While Saturn's moon Titan appears to support an active methane hydrological cycle, no direct evidence for surface-atmosphere exchange has yet appeared. The indirect evidence, while compelling, could be misleading. It is possible, for example, that the identified lake features could be filled with ethane, an involatile long-term residue of atmospheric photolysis; the apparent stream and channel features could be ancient remnants of a previous climate; and the tropospheric methane clouds, while frequent, could cause no rain to reach the surface. We report here the detection of fog at the south pole of Titan during late summer using observations from the VIMS instrument on board the Cassini spacecraft. While terrestrial fog can form from a variety of causes, most of these processes are inoperable on Titan. Fog on Titan can only be caused by evaporation of nearly pure liquid methane; the detection of fog provides the first direct link between surface and atmospheric methane. Based on the detections presented here, liquid methane appears widespread at the south pole of Titan in late southern summer, and the hydrological cycle on Titan is currently active

Brown, M. E.

Chen, C.

Smith, A. L.

Ádámkovics, M.

Caltech Authors - Main

The Astrophysical Journal, 706:L110–L113, 2009 November 20 doi:10.1088/0004-637X/706/1/L110
C© 2009. The American Astronomical Society. All rights reserved. Printed in the U.S.A.
DISCOVERY OF FOG AT THE SOUTH POLE OF TITAN
M. E. Brown
1
, A. L. Smith
1
, C. Chen
1
, and M. Ádámkovics
2
1 Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA; mbrown@caltech.edu
2 Department of Astronomy, University of California, Berkeley, CA 94720, USA
Received 2009 August 27; accepted 2009 October 16; published 2009 November 2
ABSTRACT
While Saturn’s moon Titan appears to support an active methane hydrological cycle, no direct evidence for surface–
atmosphere exchange has yet appeared. The indirect evidence, while compelling, could be misleading. It is possible,
for example, that the identified lake features could be filled with ethane, an involatile long-term residue of atmo-
spheric photolysis; the apparent stream and channel features could be ancient remnants of a previous climate;
and the tropospheric methane clouds, while frequent, could cause no rain to reach the surface. We report here the
detection of fog at the south pole of Titan during late summer using observations from the VIMS instrument on
board the Cassini spacecraft. While terrestrial fog can form from a variety of causes, most of these processes are
inoperable on Titan. Fog on Titan can only be caused by evaporation of nearly pure liquid methane; the detection
of fog provides the first direct link between surface and atmospheric methane. Based on the detections presented
here, liquid methane appears widespread at the south pole of Titan in late southern summer, and the hydrological
cycle on Titan is currently active.
Key words: infrared: solar system – planets and satellites: individual (Titan)
1. INTRODUCTION
Saturn’s moon Titan appears to support an active methane hy-
drological cycle, with evidence for tropospheric clouds (Griffith
et al. 1998; Brown et al. 2002; Roe et al. 2005), polar lakes
(Stofan et al. 2007), surface changes (Turtle et al. 2009), and
liquid carved channels (Tomasko et al. 2005; Soderblom et al.
2007; Lorentz et al. 2008). Circulation models suggest that liq-
uid methane could be predominantly confined to high latitudes
and that methane should be seasonally transported from summer
pole to summer pole (Mitchell 2008). Yet little concrete evidence
for the surface–atmosphere interactions required for this cycle
has been seen. The clouds may produce no rain (Schaller et al.
2006), the large polar lakes could be filled with non-evaporating
ethane, rather than methane (Brown et al. 2008), the cause of
surface albedo changes is totally unknown (Turtle et al. 2009),
and the channels could be a record of an ancient climate (Griffith
et al. 2008).
One signature of a currently active hydrological cycle on
Titan would be the detection of localized evaporation of liquid
methane from the surface. Though evaporation itself is not
directly visible, its effects could possibly be detected. While air
on Titan has a methane relative humidity of ∼50% (Niemann
et al. 2005), near-surface air in direct contact with evaporating
methane could locally acquire humidities surface near 100%,
allowing the methane to condense into surface-level fog. While
methane condensation is clearly seen by the presence of clouds
high in the troposphere (where air with 50% humidity can
become saturated through lifting), no surface-level fog has yet
been reported.
Here we describe a search for surface fog on Titan, discuss the
details of possible formation mechanisms for fog, and consider
the implications for Titan’s hydrological cycle.
