Space Sci Rev (2019) 215:33
https://doi.org/10.1007/s11214-019-0598-6

Titan as Revealed by the Cassini Radar
R.M.C. Lopes1 · S.D. Wall1 · C. Elachi2 · S.P.D. Birch3 · P. Corlies3 · A. Coustenis4 ·A.G. Hayes3 · J.D. Hofgartner1 · M.A. Janssen1 · R.L. Kirk5 · A. LeGall6 ·R.D. Lorenz7 · J.I. Lunine2,3 · M.J. Malaska1 · M. Mastroguiseppe8 · G. Mitri9 ·C.D. Neish10 · C. Notarnicola11 · F. Paganelli12 · P. Paillou13 · V. Poggiali3 ·J. Radebaugh14 · S. Rodriguez15 · A. Schoenfeld16 · J.M. Soderblom17 ·A. Solomonidou18 · E.R. Stofan19 · B.W. Stiles1 · F. Tosi20 · E.P. Turtle7 · R.D. West1 ·C.A. Wood21 · H.A. Zebker22 · J.W. Barnes23 · D. Casarano24 · P. Encrenaz4 · T. Farr1 ·C. Grima25 · D. Hemingway26 · O. Karatekin27 · A. Lucas28 · K.L. Mitchell1 · G. Ori9 ·R. Orosei29 · P. Ries1 · D. Riccio30 · L.A. Soderblom5 · Z. Zhang2
Received: 13 July 2018 / Accepted: 27 April 2019 / Published online: 21 May 2019
© Springer Nature B.V. 2019

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 liquidmethane 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.

B R.M.C. Lopes
1

Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena,
CA 91109, USA

2

Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena,
CA 91125, USA

3

Department of Astronomy, Cornell University, Ithaca, NY 14853, USA

4

LESIA – Observatoire de Paris, CNRS, UPMC Univ. Paris 06, Univ. Paris-Diderot, Meudon,
France5Astrogeology Science Center, U.S. Geological Survey, 2255 N. Gemini D., Flagstaff, AZ 86001,
USA

6

Laboratoire Atmosphères, Milieux, Observations Spatiales (LATMOS), Universite Versaills Saint
Quentin, Guyancourt, France

7

JHU Applied Physics Lab, Laurel, MD 20723, USA

8

University 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

10

Department of Earth Sciences, the University of Western Ontario, 1151 Richmond Street,
N. London, Ontario N6A 5B, Canada

11

Eurac Research, Institute for Earth Observation, Viale Druso 1, 39100 Bolzano, Italy

12

SETI Institute, 189 Bernardo Ave., Mountain View, CA, USA13

UMR 5804 LAB, Batiment B18N, University of Bordeaux, allée Geoffroy Saint-Hilaire,
33615 Pessac Cedex, France

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R.M.C. Lopes et al.

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 areprime targets for astrobiological research.
Keywords Titan · Cassini · Radar

1 Introduction
Prior to Cassini, Titan’s surface was expected to be strongly influenced by the known presence of methane in a heavy nitrogen atmosphere. Organic compounds and liquid methane,
perhaps in the form of vast oceans, were expected in some as yet to be determined relationship with its anticipated ice shell. While the bulk of observational and theoretical work in
the decade or so prior to Cassini’s arrival emphasized studies of the atmosphere, a numberof theoretical investigations were made pertaining to the surface, which were summarized
by Lorenz and Lunine (2005). Radar observations using Goldstone and the Very Large Array pointed to an icy (but dirty) surface, but ruled out a deep global ocean of ethane andmethane, which were known to be liquids at Titan surface conditions. In addition, HST dataand ground-based observations using Adaptive Optics yielded important constraints on thesurface, including low resolution maps (Smith et al. 1996) from HST’s Wide-Field Planetary
Camera 2 that showed a bright region that was subsequently named Xanadu. Titan’s surface
features and composition were largely unknown, though the surface was thought to be water
ice covered with, or mixed with, darker materials such as heavy hydrocarbons and possibly
14

Department of Geological Sciences, Brigham Young University, Provo, UT 84602, USA

15

Institut de Physique du Globe de Paris (IPGP), CNRS-UMR 7154, Université Paris-Diderot,
USPC, Paris, France

16

UCLA Department of Earth, Planetary, and Space Sciences, 595 Charles E. Young Dr.,
Los Angeles, CA 90095, USA

17

Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology,
77 Massachusetts Avenue, Boston, USA

18

European Space Agency (ESA), ESAC, Madrid, Spain

19

Smithsonian National Air and Space Museum, Washington DC, USA

20

INAF – IAPS National Institute for Astrophysics, Via del Fosso del Cavaliere, 100, 00133 Rome,
Italy

21

Planetary Science Institute, 1700 East Fort Lowell, Suite 106, Tucson, AZ 85719-2395, USA

22

Department of Electrical Engineering, Stanford University, Stanford, CA 94305-9515, USA

23

Department of Physics, University of Idaho, Moscow 83843, USA

24

CNR IRPI, Via Amendola 122/I, 70125 Bari, Italy

25

Institute of Geophysics, University of Texas at Austin, J.J. Pickle Research Campus,
10100 Burnet Road, Austin, TX 78758, USA

26

Miller Institute for Basic Research in Science, Department of Earth & Planetary Science,
University of California Berkeley, 307 McCone Hall, Berkeley, CA 94720, USA

27

Royal Observatory of Belgium, 3 Avenue Circulaire, 1180 Brussels, Belgium

28

Institut de Physique du Globe de Paris, CNRS, 1 rue Jussieu, Paris, FranceTitan as Revealed by the Cassini Radar

Page 3 of 50

33

silicates (Coustenis et al. 1997). Indications of surface liquids were provided by radar observations from Arecibo (Campbell et al. 2003). SAR images from the RADAR instrument
were expected to reveal surface geology and location of liquids.
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).
While the utility of optical and IR wavelengths was uncertain for a surface investigation
due to the atmospheric opacity at short wavelengths, the RADAR instrument was sure tohave the capability of penetrating the atmosphere. However, it was selected in the Cassini
competition solely as an active experiment with no passive channels. Two passive microwave
instruments had been proposed for Cassini with general objectives for observing within the
Saturn system, a millimeter/submillimeter spectrometer (MSAR, or Microwave Spectrometer And Radiometer) and a multi-frequency centimeter-wavelength radiometer (SAM, orSaturn Atmospheric Mapper). These experiments were highly rated scientifically but did not
make the final selection. Post selection in 1990, the leaders of these proposed investigations,P. Encrenaz, M. Janssen and D. Muhleman, with the help of the RADAR PI C. Elachi, petitioned NASA and ESA to consider adding radiometric capability to the RADAR instrument.
The rationale was strictly to support the RADAR investigation of Titan by adding significant
capability to the RADAR experiment. A key argument was that this would both support andaugment the interpretation of the active data in terms of Titan’s surface properties. For example, it was pointed out that the measurement of surface thermal characteristics would allow
a new dimension of interpretation. In particular, using the combined passive/active data, theway was opened to obtaining a host of surface thermophysical properties such as composition, density, and structure at depths of a few wavelengths to meters. Among the anticipatedresults was a surface emissivity map of Titan, from which could be inferred variations in the
distribution of oceans and solid surfaces.
The RADAR instrument had therefore four operating modes—synthetic aperture radar(SAR) imaging, altimetry, scatterometry, and radiometry. In its SAR mode, used at altitudes
under ∼ 4000 km, RADAR imaged Titan’s surface at incidence angles from 15 to 35 degrees
and spatial resolutions varying as low as 350 m, sufficient to identify major structures and to
make geologic maps. SAR mode was occasionally used from higher altitudes (“HiSAR”),
yielding coarser spatial resolution (> 1.5 km). SAR images respond to surface roughness
scales near the wavelength, to surface slopes at the scale of its resolution, and to dielectric
properties of the materials; and they can penetrate many wavelengths into some materials as
well, the latter a significant issue on Titan. In addition, radar can penetrate many wavelengths
into some materials on Titan, measuring for example the depths of some Titan lakes and seas.
SAR images are observationally analogous to optical images and allow us to identify major
structures and geomorphologic units.
Altimetry with a range resolution of approximately 30 m results when the antenna is
pointed within a fraction of a beam width (0.35 deg) of nadir, at a spatial resolution of
the order of 30 km, depending on altitude. The altimetry data have two broad applications.
29

Istituto di Radioastronomia, Istituto Nazionale di Astrofisica, Via Piero Gobetti, 101,
40129 Bologna, Italy30

Dept. Electrical Engineering ad Information Technology, University of Napoli Federico II, Napoli,
Italy

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First, the aggregate of all data defines an absolute geoid for Titan and constrains any rotational or tidal bulge. Second, relative topographic profiles from each radar pass characterize
landforms (e.g., impact craters, mountains), yielding important constraints on geophysicalmodels. Altimetry was obtained over 2% of Titan’s surface. In addition, altimetry was able
to obtain depths of several lakes and seas (Mastrogiuseppe et al. 2014).
An amplitude monopulse comparison technique called “SARTopo” (Stiles et al. 2009)
added an additional 5% topographic coverage (Corlies et al. 2017) of somewhat coarse resolution (∼ 10 km horizontal, ∼ 100 m vertical). The SARTopo technique depends on the
precise manner in which the location and magnitude of each SAR pixel are affected by variation in surface height between each of the five radar beams (Elachi et al. 2004). Cassini’s
antenna has multiple beams (feeds), so that specific locations on the ground are observed
from two different beams. For different beams, the mislocation error due to surface height isthe same, but the magnitude error is different and of opposite sign. The apparent differencesin pixel magnitudes between overlapping single beam SAR images are then used to infer
height. Nonzero surface heights, therefore, result in apparent differences in pixel positions
between overlapping single beam SAR images.
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 toradiometry data in order to infer surface composition. In radiometer mode, RADAR collected global measurements of Ku-band brightness temperature with a precision of ∼ 0.1 K
and an absolute accuracy approaching 1 K (Janssen et al. 2016). Radiometry data consist ofantenna temperature measurements that are processed to produce polarized brightness temperatures of the observed scene. 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. Making use of surface
temperatures independently measured by CIRS (Jennings et al. 2009, 2016), global maps ofderived surface properties such as volume scattering and emissivity have been obtained asshown in Figs. 1 and 2. These are described in detail in Janssen et al. (2016).
In its 13-year presence at Saturn, Cassini made a total of 127 passes by Titan, someas close as 880 km. 48 of these were used by RADAR, with one lost in downlink. 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 (Fig. 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. Coverageincreases 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 fouror more times. Real-aperture scatterometry and passive radiometry data exist over the entire
surface.
RADAR data, together with near- and mid-infrared images taken by Cassini’s Visual andInfrared 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 thatis both Earth-like and unearthly. We find familiar geomorphology in its mountains, dunes,Titan as Revealed by the Cassini Radar

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Fig. 1 Predicted and observed maps of scattering from Titan’s surface. The upper panel is a map of thevolume 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 thata photon entering the surface (as determined by the effective dielectric constant) scatters and escapes fromthe subsurface before it is absorbed. The map of fvol reconciles the maps of effective dielectric constant andemissivity 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 2011), showing the actuallyobserved scattering. The white arrows in the upper left-hand corner indicate two small regions that were notmappedrivers and seas, but note that these features are composed of unfamiliar materials. In thispaper, we review RADAR’s contributions to geology, hydrology, and surface-atmosphericinteraction in an attempt to summarize our knowledge of Titan as obtained from the RADAR
instrument at the end of the Cassini mission.

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 beenused to infer the interior structure of Titan. Magnetic field measurements have shown thatan 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 Ti-

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Fig. 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, theequatorial region centered on 100 W longitude, is extremely cold and non-emissive, characteristic of a highcontent of highly fractured water ice bedrocktan 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 inboth 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 toprevious 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) refer-Titan as Revealed by the Cassini Radar

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Fig. 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

ence frame. Pixels chosen in each SAR image, corresponding as closely as possible to thesame 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 exceptthat the general direction (within 10 degrees or so) must be known to exclude ambiguous
returns from the opposite side of the spacecraft. 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 polelocation, the spin rate, the precession and the nutation. Further, these authors show that thepole location is compatible with the Cassini state 1 (a dynamical equilibrium wherein thespin 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 decoupledfrom the outer ice shell by a global subsurface ocean. 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) subsystem of the
HASI (Huygens Atmospheric Structure Instrument) on board the Huygens probe measured
a Schumann-like resonance, also suggesting the presence of a subsurface ocean (Béghin

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Fig. 4 Probable present-day
structure of Titan’s interior
showing that Titan is internallydifferentiated, with a deep rockyor rock-ice mix deep interior

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 theocean, and a deep rocky or rock-ice mix interior (Hemingway et al. 2013; Mitri et al. 2014;
Tobie et al. 2014).

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 lessthan, and the equatorial radii are greater than, predicted by its gravity field. Best-fitting
solutions are shown in Table 1 below.
The hydrostatic ratio (a − c)/(b − c), which is exactly 4 for a spin-locked satellite inhydrostatic equilibrium (Hemingway et al. 2018), 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 is 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 toa 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 fromtidal interaction with Saturn or its other moons.
Another possible explanation for the overly oblate topographic figure is that there isgreater precipitation of hydrocarbon snow or loosely packed hydrocarbon particulates at theTitan as Revealed by the Cassini Radar

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Fig. 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 arelower than expected, suggesting that the depth to the subsurface ocean is smaller at the poles than at theequatorTable 1 Titan gravity and shape
triaxial ellipsoids (m)Gravity

Elevations

Differencea axis2574875 ± 7.3

2575124 ± 26−249

b axis2574716 ± 7.4

2574746 ± 45−30

c axis2574660 ± 4.52574414 ± 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.

25747952574761

Hydrostatic ratio(a − c)/(b − c)

3.83

2.14

−34

equator than at the poles, perhaps with a net equatorward transport from the poles, which
would also be consistent with the data (Zebker and Wong 2016). This could account forincreased 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 Sect. 5.3), so that poorly consolidated material may
indeed collect there to a greater extent. Several hundred meters of deposition at the equatorwould match the mean equatorial radii of both models, but precipitation would have topreferentially occur in the sub- and anti-Saturnian directions, which seems unlikely.
Further comparisons of the gravity and shape observations can constrain the depth of
any outer ice shell enclosing a global subsurface ocean (Zebker and Wong 2016). While theexact 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 preliminary results suggest ice-shell thicknesses of 327 and
187 km, respectively, a factor of ≥ 2 more than the 100 km expected from thermal models(Sohl et al. 2003; Nimmo and Bills 2010) and interpretation of the Schumann resonance33

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Fig. 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

observations by the Huygens probe (Béghin et al. 2012). 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 deep interior is lower than often assumed, hence theamount of radiogenic material in the deep interior is likely less as well. These data also
constrain the depth of Titan’s subsurface ocean and density of its deep interior, placing added
restrictions on its composition and evolution. Supposing that Titan has an undifferentiatedice/rock deep interior beneath the ocean, and that the moment of inertia is most likely inthe range 0.33–0.34 (Iess et al. 2010), then the ∼ 200-km-thick ice shell estimate above andTitan’s well-known mean density of 1.88 g cm−3 implies an ocean depth (thickness of thesubsurface ocean layer) and deep interior 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 (Sect. 5), constrain therelative 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 atmosphericscattering and absorption that hamper the observations and the presence of complex organic
molecules on the surface. The extended, dense, and hazy N2 -CH4 dominated atmosphereshields the surface from direct optical observations, except at certain wavelengths where themethane 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 Sect. 4.2 below, while RADAR can probe the surface
and subsurface scattering properties, and hence place additional constraints on composition.

