6
T8
T9
T10
T14
T15
T17
T18
T18
T19
T20
T21
T22
T23
T23
T23
T24
T24
T26
T27
T28
T29
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
K67
K9
K3
K8
0
0
K0
K0
0
9
13
12
21
28
31
38
45
45
K50
K56
K54
K49
K43
30
22
K83
K67
K62
K67
K57
K58
K58
K56
K58
K51
K47
K47
K40
K32
K28
K24
K17
39
K84
K83
K83
K83
K83
K28
K37
16
45
56
51
58
57
58
61
61
68
72
71
81
84
84
84
51
84
8
1
2
K44
K34
84
81
4618
2424
2224
2123
1910
3941
2747
3256
1862
2096
1705
3420
4542
3640
1251
3274
870
1905
1477
3637
2844
1733
2165
2356
1480
T12
T19
T20
(Continued.)
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Titan’s stratospheric composition
Table 1. (Continued.)
date
orbit
flyby
Res. (cmK1)
S.S.Lat. (8 N)
lat. range (8 N)
12
13
13
14
18
17
31
31
02
03
21
19
17
05
21
044
044
044
046
048
048
049
049
050
050
051
052
052
053
054
T30
T30
T30
T32
T34
T34
T35
T35
T36
T36
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
K21
15
14
2
0
0
K1
K3
2
2
15
5
K2
12
19
K81
K43
K46
K4
K58
K58
K31
K62
K57
K57
K46
K56
K62
K47
K39
May 2007
May 2007
May 2007
Jun 2007
Jul 2007
Jul 2007
Aug 2007
Aug 2007
Oct 2007
Oct 2007
Oct 2007
Nov 2007
Nov 2007
Dec 2007
Dec 2007
T37
T37
T38
T39
38
72
73
61
58
57
31
56
62
56
76
66
57
71
71
N
1199
1124
1883
2083
2274
1325
679
1825
2881
3926
5478
3858
2340
6913
2654
3. Latitude variations
Figure 5 shows the variation in trace gas abundance as a function of latitude. In this
plot, the results from all flybys have been averaged together using a cubic spline fit
(Teanby 2007). All species (except CO2) show an increase in abundance towards the
northern winter pole. This is in agreement with previous Cassini CIRS results and is
consistent with subsiding air over the pole. Composition cross sections (Teanby et al.
2007) along with derived thermal winds (Flasar et al. 2005; Achterberg et al. 2008)
indicate the presence of a polar vortex at 25–558 N encircling the North Pole at this
time. The vortex acts to separate enriched from unenriched air. As would be
expected, this separation is most effective for short lifetime gases such as HC3N and
C4H2—long lifetime gases such as HCN have a more diffuse variation as they have
time to escape the vortex. The fact that enrichments are strongest at 908 N implies
that subsidence is strongest in the vortex core.
One striking difference between model predictions and the CIRS observations is
that the massive enrichment of trace species predicted by numerical models in the
south (e.g. Rannou et al. 2005) is not present in the data. In the models, enrichment
is caused by a residual ‘cap’ left over from southern winter combined with a small
secondary stratospheric circulation cell rising at approximately 608 S and
descending at the South Pole. The discrepancy between models and measurements
indicates the following. (i) There is no secondary stratospheric circulation cell near
the South Pole at this time for levels above 10 mbar (where our data probe), and
Titan’s stratospheric circulation is best described by a single south–north Hadley
cell. This is consistent with the lower-than-expected stratospheric temperatures at
the South Pole observed by Flasar et al. (2005), which also suggests upwelling in
this region. (ii) Any residual South Polar enrichment left over from southern winter
has mostly dissipated. Gross offsets between the data and the model are not critical
and can be reduced by adjusting the haze distribution and photochemical
production rates (Crespin et al. 2008).
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N. A. Teanby et al.
