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Phil. Trans. R. Soc. A (2009) 367, 713–728
doi:10.1098/rsta.2008.0245
Published online 21 November 2008
Storms, polar deposits and the methane
cycle in Titan’s atmosphere
B Y C AITLIN A NN G RIFFITH *
Department of Planetary Sciences, University of Arizona,
1629 East University Boulevard, Tucson, AZ 85721, USA
In Titan’s atmosphere, the second most abundant constituent, methane, exists as a gas,
liquid and solid, and cycles between the atmosphere and the surface. Similar to the Earth’s
hydrological cycle, Titan sports clouds, rain and lakes. Yet, Titan’s cycle differs dramatically
from its terrestrial counterpart, and reveals the workings of weather in an atmosphere that
is 10 times thicker than the Earth’s atmosphere, that is two orders of magnitude less
illuminated, and that involves a different condensable. While ongoing measurements by the
Cassini–Huygens mission are revealing the intricacies of the moon’s weather, circulation,
lake coverage and geology, knowledge is still limited by the paucity of observations. This
review of Titan’s methane cycle therefore focuses on measured characteristics of the
lower atmosphere and surface that appear particularly perplexing or alien.
Keywords: planetary atmospheres; Titan; weather
1. Summary of current observations
Titan’s methane cycle plays out largely below the tropopause at 45 km (figure 1).
As on the Earth, the tropopause’s low temperature limits the abundance of the
surface-supplied condensable at higher altitudes. Yet, on Titan, the tropopause
saturation pressure of methane, PCH4 w0.015, is relatively high, well exceeding
that, PH2 O w10K5, of water on Earth. Sufficient methane therefore mixes into the
stratosphere where it is destroyed irreversibly by ultraviolet solar radiation
to produce higher order organic material. These by-products mix down to the
lower atmosphere and precipitate, with a few exceptions, onto the surface. As
a result, organic sediments accumulate on Titan’s surface at a high enough
rate to form a layer approximately 0.5 km thick, most of which is liquid
ethane, over the course of Titan’s lifetime of approximately 4.5 Gyr (Lunine et al.
1983; Yung et al. 1984). Thus, Titan’s climate continually evolves. Sediments
accumulate on the surface to be buried through cryo-volcanic resurfacing
(Mousis & Schmitt 2008) and potentially chemically altered (Atreya et al. 2006).
The moon’s slow drying of methane follows periodic resupply episodes of cryovolcanic outgassing (Lunine & Stevenson 1985; Sotin et al. 2005; Tobie et al.
2006; Lopes et al. 2007).
*griffi[email protected]
One contribution of 14 to a Discussion Meeting Issue ‘Progress in understanding Titan’s
atmosphere and space environment’.
713
This journal is q 2008 The Royal Society
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C. A. Griffith
10–7
10–5
(a)
(b)
mesosphere
10–6
10–4
pressure (bar)
10–5
10–3
10–2
stratopause
10–4
10–3
stratosphere
10–2
10–1
tropopause
10–1
troposphere
1
200
250
300
temperature (K)
160
120
temperature (K)
1
200
Figure 1. Titan’s methane cycle occurs below the tropopause, which serves as a cold trap, albeit a
leaky one, since the methane saturation mixing ratio at the temperature minimum is not low
enough to strongly restrict the mixing of methane into the stratosphere. (a) Earth and (b) Titan.
Measurements below Titan’s troposphere are limited largely to radar surface
observations (Elachi et al. 2005), near-infrared spectral images of Titan’s surface
and clouds (e.g. Griffith et al. 1991; Brown et al. 2002, 2006; Roe et al. 2002;
Porco et al. 2005), three temperature profiles of Titan’s tropical atmosphere
(Lindal et al. 1983; Fulchignoni et al. 2005) and one measurement of Titan’s
methane abundance profile (figure 2) at the probe landing site (Niemann et al.
2005). The drift of clouds and of the Huygens probe provide a few spotted
measurements of tropospheric zonal winds, and indicate prograde winds of
10–35 m sK1 at 20–40 km that decrease with altitude to speeds less than 1 m sK1
below 5 km altitude (Bird et al. 2005; Griffith et al. 2005; Porco et al. 2005;
Folkner et al. 2006). Upper atmosphere measurements also bear on the topic,
particularly temperature maps (Flasar et al. 2005), and the latitude profiles of
minor species (e.g. Coustenis & Bézard 1995; Lebonnois et al. 2001; Vinatier
et al. 2006), wind measurements through occultation and Doppler observations
(e.g. Hubbard et al. 1993; Sicardy et al. 1999, 2006; Bouchez 2004; Kostiuk et al.
