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Titan as Revealed by the Cassini Radar

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Titan was a mostly unknown world prior to the Cassini spacecraft’s arrival in July 2004. We review the major scientific advances made by Cassini’s Titan Radar Mapper (RADAR) during 13 years of Cassini’s exploration of Saturn and its moons. RADAR measurements revealed Titan’s surface geology, observed lakes and seas of mostly liquid methane in the polar regions, measured the depth of several lakes and seas, detected temporal changes on its surface, and provided key evidence that Titan contains an interior ocean. As a result of the Cassini mission, Titan has gone from an uncharted world to one that exhibits a variety of Earth-like geologic processes and surface-atmosphere interactions. Titan has also joined the ranks of “ocean worlds” along with Enceladus and Europa, which are prime targets for astrobiological research.
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Space Science Reviews
TITAN AS REVEALED BY THE CASSINI RADAR
--Manuscript Draft--
Manuscript Number:
Full Title: TITAN AS REVEALED BY THE CASSINI RADAR
Article Type: Regular review paper
Keywords: Titan; Cassini; radar
Corresponding Author: Rosaly M.C. Lopes, Ph.D.
Jet Propulsion Laboratory, Caltech
Pasadena, CA UNITED STATES
Corresponding Author Secondary
Information:
Corresponding Author's Institution: Jet Propulsion Laboratory, Caltech
Corresponding Author's Secondary
Institution:
First Author: Rosaly M.C. Lopes, Ph.D.
First Author Secondary Information:
Order of Authors: Rosaly M.C. Lopes, Ph.D.
Stephen D Wall
Charles Elachi
Samuel Birch
Paul Corlies
Athena Coustenis
Alexander Hayes
Jason Hofgartner
Michael Janssen
Randolph Kirk
Alice LeGall
Ralph Lorenz
Jonathan Lunine
Michael Malaska
Marco Mastrogiuseppe
Giuseppe Mitri
Catherine Neish
Claudia Notarnicola
Flora Paganelli
Philippe Paillou
Valerio Poggiali
Jani Radebaugh
Sebastien Rodriguez
Ashley Schoenfeld
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Jason Soderblom
Anezina Solomonidou
Ellen Stofan
Bryan Stiles
Frederico Tosi
Elizabeth Turtle
Richard West
Charles Wood
Howard Zebker
Jason Barnes
Domenico Casarano
Pierre Encrenaz
Thomas Farr
Cyril Grima
Douglas Hemingway
Ozgur Karatekin
Antoine Lucas
Karl Mitchell
Gian Ori
Roberto Orosei
Paul Ries
Daniele Riccio
Laurence Soderblom
Zhimeng Zhang
Order of Authors Secondary Information:
Funding Information: NASA Charles Elachi
Abstract: Titan was a mostly unknown world prior to the Cassini spacecraft's arrival in July 2004.
We review the major scientific advances made by Cassini's Titan Radar Mapper
(RADAR) during 13 years of Cassini's exploration of Saturn and its moons. RADAR
measurements revealed Titan's surface geology, observed lakes and seas of mostly
liquid methane in the polar regions, measured the depth of several lakes and seas,
detected temporal changes on its surface, and provided key evidence that Titan
contains an interior ocean. As a result of the Cassini mission, Titan has gone from an
uncharted world to one that can be considered as the Earth of the outer solar system,
exhibiting a variety of Earth-like geologic processes and surface-atmosphere
interactions. Titan has also joined the ranks of "ocean worlds" along with Enceladus
and Europa, which are prime locations for astrobiological research.
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1
TITAN AS REVEALED BY THE CASSINI RADAR
R.M.C. Lopes1, S. D. Wall1, C. Elachi2, S. Birch3, P. Corlies3, A. Coustenis4, A. Hayes3, J.
Hofgartner1, M. Janssen1, R. Kirk5, A. LeGall6, R. Lorenz7, J. Lunine2,3, M. Malaska1, M.
Mastroguiseppe8, G. Mitri9, C. Neish10, C. Notarnicola11, F. Paganelli12, P. Paillou13, V. Poggiali3,
J. Radebaugh14, S. Rodriguez15, A. Schoenfeld16, J. Soderblom17, A. Solomonidou18, E. Stofan19,
B. Stiles1, F. Tosi20, E. Turtle7, R. West1, C. Wood21, H. Zebker22, J. Barnes23, D. Casarano24,
P.Encrenaz4, T. Farr1, C. Grima25, D. Hemingway26, O. Karatekin27, A. Lucas28, K. Mitchell1, G.
Ori29, R. Orosei30, P. Ries1, D. Riccio31, L. Soderblom5, Z. Zhang2.
1Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena,
CA 91109, USA.
2Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena,
CA 91125, USA.
3Department of Astronomy, Cornell University, 14853 Ithaca NY, USA.
4LESIA - Observatoire de Paris, CNRS, UPMC Univ. Paris 06, Univ. Paris-Diderot, Meudon,
France.
5Astrogeology Science Center U.S. Geological Survey 2255 N. Gemini D. Flagstaff, AZ 86001
USA.
6Laboratoire Atmosphères, Milieux, Observations Spatiales (LATMOS), Universite Versaills
Saint Quentin, Guyancourt, France.
7JHU Applied Physics Lab, Laurel, MD 20723, USA.
8University of Rome “La Sapienza”, Via Eudossiana 18, 00184 Rome, Italy.
9International Research School of Planetary Sciences, Università d’Annunzio, Via Pindaro 42,
65127 Pescara Italy.
10Department of Earth Sciences. The University of Western Ontario, 1151 Richmond Street N.
London, Ontario, Canada, N6A 5B .
11Eurac research, Institute for Earth Observation, Viale Druso 1, 39100, Bolzano, Italy.
12SETI Institute, 189 Bernardo Ave., Mountain View, CA, USA.
13University of Bordeaux UMR 5804 – LAB Batiment B18N allée Geoffroy Saint-Hilaire 33615
Pessac Cedex, France.
14Department of Geological Sciences, Brigham Young University, Provo, UT 84602.
15Institut de Physique du Globe de Paris (IPGP), CNRS-UMR 7154, Université Paris-Diderot,
USPC, Paris, France.
16UCLA Department of Earth, Planetary, and Space Sciences595 Charles E Young Dr E Los
Angeles, CA, 90095.
17Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of
Technology 77 Massachusetts Avenue, Boston, USA.
18European Space Agency (ESA), ESAC, Madrid, Spain.
19National Air and Space Museum, Smithsonian Institution, Washington, DC 20560.
20INAF - IAPS National Institute for Astrophysics, Via del Fosso del Cavaliere, 100, I-00133
Rome (Italy).
21Planetary Science Institute,1700 East Fort Lowell, Suite 106 * Tucson, AZ 85719-2395.
22Department of Electrical Engineering, Stanford University, California 94305-9515, USA.
23Department of Physics University of Idaho Moscow, ID 83843.
Manuscript Click here to download Manuscript TITAN SSReviews July
11_AS.pdf
Click here to view linked References
2
24CNR IRPI Via Amendola 122/I I-70125 Bari, Italy.
25Institute of Geophysics University of Texas at Austin J.J. Pickle Research Campus
10100 Burnet Road, Austin, TX 78758.
26Miller Institute for Basic Research in Science, University of California Berkeley, Department
of Earth & Planetary Science, 307 McCone Hall, Berkeley, California, 94720, USA
27Royal Observatory of Belgium, 3 Avenue Circulaire 1180, Brussels.
28Institut de Physique du Globe de Paris, CNRS, 1 rue Jussieu, Paris, France.
29 International Research School of Planetary Sciences, DÁnnunzio University of Chieti-Pescara,
INGEO, Pescara, Italy.
30Istituto di Radioastronomia, Istituto Nazionale di Astrofisica, Via Piero Gobetti, 101 40129,
Bologna, Italy.
31Dept. Electrical Engineering ad Information Technology, University of Napoli Federico II,
Napoli, Italy.
ABSTRACT
Titan was a mostly unknown world prior to the Cassini spacecraft’s arrival in July 2004. We
review the major scientific advances made by Cassini’s Titan Radar Mapper (RADAR) during
13 years of Cassini’s exploration of Saturn and its moons. RADAR measurements revealed
Titan’s surface geology, observed lakes and seas of mostly liquid methane in the polar regions,
measured the depth of several lakes and seas, detected temporal changes on its surface, and
provided key evidence that Titan contains an interior ocean. As a result of the Cassini mission,
Titan has gone from an uncharted world to one that can be considered as the Earth of the outer
solar system, exhibiting a variety of Earth-like geologic processes and surface-atmosphere
interactions. Titan has also joined the ranks of “ocean worlds” along with Enceladus and Europa,
which are prime locations for astrobiological research.
1. INTRODUCTION
The Titan Radar Mapper (henceforth “RADAR”, Elachi et al., 2004) was one of twelve
instruments aboard the Cassini spacecraft. It was developed by NASA’s Jet Propulsion
Laboratory (JPL), the Italian Space Agency (ASI) and the Cassini Radar Science Team primarily
to reveal the surface of Titan but also to explore Saturn, its rings and the icy satellites. Nearly
opaque at most visible and near-infrared wavelengths, Titan’s atmosphere is transparent at
RADAR’s operating wavelength of 2.17 cm (Ku Band).
RADAR had four operating modes – synthetic aperture radar (SAR) imaging, altimetry,
scatterometry, and radiometry. In its SAR mode, used at altitudes under ~4,000 km, RADAR
imaged Titan’s surface at incidence angles from 15-35° degrees and spatial resolutions varying
as low as 350 m, sufficient to identify major structures and to make geologic maps. SAR images
characterize the surface roughness at scales near the radar wavelength, to surface slopes at the
scale of the radar resolution, and to dielectric properties of the constituent materials. In addition,
radar can penetrate many wavelengths into some materials on Titan, measuring for example the
3
depths of some Titan lakes and seas. SAR images are observationally similar to optical images
and allow us to identify major structures and geomorphologic units.
Altimetry at a resolution of approximately 30 m was obtained when the antenna was pointed
within a fraction of a beam width (0.35 deg) of nadir resulting in a range resolution of
approximately 30 m, with spatial resolution depending on altitude. The altimetry data have two
broad applications. First, the aggregate of all data was used to define an absolute geoid for Titan
and constrain any rotational or tidal bulge. Second, relative topographic profiles from each radar
pass were used to characterize landforms (e.g., impact craters, mountains), yielding important
constraints on geophysical models. Altimetry was obtained over 2% of Titan’s surface, and an
amplitude monopulse comparison technique called “SARTopo” (Stiles et al., 2009) added an
additional 5% topographic coverage (Corlies et al., 2017). The SARTopo technique depends on
the precise manner in which the location and magnitude of each SAR pixel are affected by the
variation in surface height between each of the five radar beams (Elachi et al., 2004). The
technique estimates surface heights from observed SAR miscalibrations. Cassini’s antenna has
multiple beams (feeds), so that some points on the ground are observed from two different beams.
For different beams, the mislocation error due to surface height is the same, but the
miscalibration error is different and of opposite sign. Nonzero surface heights, therefore, result in
apparent differences in pixel positions between overlapping single beam SAR images. In
addition, altimetry was able to obtain depths of several lakes and seas (Mastrogiuseppe et al.,
2014).
Scatterometry is obtained by pointing the transmitter off-nadir, but processing the echos
using the real aperture rather than the synthetic aperture. The data indicate the backscatter
efficiency at various incidence angles at relatively low resolution (real aperture resolution
depends on range to surface). Scatterometry coverage is near-global (Fig. 1); it can be used as a
“basemap” for SAR and as a broad characterization of global terrains. As an independent
constraint on surface roughness, scatterometry data are an important complement to radiometry
data in order to constrain surface composition. Used as a passive microwave radiometer,
RADAR collected near-global measurements (Fig. 2) of Ku-band brightness temperature with an
accuracy of 1K (Janssen et al., 2016). Radiometry data produces polarized brightness
temperatures of the observed scene, obtained from the antenna temperature measurements. The
microwave brightness temperature of a solid surface depends on many properties besides
physical temperature; e.g., emission angle, polarization, dielectric constant, porosity, surface and
subsurface roughness, etc.
4
Figure 1. Predicted and observed maps of scattering from Titan’s surface. The upper panel is a
map of the volume scattering parameter fvol for a simple surface model composed of a smooth
dielectric interface separating free space from an inhomogeneous and isotropically scattering
subsurface. fvol is the probability that a photon entering the surface (as determined by the
effective dielectric constant) scatters and escapes from the subsurface before it is absorbed. The
map of fvol reconciles the maps of effective dielectric constant and emissivity and predicts the
overall magnitude of the scattering. The lower panel is a global mosaic of Titan's normalized
radar cross-section from all real aperture data through T71 (Wye et al., 2011), showing the
actually observed scattering. The white arrows in the upper left-hand corner indicate two small
regions that were not mapped.
5
Figure 2. Global mosaic of emissivity of Titan at the 2-cm wavelength of the Cassini RADAR.
This is the final result of the brightness temperature mapping of Titan incorporating all
radiometric data obtained from Titan passes, a nearly ten-year span. The brightness temperature
was measured as a function of position, polarization and time over a wide range of geometries
and ranges, and calibrated to about 1% absolute accuracy using Huygens probe and CIRS
temperature measurements as described in Janssen et al. (2016). The polarization data were
used to construct a global mosaic of effective dielectric constant, enabling the conversion of all
measured brightness temperatures to their equivalent values at normal incidence. The data were
then folded into a massive least-squares solution for the seasonally varying brightness
temperature distribution of Titan over the time scale of the observations. Comparison with
surface temperature measurements obtained in the IR using Kirchhoff’s law then enabled the
construction of the emissivity map shown. Note that Xanadu, the equatorial region centered on
100 W° longitude, is extremely cold and non-emissive, characteristic of a high content of highly
fractured water ice bedrock.
6
In its 13-year presence at Saturn, Cassini made a total of 127 passes by Titan, some as close
as 950 km. 48 of these were used by RADAR, with one lost in downlink (for a detailed
description of results from individual flybys, see Lorenz et al. 2018). SAR coverage was limited
by both Cassini’s orbital path, which used Titan for gravity assists, and by intense competition
among instruments for observation time – resulting in a seemingly random set of strips 100 – 200
km wide (Figure 3). However, the swaths were chosen to be well distributed in latitude and
longitude, allowing for a global picture of the surface to emerge. Areal SAR coverage at the end
of mission was 46% using nominal SAR imaging swaths obtained at altitudes below 5000 km
with resolutions around 500 m. Coverage increases to 74% when high altitude imaging scans
obtained at altitudes above 9000 km are included. High altitude imaging scans have lower
resolutions that vary from 1 km to 5 km. Repeat images were obtained over 14% of the surface,
and 1.5% was covered four or more times. Real-aperture scatterometry and passive radiometry
data exist over the entire surface.
