We present the first long-term study of the behavior of the sporadically observed tropospheric clouds recently discovered near Titan's south pole. We find that one or more small individual cloud systems is present in the 70°-80° south region during every night of observation. These clouds account for 0.5%-1% of Titan's 2.0 μm flux, consistent with a global cloud cover fraction of 0.2%-0.6%. Clouds observed over multiple-night observing periods remained nearly fixed in brightness and position with respect to Titan's surface. The continual presence of south polar clouds is consistent with the hypothesis that surface heating during the long period of continuous polar sunlight at the time of Titan's southern summer solstice drives seasonal convection and cloud formation at the pole

Bouchez, Antonin H.

Brown, Michael E.

English

Caltech Authors - Main

L53
The Astrophysical Journal, 618:L53–L56, 2005 January 1
 2005. The American Astronomical Society. All rights reserved. Printed in U.S.A.
STATISTICS OF TITAN’S SOUTH POLAR TROPOSPHERIC CLOUDS
Antonin H. Bouchez1 and Michael E. Brown
Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125
Received 2004 October 13; accepted 2004 November 23; published 2004 December 3
ABSTRACT
We present the first long-term study of the behavior of the sporadically observed tropospheric clouds recently
discovered near Titan’s south pole. We find that one or more small individual cloud systems is present in the
70–80 south region during every night of observation. These clouds account for 0.5%–1% of Titan’s 2.0 mm
flux, consistent with a global cloud cover fraction of 0.2%–0.6%. Clouds observed over multiple-night observing
periods remained nearly fixed in brightness and position with respect to Titan’s surface. The continual presence
of south polar clouds is consistent with the hypothesis that surface heating during the long period of continuous
polar sunlight at the time of Titan’s southern summer solstice drives seasonal convection and cloud formation
at the pole.
Subject headings: infrared: solar system — planets and satellites: individual (Titan)
1. INTRODUCTION
The discovery of transient clouds in Titan’s troposphere
(Griffith et al. 1998) was unexpected. While high-resolution
imaging studies of Titan’s surface in the visible (Smith et al.
1996; Richardson et al. 2001) and near-infrared (Combes et al.
1997; Gibbard et al. 1999; Meier et al. 2000; Coustenis et al.
2001) gradually uncovered Titan’s fixed surface albedo fea-
tures, careful searches failed to conclusively detect any transient
cloud features. Instead, the existence of tropospheric clouds on
Titan was first confirmed indirectly by disk-integrated infrared
spectra that showed that Titan had brightened dramatically in
spectral regions sensitive to scattering in the lower atmosphere
while remaining unchanged in spectral regions sensitive only
to scattering at higher altitudes in the atmosphere (Griffith et
al. 1998). Increasingly precise disk-integrated spectra soon after
demonstrated that smaller clouds, covering ∼1% of Titan’s disk,
come and go on a daily basis (Griffith et al. 2000), yet the
location of both the small daily-varying and the occasional
larger clouds remained unknown.
Adaptive optics (AO) observations with the Keck and Gem-
ini telescopes in 2001 December clearly located small transient
clouds near the south pole (Brown et al. 2002; Roe et al. 2002).
Individual clouds were observed to persist at least 24 hr in one
case (Brown et al. 2002) and vary in flux by 50% over 3 hr
in another (Roe et al. 2002), but little other information on
their lifetimes, formation frequency, and size spectrum can be
gleaned from these few observations.
Based on these limited observations, Brown et al. (2002)
hypothesized that the presence of clouds forming near the south
pole of Titan was a result of local surface heating, which drives
atmospheric convection. In the time period around Titan’s sum-
mer solstice (which occurred in 2002 October), the south polar
regions remain in continuous sunlight and receive more daily
average insolation than any other spot on the globe. Titan’s
lower atmosphere is sufficiently close to being convectively
unstable that even a small amount of additional heat input at
the bottom of the atmosphere is sufficient to cause convection,
which in turn can lead to runaway condensation and cloud
formation. This hypothesis predicts that clouds will follow the
1 Current address: W. M. Keck Observatory, 65-1120 Mamalahoa Highway,
Kamuela, HI 96743.
point of maximum average daily insolation, which will remain
at the south pole until 2005.
Frequent resolved imaging over many rotations provides the
most promising means of constraining basic properties of Ti-
tan’s tropospheric clouds. Here we present the results of an
intensive imaging campaign performed with the Palomar Hale
5 m telescope in the fall and winter of 2002–2003. These
observations shed light on the typical locations, lifetimes, for-
mation frequency, and size spectrum of Titan’s south polar
clouds and constrain the wind speed in the region of Titan’s
troposphere in which the clouds appear.