2005; Zalucha et al. 2007) and the haze density (Lorenz et al. 1999; Rannou et al.
2006) that manifest Titan’s general circulation. The atmospheric opacity
structure further establishes the solar energy partitioning, including the surface
insolation (figure 3).
With only one humidity profile and a handful of temperature and wind
soundings, the dearth of observations curbs the present knowledge of Titan’s
atmosphere. Thus, this review stays close to the data. We focus on three
characteristics, or ‘curiosities’, of Titan’s atmosphere and surface that distinctly
differ from their terrestrial equivalents. Thermodynamic, radiative transfer and
general circulation studies are invoked to explain Titan’s and the Earth’s weather
as manifestations of the different fundamental properties of these atmospheres.
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Titan’s weather
(a) 30
(b)
altitude (km)
20
10
0
20
40
humidity
60
80
0
0.02
0.04 0.06
mixing ratio
0.08
0.10
Figure 2. (a) Two randomly chosen humidity profiles above Tucson indicate the temporal and
altitude variability of the water content of the Earth’s atmosphere (solid line, 24 November 2007;
dashed line, 26 November 2007). (b) The mixing ratio of methane in Titan’s atmosphere follows
a simple humidity profile (solid line, measured; dot-dashed line, CH4–N2 saturation; dashed line,
CH4 saturation). The constant mixing ratio below approximately 7 km indicates a constantly
increasing humidity up to saturation at approximately 8 km. Above this level, the mixing ratio
follows the saturation profile for CH4–N2 condensation up to approximately 15 km. Above this
level, the mixing ratio approaches that of the saturation of pure CH4 ice, the primary condensate
above 15 km.
(a ) Three curiosities
The ground-based detection of methane clouds in Titan’s atmosphere (Griffith
et al. 1998) provided the first indications of active weather and deep convection
(Griffith et al. 2000). Subsequent images revealed that clouds resided near the
South Pole (Brown et al. 2002; Roe et al. 2002) and later, unexpectedly, in a band
centred at K408 latitude (Roe et al. 2005a). These two specific cloud locations,
now confirmed by many additional observations (Gendron et al. 2004; Gibbard
et al. 2004; Adamkovics et al. 2005; Griffith et al. 2005; Porco et al. 2005; Hirtzig
et al. 2006), contain the majority of clouds, which cover usually approximately 1
per cent of Titan’s globe, although occasionally a single system can cover
approximately 10 per cent of the globe (Griffith et al. 1998; Schaller et al. 2006b).
By contrast, water clouds cover approximately 50 per cent of the Earth. The
infrequency of clouds on Titan probably stems from the limited power available
at the surface, approximately 1 per cent (figure 3), to drive convection (McKay
et al. 1991; Lorenz et al. 2005). Images of Titan have been recorded from 1998 to
2008, which span a sizeable fraction of Titan’s year (29.5 Earth years),
surrounding the south summer solstice, in October 2002. Comparisons of these
images indicate a diminishing incidence of South Polar clouds coupled with a
greater occurrence of mid-latitude clouds (Bouchez & Brown 2005; Schaller et al.
2006a), and suggest a coupling of the cloud locations to seasonal circulation.
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C. A. Griffith
(a)
(b)
100 %
30 %
25 %
45 %
100% 23%
12%
57%
stratosphere
15 %
45 %
40 %
57%
10 %
troposphere
22%
10 %
113 %
2%
1%
94%
57%
120 %
8%
7%
102%
surface
sunlight
convection
IR radiation
sunlight convection evaporation IR radiation
Figure 3. Radiative balance of (a) Titan’s tropical atmosphere (McKay et al. 1991) differs from that
of (b) the Earth, largely owing to the moon’s stratospheric haze that absorbs 40% of the sunlight.
Consequently, only 10% of the radiation reaches Titan’s surface, providing little power for
convection (wiggly line) and evaporation (light line). Both atmospheres have strong greenhouse
effects. The evaporative flux is unknown for Titan and therefore missing in the figure. The solar
fluxes at Earth and Titan are 1370 and 15 W mK2; one-quarter of which, 342 and 3.7 W mK2,
respectively, are the globally averaged insolations at the top of these atmospheres.