Figure 3. Combined Titan SAR coverage from the entire Cassini mission. Swaths are shown over
an ISS basemap. The swaths colors, displayed on the right-hand side, represent Titan flyby
numbers.
RADAR data, together with near- and mid-infrared images taken by Cassini’s Visual and
Infrared Mapping Spectrometer (VIMS, Brown et al., 2004) and the Imaging Science Subsystem
(ISS, Porco et al., 2004), have given us an opportunity to discover a new world that is both
Earth-like and unearthly. We find familiar geomorphology in its mountains, dunes, rivers and
seas, but note that these features are composed of unfamiliar materials. In this paper, we review
RADAR’s contributions to geology, hydrology, and surface-atmospheric interaction in an
attempt to summarize our knowledge of Titan as obtained from the RADAR instrument at the
end of the Cassini mission.
7
2. INTERIOR
Static and time-varying gravity field measurements (Iess et al., 2010, 2012), rotational
dynamics measurements (Stiles et al., 2008, 2010; Lorenz et al., 2008a; Meriggiola et al., 2016)
and shape models (Zebker et al., 2009; Mitri et al., 2014; Corlies et al., 2017) have been used to
infer the interior structure of Titan. Magnetic field measurements have shown that an intrinsic
magnetic field is not present on Titan (Wei et al., 2010). The determination of the rotational
dynamics from Cassini RADAR SAR images has been key to constraining the interior structure
of Titan. As described above, Cassini RADAR SAR coverage of Titan is dependent on orbit
dynamics and competition for observation time. As a result, SAR swaths appear somewhat
randomly distributed with coincidental overlaps (with only a few exceptions). Where overlap
occurs, and assuming that the same surface features appear in both images, it is possible to
determine how quickly Titan has rotated between observations and thus estimate Titan's pole
location and spin rate (Stiles et al., 2008, 2010), analogous to previous work using Magellan
SAR imagery to estimate the spin model of Venus (Davies et al., 1992). First, a set of
recognizable landmarks observed in two different SAR images obtained at different times is
located in a Titan-centered inertial, non-rotating (J2000) reference frame. Pixels chosen in each
SAR image, corresponding as closely as possible to the same point on the landmark, are aligned
using several techniques to minimize landmark mismatches. The spin state parameters are
estimated by minimizing the misregistration error that is, the apparent movement in Titan
body-fixed coordinates of the landmarks between observation times. Feature mapping is much
more robust for radar than for passive optimal imagery, because radar geolocation is independent
of spacecraft pointing knowledge except that the general direction (within 10° or so) must be
known to exclude mirror ambiguities. Accurate radar pixel locations in inertial space depend
only on the accuracy of the measurement of delay, Doppler shift, spacecraft velocity and position,
and presumed target body topography. Nearby SARTopo (Stiles et al., 2009) measurements of
topography help determine more precisely variations in feature locations in inertial space. Using
similar methodology and a more extensive set of observations, Meriggiola et al. (2016) also
provided a rotational model of Titan estimating the spin pole location, the spin rate, the
precession and the nutation. Further, these authors show that the pole location is compatible with
the Cassini state 1 (a dynamical equilibrium wherein the spin axis, the orbit normal and the
normal to the invariable plane are coplanar).
Stiles et al. (2008) and Meriggiola et al. (2016) have provided an estimate of Titan’s
obliquity (0.31°). The obliquity together with the quadrupole moment of the gravity field (J2 and
C22) measurements (Iess et al., 2010, 2012) constrain the moment of inertia of Titan. These
results were used by Bills and Nimmo (2011) to estimate the radial mass distribution of Titan
and, when combined with the low-degree gravity field derived from Cassini spacecraft Doppler
tracking data (Iess et al., 2010), suggest that Titan’s outer shell is mechanically decoupled from
the deeper interior. Bills and Nimmo (2011) and Meriggiola et al. (2016) find that the estimated
obliquity is compatible with a deep interior decoupled from the outer ice shell by a global
subsurface ocean. Note that Lorenz et al. (2008a) interpreted the initial indication in Stiles et al.
(2008) of nonsynchronous rotation as indicating a subsurface ocean, drawing on predictions of
atmospheric angular momentum exchange with a decoupled shell by Tokano and Neubauer
(2005). However, the spin state determination has significant degeneracy between
nonsynchronous rotation and precession of the spin pole, and subsequent observations seem to
8
favor synchronous rotation. Thus, the conclusion of Lorenz et al. (2008a) of an internal global
ocean indicated by RADAR spin measurements remains correct, but for perhaps the wrong
reasons. The presence of a subsurface ocean inferred from the rotational dynamics of Titan is
consistent with the large tidal response of Titan (tidal Love number k2=0.589±0.150) (Iess et al.,
2012; see also Mitri et al., 2014). In agreement with this scenario, the Permittivity, Wave and
Altimetry (PWA) instrument on board Huygens’ probe measured a Schumann-like resonance,
also suggesting the presence of a subsurface ocean (Bèghin et al., 2012). In summary, the gravity,
topography and rotational dynamics measurements in combination with thermal-evolution
models indicate that Titan is internally differentiated (Fig. 4), and has an outer ice shell, a
subsurface ocean, a high pressure layer at the base of the ocean, and a deep rocky or rock-ice mix
interior (Hemingway et al., 2013; Mitri et al., 2014; Tobie et al., 2014).
Figure 4. Possible present-day structure of Titan’s interior showing that that Titan is internally
differentiated, with a deep rocky or rock-ice mix core.
3. GLOBAL SHAPE
Cassini radar observations using both the altimetry and SARtopo modes reveal Titan’s
global shape and yield insights to its interior structure. The roughly 60 satellite-derived elevation
surface traces (Fig. 5; Zebker et al., 2009) show that Titan’s polar radius is less than, and the
equatorial radii are greater than, predicted by its gravity field. Best-fitting solutions are shown in
Table 1 below.
9
Figure 5. Cassini measurements of Titan surface height above a reference sphere of 2575 km
radius. As expected, the shape is dominantly triaxial ellipsoidal, with topographic lows near the
poles and topographic highs at the prime- and anti-meridians. However, the polar flattening is
greater than expected for a hydrostatic body, meaning the equatorial topography stands higher
than expected and the polar depressions are lower than expected, suggesting that the depth to
the subsurface ocean is smaller at the poles than at the equator.
Table 1. Titan Gravity and Shape Triaxial Ellipsoids (m)
Gravity Elevations Difference
a axis 2574875+/-7.3 2575124+/-26 -249
b axis 2574716+/-7.4 2574746+/-45 -30
c axis 2574660+/-4.5 2574414+/-28 246
a translation 0.8+/-5.6 69.8+/-6.2
b translation 0.1+/-5.6 68.3+/-8.0
c translation 1.7+/-3.9 45.8+/-6.4
Mean eq. rad. 2574795 2574761 -34
Hydrostatic 3.83 2.14
ratio (a-c)/(b-c)
The hydrostatic ratio (a-c)/(b-c), which is exactly 4 for a spin-locked satellite in hydrostatic
equilibrium, for Titan’s figure is observed to be only 2.14 (Zebker et al., 2009; Corlies et al.,
2017). The hydrostatic ratio obtained from the third degree gravity observations as 3.83, that is
the gravity field reflects approximate hydrostatic equilibrium. While the gravity field is
consistent with a hydrostatically relaxed body, Titan’s figure is not. If both of these data
constraints pertain, Titan’s interior does not conform to a set of spherically symmetric shells, as
these data imply that the average satellite density at the equator is less than at the poles. If Titan
has a vast interior ocean of liquid water, then some ice layers (less dense than liquid water) are
thinner at the poles than the equator. A simple model satisfying both sets of data and assuming
isostatic compensation (Fig. 6) could be the result of uneven heat dissipation in Titan’s interior,
such as may result from tidal interaction with Saturn or its other moons.
Figure 6. A model consistent with Cassini gravity and figure measurements, assuming isostatic
compensation. The thinner ice shell (exaggerated for visibility in the figure) at the poles could
result from uneven heat dissipation within Titan from tidal interactions, and the relatively
shallower “geoid” at the poles is one explanation for the preponderance of lakes at the most
northern and southern latitudes. The shallower geoid allows mobile liquids to lie closer to the
surface.
Another simple model explaining the overly oblate topographic figure assumes that there is
greater precipitation of hydrocarbon snow or loosely packed hydrocarbon particulates at the
equator than at the poles, perhaps with a net equatorward transport from the poles, which would
also be consistent with the data. This could account for increased distance from the surface to
the planet center at the equator if sufficient material were so transported. We note that this is
consistent with the observation of 100-m tall dune structures near Titan’s equator (see Section
5.3), so that poorly consolidated material may indeed collect there to a greater extent. Several
hundred meters of deposition at the equator would match the mean equatorial radii of both
models, but precipitation would have to preferentially occur in the sub- and anti-Saturnian
directions, which seems unlikely.
Further comparisons of the gravity and shape observations constrain the depth of any outer
ice shell enclosing a global subsurface ocean (Zebker and Wong, 2016). While the exact value
varies depending on how the spherical expansion is constrained, the observed gravity to
topography ratio is 0.070 for the third order terms and 0.042 for fourth order (Zebker and Wong,
2016). These imply ice-shell thicknesses of 327 and 187 km, respectively, a factor of 2 more
than the 100 km expected from thermal models (Sohl at al., 2003; Nimmo and Bills, 2010). The
Zebker and Wong solution yields a tidal Love number h2t of about 0.5, and a basal heat flow of
2.5 mW/m2. This would suggest that heat from Titan’s core is lower than often assumed, hence
the amount of radiogenic material in the core is likely less as well. These data also constrain the
depth of Titan’s mantle and density of its core, placing added restrictions on its composition and
evolution. Supposing that Titan has an undifferentiated ice/rock core beneath the ocean, and that
the moment of inertia is most likely in the range 0.33-0.34 (Iess et al., 2010), then the 200-km-
thick crust estimate above and Titan’s well-known mean density of 1.88 g cm-3 implies an ocean
depth and core density ranging from 308 km and 2.74 g cm-3 (MOI=0.33), to 226 km and 2.55 g
cm-3 (MOI=0.34).
4. COMPOSITION AND SURFACE AND SUBSURFACE PROPERTIES
The Cassini-Huygens mission has revealed the surface of Titan in unprecedented detail,
enabling us to discern different geomorphological units on the surface (Section 5), constrain the
relative times of emplacement of these units, and place constraints on composition. Titan has an
ice shell (Fig. 4) but water-ice signatures are not easily detected due to atmospheric scattering
and absorption that hamper the observations and the presence of complex organic molecules on
the surface. The extended, dense, and hazy N2-CH4 dominated atmosphere shields the surface
from direct optical observations, except at certain wavelengths where the methane absorption is
weak. These methane “atmospheric windows” (McCord et al., 2006) are exploited by the Cassini
VIMS to obtain compositional information of the top few microns of the surface, as discussed in
Section 4.2. below, while RADAR can probe the surface and subsurface scattering properties,
and hence place additional constraints on composition.
4.1 Surface and Subsurface scattering properties
Compared to the surfaces of the Moon or the Earth, the off-nadir radar response from most
of Titan’s surface is quite strong at Ku-band (2 cm-l). This indicates that more complex
processes than simple surface scattering, such as a significant volume scattering component,
have to be considered (Elachi et al., 2005; Wye et al., 2007; Paganelli et al., 2007; Janssen et al.,
2016). Indeed, given the low Titan surface temperatures and the low loss tangent of analogs of
materials relevant to Titan’s surface, signals from Cassini RADAR’s Ku-band instrument should
penetrate the surface down to a depth ranging from a few decimeters for an organic and
compacted near-surface, to several meters for a pure water ice near-surface (Paillou et al., 2008)
and thus have multiple opportunities to be scattered. The above referenced observations of
Titan’s surface are consistent with sub-surface volume scattering processes, in addition to pure
surface scattering. Analysis of Cassini scatterometer and radiometer measurements obtained
simultaneously (see Fig. 7) are best fit using models where volume scattering, enabled by the
low material losses, is enhanced by coherent backscatter processes (Zebker et al., 2008; Janssen
et al., 2011).
Janssen et al. (2016) further advance that a regionally enhanced degree of volume scattering
is indicative of a higher abundance of water ice in the near-surface. This is because water ice is
more transparent to microwaves than common organic materials, allowing for more opportunities
for scattering. This would be consistent with about 10% of Titan’s near-surface being water ice-
rich while the composition of the remaining terrains is dominated by more absorbing organic
materials, likely by-products of the intense atmospheric photochemistry (Lorenz et al., 2008b;
Hörst, 2017). The regions that contain a high degree of volume scattering include mountainous
terrains, impact craters, fluvial and fan-like features, all of which possibly correspond to highly
fractured or unconsolidated sedimentary materials derived from erosion. Many of these materials
could also have originated from cryovolcanism, in which the radar signature could be explained
by a strong volume scattering effect in a thick water-ammonia ice layer using a two-layer
scattering model (Paillou et al., 2006). Radar-bright sinuous channels in the southwest of Xanadu
(Section 5.8), showing very large radar cross-sections, are also consistent with the presence of
icy, low-loss, rounded scatterers, acting as efficient natural retro-reflectors (Le Gall et al., 2010).
Weaker radar reflectors such as Titan’s dunes are most likely organic in nature (as also
supported by VIMS (e.g., Barnes et al., 2008; Soderblom et al., 2009; Clark et al., 2010).
However, we note that these features exhibit somewhat high backscatter at large incidence angles
compared to Earth analogs, which suggests even aeolian sediments may contain centimeter-scale
gravels producing a significant volume scattering component (Paillou et al., 2014). Features
interpreted as mega-yardangs were observed on Titan, and they also exhibit a much brighter
radar signature than their terrestrial analogs, indicating that additional scattering processes, such
as volume scattering, occur in those materials as well (Paillou et al., 2016).