(a)
(b)
60
40
20
radiance (nW cm –2 sr –1 / cm –1)
0
1250
(c)
1300
1350
1250
(d )
1300
1350
400
200
0
200
630
(e)
660
690
720
630
(f)
660
690
720
100
0
800
850
900
950
wavenumber (cm –1)
1000
800
850
900
950
wavenumber (cm –1)
1000
Figure 4. Example fits (solid line) to measured spectra (grey error envelope) at 508 N. The lines at the
top of each figure show the difference between measured and fitted spectra along with the error
envelope—shifted for clarity. (a, c, e) The 0.5 cmK1 resolution data; (b, d, f ) the 2.5 cmK1 resolution
data. First, the 1240–1360 cmK1 spectral region was used to determine temperature. Second,
composition was determined by fixing temperature and using the 600–1000 cmK1 region, which is rich
in trace species emission features.
Trace gas enrichments allow us to test our assumption that vertical profiles
are a result of photochemical production and vertical mixing. If this assumption is
correct then species with shorter photochemical lifetimes should have steeper
gradients, as they decay before mixing to lower altitudes occurs. This implies that
the species with the shortest lifetime should be enriched by the largest factor in the
north relative to equatorial regions. We test this simple idea by plotting the
photochemical lifetime from Wilson & Atreya (2004) against the relative enrichment at 60, 70 and 808 N in figure 6. The hydrocarbons and carbon dioxide do
indeed display the expected behaviour—with an almost linear trend in log–log
space. However, the nitriles HCN and HC3N have relatively greater enrichment,
which implies steeper profiles than expected from the photochemistry alone.
Vinatier et al. (2007) also found that the HCN vertical profile was too steep to be
explained by photochemical production and eddy mixing. This suggests that there
is an additional sink for nitriles in the lower atmosphere—perhaps incorporation into photochemical hazes (Lebonnois et al. 2002; Wilson & Atreya 2003).
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Titan’s stratospheric composition
8 ×10 –6
5×10 –8
C2H2
4×10 –8
6 ×10 –6
3×10 –8
4 ×10 –6
2×10 –8
2 ×10 –6
1×10 –8
0
1.5×10 –6
0
1.5×10 – 6
C2H4
1.0×10 –6
1.0×10 – 6
5.0×10 –7
5.0×10 –7
0
2.0×10 –5
C4H2
HCN
0
C2H6
6×10 –8
HC3N
1.5 ×10 –5
4×10 –8
1.0×10 –5
2×10 –8
5.0×10 –6
0
4×10 –8
0
C3H4
CO2
2×10 –8
3×10 –8
2×10 –8
1×10 –8
1×10 –8
0
–90 –60
–30
0
30
latitude
60
90
0
–90 –60 –30
0
30
latitude
60
90
Figure 5. Latitude variations and error envelopes of trace species abundance averaged over the
whole 3.5-year observation period. The 0.5 (black solid curve) and 2.5 cmK1 (grey solid curve)
results are consistent to within errors, acting as a consistency check. All species show an increase in
abundance towards the North Pole, due to subsidence within the North Polar vortex. Short lifetime
species with steep vertical profiles, such as HC3N, show the greatest enrichment. Triangles show
the Voyager I results of Coustenis & Bézard (1995) and dashed lines show predictions of the
Rannou et al. (2005) numerical model for comparison.
4. Seasonal variations
The 4 years of Cassini’s prime mission provides us with a self-consistent dataset
with which to probe seasonal variations in atmospheric composition covering just
over half a Titan season (early–mid northern winter). However, due to Cassini’s
changing orbital geometry the temporal coverage is not identical for all latitudes.