In addition, the latitude of these clouds contrasts that of the cloud detected in
the 1995 Hubble Space Telescope data at 408 latitude during the northern
autumnal equinox (Lorenz 2008; Lorenz et al. 2008). To date, few tropical clouds
have been detected. These include the tenuous cloud observed at 21 km altitude
by the Huygens probe (Tomasko et al. 2005), a handful of optically thick small
clouds (Porco et al. 2005). The first curiosity is the cloud locations on Titan’s
globe, their regularities and exceptions.
Radar observations suggest that deep (non-transparent) lakes are confined to
the polar regions, while at equatorial latitudes, vast dunes are interspersed with
dendritic features (Lorenz et al. 2006; Barnes et al. 2007; Stofan et al. 2007). The
Huygens probe fortuitously landed in a damp lake bed fed by washes, which
sits in a dune field (Tomasko et al. 2005; Soderblom et al. 2007a,b). Near-IR
spectroscopy indicates further that water ice, which makes up half of Titan’s
bulk material, is exposed despite the sediments of organic material produced in
Titan’s stratosphere (Coustenis et al. 1995; Griffith et al. 2003; Soderblom et al.
2007a). The second curiosity is the latitudinal pattern of surface features, with,
for example, lakes confined to the poles and washes coexistent with dunes in
the tropics.
Titan’s temperature and methane abundance profiles reveal the stability of
its atmosphere to convection and consequent cloud formation. The Huygens
gas chromatograph mass spectrometer (GCMS) instrument measured an
atmospheric humidity of 45 per cent above a damp surface. The mixing ratio
is constant at 0.049 up to the level, approximately 8 km, where the temperature
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717
is cold enough to be saturated. The atmosphere is saturated with respect to
CH4–N2 condensation at 8–16 km, and pure CH4 ice at 25–30 km, with an
intermediate mixing ratio in between these two regions (Tokano et al. 2006a), as
shown in figure 2. This profile is markedly simple, and, in fact, it is just that
assumed in simple models of Titan’s atmosphere prior to Cassini (McKay et al.
1989; Thompson et al. 1992). But such profiles are rarely seen on Earth (figure 2).
The third curiosity is the perfect methane profile, and the damp surface despite
a cloudless sky.
2. Titan’s temperature and methane profiles
Titan’s temperature profile has been measured, to date, only three times. On 12
November 1980, near the northern spring equinox, Voyager determined the
temperature structure at dawn (at K8.58 latitude, 2568 west longitude) and at
dusk (at 6.28 latitude, 778 west longitude). The thermal profile was measured
through the refraction of radio waves relayed to the Earth and occulted by
Titan’s atmosphere as Voyager went behind Titan and re-emerged (Lindal et al.
1983). On 15 January 2005, a little over 2 years past the south summer solstice,
the Huygens atmospheric structure instrument (HASI) measured, in situ, the
temperature profile at the probe landing site, at K108 latitude, 1928 west
longitude (Fulchignoni et al. 2005). The profiles match each other to within the
error of the data of approximately 0.5 K. The HASI measurements revealed for
the first time the details of the profile at low altitudes. Temperatures follow those
of an adiabatically rising parcel (i.e. the dry lapse rate) below 300 m; this
lower region thus defines the current planetary boundary layer (Tokano et al.
2006b). A couple of additional altitude regions below 2 km approach the dry
lapse rate, suggesting that dry convection has occurred up to 2 km altitude in
the past (Griffith et al. 2008). Above 2 km, the atmosphere is stable with respect
to dry convection.
Titan’s methane abundance profile has been measured only once; these data
were recorded by Huygens’ GCMS at the probe landing site (Niemann et al.
2005). While Cassini will record additional temperature profiles at a variety of
latitudes using radio occultation measurements similar to Voyager, another
methane profile requires another trip to Titan. This unique measurement therefore deserves careful analysis in concert with the temperature profile, both
recorded on a summer afternoon in Titan’s tropics.
The humidity at Titan’s surface indicates that surface parcels, if lifted, become
saturated at 6 km, the lifting condensation level (LCL; Tokano et al. 2006a).
Condensation and the release of latent heat aids convection above this altitude.
The measured lapse rate is slightly steeper than the wet lapse rate with respect
to CH4–N2 condensation at several altitude regions below approximately 15 km
(Griffith et al. 2008). Since the atmosphere at 8–15 km is saturated (Tokano et al.
2006a), these regions are unstable with respect to wet convection (Griffith et al.
2008). Parcels that are nudged upwards convect and rise to altitudes anywhere
between 15 and 26 km, depending on the microphysics. Such convection forms
clouds, and may explain the thin cloud layer detected by the Huygens Descent
Imager/Spectral Radiometer instrument at 21 km altitude (Griffith et al. 2008).