Lastly, and of particular interest, is the case of Titan’s methane-dominated lakes, where
radar waves can penetrate down to several thousand wavelengths (at least 150 m) and be
subsequently backscattered by the bottoms of a lake or seabed (Mastrogiuseppe et al., 2014).
These results are consistent with recent laboratory investigations of the electrical properties of
liquid hydrocarbons (Mitchell et al., 2015).
Figure 7. Angular dependence of radiometer (left) and scatterometer (right) measurements of a
portion of Titan's surface. Grey x's are observations, while black circles are modeled values
assuming both surface and volume scattering terms. Both sets of curves fall off slowly with
incidence angle, indicative of significant volume scattering.
4.2 Surface Composition from VIMS and RADAR
A combination of RADAR and near-infrared multispectral imaging data (VIMS) is a
powerful way to distinguish and categorize geomorphological features into units with distinct
chemical compositions (that remain to be identified). Spectroscopic observations of Titan’s
surface are severely hindered by the presence of an optically thick, scattering and absorbing
atmosphere, allowing direct investigation of the surface within only a few spectral windows in
the near-infrared. Based on the 1.29/1.08 µm, 2.03/1.27 µm, and 1.59/1.27 µm band ratios
measured by VIMS at low to moderate latitudes, three main spectral units were initially
distinguished on the surface of Titan: bright material mainly distributed in the topographically
high and mid-latitude areas; ‘blue’ material adjacent to the bright-to-dark boundaries; and
‘brown’ material that correlates with RADAR-dark dune fields (Barnes et al., 2007; Soderblom
et al., 2007; Jaumann et al., 2008).
Even though these spectral units are distinct, their actual compositions remain elusive. A
number of chemical species were proposed to exist on the surface of Titan, but only a few
absorptions were unambiguously detected from remote-sensing observations carried out by
VIMS during Cassini flybys. A methane-ethane dominated composition seems to be present in
the polar lakes and seas of Titan (e.g., Brown et al., 2008, Lunine and Lorenz, 2009). IR
spectroscopy, microwave radiometry and scatterometry are sensitive to the physical structure of
the surface to a different extent and at different scales. IR spectroscopy measurements are used to
determine surface composition, but they are also affected, down to depths of micrometers, by the
physical properties of the surface material like roughness, photometric geometry, and porosity.
Correlations between near-infrared and microwave data of Titan’s surface are useful to
gather a broader understanding of surface properties. These were quantified at coarse spatial
resolution by Tosi et al. (2010), who applied a multivariate statistical analysis to an aggregated
data set made up of infrared spectra acquired by VIMS at spatial resolution of tens of km
together with scatterometry and radiometry data measured by RADAR. This technique allowed
for the identification of regional surface units at equatorial to mid-latitudes. Some of these units
matched both the major dark and bright features seen in the ISS mosaic of Titan (Porco et al.,
2005; Turtle et al., 2009), whereas other units showed boundaries not apparent from the visible
and near-infrared remote-sensing data set. In particular, while dark equatorial basins are very
similar to each other in terms of infrared and microwave reflectance at this spatial scale, the
major bright features do not share the same characteristics.
A comprehensive investigation of Titan's surface features using the VIMS and SAR datasets
at the best available spatial resolution is still the best approach to characterize geomorphologic
units using both spectral and morphologic characteristics. For example, the correlation between
the 5-µm-bright materials and SAR empty lakes suggests the presence of sedimentary or organic-
rich evaporitic deposits in dry polar lakebeds (Barnes et al., 2011; MacKenzie et al., 2014).
Langhans et al. (2012) extensively studied the morphology, distribution, and spectral properties
of Titan’s fluvial valleys showing that these are mostly associated with the bright surface unit.
In recent years, several investigators have applied radiative transfer models in addition to
comparison between data sets (e.g., Hirtzig et al., 2013; Solomonidou et al., 2014; Lopes et al.,
2016). These studies allow definition of both the surface and the atmospheric contributions from
VIMS spectral imaging data after performing the appropriate pixel selection of areas of interest
with the help of SAR data.
The application of radiative transfer analyses to the VIMS Titan data yields extracted
weighted surface albedos in the seven methane windows, which have been tested against a
variety of Titan candidate ice and organic constituents to provide constraints on the possible
material present in various geomorphological units (Solomonidou et al., 2018). An updated
material library is being used based on Bernard et al. (2006), Brassé et al. (2015) and the
GhoSST database (http://ghosst.osug.fr). This library includes several materials at different grain
sizes, such as ices of acetylene (C2H2), ethylene (C2H4), ethane (C2H6), propane (C3H8),
cyanoacetylene (HC3N), water (H2O), ammonia (NH3), methane (CH4) and carbon dioxide (CO2),
in addition to spectra of laboratory tholins (Bernard et al., 2006; Brassé et al., 2015), and spectra
of dark materials such as asphalite, kerite, different types of anthraxolite and amorphous carbon,
which have been proposed to lower the total surface albedo of Titan’s surface (Lellouch et al.,
2006; GhoSST database). Considering the different grain sizes, the library consists of 148
different constituent possibilities that can also be mixed. By using this constituent library,
spectral simulations are made, and an iterative process is used to obtain the best fit to the
observations, bearing in mind that there is no unique solution for the whole mixtures. With these
and other caveats, Solomonidou et al. (2018) have derived constraints on the possible major
constituent for each geological unit and reported a latitudinal dependence of Titan’s surface
composition, with water ice being the major constituent at latitudes poleward of 30°N and 30°S,
while Titan’s equatorial region appears to be dominated by a very dark organic material, possibly
aromatic in character (Clark et al., 2010). The surface albedo differences and similarities among
the various geomorphological units also have implications for the geological processes that
govern Titan’s surface and interior (e.g. aeolian, cryovolcanic, tectonic).
5. SURFACE GEOLOGY
5.1. Major geologic units and mapping
SAR data have been used since the early days of the mission to interpret different types of
terrains (Fig. 8; Stofan et al., 2006; Paganelli et al., 2007, 2008; Lopes et al. 2010). More recent
work used not only SAR data but added correlations with data from other RADAR modes
(altimetry, SARTopo, scatterometry and radiometry), and also from VIMS and ISS to provide
sufficient information on Titan’s surface to distinguish the major types of terrain units (Malaska
et al., 2016a; Lopes et al., 2016). These data were also used to establish the major
geomorphologic unit classes on Titan and their relative ages using contacts between units. In
order of total surface area, the classes of units are: plains, dunes, mountainous/hummocky
terrains, labyrinth terrains, lakes, and impact craters. The oldest units are the
mountainous/hummocky and the labyrinth terrains; it is not possible with currently available data
to differentiate the relative ages of these two oldest types of terrain. The mountainous/hummocky
terrains consist of mountain chains and isolated radar-bright terrains. The labyrinth terrains
consist of highly incised and dissected plateaus with medium radar backscatter. The plains are
younger than both mountainous/hummocky and labyrinthic unit classes. Dunes and lakes of
liquid hydrocarbons are the youngest unit classes on Titan. Additionally, we have identified
individual features such as crater rims, channels, and candidate cryovolcanic features. Crater
rims and candidate cryovolcanic features are locations more likely to expose materials from the
icy crust, while the hummocky/mountainous materials are thought to be exposed remnants of the
icy crust.
Characterization and comparison of the properties of the unit classes and individual features with
data from radar radiometry, ISS, and VIMS provide information on their composition and
possible provenance and shed light on the interconnection between the interior, the surface, and
the atmosphere. Both microwave emissivity and VIMS are helpful in characterizing the units,
although both have different penetration depths, with microwave emissivity penetrating 10s of
cm into the surface, while infrared-based responses, such as VIMS and ISS, penetrate only the
top surface coating on the order of microns. From microwave emissivity data described above,
the hummocky/mountainous terrains and impact crater rim features have relatively low
emissivity (and greater radar scattering) in radiometric data, consistent with more water ice near
the surface (Fig. 2). The undifferentiated plains, dunes, labyrinth terrains, and lakes all have high
emissivity (lower radar scattering) in radiometric data which is consistent with low-dielectric
organic materials.
Figure 8. Examples of geologic features on Titan: Top row from left: Sinlap crater with its well
defined ejecta blanket. Middle: Crateriform structure (suspected impact crater) on Xanadu.
Right: Ontario Lacus near Titan's south pole. Middle row, left: Bright (high SAR backscatter)
deposit Winia Fluctus abutting darker undifferentiated plains, the dunes indicated by arrows
overlie part of deposit. Middle row, right: Dunes abutting against hummocky and mountainous
terrain. Bottom left: mountains (SAR bright) in the equatorial region. Right: Elivagar Flumina,
interpreted as a large fluvial deposit, showing braided SAR-bright channels which overlie SAR-
dark undifferentiated plains.
Spectral coatings of the terrain units were described in Solomonidou et al. (2018) and
discussed above. From this analysis, three groups of compositional mixtures are reported, which
include the major geomorphological units mentioned in the previous section, among three
candidates (water ice, tholin, and a dark component). The units with spectral responses most
similar to water ice are the labyrinth terrains and a number of different types of plains such as the
streak-like, the scalloped, and the undifferentiated plains that are concentrated at the higher parts
of the mid-latitudes. The impact crater ejecta examined (6 in Solomonidou et al., 2018) and the
alluvial fans are part of a different compositional group in which a tholin-like material is
dominant. Furthermore, the units covered with an unknown dark constituent are the
hummocky/mountainous terrains, the variable plains, the dunes, and the undifferentiated plains
that are close to the equator and possibly contaminated by dune material (Lopes et al., 2016;
Solomonidou et al., 2018). Since microwave radiometry and VIMS are global datasets, although
at lower spatial resolution than SAR, we can also use these correlations to extrapolate to regions
not covered by SAR. This is particularly important as SAR data did not provide complete
coverage of Titan at the end of the Cassini mission.
5.2. Plains
Plains are the most widespread type of terrain on Titan (Lopes et al., 2016). Although there
are several different types of plains, by far the most extensive are the Undifferentiated Plains,
first mapped by Lopes et al. (2010). These are vast expanses of SAR-dark terrains that appear
fairly uniform in SAR images, with no major topographic units and are for this reason often
referred to as “blandlands”. Lopes et al. (2016) mapped the distribution of the Undifferentiated
Plains using SAR swaths up to flyby T92 (July 2013) and found that these terrains are located
mainly at mid-latitudes. Their gradational boundaries and paucity of recognizable features in
SAR data make geologic interpretation challenging, so Lopes et al. (2016) used all the RADAR
datasets available, plus VIMS and ISS, to examine and evaluate different formation mechanisms
including (i) cryovolcanic origin, consisting of overlapping flows of low relief or (ii)
sedimentary origins, resulting from fluvial/lacustrine or aeolian deposition, or accumulation of
photolysis products created in the atmosphere. Their analysis showed that exposures of
Undifferentiated Plains in the lower mid-latitudes are consistent with a composition
predominantly containing organic rather than icy materials and formed by depositional processes,
unlike the undifferentiated plains in the higher mid-latitudes, which are consistent with water ice
(Solomonidou et al., 2018). The study concluded that aeolian processes played a major part in
the formation of the Undifferentiated Plains, though other processes (fluvial, deposition of
photolysis products) are likely to have contributed, possibly in differing proportions depending
on location. However, the distribution of Undifferentiated Plains, both at local and global scales,
is consistent with aeolian deposition being the major process contributing to their formation.
Spectral differences between the Plains and Dunes seen in VIMS data imply that the
materials, at least on the top layers of the surface, are not exactly the same. Spectral differences
in terms of surface albedo values between locations of Undifferentiated Plains (Lopes et al.,
2016; Solomonidou et al., 2018) show that Plains at lower latitudes (closer to the dune seas) are
more spectrally similar to dune materials, suggesting that they are related and supporting the idea
that dune materials are transported by wind to mid-latitudes (Malaska et al., 2016a). The
Undifferentiated Plains located at lower mid-latitudes (and therefore closer to the equatorial
dunes) appear to be composed predominantly of organic materials, which may have been
cemented by an organic substance and/or wetted by methane, causing them to become spectrally
different from dune materials, at least at a surficial level. Work by Malaska et al. (2016a) and
Lopes et al., (2016) suggests that the plains deposits may be derived from modified dune
materials thus tying two of the major geomorphologic units together. If the Undifferentiated
Plains materials are mainly the result of aeolian deposition but contain liquids due to methane
rain or fluids transported by channels, this could explain why they show relatively high
emissivity (lower global dielectric constant, less efficient volume scattering) as well as why they
are free of observable dunes (reduced sediment mobility). It would also be consistent with the
high level of degradation of craters found at mid-latitudes, potentially due to efficient erosion by
fluvial, pluvial, or subsurface flow activity (Neish et al., 2016).
5.3. Dunes
One of the youngest and most areally extensive geomorphologic units on Titan consists of
sand dunes (Lorenz et al., 2006). These appear as long, narrow, and SAR-dark features against a
SAR-brighter substrate or interdune (Fig. 9), presumably because dune sands are smooth to
RADAR at the 2.17 cm Cassini SAR wavelength. The dunes are generally 1-2 km wide, spaced
by 1-4 km and can be over 100 km long (Lorenz et al., 2006; Radebaugh et al., 2008). Limited
measurements of heights from radarclinometry suggest they are 80-130 m tall (Neish et al.,
2010). They are grouped together in large dune fields, or sand seas, equatorward of ±30 degrees
latitude. Titan’s dunes interact with topographic obstacles, seen as SAR-bright and generally
isolated mountains, in a way that indicates general W-E transport of sand; they pile up on the
west side of obstacles, divert in their azimuth around the obstacles, and are sparser on the east
side (Radebaugh et al. 2010). Their size, general morphology and relationship with underlying
terrain and obstacles, and their style of collection are nearly identical to large, linear dunes in
Earth’s sand seas of the Sahara, Arabia and Namibia (Lorenz et al., 2006; Radebaugh et al.,
2008; Le Gall et al., 2011; Le Gall et al., 2012). Such dunes on Earth typically form under
bimodal winds (Fryberger and Dean, 1979; Tsoar, 1983). A more recent model calls on a
dominant, slightly off-axis wind and a secondary wind causing sand flux down the dune long
axis (Courrech du Pont et al., 2014; Lucas et al., 2014).