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Fig. 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 andvolume scattering terms. Both sets of curves fall off slowly with incidence angle, indicative of significant
volume scattering

4.1 Surface and Subsurface Scattering Properties
Compared to the surfaces of the Moon or the Earth, the off-nadir radar response from mostof Titan’s surface is quite strong at Ku-band (λ = 2 cm). 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 Kuband 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 nearsurface (Paillou et al. 2008) and thus have multiple opportunities to be scattered. The abovereferenced 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 wherevolume scattering, enabled by the low material losses, is enhanced by coherent backscatterprocesses (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 moreabsorbing 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. Some of these materials could also have originated from cryovolcanism, inwhich the radar signature could be explained by a strong volume scattering effect in a thickwater-ammonia ice layer using a two-layer scattering model (Paillou et al. 2006). Radarbright sinuous channels in the southwest of Xanadu (Sect. 5.8), showing very large radarcross-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 containcentimeter-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33Page 12 of 50

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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, whereradar 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 propertiesof liquid hydrocarbons (Mitchell et al. 2015).

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’ssurface 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 longward of 1 µm. 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, several main
spectral units were initially distinguished on the surface of Titan. These include material, informally referred to as, the “equatorial bright” material, mainly distributed in the topographically high and mid-latitude areas, the “dark blue” material adjacent to the bright-to-darkboundaries, the “dark brown” material, which correlates with RADAR-dark dune fields, andthe “Xanadu” material, which comprises the bright regions of Xanadu (Barnes et al. 2007a;
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 structureof 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, photometricgeometry, and porosity.
Correlations between near-infrared and microwave data of Titan’s surface are useful togather 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 techniqueallowed for the identification of regional surface units at equatorial to mid-latitudes. Some ofthese 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 atthis spatial scale, the major bright features do not share the same characteristics.
A comprehensive investigation of Titan’s surface features using the VIMS and RADARSAR datasets at the best available spatial resolution is still the best approach to characterize
geomorphologic units using both spectral and morphologic characteristics. For example, thecorrelation between the 5-µm-bright materials and RADAR empty lakes suggests the presence of sedimentary or organic-rich evaporitic deposits in dry polar lakebeds (Barnes et al.

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2011a; 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; Bonnefoy et al. 2016). These studies allow definition of both the surface
and the atmospheric contributions from VIMS spectral imaging data after performing theappropriate 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 (weighted averages of the VIMS
channels per window; Solomonidou et al. 2016), 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 (C2 H2 ), ethylene (C2 H4 ), ethane (C2 H6 ),propane (C3 H8 ), cyanoacetylene (HC3 N), water (H2 O), 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 typesof anthraxolite and amorphous carbon, which have been proposed to lower the total surface
albedo of Titan’s surface (Lellouch 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 ofterrains (Fig. 8; Stofan et al. 2006; Paganelli et al. 2007, 2008; Lopes et al. 2010). Morerecent 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, hummocky terrains, labyrinth terrains, lakes, and impact craters. The oldest units are the hummocky andthe labyrinth terrains; it is not possible with currently available data to differentiate the relative ages of these two oldest types of terrain. The hummocky terrains consist of mountain

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Fig. 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’ssouth pole. Middle row, left: Bright (high radar 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 (radar bright) in the equatorial region. Right:
Elivagar Flumina, interpreted as a large fluvial deposit, showing braided radar-bright channels which overlie
radar-dark undifferentiated plains

chains and isolated radar-bright terrains. The labyrinth terrains consist of highly incised anddissected 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 suchas crater rims, channels, and candidate cryovolcanic features. Crater rims and candidate cryovolcanic features are locations more likely to expose materials from the icy shell, while
the hummocky/mountainous materials are thought to be exposed remnants of the icy shell’ssurface.
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,Titan as Revealed by the Cassini Radar

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the surface, and the atmosphere. Both microwave emissivity and VIMS are helpful in characterizing the units, although they have different penetration depths, with microwave emissivity penetrating tens 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 radiometricdata which is consistent with low-dielectric organic materials.
Spectral coatings of the terrain units, following early studies by McCord et al. (2008),
Barnes et al. (2007b), were described in Solomonidou et al. (2018) and discussed above.
From this analysis, three groups of compositional mixtures are reported, which include themajor 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 thestreak-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 (5 in Solomonidou et al. 2018) are part of a different compositionalgroup 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, thedunes, and the undifferentiated plains that are close to the equator and possibly contaminatedby 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 radar-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 theseterrains are located mainly at mid-latitudes. Their gradational boundaries and paucity ofrecognizable 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 andformed by depositional and/or sedimentary 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, thedistribution of Undifferentiated Plains, both at local and global scales, is consistent withaeolian deposition being the major process contributing to their formation.

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Fig. 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-darklines 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 suchas to give bright glints (specular reflection) from dune slopes. The open arrows indicate the direction of SAR
illumination and incidence angle. From Radebaugh (2013)

Spectral differences between the Plains and Dunes seen in VIMS data imply that thematerials, 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 duneseas) are more spectrally similar to dune materials, suggesting that they are related andsupporting the idea that dune materials are transported by wind from equatorial to midlatitudes (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 bymethane, 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 plainsdeposits 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 ofaeolian 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 consistsof 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;

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Radebaugh et al. 2008). Limited measurements of heights from radarclinometry suggests
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 ofcollection 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, 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 theeastward 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).
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 that the dunes cannot be composed primarilyof water ice, but rather must be dominated by organics, presumably 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); or the midlatitude blandlands, though it’s also possible 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 sands 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 (Radebaugh2013). Bonnefoy et al. (2016) extracted distinct dune and interdune spectra and emissivities from most of Titan’s major dune fields. Their results indicate sand-free interdune areasof varying composition, implying that the sand dunes have been active on geologic recent
timescales.
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) andmore 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

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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 solidorganics (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 estimatesfrom Lopes et al. (2010). A simple calculation of the volume (e.g. Lorenz et al. 2008b;
Rodriguez et al. 2014) 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 ofkilometers 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. 2014). These appear tohave 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 fromdirect observations, though it was generally assumed that Titan had a similar impact crater
production history as the other Saturnian satellites. Estimates of the cratering rate weremade 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 tobe disrupted by Titan’s atmosphere and not result in craters (Artemieva and Lunine 2005;
Korycansky and Zahnle 2005). Cassini RADAR observations show an extreme paucity ofcraters. 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 to a billion years (Woodet al. 2010; Neish and Lorenz 2012), with the oldest surfaces located near the equator andthe youngest surfaces located near the poles (Neish et al. 2016).

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Fig. 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)

The craters that are observed on Titan all show evidence for extensive modification
by erosional processes (Fig. 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 (Neishet al. 2016). Modification by viscous relaxation is expected to be minimal given the cold
lithospheric temperatures on Titan (∼ 90 K), although insulation by sand could enable somerelaxation in Titan’s larger craters (Schurmeier and Dombard 2018).
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 thisobservation. 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 offluvial 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 isan extremely dynamic world, and studying its impact structures can reveal much about theprocesses that have shaped it.

5.5 Mountains and Tectonics
Features of relatively high topography, termed mountains, have been observed across Titan(Fig. 11). Large topographic relief 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33

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Fig. 11 Some examples of mountains on Titan. Radar illumination is from the top in all cases, and northis up. Bright, possibly erosional, blankets surround the central, high elevation features visible as bright/darkpairs. (a) Sinlap Crater, shown to demonstrate the effects of radar 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)

is in part because water ice generally loses strength with increasing depth, making hightopographies difficult to support rigidly. However, the exceptional amount of erosion on
Titan may also be responsible for terrain height reduction. Overall, topography on Titan israther 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 thatare 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 andfractured 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 havea high emissivity as seen by RADAR (Janssen et al. 2009, 2016), and are part of the VIMS
bright blue unit (Barnes et al. 2007b), indicating a higher water-ice component, the dominantmaterial 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. In terms of area, mountain chains are the most extensive rugged fea-

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tures (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, 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 withlineations 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 othericy satellites: they sit on elevated topography (Radebaugh et al. 2016), they are arcuate inmorphology, 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 ice
shell, 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 wereformed by contractional tectonism (Mitri et al. 2010). Some very large-scale tectonic rises,
several hundred kilometers across, may be as uplifted and later dissected plateaus composedof organic sedimentary materials (Undifferentiated Plains) that are the surface expression oflarge subsurface laccoliths (Schurmeier et al. 2017).

5.6 Cryovolcanism
Cryovolcanism involves the eruption of materials that would ordinarily be solid at the surface temperature of the icy bodies of the outer Solar System: cryomagmas therefore include
liquid or gaseous water or aqueous solutions, possibly mixed with solid fragments (Lopes
et al. 2013a). The existence of features on Titan formed by cryovolcanic activity has been
the subject of some controversy, which is unlikely to be resolved by Cassini data, mainly due
to imagery and spatial coverage limitations (Nixon et al. 2018). 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 Ar) in Titan’s atmosphere (Niemann et al. 2005) in concentrations
suggesting that the atmosphere was in communication with a reservoir of the parent atom,
40
K. 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. 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.
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. 2013b) and not a candidate
for a cryovolcanic feature.
Cassini RADAR and VIMS revealed several features interpreted as formed by cryovolcanism (Barnes et al. 2006; Sotin et al. 2005; Lopes et al. 2007; Nelson et al. 2009a, 2009b;
Soderblom et al. 2009; Wall et al. 2009; Lopes et al. 2013b; Solomonidou et al. 2016).
However, the interpretation has been debated (e.g., Moore and Pappalardo 2011) and has
not been entirely resolved by Cassini data (Nixon et al. 2018), primarily due to limitations

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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 possible temporal variations detected by VIMS (Barnes et al. 2005; Nelson et al. 2009a, 2009b;
Solomonidou et al. 2014, 2016, 2018). However, the Cassini mission had limited repeatedcoverage of the cryovolcanic candidate features and those observations did not reveal any
hot spots (thermal enhancements) that would have provided conclusive proof of active cryovolcanism. The detection of thermal activity at Titan’s surface using radiometry data (which
is sensitive to variations of ∼ 1 K) 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, 2013b; Nixon et al. 2018).
Landforms considered as possibly cryovolcanic include flow-like terrains seen on thewestern margin of Xanadu (Wall et al. 2009), spectrally different regions in Tui Regio(Barnes et al. 2005) and tangled flow regions in Hotei Regio (Soderblom et al. 2009;
Wall et al. 2009). These flow-like morphologies even exhibit elevated, lobate margins typicalof flows. Other possible cryovolcanic features are the steep-sided, small lakes at the northpolar region (see Sect. 5.9). These landforms have slightly elevated rims, steep sides, flat
floors, and deposits diffuse to SAR surrounding them. All of these characteristics are typicalof 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.
(2013b) who combined SAR imaging (including stereogrammetry, Fig. 12) and VIMS data
(Fig. 13) for a region that includes two mountains, Doom Mons (40.4°W, 14.7°S) and EreborMons (36.2°W, 5.0°S), as well as a depression, Sotra Patera (40.0°W, 14.5°S), and a region
consisting of flow-like features, Mohini Fluctus (centered at 38.5°W, 11.8°S). Doom andErebor Montes are tall mountains (Doom being ∼ 70 km in diameter and 1.5 ± 0.2 km high,Fig. 12), 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 noncircular, collapsed depressions are located between the two Montes, and flow-like features
also surround Erebor Mons.
A criticism by Moore and Pappalardo (2011) of initial interpretations by RADAR of cryovolcanic candidates reported by Lopes et al. (2007) is that flow-like features could havebeen produced by fluvial activity, since channels are seen in areas such as Hotei Regio andGanesa Macula (where topography later obtained by RADAR showed was not a shield ordome as initially interpreted). However, the Doom Mons–Sotra Patera–Erebor Mons regionis totally devoid of visible fluvial channels, making a fluvial origin for Mohini Fluctus andother 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. 2013b).
VIMS data analysis has contributed to the cryovolcanic interpretation via two different typesof 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 again using a radiative transfer model on a large selection of VIMS
data, Solomonidou et al. (2016) revealed temporal changes for the Sotra Patera and Mohini

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Fig. 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), withthe 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. 2013b)

Fluctus area, which became brighter up to a factor of 2 in terms of pure surface albedo andbrightness during one year (2005–2006), while surrounding areas and the undifferentiatedplains 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 featuresconsistent with volcanism, suggest that the regions might be active and possibly connectedto the deep interior via cryovolcanic processes. Additional support for cryovolcanic originof 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 thatthe Doom Mons–Sotra Patera–Erebor Mons area is a likely region for cryovolcanic activity.
Other putative cryovolcanic regions on Titan were discussed by Lopes et al. (2013b).

5.7 Labyrinth
The enigmatic labyrinthic terrains of Titan are defined as elevated highly dissected plateauswith 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, 2017a).
The Katain Labyrinth (Fig. 14) is a typical example. SARTopo data suggest that labyrinth
terrains are among the locally highest units on Titan (Stiles et al. 2009). Often the valley
floors contain radar-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, theterrain appears in the form of a dissected plateau; when the widths of the valleys and thewidths of the intervening plateaus are about equal, the terrain appears as a series of valleys

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Fig. 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 have a vertical exaggeration of 10:1. A movie showing the whole region can be seen at:
http://photojournal.jpl.nasa.gov/catalog/PIA13695

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 (having no external drainage) may also be present, although this may be a result of the coarse
resolution of the Cassini RADAR.
Wide valleys are 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 amountsof structural and topographic control. Some of the more circular planform labyrinths havevalley 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 (Schurmeier et al. 2017, 2018). At the terminus of the valley networks
and in contact with the labyrinths, undifferentiated plains units are found, suggesting a close
connection between the two types of units, at least at the local scale.
Radiometry data suggest that the labyrinth terrains consist of a possibly uplifted thickplateau of organic materials. The emissivity data show 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. However, VIMS analysis of one labyrinth region

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Fig. 14 SAR image of Katain Labyrinth. (52N, 349W) 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(Solomonidou et al. 2018) show that, at least on the top surface, the material is compatible
with water ice as the major constituent.
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 tothe 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, 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 ofTitan soluble materials could dissolve under Titan conditions to form the features observedby 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 southpolar terrain and are shown in Fig. 15. However, other formation scenarios are possible,
including differential hardening and deflation, aeolian deflation, or other phase change andremoval processes such as sublimation. From superposition relations, the labyrinths represent an ancient terrain (Malaska et al. 2016b), emplaced prior to the undifferentiated plains(Lopes et al. 2016).