Yet, convection at the probe landing site is weak, with a small convective
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C. A. Griffith
available potential energy (CAPE)%120 J kgK1 (Griffith et al. 2008). The
temperature and methane profiles indicate an atmosphere too stable to produce
enough rainfall presently (Tokano et al. 2006a; Barth & Rafkin 2007),
or with changes in seasons (Griffith et al. 2008), to form the washes at the
landing site.
Conditions at the probe landing site are not conducive to the convection of
surface air to the upper troposphere (Tokano et al. 2006a), because updrafts
are too weak to raise parcels up to the level of free convection (LFC) of
approximately 9 km, where they become buoyant and convect freely. Surface
heating at the equinox (in 2010) may raise the parcels to the LFC. However,
the resultant convection would also be weak, CAPE%180 J kgK1, and heavy
rainstorms unlikely (Griffith et al. 2008). The strength of convection and
the CAPE of Titan’s tropical atmosphere can only be enhanced with an
increase in humidity (Griffith et al. 2008). Detailed convection models for
Titan indicate that rainfall will occur if the atmospheric humidity is increased
to at least 60 per cent at the surface (Hueso & Sánchez-Lavega 2006; Barth &
Rafkin 2007).
(a ) Temporal humidity variations and the stability of polar lakes
On Earth, the humidity in the lower troposphere varies throughout the day
and season from dry conditions to a nearly saturated atmosphere, as shown for
two randomly chosen temperature profiles for Tucson, Arizona (figure 2). These
rapid variations are powered ultimately from the solar insolation (342 W mK2,
globally averaged), of which 57 per cent reaches the surface, 7 per cent is
available for convection and 22 per cent, on average, i.e. 75 W mK2, for
evaporation (figure 3). This power well exceeds that, approximately 7 W mK2,
needed to evaporate an entire atmospheric column of water in a three-month
season (figure 3). The Earth’s surface contains 2.7 km, globally averaged, of
liquid water, sufficient to supply the entire inventory of vapour, approximately
2.5 cm, in the atmosphere.
Variations in the atmospheric humidity on Titan are, by contrast, limited by
the low power, the low methane surface supply and the high methane vapour
pressure (Griffith et al. 2008). Titan’s atmospheric methane content, equivalent
to 5 m of liquid at the probe landing site (Tokano et al. 2006a), is high owing to
the large saturation vapour pressure and scale height. Thus, a large mass of
methane must evaporate, by terrestrial standards, to significantly change the
humidity. Ample power is needed to this effect. On Titan, however, 10 per cent of
the incident sunlight reaches the surface (McKay et al. 1991; Tomasko et al.
2008), and only 1 per cent, or 0.037 W mK2, powers convection (figure 3). The
evaporation rate is unknown, and probably depends on the terrain. Yet, even the
entire globally averaged surface flux, i.e. 0.37 W mK2, falls short of the 4 W mK2
power needed to evaporate a full column of Titan’s methane in its long 7.4 Earth
year summer. If 1 per cent of the power is available for evaporation, then only
7 cm of methane evaporates in half a Titan year. By contrast, changes in the
ethane humidity, requiring little energy, are probably more common (table 1).
These numbers suggest that large-scale variations in Titan’s methane humidity
in the lower troposphere may be energy limited.
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Titan’s weather
Table 1. Earth and Titan condensables.
condensable
Earth: H2O
Titan: CH4
Titan: C2H6
surface saturated mixing ratio
atmosphere abundance (m)
LCL (km)
heat of vaporization (J kgK1)
density (kg mK3)
Tevaporationb (years)
Tpblc (years)
latent heat content (J mK2)
0.019
0.025
1.8
2.26!106
1000
0.009
0.0005
6!107
0.12
w4
6
5.4!105
456
84
1.4
109
1.7!10K5
0.0001a
2.3a
4.9!105
547
0.016
0.2
2!105a
a
If 50% humidity at the surface.bThe time to evaporate the atmosphere’s condensable complement
from the globally averaged surface insolation, i.e. 0.37 W mK2 for Titan and 195 W mK2 for the
Earth.c Time scale to heat the atmosphere to form a planetary boundary layer up to the LCL, as
estimated in appendix A.