2. OBSERVATIONS
To search for fog on Titan we examined all data publicly
available through the NASA Planetary Data System database
from the VIMS (Brown et al. 2004) instrument on the Cassini
spacecraft. VIMS is a hyperspectral imager, obtaining near-
simultaneous images in up to 256 channels between 1 μm and
5 μm. This capability, coupled with several strong methane ab-
sorption features in Titan’s atmosphere throughout this spectral
region, allows us to sum multiple wavelength images to con-
struct synthetic filters which probe to different depths in Titan’s
atmosphere. In Brown et al. (2009a, 2009b), we created three
synthetic filters which allowed us to probe to the surface, to
the troposphere, and to the stratosphere separately. Using the
three filters, we could quickly discern which features were sur-
face features, tropospheric clouds, or stratospheric hazes. Here,
we use the same three synthetic filters, and we also create an ad-
ditional filter from the sum of all channels from 4.95 to 5.12 μm.
This region of the spectrum is transparent all the way to the sur-
face and is sensitive to scattering from cloud particles.
Fog, if present, would appear as a bright feature visible in the
synthetic filter that probes to the surface, but it would not appear
in the filters that probe only to the troposphere and stratosphere.
(The many tropospheric clouds, in contrast, appear bright in
both the surface and tropospheric filters.) Fog (and clouds)
would also appear bright in the 5 μm filter where the surface
is generally dark but cloud particles are bright. Fog would be
indistinguishable from a bright albedo mark on the surface of
Titan except that fog could be highly variable. Our strategy to
search for fog on Titan was thus to search for features which
appear bright in the surface and 5 μm filters, which do not
appear in the troposphere and stratosphere filters, and which are
temporally variable.
For each of the ∼9000 VIMS hyperspectral observation of
Titan available to date, we created the four synthetic filter
images and redisplayed them in a south polar projection. We
then visually inspected each set of images to search for features
which had the expected characteristics of fog. While examining
these images, we found that multiple images of a single location
viewed at different solar and spacecraft geometries appear
subtly different; we thus only considered a detection to be
significant when the surface filter and 5 μm filter simultaneously
showed a feature as bright as any feature ever observed at the
south pole. Four such features, which we will temporarily call
“fog-like,” were identified. The best examples are shown in
L110
No. 1, 2009 DISCOVERY OF FOG AT THE SOUTH POLE OF TITAN L111
Figure 1. Views of the south pole of Titan on four separate dates. Fog-like features, marked with green arrows, can be seen in the synthetic filter which probes to
Titan’s surface, and in the 5 μm filter, which is particularly sensitive to large cloud particles. These variable surface features do not appear in the synthetic troposphere
filter, which is insensitive to scattering below ∼10 km. The images are all shown as identical polar projections with lines of latitude between −60 and −90 shown
every 10 degrees and with 0 degree longitude at the top.
Figure 1. These images reveal the typical wispy appearance
of these features when seen at moderate spatial resolution.
Indeed, the morphological appearance of the 2007 March 25
image is particularly suggestive: features which appear in the
troposphere-probing filter appear related to those which appear
only in the surface filter, suggesting, perhaps, surface fog which
is also rising into the troposphere in places.
A more detailed examination of the full 1–5 μm spectra of
these features can provide more clues to their origin. Figure 2
shows an example of comparisons between the spectra of
the surface, a tropospheric cloud, and a fog-like feature. The
fog-like features appear spectrally unlike any surface unit at
the south pole. In spectral regions that are transparent all
the way to the surface, the fog-like feature appears identical
to the tropospheric cloud, including, most dramatically, the
high reflectivity near 5 μm when compared to the surface.
Tropospheric clouds are bright at these wavelengths because
they are composed of bright single scattering particles with
sizes larger than 5 μm (Barnes et al. 2005; Griffith et al.
2005), as expected for condensation of an abundant species
like methane. Higher elevation ethane clouds, in contrast, are
dark at 5 μm because they are made from smaller particles, as
expected for condensation from a minor constituent (Griffith
et al. 2006). The spectral similarity of the fog-like feature
to tropospheric methane clouds suggests a similar particle
size and thus similar high atmospheric abundance. Methane is
the only condensible with such a high abundance in Titan’s
atmosphere. In spectral regions where transmission to the
surface is significantly attenuated, but where transmission to
the troposphere is high (the 2.1 μm region, for example),
tropospheric clouds appear bright while the surface and fog-
like features are dark, suggesting again that the fog-like features
originate from close to the surface.