5.8 Xanadu
Much of Titan’s geology has a regional organization; dunes and mountain belts are foundnear the equator, lakes and seas are found near the poles, and relatively bland regions are

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Fig. 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

Fig. 16 Xanadu region. All Cassini SAR and HiSAR image swaths covering the Xanadu region, overlain
on a VIMS basemap. Xanadu is generally SAR-bright because of rough terrain and fractured ice, which isinterspersed 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

located in the midlatitudes (Lopes et al. 2010). Xanadu (Fig. 16), however, is a continentsized region that breaks with all predictions. It is 4000 × 2500 km wide, is located on Titan’s leading hemisphere, and interrupts the equator-encircling sand seas. The feature wasobserved before Cassini arrived (Lemmon et al. 1993; Smith et al. 1996), and efforts weremade 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 emissivity as indicated in scatterometry and radiometry measurements, which is consistent with a water-ice composition

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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 isa higher than average percentage of water-ice exposed at the surface (Barnes et al. 2007a;
Tosi et al. 2010).
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-scalelinear 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 andhigh local topographic variations, the region as a whole appears to be regionally lower inelevation than anywhere else near the equator (Zebker et al. 2009; Stiles et al. 2009). This isevidenced 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 shell (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 might have resulted
from global contraction related to interior cooling, tidal spinup or spindown, or internalconvection (Radebaugh et al. 2011).
Several outstanding questions about Xanadu remain, in addition to its origin: why is
it regionally depressed? Why the dunes of the equatorial sand seas do not 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.

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 observations of Titan’s hydrocarbon lakes were acquiredby the RADAR in July 2006 (T16), when ∼ 75 features with exceptionally low backscatter,high emissivity, and distinctly lacustrine morphology were identified in SAR images (Stofan et al. 2007). The confirmation that the lakes contained liquids was obtained by VIMS
observations showing specular reflection from one of the lakes (Stephan et al. 2010). Subsequent observations by the RADAR revealed more than 650 such features, both dry and33

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Fig. 17 Map of lakes and seas on the northern (top) and southern (bottom) polar regions of Titan

filled, scattered throughout Titan’s polar terrain (Hayes et al. 2008; Birch et al. 2017). Thesefeatures have diameters that follow a log-normal distribution with a median of 77 ± 20 km(Hayes 2016). The morphology of both dry and filled lakes and seas on Titan provides
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 moreconsistent 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 ofCassini’s exploration of Titan’s lakes and seas, see Hayes (2016).
Lakes and seas encompass 1% of Titan’s total surface area (Hayes 2016). The majorityof surface liquids (97% by area) reside in the north polar region, with 80% of all liquidfilled surface area contained in three large seas; Kraken Mare, Ligeia Mare, and Punga Mare(Fig. 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Titan as Revealed by the Cassini Radar

Page 29 of 50 33(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 ∼ 5 × 1014 kg of methane per Titan year from the south pole to the north, driven
by a seasonal asymmetry in insolation that is the result of Saturn’s eccentric orbit aroundthe Sun (Aharonson et al. 2009; Lora and 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 northernsummer. Long timescale (∼ 100000 yr) orbital cycles can switch the direction and magnitude of the seasonal asymmetry driving this transport, moving liquid deposits between thepoles 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 theterminus 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. 2010) support the theory of rising and falling liquid levels as the magnitude anddirection 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 northand 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 and the lack of comparable repeated SAR images, however, this is not surprising.
Although confirmed and stable liquid deposits are currently restricted to polar terrain, theequatorial features Hotei and Tui Regiones have been interpreted as possible low-latitude
paleoseas (Moore et al. 2014). 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 et al. 2014). Dark flow-like features identified adjacent to theradar-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. 2013b). 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, the RADAR acquired nadir-pointed altimetry over the Ligeia Mare. The
resulting altimetry echoes revealed waveforms that displayed two distinct returns, one fromthe surface of Ligeia Mare and one from its seabed (Mastrogiuseppe et al. 2014). The difference in the received timing between these returns was a 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 thedepth 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 overPunga Mare and Kraken Mare (Mastrogiuseppe et al. 2016). A reprocessing of altimetry

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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 maredata acquired over Ontario Lacus in December 2008 allowed the technique to be applied
to Ontario Lacus as well (Mastrogiuseppe et al. 2018). Ligeia Mare was determined tohave 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 ∼ 14000 km3 of liquid (Hayes 2016). The estimated Ku-band loss tangent was
4.4 ± 1 × 10−5 (Mastrogiuseppe et al. 2016). Assuming a methane-ethane-nitrogen composition and using the laboratory measurements of Mitchell et al. (2015) with the LorentzLorenz formulation, the best-fit loss tangent is consistent with a methane-dominated composition of 71% CH4 : 12% C2 H6 : 17% N2 . As large quantities of liquid ethane should havebeen produced by photolysis of methane in the upper atmosphere, and at least trace amountsof 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 seasrequires 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 composition similar to Ligeia (Mastrogiuseppe et al. 2018). Alongmost of Kraken Mare the seafloor was not detected, indicating that the seas are either too
deep or too absorptive in these areas. Within Punga Mare, a clear detection of the subsurface was observed to reach a maximum depth of up 120 m, along tens of km of track(Mastrogiuseppe et al. 2018). At Ontario Lacus, however, the best fit loss tangent is greater(7 ± 3 × 10−5 ), consistent with a composition of ∼ 47% CH4 , ∼ 40% C2 H6 , and ∼ 13% N2 ,possibly suggesting an increased abundance of high-order hydrocarbons as compared to thenorthern 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)

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Fig. 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 Marehydrocarbons 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, tothe 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 andseas is ∼ 70000 km3 (Hayes 2016). It is interesting to note that this represents only 1/7 theamount of methane that currently resides 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 notplanned initially and represents an exciting discovery made during the Cassini spacecraft
lifetime.

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, andthe 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. 2013) has thus provided similar constraints
on the nature of Titan’s surface and the climatic conditions that proved favorable for theformation of these features, albeit in a far more limited fashion.
Cassini’s RADAR imaged large portions of Titan and showed that valley networks are
distributed at all latitudes (Lorenz et al. 2008c; Burr et al. 2009; Lopes et al. 2010) andhave a wide variety of surface morphologies (Fig. 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. 2013),
and even a meandering-like feature in the south polar region (Malaska et al. 2011b;

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Birch et al. 2018). The presence of canyons implies a vertically weak bedrock, which
may be influenced by fractures in the ice shell and/or a relatively highly erodible material. Similarly, rectilinear channels imply a fractured bedrock, where channels are forced tofollow 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 intransport 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 thisis the region where the Huygens lander descended, where descent images, with an orderof-magnitude higher resolution (Tomasko et al. 2005), showed a highly-dissected networkof dendritic valleys (Perron et al. 2006). It is likely that Titan is dissected everywhere at thescale 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 (10’s of meters).
While not surprising that Titan has a global network of fluvial valleys, 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. 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.
In some locations, fluvial channels terminate in alluvial or fluvial fans, distributary landforms that indicate a transition from a high to a low elevation (Radebaugh et al. 2016; Birchet 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. 2016). These landforms are widely distributed across the surface, but they are notabundant (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 theselandforms are not readily generated.

6 Temporal Change
Temporal changes were detected on Titan during the course of the Cassini mission, due toseasonal or other effects. Data from more than one instrument are key for determining thepossible causes of change. For example, as mentioned in Sect. 5.6, the radiative transfer codeanalysis of VIMS data from Tui Regio (2005–2009) and Sotra Patera (2005–2006) showedtemporal 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, possibly cryovolcanism or degassing, orelse meteorological or atmospheric processes, such as 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Titan as Revealed by the Cassini Radar

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of millimeters (Barnes et al. 2011b; 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 toa 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. Specularreflections 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 radar brightfeatures were observed in Kraken Mare (Hayes 2016).
The transient bright features were nicknamed “Magic Islands” due to their appearing/disappearing act and similarity in appearance to islands in SAR images. The features
are not islands, however, and are most consistent with waves, or floating and/or suspended
solids and bubbles. Based on the frequency of these phenomena in analogous terrestrial
settings, surface waves (intended to mean roughness of the liquid surface regardless of theprocess 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.
The wave properties (e.g., height, wavelength, and driving wind speed) cannot be uniquelydetermined from the Magic Island observations alone. Surface roughness on scales of approximately 1 cm or greater would be sufficient to cause the observed brightening. A model
for waves on Ligeia Mare suggests that for the expected maximum wind speeds of 1 m/s,
a significant wave height of 0.2 m and wavelength of 4 m can be expected (Lorenz andHayes 2012). Magic Islands were observed in three regions; two in Ligeia Mare (Hofgartner
et al. 2014, 2016) and one in Kraken Mare (Hayes 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 brightfeatures 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 notdetect Magic Islands, however, the possibility that the “Magic Islands” were present but notdetected in these observations could not be ruled out. “Magic Islands” were definitely notpresent in SAR observations before the first appearance and after the second appearance(Fig. 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 radar 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).

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 observationacquired 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 SAR-Topo 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 observationsof this area acquired between 2007 and 2009 show that the dark splotches had becomebrighter 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33

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Fig. 20 The large panel on the right is a SAR mosaic of Titan’s hydrocarbon sea, Ligeia Mare. The panelson 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 definitely 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)

same topographic depressions, this time in the microwave, that were interpreted as either
the evaporation or infiltration of ponded liquid (Hayes et al. 2011).
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 change occurred in 2010 when an equatorial areaof over 500000 km in size was observed to darken, presumably by methane precipitation,
after a chevron shaped cloud pass over the region (Turtle et al. 2011a). SAR images of thearea suggest that the darkened region represented local topographic lows. Showing a similar
pattern to the changes observed at Arrakis Planatia, the area was later observed to brighten
relative to surrounding terrain and then return to its original albedo (Barnes et al. 2013).
Whereas there have been no definitive changes observed in the shorelines of the northernlakes and seas through April 2017 (flyby T126), there have been several surface changes

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reported for lacustrine features in the south polar region. Turtle et al. (2011b) argued forshoreline recession at Ontario Lacus between ISS images acquired June 2005 and March2009), although the poor resolution of T51 makes quantitative measures 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 and2008/2009 that contain lacustrine features that seem to disappear between subsequent SAR
observations. The observed 10-fold increase in SAR backscatter cannot easily be explainedby 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” have been observed within the northern seas. These changes are discussed
above in Sect. 6.1.
Although the Cassini mission’s exploration of Titan’s methane cycles has ended, groundbased 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 tomonitor the changing seasons on Titan. If any change were to occur, it should be primarily inTitan’s polar regions where the most important temperature variations are expected (though
limited to 2–4 K over the course of a year).
On board 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
GCMs, 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 whichsuggests that the solid surface surroundings the lakes and seas are saturated with liquid andbehave thermally like the liquids. They may as well experience evaporative cooling which
would explain the low measured temperatures in the north polar region and have important
implication 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 (seeSect. 6.1), the widespread observations of dunes on Titan attest to winds that have acted
over significant periods in the past, and indeed the possibility that aeolian landforms mightshed 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

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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 andIversen 1987; Lorenz et al. 1995; Lorenz 2014). This estimate is based on an assumptionthat interparticle cohesion is not too different from terrestrial sands: some laboratory measurements suggest they could be slightly larger (Burr et al. 2015) and it is possible or evenlikely that (methane/ethane) moisture (Yu et al. 2017) and/or electrostatic charging (Lorenz
2014; Méndez-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 (∼ 100 m tall) observed
by Cassini is substantial, of the order of 50000 years (Ewing et al. 2015; see also Lorenz
et al. 1995; Lorenz 2014) and thus not only does the presence of dunes require that thewinds have been above the saltation threshold for a substantial integrated period, but alsothat the dune pattern observed today retains some memory of winds extending into the pastby a substantial part or multiple of an astronomical (Croll-Milankovitch) climate cycle (seee.g. Aharonson et al. 2009; Lora et al. 2014). In particular, Ewing et al. (2015) noted thatsome 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 Radebaugh2009) 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, thesetypical 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 saltationthreshold 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 winddiversity 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 suggestthat there is a divergence of material transport in the equatorial regions, and a convergencein the mid-latitude regions around latitude 35°. This suggests a relationship exists between
two major land units on Titan: the longitudinal dunes and the undifferentiated plains, as
discussed in Sect. 5.2. It has been suggested (Rubin and Hesp 2009) that “sticky” sand may
yield longitudinal features, and laboratory results with organic material have shown thatelectrostatic charging may be significant for Titan organics under cryogenic, dry conditions(Méndez-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 wetness. Early Titan GCM studies (e.g. Rannou et al. 2006; Mitchell 2008) indicated that Titan’s low latitudes should be dried out by

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the general circulation, as a result of the meridional (Hadley) cells on this slowly rotatingworld. Thus, the dunes form a broad equatorial belt on Titan, whereas they form belts at
about 20 degrees 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 thatthe 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 only minimal variations with latitude.
Charnay and Lebonnois (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 ofmoisture 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 interpretationof 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 ∼ 1 K/km lapse rate may havea 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 activehydrologic cycle. Titan’s methane-based hydrologic cycle is an extreme analog to Earth’s
water cycle. 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 thirteenyear exploration of the Saturn system, Cassini has revealed that Titan’s hydrocarbon-basedhydrology is driven by nested methane cycles that operate over a range of timescales including geologic, orbital, seasonal, and that of a single convective storm. A fast physical (phasechange) 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. For a recent review of Titan’s hydrologic cycle, seeHayes 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

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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 sucha crucial tool. However, by the irreversible deposition of heavy hydrocarbons, nitriles, andother photochemical products from methane and nitrogen, the atmosphere also obscuresthe underlying surface geology to some extent. Were the current inventory of methane tocondense 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 solidand liquid deposits of organics on and within the ice shell (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 recordedon the surface. Observations relevant to its history include:(1) The low observed numbers of impact craters (Porco et al. 2005) yield an age of hundredsof 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 deep interior, a high-pressure ice layer of uncertain thickness, a liquid water ocean (Iess et al.
2010) perhaps with salts and ammonia (Mitri et al. 2014), and an ice shell 50 to 150 km
thick. The deep interior is either significantly hydrated (Castillo-Rogez and Lunine2010), 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 thata significant event, or change in the way Titan evolves, occurred sometime between a
few hundred million and a billion years ago (Hörst 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 in 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 to answer 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, ice shell, and atmosphere about
500 million years ago (Wood 2018). In the Tobie et al. (2006) model, Titan had a thin andrigid clathrate ice shell—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 methanefrom 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 shell to form underneath thebuoyant clathrate hydrate ice shell, with diapirism in the thickening ice I shell 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 (ice shell) the “Great Crustal Thickening
Event” and noted that the mode of geologic processes would change dramatically as Titantransitioned from a body with a thin rigid conductive ice shell over the ocean to one with athicker and rheologically heterogenous ice shell.