Regardless of whether the classical (bimodal) or fingering-mode dune growth mechanisms
apply, a fundamental challenge raised by the RADAR observations of the dunes is the eastward
direction of growth and sand transport (Lorenz et al., 2006; Radebaugh et al., 2010). This
contrasts with expectations that low-latitude near-surface winds should generally blow to the
west. The solution appears to be that the dunes reflect strong but infrequent eastward winds,
either associated with vertical mixing in the atmosphere at equinox leading to strong westerly
gusts (Tokano, 2010) or methane rainstorms having a similar effect (Charnay et al., 2015).
Additionally, convergence of the meridional transport predicted in models (e.g. Lucas et al.,
2014) can further explain why Titan’s dunes are confined within ±30 latitudes, where sediment
fluxes converge (see also Malaska et al., 2016a).
Figure 9. Dunes on Titan as seen by the Cassini SAR (wavelength 2.17 cm, with spatial
resolution of 350 m) in the Belet Sand Sea, from the T61 (Aug 2009) swath on the equatorial
leading hemisphere, at 11
°
S, 255
°
W. Dune surfaces are generally smooth and absorbing to
SAR and thus typically, as here, appear as SAR-dark lines against a rougher and/or fractured,
and thus radar-bright substrate, unless the RADAR is pointed directly at a dune face, in which
case it appears as a thin, SAR-bright line. Occasionally the radar incidence is such as to give
bright glints from dune slopes. The open arrows indicate the direction of SAR illumination and
incidence angle. From Radebaugh et al. (2013).
Titan’s dune sands are not only dark to SAR but they are some of the darkest materials seen
by ISS (Porco et al., 2005; Karkoschka et al., 2017) and have a low albedo and red slope as seen
by VIMS, thus comprising the VIMS dark brown spectral unit (Soderblom et al., 2007; Barnes et
al., 2008; Clark et al., 2010; Rodriguez et al., 2014). Volume scattering within the dunes is very
low, consistent with smooth, homogeneous surfaces in general, and lacking large voids or clasts
(Janssen et al., 2009; Le Gall et al., 2011), although modeling by Paillou et al. (2014) suggests
shallow surface ripples and some volume scattering. The observations indicate the dunes cannot
be composed of water ice, but rather must be made of organics, ultimately derived from
photolytic processing of methane in the upper atmosphere, and precipitation to the surface
(Lorenz et al., 2006, 2008a; Soderblom et al., 2007). Sand sources could include river channels,
as on Earth (Radebaugh, 2013); low-latitude evaporite deposits, which can show similar VIMS
properties (Barnes et al., 2011); or the midlatitude blandlands, though it’s more likely that sands
are being transported there from the equatorial regions (Lopes et al., 2016, Malaska et al., 2016a).
The extent of the dunes indicates that sand have been generated on Titan in great volumes and
transported by wind, and that processes have acted on the surface long enough to produce
extensive and morphologically consistent landforms (Radebaugh, 2013).
Gathering all exploitable SAR and HiSAR images since the start of the Cassini mission,
Rodriguez et al. (2014) built a global map of the dune coverage available at that time, accounting
for observations from TA to T92 flybys (from October 2004 to July 2013) and more than 30
individual RADAR SAR and HiSAR swaths. They evaluated that dunes cover 13 ± 2% of the
58.1% of Titan’s surface observed with SAR and Hi-SAR, considering only those images having
sufficient spatial resolution to identify individual dunes (i.e., excluding HiSAR swaths with a
resolution coarser than 2 km/pixel). In terms of latitudinal distribution, 99.6% of the imaged
dunes are found within the equatorial belt (within ±30° latitudes), 61.3% of which has been
imaged after flyby T92. The overlapping of a VIMS global mosaic and the global distribution of
dunes as seen by the RADAR highlights the strong correlation between the dunes and a specific
infrared unit spectrally compatible with complex solid organics (the “dark brown” unit). This
allowed an extrapolation of the dune geographic distribution to the entire spatial extent of the
VIMS dark brown unit, even in locations where dunes are not seen because of lack of RADAR
coverage, and extending the previously estimated total surface area of Titan covered by dunes up
to »17% (»14.106 km2, 1.5 times the surface area of the Sahara desert on Earth), the same as
early estimates from Lopes et al. (2010). A simple calculation of the volume (e.g. Lorenz et al.,
2008b) indicates that dunes are a major surface reservoir of organics, probably originating from
the atmosphere.
In addition to dunes, there are other aeolian features and landforms on Titan’s surface. These
are wind streaks and yardangs, or wind-carved ridges. The wind streaks are visible in ISS images
as bright features that extend in the downwind direction from obstacles (e.g. Porco et al., 2005;
Lorenz et al., 2006; Malaska et al., 2016a). They can be several tens of kilometers wide and long,
can have flow-like, teardrop shapes, and appear as though wind has shaped the bright landscapes,
and has deposited dark materials, likely sand, in the low regions downwind of the obstacles.
These features help indicate the direction of the winds, which also broadly parallels the linear
dunes seen in Cassini SAR images (Malaska et al., 2016a).
Deposits that are SAR-bright, circular in planform and likely elevated into small mounds or
domes are found in some regions in the northern midlatitudes (Lopes et al. 2016). These domes
are easily eroded as revealed by deep, badlands-like river channels that flow outwards from their
centers. Cutting across the channels and the domes are a series of parallel, long lineations ~1 km
wide, spaced by a few km, and tens of kilometers long (Paillou et al., 2016; Northrup et al.,
2018). They are similar in appearance and SAR brightness, radiometry and scatterometry to
yardangs, or wind-carved ridges (Paillou et al., 2015). These appear to have formed in easily
eroded materials, similar to yardangs on Earth and Mars and further indicate the action of wind
at moderate to high latitudes now or in the past (Northrup et al., 2018).
5.4. Impact Craters
Before Cassini arrived at Saturn, the impact cratering history on Titan was unknown from
direct observations. Estimates of the cratering rate were made by extrapolating the crater
distributions observed on other Saturnian satellites, or by predicting impact rates by comet
populations. Such estimates suggested that at least several hundred craters larger than 20 km in
diameter should be present on Titan (Zahnle et al., 2003). Impactors that create craters smaller
than 20 km in diameter are expected to be disrupted by Titan’s atmosphere (Artemieva and
Lunine, 2005; Korycansky and Zahnle, 2005). Cassini RADAR observations show an extreme
paucity of craters. Only 23 certain or nearly certain craters and ~10 probable craters >20 km in
diameter have been observed on Titan, with a handful of smaller crater candidates (Wood et al.,
2010; Neish and Lorenz, 2012; Buratti et al., 2012; Neish et al., 2016). This suggests that Titan
has a crater retention age of several hundred million years (Wood et al., 2010; Neish and Lorenz,
2012), with the oldest surfaces located near the equator and the youngest surfaces located near
the poles (Neish et al., 2016).
The craters that are observed on Titan all show evidence for extensive modification by
erosional processes (Figure 10). Channels are observed to cut through the ejecta blankets and
floors of several impact craters (Wood et al., 2010; Soderblom et al., 2010; Neish et al., 2015).
Many of Titan’s impact craters are located in its equatorial sand seas, and also show evidence for
extensive infilling by sand (Wood et al., 2010; Le Mouelic et al., 2008; Neish et al., 2013). In
addition to the morphologic evidence for erosion and burial, Titan’s craters are consistently
shallower than similarly sized fresh craters on Ganymede (often by many hundreds of meters),
suggestive of infill (Neish et al., 2013). Given the distribution of depths, aeolian infilling appears
to be the dominant modification process on Titan (Neish et al., 2013), but fluvial erosion seems
to play an important secondary role (Neish et al., 2016). Modification by viscous relaxation is
expected to be minimal given the cold lithospheric temperatures on Titan, although insulation by
sand could enable some relaxation in Titan’s larger craters (Schurmeier and Dombard, 2017).
In addition to being highly modified, Titan’s impact craters are not uniformly distributed
across the moon. There is an almost complete absence of impact craters near Titan’s poles, with
the majority of the craters found in the topographically high, equatorial sand seas (Neish and
Lorenz, 2014). There have been several hypotheses advanced to explain this observation. Neish
and Lorenz (2014) proposed the lack of craters near Titan’s poles might be indicative of marine
impacts into a former ocean in this region. Moore et al. (2014) suggested that extreme climate
change occurred in Titan’s recent past, causing global methane rainfall that produced sediment
that settled in Titan’s topographically low polar regions, burying any craters there. Finally, Neish
et al. (2016) suggested that an increased rate of fluvial erosion near the poles could degrade
Titan’s craters to the point where they would be unrecognizable from orbit. In any case, Titan's
cratering record demonstrates that it is an extremely dynamic world, and studying its impact
structures can reveal much about the processes that have shaped it.
Figure 10. Nine probable impact craters observed on Titan by Cassini RADAR, from smallest (a)
to largest (i). Impact craters on Titan show modification by exogenic processes such as fluvial
erosion and infilling by sand. Figure from Neish et al. (2013).
5.5. Mountains and tectonics
Features of relatively high topography, termed mountains, have been observed across Titan
(Fig. 11). Topography on icy satellites is rare, taking the form of, for example, impact crater rims,
grooved terrain on Ganymede, or the towering scarps of Miranda. This is in part because water
ice generally loses strength with increasing depth, making high topographies difficult to support.
However, the exceptional amount of erosion on Titan may also be responsible for terrain height
reduction. Overall, topography on Titan is rather subdued, having a range of just a few
kilometers (Lorenz et al., 2013; Corlies et al., 2017); nevertheless, there are features on the
surface aside from impact crater rims that are elevated. These take the form of isolated blocks,
chains, ridges, and elevated plateaus (Radebaugh et al., 2007). Mountains of all types are SAR
bright due to the roughness and fractured nature of the materials, as well as the slope geometry
with respect to the SAR antenna look direction and incidence angle. Mountains also exhibit high
scattering and have a high emissivity as seen by RADAR (Janssen et al., 2009; 2016), and are
part of the VIMS bright blue unit (Barnes et al., 2007), indicating a higher water-ice component,
the dominant material of Titan’s lithosphere.
Isolated mountain blocks are found only in a few select regions and tend to be several km
across and a few hundred meters high, as observed by radarclinometry (Radebaugh et al., 2007).
More frequently, mountains are found in small belts near the middle and high latitudes, and are
only few to tens of kilometers in length, and also a few hundred meters high. Mountain chains
are the most a really extensive rugged features (aside from Xanadu) and are the most dramatic on
the surface, being found dominantly at the equatorial regions and generally aligned E-W. They
are up to several hundred kilometers in length, are arcuate in planform, and can be up to several
kilometers high (Radebaugh et al., 2007; Radebaugh et al., 2016). In Xanadu, there are extensive,
mountainous and rugged terrains. These appear crenulated to SAR and exhibit multiple
overlapping peaks concentrated in regions over tens to hundreds of square kilometers. They are
interspersed with lineations that indicate a regional tectonic fabric, likely extensional, given the
straight nature of the lineations (Radebaugh et al., 2011). All mountains on Titan are highly
rugged and exhibit signs of extensive erosion from methane rainfall and possibly mass wasting.
All mountain ridges and chains exhibit a preferential orientation (Cook-Hallett et al., 2015; Liu
et al., 2016a) indicating internal tectonic forces operating on their formation. The equatorial
mountain chains differ in morphology from long, narrow tectonic features on other icy satellites:
they sit on elevated topography (Radebaugh et al., 2016), they are arcuate in morphology, and
they have low slopes (Liu et al. 2016b). This indicates they are more likely to have been created
by contractional tectonism, by N-S directed forces in the interior and at the equator. Such thrust
faulting could have been enabled by liquid methane within the crust, which could act to lubricate
fault zones, much as occurs on Earth with water (Liu et al., 2016b). A similar conclusion is
reached for the mountain ridges north of Xanadu, which contain the highest peak on Titan at just
over 3300 m (Radebaugh et al., 2016), that they were formed by contractional tectonism (Mitri et
al., 2010). Some very large-scale tectonic rises, several hundred kilometers across, may be
topographic rises related to laccolithic activity (Schurmeier et al., 2018).
Figure 11. Some examples of mountains on Titan. SAR illumination is from the top in all cases,
and north is up. Bright, possibly erosional, blankets surround the central, high elevation
features visible as bright/dark pairs. (a). Sinlap Crater, shown to demonstrate the effects of SAR
illumination on a rimmed depression. (b). Individual mountain blocks in the T3 swath south of
Sinlap crater. (c). Rugged mountain from the T3 swath. (d). A portion of mountain ranges west of
the Huygens landing site from the T8 swath. From Radebaugh et al. (2007).
5.6. Cryovolcanism
The possibility of finding cryovolcanic features on Titan had been discussed prior to Cassini
(e.g. Lorenz, 1993; 1996). The case for cryovolcanism was strengthened by results from the Gas
Chromatograph Mass Spectrometer (GCMS) instrument on board the Huygens probe, which
detected the radiogenic isotope of Argon 40 in Titan's atmosphere (Niemann et al., 2005) in
concentrations suggesting that the atmosphere was in communication with a reservoir of the
parent atom. Prior to the first Titan flyby using RADAR, VIMS imaged a bright feature (later
named Tortola Facula) that Sotin et al. (2005) proposed to be cryovolcanic in origin. However,
SAR images obtained later in the mission showed Tortola Facula to be a local topographic high
similar to others elsewhere on Titan (Lopes et al., 2013) and not a candidate for a cryovolcanic
feature. Sotin et al (2005) further suggested that the upwelling of large cryovolcanic plumes
might be releasing sufficient methane into the atmosphere to account for the known atmospheric
composition.
Cassini RADAR and VIMS revealed several features interpreted as formed by
cryovolcanism (Barnes et al., 2006; Sotin et al., 2005; Lopes et al., 2007; Soderblom et al., 2009;
Wall et al., 2009; Lopes et al., 2013; Solomonidou et al., 2016). However, the interpretation has
been the subject of some debate (e.g., Moore and Pappalardo, 2011) and has not been entirely
resolved by Cassini data (Nixon et al., 2018), primarily due to limitations in spatial resolution
and coverage. Cryovolcanic interpretations by RADAR (using several data sets including SAR,
stereogrammetry (Kirk et. al., 2010), SARTopo, and radiometry) and VIMS (surface albedo
retrievals, surface composition constraints, and temporal variations) are based on morphology;
differences in surface albedo between the cryovolcanic areas, their surrounding terrains, and
several other geomorphological features; and temporal variations detected by VIMS (Barnes et
al., 2005, Solomonidou et al., 2014; 2016; 2018). However, the Cassini mission did not reveal
any hot spots (thermal enhancements) that would have provided conclusive proof. The detection
of thermal activity at Titan’s surface using radiometry data (which is sensitive to variations of
~1K) or VIMS, would require Cassini’s instruments to be observing the right locations at the
right times and in multiple occasions, an unlikely scenario given the consensus that cryovolcanic
candidate features are not ubiquitous on Titan (Lopes et al., 2010).