2. OBSERVATIONS
Images of Titan were acquired on 16 nights between 2001
December 20 and 2003 January 14, using the JPL AO system
and the Cornell-built PHARO camera/spectrograph at the
Cassegrain focus of the Palomar Hale telescope (Table 1). All
images were taken through a filter (1.990–2.300 mm) andKs
recorded at a plate scale of 0.02517 pixel1, with an integration
time of 9.085 s. A minimum of 16 images were taken each
night, with the telescope offset by 5 between each set of four
to minimize the effect of bad pixels and uncertain flat-field
calibration. Observing conditions ranged from photometric to
thin high clouds.
Images were processed using standard near-infrared data im-
age reduction techniques (sky frame subtraction, division by a
flat-field map, and correction for bad pixels). Each night’s images
were then sorted by quality of the AO performance, and only
the best 8–16 frames were retained. These images were then
shifted to a common center and averaged to produce a single
mean image of Titan for each night of observation (Fig. 1) and
an associated uncertainty map given by the variance of each
pixel across the stack of shifted frames.
3. ANALYSIS
In the initial cloud imaging observations of Brown et al.
(2002) and Roe et al. (2002), the identification of transient
features near the south pole as tropospheric clouds was con-
firmed by multiwavelength measurement of the height of the
features in the atmosphere. The current observations consist of
images at only a single wavelength, so the clouds can be neither
located nor confirmed by their height in the atmosphere. In-
L54 BOUCHEZ & BROWN Vol. 618
TABLE 1
Observations
Date UT
CMLa
(deg) Observersb
2001 Dec 20 . . . . . . 06:31–06:43 92.5 1
2002 Sep 23 . . . . . . 11:42–11:45 210.5 1
2002 Sep 24 . . . . . . 12:08–12:15 233.5 1
2002 Sep 25 . . . . . . 11:29–11:36 255.4 1
2002 Sep 26 . . . . . . 12:08–12:24 278.6 2
2002 Sep 27 . . . . . . 11:50–12:05 300.9 2
2002 Nov 14 . . . . . . 10:21–10:38 305.2 3
2002 Nov 15 . . . . . . 10:30–10:54 327.0 4
2002 Nov 16 . . . . . . 09:01–09:16 348.2 5
2002 Nov 17 . . . . . . 09:59–10:06 11.7 5
2002 Nov 18 . . . . . . 09:35–09:42 33.9 5
2002 Dec 24 . . . . . . 08:36–09:07 129.1 6
2002 Dec 27 . . . . . . 08:03–08:10 196.4 7
2002 Dec 28 . . . . . . 07:52–07:59 218.9 7
2003 Jan 12 . . . . . . . 07:00–07:14 198.0 5
2003 Jan 14 . . . . . . . 05:31–05:38 241.9 5
a West longitude of Titan’s central meridian.
b Observers: (1) A. Bouchez, M. Brown, M. Troy, R. De-
kany. (2) C. Dumas. (3) M. Konacki. (4) C. Trujillo, A.
Bouchez. (5) S. Metchev, C. Bian, J. Eisner. (6) J. Carson,
E. Furlan, W. Forrest, K. Uchida. (7) E. Furlan, L. Keller,
K. Uchida, D. Watson.
Fig. 1.—Palomar Hale telescope images of Titan in 2001–2003. Transient clouds can be identified near Titan’s south pole (white arrows) on all but one night
(2003 November 15, when the AO correction is poor). Each image is 1.2 across.
stead, south polar clouds were located in these images by their
transient appearance on an otherwise time-invariant rotating
surface.
In almost all images, one or more distinct sources is visible
near Titan’s south pole. Comparison with high-resolution maps
of Titan’s surface now available (Roe et al. 2004; Bouchez et
al. 2004) confirm that these brightenings’ sources are indeed
transient. We have identified 19 unresolved transient bright
features in the images, corresponding to 12 distinct transient
features visible on 15 of the 16 nights (Table 2). On 2002
November 15—the single night when no obvious south polar
transient is visible—the AO correction is poor, and we observe
a transient feature both the day before and the day after this
night. We conclude that this feature was also present on No-
vember 15 but was not detected because of the poor correction.
While the height in the atmosphere of these transient features
cannot be measured from these data, the near-continuous ap-
pearance of transient features near Titan’s south pole in these
observations strongly suggests that we are viewing the same
phenomenon earlier identified as tropospheric clouds.