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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 deep interior (core) hydration (Tobie et al. 2014). How much this affects the surface evolution is unclear. While theidea is commonly held that the source of the methane to resupply the atmosphere is in crustal
clathrate hydrate (such a crust/ice shell was predicted in pre-Voyager days; Lewis 1971),
how the resupply works is unclear. Simple forcing out of the methane from the clathrateby the photochemically produced ethane eventually fails because of the stoichiometry (twomethane molecules making one ethane), although this replacement could eventually weighdown the ice shell and cause an overturn because clathrate with predominantly ethane isheavier 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 ofexternal 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 andnitrogen might have been preceded by one with a widespread ocean of methane and otherhydrocarbons. The longevity of such an ocean, in particular its decline, may be constrainedby the limits on tidal dissipation of the orbital eccentricity during ocean shrinkage (Saganand 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.
While major progress on understanding Titan was made by the Cassini mission, the limited opportunities for observations with all Cassini’s instruments, not just RADAR, have leftmany questions for future missions to answer. Cassini was a Saturn orbiter, and Titan flybys were limited and brief. Only about half of Titan’s surface was mapped with SAR, andcoverage in the near-infrared was similarly limited by observation time and spatial resolution. The outstanding questions after the Cassini-Huygens mission were discussed by Nixon
et al. (2018) as Titan’s “cold case files”. Among these are some related to topics discussed inthis paper, such as the age of Titan’s surface, the thickness of the ice shell, the depth of theinterior ocean, whether cryovolcanic features are present and whether cryovolcanism is still
occurring, the composition of the surface, whether there are or were lakes at mid-latitudes,
what is the variability of composition between the lakes and seas, and where lakes and seas
existed in the past. Titan after Cassini has also become a key world in which to study astrobiology and to investigate what complex chemistry, and possible biological activity, may be
happening in its surface and subsurface ocean.
Acknowledgements We thank Robert M. Nelson and an anonymous reviewer for excellent and detailed
reviews that greatly improved the manuscript, and Phil Callahan for an informal review and helpful suggestions for improvement. Part of this work was carried out at the Jet Propulsion Laboratory, California Instituteof technology, under contract with NASA. Copyright 2018, California Institute of Technology. Government
sponsorship is acknowledged.

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Publisher’s Note Springer Nature remains neutral with regard to jurisdictional claims in published maps
and institutional affiliations.

References
O. Aharonson, A.G. Hayes, J.I. Lunine, R.D. Lorenz, M.D. Allison, C. Elachi, An asymmetric distribution oflakes on Titan as a possible consequence of orbital forcing. Nat. Geosci. 2(12), 851 (2009)
B. Andreotti, A. Fourriere, F. Ould-Kaddour, B. Murray, P. Claudin, Giant aeolian dune size determined by
the average depth of the atmospheric boundary layer. Nature 457(7233), 1120 (2009)
N. Artemieva, J.I. Lunine, Numerical calculations of the longevity of impact oases on Titan. Icarus 173,
243–253 (2005)
J.W. Barnes, R.H. Brown, E.P. Turtle, A.S. McEwen, R.D. Lorenz, M. Janssen, E.L. Schaller, M.E. Brown,
B.J. Buratti, C. Sotin, C. Griffith, R. Clark, J. Perry, S. Fussner, J. Barbara, R. West, C. Elachi,
A.H. Bouchez, H.G. Roe, K.H. Baines, G. Bellucci, J-P. Bibring, F. Capaccioni, P. Cerroni, M. Combes,
A. Coradini, D.P. Cruikshank, P. Drossart, V. Formisano, R. Jaumann, Y. Langevin, D.L. Matson,
T.B. McCord, P.D. Nicholson, B. Sicardy, A 5-micron-bright spot on Titan: evidence for surface diversity. Science 310, 92–95 (2005). https://doi.org/10.1126/science.1117075
J.W. Barnes, R.H. Brown, J. Radebaugh, B.J. Buratti, C. Sotin, S. Le Mouelic, S. Rodriguez, E.P. Turtle,
J. Perry, R. Clark, K.H. Baines, P.D. Nicholson, Cassini observations of flow-like features in western
Tui Regio, Titan. Geophys. Res. Lett. 33, L16204 (2006). https://doi.org/10.1029/2006GL026843
J. Barnes, J. Radebaugh, R.H. Brown, S. Wall, L. Soderblom, J. Lunine, D. Burr, C. Sotin, S. Le Mouelic,
S. Rodriguez, B.J. Buratti, R. Clark, K.H. Baines, R. Jaumann, P.D. Nicholson, R.L. Kirk, R. Lopes,
R. Lorenz, K. Mitchell, C.A. Wood, Near-infrared spectral mapping of Titan’s mountains and channels.
J. Geophys. Res. 112, E11006 (2007a). https://doi.org/10.1029/2007JE002932
J.W. Barnes, R.H. Brown, L. Soderblom, B.J. Buratti, C. Sotin, S. Rodriguez et al., Global-scale surface
spectral variations on Titan seen from Cassini/VIMS. Icarus 186, 242–258 (2007b)
J.W. Barnes, R.H. Brown, L. Soderblom, C. Sotin, S. Le Mouelic, S. Rodriguez, R. Jaumann, R.A. Beyer,
R. Clark, P. Nicholson, Spectroscopy, morphometry, and photoclinometry of Titan’s dunefields from
Cassini/VIMS. Icarus 195, 400–414 (2008)
J.W. Barnes, J. Bow, J. Schwartz, R.H. Brown, J.M. Soderblom, A.G. Hayes, G. Vixie, S. Le Mouelic, S. Rodriguez, C. Sotin, R. Jaumann, K. Stephan, L.A. Soderblom, R.N. Clark, B.J. Buratti, K.H. Baines,
P.D. Nicholson, Organic sedimentary deposits in Titan’s dry lakebeds: probable evaporate. Icarus 216,
136–140 (2011a)
J.W. Barnes, J.M. Soderblom, R.H. Brown, Wave constraints for Titan’s Jingpo Lacus and Kraken Marefrom VIMS specular reflection lightcurves. Icarus 211, 722–731 (2011b). https://doi.org/10.1016/
j.icarus.2010.09.022
J.W. Barnes, B.J. Buratti, E.P. Turtle, J. Bow, P.A. Dalba, J. Perry, R.H. Brown, S. Rodriguez, S. Le Mouélic,K.H. Baines, C. Sotin, R.D. Lorenz, M.J. Malaska, T.B. McCord, R.N. Clark, R. Jaumann, P.O. Hayne,
P.D. Nicholson, J.M. Soderblom, L.A. Soderblom, Precipitation-induced surface brightenings seen on
Titan by Cassini VIMS and ISS. Planet. Sci. 2, 1 (2013). https://doi.org/10.1186/2191-2521-2-1
J.W. Barnes, C. Sotin, J.M. Soderblom et al., Cassini/VIMS observes rough surfaces on Titan’s Punga marein specular reflection. Planet. Sci. 3, 17 (2014). https://doi.org/10.1186/s13535-014-0003-4
C. Béghin, O. Randriamboarison, M. Hamelin, E. Karkoschka, C. Sotin, R.C. Whitten, J.-J. Berthelier,R. Grard, F. Simoes, Analytic theory of Titan’s Schumann resonance: constraints on ionospheric conductivity and buried water ocean. Icarus 218, 1028–1042 (2012)
J.-M. Bernard, E. Quirico, O. Brissaud, G. Montagnac, B. Reynard, B.P. McMillan, P. Coll, M.-J. Nguyen,
F. Raulin, B. Schmitt, Reflectance spectra and chemical structure of Titan’s tholins: application to theanalysis of Cassini Huygens observations. Icarus 185, 301–307 (2006)
B.G. Bills, F. Nimmo, Rotational dynamics and internal structure of Titan. Icarus 214(1), 351–355 (2011)
S.P.D. Birch, A.G. Hayes, P. Corlies, E.R. Stofan, J.D. Hofgartner, R.M.C. Lopes, R.D. Lorenz, J.I. Lunine,
S.M. MacKenzie, M.J. Malaska, C.A. Wood (the Cassini RADAR Team), Morphologic evidence that
Titan’s southern hemisphere basins are paleoseas. Icarus 310, 140–148 (2017). https://doi.org/10.1016/
j.icarus.2017.12.016
S.P.D. Birch, A.G. Hayes, J.D. Hofgartner, The raised rims of Titan’s small lakes, in 49th Lunar and Planetary
Science Conference, 19–23 March, 2018, The Woodlands, Texas. LPI Contribution, vol. 2083 (2018)
B.A. Black, J.T. Perron, D.M. Burr, S.A. Drummond, Estimating erosional exhumation on Titan fromdrainage network morphology. J. Geophys. Res. 117, E08006 (2012)
B.A. Black, J.T. Perron, D. Hemingway, E. Bailey, F. Nimmo, H. Zebker, Global drainage patterns and theorigins of topographic relief on Earth, Mars, and Titan. Science 356, 727–731 (2017)

Titan as Revealed by the Cassini Radar

Page 41 of 50 33

L.E. Bonnefoy, A.G. Hayes, P.O. Hayne, M.J. Malaska, A. Le Gall, A. Solomonidou, A. Lucas, Compositional and spatial variations in Titan dune and interdune regions from Cassini VIMS and RADAR. Icarus
270, 222–237 (2016)
C. Brassé, O. Munoz, P. Coll, F. Raulin, Optical constants of Titan aerosols and their tholins analogs: experimental results and modeling/observational data. Planet. Space Sci. 109–110, 159–174 (2015)
R.H. Brown, K.H. Baines, G. Bellucci, J.-P. Bibring, B.J. Buratti, F. Capaccioni et al., The Cassini Visual and Infrared Mapping Spectrometer (VIMS) investigation. Space Sci. Rev. 115, 111–168 (2004).
https://doi.org/10.1007/s11214-004-1453-x
R.H. Brown, L.A. Soderblom, J.M. Soderblom, R.N. Clark, R. Jaumann, J.W. Barnes, C. Sotin, B. Buratti,
K.H. Baines, P.D. Nicholson, The identification of liquid ethane in Titan’s Ontario Lacus. Nature 454,
607–610 (2008). https://doi.org/10.1038/nature07100
B.J. Buratti, C. Sotin, K. Lawrence, R.H. Brown, S. Le Mouélic, J.M. Soderblom et al., A newly discovered
impact crater in Titan’s Senkyo Cassini VIMS observations and comparison with other impact features.
Planet. Space Sci. 60, 18–25 (2012)
D.M. Burr, J.P. Emery, R.D. Lorenz, G.C. Collins, P.A. Carling, Sediment transport by liquid surficial flow:
application to Titan. Icarus 181, 235–242 (2006)
D.M. Burr, R.E. Jacobsen, D.L. Roth, C.B. Phillips, K.L. Mitchell, D. Viola, Fluvial network analysis on
Titan: evidence for subsurface structures and west-to-east wind flow, southwestern Xanadu. Geophys.
Res. Lett. 36, L22203 (2009)
D.M. Burr, S.A. Drummond, R. Cartwright, B.A. Black, J.T. Perron, Morphology of fluvial networks on
Titan: evidence for structural control. Icarus 226, 742–759 (2013)
D.M. Burr, N.T. Bridges, J.R. Marshall, J.K. Smith, B.R. White, J.P. Emery, Higher-than-predicted saltationthreshold wind speeds on Titan. Nature 517(7532), 60 (2015)
D.B. Campbell, G.J. Black, L.M. Carter, S.J. Ostro, Radar evidence for liquid surfaces on Titan. Science
302(5644), 431–434 (2003)
R. Cartwright, J.A. Clayton, R.L. Kirk, Channel morphometry, sediment transport, and implications for tectonic activity and surficial ages of Titan basins. Icarus 214(2), 561–570 (2011)
J.C. Castillo-Rogez, J.I. Lunine, Evolution of Titan’s rocky core constrained by Cassini observations. Geophys. Res. Lett. 37, L20205 (2010). https://doi.org/10.1029/2010GL044398
B. Charnay, S. Lebonnois, Two boundary layers in Titan’s lower troposphere inferred from a climate model.
Nat. Geosci. 5(2), 106–109 (2012)
B. Charnay, F. Forget, G. Tobie, C. Sotin, R. Wordsworth, Titan’s past and future: 3D modeling of a purenitrogen atmosphere and geological implications. Icarus 241, 269–279 (2014)
B. Charnay, E. Barth, S. Rafkin, C. Narteau, S. Lebonnois, S. Rodriguez, S.C. Du Pont, A. Lucas, Methanestorms as a driver of Titan’s dune orientation. Nat. Geosci. 8(5), 362 (2015)
M. Choukroun, S. Sotin, Is Titan’s shape caused by its meteorology and carbon cycle? Geophys. Res. Lett.
39, L04201 (2012). https://doi.org/10.1029/2011GL050747
R.N. Clark, J.M. Curchin, J.W. Barnes, R. Jaumann, L. Soderblom, D.P. Cruikshank, R.H. Brown, S. Rodriguez, J. Lunine, K. Stephan, T.M. Hoefen, Detection and mapping of hydrocarbon deposits on Titan.
J. Geophys. Res., Planets 115(E10), E10005 (2010). https://doi.org/10.1029/2009JE003369
G.C. Collins, Relative rates of flucial bedrock incision on Titan and Earth. Geophys. Res. Lett. 32(22), L22202(2005)
C. Cook-Hallett, J.W. Barnes, S.A. Kattenhorn, T. Hurford, J. Radebaugh, B. Stiles, M. Beuthe, Global contraction/expansion and polar lithospheric thinning on Titan from patterns of tectonism. J. Geophys. Res.,
Planets 120(6), 1220–1236 (2015)
D. Cordier, O. Mousis, J.I. Lunine, P. Lavvas, V. Vuitton, An estimate of the chemical composition of Titan’slakes. Astrophys. J. 707, L128–L131 (2009). https://doi.org/10.1088/0004-637X/707/2/L128
D. Cordier, O. Mousis, J.I. Lunine, S. Lebonnois, P. Rannou, P. Lavvas, L.Q. Lobo, A.G.M. Ferreira, Titan’slakes chemical composition: sources of uncertainties and variability. Planet. Space Sci. 61(1), 99–107(2012)
D. Cordier, F. García-Sánchez, D.N. Justo-García, G. Liger-Belair, Bubble streams in Titan’s seas a productof liquid N2 + CH4 + C2 H6 cryogenic mixture. Nat. Astron. 1, 1–4 (2017). https://doi.org/10.1038/
s41550-017-0102
P. Corlies et al., Titan’s topography and shape at the end of the Cassini mission. Geophys. Res. Lett. 44(23),11754–11761 (2017)
T. Cornet, O. Bourgeois, S. Le Mouélic, S. Rodriguez, T. Lopez Gonzalez, C. Sotin, G. Tobie, C. Fleurant,
J.W. Barnes, R.H. Brown, Geomorphological significance of Ontario Lacus on Titan: integrated interpretation of Cassini VIMS, ISS and RADAR data and comparison with the Etosha Pan (Namibia). Icarus
218(2), 788–806 (2012)
T. Cornet, D. Cordier, T. Le Bahers, O. Bourgeois, C. Fleurant, S. Le Mouélic, N. Altobelli, Dissolution
on Titan and on Earth: toward the age of Titan’s karstic landscapes. J. Geophys. Res., Planets 120,
1044–1074 (2015). https://doi.org/10.1002/2014JE004738

33

Page 42 of 50

R.M.C. Lopes et al.

