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Phil. Trans. R. Soc. A (2009) 367, 683–695
doi:10.1098/rsta.2008.0186
Published online 18 November 2008
REVIEW
Composition and chemistry of
Titan’s stratosphere
B Y B RUNO B ÉZARD *
LESIA, Observatoire de Paris, CNRS, 92195 Meudon, France
Our present knowledge of the composition and chemistry of Titan’s stratosphere is
reviewed. Thermal measurements by the Cassini spacecraft show that the mixing ratios
of all photochemical species, except ethylene, increase with altitude at equatorial and
southern latitudes, reflecting transport from a high-altitude source to a condensation
sink in the lower stratosphere. Most compounds are enriched at latitudes northward of
458 N, a consequence of subsidence in the winter polar vortex. This enrichment is
much stronger for nitriles and complex hydrocarbons than for ethane and acetylene.
Titan’s chemistry originates from breakdown of methane due to photodissociation in
the upper atmosphere and catalytical reactions in the stratosphere, and from destruction
of nitrogen both by UV photons and electrons. Photochemistry also produces haze
particles made of complex refractory material, albeit at a lower rate than ethane,
the most abundant gas product. Haze characteristics (vertical distribution, physical and
spectral properties) inferred by several instruments aboard Cassini/Huygens are
discussed here.
Keywords: Titan; stratosphere; composition; haze; chemistry
1. Introduction
Titan’s stratosphere harbours a suite of hydrocarbons, nitriles and oxygen
compounds produced by a complex photochemistry. It also contains most of the
mass of organic haze, an important component of Titan’s chemistry. For many
photochemical species, the stratosphere is a transition region between the upper
atmosphere (above 600 km) where they are formed and their condensation sink in
the region 60–100 km. However, some chemistry is still at work in the stratosphere,
in particular due to long-wavelength UV photons, which can penetrate down to
these levels, photodissociate some compounds (e.g. C2H2, HC3N), and produce
reactive radicals.
The stratosphere extends from the tropopause (approx. 40 km) up to the
stratopause, a temperature maximum located near 320 km at mid-latitudes. Its
radiative balance results from absorption of solar radiation by methane and
*[email protected]
One contribution of 14 to a Discussion Meeting Issue ‘Progress in understanding Titan’s
atmosphere and space environment’.
683
This journal is q 2008 The Royal Society
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684
radiance (mW m–2 sr –1/cm–1)
B. Bézard
7 (a)
C2H2
6
5
C4H2
4
HCN
HC3N
3
C3H8
CO
2
C3H4
2
C6H6
1
0
600
650
700
750
1.0
C2H6
C2H4
800
850
900
950
1000
(b)
CH4
0.8
0.6
0.4
CH3D
C2H6
0.2
0
1050
1100
1150
1200
1250 1300 1350
wavenumber (cm–1)
1400
1450
1500
Figure 1. An average of CIRS limb spectra recorded at altitudes between 100 and 200 km and
latitudes in the range 55–908 N. Emission bands from various compounds are indicated. (a) FP3
and (b) FP4.
haze particles and thermal cooling by these two agents and mostly C2H6, C2H2
and hydrogen cyanide (HCN). This region is probed mainly through thermalinfrared measurements beyond approximately 7 mm.
2. Gas composition
The Composite InfraRed Spectrometer (CIRS) aboard the Cassini spacecraft is
currently mapping Titan’s thermal emission using three focal planes (FP1, FP3
and FP4) that cover altogether the 10–1500 cmK1 range (1000–7 mm). Figure 1
shows the spectral signatures that can be detected in FP3 and FP4. Note the n4
band of methane centred at 1305 cmK1 in FP4, which is used to retrieve the
stratospheric temperature profile. Nadir (or more generally surface-intercepting)
observations provide information on the temperature profile from 130 to 250 km
and on the mean gas abundances in a broad region usually located near 120 km.