Cloud identification is aided by the relatively dark and bland
appearance of Titan’s south polar surface regions; all bright
spots near the south pole have been subsequently identified as
clouds. The remainder of Titan, however, has complex and
bright albedo variations on the surface. The variable perfor-
mance of AO on different nights of observation makes con-
fident identification of transient features in these regions dif-
ficult. The midlatitude clouds recently reported by Roe et al.
(2005), for example, would likely not have been detectable in
these observations.
In order to measure the flux and location of the transient
clouds that were identified, we first attempt to isolate the cloud
emission in each image by subtracting the contribution to the
images from the haze and surface of Titan. The contribution
of sunlight scattered off of Titan’s relatively invariant strato-
No. 1, 2005 TITAN’S SOUTH POLAR TROPOSPHERIC CLOUDS L55
Fig. 2.—Polar stereographic projection of the locations of the transient
clouds detected. Relative flux is indicated by the area of the filled circles, and
1 j uncertainties in the positions are shown with error bars.
TABLE 2
Observed Point-Link Tropospheric Clouds
Date
Fluxa
(%)
West Longitude
(deg)
Latitude
(deg) Identificationb
2001 Dec 20 . . . . . . 0.38  0.06 46  12 77  3 1
0.32  0.06 126  17 78  4 2
0.09  0.05 156  8 63  3 3
2002 Sep 23 . . . . . . 1.05  0.08 183  11 72  5 4
2002 Sep 24 . . . . . . 0.89  0.08 208  12 73  5 4
2002 Sep 25 . . . . . . 0.77  0.08 194  8 72  2 4
0.52  0.08 279  23 81  5 5
2002 Sep 26 . . . . . . 0.60  0.07 216  9 75  2 4
0.62  0.07 292  21 80  5 5
2002 Nov 16 . . . . . . 0.67  0.11 38  46 83  3 6
2002 Nov 17 . . . . . . 0.67  0.11 37  24 79  4 6
2002 Nov 18 . . . . . . 0.67  0.11 35  21 81  5 6
2002 Dec 24 . . . . . . 0.80  0.23 95  23 82  3 7
2002 Dec 27 . . . . . . 0.39  0.10 241  11 72  3 8
2002 Dec 28 . . . . . . 0.58  0.10 229  15 77  5 8
2003 Jan 12 . . . . . . . 0.88  0.11 155  27 82  3 9
2003 Jan 14 . . . . . . . 0.52  0.10 179  7 71  2 10
0.49  0.10 248  40 83  2 11
0.18  0.06 299  7 58  2 12
a Flux of cloud, compared to total disk flux at orbital longitude 253 west.
b Identification number of individual clouds viewed on multiple occasions.
spheric haze is removed in an ad hoc fashion by subtracting a
scaled Brackett-g (Brg, 2.16–2.18 mm) image, obtained on
2002 September 23, from each. The difference images record
only sunlight scattered below the altitude at which fort ≈ 1CH4
the Brg filter, which corresponds to approximately 50 km (Roe
et al. 2002).
Next, we attempt to model and remove the contribution from
Titan’s surface. The surface contribution is represented by the
convolution of a uniform sphere and a variable point-spread
function (PSF). We model the PSF as the sum of a diffraction-
limited Airy pattern and two Gaussian functions. Such a model
PSF is a reasonable approximation of the diffraction-limited
core plus seeing-limited halo PSF of an AO system. The width
and strength of the Gaussian contributions to each night’s PSF
are iteratively adjusted until the residuals in the subtraction of
the data and the model image are minimized. The halo of
emission around Titan’s sharp limb provides an excellent mea-
sure of the non–diffraction-limited component of the PSF in
each image. While it is clear that a uniform disk little resembles
the complex surface features seen on Titan, the surface near
the south polar regions where the clouds have been identified
is indeed one of the more uniform regions of the disk, so locally
the subtraction remains good (Roe et al. 2004; Bouchez et al.
2004). Finally, the model PSF-convolved surface image is sub-
tracted from the haze-subtracted image, leaving the south polar
clouds isolated from surface and haze contributions.
The location and flux of the unresolved clouds are deter-
mined by fitting scaled PSFs to the residual images, and un-
certainties in these measurements are determined by adding 20
different realizations of the background and photon noise to
the images and repeating the entire forward-modeling process.
The flux of each cloud is then normalized to Titan’s total flux
at an orbital longitude of 253, using the relative disk-averaged
surface brightness found in the high-resolution maps of
Bouchez et al. (2004). The clouds of September 27 and No-
vember 14 do not appear to be single point sources, so their
fluxes and positions are not well measured, but in both cases
the same clouds are observed on previous or subsequent nights.