T. Cornet, C. Fleurant, B. Signovert, D. Cordier, O. Bourgois, S. Le Mouélic, S. Rodriguez, A. Lucas, Landscape formation through dissolution on Titan: a 3D landscape evolution model, in Lunar and Planetary
Science 2017 (2017). Abstract 1835
V. Cottini, C.A. Nixon, D.E. Jennings, R. de Kok, N.A. Teanby, P.G.J. Irwin, F.M. Flasar, Spatial and temporalvariations in Titan’s surface temperatures from Cassini CIRS observations. Planet. Space Sci. 60(1), 62–
71 (2012). https://doi.org/10.1016/j.pss.2011.03.015
S. Courrech du Pont, C. Narteau, X. Gao, Two modes for dune orientation. Geology (2014). https://doi.org/
10.1130/G35657.1
A. Coustenis, E. Lellouch, R. Wittemberg, J.-P. Maillard, C.P. Mckay, Modelling Titan’s surface from near-IR
FTS/CFHT1995–1996 spectra, in 22nd General Assembly of the European Geophysical Society, 21–25
April, Vienne (1997)
M.E. Davies, T.R. Colvin, P.G. Rogers, P.W. Chodas, W.L. Sjogren, E.L. Akim, V.A. Stepaniants,
Z.P. Vlasova, A.I. Zakharov, The rotation period, direction of the North Pole, and geodetic control
network of Venus. J. Geophys. Res. 97(E8), 13141–13151 (1992)
C. Elachi, M.D. Allison, L. Borganelli, P. Encrenaz, E. Im, M.A. Janssen, W.T.K. Johnson, R.L. Kirk,
R.D. Lorenz, J.I. Lunine, D.O. Muhleman, S.J. Ostro, G. Picardi, F. Posa, C.G. Rapley, L.E. Roth,S. Seu, L.A. Soderblom, S. Vetrella, S.D. Wall, C.A. Wood, H.A. Zebker, Radar: The Cassini Titan
Radar Mapper. Space Sci. Rev. 115, 71–110 (2004)
E. Elachi et al., Cassini Radar views the surface of Titan. Science 308, 970–974 (2005)
R.C. Ewing, A.G. Hayes, A. Lucas, Sand dune patterns on Titan controlled by long-term climate cycles. Nat.
Geosci. 8(1), 15–19 (2015)
S.G. Fryberger, G. Dean, Dune forms and wind regime, in A Study of Global Sand Seas, ed. by E.D. McKee.
U.S. Geol. Surv. Prof. Pap., vol. 1052 (1979), pp. 137–169
C.R. Glein, E.L. Shock, A geochemical model of non-ideal solutions in the methane-ethane-propanenitrogen-actylene system on Titan. Geochim. Cosmochim. Acta 115, 217–240 (2013). https://doi.org/
10.1016/j.gca.2013.03.030
R. Greeley, J.D. Iversen, Wind as a Geological Process: On Earth, Mars, Venus and Titan, vol. 4 (Cambridge
University Press, Cambridge, 1987)
C.A. Griffith, L. Doose, M.G. Tomasko, P.F. Penteado, C. See, Radiative transfer analyses of Titan’s tropicalatmosphere. Icarus 218(2), 975–988 (2012)
C. Grima, M. Mastrogiuseppe, A.G. Hayes, S.D. Wall, R.D. Lorenz, J.D. Hofgartner, B.C. Stiles, C. Elachi(The Cassini RADAR Team), Surface roughness of Titan’s hydrocarbon seas. Earth Planet. Sci. Lett.
474, 20–24 (2017)
A.G. Hayes, The lakes and seas of Titan. Annu. Rev. Earth Planet. Sci. 44, 57–83 (2016)
A. Hayes, O. Aharonson, P. Callahan, C. Elachi, Y. Gim, R. Kirk, K. Lewis, R. Lopes, R. Lorenz, J. Lunine etal., Hydrocarbon lakes on Titan: distribution and interaction with a porous regolith. Geophys. Res. Lett.
35(9), L09204 (2008)
A.G. Hayes, A.S. Wolf, O. Aharonson, H. Zebker, R. Lorenz, R.L. Kirk, P. Paillou, J. Lunine, L. Wye,
P. Callahan et al., Bathymetry and absorptivity of Titan’s Ontario Lacus. J. Geophys. Res. 115(E9),E09009 (2010)
A.G. Hayes, O. Aharonson, J.I. Lunine, R.L. Kirk, H.A. Zebker, L.C. Wye, R.D. Lorenz, E.P. Turtle, P. Paillou, G. Mitri et al., Transient surface liquid in Titan’s polar regions from Cassini. Icarus 211(1), 655–671(2011)
A.G. Hayes, R.D. Lorenz, M.A. Donelan et al., Wind driven capillary-gravity waves on Titan’s lakes: hard to
detect or non-existent? Icarus 225, 403–412 (2013). https://doi.org/10.1016/j.icarus.2013.04.004
A.G. Hayes, S.P.D. Birch, W.E. Dietrich, A.D. Howard, R.L. Kirk, V. Poggiali, M. Mastrogiuseppe,
R.J. Michaelides, P.M. Corlies, J.M. Moore et al., Topographic constraints on the evolution and connectivity of Titan’s lacustrine basins. Geophys. Res. Lett. 44(23), 11,745–11,753 (2017)
A.G. Hayes, R.D. Lorenz, J.I. Lunine, A post-Cassini view of Titan’s methane-based hydrologic cycle. Nat.
Geosci. 11, 306–313 (2018)
D.J. Hemingway, F. Nimmo, H. Zebker, L. Iess, A rigid and weathered ice shell on Titan. Nature 500, 550–552 (2013)
D.J. Hemingway, L. Iess, R. Tajeddine, G. Tobie, The interior of Enceladus, in Enceladus and the Icy Moonsof Saturn, ed. by P.M. Schenk, R.N. Clark, C.J.A. Howett, A.J. Verbiscer (University of Arizona Press,
Tucson, 2018), pp. 57–77
M. Hirtzig, B. Bézard, E. Lellouch, A. Coustenis, C. de Bergh, P. Drossart, A. Campargue, V. Boudon,
V. Tyuterev, P. Rannou, T. Cours, S. Kassi, A. Nikitin, D. Mondelain, S. Rodriguez, S. Le Mouélic,Titan’s surface and atmosphere from Cassini/VIMS data with updated methane opacity. Icarus 226,
470–486 (2013)
J.D. Hofgartner, A.G. Hayes, J.I. Lunine et al., Transient features in a Titan sea. Nat. Geosci. 7, 493–496(2014). https://doi.org/10.1038/ngeo2190

Titan as Revealed by the Cassini Radar

Page 43 of 50 33

J.D. Hofgartner, A.G. Hayes, J.I. Lunine et al., Titan’s “Magic Islands”: transient features in a hydrocarbon
sea. Icarus 271, 338–349 (2016)
S. Hörst, Titan’s atmosphere and climate. J. Geophys. Res., Planets 122, 432–482 (2017). https://doi.org/
10.1002/2016JE005240
M. Howard, S. Bastea, L.E. Fried, B. Khare, C.P. McKay, Titans’ interior chemical composition: a thermo
chemical assessment, in American Astronomical Society, DPS Meeting, vol. 41 (2009)
A.D. Howard, S. Breton, J.M. Moore, Formation of gravel pavements during fluvial erosion as an explanation for persistence of ancient cratered terrain on Titan and Mars. Icarus 270, 100–113 (2016).
https://doi.org/10.1016/j.icarus.2015.05.034
L. Iess et al., Gravity field, shape, and moment of inertia of Titan. Science 327, 1367 (2010). https://doi.org/
10.1126/science.1182583
L. Iess et al., The tides of Titan. Science 337, 457 (2012). https://doi.org/10.1126/science.1219631
M.A. Janssen, R.D. Lorenz, R. West, F. Paganelli, R.M. Lopes, R.L. Kirk, C. Elachi, S.D. Wall, W.T.K. Johnson, Y. Anderson, R.A. Boehmer, P. Callahan, Y. Gim, G.A. Hamilton, K.D. Kelleher, L. Roth,
B. Stiles, A. Le Gall (the Cassini Radar Team), Titan’s surface at 2.2-cm wavelength imaged by the
Cassini RADAR Radiometer: calibration and first results. Icarus 200, 222–239 (2009). https://doi.org/
10.1016/j.icarus.2008.10.017
M.A. Janssen, A. Le Gall, L.C. Wye, Anomalous radar backscatter from Titan’s surface? Icarus 212(1), 321–
328 (2011)
M.A. Janssen, A. Le Gall, R.M. Lopes, R.D. Lorenz, M.J. Malaska, A.G. Hayes, C.D. Neish, A. Solomonidou,
K.L. Mitchell, J. Radebaugh, S.J. Keihm, Titan’s surface at 2.18-cm wavelength imaged by the Cassini
RADAR radiometer: results and interpretations through the first ten years of observation. Icarus 270,
443–459 (2016)
R. Jaumann, R.H. Brown, K. Stephan, J.W. Barnes, L.A. Soderblom, C. Sotin, S. Le Mouélic, R.N. Clark,
J. Soderblom, B.J. Buratti et al., Fluvial erosion and post-erosional processes on Titan. Icarus 197(2),526–538 (2008)
D.E. Jennings et al., Titan’s surface brightness temperatures. Astrophys. J. 691, L103–L105 (2009)
D.E. Jennings et al., Surface temperatures on Titan during Northern winter and spring. Astrophys. J. Lett.
816, L17 (2016)
E. Karkoschka, A. McEwen, J. Perry, Creating the Best Global Mosaic of Titan’s Surface Albedo Using
Cassini Images. AAS/Division for Planetary Sciences Meeting Abstract #49 (2017)
R.L. Kirk, E. Howington-Kraus, A.G. Hayes, R.M.C. Lopes, R.D. Lorenz, J.I. Lunine, K.L. Mitchell,
E.R. Stofan, S.D. Wall, La Sotra y los otros: topographic evidence for (and against) cryovolcanism
on Titan, in Eos Trans. AGU, 91(52), Fall Meet. Suppl. (2010). Abstract P22A-03
D.G. Korycansky, K.J. Zahnle, Modeling crater populations on Venus and Titan. Planet. Space Sci. 53, 695–
710 (2005)
M.H. Langhans, R. Jaumann, K. Stephan, R.H. Brown, B.J. Buratti, R.N. Clark, K.H. Baines, P.D. Nicholson, R.D. Lorenz, L.A. Soderblom et al., Titan’s fluvial valleys: morphology, distribution, and spectral
properties. Planet. Space Sci. 60(1), 34–51 (2012)
A. Le Gall, M.A. Janssen, P. Paillou, R.D. Lorenz, S.D. Wall, Radar-bright channels on Titan. Icarus 207,
948–958 (2010)
A. Le Gall, M.A. Janssen, L.C. Wye, A.G. Hayes, J. Radebaugh, C. Savage, H. Zebker, R.D. Lorenz, J.I. Lunine, R.L. Kirk, R.M.C. Lopes, S. Wall, P. Callahan, E.R. Stofan, T. Farr (the Cassini Radar Team),
Cassini SAR, radiometry, scatterometry and altimetry observations of Titan’s dune fields. Icarus 213,
608–624 (2011)
A. Le Gall, A.G. Hayes, R. Ewing, M.A. Janssen, J. Radebaugh, C. Savage, P. Encrenaz (the Cassini Radar
Team), Latitudinal and altitudinal controls on Titan’s dune field morphometry. Icarus 217, 231–242(2012)
A. Le Gall, M.J. Malaska, R.D. Lorenz, M.A. Janssen, T. Tokano, A.G. Hayes, M. Mastrogiuseppe, J.I. Lunine, G. Veyssière, P. Encrenaz, O. Karatekin, Composition, seasonal change and bathymetry of Ligeia
Mare, Titan, derived from its microwave thermal emission. J. Geophys. Res., Planets 121, 233–251(2016)
S. Le Mouelic, P. Paillou, M.A. Janssen, J.W. Barnes, S. Rodriguez, C. Sotin, R.H. Brown, K.H. Baines,
B.J. Buratti, R.N. Clark, M. Crapeau, P.J. Encrenaz, R. Jaumann, D. Geudtner, F. Paganelli,
L. Soderblom, G. Tobie, S. Wall, Mapping and interpretation of Sinlap crater on Titan using Cassini
VIMS and RADAR data. J. Geophys. Res. 113, E04003 (2008). https://doi.org/10.1029/2007JE002965
E. Lellouch, Titan’s zoo of clouds. Science 311, 186–187 (2006)
M.T. Lemmon, E. Karkoschka, M. Tomasko, Titan’s rotation—surface feature observed. Icarus 103(2), 329–
332 (1993)
J.S. Lewis, Satellites of the outer planets: their physical and chemical nature. Icarus 15, 174–185 (1971)

33

Page 44 of 50

R.M.C. Lopes et al.

