Until now, all the clouds imaged in Titan's troposphere have been found at far southern latitudes (60°-90° south). The occurrence and location of these clouds is thought to be the result of convection driven by the maximum annual solar heating of Titan's surface, which occurs at summer solstice (2002 October) in this south polar region. We report the first observations of a new recurring type of tropospheric cloud feature, confined narrowly to ~40° south latitude, which cannot be explained by this simple insolation hypothesis. We propose two classes of formation scenario, one linked to surface geography and the other to seasonally evolving circulation, which will be easily distinguished with continued observations over the next few years

Bouchez, A. H.

Brown, M. E.

Roe, H. G.

Schaller, E. L.

Trujillo, C. A.

English

Caltech Authors - Main

L49
The Astrophysical Journal, 618:L49–L52, 2005 January 1 
 2005. The American Astronomical Society. All rights reserved. Printed in U.S.A.
DISCOVERY OF TEMPERATE LATITUDE CLOUDS ON TITAN1,2
H. G. Roe,3,4 A. H. Bouchez,5 C. A. Trujillo,6 E. L. Schaller,3 and M. E. Brown3
Received 2004 October 6; accepted 2004 November 17; published 2004 November 29
ABSTRACT
Until now, all the clouds imaged in Titan’s troposphere have been found at far southern latitudes (60–90
south). The occurrence and location of these clouds is thought to be the result of convection driven by the
maximum annual solar heating of Titan’s surface, which occurs at summer solstice (2002 October) in this south
polar region. We report the first observations of a new recurring type of tropospheric cloud feature, confined
narrowly to ∼40 south latitude, which cannot be explained by this simple insolation hypothesis. We propose
two classes of formation scenario, one linked to surface geography and the other to seasonally evolving circulation,
which will be easily distinguished with continued observations over the next few years.
Subject headings: infrared: solar system — planets and satellites: individual (Titan)
Online material: color figure
1. INTRODUCTION
The first detection of meteorological activity on Titan was
reported by Griffith et al. (1998), who attributed global bright-
ening in several spectral windows to a large cloud covering
∼5%–7% at unknown latitude. This was followed by Griffith
et al. (2000), who inferred daily clouds covering ∼0.1%–0.5%
of Titan’s disk, again at unknown latitude. The first spatially
resolved detections of Titan’s tropospheric clouds were re-
ported by Brown et al. (2002) and Roe et al. (2002), who used
high spatial resolution spectroscopy and narrowband filter im-
aging to observe clouds near Titan’s south pole. Since that
time, clouds have regularly been observed in the south polar
region (Bouchez 2003; Bouchez & Brown 2005), and the for-
mation hypothesis of Brown et al. (2002) remains the most
plausible scenario, in which maximum surface heating occurs
on Titan at the south pole near the summer solstice (2002
October), driving atmospheric convection that leads to the for-
mation of moist convective clouds. Until now, no clouds have
been observed north of ∼60 south latitude, and the vast ma-
jority of clouds have been found south of 70 south latitude.
We report a distinct new type of cloud feature on Titan,
discovered recently at temperate southern latitudes. These
clouds are unlikely to form via the same surface heating hy-
pothesis that is suggested for the south polar clouds, and we
discuss possible formation mechanisms for these new clouds.
1 Some of the data presented herein were obtained at the W. M. Keck
Observatory, which is operated as a scientific partnership among the California
Institute of Technology, the University of California, and the National Aero-
nautics and Space Administration. The Observatory was made possible by the
generous financial support of the W. M. Keck Foundation.
2 Based on observations obtained at the Gemini Observatory, which is op-
erated by the Association of Universities for Research in Astronomy, Inc.,
under a cooperative agreement with the NSF on behalf of the Gemini part-
nership: the National Science Foundation (US), the Particle Physics and As-
tronomy Research Council (UK), the National Research Council (Canada),
CONICYT (Chile), the Australian Research Council (Australia), CNPq
(Brazil), and CONICET (Argentina).
3 Division of Geological and Planetary Sciences, California Institute of Tech-
nology, 1200 East California Boulevard, Pasadena, CA 91125; hroe@
gps.caltech.edu, emily@gps.caltech.edu, mbrown@gps.caltech.edu.
4 NSF Astronomy and Astrophysics Postdoctoral Fellow.
5 W. M. Keck Observatory, 65-1120 Mamalahoa Highway, Kamuela, HI
96743; abouchez@keck.hawaii.edu.
6 Gemini Observatory, 670 North A‘ohoku Place, Hilo, HI 96720;
trujillo@gemini.edu.
2. OBSERVATIONS
Data presented here were taken at the Gemini North 8 m
telescope using the Altair adaptive optics system and the facility
near-infrared camera (Herriot et al. 2000; Hodapp et al. 2003)
and the W. M. Keck 10 m telescope using the facility adaptive
optics system and the NIRC2 near-infrared camera (Wizinowich
et al. 2000). Variations in methane and hydrogen opacity with
wavelength allow us to discriminate surface, tropospheric, and
stratospheric features. The filter (2.03–2.36 mm) probes down′K