3. ANALYSIS
To determine the altitude of the fog-like feature, we perform
full calculations of the radiative transfer through Titan’s atmo-
sphere using the method of Ádámkovics et al. (2009; see also
Ádámkovics et al. 2007) which takes Huygens measurements of
temperature, pressure, composition, and haze profiles as initial
starting points and solves the radiative transfer equation for 16
pseudo-plane-parallel layers from 0–200 km altitude. While ac-
curate radiative transfer calculations through the poorly known
L112 BROWN ET AL. Vol. 706
Figure 2. VIMS images of the 2006 October 25 fog. The surface, 5 μm, and troposphere images use the same synthetic filters as in Figure 1. Areas of surface, fog,
and tropospheric cloud are shown by green, red, and black ovals, respectively. The full spectra of each of these selected regions are shown in the same colors. The
surface, fog, and cloud spectra clearly differ. Both the fog and the cloud are bright at 5 μm, while the fog appears intermediate between the surface and troposphere in
the 2.5–3 μm region.
south polar atmosphere on Titan are fraught with uncertainty, we
side-step many of these difficulties by instead performing simple
comparisons of adjacent areas of the image with and without
fog-like features. Assuming that the large-scale atmospheric
scattering and opacity do not change significantly between
these regions, which are only 400 km apart, we can accurately
model the relative effect of adding fog at varying heights to the
spectrum.
First, we attempt to reproduce the spectrum of the surface
immediately adjacent to the fog-like feature. We model the upper
atmosphere haze profile by scaling the Ádámkovics et al. (2009)
Huygens-derived aerosol opacity profile until the spectrum
beyond 2.15 μm (which is only sensitive to regions above the
tropopause) is reproduced. Next we model the spectrum below
2.15 μm by first scaling the modeled surface reflectivity until
the modeled spectrum matches the observations in the peak
of the 2 μm window and determining the apparent reflectivity
of the remainder of the spectral region. At this point, remaining
discrepancies between the model and measured spectrum are
assumed to be due to the wavelength dependence of the surface
reflectivity. This modeling of the spectrum of the surface as seen
through Titan’s atmosphere is not unique, but provides a basis
for comparing the spectra at adjacent locations.
After matching the surface spectrum, we calculate the spectral
effect of clouds and fog by adding a series of scattering
layers of varying elevations and opacities to the model. In
these models, cloud particles are assumed to be large and thus
uniformly scattering at all wavelengths, with albedos of 0.99
and Henyey–Greenstein scattering parameters of 0.85 (varying
these parameters over wide ranges had little impact on the final
spectrum). Figure 3 shows modeled spectrum for a series of
optically thin clouds (τ = 0.25) of varying altitudes overlying a
surface of reflectivity 0.10. The spectrum of the fog-like feature
is best fit by a cloud with an altitude of 750 m. Models with
cloud heights of 3 km or higher differ significantly from the
data in the 1.98 μm and the 2.09–2.14 μm regions which are
particularly sensitive to the lower troposphere. We thus conclude
that the fog-like feature is indeed best described as surface fog
or perhaps near-surface fog on Titan. No cloud or fog at such a
low elevation has ever before been identified on Titan.
2.0 2.1 2.2 2.3 2.4
Wavelength
0.00
0.01
0.02
0.03
0.04
0.05
0.06
0.07
I/F
obs (clear)
obs (fog)
model (clear)
0.25 km
0.75 km
1.5 km
3 km
5.5 km
6.5 km
7.5 km
8.5 km
9.5 km
11 km
13 km
15 km
17 km
20 km
Figure 3. Spectra of the surface (filled circles) and fog (black open squares) in
the 2 μm spectral window where altitude is best constrained. The purple line
shows the best-fit radiative transfer model which matches the surface. Colored
lines show fits to the fog spectra that include an increased surface reflectivity
and a scattering cloud layer at altitudes between 0.25 km and 20 km. Cloud
altitudes near 750 m most closely match the observations, whereas models with
cloud tops above 3 km altitude are inconsistent with the observed spectra.