Z.Y-C. Liu, J. Radebaugh, R.A. Harris, E.H. Christiansen, C.D. Neish, R.L. Kirk, R.D. Lorenz (the Cassini
RADAR Team), The tectonics of Titan: global structural mapping from Cassini RADAR. Icarus 270,
14–29 (2016a)
Z.Y.-C. Liu, J. Radebaugh, R.A. Harris, E.H. Christiansen, S. Rupper, Role of fluids in the tectonic evolutionof Titan. Icarus 270, 2–13 (2016b)
R.M.C. Lopes, K.L. Mitchell, E.R. Stofan, J.I. Lunine, R. Lorenz, F. Paganelli, R.L. Kirk, C.A. Wood,
S.D. Wall, L.E. Robshaw, A.D. Fortes, C.D. Neish, J. Radebaugh, E. Reffet, S.J. Ostro, C. Elachi,
M.D. Allison, Y. Anderson, R. Boehmer, G. Boubin, P. Callahan, P. Encrenaz, E. Flamini,
G. Francescetti, Y. Gim, G. Hamilton, S. Hensley, M.A. Janssen, W.T.K. Johnson, K. Kelleher,
D.O. Muhleman, G. Ori, R. Orosei, G. Picardi, F. Posa, L.E. Roth, R. Seu, S. Shaffer, L.A. Soderblom,
B. Stiles, S. Vetrella, R.D. West, L. Wye, H.A. Zebker, Cryovolcanic features on Titan’s surface asrevealed by the Cassini Titan Radar Mapper. Icarus 186, 395–412 (2007)
R.M.C. Lopes, E.R. Stofan, R. Peckyno, J. Radebaugh, K.L. Mitchell, G. Mitri, C.A. Wood, R.L. Kirk,
S.D. Wall, J.I. Lunine, A. Hayes, R. Lorenz, T. Farr, L. Wye, J. Craig, R.J. Ollerenshaw, M. Janssen,
A. LeGall, F. Paganelli, R. West, B. Stiles, P. Callahan, Y. Anderson, P. Valora, L. Soderblom (the Cassini
RADAR Team), Distribution and interplay of geologic processes on Titan from Cassini RADAR data.
Icarus 205, 540–588 (2010). https://doi.org/10.1016/j.icarus.2009.08.010
R.M.C. Lopes, S.A. Fagents, K.L. Mitchell, T.K.P. Gregg, Planetary volcanism, in Modeling Volcanic Processes, ed. by S.A. Fagents, T.K.P. Gregg, R.M.C. Lopes (Cambridge University Press, Cambridge,
2013a), pp. 384–413
R.M.C. Lopes, R.L. Kirk, K.L. Mitchell, A. LeGall, J.W. Barnes, A. Hayes, J. Kargel, L. Wye, J. Radebaugh,
E.R. Stofan, M. Janssen, C. Neish, S. Wall, C.A. Wood, J.I. Lunine, M. Malaska, Cryovolcanism on
Titan: new results from Cassini RADAR and VIMS. J. Geophys. Res., Planets 118, 1–20 (2013b).
https://doi.org/10.1002/jgre.20062
R.M.C. Lopes, M.J. Malaska, A. Solomonidou, A. LeGall, M.A. Janssen, C. Neish, E.P. Turtle, S.P.D. Birch,
A.G. Hayes, J. Radebaugh, A. Coustenis, A. Schoenfeld, B.W. Stiles, R.L. Kirk, K.L. Mitchell, E.R. Stofan, K.J. Lawrence (the Cassini RADAR Team), Nature, distribution, and origin of Titan’s undifferentiated plains (“blandlands”). Icarus 270, 162–182 (2016)
J.M. Lora, J.I. Mitchell, Titan’s asymmetric lake distribution mediated by methane transport due to atmospheric eddies. Geophys. Res. Lett. 42(15), 6213–6220 (2015)
J.M. Lora, J.I. Lunine, J.L. Russell, A.G. Hayes, Simulations of Titan’s paleoclimate. Icarus 243, 264–273(2014)
R.D. Lorenz, The surface of Titan in the context of ESA’s Huygens probe. ESA J. 17, 275–292 (1993)
R.D. Lorenz, Pillow lava on Titan: expectations and constraints on cryovolcanic processes. Planet. Space Sci.
44(9), 1021–1028 (1996)
R.D. Lorenz, Physics of saltation and sand transport on Titan: a brief review. Icarus 230, 162–167 (2014)
R.D. Lorenz, A.G. Hayes, The growth of wind-waves in Titan’s hydrocarbon seas. Icarus 219(1), 468–475(2012). https://doi.org/10.1016/j.icarus.2012.03.002
R.D. Lorenz, J.I. Lunine, Erosion on Titan: past and present. Icarus 196, 79–91 (1996)
R.D. Lorenz, J.I. Lunine, Titan’s surface before Cassini. Planet. Space Sci. 53, 557–576 (2005)
R.D. Lorenz, J. Radebaugh, Global pattern of Titan’s dunes: radar survey from the Cassini prime mission.
Geophys. Res. Lett. 36(3), L03202 (2009)
R.D. Lorenz, J.R. Zimbelman, Dune Worlds: How Windblown Sand Shapes Planetary Landscapes (Springer,
Berlin, 2014)
R.D. Lorenz, J.I. Lunine, J.A. Grier, M.A. Fisher, Prediction of aeolian features on planets: application to
Titan paleoclimatology. J. Geophys. Res., Planets 100(E12), 26377–26386 (1995)
R.D. Lorenz, C.P. McKay, J.I. Lunine, Photochemically driven collapse of Titan’s atmosphere. Science 275,
642–644 (1997)
R.D. Lorenz, G. Biolluz, P. Encrenaz, M.A. Janssen, R.D. West, D.O. Muhleman, Cassini RADAR: prospects
for Titan surface investigations using the microwave radiometer. Planet. Space Sci. 51(4–5), 353–364(2003)
R.D. Lorenz, C.A. Griffith, J.I. Lunine, C.P. McKay, N.O. Rennò, Convective plumes and the scarcity ofTitan’s clouds. Geophys. Res. Lett. 32(1), L01201 (2005)
R.D. Lorenz, S. Wall, J. Radebaugh, G. Boubin, E. Reffet, M. Janssen, E. Stofan, R. Lopes, R. Kirk,
C. Elachi, J. Lunine, The sand seas of Titan: Cassini RADAR observations of longitudinal dunes. Science 312(5774), 724–727 (2006)
R.D. Lorenz, C.A. Wood, J.I. Lunine, S.D. Wall, R.M. Lopes, K.L. Mitchell, F. Paganelli, Y.Z. Anderson,
L. Wye, C. Tsai, H. Zebker, E.R. Stofan, Titan’s young surface: initial impact crater survey by Cassini
RADAR and model comparison. Geophys. Res. Lett. 34, L07204 (2007)
R.D. Lorenz, B. Stiles, R.L. Kirk, M. Allison, P. Persi del Marmo, L. Iess, J.I. Lunine, S.J. Ostro, S. Hensley,
Titan’s rotation reveals an internal ocean and changing zonal winds. Science 319, 1649–1651 (2008a)

Titan as Revealed by the Cassini Radar

Page 45 of 50 33

R.D. Lorenz, K.L. Mitchell, R.L. Kirk, A.G. Hayes, H.A. Zebker, P. Paillou, J. Radebaugh, J.I. Lunine,
M.A. Janssen, S.D. Wall, R.M. Lopes, B. Stiles, S. Ostro, G. Mitri, E.R. Stofan (the Cassini RADAR
Team), Titan’s inventory of organic surface materials. Geophys. Res. Lett. 35, L02206 (2008b). https://doi.org/10.1029/2007GL032118
R.D. Lorenz, R.M. Lopes, F. Paganelli, J.I. Lunine, R.L. Kirk, K.L. Mitchell, L.A. Soderblom, E.R. Stofan,
G. Ori, M. Myers, H. Miyamoto, J. Radebaugh, B. Stiles, S.D. Wall, C.A. Wood (the Cassini RADAR
Team), Fluvial channels on Titan: initial Cassini RADAR observations. Planet. Space Sci. 56, 1132–
1144 (2008c)
R.D. Lorenz, P. Claudin, B. Andreotti, J. Radebaugh, T. Tokano, A 3 km atmospheric boundary layer on
Titan indicated by dune spacing and Huygens data. Icarus 205, 719–721 (2010). https://doi.org/10.1016/
j.icarus.2009.08.002
R.D. Lorenz et al., A global topographic map of Titan. Icarus 255, 367–377 (2013). https://doi.org/10.1016/
j.icarus.2013.04.002
A. Lucas, S. Rodriguez, C. Narteau, B. Charnay, S.C. Pont, T. Tokano, A. Garcia, M. Thiriet, A.G. Hayes,
R.D. Lorenz, O. Aharonson, Growth mechanisms and dune orientation on Titan. Geophys. Res. Lett.
41(17), 6093–6100 (2014). https://doi.org/10.1002/2014GL060971
J.I. Lunine, R.D. Lorenz, Rivers, lakes, dunes, and rain: crustal processes in Titan’s methane cycle. Annu.
Rev. Earth Planet. Sci. 37(1), 299–320 (2009)
J.I. Lunine, R.D. Lorenz, W.K. Hartmann, Some speculations on Titans past, present and future. Planet. Space
Sci. 46(9–10), 1099–1107 (1998)
J.I. Lunine, C. Elachi, S.D. Wall, M.A. Janssen, M.D. Allison, Y. Anderson et al., Titan’s diverse landscapes
as evidenced by Cassini RADAR’s third and fourth looks. Icarus 195, 415–433 (2008)
S.M. MacKenzie, J.W. Barnes, C. Sotin, J.M. Soderblom, S. Le Mouélic, S. Rodriguez, K.H. Baines, B.J. Buratti, R.N. Clark, P.D. Nicholson, T.B. McCord, Evidence of Titan’s climate history from evaporite distribution. Icarus 243, 191–207 (2014)
M.J. Malaska, R. Hodyss, Dissolution of benzene, naphthalene, and biphenyl in a simulated Titan lake. Icarus
242, 74–81 (2014). https://doi.org/10.1016/j.icarus.2014.07.022
M. Malaska, J. Radebaugh, R. Lorenz, K. Mitchell, T. Farr, E. Stofan, Identification of karst-like terrain on
Titan from valley analysis, in Lunar and Planetary Science Conference, vol. 41 (2010). Abstract 1544
M. Malaska, J. Radebaugh, K. Mitchell, R. Lopes, S. Wall, R. Lorenz, Surface dissolution model for Titan
karst, in First International Planetary Cave Research Workshop, October 2011 (2011a). Abstract 8018
M. Malaska, J. Radebaugh, A. Le Gall, K. Mitchell, R. Lopes, S. Wall, High-volume meandering channels in
Titan’s south polar region, in Lunar and Planetary Science Conference (2011b). Abstract 1562
M.J. Malaska, R.M. Lopes, A.G. Hayes, J. Radebaugh, R.D. Lorenz, E.P. Turtle, Material transport map ofTitan: the fate of dunes. Icarus 270, 183–196 (2016a). https://doi.org/10.1016/j.icarus.2015.09.029
M.J. Malaska, R.M.C. Lopes, D.A. Williams, C.D. Neish, A. Solominidou, J.M. Soderblom, A.M. Schoenfeld, S.P.D. Birch, A.G. Hayes, A. Le Gall, M.A. Janssen, T.G. Farr, R.D. Lorenz, J. Radebaugh, E.P. Turtle, Geomorphological map of the Afekan Crater region, Titan: terrain relationships
in the equatorial and mid-latitude regions. Icarus 270, 130–161 (2016b). https://doi.org/10.1016/
j.icarus.2016.02.021
M.J. Malaska, R. Hodyss, J.I. Lunine, A.G. Hayes, J.D. Hofgartner, G. Hollyday, R.D. Lorenz, Laboratory
measurements of nitrogen dissolution in Titan lake fluids. Icarus 289, 94–105 (2017a). https://doi.org/
10.1016/j.icarus.2017.01.033
M.J. Malaska, R.M.C. Lopes, K.L. Mitchell, J. Radebaugh, T. Verlander, A. Schoenfeld, Classification oflabyrinth terrains on Titan, in Lunar and Planetary Science Conference (2017b). Abstract 2406
M. Mastrogiuseppe, V. Poggiali, A. Hayes, R. Lorenz, J. Lunine, G. Picardi, R. Seu, E. Flamini, G. Mitri,
C. Notarnicola, Ph. Paillou, H. Zebker, The bathymetry of a Titan sea. Geophys. Res. Lett. 41(5), 1432–
1437 (2014)
M. Mastrogiuseppe, A. Hayes, V. Poggiali, R. Seu, J.I. Lunine, J.D. Hofgartner, Radar sounding using the
Cassini altimeter: waveform modeling and Monte Carlo approach for data inversion of observations ofTitan’s seas. IEEE Trans. Geosci. Remote Sens. 54(10), 5646–5656 (2016)
M. Mastrogiuseppe, A.G. Hayes, V. Poggiali, J.I. Lunine, R.D. Lorenz, R. Seu, A. Le Gall, C. Notarnicola,
K.L. Mitchell, M. Malaska, S.P.D. Birch, Bathymetry and composition of Titan’s Ontario Lacus derived
from Monte Carlo-based waveform inversion of Cassini RADAR altimetry data. Icarus 300, 203–209(2018)
T.B. McCord, G.B. Hansen, B.J. Buratti, R.N. Clark, D.P. Cruikshank, E. D’Aversa, C.A. Griffith,
E.K.H. Baines, R.H. Brow, C.M. Dalle Ore, G. Filacchione, V. Formisano, C.A. Hibbitts, R. Jaumann,
J.I. Lunine, R.M. Nelson, C. Sotin (the Cassini VIMS Team), Composition of Titan’s surface from
Cassini VIMS. Planet. Space Sci. 54, 1524–1539 (2006). https://doi.org/10.1016/j.pss.2006.06.007
T.B. McCord, P. Hayne, J.-P. Combe, G.B. Hansen, J. Barnes, S. Rodriguez, S. Le Mouelic, K. Baines, B. Buratti, C. Sotin, P. Nicholson, R. Jaumann, R. Nelson (Cassini VIMS Team), Titan’s surface: search forspectral diversity and composition using the Cassini VIMS investigation. Icarus 194, 212–242 (2008)

33

Page 46 of 50

R.M.C. Lopes et al.