In particular, high northern latitudes were only visible during the 1808 orbital
transfer (mid-2006 to mid-2007). Looking for seasonal variations at northern
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enrichment factor
10 3
10 2
HC3N
C4H2
10
HCN
1
103
C2H6
CO2
102
C2H2
C3H4
C2H4
10
1
lifetime (years)
10 –1
10 –2
Figure 6. Enrichment at three latitudes within the polar vortex with respect to the equator plotted
against species lifetime from Wilson & Atreya (2004). Hydrocarbons and CO2 show a roughly
linear trend, indicating that the vertical gradient of these species is inversely related to the species
lifetime, as expected. The nitriles (HCN and HC3N) show greater enrichment, indicating that the
actual gradient is steeper than expected. C2H4 lies slightly off-trend as it does not condense at the
tropopause, so it is not expected to have a simple source–sink type profile. Squares, 808 N/08 N;
triangles, 708 N/08 N; circles, 608 N/08 N.
latitudes is also complicated by the presence of large temperature and
composition gradients. Therefore, the best place to look for seasonal variations
with the current CIRS dataset is at southern latitudes. Figure 7 shows the
variation of each species at 558 S along with those predicted from the numerical
model of Rannou et al. (2005). This latitude is the furthest south we can probe
while maintaining a well-sampled time series.
The data show a steady decrease in abundance of C2H2, C2H4, C3H4 and HCN until
early 2006, when the southern stratospheric composition stabilizes. C4H2 and HC3N
have very low abundance at southern latitudes and are hence very noisy. Over the same
period, the temperature at 3 mbar increases by approximately 2 K. This proves that
Titan’s atmospheric temperature and composition are not in a steady state. The
reduction in abundances could be due to dissipation of trace species built up at the
South Pole by subsidence during southern winter and/or current upwelling at southern
latitudes. Models also predict that abundances and temperatures should decrease and
increase, respectively. However, for most species, the observed abundance decreases
are less than predicted by the model, especially after early 2006, when the data show
Titan’s atmospheric composition to be stable. This implies that residual southern
winter build-up had largely dissipated before the arrival of Cassini, with the last stage
occurring around early 2006. Note that, as would be expected, long lifetime species
such as CO2 and C2H6 show no significant seasonal variation.
Figure 5 shows that for most gases the variations with latitude are similar to
those derived from the Voyager flyby, although greater enrichment was seen at
the Voyager epoch for HCN and C2H4. However, CIRS is observing Titan earlier
in the seasonal cycle and gas concentrations may continue to increase as the
winter season progresses and subsidence continues. Another possibility is that
the difference is due to the changes in the haze vertical distribution between
Voyager and Cassini epochs (Porco et al. 2005)—if HCN is indeed important for
haze formation then haze and HCN abundances will be linked.
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Titan’s stratospheric composition
160
(b)
80
70
60
50
40
30
20
10
0
emission angle (deg.)
temperature (K)
(a)
155
150
(c)
(d )
2×10 –9
(e)
C4H2
C2H2
2×10 – 6
10 – 6
0
0
(f)
2×10 – 8
CO2
3×10 –7
C2H4
10 –9
2×10 –7
10 – 8
10 –7
1.5×10 –7
0
C3H4
(i )
(h)
10 –5
HCN
10 –7
5×10 – 6
0
( j)
10 –8
5×10 – 8
0
10 –9
HC3N
C2H6
(g)
5×10 –9
0
5×10 –10
0
2005
2006
2007
year
2008
2005
2006
2007
year
2008
Figure 7. Time variations of (a) temperature (at 3 mbar), (b) observation emission angle and
(c–j ) composition at 558 S covering 3.5 years of Cassini’s 4-year prime mission. In this time, the
temperature has increased by approximately 2 K and moderate lifetime gases (c) C2H2,
(e) C2H4, (i ) C3H4 and (h) HCN have reduced in abundance. Short lifetime gases (d ) C4H2 and
( j ) HC3N have low abundance at this latitude and variations are noisy. Long lifetime species
(g) C2H6 and ( f ) CO2 do not show significant variation. Depletion could be due to upwelling or
dissipation of polar enrichment built up during southern winter. Dashed line shows the model of
Rannou et al. (2005) for comparison, normalized to coincide with the first datapoint. Squares,
0.5 cmK1 data; circles, 0.5 cmK1 data.