4. DISCUSSION
Fog forms when the vapor in ground-level air saturates with
some species which then condenses. On the Earth, this saturation
commonly occurs when air radiatively cools overnight until
it reaches the water dew point. On Titan, such a formation
mechanism is impossible. Titan’s lower atmosphere has a
radiative time constant of ∼100 yr (Hunten et al. 1984) and
94 K air that has a relative methane humidity of ∼50% must be
cooled ∼7 K before methane condensation will initiate (mixing
ratios of any other condensible species are negligible). Similarly,
advection of typical Titan air over even the coldest locations
on Titan provides insufficient cooling for any composition fog
to form. Fog on Titan instead requires elevating the mixing
No. 1, 2009 DISCOVERY OF FOG AT THE SOUTH POLE OF TITAN L113
Figure 4. Locations of all identified clouds features. Blue, purple, red, and green
are from 2006 October 25, 2007 January 29, 2007 March 9, and 2007 March 25,
respectively. The background shows a south polar base map of Titan as derived
from the visible imager on the ISS instrument.
ratio of the fog-forming condensible species rather than simply
decreasing temperature. If the fog is near ground level, the
surface relative humidity of the condensible species must be
nearly 100%.
There are few ways to produce such a locally elevated
relative humidity of a condensible species on Titan, short of
some unphysically motivated mechanical injection mechanism.
Indeed, we find no physically plausible explanation other than to
hypothesize that saturation occurs where surface air is in direct
contact with nearly pure evaporating liquid methane. Liquid
methane appears clearly implicated: the fog is composed of large
particles, like those of the higher level tropospheric methane
clouds, suggesting condensation of an abundant species, and
methane is the only major surface constituent with a non-
negligible vapor pressure. Methane evaporation alone can lead
to significant condensible concentrations. The liquid methane
must be nearly pure because evaporation into overlying air can
only elevate the humidity of the near-surface air to a value as
high as the mixing ratio of the methane in the liquid, thus, to
raise the humidity to nearly 100% requires nearly pure liquid
methane.
While fog requires saturated near-surface air, methane satu-
rated near-surface air on Titan is unstable to moist convection for
a typical Titan thermal profile (Griffith et al. 2008). Rather than
forming fog, such air will simply rise through the troposphere.
Fog can only persist at the surface if the surface air is both satu-
rated in methane and colder (and thus more stable) than the typ-
ical thermal profile. Pools of evaporating liquid methane could
indeed be cooler than their surroundings (Mitri et al. 2007) and,
under the right meteorological conditions, will add humidity to
and drain heat from overlying air parcels. No other formation ex-
planation can naturally explain both the increased humidity and
decreased temperature required. This formation mechanism also
naturally explains the occasional correlation between fog and
overlying tropospheric clouds. If surface humidity is raised with-
out a sufficient decrease in temperature, saturated air parcels will
convect into the upper troposphere (Griffith et al. 2000). Much of
the continued south polar cloudiness (Brown et al. 2009b) may
be tied directly to surface liquid methane evaporating and rising
unstably.
The locations of the identified fog features and a comparison
to the ISS base map3 are shown in Figure 4. All identified fog
features are southward of 65 S. No correlation is seen between
the locations of fog and the location of the one suspected large
lake, Ontario Lacus—which perhaps is a reservoir of ethane only
or mixed ethane–methane (Brown et al. 2008)—the locations of
dark albedo features, or the location of the large observed albedo
change (Turtle et al. 2009). No temporal association with known
south polar tropospheric cloud outburst appears (Schaller et al.
2006, 2009).
Fog is likely a more common occurrence than shown here; it is
difficult to identify in the typical low-resolution images obtained
by VIMS, but seen in nearly all high-resolution south polar
images. Liquid methane and evaporation are likely distributed
even more widely than the observed fog; all evaporation need not
cause fog: special meteorological conditions such as low winds
are also likely required. Liquid methane appears widespread at
the south pole of Titan in the late southern summer.
This research has been supported by a grant from the NSF
Planetary Astronomy program.