G.D. McDonald, A.G. Hayes, R.C. Ewing, J.M. Lora, C.E. Newman, T. Tokano, A. Lucas, A. Soto, G. Chen,
Variations in Titan’s dune orientations as a result of orbital forcing. Icarus 270, 197–210 (2016)
J. Méndez-Harper, G.D. McDonald, J. Dufek, M.J. Malaska, D.M. Burr, A.G. Hayes, J. McAdams, J.J. Wray,
The electrified sands of Titan. Nat. Geosci. 10, 260–265 (2017). https://doi.org/10.1038/ngeo2921
R. Meriggiola, L. Iess, B.W. Stiles, J.I. Lunine, G. Mitri, The rotational dynamics of Titan from Cassini
RADAR images. Icarus 275, 183–192 (2016)
J.L. Mitchell, The drying of Titan’s dunes: Titan’s methane hydrology and its impact on atmospheric circulation. J. Geophys. Res., Planets 113(E8), E08015 (2008)
J.L. Mitchell, J.M. Lora, The climate of Titan. Ann. Rev. Earth Planet. Sci. 44, 353–380 (2016). https://doi.org/10.1146/annurev-earth-060115-012428
K.L. Mitchell, M.B. Barmatz, C.S. Jamieson, R.D. Lorenz, J.I. Lunine, Laboratory measurements of cryogenic liquid alkane microwave absorptivity and implications for the composition of Ligeia Mare, Titan.
Geophys. Res. Lett. 42, 1340–1345 (2015)
G. Mitri, M.T. Bland, A.P. Showman, J. Radebaugh, B. Stiles, R.M.C. Lopes, J.I. Lunine, R.T. Pappalardo,
Mountains on Titan: modeling and observations. J. Geophys. Res. 115, E10002 (2010). https://doi.org/
10.1029/2010JE003592
G. Mitri et al., Shape, topography, gravity anomalies and tidal deformation of Titan. Icarus 236, 169–177(2014)
J.M. Moore, R.T. Pappalardo, Titan: an exogenic world? Icarus 212, 790–806 (2011)
J.M. Moore, A.D. Howard, A.M. Morgan, The landscape of Titan as witness to its climate evolution. J. Geophys. Res., Planets 119, 2060–2077 (2014). https://doi.org/10.1002/2014JE004608
C.D. Neish, R.D. Lorenz, Titan’s global crater population: a new assessment. Planet. Space Sci. 60(1), 26–33(2012)
C.D. Neish, R.D. Lorenz, Elevation distribution of Titan’s craters suggests extensive wetlands. Icarus 228,
27–34 (2014)
C.D. Neish, R.D. Lorenz, R.L. Kirk, L.C. Wye, Radarclinometry of the sand seas of Africa’s Namibia andSaturn’s moon Titan. Icarus 208, 385–394 (2010)
C.D. Neish, R.L. Kirk, R.D. Lorenz, V.J. Bray, P. Schenk, B.W. Stiles et al., Crater topography on Titan:
implications for landscape evolution. Icarus 223, 82–90 (2013)
C.D. Neish, J.W. Barnes, C. Sotin, S. MacKenzie, J.M. Soderblom, S. Le Mouélic et al., Spectral properties
of Titan’s impact craters imply chemical weathering of its surface. Geophys. Res. Lett. 42, 3746–3754(2015)
C.D. Neish, J.L. Molaro, J.M. Lora, A.D. Howard, R.L. Kirk, P. Schenk et al., Fluvial erosion as a mechanism
for crater modification on Titan. Icarus 270, 114–129 (2016)
R.M. Nelson, L.W. Kamp, D.L. Matson, P.G.J. Irwin, K.H. Baines, M.D. Boryta, F.E. Leader, R. Jaumann,
W.D. Smythe, C. Sotin, R.N. Clark, D.P. Cruikshank, P. Drossart, J.C. Pearl, B.W. Hapke, J. Lunine,
M. Combes, G. Bellucci, J.-P. Bibring, F. Capaccioni, P. Cerroni, A. Coradini, V. Formisano, G. Filacchione, R.Y. Langevin, T.B. McCordo, V. Mennella, P.D. Nicholson, B. Sicardy, Saturn’s Titan: surfacechange, ammonia, and implications for atmospheric and tectonic activity. Icarus 199, 429–441 (2009a).
https://doi.org/10.1016/j.icarus.2008.08.013
R.M. Nelson, L.W. Kamp, R.M.C. Lopes, D.L. Matson, R.L. Kirk, B.W. Hapke, S.D. Wall, M.D. Boryta,
F.E. Leader, W.D. Smythe, K.L. Mitchell, K.H. Baines, R. Jaumann, C. Sotin, R.N. Clark, D.P. Cruikshank, P. Drossart, J.I. Lunine, M. Combes, G. Bellucci, J-P. Bibring, F. Capaccioni, P. Cerroni, A. Coradini, V. Formisano, G. Filacchione, Y. Langevin, T.B. McCord, V. Mennella, P.D. Nicholson, B. Sicardy,
P.G.J. Irwin, Photometric changes on Saturn’s moon Titan: evidence for cryovolcanism. Geophys. Res.
Lett. 36, L04202 (2009b). https://doi.org/10.1029/2008GL036206
H.B. Niemann, S.K. Atreya, S.J. Bauer, G.R. Carignan, J.E. Demick, R.L. Frost, D. Gautier, J.A. Haberman, D.N. Harpold, D.M. Hunten, G. Israel, J.I. Lunine, W.T. Kasprzak, T.C. Owen, M. Paulkovich,
F. Raulin, E. Raaen, S.H. Way, Huygens Probe Gas Chromatograph Mass Spectrometer: the atmosphereand surface of Titan. Nature (2005). https://doi.org/10.1038/nature04122
F. Nimmo, B.G. Bills, Shell thickness variations and the long-wavelength topography of Titan. Icarus 208,
896–904 (2010)
C. Nixon, R. Clark, R. Courtin, A. Hayes, R. Lopes, R. Yelle, C. Sotin, A. Rymer, R. Johnson, R. Lorenz,
M. Mastrogiuseppe, H. Smith, D. Strobel, R. Achterberg, A. Buch, K. Mandt, D. Mitchell, F. Raulin,
E. Turtle, L. Iess, V. Vuitton, A. Solomonidou, R. West, P. Coll, Titan’s cold case files—outstanding
questions after Cassini-Huygens. Planet. Space Sci. 155, 50–72 (2018)
D. Northrup, J. Radebaugh, E.H. Christiansen, S. Tass, L. Kerber, Yardang and dune classification on Titan
through length, width, and Sinuosity, in 49th Lunar and Planetary Science Conference, 19–23 March,
2018, The Woodlands, Texas. LPI Contribution, vol. 2083 (2018)

Titan as Revealed by the Cassini Radar

Page 47 of 50 33

F. Paganelli, M.A. Janssen, B. Stiles, R. West, R.D. Lorenz, J.I. Lunine, S.D. Wall, P. Callahan, R.M. Lopes,
E. Stofan, R.L. Kirk, W.T.K. Johnson, L. Roth, C. Elachi (the Cassini Radar Team), Titan’s surface fromthe Cassini Radar SAR and high resolution radiometry data of the first five flybys. Icarus 191, 211–222(2007)
F. Paganelli, M.A. Janssen, R.M. Lopes, E. Stofan, S.D. Wall, R.D. Lorenz, J.I. Lunine, R.L. Kirk, L. Roth,
C. Elachi (the Cassini Radar Team), Titan’s surface from the Cassini RADAR radiometry data during
SAR mode. Planet. Space Sci. 56, 100–108 (2008)
P. Paillou, M. Crapeau, C. Elachi, S. Wall, P. Encrenaz, Models of SAR backscattering for bright flows anddark spots on Titan. J. Geophys. Res. 111, E11011 (2006). https://doi.org/10.1029/2006JE002724
P. Paillou, J.I. Lunine, G. Ruffié, P. Encrenaz, S.D. Wall, R.D. Lorenz, M.A. Janssen, Microwave dielectric
constant of Titan-relevant materials. Geophys. Res. Lett. 35, L18202 (2008)
P. Paillou, D. Bernard, J. Radebaugh, R. Lorenz, A. Le Gall, T. Farr, Modeling the SAR backscatter of linear
dunes on Earth and Titan. Icarus 230, 208–214 (2014)
P. Paillou, B. Seignovert, J. Radebaugh, S. Wall, Radar scattering of linear dunes and mega-yardangs: application to Titan. Icarus 270, 211–221 (2016)
J.T. Perron, M.P. Lamb, C.D. Koven, I.Y. Fung, E. Yager, M. Adamkovics, Valley formation and methane
precipitation rates on Titan. J. Geophys. Res. 111, E11001 (2006)
V. Poggiali, M. Mastrogiuseppe, A.G. Hayes, R. Seu, S.P.D. Birch, R. Lorenz, C. Grima, J.D. Hofgartner,
Liquid-filled canyons on Titan. Geophys. Res. Lett. 43, 7887–7894 (2016)
C.C. Porco, R.A. West, S. Squyres, A. McEwen, T. Peter, C.D. Murray, A. Delgenio, A.P. Ingersoll,
T.V. Johnson, G. Neukum et al., Cassini imaging science: instrument characteristics and anticipatedscientific investigations at Saturn. Space Sci. Rev. 115(1–4), 363–497 (2004). https://doi.org/10.1007/
s11214-004-1456-7
C.C. Porco, E. Baker, J. Barbara, K. Beurle, A. Brahic, J.A. Burns, S. Charnoz, N. Cooper, D.D. Dawson, A.D. Del Genio, T. Denk, L. Dones, U. Dyudina, M.W. Evans, S. Fussner, B. Giese, K. Grazier,P. Helfenstein, A.P. Ingersoll, R.A. Jacobson, T.V. Johnson, A. McEwen, C.D. Murray, G. Neukum,
W.M. Owen, J. Perry, T. Roatsch, J. Spitale, S. Squyres, P. Thomas, M. Tiscareno, E.P. Turtle,
A.R. Vasavada, J. Veverka, R. Wagner, R. West, Imaging of Titan from the Cassini spacecraft. Nature
434, 159–168 (2005). https://doi.org/10.1038/nature03436
J. Radebaugh, Dunes on Saturn’s moon Titan at the end of the Cassini Equinox Mission. Aeolian Res. 11,
23–41 (2013)
J. Radebaugh, R.D. Lorenz, R.L. Kirk, J.I. Lunine, E.R. Stofan, R.M.C. Lopes, S.D. Wall (the Cassini
Radar Team), Mountains on Titan observed by Cassini Radar. Icarus 192, 77–91 (2007). https://doi.org/
10.1016/j.icarus.2007.06.020
J. Radebaugh, R. Lorenz, J. Lunine, S. Wall, G. Boubin, E. Reffet, R. Kirk, R. Lopes, E. Stofan, L. Soderblom,
M. Allison, M. Janssen, P. Paillou, P. Callahan (the Cassini Radar Team), Dunes on Titan observed by
Cassini Radar. Icarus 194, 690–703 (2008). https://doi.org/10.1016/j.icarus.2007.10.015
J. Radebaugh, R. Lorenz, T. Farr, P. Paillou, C. Savage, C. Spencer, Linear dunes on Titan and Earth:initial remote sensing comparisons. Geomorphology 121, 122–132 (2010). https://doi.org/10.1016/
j.geomorph.2009.02.022
J. Radebaugh, R.D. Lorenz, S.D. Wall, R.L. Kirk, C.A. Wood, J.I. Lunine, E.R. Stofan, R.M.C. Lopes, P. Valora, T.G. Farr, A.G. Hayes, B. Stiles, G. Mitri, H. Zebker, M. Janssen, L. Wye, A. Le Gall, K.L. Mitchell,
F. Paganelli, R.D. West, E.L. Schaller (the Cassini RADAR Team), Regional geomorphology and historyof Titan’s Xanadu province. Icarus 211, 672–685 (2011)
J. Radebaugh, D. Ventra, R.D. Lorenz, T. Farr, R. Kirk, A. Hayes, M.J. Malaska, S. Birch, Z.Y-C. Liu,
J. Lunine, J. Barnes, A. Le Gall, R. Lopes, E. Stofan, S. Wall, P. Paillou, Alluvial and fluvial fans on
Saturn’s moon Titan reveal processes, materials and regional geology, in Geology and Geomorphologyof Alluvial and Fluvial Fans: Terrestrial and Planetary Perspectives, ed. by D. Ventra, L.E. Clarke.
Special Publications, vol. 440 (Geological Society, London, 2016). https://doi.org/10.1144/SP440.6
P. Rannou, F. Montmessin, F. Hourdin, S. Lebonnois, The latitudinal distribution of clouds on Titan. Science311(5758), 201–205 (2006)
F. Raulin, Organic chemistry in the oceans of Titan. Adv. Space Res. 7(5), (5)71–(5)81 (1987)
L.A. Richardson, J.W. Hartwig, J.W. Leachman, Experimental PρT-x measurements of liquidmethane-ethane-nitrogen mixtures. Fluid Phase Equilib. 462, 38–43 (2018). https://doi.org/10.1016/
j.fluid.2018.01.023
S. Rodriguez, A. Garcia, A. Lucas, T. Appéré, A. Le Gall, E. Reffet, L. Le Corre, S. Le Mouélic, T. Cornet,
S. Courrech du Pont, C. Narteau, O. Bourgeois, J. Radebaugh, K. Arnold, J.W. Barnes, K. Stephan,
R. Jaumann, C. Sotin, R.H. Brown, R.D. Lorenz, E.P. Turtle, Global mapping and characterization of
Titan’s dune fields with Cassini: correlation between RADAR and VIMS observations. Icarus 230, 168–179 (2014)
D.M. Rubin, P.A. Hesp, Multiple origins of linear dunes on Earth and Titan. Nat. Geosci. 2(9), 653 (2009)33

Page 48 of 50

R.M.C. Lopes et al.

