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On both Earth and Titan, some linear dunefields are characterized by curvilinear patterning atypical of the regularity and straightness of typical longitudinal dunefields. We use remotely sensed imagery and an automated dune crestline detection algorithm to analyze the controls on spatial patterning. Here it is shown that topography can influence the patterning, as dune alignments bend to deflect downslope under the influence of gravity. The effect is pronounced in a terrestrial dunefield (the Great Sandy desert, Australia) where substantial topography underlies, but is absent where the dunefield is underlain by subdued relief (southwestern Kalahari). This knowledge allows the inference of subtle topographic changes underlying dunefields from dunefield patterning, where other sources of elevation data may be absent. This methodology is explored using the Belet Sand Sea of Titan, where likely areas of topographic change at resolutions finer than those currently available from radar altimetry are inferred. Plain Language Summary: Linear dunes form large dunefields both on Earth and Saturn's moon Titan, and look remarkably similar on both worlds. They are characterized by repeated ridges of sand which extend approximately parallel to the wind, and may continue unbroken for tens or even hundreds of kilometers. Perhaps their most remarkable feature is their regularity, and consistent orientation of the dunes. In a few locations, however, the dunes form distinctive curved patterns. This study investigates the causes of this phenomenon, by comparing two dunefields on Earth: Australia's Great Sandy desert, where the curved dunes are abundant, and the Kalahari of southern Africa, where they are absent. The cause of the curved dunes is shown to be underlying topography. The Kalahari is very flat, and thus, the dunes form straight lines. But the Great Sandy desert lies over a long‐dry river valley system, and where the dunes encounter slopes, they deflect downslope. On Titan, knowledge of surface elevations and topography is patchy, and with lander missions planned better understanding is important. The method of analysis proposed here is demonstrated on radar data from the Belet dunefield of Titan, and we show that topography can be inferred from dune patterning alone. Key Points: Local variations in dune trend are identified in some linear dunefields on Earth and TitanThe cause is identified as underlying topographic relief resulting in downslope deflection of dunesDunefield patterning offers the potential to infer topographic relief, with implications for identifying planetary lander sites [ABSTRACT FROM AUTHOR]

Abstract: Thousands of dunes found in Cassini Radar images of the equatorial regions of Titan, a moon around Saturn, are similar in size and morphology to linear dunes on Earth. We present remote sensing images of terrestrial analogues to the dunes on Titan obtained by Landsat and radar, both at considerably higher resolution than are available at Titan, that provide information about dune landforms and processes. Dunes are generally dark to radar, indicating smooth surfaces and signal absorbing materials, but at certain incidence angles, dune surfaces can reflect the radar signal and lead to a bright return. Linear dunes on Titan and Earth diverge around topographic obstacles, creating teardrop patterns that indicate mean direction of wind flow and sand transport. When sand supply or wind conditions change in linear dune fields, such as behind an obstacle or near the margin of the dune field, dunes disappear, change size and spacing, or change dune type. These comparisons of features on Titan and Earth provide a better understanding of the global, sand-transporting wind directions, sand properties and supply, and the nature of the underlying substrate, on Titan. [Copyright &y& Elsevier]

Abstract: Thousands of longitudinal dunes have recently been discovered by the Titan Radar Mapper on the surface of Titan. These are found mainly within ±30° of the equator in optically-, near-infrared-, and radar-dark regions, indicating a strong proportion of organics, and cover well over 5% of Titan''s surface. Their longitudinal duneform, interactions with topography, and correlation with other aeolian forms indicate a single, dominant wind direction aligned with the dune axis plus lesser, off-axis or seasonally alternating winds. Global compilations of dune orientations reveal the mean wind direction is dominantly eastwards, with regional and local variations where winds are diverted around topographically high features, such as mountain blocks or broad landforms. Global winds may carry sediments from high latitude regions to equatorial regions, where relatively drier conditions prevail, and the particles are reworked into dunes, perhaps on timescales of thousands to tens of thousands of years. On Titan, adequate sediment supply, sufficient wind, and the absence of sediment carriage and trapping by fluids are the dominant factors in the presence of dunes. [Copyright &y& Elsevier]