Landforms considered as possibly cryovolcanic include flow-like terrains seen on the
western margin of Xanadu (Wall et al., 2010), spectrally different regions in Tui Regio (Barnes
et al. 2005) and tangled flow regions in Hotei Regio (Soderblom et al., 2009; Wall et al., 2010).
These flow-like morphologies even exhibit elevated, lobate margins typical of flows. Other
possible cryvolcanic features are the steep-sided, small lakes at the north polar region (see
section 5.9). These landforms have slightly elevated rims, steep sides, flat floors, and deposits
diffuse to SAR surrounding them. All of these characteristics are typical of maar craters on Earth
(Wood et al., 2007) though they are also generally consistent with dissolution and sublimation-
related features (Hayes et al., 2017).
The strongest evidence for cryovolcanic features on Titan was put forward by Lopes et al.
(2013) who combined SAR imaging (including stereogrammetry, Fig. 12) and VIMS data (Fig.
13) for a region that includes two mountains, Doom Mons (40.4oW, 14.7oS) and Erebor Mons
(36.2oW, 5.0oS), as well as a depression, Sotra Patera (40.0oW, 14.5oS), and a region consisting
of flow-like features, Mohini Fluctus (centered at 38.5oW, 11.8oS). Doom and Erebor Montes are
tall mountains (Doom being ~70 km in diameter and 1.5± 0.2 km high, Fig. 5.6.1), Sotra Patera
is the deepest depression found on Titan (1.7 ± 0.2 km deep, relative to surrounding terrain). It is
non-circular and interpreted as a collapse feature adjacent to Doom Mons (Fig. 13). Mohini
Fluctus appears to emerge from Doom Mons. Other non-circular, collapsed depressions are
located between the two Montes, and flow-like features also surround Erebor Mons.
A criticism of Moore and Pappalardo (2011) of initial interpretations by RADAR of
cryovolcanic candidates reported by Lopes et al. (2007) is that flow-like features could have
been produced by fluvial activity, since channels are seen in areas such as Hotei Regio and
Ganesa Macula (which topography later obtained by RADAR showed was not a shield or dome
as initially interpreted). However, the Doom Mons – Sotra Patera – Erebor Mons region is totally
devoid of visible fluvial channels, making a fluvial origin for Mohini Fluctus and other flows
unlikely. A vast dune field is located between Doom and Erebor Montes, indicating a dry region.
The depressions seen in the region, including Sotra Patera, are not circular, are very deep, and
are therefore unlikely to have had an impact origin (Lopes et al. 2013). VIMS data analysis has
contributed to the cryovolcanic interpretation via two different types of investigation. First,
analysis of VIMS data using a radiative transfer model (Solomonidou et al., 2014) showed that
the surface albedo of the candidate cryovolcanic features is different from that of plains or dunes,
indicating differences in composition (Solomonidou et al., 2014). Following this and using again
a radiative transfer model on a large selection of VIMS data, Solomonidou et al. (2016) revealed
temporal changes for the Sotra Patera and Mohini Fluctus area, which became brighter up to a
factor of 2 in terms of pure surface albedo and brightness during one year (2005-2006), while
surrounding areas and the undifferentiated plains and dunes did not present any significant
change for the same period of time. The surface albedo variations, together with the presence of
volcanic-like morphological features consistent with volcanism, suggest that the regions might
be active and possibly connected to the deep interior via cryovolcanic processes. Additional
support for cryovolcanic origin of these features comes from interior structure models of Titan
and corresponding calculations of the spatial pattern of maximum tidal stresses (Sohl et al.,
2014), which indicate that the Doom Mons – Sotra Patera Erebor Mons area is a likely region
for cryovolcanic activity.
Figure 12. Topography of Doom Mons, Sotra Patera and (at the top) Erebor Mons region from
SAR stereo. The image on the left is SAR over the region. The central image shows a color-coded
DTM (scale shown), with the tops of Doom and Erebor Montes being the highest points, the
white lines show the locations of the three profiles shown. The SAR and DTM are merged at the
right. (From Lopes et al., 2013).
Figure 13. Perspective view of Doom Mons and Sotra Patera, obtained by combining a Digital
Topographic Model (produced from two SAR swaths) and VIMS data showing compositional
differences in representative color, which shows that the dune fields (in blue) are of a different
composition from the candidate cryovolcanic materials in shades of green and yellow. The image
shows one of the tallest peaks on Titan, Doom Mons, which is ~70 km in diameter and 1.5 +/-
0.2 km high. Doom Mons is adjacent to the deepest depression so far found on Titan, Sotra
Patera, an elongated pit ~30 km in diameter and 1.7 ±0.2 km deep. The DTM data has a vertical
exaggeration of 10:1. A movie showing the whole region can be seen at:
http://photojournal.jpl.nasa.gov/catalog/PIA13695
5.7. Labyrinth
The enigmatic labyrinthic terrains of Titan are defined as elevated, highly dissected plateaus
with intersecting valleys or remnant ridges of low to medium backscatter with a generally >5000
km2 extent (Malaska et al., 2010; Moore et al., 2014; Malaska, et al. 2016b; Malaska et al.,
2017a). The Katain Labyrinth (Fig. 14) is a typical example. SARTopo and suggests that
labyrinth terrains are among the locally highest units on Titan (Stiles et al., 2009). Often the
valley floors contain SAR-dark floors or fill. The valley and upland widths are variable: in areas
where the valleys are narrow and the intervening uplands (or valley spacing) are wide, the terrain
appears in the form of a dissected plateau; when the widths of the valleys and the widths of the
intervening plateaus are about equal, the terrain appears as a series of valleys and intervening
plateaus; and when the valleys are wide and the widths of the intervening plateaus are small, the
terrain can appear as a series of remnant ridges. Closed valleys may also be present, although this
may be a result of the coarse resolution of the Cassini SAR.
Figure 14. SAR image of Katain Labyrinth. (52 N, 349 W) The highly dissected feature at center
is Katain labyrinth. SAR illumination is from right (large straight lines through center are SAR
artifact due to beam overlap). Sinusoidal projection centered at 349W. North is at top.
Wide valleys may appear suddenly in the plateaus, suggestive of amphitheater-headed
valleys. The overall planforms of the labyrinth terrain units are circular, ovoid, or tabular. The
valley density at a scale of 300 m/pixel is above 0.02, significantly higher than the density of
valley networks described elsewhere on Titan such as the network in western Xanadu (Burr et al.,
2009). The valley networks inside the labyrinth units are rectangular to dendritic, suggesting
varying amounts of structural and topographic control. Some of the more circular planform
labyrinths have valley or ridge networks that are radial, extending away from the center of the
region, suggesting that doming occurred prior to erosion, possibly as a result of a liquid water
laccolith injection at depth (Shurmeier et al., 2017; 2018). At the terminus of the valley networks
and in contact with the labryinths, undifferentiated plains units are found, suggesting a close
connection between the two types of units, at least at the local scale.
The labyrinth terrains are composed almost wholly of a possibly uplifted thick plateau of
organic materials, with very little water ice materials present. The emissivity data shows that
labyrinth terrains have significantly higher emissivity than mountain and hummocky terrains and
have similar emissivities to dunes or undifferentiated plains. The microwave data are consistent
with the labyrinths being composed of low dielectric organic materials and are not consistent
with materials containing significant amounts of water ice.
The labyrinths were likely formed from the uplifted plateaus through a combination of
dissolution coupled with mechanical erosion, or other phase changes that could allow the
development of closed valleys with transport of the remaining materials to the outlying plains.
Karstic dissolution is a likely scenario, and many of the labyrinth terrains have morphologic
analogs with terrestrial karst terrains (Malaska et al., 2010). Theoretical predictions and
laboratory work have shown that organic materials on Titan may dissolve when exposed to Titan
hydrocarbon rainfall or liquids. (Raulin, 1987; Lorenz and Lunine, 1996; Cordier et al., 2009;
Malaska et al., 2011a; Glein and Shock, 2013; Malaska and Hodyss 2014; Cornet et al., 2015;
Cornet et al., 2017). Dissolution geology on Titan may create a landscape that is similar to
terrestrial karst terrain present in water-soluble materials such as limestone and gypsum on Earth
(Malaska et al., 2011a, Malaska and Hodyss, 2014; Cornet et al., 2015). Preliminary modelling
by Cornet et al. (2017) suggests that blocks of Titan soluble materials could dissolve under Titan
conditions to form the features observed by SAR that are similar to polygonal karst. The
evolution sequence begins with incised valleys in a plateau, then widening to form the end-stage
wide-floored remnant ridges. Type examples of each stage are found in close proximity near
Sikun Labyrinth in Titan’s south polar terrain and are shown in Fig. 15. However, other
formation scenarios are possible, including differential hardening and deflation, aeolian deflation,
or other phase change and removal processes such as sublimation. From superposition relations,
the organic labyrinths represent an ancient terrain (Malaska et al., 2016b). Thus, the labyrinth
terrains unit represents a significant deposit of organic materials on Titan, which suggests that
significant organic deposition, lithification, and uplift/exposure occurred early in Titan’s history.
Figure 15. SAR images of different types of labyrinth terrains found in close proximity in Titan’s
south polar terrain. Left: Naraj Labyrinth, represents thin valleys incised in a plateau, Center:
Sikun Labyrinth, represents valleys and plateaus of near equal width, Right: Tupile Labyrinth, a
type example of remnant ridge. Sinusoidal projection. North is at top.
5.8. Xanadu
Much of Titan’s geology has a regional organization; dunes and mountain belts are found
near the equator, lakes and seas are found near the poles, and relatively bland regions are located
in the midlatitudes (Lopes et al. 2010). Xanadu (Fig. 16), however, is a continent-sized region
that breaks with all predictions. It is 4000 x 2500 km wide, is located on Titan’s leading
hemisphere, and interrupts the equator-encircling sand seas. The feature was observed before
Cassini arrived (Lemmon et al., 1993; Smith et al., 1996), and efforts were made to observe and
understand Xanadu by Cassini SAR and other instruments. Xanadu is generally SAR-bright,
which indicates it is composed of rough and fractured terrains (Radebaugh et al., 2011). It has a
high backscatter but low brightness temperature as indicated in scatterometry and radiometry
measurements, which is consistent with a water-ice composition in the bedrock (Wye et al. 2007;
Zebker et al. 2008; Janssen et al. 2009; 2016). Furthermore, some portions of Xanadu are
correlated with the VIMS dark blue unit, indicating there is a higher than average percentage of
water-ice exposed at the surface (Barnes et al., 2007).
Xanadu is unique in several respects to other regions on Titan. While geologically diverse,
many regions in Xanadu have extremely rugged terrains, manifest as many adjacent, deeply
eroded, and incised mountain ranges (Radebaugh et al., 2011). There are broad-scale linear
features characteristic of NE-SW and NW-SE extensional tectonism, and broadly arcuate
mountain ranges indicative of N-S directed contractional tectonism (Radebaugh et al., 2011).
These features all indicate a long and complicated tectonic history for Xanadu. Extensive,
dendritic networks of varying morphologies (Burr et al., 2009), large channels that distribute fans
and cobbles to the south (Le Gall et al., 2010), and the extensive erosion of the mountains also
reveal a long erosional history. Over twice as many impact craters or possible impact craters can
be found in Xanadu as on the rest of Titan (Wood et al. 2010; Radebaugh et al., 2011), which is
further evidence of the generally old nature of Xanadu compared with the rest of the surface of
Titan (Radebaugh et al., 2011; Wood et al., 2010). A region of mottled terrain is found on the
western margin, with arcuate depositional morphologies and lack of integrated drainage,
postulated to be a possible cryovolcanic deposit (Wall et al., 2009). These morphologies are
consistent with other possible cryovolcanic zones in Hotei Regio, bordering Xanadu’s southern
margin (Soderblom et al., 2009). These landforms may instead be related to swamp-like deposits
entirely fluvial in origin, but they are unique and not widespread.
Xanadu’s most puzzling characteristic is that despite the abundance of mountains and high
local topographic differences, the region as a whole appears to be regionally lower in elevation
than anywhere else near the equator (Zebker et al., 2009; Stiles et al., 2009). This is evidenced by
radar altimetry measurements as well as SARTopo observations (Stiles et al., 2009). It is
possible that after a time period of mountain-building and contraction, there was gravitational
collapse of the water ice crust (Mitri et al., 2010), resulting in broad extensional tectonism and
down-dropping of Xanadu (Radebaugh et al., 2011). This scenario might have led to Xanadu-
bounding fault zones, along which the possible cryolavas of western Xanadu and Hotei in the
south could have ascended (Radebaugh et al., 2011). What could have driven this N-S directed
contractional tectonism is unknown; but it might have resulted from interior cooling-related
global contraction related to interior cooling, tidal spinup or spindown, or internal convection
(Radebaugh et al., 2011). Another scenario hypothesized for the formation of Xanadu involves a
large impact event early in Titan’s history, leaving behind a disrupted terrain (Brown et al.,
2011).
Several outstanding questions about Xanadu remain, in addition to its origin: Why is it
regionally depressed? Why don’t the dunes of the equatorial sand seas cover and fill Xanadu?
Why is water ice more exposed here than in most other places on Titan? Much remains to be
learned about this unusual region.
Figure 16. Xanadu region. All Cassini SAR and HiSAR image swaths covering the Xanadu
region, overlain on a VIMS basemap (from Le Mouelic et al., 2012). Xanadu is generally SAR-
bright because of rough terrain and fractured ice, which is interspersed with valleys filled with
SAR-darker sediment. Some regions of Xanadu are VIMS dark blue, consistent with the presence
of exposed water ice. The southwest margin of Xanadu has large river channels emptying to the
south.