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Ádámkovics, M., de Pater, I., Hartung, M., & Barnes, J. W. 2009, Planet. Space
Sci., in press (arXiv:0907.2255A)
Ádámkovics, M., Wong, M. H., Laver, C., & de Pater, I. 2007, Science, 318,
962
Barnes, J. W., et al. 2005, Science, 310, 92
Brown, M. E., Bouchez, A. H., & Griffith, C. A. 2002, Nature, 420,
795
Brown, M. E., Roberts, J., & Schaller, E. L. 2009a, Icarus, in press
Brown, M. E., Schaller, E. L., Roe, H. G., Chen, C., Roberts, J., Brown, R. H.,
Baines, K. H., & Clark, R. N. 2009b, Geophys. Res. Lett., 36, 01103
Brown, R. H., et al. 2004, Space Sci. Rev., 115, 111
Brown, R. H., et al. 2008, Nature, 454, 607
Griffith, C. A., Hall, J. L., & Geballe, T. R. 2000, Science, 290, 509
Griffith, C. A., McKay, C. P., & Ferri, F. 2008, ApJ, 687, L41
Griffith, C. A., Owen, T., Miller, G. A., & Geballe, T. 1998, Nature, 395,
575
Griffith, C. A., et al. 2005, Science, 310, 474
Griffith, C. A., et al. 2006, Science, 313, 1620
Hunten, D. M., Tomasko, M. G., Flasar, F. M., Samuelson, R. E., Strobel, D. F.,
& Stevenson, D. J. 1984, Saturn (Tucson, AZ: Univ. Arizona Press), 671
Lorentz, R. D., et al. & Cassini Radar Team 2008, Planet. Space Sci., 56, 1132
Mitchell, J. L. 2008, JGR-Planets, in press
Mitri, G., Showman, A. P., Lunine, J. I., & Lorenz, R. D. 2007, Icarus, 186,
385
Niemann, H. B., et al. 2005, Nature, 438, 779
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2005, ApJ, 618, L49
Schaller, E. L., Brown, M. E., Roe, H. G., & Bouchez, A. H. 2006, Icarus, 182,
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Soderblom, L. A., et al. 2007, Planet. Space Sci., 55, 2015
Stofan, E. R., et al. 2007, Nature, 445, 61
Tomasko, M. G., et al. 2005, Nature, 438, 765
Turtle, E. P., Perry, J. E., McEwen, A. S., DelGenio, A. D., Barbara, J., West,
R. A., Dawson, D. D., & Porco, C. C. 2009, Geophys. Res. Lett., 36, 02204
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English

Discovery of Fog at the South Pole of Titan

The Astrophysical Journal, 706:L110–L113, 2009 November 20 doi:10.1088/0004-637X/706/1/L110
C© 2009. The American Astronomical Society. All rights reserved. Printed in the U.S.A.
DISCOVERY OF FOG AT THE SOUTH POLE OF TITAN
M. E. Brown1, A. L. Smith1, C. Chen1, and M. A´da´mkovics2
1 Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA; mbrown@caltech.edu
2 Department of Astronomy, University of California, Berkeley, CA 94720, USA
Received 2009 August 27; accepted 2009 October 16; published 2009 November 2
ABSTRACT
While Saturn’s moon Titan appears to support an active methane hydrological cycle, no direct evidence for surface–
atmosphere exchange has yet appeared. The indirect evidence, while compelling, could be misleading. It is possible,
for example, that the identified lake features could be filled with ethane, an involatile long-term residue of atmo-
spheric photolysis; the apparent stream and channel features could be ancient remnants of a previous climate;
and the tropospheric methane clouds, while frequent, could cause no rain to reach the surface. We report here the
detection of fog at the south pole of Titan during late summer using observations from the VIMS instrument on
board the Cassini spacecraft. While terrestrial fog can form from a variety of causes, most of these processes are
inoperable on Titan. Fog on Titan can only be caused by evaporation of nearly pure liquid methane; the detection
of fog provides the first direct link between surface and atmospheric methane. Based on the detections presented
here, liquid methane appears widespread at the south pole of Titan in late southern summer, and the hydrological
cycle on Titan is currently active.
Key words: infrared: solar system – planets and satellites: individual (Titan)
1. INTRODUCTION
Saturn’s moon Titan appears to support an active methane hy-
drological cycle, with evidence for tropospheric clouds (Griffith
et al. 1998; Brown et al. 2002; Roe et al. 2005), polar lakes
(Stofan et al. 2007), surface changes (Turtle et al. 2009), and
liquid carved channels (Tomasko et al. 2005; Soderblom et al.
2007; Lorentz et al. 2008). Circulation models suggest that liq-
uid methane could be predominantly confined to high latitudes
and that methane should be seasonally transported from summer
pole to summer pole (Mitchell 2008). Yet little concrete evidence
for the surface–atmosphere interactions required for this cycle
has been seen. The clouds may produce no rain (Schaller et al.
2006), the large polar lakes could be filled with non-evaporating
ethane, rather than methane (Brown et al. 2008), the cause of
surface albedo changes is totally unknown (Turtle et al. 2009),
and the channels could be a record of an ancient climate (Griffith
et al. 2008).