4. DISCUSSION
Transient clouds were detected on 15 of 16 nights in Palomar
images of Titan taken between 2001 December 20 and 2003
January 14. The one night on which no clouds were apparent
suffered from poor AO correction, and the presence of clouds
both the previous and subsequent nights suggests that clouds
were present but not resolved even on that night. It therefore
appears likely that the clouds were continually present poleward
of 70 south during the period of 2001 December to 2003
January. Occasional clouds were also detected at lower lati-
tudes, the furthest north being located at south on58  2
2003 January 14 (Fig. 2).
Individual unresolved clouds observed with the Palomar AO
system contribute between 0.1% and 1.1% of Titan’s flux at
2.0 mm. This flux corresponds to the brightness expected for
optically thick, foreshortened clouds with diameters between
L56 BOUCHEZ & BROWN Vol. 618
100 and 400 km (Brown et al. 2002), somewhat smaller than
the projected diffraction limit of km. Thus, the500# 1000
brightness of the unresolved clouds is consistent with filling
factors of 5%–20%.
When we integrate the light of all clouds identified each
night, the corresponding areal coverage of optically thick
clouds over Titan’s visible disk varied from 0.2% on 2002
December 27 to 0.6% on 2002 September 23. These figures
are similar to those derived by Griffith et al. (2000) for the
areal coverage of the clouds responsible for Titan’s daily spec-
tral variations at 2.12–2.16 mm. However, no clouds were de-
tected that rival the 7% coverage event observed spectroscop-
ically on 1995 September 5 (Griffith et al. 1998).
No day-to-day variation in the flux of individual clouds was
detected that could not be attributed to their fading due to their
reduced projected area and increased methane absorption as
they approach Titan’s limb. Such geometric fading is clearly
seen in the cloud that persisted for four nights from 2002 Sep-
tember 23 to 2002 September 26 (cloud identification number
4), gradually decreasing in flux by 40%. We see no evidence
of the rapid cloud formation or dissipation as observed in more
sensitive Keck AO observations (Roe et al. 2002).
Tracking the motion of tropospheric clouds is one of the few
available methods for measuring the tropospheric wind speeds
on Titan. The longevity of the clouds that we have detected
(two persist for at least three nights, four for at least two nights)
suggests that this measurement might be possible. No cloud
shows any significant motion. From these lack of motions, we
place 3 j upper limits of 4.0 m s1 westward and 2.0 m s1
eastward on the wind speeds at 72–83 south, the location of
the four multiple-night clouds. This upper limit applies to al-
titudes of 10–20 km above the surface, the region in which
spectra have placed the cloud tops (Griffith et al. 2000; Brown
et al. 2002), and is consistent with the low tropospheric winds
predicted by general circulation models of Titan’s atmosphere
(Hourdin et al. 1995; Tokano et al. 2001).
5. CONCLUSIONS
The 16-night study presented here provides the first long-
term look at the tropospheric clouds recently discovered near
Titan’s south pole. The images show that tropospheric clouds
are continually present in the 70–80 south region and occa-
sionally as far north as 58 south. The constant presence of
southern clouds during the current season is consistent with
the hypothesis that surface heating during the long period of
continuous polar sunlight at the time of Titan’s southern sum-
mer solstice drives seasonal convection and cloud formation
near the pole (Brown et al. 2002). As summer solstice passes,
the peak of mean daily insolation will eventually jump from
the pole and begin moving northward with the subsolar point.
The surface-heating hypothesis predicts that as the polar regions
cool, cloud activity there will cease and that new cloud systems
will emerge following the subsolar point northward. Obser-
vations of Titan and its clouds over the coming years will test
this long-term prediction and continue to improve our under-
standing of Titan’s dynamic cloud systems.
We thank the many observers who took time out of their
own programs to observe Titan. This research is supported by
NSF planetary astronomy grant AST-0401559. We thank Henry
Roe and Emily Schaller for interesting comments and conver-
sations.
REFERENCES
Bouchez, A. H., Brown, M. E., Roe, H. G., & Schaller, E. L. 2004, BAAS,
in press
Brown, M. E., Bouchez, A. H., & Griffith, C. A. 2002, Nature, 420, 795
Combes, M., Vapillon, L., Gendron, E., Coustenis, A., Lai, O., Wittemberg,
R., & Sirdey, R. 1997, Icarus, 129, 482
Coustenis, A., et al. 2001, Icarus, 154, 501
Gibbard, S. G., Macintosh, B., Gavel, D., Max, C. E., de Pater, I., Ghez,
A. M., Young, E. F., & McKay, C. P. 1999, Icarus, 139, 189
Griffith, C. A., Hall, J. L., & Geballe, T. R. 2000, Science, 290, 509
Griffith, C. A., Owen, T., Miller, G. A., & Geballe, T. 1998, Nature, 395, 575
Hourdin, F., Talagrand, O., Sadourny, R., Courtin, R., Gautier, D., & McKay,
C. P. 1995, Icarus, 117, 358
Meier, R., Smith, B. A., Owen, T. C., & Terrile, R. J. 2000, Icarus, 145, 462
Richardson, J. E., Lorenz, R. D., & McEwen, A. 2001, BAAS, 33, 1110
Roe, H. G., Bouchez, A. H., Trujillo, C. A., Schaller, E. L., & Brown, M. E.