Limb-viewing geometry allows us to extend the sounding up to approximately
500 km, i.e. in the lower mesosphere. Gas mixing ratio profiles are retrieved in
two steps (Vinatier et al. 2007a). First, the temperature profile above the 5 mbar
level is derived from the inversion of the n4 methane band observed in nadir and
limb spectra. At levels deeper than approximately 10 mbar, temperatures
measured in situ by the Huygens/HASI experiment (Fulchignoni et al. 2005) are
used. This temperature profile is then incorporated into the atmospheric model
and gas vertical profiles are retrieved by inversion of nadir and limb spectra in
specific spectral intervals that contain their spectral signatures.
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Review. Composition of Titan’s stratosphere
685
Table 1. Stratospheric composition: mid-latitude mixing ratios at approximately 120 km.
hydrocarbons
CH4
C2H6
C2H2
C2H4
C3H8
CH3C2H
C4H2
C6H6
H2
nitriles
1.4%; Niemann
et al. (2005)
10 ppm; Vinatier
et al. (2007a)
2 ppm; Vinatier
et al. (2007a)
0.4 ppm; Vinatier
et al. (2007a)
0.5 ppm; Vinatier
et al. (2007a)
8 ppb; Vinatier
et al. (2007a)
1 ppb; Vinatier
et al. (2007a)
0.4 ppb; Coustenis
et al. (2007)
0.1%; Courtin
et al. (2007)
HCN
CH3CN
HC3N
C2N2
oxygen
compounds
0.1 ppm; Vinatier CO
et al. (2007a)
20 ppb; Marten
CO2
et al. (2002)
1 ppba; Teanby
H2O
et al. (2006)
1 ppbb; Teanby
et al. (2006)
47 ppm; de Kok
et al. (2007a)
16 ppb; de Kok
et al. (2007a)
0.4 ppb; Coustenis
et al. (1998)
noble gases
36
Ar
40
Ar
0.3 ppm; Niemann
et al. (2005)
43 ppm; Niemann
et al. (2005)
a
At 300 km (disc-averaged).bAt 608 N.
Table 1 lists the compounds that have been detected in Titan’s stratosphere.
The most abundant hydrocarbon after methane is ethane (C2H6) with a mixing
ratio of 10 ppm at 120 km and mid-latitudes. As expected, the abundance
decreases with the number of C-atoms and thus the complexity of the molecule.
Also, for a given number of C-atoms, saturated species are more abundant than
unsaturated ones. The heaviest compound detected is benzene (C6H6). Hydrogen
is a by-product of methane photodissociation and its abundance probably results
from a balance between production and loss by atmospheric escape. HCN is the
most abundant nitrile, followed by acetonitrile (CH3CN), which has only been
observed from ground-based radiotelescopes. Cyanogen (C2N2) is only seen at
high northern latitudes. Oxygen compounds are also present, carbon monoxide
(CO) being by far the most abundant. Water vapour was detected from
rotational lines observed with the Infrared Space Observatory (ISO).
Figure 2 displays the vertical profiles of HCN and hydrocarbons that have
been inferred from two CIRS observational sequences at 158 S and 808 N. This
work has recently been extended to two additional sequences at 548 N and S and
two additional molecules (HC3N and CO2; Vinatier 2007). At southern and midnorthern latitudes, the mixing ratios of all species except ethylene (C2H4)
increase with altitude. This simply results from their production in the upper
atmosphere (higher than 500 km) and condensation loss in the lower stratosphere
(60–100 km) that together maintain a positive vertical concentration gradient.
This gradient is steeper for HC3N, C4H2 and HCN than for other molecules
(C2H2, C2H6), which probably results from chemical losses in the stratosphere.