5.9. Lakes
While ISS revealed ~50 dark features poleward of 70°S during one of Cassini’s first
observations of Titan, these features were not initially referred to as lakes as they could not be
distinguished from dark equatorial dune fields at optical wavelengths (Porco et al., 2004). The
first high-resolution and definitive observation of Titan’s hydrocarbon lakes were acquired by
the RADAR in July 2016 (T16), when ~75 features with exceptionally low backscatter, high
emissivity, and distinctly lacustrine morphology were identified in SAR images (Stofan et al.,
2007). Subsequent observations have revealed more than 650 such features, both dry and filled,
scattered throughout Titan’s polar terrain (Hayes et al., 2008; Birch et al., 2017). These features
have diameters that follow a log-normal distribution with a median of 77 ± 20 km (Hayes et al.,
2016). The morphology of both dry and filled lakes/seas on Titan provide a record of past and
current climatic conditions and surface evolution processes. For example, while Titan’s large
seas have complex shorelines that are consistent with drowning pre-existing topography, most of
the smaller lakes are steep-sided depressions that are more consistent with dissolution-based
erosion driven by karstic processes (Hayes et al., 2017; Cartwright et al., 2011; Langhans et al.,
2012). For a recent and more detailed review of Cassini’s exploration of Titan’s lakes and seas,
see Hayes et al. (2016).
Figure 17. Map of lakes and seas on the northern (top) and southern (bottom) polar regions of
Titan.
Lakes and seas encompass 1% of Titan’s total surface area (Hayes, 2016). The majority
of surface liquids (97% by area) reside in the north polar region, with 80% of all liquid-filled
surface area contained in three large seas; Kraken Mare, Ligeia Mare, and Punga Mare (Figure
17). The largest modern liquid body in the south polar region is Ontario Lacus, although several
large empty basins that encompass an area similar to the northern Maria (7.6 ´ 105 km2) have
been identified and interpreted as paleoseas (Hayes et al., 2010; 2011; Birch et al., 2018). The
observed dichotomy in lake distribution has been attributed to a net transport of ~5x1014 kg of
methane per Titan year from the south pole to the north, driven by a seasonal asymmetry in solar
insolation that is the result of Saturn’s eccentric orbit around the sun (Aharonson et al., 2009;
Lora & Mitchell, 2015). Summer solstice in Titan’s southern hemisphere occurs near perihelion
while northern summer solstice occurs at aphelion, resulting in 25% higher peak insolation
during southern summer as compared to northern summer. Long timescale (~100,000 yr) orbital
cycles can switch the direction and magnitude of the seasonal asymmetry driving this transport,
moving liquid deposits between the poles similar to the way Croll-Milankovitch cycles drive ice
ages and other long-term climate effects on Earth (Aharonson et al., 2009). The presence of
drowned river valleys at the terminus of channels flowing into the northern seas (Stofan et al.,
2006), as well as the presence of exposed and abandoned river deltas adjacent to the shores of
southern Ontario Lacus (Wall et al., 2009) support the theory of rising and falling liquid levels as
the magnitude and direction of net pole-to-pole methane transport varies with Titan’s orbital
cycles (Lora et al., 2014).
While a few small lakes have been observed to disappear or brighten in both the north and
south over the thirteen years of Cassini observations, no large-scale changes in the sea shorelines
have been observed over the course of the mission. Given the resolution of the RADAR,
however, this is not surprising. Although confirmed and stable liquid deposits are currently
restricted to polar terrain, the equatorial features Hotei and Tui Regiones have been interpreted
as possible low-latitude paleoseas (Moore and Howard, 2011). Both regions are surrounded by
fluvial networks that appear to converge on a field of radar-bright, lobate, depressions that are
morphologically similar to high-latitude lakes (Moore and Howard, 2011). Dark flow-like
features identified adjacent to the SAR-bright depressions have been interpreted as cryovolcanic
deposits (Barnes et al., 2006; Wall et al., 2009), suggesting that both paleo-lakes and
cryovolcanic flows may be present at Tui Regio and Hotei Regio (Lopes et al., 2013). The
existence of modern equatorial lakes has been proposed based on the longevity of low albedo
localities observed by Cassini VIMS (Griffith et al., 2012; Vixie et al., 2015), although none of
these features have been observed in higher-resolution SAR or altimetry datasets of the regions.
Ample evidence exists (e.g. at the Huygens landing site) for at least transient liquids at low
latitudes, and very flat areas exist which may be lake beds. Indeed, strong specular reflections
were observed at low latitude by the Arecibo radar on Earth (the longest-range radar astronomy
experiment conducted to date) before Cassini's arrival, and the favored interpretation at the time
was as extant bodies of smooth liquid hydrocarbons (Campbell et al., 2003).
In May 2014, RADAR acquired nadir-pointed altimetry over the Ligeia Mare. The resulting
altimetry echoes revealed waveforms that displayed two distinct returns, one from the surface of
Ligeia Mare and one from its seabed (Mastrogiuseppe et al., 2014). The difference in the
received timing between these returns was direct measure of Ligeia’s depth (Fig. 18), while the
relative intensity difference between the surface and subsurface return was a measurement of the
liquid’s loss-tangent (i.e., absorbance). While several studies (e.g., Brown et al., 2008; Lorenz et
al., 2008a, Lunine and Lorenz, 2009; Cordier et al., 2012, Cornet et al., 2012, Ventura et al.,
2012) have used indirect measurements to constrain the depth of Titan’s lakes and seas, the
altimetry observations over Ligeia represent the first direct measurement of extraterrestrial
bathymetry profiles. Following the identification of Ligeia Mare’s seabed, several passes of
Titan were modified to repeat the experiment over Punga Mare and Kraken Mare
(Mastrogiuseppe et al., 2016). A reprocessing of altimetry data acquired over Ontario Lacus in
December 2008 allowed the technique to be applied to Ontario Lacus as well (Mastrogiuseppe et
al., 2017). Ligeia Mare was determined to have a depth of 170 m at the deepest point along the
observed track (Mastrogiuseppe et al., 2014) and in combination with SAR images, the total
volume of the basins was found to be around ~14,000 km3 of liquid (Hayes et al., 2016). The
estimated Ku-band loss tangent was 4.4±1x10-5 (Mastrogiuseppe et al., 2016). Assuming a
methane-ethane-nitrogen composition and using the laboratory measurements of Mitchell et al.
(2015) with the Lorentz-Lorenz formulation, the best-fit loss tangent is consistent with a
methane-dominated composition of 71% CH4: 12% C2H6: 17% N2. As large quantities of liquid
ethane should have been produced by photolysis of methane in the upper atmosphere, and at least
trace amounts of ethane have been detected in Ontario Lacus (Brown et al., 2008), the lack of
significant ethane (and other higher order hydrocarbons such as propane) in Titan’s lakes and
seas requires that the ethane is sequestered in reservoirs (e.g., subsurface liquid deposits or
sequestration in crustal clathrate hydrate).
To within error, the loss tangents of Punga Mare and the shallower parts of Kraken Mare
(Fig. 18) suggested a similar composition to Ligeia (Mastrogiuseppe et al., 2018). Along most of
Kraken Mare the seafloor was not detected, indication that the seas are either too deep or too
absorptive in these areas. Within Punga Mare, a clear detection of the subsurface was observed
up to 120 m along-track (Mastrogiuseppe et al., 2018). At Ontario Lacus, however, the loss
tangent was observed to be greater (7±3 x 10-5), consistent with a composition of ~47% CH4,
~40% C2H6, and ~13% N2, suggesting an increased abundance of high-order hydrocarbons as
compared to the northern seas. This higher loss tangent could be related to an increased
abundance of more involatile hydrocarbons and/or nitriles or suspended particulates that
represent a lag deposit generated as methane is transported from the south to the north over
multiple seasonal cycles. The final radar observation of the bathymetry campaign was the T126
(22 April 2017) final fly-by at Titan. The Cassini RADAR observed several small - medium size
(10 - 50 km) hydrocarbon lakes present at the northern polar terrain, revealing that such lakes
can exceed one hundred meters of depth and have similar loss tangents, and therefore
composition, to the northern seas. When the bathymetry measurements are used to anchor
models of sea and lake depth from SAR images, the estimated volume of all Titan’s observed
lakes and seas is ~70,000 km3 (Hayes, 2016). It is interesting to note that this represents only 1/7
the amount of methane that currently resided in Titan’s atmosphere, suggesting that the lakes and
seas do not drive global-scale heat transport or meteorology. It is also worthwhile mentioning
that measuring the bathymetry and microwave absorptivity of Titan’s seas was not planned
initially and represents an exciting discovery made during the Cassini spacecraft lifetime.
Fig. 18: SAR mosaics, radargrams and relative bathymetries (respectively in the upper, middle
and bottom panels) relative to the flyby T91 over Ligeia Mare (left), the flyby T104 over Kraken
Mare (center), and the flyby T108 over Punga Mare (right) altimetry observations. Note that
seafloor echoes have been detected for Ligeia and Punga maria, while only surface returns are
present over the open sea of Kraken mare.
5.10. Rivers/channels
When water falls to the surface of the Earth, the most visible conduits on its journey
downslope are networks of fluvial channels that slowly transport it toward the oceans. These
networks take on many different forms that are the result of the mechanical and chemical
properties of the surface (Burr et al., 2006), the climate and weather that generate fluid flow, and
the mechanisms that produce topographic relief (Black et al., 2017). The observation of channels
on Titan by Cassini and Huygens (Collins, 2005; Lorenz et al., 2008c; Lunine et al., 2008; Burr
et al., 2009; Black et al., 2012; Burr et al., 2013a) has thus provided similar constraints on the
nature of Titan’s surface and the climatic conditions that proved favorable for the formation of
these features, albeit in a far more limited fashion.
Cassini’s RADAR imaged large portions of the moon and showed that valley networks are
distributed at all latitudes (Lorenz et al., 2008c; Burr et al., 2009; Lopes et al., 2010) and have a
wide variety of surface morphologies (Figure 19). With a diversity of valley networks analogous
to the Earth, Cassini has observed canyon networks at the poles (Poggiali et al., 2016), both
dendritic and rectilinear networks globally (Burr et al., 2013b), and even a meandering-like
feature in the south polar region (Malaska et al., 2011b; Birch et al., 2018). The presence of
canyons implies a vertically weak bedrock, which may be influenced by fractures in the crust
and/or a relatively highly erodible material. Similarly, rectilinear channels imply a fractured
bedrock, where channels are forced to follow tectonically-controlled paths of weakness.
Meandering networks, meanwhile, imply the presence of a cohesive substrate (Howard et al.,
2009). A critical unknown following the Cassini mission, however, is whether there are any
systematic variations in morphologic type that may be indicative of crustal heterogeneities (Burr
et al., 2013) and/or variations in transport efficiencies/climate change (Moore et al., 2014).
Due to the coarse resolution of the Cassini RADAR, we have been limited to studying only
the largest valley networks on Titan. We therefore have a limited idea about the extent to which
Titan’s landscapes are dissected by fluvial networks. The one exception to this was the region
where the Huygens lander descended, where descent images, with an order-of-magnitude higher
resolution (Tomasko et al., 2005), showed a highly-dissected network of dendritic valleys
(Perron et al., 2006). It is likely that Titan is dissected everywhere at the scale observed by
Huygens, however, a definitive answer to this question requires image data and topography with
a resolution finer than the scale of fluvial dissection (10s of meters).
The mere presence of channelized flow conduits implies that the surface material can be
eroded either physically or chemically, and that flows of sufficient magnitude, either from
precipitation or groundwater, are able to erode Titan’s surface (Collins 2005). However, using
estimates for the initial topography and erodibility of the substrate, channels may be very
inefficient agents of erosion on Titan (Black et al., 2012) or there may be a gravel lag deposit
that inhibits erosion under Titan’s current climate (Howard et al. 2016). Better estimates for the
physical and chemical properties of both the bedrock and the fluid(s) (Burr et al., 2006; Cordier
et al., 2017; Malaska et al., 2017b; Richardson et al., 2018) are needed to provide better
understanding about the role that fluvial channels have played in sculpting Titan’s surface.
Figure 19. (a) Rectilinear networks in eastern Xanadu; (b) Celadon Flumina, a meandering
network near the south pole; (c) Elvigar Flumina, a braided network that deposits into an
alluvial fan; (d) Vid Flumina, a dendritic canyon network up to 500 m deep that drains into
Ligeia Mare.
In some locations, fluvial channels terminate in distributary channel patterns suggestive of
the transition from erosion-dominated channel systems to flat surfaces, named in studies as
alluvial or fluvial fans (Burr et al. 2013a; Radebaugh et al. 2018; Birch et al. 2017). These are
fairly low in slope, and in some cases can run out to large distances, indicating the carrying
power by methane fluid of organic sedimentary rock (Radebaugh et al., 2017). These landforms
are widely distributed across the surface, but they are not abundant (Birch et al., 2017). This may
indicate there is not frequent rainfall that can generate surface erosion, or that topographic
gradients are gentle on a global scale such that these landforms are not readily generated.
6. TEMPORAL CHANGE
Temporal changes were detected on Titan during the course of the Cassini mission, due to
seasonal or other effects. Data from more than one instrument is key for determining the possible
causes of change. For example, as mentioned in section 5.6, the analysis of VIMS data from Tui
Regio (2005-2009) and Sotra Patera (2005-2006) using radiative transfer showed temporal
surface albedo changes in two areas identified by SAR as cryovolcanic candidates: Tui Regio
darkened by 50% and Sotra Patera brightened by a factor of 2 (Solomonidou et al., 2016). These
changes could be due to endogenic and/or exogenic processes, possibly cryovolcanism or
atmospheric deposition.
6.1.“Magic islands”
For the majority of the Cassini Mission, Titan’s lakes and seas were observed to be
quiescent, with no temporal changes and maximum vertical surface roughness on the order of
millimeters (Barnes et al., 2011; Stephan et al., 2010; Wye et al., 2009; Zebker et al., 2014;
Grima et al., 2017). This lack of observable surface roughness has been attributed to a seasonal
effect in which polar winds were too weak to create waves or other dynamic features (Hayes et
al., 2013). As the northern hemisphere transitioned from spring equinox to summer solstice,
temporal changes were observed in all three of Titan’s seas. Specular reflections offset from the
geometric specular point were observed by VIMS in Punga Mare (Barnes et al., 2014), transient
bright features were observed by RADAR in Ligeia Mare (Hofgartner et al., 2014; 2016) and
both offset specular reflections and transient SAR-bright features were observed in Kraken Mare
(Hayes et al., 2016).