The polar lakes are estimated to contain at least 3!104 km3 of methane
(Lorenz 2008; Lorenz et al. 2008). While their compositions (i.e. the methane and
ethane content) are unknown, it is likely that they contain enough methane
(above 2!104 km3) to increase the humidity in the Northern Hemisphere by 15
per cent. Yet radiative transfer derivations of the summer heating of the North
Pole indicate that the power is sufficient to evaporate only 2.8!103 km3 of
methane, enough to increase the humidity of the hemisphere below 8 km to only
2 per cent (Griffith et al. 2008). This is an upper limit, since it is not clear that
polar methane efficiently transports to the equator over the time scale of a
season. The methane column abundance in the tropics therefore does not change
significantly with season, barring cryo-volcanism, and the measured abundance
by Huygens is probably representative of Titan’s tropical atmosphere.
Vertical redistribution of methane could result from seasonal cooling and
condensation in the upper atmosphere. For example, a cooling of 2 K above
23 km would condense enough methane to saturate the atmosphere between 4
and 8 km (Griffith et al. 2008). Yet such a redistribution of methane, even if
seasonal, is not indicated by the Huygens profile. Instead, the steady decrease in
methane with height, below 8 km, points to the surface, where methane is most
abundant, as the atmosphere’s source.
(b ) Damp landing site
Huygens measured a damp surface (Niemann et al. 2005), which, given the
relatively dry atmosphere (45% humidity), poses the question of whether there
was recent rainfall. Tenuous clouds predicted by general circulation models
(GCMs) to result from temperature variations create methane cirrus of radii less
than 0.9 mm and thus virga rather than rainfall (Barth & Toon 2004; Rannou
et al. 2004; Graves et al. 2008). Microphysical models of convective systems
indicate the production of virga, with few larger surface-prone particles (Barth &
Rafkin 2007; Graves et al. 2008; Griffith et al. 2008). Possibly this drizzle
replenishes the surface liquids at a rate of 5–50 mm yrK1 (Rannou et al. 2006;
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C. A. Griffith
Tokano et al. 2006a; Barth & Rafkin 2007) comparable with the evaporation rate
(Mitri et al. 2007), which is slow, on average, owing to the low insolation (Griffith
et al. 2008).
Alternatively, methane is in near vapour pressure equilibrium above a surface
solution, a condition that Titan’s atmosphere tends towards as it dries of
methane in the absence of volcanism. Such a concoction would contain the
principal by-product of methane photolysis, ethane (Lunine et al. 1983), which
was detected in the South Polar lake-like feature, Ontario Lacus (Brown et al.
2008). In vapour pressure equilibrium, a liquid containing 39 per cent methane,
54 per cent ethane and 7 per cent nitrogen (Thompson et al. 1992) provides an
atmosphere with the measured methane mixing ratio of 0.049 (Niemann et al.
2005) and an ethane mixing ratio, 1.4!10K5, close to that, 10K5, measured at
160 km altitude (Vinatier et al. 2006). The Huygens GCMS detected evidence
for the presence of ethane on Titan’s surface, from a rise in the count rate
following landing (Niemann et al. 2005). Yet, whether equilibrium conditions
persist will be tested with Huygens GCMS measurements of the atmospheric
ethane content.
3. Cloud locations
Since the year 2000, Titan’s clouds frequent primarily the South Polar region and
a thin band centred at K408 latitude. Their time variability, cumuli morphology
and altitudes indicate a convective origin (Griffith et al. 2000, 2005; Porco et al.
2005). Initially, the presence of highly variable clouds in an atmosphere whose
radiative time constant of approximately 138 years (Smith et al. 1981; Flasar
1983) exceeds a Titan year was somewhat surprising.1 The first images provided
an explanation as they indicated that clouds reside at South Polar latitudes,
which receive the highest daily averaged solar insolation (Brown et al. 2002; Roe
et al. 2002). These data suggested that cloud formation results from the solar
heating of the surface, which, unlike the atmosphere, responds within the time
scale of a Titan season (Brown et al. 2002). Thereby, warmed, surface parcels
convect, cool and condense in the upper troposphere.
The detection of the K408 latitude clouds (Roe et al. 2005a) was unexpected
since the daily surface insolation is not exceptional at this latitude. These midlatitude clouds furthermore, unlike the South Polar clouds, were initially found
to prefer several longitudes, centred at 08 and 908 (Roe et al. 2005b). These
longitudes do not point to forcing by Saturn’s tides, which significantly affect
Titan’s dynamics (Tokano & Neubauer 2002). One possible explanation for
these clouds is that they represent the ‘smoking gun’ of volcanoes (Roe et al.