2005, ApJ, 618, L49
Roe. H. G., de Pater, I., Gibbard, S. G., Macintosh, B. A., Max, C. E., Young,
E. F., Brown, M. E., & Bouchez, A. H. 2004, Geophys. Res. Lett., 31,
L17S03
Roe, H. G., de Pater, I., Macintosh, B. A., & McKay, C. P. 2002, ApJ, 581,
1399
Smith, P. H., Lemmon, M. T., Lorenz, R. D., Sromovsky, L. A., Caldwell,
J. J., & Allison, M. D. 1996, Icarus, 119, 336
Tokano, T., Neubauer, F. M., Laube, M., & McKay, C. P. 2001, Icarus, 153,
130


Statistics of Titan's South Polar Tropospheric Clouds

L53
The Astrophysical Journal, 618:L53–L56, 2005 January 1
 2005. The American Astronomical Society. All rights reserved. Printed in U.S.A.
STATISTICS OF TITAN’S SOUTH POLAR TROPOSPHERIC CLOUDS
Antonin H. Bouchez1 and Michael E. Brown
Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125
Received 2004 October 13; accepted 2004 November 23; published 2004 December 3
ABSTRACT
We present the first long-term study of the behavior of the sporadically observed tropospheric clouds recently
discovered near Titan’s south pole. We find that one or more small individual cloud systems is present in the
70–80 south region during every night of observation. These clouds account for 0.5%–1% of Titan’s 2.0mm
flux, consistent with a global cloud cover fraction of 0.2%–0.6%. Clouds observed over multiple-night observing
periods remained nearly fixed in brightness and position with respect to Titan’s surface. The continual presence
of south polar clouds is consistent with the hypothesis that surface heating during the long period of continuous
polar sunlight at the time of Titan’s southern summer solstice drives seasonal convection and cloud formation
at the pole.
Subject headings: infrared: solar system — planets and satellites: individual (Titan)
1. INTRODUCTION
The discovery of transient clouds in Titan’s troposphere
(Griffith et al. 1998) was unexpected. While high-resolution
imaging studies of Titan’s surface in the visible (Smith et al.
1996; Richardson et al. 2001) and near-infrared (Combes et al.
1997; Gibbard et al. 1999; Meier et al. 2000; Coustenis et al.
2001) gradually uncovered Titan’s fixed surface albedo fea-
tures, careful searches failed to conclusively detect any transient
cloud features. Instead, the existence of tropospheric clouds on
Titan was first confirmed indirectly by disk-integrated infrared
spectra that showed that Titan had brightened dramatically in
spectral regions sensitive to scattering in the lower atmosphere
while remaining unchanged in spectral regions sensitive only
to scattering at higher altitudes in the atmosphere (Griffith et
al. 1998). Increasingly precise disk-integrated spectra soon after
demonstrated that smaller clouds, covering∼1% of Titan’s disk,
come and go on a daily basis (Griffith et al. 2000), yet the
location of both the small daily-varying and the occasional
larger clouds remained unknown.
Adaptive optics (AO) observations with the Keck and Gem-
ini telescopes in 2001 December clearly located small transient
clouds near the south pole (Brown et al. 2002; Roe et al. 2002).
Individual clouds were observed to persist at least 24 hr in one
case (Brown et al. 2002) and vary in flux by 50% over 3 hr
in another (Roe et al. 2002), but little other information on
their lifetimes, formation frequency, and size spectrum can be
gleaned from these few observations.
Based on these limited observations, Brown et al. (2002)
hypothesized that the presence of clouds forming near the south
pole of Titan was a result of local surface heating, which drives
atmospheric convection. In the time period around Titan’s sum-
mer solstice (which occurred in 2002 October), the south polar
regions remain in continuous sunlight and receive more daily
average insolation than any other spot on the globe. Titan’s
lower atmosphere is sufficiently close to being convectively
unstable that even a small amount of additional heat input at
the bottom of the atmosphere is sufficient to cause convection,
which in turn can lead to runaway condensation and cloud
formation. This hypothesis predicts that clouds will follow the
1 Current address: W. M. Keck Observatory, 65-1120 Mamalahoa Highway,
Kamuela, HI 96743.
point of maximum average daily insolation, which will remain
at the south pole until 2005.