to Titan’s surface, while the H2 (1–0) filter (2.111–2.137 mm)
is sensitive to ∼10 km altitude, and the Brg filter (2.154–
2.183 mm) probes to approximately the tropopause (50 km; Roe
et al. 2002). Thus, with these three filters we can determine if
a feature is on the surface, in the troposphere, or in the strato-
sphere. Data reduction was limited to correction for pixel-to-
pixel sensitivity (“flat-fielding”), shifting and adding two to four
images from each filter and night, and rotating the final image
to align Titan’s north pole with the vertical axis.
Figure 1 shows data from 13 nights in 2003–2004. Obser-
vations in 2003 December and 2004 September are from Keck,
while all other data are from Gemini. On nearly every night, a
cloud is apparent near the south pole. The Brg images show no
night-to-night changes, except variation that is attributable to
variable seeing, instrument performance, and phase angle. The
new temperate latitude clouds are most apparent in the tropo-
sphere probing images on 2004 April 8 and 9, 2004 May 4, and
2004 September 2, although they appear with lower contrast near
the limb on 2003 December 18 and 2004 May 5.
The locations of the clouds in our data, as well as earlier
clouds (Brown et al. 2002; Roe et al. 2002; Bouchez & Brown
2005), are shown in Figure 2, showing the south polar clouds
clustered south of ∼70 south and the new clouds at a latitude
of ∼40 south. We have not yet observed enough of these new
clouds to say whether any clustering in longitude is statistically
significant.
3. DISCUSSION
The sudden appearance of temperate clouds on Titan cannot
be explained easily by the same surface heating hypothesis
suggested for the south polar clouds; even 2 yr after southern
summer solstice, the pole is still the point of maximum inso-
lation (see Fig. 3). Any successful hypothesis for the appear-
L50 ROE ET AL. Vol. 618
Fig. 1.—Observations of Titan in the three filters. Subobserver longitude and latitude are given. From left to right are images in (surface probing), H2 (1–0)′K
(troposphere probing), and Brg (stratosphere probing). The wire frame at the lower right is for a subobserver latitude of 25.6 south. All images are scaled to show
Titan at the same size, although its angular diameter ranged over 0.73–0.88. In the stratospheric probing images, we see only the limb-brightened stratospheric haze,
with a seasonal north-south asymmetry. In the tropospheric probing images, we see the stratospheric hazes limb-brightened, a general brightening in the south due to
the tropopause cirrus, the distinct south polar clouds (see especially April 9, although a cloud is near the south pole in every one of these images), and the new ∼40
south clouds, which are especially apparent on April 8–9, May 4, and September 2. The new temperate latitude clouds are indicated with white arrows. In the surface
probing images, Titan’s 22.5 day1 rotation rate is apparent and the tropospheric clouds also appear.
ance of these new clouds will need to explain several distinct
characteristics: (1) The clouds are clustered in latitude (37–
44 south), although they are not at identical latitudes. For
example, the point-source cloud of September 2 is at 38 
south, while the extended cloud of the same date is at2
south. (2) We first observed the new clouds in 200344  2
December and have seen them regularly, although not con-
stantly, since that time. In spite of numerous earlier observa-
tions that would have resolved these clouds, they were not
observed earlier, suggesting they are a new or irregularly oc-
curring phenomenon. (3) Like the south polar clouds, these
clouds are tropospheric and do not extend into the stratosphere.
(4) Most of the clouds extend in longitude for several hundred
to several thousand kilometers.
After considering many conceivable methods for forming
clouds on Titan, we find two plausible classes of cloud for-
mation models that satisfy all of the constraints above and that
could possibly explain these new recurring clouds. The first
class of model for spatially confined recurring cloud formation
attempts to tie the location of clouds directly to geographic
features on the surface of Titan. On the Earth, examples of
such spatially confined clouds include orographic, marine, and
volcanic clouds. On Titan, the recent sudden onset of temperate
latitude cloud activity argues against a permanent surface char-
acteristic—such as the presence of mountain or an ocean—
being the sole cloud instigator. We are led to examine instead
potential transient events that might be occurring on Titan’s
surface as a possible source of the recurring but transient
clouds. Some conceivable sporadic events tied to Titan’s sur-
face involve hypothetical geyser or cryovolcanic activity. Gey-
sers on Titan could not rise high enough to directly form tro-
pospheric clouds (Lorenz 2002), but injection of methane even
at the base of the troposphere could cause enough condensation
and latent heat release to drive runaway convection and sub-
sequent cloud formation. Cryovolcanism on Titan would like-
wise not be expected to inject material high into the troposphere
(Lorenz 1996) but could release methane and again drive con-
vection and cloud formation. An interesting possibility with no
Earth analog is that a small amount of geothermal heating in
one location could raise the surface temperature of Titan by