C. Sagan, S.F. Dermott, The tide in the seas of Titan. Nature 300, 731–733 (1982)
C.J. Savage, J. Radebaugh, E.H. Christiansen, R.D. Lorenz, Implications of dune pattern analysis for Titan’ssurface history. Icarus 230, 180–190 (2014)
E.L. Schaller, M.E. Brown, H.G. Roe, A.H. Bouchez, A large cloud outburst at Titan’s south pole. Icarus
182(1), 224–229 (2006)
L.R. Schurmeier, A.J. Dombard, Crater relaxation on Titan aided by low thermal conductivity sand infill.
Icarus 305, 314–323 (2018). https://doi.org/10.1016/j.icarus.2017.10.034
L. Schurmeier, A.J. Dombard, M. Malaska, J. Radebaugh, Are Titan’s radial labyrinth terrains surface expressions of large laccoliths? in American Geophysical Union Fall Meeting, New Orleans (2017)
L. Schurmeier, A.J. Dombard, J. Radebaugh, M. Malaska, Intrusive and extrusive cryovolcanism and thecomposition of Titan’s icy crust, in Lunar and Planetary Science Conference, vol. 49 (2018). Abstract2934
W.D. Sears, Tidal dissipation in oceans on Titan. Icarus 113, 39–56 (1995)
P.H. Smith, M.T. Lemmon, R.D. Lorenz, L.A. Sromovsky, J.J. Caldwell, M.D. Allison, Titan’s surface, revealed by HST imaging. Icarus 119(2), 336–349 (1996)
L. Soderblom, J. Anderson, K. Baines, J. Barnes, J. Barrett, R. Brown, B. Buratti, R. Clark, D. Cruikshank,
C. Elachi, M. Janssen, R. Jaumann, R. Kirk, E. Karkoschka, S. Lemouelic, R. Lopes, R. Lorenz, J. Lunine, T. McCord, P. Nicholson, J. Radebaugh, B. Rizk, C. Sotin, E. Stofan, T. Sucharski, M. Tomasko,
S. Wall, Correlations between Cassini VIMS spectra and RADAR SAR images: implications for Titan’s surface composition and the character of the Huygens Probe landing site. Planet. Space Sci. 55,
2025–2036 (2007)
L.A. Soderblom, R.H. Brown, J.M. Soderblom, J.W. Barnes, R.L. Kirk, C. Sotin, R. Jaumann, D.J. MacKinnon, D.W. Mackowski, K.H. Baines, B.J. Buratti, R.N. Clark, P.D. Nicholson, The geology of Hotei
Regio, Titan: correlation of Cassini VIMS and RADAR. Icarus 204, 610–618 (2009). https://doi.org/
10.1016/j.icarus.2009.07.033
J.M. Soderblom, R.H. Brown, L.A. Soderblom, J.W. Barnes, R. Jaumann, S. Le Mouélic et al., Geology of
the Selk crater region on Titan from Cassini VIMS observations. Icarus 208, 905–912 (2010)
J.M. Soderblom, J.W. Barnes, R.H. Brown, V. Chevrier, K. Farnsworth, L.A. Soderblom, Evidence for frozenhydrocarbons on Titan, in American Astronomical Society, DPS Meeting, vol. 48 (2016)
F. Sohl, H. Hussmann, B. Schwentker, T. Spohn, R.D. Lorenz, Interior structure models and tidal Love numbers of Titan. J. Geophys. Res. 108(E12), 5130 (2003). https://doi.org/10.1029/2003JE002044
F. Sohl, A. Solomonidou, F.W. Wagner, A. Coustenis, H. Hussmann, D. Schulze-Makuch, Tidal stresses on
Titan and implications for its geology and habitability. J. Geophys. Res. 119, 1013–1036 (2014)
A. Solomonidou et al., Surface albedo spectral properties of geologically interesting areas on Titan. J. Geophys. Res. 119, 1729–1747 (2014)
A. Solomonidou, A. Coustenis, M. Hirtzig, S. Rodriguez, K. Stephan, R.M.C. Lopes, P. Drossart, C. Sotin, S.
Le Mouélic, K. Lawrence, E. Bratsolis, R. Jaumann, R.H. Brown, Temporal variations of Titan’s surface
with Cassini/VIMS. Icarus 270, 85–99 (2016)
A. Solomonidou, A. Coustenis, R.M.C. Lopes, M. Malaska, S. Rodriguez, P. Drossart, C. Elachi, B. Schmitt,
S. Philippe, M. Janssen, M. Hirtzig, S. Wall, C. Sotin, K. Lawrence, N. Altobelli, E. Bratsolis,
J. Radebaugh, K. Stephan, R.H. Brown, S. Le Mouélic, A. Le Gall, E.V. Villanueva, J.F. Brossier,
A.A. Bloom, O. Witasse, C. Matsoukas, A. Schoenfeld, The spectral nature of Titan’s major geomorphological units: constraints on surface composition. J. Geophys. Res., Planets (2018). https://doi.org/
10.1002/2017JE005477
C. Sotin, R. Jaunmann, B.J. Buratti, R.H. Brown, R.N. Clark, L.A. Soderblom, K.H. Baines, G. Bellucci,
J-P. Bribing, F. Capaccioni, P. Cerroni, A. Coradini, D.P. Cruikshank, P. Drossart, V. Formisano,Y. Langevin, D.L. Matson, T.B. McCord, R.M. Nelson, P.D. Nicholson, B. Sicardy, S. LeMouelic,
S. Rodriguez, K. Stephan, C.K. Scholz, Release of volatiles from a possible cryovolcano from nearinfrared imaging of Titan. Nature 435, 786–789 (2005)
K. Stephan et al., Specular reflection on Titan: liquids in Kraken Mare. Geophys. Res. Lett. 37, L07104(2010). https://doi.org/10.1029/2009GL042312
B.W. Stiles, R.L. Kirk, R.D. Lorenz, S. Hensley, E. Lee, S.J. Ostro, M.D. Allison, P.S. Callahan, Y. Gim,
L. Iess, Determining Titan’s spin state from Cassini radar images. Astron. J. 135(5), 1669–1680 (2008)
B.W. Stiles, S. Hensley, Y. Gim, D.M. Bates, R.L. Kirk, A. Hayes, J. Radebaugh, R.D. Lorenz, K.L. Mitchell,
P.S. Callahan, H. Zebker, W.T.K. Johnson, S.D. Wall, J.I. Lunine, C.A. Wood, M. Janssen, F. Pelletier,
R.D. West, C. Veeramacheneni, Determining Titan surface topography from Cassini SAR data. Icarus
102, 584–598 (2009). https://doi.org/10.1016/j.icarus.2009.03.032
B. Stiles, R. Kirk, R. Lorenz, S. Hensley, E. Lee, S. Ostro, M. Allison, P. Callahan, Y. Gim, L. Iess, P. Percidel Marmo, G. Hamilton, W. Johnson, R. West (Cassini RADAR Team), Determining Titan’s spin statefrom Cassini Radar images. Astron. J. 139(1), 311 (2010)

Titan as Revealed by the Cassini Radar

Page 49 of 50 33

E.R. Stofan, J.I. Lunine, R. Lopes, F. Paganelli, R.D. Lorenz, C.A. Wood, R. Kirk, S. Wall, C. Elachi,
L.A. Soderblom, S. Ostro, M. Janssen, J. Radebaugh, L. Wye, H. Zebker, Y. Anderson, M. Allison,
R. Boehmer, P. Callahan, P. Encrenaz, E. Flamini, G. Francescetti, Y. Gim, G. Hamilton, S. Hensley,
W.T.K. Johnson, K. Kelleher, D. Muhleman, G. Picardi, F. Posa, L. Roth, R. Seu, S. Shaffer, B. Stiles,
S. Vetrella, R. West, Mapping of Titan: results from the first Titan Radar passes. Icarus 185, 443–456(2006)
E.R. Stofan, C. Elachi, J.I. Lunine, R.D. Lorenz, B. Stiles, K.L. Mitchell, S. Ostro, L. Soderblom, C. Wood,
H. Zebker, S. Wall, M. Janssen, R. Kirk, R. Lopes, F. Paganelli, J. Radebaugh, L. Wye, Y. Anderson,
M. Allison, R. Boehmer, P. Callahan, P. Encrenaz, E. Flamini, G. Francescetti, Y. Gim, G. Hamilton, S. Hensley, W.T.K. Johnson, K. Kelleher, D. Muhleman, P. Paillou, G. Picardi, F. Posa, L. Roth,
R. Seu, S. Shaffer, S. Vetrella, R. West, The lakes of Titan. Nature 445, 61–64 (2007). https://doi.org/
10.1038/nature05438
G. Tobie, J.I. Lunine, C. Sotin, Episodic outgassing as the origin of atmospheric methane on Titan. Nature
440, 61–64 (2006)
G. Tobie, J.I. Lunine, J. Monteux, O. Mousis, F. Nimmo, The origin and evolution of Titan, in Titan: Interior, Surface, Atmosphere, and Space Environment, ed. by I. Müller-Wodarg, C.A. Griffith,
E. Lellouch, T.E. Cravens (Cambridge University Press, Cambridge, 2014). https://doi.org/10.1017/
CBO9780511667398
T. Tokano, Meteorological assessment of the surface temperatures on Titan: constraints on the surface type.
Icarus 173, 222–242 (2005)
T. Tokano, Dune-forming winds on Titan and the influence of topography. Icarus 194(1), 243–262 (2008)
T. Tokano, Relevance of fast westerlies at equinox for the eastward elongation of Titan’s dunes. Aeolian Res.
2(2–3), 113–127 (2010)
M. Tomasko, B. Archinal, T. Becker, B. Bézard, M. Bushroe, M. Combes, D. Cook, A. Coustenis, C.
de Bergh, L. Dafoe, L. Doose, S. Douté, A. Eibl, S. Engel, F. Gliem, B. Grieger, K. Holso, E. HowingtonKraus, E. Karkoschka, H. Keller, R. Kirk, R. Kramm, M. Küppers, P. Lanagan, E. Lellouch, M. Lemmon, J. Lunine, E. McFarlane, J. Moores, M. Prout, B. Rizk, M. Rosiek, P. Rueffer, S. Schröder,B. Schmitt, C. See, P. Smith, L. Soderblom, N. Thomas, R. West, Rain, winds, and haze during the
Huygens probe descent to Titan’s surface. Nature 438, 765–778 (2005)
F. Tosi, R. Orosei, R. Seu, A. Coradini, J.I. Lunine, G. Filacchione, A.I. Gavrishin, F. Capaccioni, P. Cerroni,
A. Adriani et al., Correlations between VIMS and RADAR data over the surface of Titan: implications
for Titan’s surface properties. Icarus 208(1), 366–384 (2010)
H. Tsoar, Dynamic processes acting on a longitudinal (seif) dune. Sedimentology 30, 567–578 (1983)
E.P. Turtle, J.E. Perry, A.S. McEwen, A.D. Del Genio, J. Barbara, R.A. West, D.D. Dawson, C.C. Porco,
Cassini imaging of Titan’s high-latitude lakes, clouds, and south-polar surface changes. Geophys. Res.
Lett. 36(2), L02204 (2009)
E.P. Turtle, J.E. Perry, A.G. Hayes, R.D. Lorenz, J.W. Barnes, A.S. McEwen, R.A. West, A.D. Del Genio, J.M. Barbara, E.L. Schaller, T.L. Ray, J.I. Lunine, R.M.C. Lopes, E.R. Stofan, Rapid and extensive surface changes near Titan’s equator: evidence of April showers. Science 331, 1414 (2011a).
https://doi.org/10.1126/science.1201063. 2011
E.P. Turtle, J.E. Perry, A.G. Hayes, A.S. McEwen, Shoreline retreat at Titan’s Ontario Lacus and Arrakis
Planitia from Cassini Imaging Science Subsystem observations. Icarus 212, 957–959 (2011b)
B. Ventura et al., Electromagnetic models and inversion techniques for Titan’s Ontario Lacus depth estimationfrom Cassini RADAR data. Icarus 221, 960–969 (2012). https://doi.org/10.1016/j.icarus.2012.09.011
G. Vixie, J.W. Barnes, B. Jackson, S. Rodriguez, S. Le Mouélic, C. Sotin, S. MacKenzie, P. Wilson, Possible
temperate lakes on Titan. Icarus 257, 313–323 (2015)
S.D. Wall, R.M. Lopes, E.R. Stofan, C.A. Wood, J.L. Radebaugh, B.W. Stiles, R.M. Nelson, L.W. Kamp,
M.A. Janssen, R.L. Lorenz, J.I. Lunine, T.G. Farr, G. Mitri, P. Paillou, F. Paganelli, K.L. Mitchell,
Cassini RADAR images at Hotei Arcus and Western Xanadu, Titan: evidence for recent cryovolcanic
activity. Geophys. Res. Lett. 36, L04203 (2009). https://doi.org/10.1029/2008GL036415
S. Wall, A. Hayes, C. Bristow, R. Lorenz, E. Stofan, J. Lunine, A. Le Gall, M. Janssen, R. Lopes, L. Wye,
L. Soderblom, P. Paillou, O. Aharonson, H. Zebker, T. Farr, G. Mitri, R. Kirk, K. Mitchell, C. Notarnicola, D. Casarano, B. Ventura, Active shoreline of Ontario Lacus, Titan: a morphological studyof the lake and its surroundings. Geophys. Res. Lett. 37, L05202 (2010). https://doi.org/10.1029/
2009GL041821
H.Y. Wei, C.T. Russell, M.K. Dougherty, F.M. Neubauer, Y.J. Ma, Upper limits on Titan’s magnetic moment
and implications for its interior. J. Geophys. Res. 115, E10007 (2010)
C.A. Wood, Titan’s great crustal thickening event and recent geologic history, in LPSC, vol. 49 (2018) p. 1343
C.A. Wood, K.L. Mitchell, R.M.C. Lopes, J. Radebaugh, E. Stoffan, J. Lunine (the Cassini RADAR Team),
Volcanic Calderas in the North Polar Region of Titan, in Lunar Planet. Sci. Conf. XXXVIII, Houston,
Texas (2007). Abstract 1454

33

Page 50 of 50

R.M.C. Lopes et al.

C.A. Wood, R. Lorenz, R. Kirk, R. Lopes, K. Mitchell, E. Stofan, Impact craters on Titan. Icarus 206, 334–
344 (2010)
L.C. Wye, Radar Scatering from Titan and Saturn’s Icy Satellites using the Cassini Spacecraft. Ph.D. thesis,
Stanford University (2011)
L.C. Wye, H.A. Zebker, S.J. Ostro, R.D. West, Y. Gim, R.D. Lorenz (the Cassini Radar Team), Electricalproperties of Titan’s surface from Cassini RADAR scatterometer measurements. Icarus 188, 367–385(2007)
L.C. Wye, H.A. Zebker, R.D. Lorenz, Smoothness of Titan’s Ontario Lacus: constraints from Cassini RADAR
specular reflection data. Geophys. Res. Lett. 36, 16201 (2009). https://doi.org/10.1029/2009GL039588
X. Yu, S.M. Hörst, C. He, N.T. Bridges, D.M. Burr, J.A. Sebree, J.K. Smith, The effect of adsorbed liquidand material density on saltation threshold: insight from laboratory and wind tunnel experiments. Icarus
297, 97–109 (2017)
Y.L. Yung, M. Allen, J.P. Pinto, Photochemistry of the atmosphere of Titan: comparison between model andobservations. Astrophys. J. Suppl. Ser. 55, 465–506 (1984)
K. Zahnle, P. Schenk, H. Levison, L. Dones, Cratering rates in the outer Solar System. Icarus 163, 263–289(2003)
K. Zebker, Y-Q. Wong, Shape of Titan from Cassini radar elevation measurements and implications for interior structure and composition, in AGU Fall Meeting, Dec. 12–16, San Francisco (2016). Abstract
P33F-01
H. Zebker, L. Wye, M. Janssen (Cassini Radar Team), Titan’s surface from reconciled Cassini microwave
reflectivity and emissivity observations. Icarus 194(2), 704–710 (2008)
H.A. Zebker, B. Stiles, S. Hensley, R. Lorenz, R.L. Kirk, J.I. Lunine, Size and shape of Saturn’s moon Titan.
Science 324, 921–923 (2009)
H. Zebker, A. Hayes, M. Janssen, A. Le Gall, R. Lorenz, L. Wye, Surface of Ligeia Mare, Titan, from Cassini
altimeter and radiometer analysis. Geophys. Res. Lett. 41, 308–313 (2014). https://doi.org/10.1002/
2013GL058877