The unique behaviour of ethylene, whose mixing ratio decreases with height,
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686
B. Bézard
500
(b) 10–3
10–2
400
10–2
400
10–1
300
10–1
300
100
10
10 –7 10 –6 10 –5
mixing ratio
50
10 –4
pressure (mbar)
10–3
400
10–1
300
C4 H 2
1
CH3C2H
C3 H8
10
200
100
102
50
10–7 10–6 10–5
mixing ratio
10–2
500
(ii)
C3 H8
10–1
C4 H2
CH3C2H
400
300
200
1
10
100
10–8
10–7
10–6
mixing ratio
10–3
10–2
400
10–2
400
10–1
300
10–1
300
100
10
10 2 –9
10
200
10–8
10–7
10–6
mixing ratio
50
pressure (mbar)
C2 H4
500
(iii)
altitude (km)
1
altitude (km)
10–4
500
(iii)
pressure (mbar)
100
10
102
10–10 10–9 10–8 10–7 10–6
mixing ratio
10–3
HCN C2H2 C2H6
10–3
500
(ii)
200
1
10 2
10 –8
pressure (mbar)
10 2
10 –8
10–2
200
altitude (km)
C 2 H 2 C 2 H6
1
200
C6 H6
C2 H4
100
10
102 –9
10
altitude (km)
HCN
pressure (mbar)
1
500
(i)
altitude (km)
pressure (mbar)
(i)
altitude (km)
(a) 10–3
10–8
10–7
10–6
mixing ratio
Figure 2. Vertical profiles of hydrocarbons and HCN retrieved at (a(i)–(iii)) 158 S and (b(i)–(iii))
808 N from Cassini/CIRS spectra. The altitude resolution of the data is approximately 40 km.
From Vinatier et al. (2007a).
could be related to its failure to condense in the atmosphere and southward
transport in the lower atmosphere through the return branch of the Hadley cell
(Crespin et al. 2008).
Large compositional changes occur at high northern latitudes, beyond 458 N.
In the lower stratosphere, mixing ratios of all species with the possible exception
of CO2 exhibit larger concentrations. This enhancement is the strongest for
nitriles, C4H2 and CH3C2H. The most dramatic variation is seen in HC3N with an
increase of its mixing ratio at 200 km by three orders of magnitude from 158 S to
808 N. At 808 N, a concentration minimum, more pronounced for HC3N, C4H2
and CH3C2H, takes place at approximately 300 km. This minimum is possibly a
bit higher (350–400 km) and sharper at 548 N than at 808 N. An observational
sequence at 828 N with very high (10 km) height resolution has even revealed a
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687
Review. Composition of Titan’s stratosphere
volume mixing ratio
(a)
10–5
10–6
C2H6
(b)
(c)
10–6
10–6
C2H2
10–7
C6H6 × 100
10–7
C2H4
10–8
CH3C2H
10–8
CO2
10–9
C3H8
10–7
10–9
10–8
–90 –60 –30 0 30 60 90
latitude
C4H2
–90 –60 –30 0 30 60 90
latitude
10–10
HCN
HC3N
C2N2
–90 –60 –30 0 30 60 90
latitude
Figure 3. (a–c) Meridional variation of composition in the lower stratosphere at approximately
120 km. Filled symbols and solid lines represent data from Cassini/CIRS spectra recorded between
December 2004 and January 2006 (Coustenis et al. 2007). Empty symbols and dashed lines correspond
to mixing ratios derived from Voyager 1 spectra in November 1980 (Coustenis & Bézard 1995).
layered structure in the profiles of HCN and HC3N, with variations of the latter
exceeding one order of magnitude over a few tens of km (Teanby et al. 2007). The
pattern is reminescent of the discrete haze layers seen in Cassini images of the
northern polar hood (Porco et al. 2005). If an altitude correlation does exist, it
would imply a strong chemical connection between haze and nitriles.
More detailed information on the latitudinal variation of composition is available
from nadir spectra. However, this information is generally limited to mean mixing
ratios in the lower stratosphere, typically at approximately 120 km. Figure 3 shows
the composition retrieved by Coustenis et al. (2007) from 108 latitude bins. All gases
(except possibly CO2) exhibit an increase of their mixing ratios at high northern
(winter) latitudes. As mentioned above, this enrichment is larger for nitriles, C4H2
and CH3C2H than for gases such as C2H6 and C2H2. Mapping the abundances at
higher spatial resolution shows that a sharp increase in the mixing ratios takes place at
45G58 N. By contrast, HCN shows a more gradual increase from south to north. Note
that the species enrichment in the north was more pronounced at the Voyager
encounter time (Coustenis & Bézard 1995), a season close to spring equinox (figure 3).
The enrichment of the species at winter latitudes seems to be correlated
with their vertical concentration gradient at low latitudes, which itself is
approximately anti-correlated with their lifetime in the stratosphere (see Teanby
et al. (2009) for details). This can be explained by subsidence in the winter polar
vortex, bringing air enriched in photochemical compounds from the upper
atmosphere where they are formed down to the stratosphere (Lebonnois et al.