The transient bright features were nicknamed “Magic Islands” due to their
appearing/disappearing action and similarity in appearance to islands in SAR images. The
features are not islands, however, because they are transient, but are consistent with waves, or
floating and/or suspended solids and bubbles. Based on the frequency of these phenomena in
analogous terrestrial settings, wind-driven waves (intended to mean roughness of the liquid
surface regardless of the process causing the roughness) are the most probable hypothesis. Tides,
changes of sea level and changes of the seafloor are unlikely to be the primary cause of the
temporal changes. Magic Islands were observed in three regions; two in Ligeia Mare
(Hofgartner et al., 2014, 2016) and one in Kraken Mare (Hayes et al., 2016).
Figure 20 shows the time evolution of the first and most observed Magic Island region. This
region was observed to have Magic Islands on two occasions; the transient bright features were
in the same location on both occasions but differed in areal extent and morphology. HiSAR and
VIMS observations acquired between the two SAR detections did not detect Magic Islands,
however, the possibility that the Magic Islands were present but not detected in these
observations could not be ruled out. Magic Islands were definitely not present in SAR
observations before the first appearance and after the second appearance (Figure 20 only
includes a subset of the images of the region).
The Kraken Magic Island was also observed as a five-micron sunglint by VIMS within two
hours after the SAR detection. The co-detection of the Kraken Magic Island by both RADAR
and VIMS suggests that it is likely caused by surface waves, as the reflecting facets must be
smooth at both microwave and micron length scales (Hayes, 2016).
Figure 20. The large panel on the right is a SAR mosaic of Titan’s hydrocarbon sea, Ligeia Mare.
The panels on the left show the temporal variation of a region observed to have Magic Islands.
Transient bright features (Magic Islands) are observed in the images from July 10, 2013 and
August 21, 2014 that are not present in any other images of this region. The VIMS observation
on July 26, 2013 (panel d) did not detect Magic Islands, however, the possibility that they were
present but not detected in these observations could not be ruled out. Magic Islands were not
present in SAR observations before the first appearance and after the second appearance. The
figure shows only a subset of Cassini observations of the region, see Hofgartner et al. (2016) for
all observations with a resolution sufficient to observe the Magic Islands. Figure from Hayes
(2016).
6.2. Arrakis Planitia precipitation and other transient events
The first Cassini observations of surface change on Titan were obtained over Arrakis
Planitia, near the south pole, where ISS observed the appearance of dark splotches (interpreted as
ponded hydrocarbon liquid) in June 2005 that were not present in the previous observation
acquired in July 2004 (Turtle et al., 2009). In October 2004, between those two observations, a
large cloud outburst was observed near Titan’s South Pole from Earth-based telescopes (Schaller
et al., 2006). SAR images and SARTopo later found that the ISS dark splotches occurred in
topographic depressions that are morphologically similar to steep-sided depressions interpreted
as empty lakes in the north (Soderblom et al., 2016). VIMS observations of this area acquired
between 2007 and 2009 show that the dark splotches had become brighter than the surrounding
terrain (Soderblom et al., 2016). SAR images of the area obtained in October 2007 and
December 2008 showed the absence of dark splotches in the same topographic depressions, this
time in the microwave, that were interpreted as either the evaporation or infiltration of ponded
liquid (Hayes et al., 2011).
Although the Cassini mission’s exploration of Titan’s methane cycles has ended, ground-
based observations can continue to monitor Titan’s weather until future missions can map fluvial
features at a higher resolution and characterize the composition of surface material (including the
lakes and seas) through in-situ exploration.
In addition to Arrakis Planitia, temporal changes have also been observed at other locations
in the south pole as well as within the northern lakes and seas and at equatorial latitudes. The
largest observed surface chance occurred in 2010 when an equatorial area of over 500,000 km in
size was observed to darken, presumably by methane precipitation, after a chevron shaped cloud
passed over the region (Turtle et al., 2011a). SAR images of the area suggest that the darkened
region coincided with local topographic lows. The area was later observed to return to its original
albedo (Barnes et al., 2012). Whereas there have been no definitive changes observed in the
shorelines of the northern lakes and seas through April 2017 (flyby T126), there have been
several surface changes reported for lacustrine features in the south polar region. Turtle et al.
(2011b) argued for shoreline recession at Ontario Lacus between ISS images acquired June 2005
and March 2009), although the poor resolution of T51 makes quantitative measures of this
difficult. Hayes et al. (2011) found that, while inter-instrument comparisons can be dangerous,
SAR images acquired in 2009 (T57/T58) and 2010 (T65) were consistent with a receded
shoreline when compared to the June 2005 images obtained by ISS. However, Cornet et al.
(2012) argued that, to within measurement error, the data are consistent with no changes at all.
Hayes et al. (2011) also discussed repeat RADAR passes of the south acquired in 2007 and
2008/2009 that contain lacustrine features that seem to disappear between subsequent SAR
observations. The observed 10-fold increase in SAR backscatter cannot easily be explained by
geometric effects and suggests that, between the observations, liquid either infiltrated into the
ground, evaporated, or did both (Hayes et al., 2011). Other temporal changes, including
roughening events interpreted as wave or fluvial activity as well as Titan’s mysterious “Magic
Islands” (discussed in 6.1) have been observed within the northern seas.
Although the Cassini mission’s exploration of Titan’s methane cycles has ended, ground-
based observations can continue to monitor Titan’s weather until future missions can map fluvial
features at a higher resolution and characterize the composition of surface material (including the
lakes and seas) through in-situ exploration.
6.3. Observation of a summer lag in the North pole (by radiometry)
One of the main scientific objectives of the Cassini extended mission (2008-2017) was to
monitor the changing seasons on Titan. If any change were to occur, it should be primarily in
Titan’s arctic regions where the most important temperature variations are expected (though
limited to 2-4 K over the course of a year).
Onboard Cassini, both CIRS and the microwave radiometer had the ability to measure the
variations of surface/near-surface temperature with time (Jennings et al., 2009; Cottini et al.,
2012; Janssen et al., 2016) and both instruments observed a lag in the summer warming of the
northern polar terrains (Jennings et al., 2016; Le Gall et al., 2016). They reported a much slower
rise of temperature in late spring (2014-2015) than predicted by GCM, even assuming a very
high thermal inertia for lakes and seas (Tokano, 2005). Further, there seems to be no significant
temperature difference between the land and the seas, which suggests that the solid surface
surroundings the lakes and seas is saturated with liquid and behaves thermally like the liquids.
These surfaces may as well experience evaporative cooling, which would explain the low
measured temperatures in the north polar region, and would have important implications for the
hydrocarbon cycle on Titan.
7. SURFACE AND ATMOSPHERE INTERACTION
7.1.Winds and Temperature
In contrast to the contemporary instantaneous winds revealed by sea surface roughness (see
section 6.1), the widespread observations of dunes on Titan attest to the presence of winds that
have acted over significant periods in the past. Indeed the possibility that aeolian landforms
might shed light on Titan's climate history was recognized before Cassini's launch (e.g. Lorenz et
al., 1995). Since so few trackable cloud features have been observed on Titan, the aeolian
features in Titan's landscape have emerged as one of the principal constraints on Titan's
meteorology. Specifically, it has been estimated that the saltation threshold for the movement of
dry sediment on Titan requires surface winds of the order of 1 m/s (e.g. Greeley and Iversen,
1987; Lorenz et al., 1995; Lorenz, 2014). This estimate is based on an assumption of
interparticle cohesion not too different from terrestrial sands: some laboratory measurements
suggest they could be slightly larger (Burr et al., 2015) and it is possible or even likely that
(methane/ethane) moisture (Yu et al., 2017) and/or electrostatic charging (Lorenz, 2014;
Mendez-Harper et al., 2017) could be responsible for stronger cohesion. In any case, the
presence of dunes requires winds sometimes exceeding this threshold in the past.
The construction or reorientation time for dunes of the size (~100m tall) observed by Cassini
is substantial, of the order of 50,000 years (Ewing et al., 2015; see also Lorenz et al., 1995;
Lorenz, 2014). Thus, not only does the presence of dunes requires that the winds have been
above the saltation threshold for a substantial integrated period, but also that the dune pattern
observed today retains some memory of winds extending into the past by a substantial part or
multiple of an astronomical (Croll-Milankovich) climate cycle (see e.g. Aharonson et al. 2009;
Lora et al. 2014). In particular, Ewing et al. (2015) noted that some of Titan's dunes are
somewhat crescentic, implying a recent dominance of a northward meridional component to the
winds (see also McDonald et al. 2016).
The generally-eastwards direction of sand transport implied by the dune morphology was
noted early (Lorenz et al., 2006; Radebaugh et al., 2008; Lorenz and Radebaugh, 2009) and was
a challenge to meteorological expectations, since low-latitude, near-surface winds should have
on average an easterly (westwards) flow, much like the trade winds on Earth. Tokano (2008)
made some of the first systematic experiments with a global circulation model (GCM) to attempt
to reproduce the observed pattern by positing the influence of Xanadu as a highland or bright
region. The vexing paradox (see e.g. Lorenz and Zimbelman, 2014) was resolved by invoking
occasional westward gusts (Tokano, 2010; Charnay et al., 2015) such that even though the
average wind direction is eastwards, these typical winds are below the threshold speed and so are
not reflected in the sand transport. Thus, the landscape is shaped only by the stronger (westward)
gusts with the saltation threshold acting like a 'diode' in an electrical analogy of alternating
winds. Tokano (2010) found that a threshold of order 1 m/s was consistent with obtaining a dune
pattern similar to that observed and suggested that stronger vertical mixing in the low-latitude
troposphere during the equinox period might cause the required westward flows. This idea has
been developed somewhat further by Charnay et al. (2015) who suggested that methane
rainstorms in particular may be responsible. Significant developments in the mapping of dune
morphology and orientation to wind diversity and sand supply/mobility have taken place in the
last decade and a half, stimulated in no small part by the Cassini discovery of large linear dunes
as well as other wind-borne features (Lorenz and Radebaugh, 2009; Malaska et al., 2016a).
Detailed observations suggest that there is a divergence of material transport in the equatorial
regions, and a convergence in the mid-latitude regions around latitude 35o. This suggests a
relationship exists between two major land units on Titan: the longitudinal dunes and the
undifferentiated plains, as discussed in Section 5.2. It has been suggested (Rubin and Hesp,
2009) that 'sticky' sand may yield longitudinal features, though morphologically the large linear
dunes are identical to the large desert dunes of Earth where sands are subround and free to move,
and dissimilar in shape to potential analogues made of sticky or clay particles. Laboratory results
with organic material have shown that electrostatic charging may be significant for Titan
organics under cryogenic, dry conditions (Mendez-Harper et al., 2017). Nonetheless, the
interaction of multiple modes of dune growth may be important in decoding Titan's winds from
dunes (e.g. Lucas et al., 2014).
In contrast to the complex wind story, the overall distribution of dunes on Titan is somewhat
straightforward from the standpoint of humidity. Early Titan GCM studies (e.g. Rannou et al.,
2006; Mitchell, 2008) indicated that Titan's low latitudes should be dried out by the general
circulation, as a result of the meridional (Hadley) cells on this slowly rotating world. Thus, the
dunes form a broad equatorial belt on Titan, whereas they form belts at about 20° north or south
on the faster-rotating Earth. The size and spacing of dunes, assuming that they have been
allowed to grow to their full extent without being limited by growth time or sand supply, has
been determined (e.g. Andreotti et al., 2009) to correlate to the thickness of the atmospheric
boundary layer. Essentially, the layer caps the dune growth once the spacing is roughly equal to
the layer thickness. Lorenz et al. (2010) showed that the Huygens descent data was consistent
with a boundary layer thickness of the order of 3 km, matching the typical dune spacing on Titan.
Extensive dune spacing measurements (e.g. Le Gall et al., 2011; Savage et al., 2014) show
minimal variations with latitude, likely related to increased humidity at higher latitudes. Charnay
et al. (2012) found that a GCM with an improved boundary layer scheme reproduced the 3 km
thickness, interpreting this as a 'seasonal' boundary layer.
The full meteorological interpretation of the dune pattern revealed by Cassini's RADAR will
require a finer scale of modeling than has been performed so far, including regional topography
and albedo effects. It may be that the dune fields, by virtue of having a low thermal inertia and
albedo, cause their own 'sea breeze' effect, modifying the local winds. The role of evolving
ground moisture remains to be elucidated, although some hints of moisture effects on ground
thermal inertia have been suggested in the RADAR radiometry data (Janssen et al., 2016).
Although the radiometer in principle is an indicator of surface temperature and could be
used to independently constrain gradients with latitude etc. (e.g. Lorenz et al., 2003), in practice
the surface temperature estimates from the Cassini Composite Infrared Spectrometer (CIRS) and
the Huygens probe have been adopted as 'ground truth' and the interpretation of the microwave
radiometry has been principally in terms of the surface dielectric properties. However, future
studies might profitably examine small-scale radiometer variations and their correlation with
surface elevation – in principle the ~ 1K/km lapse rate may have a signature in surface brightness
temperature.
7.2. Methanologic cycle
Titan is the only place in the solar system, other than Earth, that is known to have an active
hydrologic cycle. Titan’s methane-based hydrologic cycle is an extreme analog to Earth’s water-
based hydrologic cycle (Lunine and Atreya 2008). Exchange processes between atmospheric,
surface, and subsurface reservoirs produce methane and ethane cloud systems, as well as
erosional and depositional landscapes that have strikingly similar forms to their terrestrial
counterparts. Over its thirteen-year exploration of the Saturn system, Cassini has revealed that
Titan’s hydrocarbon-based hydrology is driven by nested methane cycles that operate over a
range of timescales including geologic, orbital, seasonal, and that of a single convective storm
(Lunine and Atreya 2008). A fast physical (phase change) cycle drives active weather and fluvial
processes over seasonal to orbital timescales. A medium-paced chemical cycle siphons off
methane for photochemical synthesis in the upper atmosphere, depositing the products on the
surface over timescales of millions of years. A long-term geologic cycle may sporadically inject
methane into the system from Titan’s interior over the age of the solar system (Hayes et al.,
2018).
8. TITAN AS A SYSTEM
Titan is the only moon in the solar system with an atmosphere so massive that it dominates
the total volatile inventory in the surface-atmosphere system as well as providing strong radiative
forcing and an active meteorology (Lorenz et al., 2005). It also obscures the surface from view in
both the optical and infrared, which is why the Cassini RADAR has been such a crucial tool.