One signature of a currently active hydrological cycle on
Titan would be the detection of localized evaporation of liquid
methane from the surface. Though evaporation itself is not
directly visible, its effects could possibly be detected. While air
on Titan has a methane relative humidity of ∼50% (Niemann
et al. 2005), near-surface air in direct contact with evaporating
methane could locally acquire humidities surface near 100%,
allowing the methane to condense into surface-level fog. While
methane condensation is clearly seen by the presence of clouds
high in the troposphere (where air with 50% humidity can
become saturated through lifting), no surface-level fog has yet
been reported.
Here we describe a search for surface fog on Titan, discuss the
details of possible formation mechanisms for fog, and consider
the implications for Titan’s hydrological cycle.
2. OBSERVATIONS
To search for fog on Titan we examined all data publicly
available through the NASA Planetary Data System database
from the VIMS (Brown et al. 2004) instrument on the Cassini
spacecraft. VIMS is a hyperspectral imager, obtaining near-
simultaneous images in up to 256 channels between 1 μm and
5 μm. This capability, coupled with several strong methane ab-
sorption features in Titan’s atmosphere throughout this spectral
region, allows us to sum multiple wavelength images to con-
struct synthetic filters which probe to different depths in Titan’s
atmosphere. In Brown et al. (2009a, 2009b), we created three
synthetic filters which allowed us to probe to the surface, to
the troposphere, and to the stratosphere separately. Using the
three filters, we could quickly discern which features were sur-
face features, tropospheric clouds, or stratospheric hazes. Here,
we use the same three synthetic filters, and we also create an ad-
ditional filter from the sum of all channels from 4.95 to 5.12 μm.
This region of the spectrum is transparent all the way to the sur-
face and is sensitive to scattering from cloud particles.
Fog, if present, would appear as a bright feature visible in the
synthetic filter that probes to the surface, but it would not appear
in the filters that probe only to the troposphere and stratosphere.
(The many tropospheric clouds, in contrast, appear bright in
both the surface and tropospheric filters.) Fog (and clouds)
would also appear bright in the 5 μm filter where the surface
is generally dark but cloud particles are bright. Fog would be
indistinguishable from a bright albedo mark on the surface of
Titan except that fog could be highly variable. Our strategy to
search for fog on Titan was thus to search for features which
appear bright in the surface and 5 μm filters, which do not
appear in the troposphere and stratosphere filters, and which are
temporally variable.
For each of the ∼9000 VIMS hyperspectral observation of
Titan available to date, we created the four synthetic filter
images and redisplayed them in a south polar projection. We
then visually inspected each set of images to search for features
which had the expected characteristics of fog. While examining
these images, we found that multiple images of a single location
viewed at different solar and spacecraft geometries appear
subtly different; we thus only considered a detection to be
significant when the surface filter and 5 μm filter simultaneously
showed a feature as bright as any feature ever observed at the
south pole. Four such features, which we will temporarily call
“fog-like,” were identified. The best examples are shown in
L110
No. 1, 2009 DISCOVERY OF FOG AT THE SOUTH POLE OF TITAN L111
Figure 1. Views of the south pole of Titan on four separate dates. Fog-like features, marked with green arrows, can be seen in the synthetic filter which probes to
Titan’s surface, and in the 5 μm filter, which is particularly sensitive to large cloud particles. These variable surface features do not appear in the synthetic troposphere
filter, which is insensitive to scattering below ∼10 km. The images are all shown as identical polar projections with lines of latitude between −60 and −90 shown
every 10 degrees and with 0 degree longitude at the top.
Figure 1. These images reveal the typical wispy appearance
of these features when seen at moderate spatial resolution.
Indeed, the morphological appearance of the 2007 March 25
image is particularly suggestive: features which appear in the
troposphere-probing filter appear related to those which appear
only in the surface filter, suggesting, perhaps, surface fog which
is also rising into the troposphere in places.
A more detailed examination of the full 1–5 μm spectra of
these features can provide more clues to their origin. Figure 2
shows an example of comparisons between the spectra of
the surface, a tropospheric cloud, and a fog-like feature. The
fog-like features appear spectrally unlike any surface unit at
the south pole. In spectral regions that are transparent all
the way to the surface, the fog-like feature appears identical
to the tropospheric cloud, including, most dramatically, the
high reflectivity near 5 μm when compared to the surface.
Tropospheric clouds are bright at these wavelengths because
they are composed of bright single scattering particles with
sizes larger than 5 μm (Barnes et al. 2005; Griffith et al.