2005b). Yet, visual and infrared mapping spectrometer observations indicate
that an abrupt change in the haze opacity also occurs at K408, suggesting that
Titan’s general circulation changes, giving rise to upwelling at this latitude
(Griffith et al. 2005). A general circulation model now predicts the presence
of an upwelling branch and the occurrence of clouds at K408 latitude (Rannou
et al. 2006).
1
Titan’s long radiative time constant stems from the thickness of its atmosphere (with a column
density of 92 km amagat as opposed to 8 km amagat on Earth) and its low effective temperature of
approximately 82 K.
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Yet the details of cloud formation are not understood, because the
atmospheric methane and temperature profiles are unknown at middle and
high latitudes where clouds prevail. Conditions at the Huygens landing site are
not conducive to the formation of the clouds observed at K408 latitude (Griffith
et al. 2008), which reach heights of approximately 45 km. Instead, a more humid
lower atmosphere is indicated, if these clouds are convective, as suggested by
their morphologies (Griffith et al. 2005). Detailed models of the formation of
Titan’s South Polar clouds also point to conditions exceeding 80 per cent
humidity (Hueso & Sánchez-Lavega 2006). Such a moist environment may occur
at the summer poles by lake evaporation powered by the large increase in the
surface insolation (Griffith et al. 2008). Whether this occurs or not depends on
the ethane content in the lakes, which, if abundant, would limit the vapour
pressure of methane. Yet the humidification of the K408 latitude region through
the evaporation from the lakes would require a precise transport of methane to
lower latitudes near the surface, since there is insufficient power to evaporate the
lakes and humidify the entire disc north and including K408 latitude. An
unstable lower troposphere could be produced by the evaporation of surface
liquids too shallow to have been detected by Cassini radar measurements.
Alternatively, the lower atmosphere may be humidified by the transfer of
methane from the upper to the lower troposphere, such that variations in Titan’s
upper tropospheric temperature induce condensation, virga and evaporation of
methane in the undersaturated atmosphere below 8 km. Such a possibility will be
in part tested with temperature measurements by the Cassini radio occultation
experiment, currently in progress.
Few clouds have been observed in Titan’s tropical atmosphere. These include
small isolated clouds in the Southern Hemisphere (Porco et al. 2005) and clouds
at the landing site at 21 km altitude (Tomasko et al. 2005), which can be
explained by the weak mid-troposphere convection cell evident from the Huygens
methane and temperature data (Griffith et al. 2008). Clouds above 35 km have
not been observed at tropical latitudes. Thus, currently, it seems that deep
convection, as indicated by cloud heights, occurs only at high polar latitudes in
the summer.
(a ) The importance of being ethane
Ethane, although only a minor species in Titan’s atmosphere (table 1), can have
a large effect on the methane cycle, because it also exists as a liquid, gas and solid. Its
influence does not derive from its latent heat content, which is small by comparison
with that from methane (table 1). Instead, ethane dissolves in methane liquid and, if
plentiful as surface liquids, regulates the methane humidity as well as the
distribution of liquid methane on Titan’s surface.
The detection of ethane in Ontario Lacus (Brown et al. 2008) suggests that all
of the polar lakes contain ethane. Ethane is produced in the stratosphere,
transported downwards to the winter polar region (Rannou et al. 2006),
condenses above the polar tropopause (Griffith et al. 2006) and mixes down to
the surface (Barth & Toon 2004). In addition, Huygens GCMS measurements
suggest that the tropical surface is damp with respect to ethane and methane
(Niemann et al. 2005). Possibly, there are small shallow (several metre) ethane–
methane pools in the tropics, which are transparent to radar sounding.
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C. A. Griffith
If ethane is prevalent as a liquid on Titan’s surface, methane would then
evaporate most rapidly from dry land and accumulate in the ethane-rich lakes.
These lakes would then contain the surface reservoir of methane. In addition,
they would regulate the methane content in the atmosphere. Such ethane-rich
lakes could contribute to cloud formation, such that under windy conditions,
methane evaporates from the lakes thereby forming clouds specifically over the
shallow and unseen bogs. The repeated past presence of clouds at K408 latitude
and 08 longitude, indicating a preference for clouds at a specific place, could
result from the presence of shallow lakes.