2009 and references therein). The stronger enrichment observed by Voyager at
spring equinox implies that subsidence persists throughout winter, as predicted
by global circulation models (GCMs).
3. Isotopic ratios
Cassini/CIRS observations have provided new information on the isotopic
composition of Titan’s stratosphere. Signatures from deuterated methane (CH3D)
and acetylene (C2HD) have been observed and analysed (Bézard et al. 2007;
Phil. Trans. R. Soc. A (2009)
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688
B. Bézard
K4
Coustenis et al. 2007, 2008). The D/H ratio in methane (1:3C0:15
; Bézard
K0:11 !10
et al. 2007) is expected to represent that in the primordial material that accreted
to form Titan and provides important constraints on the origin of Titan
(e.g. Owen & Niemann 2009). Within error bars, the D/H ratio determined in
acetylene is consistent with that in methane.
The 13C isotope was detected in various hydrocarbons (CH4, CH3D, C2H6,
C2H2; Bézard et al. 2007; Nixon et al. 2008) and in HCN (Vinatier et al. 2007b).
The 12C/13C ratio found in hydrocarbons (81G2) agrees with that measured in
methane by the Huygens/GCMS (82.3G1; Niemann et al. 2005) and is clearly
smaller than the terrestrial value (89), which probably points to fractionation by
atmospheric escape. This escape rate has to be comparable with the photolytic
destruction rate to produce significant fractionation over the lifetime of methane
in Titan’s atmosphere (10–100 Myr). Strobel (2008) has indeed recently argued
that hydrodynamic escape in the upper atmosphere leads a large mass loss of
essentially CH4 and H2.
The 14N/15N ratio in HCN was determined by Vinatier et al. (2007b). The
derived value 56G8 (i.e. 0.2 times the terrestrial value) agrees with Marten
et al.’s (2002) earlier determination from millimetre radio observations. It is
lower than the ratio in N2 measured in situ by the Huygens/GCMS (183G5) by a
factor of approximately three, which points to an efficient fractionation process
in the formation of HCN from N2. This process has been recently identified as
photolytic fractionation of 14N14N and 14N15N (Liang et al. 2007), arising from
the isotopic shift of the predissociation states. Solar wavelengths corresponding
to 14N14N lines are more rapidly attenuated than those corresponding to the rarer
isotopes. 14N15N can then undergo photodissociation at levels deeper than 14N14N
which is self-shielded. This process favours the production of 15N radicals in
Titan’s atmosphere and would lead to a HC14N/HC15N ratio as low as 23
according to photochemical calculations (Liang et al. 2007). The observed
nitrogen isotopic ratio in HCN therefore implies an additional flux of nonfractionated atomic N, probably from ion/electron impact. Liang et al. estimate
this source to be of the order of 109 cmK2 sK1, about twice the photolytic rate.
Finally, the Huygens/GCMS detected two isotopes of argon: primordial
36
Ar and radiogenic 40Ar, the decay product of 40K, which is contained in the
rocky component of Titan’s interior (Niemann et al. 2005). Both the detection
of 40Ar and the scarcity of primordial noble gases (36Ar, 38Ar, Kr, Xe) have
strong implications on the origin and evolution of Titan, as discussed by Tobie
et al. (2009).
4. Photochemistry
One-dimensional photochemical models have been developed to investigate
Titan’s chemistry (e.g. for the most recent ones, Wilson & Atreya 2004;
Lebonnois 2005; Lavvas et al. 2008a,b; Vuitton et al. 2008). These models
incorporate chemical sinks and losses (photodissociation, 2- and 3-body
reactions) and vertical transport parametrized by an altitude-varying eddy
mixing coefficient. Two-dimensional models that couple a GCM and photochemistry have also been developed (see Lebonnois et al. 2009) to study the
seasonal and latitudinal variations of composition.
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689
From these models, constrained by the available observational data, the
general characteristics of Titan’s photochemistry are now understood, at least as
concerns the most abundant species (figure 4). However, uncertainties clearly
remain due to unknowns in some important chemical reaction schemes.