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3×10 – 6
5×10 –7
C2H2
4×10 –7
2×10 – 6
2×10 –7
C2H6
5×10 – 6
10 –7
0
0
C3H4
3×10 –9
5×10 –9
0
–90
10 –5
3×10 –7
10 – 6
10 – 8
C2H4
– 60 –30
latitude
0
0
C4H2
HCN
1.5×10 –7
2×10 –9
10 – 7
10 –9
5×10 – 8
0
–90
– 60 –30
latitude
0
0
–90
–60 –30
latitude
0
Figure 8. Comparison of abundance from maps taken on 2 July 2004 (black solid curve) and 29
January 2007 (grey solid curve). The two maps have similar viewing geometries, so the results are
directly comparable. C2H2, C2H4, C3H4 and C4H2 show an initial enriched cap over the South Pole
in 2004, which is no longer present in 2007. HCN, having a longer lifetime, is too diffused to resolve
the cap.
During Cassini’s orbital insertion in July 2004 it flew directly over the South
Pole. This happened again in January 2007 during the 1808 orbital transfer.
These two datasets have the same viewing geometry and allow us to look for
dissipation of any South Polar build-up (figure 8). C2H2, C2H4, C3H4 and C4H2
all show a slight residual cap of enriched gas over the South Pole in 2004, which
is not present in 2007. This cap has not been recognized before in the CIRS data.
In addition, South Polar C2H4 has been measured from ground-based
measurements by Roe et al. (2004), who reported an enrichment of at least a
factor of five over the South Pole during 1999–2002. In the CIRS data, we see a
factor of two enrichment in 2004 and no enrichment by 2007. These
observations would be consistent with a slowly dissipating South Polar buildup due to upwelling or photochemical decay, which are both consistent with the
absence of a secondary circulation cell in the middle stratosphere. The time scale
of these dissipations provides a powerful constraint for dynamical and
photochemical models.
5. Conclusions
The first 3.5 years of data from Cassini CIRS has been used to determine the
composition of Titan’s atmosphere as a function of latitude and time for early–mid
northern winter. If we assume that trace gas abundances generally increase with
altitude due to high-altitude photochemical production combined with condensation at the tropopause, then subsidence leads to enrichment and upwelling leads
to depletion in the stratosphere where gas emission lines are formed. This simple
idea has been used to probe the atmospheric circulation, summarized as follows.
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Titan’s stratospheric composition
709
Trace species are more abundant in the north. This implies that subsidence is
occurring there during the present epoch.
The relative enrichment of trace species increases with decreasing photochemical lifetime. This implies that shorter lifetime species have steeper vertical
gradients, in keeping with ideas about processes that define the profile shapes.
HCN and HC3N have greater enrichment than hydrocarbons with comparable
lifetimes, implying that these gases have a steeper vertical profile than expected
from photochemistry alone. Perhaps this is due to an additional sink in the lower
atmosphere such as incorporation into hazes.
Contrary to numerical model predictions, there is no enrichment of trace gases
in the Southern Hemisphere in the time-averaged abundance measurements.
This is a new constraint from Cassini (Voyager did not cover high southern
latitudes) implying that there is no secondary stratospheric cell and that Titan’s
stratospheric circulation is best described as a single south–north Hadley cell. It
also implies that any residual trace gas build-up from southern winter has now
largely dissipated.
However, in the individual flyby data from July 2004 there is evidence for an
enriched South Polar cap, most probably a residual from subsidence in southern
winter, although the enrichment is much less than predicted by models. By early
2007, this polar cap had fully dissipated. Observations at 558 S indicate that a
stable atmospheric composition at temperate southern latitudes is reached by
early 2006.
The abundance of trace species at 558 S has decreased from 2004 to 2008,
consistent with upwelling in the south at present and/or dissipation of southern
winter build-up.
The rate of these seasonal variations provides important new constraints for
dynamical models of Titan’s atmosphere.
We gratefully acknowledge the financial support from the UK Science and Technology Facilities
Council. The authors also thank the Cassini CIRS instrument team for making this research possible.
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