Similarly, significant ethane in the polar lakes would regulate the methane
humidity, thereby controlling the atmospheric stability; albeit the presence of
South Polar clouds points to a humid atmosphere (Hueso & Sánchez-Lavega
2006). If, alternatively, the lakes contain less than approximately 40 per cent
ethane, summer heating of the polar region is sufficient to significantly humidify
the polar surface, thereby causing deep convection and cloud formation
(Griffith et al. 2008). Yet, it is unclear whether ethane plays a major role in the
methane cycle since the ethane content of the atmosphere has not been
measured. It is interesting that there have been no detections of an ethane mist,
since relatively little power is needed to evaporate ethane and raise it to its
LCL (table 1).
4. Latitude distribution of surface features
Dunes extending thousands of kilometres are ubiquitous in Titan’s tropics,
within G308 of the equator (Lorenz et al. 2005; Barnes et al. 2007; Radebaugh
et al. 2008). Poleward of 608 latitude, radar-dark patches indicate the presence of
lakes, which extend for hundreds of kilometres (Stofan et al. 2007). Optical and
radar images of the South Polar region also reveal lakes poleward of K758
latitude; however, as the South Pole has not yet been extensively mapped, the
lake coverage is poorly known. Lakes have not been detected on Titan’s surface
within K508 and 508 latitude. These observations suggest the presence of
different climates as experienced on Earth. Terrestrial sand dunes and deserts,
e.g. the Namib and Sahara deserts, prevail at specific latitudes, concentrating
around the K258 and 258 latitude bands. While oceans extend from the North to
the South Pole, the major river basins, e.g. the Amazon and Danube, are more
numerous near the equator, and approximately 508 latitude.
(a ) Methane migration
The Earth’s rivers prevail at latitudes of greater occurrence of precipitation,
which, in turn, correlates with the Earth’s circulation. The equator experiences
updrafts and condensation associated with the Hadley circulation; and slantwise
convection, connected with the polar front, develops at high latitudes. Evaporation
transpires preferentially at more arid latitudes approximately 258 north and south
of the equator, where the circulation causes subsidence and associated warming.
On average, water vapour blows out of the Southern Hemisphere and into
the Northern Hemisphere. This would lead to the accumulation of water in the
Northern Hemisphere, if the nearly pole-to-pole oceans did not counteract this
effect with a flow of water into the Southern Hemisphere.
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Titan’s weather
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For Titan, where global ocean currents do not balance the atmospheric
transport of the condensable, the question arises as to whether the methane is
transported by the circulation over long time scales to accumulate at specific
latitudes. Titan’s cloud and circulation patterns suggest that, currently, methane
moves from the Northern to the Southern Hemisphere, where rain prevails. Yet
the moon’s circulation will change dramatically, causing a Hadley cell circulation
briefly at the equinox (in 2010) before flipping the pole-to-pole circulation to
incur updrafts in the north and subsidence in the south (Hourdin et al. 1995;
Mitchel et al. 2006; Rannou et al. 2006). The long-term effect, according to a
most recent GCM calculation, is the accumulation of methane at Titan’s poles
(Rannou et al. 2006). Polar lakes are stable against evaporation because there is
insufficient daily and seasonal surface insolation variations to evaporate them
(Griffith et al. 2008). The non-detection of tropical lakes and prevalence of
tropical dunes further support the hypothesis that the poles are wet at the
expense of drying the tropics.
Yet the fluvial features in the tropics suggest rainfall. The morphology of
the south-flowing ‘washes’ north of the Huygens landing site indicates a
comprehensive drainage system in the highlands that results from rainfall
(Soderblom et al. 2007b). This terrain exhibits slopes as high as 308,
suggesting a formation more recent than the time scale for wind erosion
(Soderblom et al. 2007b). The actual landing site is a flood plain that runs
east to west, which appears to have been vigorous enough to mould rounded
cobbles (Soderblom et al. 2007b). The formation of oases such as this one,
interspersed among dunes, is unclear. Current conditions, as evident from
Titan’s temperature and methane profiles, are not conducive to rain (Tokano
et al. 2006a; Barth & Raftkin 2007; Griffith et al. 2008). Seasonal updrafts
predicted at tropical latitudes (Mitichel et al. 2006; Rannou et al. 2006) may
lead to greater cloud formation, as seen at K408 latitude and near the South
Pole. Yet, without an increase in humidity, the atmosphere will remain only
weakly unstable; thus, mildly convective clouds extending to approximately
26 km rather than deeply convective rainstorms that reach the tropopause are
expected (Griffith et al. 2008). Cryo-volcanism could locally humidify the
atmosphere and instigate rainstorms, although evidence is lacking for
their presence at the locations of the dendritic features. The washes may be
a relic of a wetter climate in the past; however, ruggedness of the terrain
suggests youth.