Highlights: [•] Use of linear dunes of the Great Sand Sea to model the radar signature of dunes. [•] Use of SIR-C/X-SAR radar images and SRTM topography. [•] Analysis of Cassini Radar T8 flyby data: Belet equatorial sand sea of Titan. [•] A surface scattering term+a volume scattering term needed to model the radar signature of Titan’s dunes. [•] Diffuse scattering term have consequences on the composition/structure of Titan’s dunes. [ABSTRACT FROM AUTHOR]

The most recent Cassini RADAR images of Titan show widespread regions (up to 1500 kilometers by 200 kilometers) of near-parallel radar-dark linear features that appear to be seas of longitudinal dunes similar to those seen in the Namib desert on Earth. The Ku-band (2.17-centimeter wavelength) images show ∼100-meter ridges consistent with duneforms and reveal flow interactions with underlying hills. The distribution and orientation of the dunes support a model of fluctuating surface winds of ∼0.5 meter per second resulting from the combination of an eastward flow with a variable tidal wind. The existence of dunes also requires geological processes that create sand-sized (100- to 300-micrometer) particulates and a Lack of persistent equatorial surface liquids to act as sand traps, [ABSTRACT FROM AUTHOR]

Highlights: [•] Titan’s dunes have been mapped at global scale with RADAR and VIMS on board the Cassini spacecraft. [•] Excellent correlation has been found at the global scale between RADAR-mapped dunes and the VIMS dark brown unit. [•] Dune material is dominated by solid organics of atmospheric origin. [•] Dune material covers 17% of Titan’s surface, corresponding to a total average mass of organic sediment of ∼230,000GT. [•] Dunes are the largest visible surface reservoir of hydrocarbons and may be less than 730-Myr old. [ABSTRACT FROM AUTHOR]

Abstract: Large expanses of linear dunes cover Titan’s equatorial regions. As the Cassini mission continues, more dune fields are becoming unveiled and examined by the microwave radar in all its modes of operation (SAR, radiometry, scatterometry, altimetry) and with an increasing variety of observational geometries. In this paper, we report on Cassini’s radar instrument observations of the dune fields mapped through May 2009 and present our key findings in terms of Titan’s geology and climate. We estimate that dune fields cover ∼12.5% of Titan’s surface, which corresponds to an area of ∼10millionkm2, roughly the area of the United States. If dune sand-sized particles are mainly composed of solid organics as suggested by VIMS observations (Cassini Visual and Infrared Mapping Spectrometer) and atmospheric modeling and supported by radiometry data, dune fields are the largest known organic reservoir on Titan. Dune regions are, with the exception of the polar lakes and seas, the least reflective and most emissive features on this moon. Interestingly, we also find a latitudinal dependence in the dune field microwave properties: up to a latitude of ∼11°, dune fields tend to become less emissive and brighter as one moves northward. Above ∼11° this trend is reversed. The microwave signatures of the dune regions are thought to be primarily controlled by the interdune proportion (relative to that of the dune), roughness and degree of sand cover. In agreement with radiometry and scatterometry observations, SAR images suggest that the fraction of interdunes increases northward up to a latitude of ∼14°. In general, scattering from the subsurface (volume scattering and surface scattering from buried interfaces) makes interdunal regions brighter than the dunes. The observed latitudinal trend may therefore also be partially caused by a gradual thinning of the interdunal sand cover or surrounding sand sheets to the north, thus allowing wave penetration in the underlying substrate. Altimetry measurements over dunes have highlighted a region located in the Fensal dune field (∼5° latitude) where the icy bedrock of Titan is likely exposed within smooth interdune areas. The hemispherical assymetry of dune field properties may point to a general reduction in the availability of sediments and/or an increase in the ground humidity toward the north, which could be related to Titan’s asymmetric seasonal polar insolation. Alternatively, it may indicate that either the wind pattern or the topography is less favorable for dune formation in Titan’s northern tropics. [Copyright &y& Elsevier]