2005), as expected for condensation of an abundant species
like methane. Higher elevation ethane clouds, in contrast, are
dark at 5 μm because they are made from smaller particles, as
expected for condensation from a minor constituent (Griffith
et al. 2006). The spectral similarity of the fog-like feature
to tropospheric methane clouds suggests a similar particle
size and thus similar high atmospheric abundance. Methane is
the only condensible with such a high abundance in Titan’s
atmosphere. In spectral regions where transmission to the
surface is significantly attenuated, but where transmission to
the troposphere is high (the 2.1 μm region, for example),
tropospheric clouds appear bright while the surface and fog-
like features are dark, suggesting again that the fog-like features
originate from close to the surface.
3. ANALYSIS
To determine the altitude of the fog-like feature, we perform
full calculations of the radiative transfer through Titan’s atmo-
sphere using the method of ´Ada´mkovics et al. (2009; see also
´Ada´mkovics et al. 2007) which takes Huygens measurements of
temperature, pressure, composition, and haze profiles as initial
starting points and solves the radiative transfer equation for 16
pseudo-plane-parallel layers from 0–200 km altitude. While ac-
curate radiative transfer calculations through the poorly known
L112 BROWN ET AL. Vol. 706
Figure 2. VIMS images of the 2006 October 25 fog. The surface, 5 μm, and troposphere images use the same synthetic filters as in Figure 1. Areas of surface, fog,
and tropospheric cloud are shown by green, red, and black ovals, respectively. The full spectra of each of these selected regions are shown in the same colors. The
surface, fog, and cloud spectra clearly differ. Both the fog and the cloud are bright at 5 μm, while the fog appears intermediate between the surface and troposphere in
the 2.5–3 μm region.
south polar atmosphere on Titan are fraught with uncertainty, we
side-step many of these difficulties by instead performing simple
comparisons of adjacent areas of the image with and without
fog-like features. Assuming that the large-scale atmospheric
scattering and opacity do not change significantly between
these regions, which are only 400 km apart, we can accurately
model the relative effect of adding fog at varying heights to the
spectrum.
First, we attempt to reproduce the spectrum of the surface
immediately adjacent to the fog-like feature. We model the upper
atmosphere haze profile by scaling the ´Ada´mkovics et al. (2009)
Huygens-derived aerosol opacity profile until the spectrum
beyond 2.15 μm (which is only sensitive to regions above the
tropopause) is reproduced. Next we model the spectrum below
2.15 μm by first scaling the modeled surface reflectivity until
the modeled spectrum matches the observations in the peak
of the 2 μm window and determining the apparent reflectivity
of the remainder of the spectral region. At this point, remaining
discrepancies between the model and measured spectrum are
assumed to be due to the wavelength dependence of the surface
reflectivity. This modeling of the spectrum of the surface as seen
through Titan’s atmosphere is not unique, but provides a basis
for comparing the spectra at adjacent locations.
After matching the surface spectrum, we calculate the spectral
effect of clouds and fog by adding a series of scattering
layers of varying elevations and opacities to the model. In
these models, cloud particles are assumed to be large and thus
uniformly scattering at all wavelengths, with albedos of 0.99
and Henyey–Greenstein scattering parameters of 0.85 (varying
these parameters over wide ranges had little impact on the final
spectrum). Figure 3 shows modeled spectrum for a series of
optically thin clouds (τ = 0.25) of varying altitudes overlying a
surface of reflectivity 0.10. The spectrum of the fog-like feature
is best fit by a cloud with an altitude of 750 m. Models with
cloud heights of 3 km or higher differ significantly from the
data in the 1.98 μm and the 2.09–2.14 μm regions which are
particularly sensitive to the lower troposphere. We thus conclude
that the fog-like feature is indeed best described as surface fog
or perhaps near-surface fog on Titan. No cloud or fog at such a
low elevation has ever before been identified on Titan.
2.0 2.1 2.2 2.3 2.4
Wavelength
0.00
0.01
0.02
0.03
0.04
0.05
0.06
0.07
I/F
obs (clear)
obs (fog)
model (clear)
0.25 km
0.75 km
1.5 km
3 km
5.5 km
6.5 km
7.5 km
8.5 km
9.5 km
11 km
13 km
15 km
17 km
20 km
Figure 3. Spectra of the surface (filled circles) and fog (black open squares) in
the 2 μm spectral window where altitude is best constrained. The purple line
shows the best-fit radiative transfer model which matches the surface. Colored
lines show fits to the fog spectra that include an increased surface reflectivity
and a scattering cloud layer at altitudes between 0.25 km and 20 km. Cloud
altitudes near 750 m most closely match the observations, whereas models with
cloud tops above 3 km altitude are inconsistent with the observed spectra.