5. Titan’s weather: a deranged version of the Earth
Titan’s methane cycle differs from the Earth’s hydrological cycle. Current
research indicates that the dissimilarities stem from the fundamental properties
of these bodies. Titan’s slow spin rate allows for a pole-to-pole circulation; its
orbital period (29.5 Earth years) affects long seasons; and its distance to the Sun
lowers the solar heating two orders of magnitude. Titan’s large atmospheric mass
contributes to a large radiative time constant. The chemistry of the atmosphere,
particularly the presence of methane, strongly differentiates Titan from the
Earth. The photolysis of methane and the production of haze causes the
atmosphere to be optically thick and limits surface insolation to 10 per cent, and
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724
C. A. Griffith
ethane
abundance
atmospheric
mass
distance to
the Sun
atmospheric
opacity
TR > TS
stability of
polar lakes
winter
polar clouds
lakes at poles
variation in
polar humidity
tropical clouds
winter mid-latitude
clouds
surface methane
depth
length of year
high CH4
vapour pressure
atmospheric
CH4 abundance
circulation
surface liquids
in tropics
variation in
tropical humidity
summer mid-latitude
clouds
summer polar
clouds
Figure 4. Hierarchy of knowledge and deduction. The top white bubbles indicate fundamental
properties of Titan’s atmosphere that determine Titan’s weather. These properties of Titan’s orbit,
and the atmosphere’s chemical, radiative and thermodynamical properties, affect (as shown by the
arrows) the dynamics, the atmospheric and the surface distribution of methane and ethane (grey
bubbles), which, in turn, control the weather. Here, TR is the radiative time constant and TS is the
duration of a season. The fundamental characteristics that are poorly constrained across the disc
(outlined in black) profoundly affect our understanding of the circulation, latent heat effects,
surface–atmosphere interface, the structure of the polar atmosphere, the formation and evolution
of clouds and the long-term transport of methane. Attributes that are currently observable are
outlined in grey. Note that local effects, such as volcanism and orographic winds, which influence
cloud formation, are poorly constrained and not included here.
the power for convection and evaporation to approximately 1 per cent of the
incident sunlight. Clouds are therefore sparse, by terrestrial standards. The
saturation vapour pressure of methane allows the atmosphere to accumulate over
two orders of magnitude more condensables than possible on the Earth, thereby
endowing the atmosphere with a greater latent heat potential. As on Earth,
clouds appear to follow updrafts in Titan’s circulation, and thereby frequent
latitudes uncommon to their terrestrial counterparts. However, studies of Titan’s
atmosphere are still young. The effects of ethane are unknown; the methane
abundance profile has only been measured at one place and one time; the depth
and composition of the oceans cannot yet be measured; the atmospheric opacity
is ill constrained at the poles; the dampness of the tropical surface is
unconstrained except at the probe landing site (figure 4). Local incongruities,
such as surface puddles, orographic winds, outgassing and methane vents, are
entirely uncharacterized. This lack of information limits our understanding of the
energy partitioning at the surface, the thermodynamic effects of ethane, the
surface forcing of the circulation, the local dynamical conditions and the local
stability of the atmosphere. Present studies may almost appear to predict the
workings of Titan’s methane cycle from an analysis of the hydrological cycle and
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Titan’s weather
725
the replacement of Earth’s fundamental properties with those of Titan. Yet, this
uncomplicated impression may be superficial as it is based on a handful of
observational data.
Appendix A. Growth of the boundary layer
The time scale, TPBL, for the growth of the planetary boundary layer is estimated
by assuming that all of the globally averaged surface insolation goes into sensible
heat, Hs. This value is an overestimate for both Titan and the Earth, as shown in
figure 3. The sensible heat warms and mixes the lower atmosphere to form a
growing layer of uniform potential temperature and composition. For a quiescent
atmosphere, one can estimate this process thermodynamically. A boundary layer
of height ZB and approximate constant density, r, and cp experiences a change in
virtual temperature, Tv, with time of dTv/dtZHs/ZBcpr. This layer grows such
that its height occurs at the intersection of its constant potential temperature
and the ambient potential temperature: dZB =dtZ ðdTv =dtÞ=ðdTv =dz C g=cp Þamb .
Based on Titan’s thermal profile (Fulchignoni et al. 2005), the difference between
the dry and measured lapse rates (the denominator) is approximately
0.26 K kmK1, roughly one-fifth of the lapse rate between the surface and 6 km.
For the Earth, the ambient lapse rate is also assumed to be 20 per cent less steep
than the dry value.
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