Frequent resolved imaging over many rotations provides the
most promising means of constraining basic properties of Ti-
tan’s tropospheric clouds. Here we present the results of an
intensive imaging campaign performed with the Palomar Hale
5 m telescope in the fall and winter of 2002–2003. These
observations shed light on the typical locations, lifetimes, for-
mation frequency, and size spectrum of Titan’s south polar
clouds and constrain the wind speed in the region of Titan’s
troposphere in which the clouds appear.
2. OBSERVATIONS
Images of Titan were acquired on 16 nights between 2001
December 20 and 2003 January 14, using the JPL AO system
and the Cornell-built PHARO camera/spectrograph at the
Cassegrain focus of the Palomar Hale telescope (Table 1). All
images were taken through a filter (1.990–2.300mm) andKs
recorded at a plate scale of 0.02517 pixel1, with an integration
time of 9.085 s. A minimum of 16 images were taken each
night, with the telescope offset by 5 between each set of four
to minimize the effect of bad pixels and uncertain flat-field
calibration. Observing conditions ranged from photometric to
thin high clouds.
Images were processed using standard near-infrared data im-
age reduction techniques (sky frame subtraction, division by a
flat-field map, and correction for bad pixels). Each night’s images
were then sorted by quality of the AO performance, and only
the best 8–16 frames were retained. These images were then
shifted to a common center and averaged to produce a single
mean image of Titan for each night of observation (Fig. 1) and
an associated uncertainty map given by the variance of each
pixel across the stack of shifted frames.
3. ANALYSIS
In the initial cloud imaging observations of Brown et al.
(2002) and Roe et al. (2002), the identification of transient
features near the south pole as tropospheric clouds was con-
firmed by multiwavelength measurement of the height of the
features in the atmosphere. The current observations consist of
images at only a single wavelength, so the clouds can be neither
located nor confirmed by their height in the atmosphere. In-
L54 BOUCHEZ & BROWN Vol. 618
TABLE 1
Observations
Date UT
CMLa
(deg) Observersb
2001 Dec 20. . . . . . 06:31–06:43 92.5 1
2002 Sep 23. . . . . . 11:42–11:45 210.5 1
2002 Sep 24. . . . . . 12:08–12:15 233.5 1
2002 Sep 25. . . . . . 11:29–11:36 255.4 1
2002 Sep 26. . . . . . 12:08–12:24 278.6 2
2002 Sep 27. . . . . . 11:50–12:05 300.9 2
2002 Nov 14. . . . . . 10:21–10:38 305.2 3
2002 Nov 15. . . . . . 10:30–10:54 327.0 4
2002 Nov 16. . . . . . 09:01–09:16 348.2 5
2002 Nov 17. . . . . . 09:59–10:06 11.7 5
2002 Nov 18. . . . . . 09:35–09:42 33.9 5
2002 Dec 24. . . . . . 08:36–09:07 129.1 6
2002 Dec 27. . . . . . 08:03–08:10 196.4 7
2002 Dec 28. . . . . . 07:52–07:59 218.9 7
2003 Jan 12. . . . . . . 07:00–07:14 198.0 5
2003 Jan 14. . . . . . . 05:31–05:38 241.9 5
a West longitude of Titan’s central meridian.
b Observers: (1) A. Bouchez, M. Brown, M. Troy, R. De-
kany. (2) C. Dumas. (3) M. Konacki. (4) C. Trujillo, A.
Bouchez. (5) S. Metchev, C. Bian, J. Eisner. (6) J. Carson,
E. Furlan, W. Forrest, K. Uchida. (7) E. Furlan, L. Keller,
K. Uchida, D. Watson.
Fig. 1.—Palomar Hale telescope images of Titan in 2001–2003. Transient clouds can be identified near Titan’s south pole (white arrows) on all but one night
(2003 November 15, when the AO correction is poor). Each image is 1.2 across.
stead, south polar clouds were located in these images by their
transient appearance on an otherwise time-invariant rotating
surface.