enough to drive south polar–style convection and cloud for-
mation. As is hypothesized for the south pole, a surface tem-
No. 1, 2005 TEMPERATE LATITUDE CLOUDS ON TITAN L51
Fig. 2.—Locations of the previously observed south polar clouds (dia-
monds). Extended temperate latitude clouds are represented by solid lines, with
an arrow indicating where the cloud may have extended beyond the visible
limb of Titan. One spatially unresolved temperate latitude cloud was observed
on 2004 September 2 (square). The uncertainty in location is 1–3. [See the
electronic edition of the Journal for a color version of this figure.]
Fig. 3.—Daily mean insolation at the top of Titan’s atmosphere. The max-
imum seasonal insolation occurs at the south pole at the time of southern
summer solstice (SSS). In early 2004, a local maximum emerges at ∼50 south
and then moves northward and becomes the global maximum in 2005 July.
perature increase of only a few degrees kelvin is thought to be
sufficient to drive cloud formation.
A second class of model for latitudinally confined cloud for-
mation suggests that, like the south polar clouds, the temperate
clouds are tied to Titan’s seasonally varying patterns of insolation
and global circulation. As Titan moves from southern summer
solstice to the spring equinox, the latitude of maximum insolation
moves from the pole to the equator, as shown in Figure 3. By
equinox (which next occurs in 2009), the global circulation will
shift from the pole-to-pole flow thought to occur currently to a
two-cell system rising at the equator and sinking at the poles
(Tokano et al. 2001). As on the Earth, cloud formation should
occur at the location of the rising branch of the circulation system,
so by equinox the clouds should shift from the pole to the equator.
Between the solstice and equinox, the cloud position has been
predicted to follow the location of maximum average insolation
(Brown et al. 2002). Yet the south pole remains the point of
maximum average insolation until 2005 July, and we expect
cooling of the south pole to take even longer, so this initial
prediction from the global circulation hypothesis cannot explain
the appearance of these temperate clouds. Nonetheless, we note
that a secondary local maximum in the average insolation began
to occur in 2004 February at ∼52 south. This secondary max-
imum moves northward quickly, passing 40 south in 2005 Jan-
uary. While the presence of a local maximum in insolation does
not lead to convection in any simple way, the shift from a single-
cell to a two-cell circulation is complicated, so we do not im-
mediately rule out this secondary insolation peak as perhaps
beginning to drive this large-scale circulation shift and thus tem-
perate cloud formation. Such complex behavior is best explored
in global circulation models (GCMs). Current Titan GCMs (see,
e.g., Tokano et al. 2001 and references therein) hold surface
temperature temporally fixed and so cannot capture these types
of behaviors (including the south polar clouds). New Titan GCMs
that allow surface temperature to seasonally vary are needed to
test the viability of this hypothesis.
While both of these classes of models fit the constraints of
the current observations, both suffer severe limitations. The geo-
logical explanation relies on hypothetical cryovolcanic activity
that has never been seen and is perhaps unlikely. The meteor-
ological explanation relies on the fact that the circulation is com-
plicated, to dismiss the initial prediction that cloud formation
should still occur exclusively at the south pole for several more
years. Nonetheless, these two classes of models do the best job
of meeting the constraints of the current observations.
Determining which one of the classes of models best explains
these new clouds is a straightforward observational problem.
In the geological models, the clouds are tied to surface features
that move only on geological timescales. These temperate
clouds will permanently remain at the locations we see them
today, even as the large-scale circulation shifts. In contrast, the
meteorological hypothesis predicts that the clouds will move
northward as the secondary maximum (and eventually the max-
imum) in solar insolation moves northward. The beginning of
the movement from 40 south to the equator in only 7 yr will
be easily seen in remote observations of these clouds over the
next few years.
H. G. R. is supported by an NSF Astronomy and Astro-
physics Postdoctoral Fellowship under award AST-0401559.
Additional support was provided by NSF award AST-0307929.
The authors wish to recognize and acknowledge the very sig-
nificant cultural role and reverence that the summit of Mauna
Kea has always had within the indigenous Hawaiian commu-
nity. We are most fortunate to have the opportunity to conduct
observations from this mountain.
L52 ROE ET AL. Vol. 618
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Note added in proof.—The Cassini spacecraft confirmed the frequent presence of these midlatitude clouds with observations
starting 2004 May 29 (C. C. Porco et al. 2005, in preparation).


Discovery of Temperate Latitude Clouds on Titan

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Discovery of Temperate Latitude Clouds on Titan

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