However, by the irreversible deposition of heavy hydrocarbons, nitriles, and other photochemical
products from methane and nitrogen, the atmosphere also obscures the underlying surface
geology to some extent. Were the current inventory of methane to condense onto the surface, it
would form a layer 5 meters thick (Mitchell and Lora, 2016), but a variety of evidence suggests
that many times that number is present in various solid and liquid deposits of organics on and
within the crust (Hayes et al., 2018).
Therefore, Titan’s geologic history is poorly constrained and in particular there is a
significant uncertainty as to what fraction of the body’s 4.5 billion years of existence is recorded
on the surface. Observations relevant to its history include:
(1) The low observed numbers of impact craters (Porco et al., 2005) yield an age of
hundreds of millions of years, not billions (Lorenz et al., 2007, Wood et al., 2010, Neish and
Lorenz, 2012).
(2) The rate of photodissociation of methane in Titan’s atmosphere implies that the current
gaseous inventory will be depleted in some tens of millions of years (Yung et al., 1984).
(3) Titan’s interior has at least partially differentiated, resulting in a rock-metal core, a high-
pressure ice mantle of uncertain thickness, a liquid water ocean (Iess et al., 2010) perhaps with
salts and ammonia (Mitri et al., 2014), and an ice crust 50 to 150 km thick. The core is either
significantly hydrated (Castillo-Rogez and Lunine, 2010), or there is a mixed rock-ice layer
somewhere in the interior (see, e.g., Tobie et al., 2014).
(4) A range of chemical and physical data from the atmosphere to the interior suggest that a
significant event, or change in the way Titan evolves, occurred sometime between a few hundred
million and a billion years ago (Horst, 2017).
The relatively youthful age of the surface, which may be the result of geologic activity, older
impacts occurring on surfaces covered by liquids (Neish and Lorenz, 2014), extensive erosion, or
substantial burial by organic matter, means that there is little if any geologic evidence of the first
3/4 of Titan’s history. Two unanswered questions are (1) what was the process/ processes that
eroded or covered older impact craters and other landforms, and (2) did the obscuration of
features older than a few hundred million years occur continuously over time, or in some
singular event?
There is no evidence of the answer to the second question, but a theoretical model of the
evolution of Titan’s interior by Tobie et al (2006) provides an intriguing scenario that implies
Titan had a significant change in the working of its interior, crust, and atmosphere about 500
million years ago (Wood, 2018). In the Tobie et al (2006) model, Titan had a thin and rigid
clathrate crust—with methane as the dominant guest species—up until 500 million to one billion
years ago. During that earlier epoch, several major heating events resulted in the release of large
(compared to the present atmospheric inventory) amounts of methane from the clathrate hydrate
into the surface-atmosphere system. Within the last 500-1000 million years the interior has
cooled sufficiently to allow an ice I crust to form underneath the buoyant clathrate hydrate crust,
with diapirism in the thickening ice I crust providing one or several episodes of further release of
methane into the surface-atmosphere system. Wood (2018) called the onset of the ice I subcrust
the “Great Crustal Thickening Event” and noted that the mode of geologic processes would
change dramatically as Titan transitioned from a body with a thin rigid conductive crust over the
ocean to one with a thicker and rheologically heterogenous crust.
Models of Titan’s interior look broadly similar but with substantial disagreements on the
thickness of the high pressure ice layer and the extent of silicate core hydration (Tobie et al.,
2014). How much this affects the surface evolution is unclear. While the idea is commonly held
that the source of the methane to resupply the atmosphere is in crustal clathrate hydrate (such a
crust predicted in pre-Voyager days; Lewis, 1971), how the resupply works is unclear. Simple
forcing out of the methane from the clathrate by the photochemically produced ethane eventually
fails because of the stoichiometry (two methane molecules making one ethane). However, this
replacement could eventually weigh down the crust and cause an overturn, because clathrate with
predominantly ethane is heavier than Ice I. (Choukroun and Sotin, 2012). This could cause
interesting geologic consequences in the present era when the clathrate is nominally underlain by
warm ice I. Whether the methane hydrological cycle that we see today shaping so many aspects
of Titan’s surface is ancient or recent, episodic (Lunine et al., 1998) or continuous, remains a
mystery that may be directly coupled to the poorly understood interior evolution. Or it may
reflect a series of external events whose record in the Saturn system has yet to be properly read.
Birch et al (2017) have pointed to geologic evidence (notably the presence of large
sedimentary deposits) suggesting that the present epoch of lakes and seas of methane, ethane and
nitrogen might have been preceded by one with a widespread ocean of methane and other
hydrocarbons. The longevity of such an ocean, in particular its decline, may be constrained by
the limits on tidal dissipation of the orbital eccentricity during ocean shrinkage (Sagan and
Dermott, 1982; Sears, 1995), since we now have a measured global Titan topography. Finally, it
is possible that Titan has run out of atmospheric and surface methane multiple times in its history,
leading to dramatic atmospheric changes (Lorenz et al., 1997) and possibly epochs in the which
the surface is worked by liquid nitrogen seas and rivers (Charnay et al., 2014).
If Titan’s geologic and atmospheric nature have changed in a secular way over its history, it
would join the “other” terrestrial planets—Venus, Earth, and Mars, in this regard. In each case,
interior and surface-atmosphere changes over time have led to present-day characteristics that are
likely to have been dramatically different from those in the past.
Acknowledgments
Part of this work was carried out at the Jet Propulsion Laboratory, California Institute of
technology, under contract with NASA. Copyright 2018, California Institute of Technology.
Government sponsorship is acknowledged.
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... Titan, Saturn's largest moon, exhibits unique characteristics when compared to other planetary bodies in the solar system: it is simultaneously an ocean world, an icy world, and an organic world (Tobie et al., 2005;Iess et al., 2012;Hayes et al., 2016;Lopes et al., 2019). It is also the only natural satellite in the solar system with a significant atmosphere. ...
... Miller et al. (2019) has also provided evidence for large amounts of organics delivered to Titan at the time of initial accretion, with likely extraction and excretion of organic molecules into Titan's deep subsurface ocean. With these characteristics, Titan has become a prime target for astrobiological research (e.g., Lopes et al., 2019). ...
Article
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Titan is unique in the solar system: it is an ocean world, an icy world, an organic world, and has a dense atmosphere. It is a geologically active world as well, with ongoing exogenic processes, such as rainfall, sediment transportation and deposition, erosion, and possible endogenic processes, such as tectonism and cryovolcanism. This combination of an organic and an ocean world makes Titan a prime target for astrobiological research, as biosignatures may be present in its surface, in impact melt deposits and in cryovolcanic flows, as well as in deep ice and water ocean underneath the outer ice shell. Impact craters are important sites in this context, as they may have allowed an exchange of materials between Titan's layers, in particular between the surface, composed of organic sediments over icy bedrock, and the subsurface ocean. It is also possible that impacts may have favored the advance of prebiotic chemical reactions themselves, by providing thermal energy that would allow these reactions to proceed. To investigate possible exchange pathways between the subsurface water ocean and the organic-rich surface, we modeled the formation of the largest crater on Titan, Menrva, with a diameter of ca. 425 km. The premise is that, given a large enough impact event, the resulting crater could breach into Titan's ice shell and reach the subsurface ocean, creating pathways connecting the surface and the ocean. Materials from the deep subsurface ocean, including salts and potential biosignatures of putative subsurface biota, could be transported to the surface. Likewise, atmospherically derived organic surface materials could be directly inserted into the ocean, where they could undergo aqueous hydrolysis to form potential astrobiological building blocks, such as amino acids. To study the formation of a Menrva-like impact crater, we staged numerical simulations using the iSALE-2D shock physics code. We varied assumed ice shell thickness from 50 to 125 km and assumed thermal structure over a range consistent with geophysical data. We analyze the implications and potential contributions of impact cratering as a process that can facilitate the exchange of surface organics with liquid water. Our findings indicate that melt and partial melt of ice took place in the central zone, reaching ca. 65 km depth and ca. 60 km away from the center of the structure. Furthermore, a volume of ca. 10² km³ of ocean water could be traced to depths as shallow as 10 km. These results highlight the potential for a significant exchange of materials from the surface (organics and ice) and the subsurface (water ocean), particularly in the crater's central area. Our studies suggest that large hypervelocity impacts are a viable and likely key mechanism to create pathways between the underground water ocean and Titan's organic-rich surface layer and atmosphere.
... Physical processes, both in Titan's atmosphere and on the surface, can lead to these organic molecules interacting. These interactions could include, but are not limited to, cocondensation within ice clouds in the atmosphere, 9,10 condensation/adsorption onto aerosols in the organic haze layers of the atmosphere, the deposition of particles onto the surface, aeolian processing of solid surface materials (erosion and dune formation), fluvial/pluvial processing of surface materials (dissolution and precipitation) by the hydrocarbon liquid phase of Titan, large-scale geological processing (lithification, subduction, and uplift), and possibly cryovolcanic activity 4,11,12 or the outgassing of volatiles such as NH 3 and CO 2 from Titan's interior. 13,14 These processes would result in many of Titan's small organic molecules crystallizing into multicomponent minerals, which will be the focus of this Account. ...
... This might explain or contribute to the scattering observed in Cassini radar measurements. 11 Furthermore, the subduction of material rich in cocrystals into the warmer interior of Titan via tectonism could lead to expansion and uplift, possibly generating the labyrinth terrain after erosion as described above. ...
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Full-text available
In this Account, we highlight recent work in the developing field of mineralogy of Saturn’s moon Titan, focusing on binary co-crystals of small organic molecules. Titan has a massive inventory of organic molecules on its surface that are formed via photochemistry in the atmosphere and likely processing on the surface as well. Physical processes both in the atmosphere and on the surface can lead to molecules interacting at cryogenic temperatures. Recent laboratory work has demonstrated that co-crystals between two or more molecules can form under these conditions. In the organic-rich environment of Titan, such co-crystals are naturally occurring minerals and a critical area of research to understand the physical, chemical, and possibly even biological and prebiotic processes occurring in this alien world.
... Spectral analysis tools are foundational to the exploration of extraterrestrial bodies. Landers, orbiting satellites, and observatories equipped with different spectrometers have provided invaluable information about planets and moons in our solar system and beyond (e.g., Giuranna et al., 2019;Lopes et al., 2019;Rapin et al., 2019;Hansen et al., 2020;Kunimoto and Matthews, 2020;Paganini et al., 2020). While NASA's previous missions to Mars have exploited various forms of spectroscopy for geological applications, these approaches have uncovered details about the history of liquid water on the planet (Schmidt et al., 2008); an essential prerequisite for life . ...
Article
Spectroscopic instruments are increasingly being implemented in the search for extraterrestrial life. However, microstructural spectral analyses of alien environments could prove difficult without knowledge on the molecular identification of individual spectral signatures. To bridge this gap, we introduce unsupervised K-means clustering as a statistical approach to discern spectral patterns of biosignatures without prior knowledge of spectral regions of biomolecules. Spectral profiles of bacterial isolates from analogous polar ice sheets were measured with Raman spectroscopy. Raman analysis identified carotenoid and violacein pigments, and key cellular features including saturated and unsaturated fats, triacylglycerols, and proteins. Principal component analysis and targeted spectra integration biplot analysis revealed that the clustering of bacterial isolates was attributed to spectral biosignatures influenced by carotenoid pigments and ratio of unsaturated/saturated fat peaks. Unsupervised K-means clustering highlighted the prevalence of the corresponding spectral peaks, while subsequent supervised permutational multivariate analysis of variance provided statistical validation for spectral differences associated with the identified cellular features. Establishing a validated catalog of spectral signatures of analogous biotic and abiotic materials, in combination with targeted supervised tools, could prove effective at identifying extant biosignatures.
... To summarize, the present work provides a facile conceptual framework on the barrierless, low temperature formation of two prototypes of NPAHs -quinoline (C 9 H 7 N, P1) and isoquinoline (C 9 H 7 N, P2) -in the gas phase of cold molecular clouds in analogy to the Hydrogen Abstraction -Vinylacetylene Addition mechanism. 2,4,5,8 Low temperature conditions also hold for hydrocarbon-rich atmospheres of planets, their moons (Titan), 63 and trans-Neptunian Objects (TNOs) such as Pluto, 64 where solar photons initiate a vigorous low temperature photochemistry of their methane-nitrogen based atmospheres. The pathways derived here require the presence of pyridine (C 5 H 5 N), which can be photolyzed to o-, m-, and p-pyridinyl radicals (C 5 H 4 N), and vinylacetylene (C 4 H 4 ), which was recently observed in the cold molecular cloud TMC-1, 65 but not (yet) in Titan's or Pluto's atmosphere (Fig. 6). ...
Article
Despite remarkable progress toward the understanding of the formation pathways leading to polycyclic aromatic hydrocarbons (PAHs) in combustion systems and in deep space, the complex reaction pathways leading to nitrogen-substituted PAHs (NPAHs) at low temperatures of molecular clouds and hydrocarbon-rich, nitrogen-containing atmospheres of planets and their moons like Titan have remained largely obscure. Here, we demonstrate through laboratory experiments and computations that the simplest prototype of NPAHs - quinoline and isoquinoline (C9H7N) - can be synthesized via rapid and de-facto barrier-less reactions involving o-, m- and p-pyridinyl radicals (C5H4N˙) with vinylacetylene (C4H4) under low-temperature conditions.
... Methane clouds in Titan's lower troposphere were first detected at low latitudes in groundbased observations (Griffith et al. 1998). The subsequent discovery and investigation of seasonal cloud coverage (Brown et al. 2002;Turtle et al. 2018) and observations of surface darkening (Turtle et al. 2011;Lopes et al. 2019) at the poles motivated predictions of methane rainfall and surface transport (Lora et al. 2015;Faulk et al. 2020). ...
... Even larger and more complex atmospheric molecules, with molecular weights of thousands of Daltons (Da), have been detected but not resolved (Coates et al. 2007(Coates et al. , 2009Waite et al. 2007) due to instrument limitations. These atmospherically produced organics coalesce into haze particles that then settle out to cover much of Titan's water-ice bedrock (Rodriguez et al. 2006; Barnes et al. 2007a;Soderblom et al. 2007;Le Mouélic et al. 2008;Janssen et al. 2009Janssen et al. , 2016Hayne et al. 2014;Neish et al. 2015;Lopes et al. 2019). ...