In almost all images, one or more distinct sources is visible
near Titan’s south pole. Comparison with high-resolution maps
of Titan’s surface now available (Roe et al. 2004; Bouchez et
al. 2004) confirm that these brightenings’ sources are indeed
transient. We have identified 19 unresolved transient bright
features in the images, corresponding to 12 distinct transient
features visible on 15 of the 16 nights (Table 2). On 2002
November 15—the single night when no obvious south polar
transient is visible—the AO correction is poor, and we observe
a transient feature both the day before and the day after this
night. We conclude that this feature was also present on No-
vember 15 but was not detected because of the poor correction.
While the height in the atmosphere of these transient features
cannot be measured from these data, the near-continuous ap-
pearance of transient features near Titan’s south pole in these
observations strongly suggests that we are viewing the same
phenomenon earlier identified as tropospheric clouds.
Cloud identification is aided by the relatively dark and bland
appearance of Titan’s south polar surface regions; all bright
spots near the south pole have been subsequently identified as
clouds. The remainder of Titan, however, has complex and
bright albedo variations on the surface. The variable perfor-
mance of AO on different nights of observation makes con-
fident identification of transient features in these regions dif-
ficult. The midlatitude clouds recently reported by Roe et al.
(2005), for example, would likely not have been detectable in
these observations.
In order to measure the flux and location of the transient
clouds that were identified, we first attempt to isolate the cloud
emission in each image by subtracting the contribution to the
images from the haze and surface of Titan. The contribution
of sunlight scattered off of Titan’s relatively invariant strato-
No. 1, 2005 TITAN’S SOUTH POLAR TROPOSPHERIC CLOUDS L55
Fig. 2.—Polar stereographic projection of the locations of the transient
clouds detected. Relative flux is indicated by the area of the filled circles, and
1 j uncertainties in the positions are shown with error bars.
TABLE 2
Observed Point-Link Tropospheric Clouds
Date
Fluxa
(%)
West Longitude
(deg)
Latitude
(deg) Identificationb
2001 Dec 20. . . . . . 0.38 0.06 46 12 77  3 1
0.32  0.06 126 17 78  4 2
0.09  0.05 156 8 63  3 3
2002 Sep 23. . . . . . 1.05 0.08 183 11 72  5 4
2002 Sep 24. . . . . . 0.89 0.08 208 12 73  5 4
2002 Sep 25. . . . . . 0.77 0.08 194 8 72  2 4
0.52  0.08 279 23 81  5 5
2002 Sep 26. . . . . . 0.60 0.07 216 9 75  2 4
0.62  0.07 292 21 80  5 5
2002 Nov 16. . . . . . 0.67 0.11 38 46 83  3 6
2002 Nov 17. . . . . . 0.67 0.11 37 24 79  4 6
2002 Nov 18. . . . . . 0.67 0.11 35 21 81  5 6
2002 Dec 24. . . . . . 0.80 0.23 95 23 82  3 7
2002 Dec 27. . . . . . 0.39 0.10 241 11 72  3 8
2002 Dec 28. . . . . . 0.58 0.10 229 15 77  5 8
2003 Jan 12. . . . . . . 0.88 0.11 155 27 82  3 9
2003 Jan 14. . . . . . . 0.52 0.10 179 7 71  2 10
0.49  0.10 248 40 83  2 11
0.18  0.06 299 7 58  2 12
a Flux of cloud, compared to total disk flux at orbital longitude 253 west.
b Identification number of individual clouds viewed on multiple occasions.
spheric haze is removed in an ad hoc fashion by subtracting a
scaled Brackett-g (Brg, 2.16–2.18mm) image, obtained on
2002 September 23, from each. The difference images record
only sunlight scattered below the altitude at which fort ≈ 1CH4
the Brg filter, which corresponds to approximately 50 km (Roe
et al. 2002).
Next, we attempt to model and remove the contribution from
Titan’s surface. The surface contribution is represented by the
convolution of a uniform sphere and a variable point-spread
function (PSF). We model the PSF as the sum of a diffraction-
limited Airy pattern and two Gaussian functions. Such a model
PSF is a reasonable approximation of the diffraction-limited
core plus seeing-limited halo PSF of an AO system. The width
and strength of the Gaussian contributions to each night’s PSF
are iteratively adjusted until the residuals in the subtraction of
the data and the model image are minimized. The halo of
emission around Titan’s sharp limb provides an excellent mea-
sure of the non–diffraction-limited component of the PSF in
each image. While it is clear that a uniform disk little resembles
the complex surface features seen on Titan, the surface near
the south polar regions where the clouds have been identified
is indeed one of the more uniform regions of the disk, so locally
the subtraction remains good (Roe et al. 2004; Bouchez et al.
2004). Finally, the model PSF-convolved surface image is sub-
tracted from the haze-subtracted image, leaving the south polar
clouds isolated from surface and haze contributions.