Hydrocarbon photochemistry is initiated by methane photodissociation that
produces CH3, CH2 (in the singlet 1CH2 and triplet 3CH2 states) and CH radicals.
This occurs either directly in the upper atmosphere above 700 km, mainly from
Lya photons, or through catalytic destruction below 500 km with C2H or C2
radicals attacking CH4. These radicals originate from the photodissociation of
C2H2 due to long-wavelength UV photons that can penetrate down to the
stratosphere. Ethane, the main photochemical product, is formed by the selfrecombination of the methyl radical (CH3) and is mostly lost by condensation
near 60–70 km. Ethylene (C2H4) can be produced by two reactions above 700 km:
3
CH2CCH3 and CHCCH4. Acetylene (C2H2) is formed at high altitudes (above
600 km) by the photodissociation of ethylene; it is then transported to lower
altitudes where it can be lost by various mechanisms: formation of diacetylene
(C4H2) and higher-order hydrocarbons, reactions with H to recycle C2H4, and
condensation near 80 km. C3 compounds are formed by insertion of a C1 radical
into a C2 compound: propane (C3H8) through CH3CC2H5, methylacetylene
(CH3C2H) through CHCC2H4.
Benzene (c-C6H6) has received a lot of attention because it is thought to be a
precursor of complex aromatic molecules that are probably incorporated in haze
particles. However, its production and loss mechanisms are still uncertain in
conditions relevant to Titan, as well as giant planets, where it has also been
detected (Bézard et al. 2001). The mechanism proposed is the recombination of
propargyl (C3H3) radicals in the stratosphere that produces C6H6 isomers (e.g.
Moses et al. 2000). Assuming that this reaction is followed by rapid isomerization
to benzene, photochemical models (Wilson & Atreya 2004; Lebonnois 2005;
Lavvas et al. 2008a) approximately reproduce the observed abundances in the
stratosphere. However, laboratory measurements at low temperature and pressure
of the relative yields of the different isomers and of the benzene photolysis are
crucially needed. Note that data from the Cassini/INMS (Ion and Neutral Mass
Spectrometer) indicate an additional source of benzene in the ionosphere, at
approximately 900 km (Waite et al. 2007; Vuitton et al. 2008). A general
discussion of the ionospheric chemistry is presented by Vuitton et al. (2009).
Nitrile photochemistry begins with the dissociation of the N2 molecule. Shortwavelength UV photons (less than 100 nm) produce N4s and N2d atoms or NC
ions and N2d atoms through photoionization. Electrons can also break N2 to yield
NC, N4s or N2d and electrons. HCN, the most abundant nitrile, is then mostly
formed (directly or indirectly) by reaction of N4s atoms with CH3 radicals (e.g.
Wilson & Atreya 2004; Lavvas et al. 2008a). Pathways to more complex nitriles
involve the CN radical, produced by the photodissociation of HCN. Most of these
CN radicals are recycled back to HCN through reactions with CH4, C2H6 or
C2H4, which explains the stability of HCN once formed. However, reaction of CN
with C2H2 produces HC3N and reaction of CN and C2H4 partly yields
acrylonitrile (C2H3CN) which has not yet been detected in the stratosphere.
Reaction of N2d with HC3N leads to C2N2 while acetonitrile (CH3CN) may be
produced by N2dCC2H4 (Wilson & Atreya 2004; Lavvas et al. 2008b).
Phil. Trans. R. Soc. A (2009)
690
C2n+2H2
C2H2
HCN
HCN
C4H2
CH4
C2H4
C3H3
C6H6
C3H6
nitriles
HC3N
N2
CH3CN
PAH
polymers
nitrile
polymers
C4N2
N2d
C2H3CN
C2N2
CHCN
C4H6
PAH
Figure 4. A simplified scheme of Titan’s hydrocarbon and nitrile photochemistry. The thick arrows show the main pathways to haze production
from polyacetylene, polycyclic aromatic hydrocarbon (PAH) and nitrile polymers. The species in bold have been detected in Titan’s stratosphere.
From Atreya et al. (2006).
