Exploring lakes and dune features on Titan surface through
SAR images and electromagnetic models
Mattia Callegari (1), Domenico Casarano (2), Claudia Notarnicola (1), Luca Pasolli (1), Bartolomeo Ventura (1)
(1) EURAC, Viale Druso 1, 39100 Bolzano (I)
(2) CNR-IRPI, Via Amendola 122/I, 70126 Bari (I)
Since 2004, Cassini RADAR has provided a vast amount of data, suggesting new scenarios for Titan’s morphology and
evolution. On Titan surface, hydrocarbon lakes and dune fields are considered among the most studied and intriguing features.
In particular, a key result for Titan exploration was the detection of lakes constituted by liquid hydrocarbons, thus supporting
the hypothesis of a methane and ethane cycle similar to water cycle on Earth. Many lakes with were observed in the northern
polar region, the largest ones with area over 100000 km2, while in the southern hemisphere only one important feature (area of
around 20000 km2), called Ontario Lacus, was detected. Moreover, vast longitudinal dune fields confined around the equatorial
region, between -30° and 30° latitude, have been discovered and it has been estimated that they cover ~12% of the total Titan's
surface. Understanding the formation process of dunes, their morphology and distribution can reveal important clues on Titan's
climatic and geological history. For instance, dunes shape could indicate the mean direction of wind flow and sand transport;
their dimension could tell the quantity of sand supply; topographic and local climate conditions favorable to sand deposits can
be predicted from a dune distribution analysis.
This work will present the main results obtained on lakes and dune by exploitation of SAR images in combination with
electromagnetic models.
For lake characterization, SAR data have been analyzed by addressing using electromagnetic modelling approaches to retrieve
the optical thickness parameter of the liquid hydrocarbon layer. IEM model was combined with a gravity-capillary wave
spectrum and integrated into a Bayesian statistical inversion. The impact of hypotheses on the dielectric constant of the surface
constituents, as well as wind speed and wave motion scenarios, on the retrieved optical thickness are addressed thus obtaining
information on the lake depth and volume. The results also allowed to constrain wind speed values, in good agreement with the
outputs of Global Circulation Models on Titan. This analysis was performed in particular on Ontario Lacus, showing the
dependence of depth maps and volume estimates on the uncertainties in the values of loss tangent and on the percentage of lake
area reaching the conditions of total absorption of the incident wave.
For longitudinal dunes, the backscattering coming from different Cassini SAR passes that acquired Titan longitudinal dunes
using different observation and off-nadir angles have been modeled thus allowing a better understanding of the dielectric
properties of dunes surface. As also mentioned in previous works [1], it comes out from this analysis that most of dunes in
Titan are not homogeneous in material composition, and the presence of bright and dark areas in the SAR image is not only
due to the geometric effect (change in local incidence angle). In particular, interdunes often likely correspond to sand-free
areas (or areas covered by a very thin layer of sand) and therefore exhibit a generally higher backscattering value with respect
to the sand covered dunes areas, since the probably icy bedrock is more reflective and less absorptive of the sand material. This
is confirmed by the visibility of longitudinal dunes in SAR parallel acquisitions with respect to dunes direction. With this
observation geometry, in fact, homogeneous dunes should be invisible by SAR, since the signal incidence angle does not vary
with dunes topography. Assuming only surface scattering and using the Geometric Optics (GO) model, ranges for dielectric
constant and roughness for the dark areas (corresponding to the sand covered part of dunes) were obtained. For the bright area,
the estimation of dielectric constant is more complicate: an opportune modeling should take into account also volume and/or
subsurface scattering contributes, due to the very low values of loss tangent of ice at the Ku-band frequencies and at very low
temperatures. Since the variability in backscattering coefficients is due not only to the dune geometry, dunes height estimation
in case of dunes fields characterized by non-homogeneous material composition is more challenging. However, exploiting
couples of acquisitions with orthogonal observations on the same area (parallel and perpendicular with respect to dunes
direction), the effect on the backscattering caused by the difference in material composition could be addressed with the main
purposes to isolate the “geometric effect” (only visible in the perpendicular acquisitions with respect to dunes direction). Once
the geometric effect is isolated, the dunes slope and thus dunes height looking at the backscattering angular trend are likely
retrieved.
[1] Le Gall, A., Janssen, M.A., Wye, L.C., Hayes, A.G., Radebaugh, J., Savage, C., Zebker, H., Lorenz, R.D., Lunine, J.I.,
Kirk, R.L., Lopes, R.M.C., Wall, S., Callahan, P., Stofan, E.R., and Farr, T., "Cassini SAR, radiometry, scatterometry and
altimetry observations of Titan's dune fields," Icarus 213(2), 608-624 (2011).