The location and flux of the unresolved clouds are deter-
mined by fitting scaled PSFs to the residual images, and un-
certainties in these measurements are determined by adding 20
different realizations of the background and photon noise to
the images and repeating the entire forward-modeling process.
The flux of each cloud is then normalized to Titan’s total flux
at an orbital longitude of 253, using the relative disk-averaged
surface brightness found in the high-resolution maps of
Bouchez et al. (2004). The clouds of September 27 and No-
vember 14 do not appear to be single point sources, so their
fluxes and positions are not well measured, but in both cases
the same clouds are observed on previous or subsequent nights.
4. DISCUSSION
Transient clouds were detected on 15 of 16 nights in Palomar
images of Titan taken between 2001 December 20 and 2003
January 14. The one night on which no clouds were apparent
suffered from poor AO correction, and the presence of clouds
both the previous and subsequent nights suggests that clouds
were present but not resolved even on that night. It therefore
appears likely that the clouds were continually present poleward
of 70 south during the period of 2001 December to 2003
January. Occasional clouds were also detected at lower lati-
tudes, the furthest north being located at south on58  2
2003 January 14 (Fig. 2).
Individual unresolved clouds observed with the Palomar AO
system contribute between 0.1% and 1.1% of Titan’s flux at
2.0 mm. This flux corresponds to the brightness expected for
optically thick, foreshortened clouds with diameters between
L56 BOUCHEZ & BROWN Vol. 618
100 and 400 km (Brown et al. 2002), somewhat smaller than
the projected diffraction limit of km. Thus, the500# 1000
brightness of the unresolved clouds is consistent with filling
factors of 5%–20%.
When we integrate the light of all clouds identified each
night, the corresponding areal coverage of optically thick
clouds over Titan’s visible disk varied from 0.2% on 2002
December 27 to 0.6% on 2002 September 23. These figures
are similar to those derived by Griffith et al. (2000) for the
areal coverage of the clouds responsible for Titan’s daily spec-
tral variations at 2.12–2.16mm. However, no clouds were de-
tected that rival the 7% coverage event observed spectroscop-
ically on 1995 September 5 (Griffith et al. 1998).
No day-to-day variation in the flux of individual clouds was
detected that could not be attributed to their fading due to their
reduced projected area and increased methane absorption as
they approach Titan’s limb. Such geometric fading is clearly
seen in the cloud that persisted for four nights from 2002 Sep-
tember 23 to 2002 September 26 (cloud identification number
4), gradually decreasing in flux by 40%. We see no evidence
of the rapid cloud formation or dissipation as observed in more
sensitive Keck AO observations (Roe et al. 2002).
Tracking the motion of tropospheric clouds is one of the few
available methods for measuring the tropospheric wind speeds
on Titan. The longevity of the clouds that we have detected
(two persist for at least three nights, four for at least two nights)
suggests that this measurement might be possible. No cloud
shows any significant motion. From these lack of motions, we
place 3j upper limits of 4.0 m s1 westward and 2.0 m s1
eastward on the wind speeds at 72–83 south, the location of
the four multiple-night clouds. This upper limit applies to al-
titudes of 10–20 km above the surface, the region in which
spectra have placed the cloud tops (Griffith et al. 2000; Brown
et al. 2002), and is consistent with the low tropospheric winds
predicted by general circulation models of Titan’s atmosphere
(Hourdin et al. 1995; Tokano et al. 2001).
5. CONCLUSIONS
The 16-night study presented here provides the first long-
term look at the tropospheric clouds recently discovered near
Titan’s south pole. The images show that tropospheric clouds
are continually present in the 70–80 south region and occa-
sionally as far north as 58 south. The constant presence of
southern clouds during the current season is consistent with
the hypothesis that surface heating during the long period of
continuous polar sunlight at the time of Titan’s southern sum-
mer solstice drives seasonal convection and cloud formation
near the pole (Brown et al. 2002). As summer solstice passes,
the peak of mean daily insolation will eventually jump from
the pole and begin moving northward with the subsolar point.
The surface-heating hypothesis predicts that as the polar regions
cool, cloud activity there will cease and that new cloud systems
will emerge following the subsolar point northward. Obser-
vations of Titan and its clouds over the coming years will test
this long-term prediction and continue to improve our under-
standing of Titan’s dynamic cloud systems.
We thank the many observers who took time out of their
own programs to observe Titan. This research is supported by
NSF planetary astronomy grant AST-0401559. We thank Henry
Roe and Emily Schaller for interesting comments and conver-
sations.
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Statistics of Titan's South Polar Tropospheric Clouds

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