B. Bézard
haze
C2H3
N4s
CN
CH3C2H
C2H5
C3H8
polyacetylene
polymers
H2CN
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C2H6
CH3
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691
Oxygen species are also present in Titan’s atmosphere. Water vapour is deposited
by an influx of micro-meteorites, as is the case for the giant planets. Hydroxyl
radicals (OH) are then formed from H2O photodissociation. While part of OH is
recycled back to H2O, some reacts with CO to yield CO2 at altitudes above 400 km.
What is the origin of the large amount of CO in the atmosphere? Meteorites or
comets are not a probable source, given the scarcity of primordial noble gases in the
atmosphere. Catalytic reactions between methane and water vapour can produce
CO; however, models indicate that the expected equilibrium CO abundance would
be as low as a few ppm (Wong et al. 2002) in contrast to the quantity observed
(47 ppm). A possible source is cryovolcanic activity releasing promordial CO
trapped in the interior (Baines et al. 2006). Another possibility is that CO is formed
from the influx of O atoms from Saturn’s magnetosphere (Hörst et al. 2008).
5. Photochemical haze
Our knowledge of Titan’s ubiquitous haze has greatly improved with the arrival
of Cassini/Huygens. The Descent Imager/Spectral Radiometer (DISR) aboard
the Huygens probe recorded spectra of the ambient sunlight at a variety of
scattering angles from 150 km down to the surface. Analysis of this complex
dataset has shown that Titan’s aerosols are aggregated with a few thousands of
monomers per particle above 60 km, the monomers’ radius being at most 0.05 mm
(Tomasko et al. 2008). Aerosols are present down to the surface, their number
density is approximately 5 cmK3 at 80 km and decreases at higher altitudes with
a scale height of 65 km. Analysis of far-infrared CIRS limb spectra at 458 S also
indicates an extinction scale height of approximately 65 km between 80 and
230 km (de Kok et al. 2007b). Above 80 km, the haze optical constants in the
DISR spectral range (480–1720 nm) are consistent with those of some laboratory
analogues (‘tholins’) of Titan’s aerosols (Khare et al. 1984). Below 80 km and
again below 30 km, the particles become brighter and bigger, possibly due to
condensation of photochemical gases and methane on the haze particles.
It is interesting to compare the mass production rate of haze with that of
ethane, the most abundant gaseous photochemical product. Their ratio is given
by the ratio of their atmospheric density divided by the ratio of their mixing
ratio (or concentration) scale height at any level below their formation regions,
assuming that their distributions are governed by diffusive transport. The
relevant quantities can be calculated at 120 km from the CIRS-derived C2H6
profile at 158 S (Vinatier 2007) and from the DISR haze particle profile assuming
a monomer density of 0.6 g cmK3. I find an ethane to haze production rate ratio
of 15, implying that the dominant product of Titan’s photochemistry is by far
ethane rather than haze. The haze mass production rate is then approximately
1.5!10K14 g cmK2 sK1 if the ethane production rate from Lavvas et al. (2008b) is
used (2.5 times less if Wilson & Atreya’s (2004) rate is used instead). This large
ratio of 15 casts doubts on Hunten’s (2006) idea that ethane is sequestrated in
haze particles to produce a dusty material (‘smust’) that could explain the
surface deposits and lack of extents of liquid ethane at mid-latitudes.
A solar occultation by Titan’s Southern Hemisphere, observed by the Cassini
Visual Infrared Mapping Spectrometer (VIMS), provides additional information
on the haze characteristics (Bellucci et al. 2007). The derived haze extinction
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profile indicates a number density that decreases with a scale height of
approximately 65 km up to an altitude of 450 km. This is similar to the scale
height derived by DISR at 108 S from 80 to 150 km. The increase of the haze
concentration with height implies a source at high altitudes (above 450 km). As
discussed by Waite et al. (2007), in situ data from the INMS and the Cassini
plasma spectrometer also give evidence for production of organic aerosols at
high altitudes (approx. 1000 km), with the formation of polycyclic aromatic
hydrocarbons (PAHs) by ion chemistry (see discussion by Vuitton et al. 2008)
and the presence of massive negatively charged molecules.