4. DISCUSSION
Fog forms when the vapor in ground-level air saturates with
some species which then condenses. On the Earth, this saturation
commonly occurs when air radiatively cools overnight until
it reaches the water dew point. On Titan, such a formation
mechanism is impossible. Titan’s lower atmosphere has a
radiative time constant of ∼100 yr (Hunten et al. 1984) and
94 K air that has a relative methane humidity of ∼50% must be
cooled ∼7 K before methane condensation will initiate (mixing
ratios of any other condensible species are negligible). Similarly,
advection of typical Titan air over even the coldest locations
on Titan provides insufficient cooling for any composition fog
to form. Fog on Titan instead requires elevating the mixing
No. 1, 2009 DISCOVERY OF FOG AT THE SOUTH POLE OF TITAN L113
Figure 4. Locations of all identified clouds features. Blue, purple, red, and green
are from 2006 October 25, 2007 January 29, 2007 March 9, and 2007 March 25,
respectively. The background shows a south polar base map of Titan as derived
from the visible imager on the ISS instrument.
ratio of the fog-forming condensible species rather than simply
decreasing temperature. If the fog is near ground level, the
surface relative humidity of the condensible species must be
nearly 100%.
There are few ways to produce such a locally elevated
relative humidity of a condensible species on Titan, short of
some unphysically motivated mechanical injection mechanism.
Indeed, we find no physically plausible explanation other than to
hypothesize that saturation occurs where surface air is in direct
contact with nearly pure evaporating liquid methane. Liquid
methane appears clearly implicated: the fog is composed of large
particles, like those of the higher level tropospheric methane
clouds, suggesting condensation of an abundant species, and
methane is the only major surface constituent with a non-
negligible vapor pressure. Methane evaporation alone can lead
to significant condensible concentrations. The liquid methane
must be nearly pure because evaporation into overlying air can
only elevate the humidity of the near-surface air to a value as
high as the mixing ratio of the methane in the liquid, thus, to
raise the humidity to nearly 100% requires nearly pure liquid
methane.
While fog requires saturated near-surface air, methane satu-
rated near-surface air on Titan is unstable to moist convection for
a typical Titan thermal profile (Griffith et al. 2008). Rather than
forming fog, such air will simply rise through the troposphere.
Fog can only persist at the surface if the surface air is both satu-
rated in methane and colder (and thus more stable) than the typ-
ical thermal profile. Pools of evaporating liquid methane could
indeed be cooler than their surroundings (Mitri et al. 2007) and,
under the right meteorological conditions, will add humidity to
and drain heat from overlying air parcels. No other formation ex-
planation can naturally explain both the increased humidity and
decreased temperature required. This formation mechanism also
naturally explains the occasional correlation between fog and
overlying tropospheric clouds. If surface humidity is raised with-
out a sufficient decrease in temperature, saturated air parcels will
convect into the upper troposphere (Griffith et al. 2000). Much of
the continued south polar cloudiness (Brown et al. 2009b) may
be tied directly to surface liquid methane evaporating and rising
unstably.
The locations of the identified fog features and a comparison
to the ISS base map3 are shown in Figure 4. All identified fog
features are southward of 65 S. No correlation is seen between
the locations of fog and the location of the one suspected large
lake, Ontario Lacus—which perhaps is a reservoir of ethane only
or mixed ethane–methane (Brown et al. 2008)—the locations of
dark albedo features, or the location of the large observed albedo
change (Turtle et al. 2009). No temporal association with known
south polar tropospheric cloud outburst appears (Schaller et al.
2006, 2009).
Fog is likely a more common occurrence than shown here; it is
difficult to identify in the typical low-resolution images obtained
by VIMS, but seen in nearly all high-resolution south polar
images. Liquid methane and evaporation are likely distributed
even more widely than the observed fog; all evaporation need not
cause fog: special meteorological conditions such as low winds
are also likely required. Liquid methane appears widespread at
the south pole of Titan in the late southern summer.
This research has been supported by a grant from the NSF
Planetary Astronomy program.
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Discovery of Fog at the South Pole of Titan

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