The general spectral variation of the haze extinction (1–5 mm) derived from
VIMS agrees with ‘tholin’ opacity (Khare et al. 1984) with the exception of two
absorption bands at 3.0 and 4.6 mm seen in the laboratory material. By contrast,
an unidentified feature is detected at 3.4 mm, which is probably due to C–H
stretching vibration of complex organics. Spectral variations of the haze
absorption can also be inferred in the thermal infrared from CIRS measurements
(de Kok et al. 2007b; Vinatier 2007). The Aerosol Collector & Pyrolyser aboard
Huygens collected two atmospheric samples during the descent. Israël et al.
(2005) found that NH3 and HCN are the main products of the atmospheric
sample taken at 25–20 km after pyrolysis at 6008C. These results would imply
that nitrogen is incorporated in different ways in the refractory core of the
aerosols, possibly through nitrile (–CN) and amino (–NH2) groups. However,
Biemann (2007) has recently raised doubts about the detection of aerosol
pyrolysis products in this experiment. Rigorous laboratory calibrations need to
be completed to confirm these preliminary results.
Vinatier et al. (2007a) have argued that most of the HCN formed at high
altitudes might be incorporated into Titan’s aerosol in the stratosphere. This
conclusion comes from the difference in the vertical gradients observed for HCN
and C2H6 at mid-latitudes (figure 2) while both compounds are supposedly
mostly formed at high altitudes with negligible chemical loss during the
transport to their condensation levels. If this is true, the steeper gradient of HCN
implies a large sink in the stratosphere (of the order of 108 cmK2 sK1), not
expected from gas phase chemistry alone. Wilson & Atreya (2003) and Lebonnois
(2005) have investigated the formation of haze from the condensation of various
polymers. They predict formation of polyacetylene, polyyne and nitrile polymers
in the upper atmosphere, above 500 km, and PAH polymers in the stratosphere.
In these models, the main source of haze is PAH polymers, with a peak near
200 km; nitrile polymerization, a 3–10 times smaller source, is not large enough
to account for Vinatier et al.’s HCN sink but it should be noted that large
uncertainties remain in the kinetics of these polymerization processes and that
other possible pathways remain to be explored.
6. Summary and conclusions
Titan’s methane–nitrogen photochemistry produces a number of hydrocarbons
and nitriles that condense in the lower stratosphere as well as haze particles.
These compounds are eventually deposited on the ground. The gas production
rate, predominantly ethane, exceeds that of haze particles by an order of
magnitude. At mid-latitudes, the mixing ratios of photochemical gases increase
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with altitude in the stratosphere (except for ethylene that does not condense) as
a result of vertical transport from formation to condensation levels. The vertical
gradient varies from species to species in response to local chemical sources and
sinks. The enhancement observed for most gases in the stratosphere northward of
458 N can be understood by downwelling in the winter polar vortex but details of
the observed vertical profiles, such as a minimum in the mixing ratio of many
species at 300–400 km, have yet to be explained.
The production of solid aerosols from the gas phase is a crucial, but poorly
known, process in Titan’s atmosphere. There is evidence from different Cassini
instruments that a source of particles is present at high altitudes, possibly
initiated by the formation of PAHs through ion chemistry as hypothesized by
Waite et al. (2007). Particle production and/or growth may also take place in the
stratosphere with the formation of PAHs by neutral chemistry and possibly the
incorporation of HCN into the aerosols. The organic material composing these
aerosols presents significant spectral differences with their laboratory analogues
as produced by different groups, the so-called ‘tholins’.
Chemical models of the atmosphere still suffer from uncertainties in important
reaction schemes, especially as concerns polymerization processes and aerosol
surface chemistry. Laboratory measurements of kinetics, branching ratios and
photolysis yields, in conditions relevant to Titan’s stratosphere, are essential to
make further progress in our understanding of Titan’s complex photochemistry
and dynamics.
I acknowledge support from the Centre National d’Etudes Spatiales (CNES) and from the Centre
National de la Recherche Scientifique (CNRS). I thank Sushil Atreya for providing figure 4 and
Caitlin Griffith for lively discussions on Titan’s chemistry and dynamics and on non-related topics.
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