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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: In 13 years, infrared observations from the Visual and Infrared Mapping Spectrometer onboard Cassini provided significant hints about the spectral and geological diversity of Titan's surface. The analysis of the infrared (IR) signature of spectral units enables constraining the surface composition, which is crucial for understanding possible interactions between Titan's interior, surface, and atmosphere. Here we investigate a selection of areas in the equatorial regions, imaged by Cassini's instruments, which exhibit an apparent transition from the Visual and Infrared Mapping Spectrometer IR‐bright to the IR‐blue and IR‐brown units (from false‐color composites using red: 1.57/1.27 μm, green: 2.01/1.27 μm, and blue: 1.27/1.08 μm). By applying an updated radiative transfer model, we extract the surface albedo of IR units identified in these regions. Then, we compare them with synthetic mixtures of two expected components on Titan's surface, namely, water ice and laboratory tholins. This allows us to reconnect the derived composition and grain size information to the geomorphology observed from Radio Detection and Ranging instrument (RADAR)/Synthetic Aperture Radar images. We interpret IR‐bright units as hills and plains coated by organic material and incised by fluvial networks. Erosion products are transported downstream to areas where IR‐blue units are seen near the IR‐bright units. These units, enriched in water ice, are most likely outwash plains hosting debris from fluvial erosion. Farther away from the IR‐bright units, the IR‐brown units are dominantly made of organics with varied grain sizes, ranging from dust‐ to sand‐sized particles that form the dune fields. The transition areas therefore exhibit trends in water ice content and grain size supported by geomorphological observations. [ABSTRACT FROM AUTHOR]

Abstract: We investigate Titan's low‐latitude and midlatitude surface using spectro‐imaging near‐infrared data from Cassini/Visual and Infrared Mapping Spectrometer. We use a radiative transfer code to first evaluate atmospheric contributions and then extract the haze and the surface albedo values of major geomorphological units identified in Cassini Synthetic Aperture Radar data, which exhibit quite similar spectral response to the Visual and Infrared Mapping Spectrometer data. We have identified three main categories of albedo values and spectral shapes, indicating significant differences in the composition among the various areas. We compare with linear mixtures of three components (water ice, tholin‐like, and a dark material) at different grain sizes. Due to the limited spectral information available, we use a simplified model, with which we find that each albedo category of regions of interest can be approximately fitted with simulations composed essentially by one of the three surface candidates. Our fits of the data are overall successful, except in some cases at 0.94, 2.03, and 2.79 μm, indicative of the limitations of our simplistic compositional model and the need for additional components to reproduce Titan's complex surface. Our results show a latitudinal dependence of Titan's surface composition, with water ice being the major constituent at latitudes beyond 30°N and 30°S, while Titan's equatorial region appears to be dominated partly by a tholin‐like or by a very dark unknown material. The albedo differences and similarities among the various geomorphological units give insights on the geological processes affecting Titan's surface and, by implication, its interior. We discuss our results in terms of origin and evolution theories. [ABSTRACT FROM AUTHOR]

The surface of Saturn’s haze-shrouded moon Titan has long been proposed to have oceans or lakes, on the basis of the stability of liquid methane at the surface. Initial visible and radar imaging failed to find any evidence of an ocean, although abundant evidence was found that flowing liquids have existed on the surface. Here we provide definitive evidence for the presence of lakes on the surface of Titan, obtained during the Cassini Radar flyby of Titan on 22 July 2006 (T16). The radar imaging polewards of 70° north shows more than 75 circular to irregular radar-dark patches, in a region where liquid methane and ethane are expected to be abundant and stable on the surface. The radar-dark patches are interpreted as lakes on the basis of their very low radar reflectivity and morphological similarities to lakes, including associated channels and location in topographic depressions. Some of the lakes do not completely fill the depressions in which they lie, and apparently dry depressions are present. We interpret this to indicate that lakes are present in a number of states, including partly dry and liquid-filled. These northern-hemisphere lakes constitute the strongest evidence yet that a condensable-liquid hydrological cycle is active in Titan’s surface and atmosphere, in which the lakes are filled through rainfall and/or intersection with the subsurface ‘liquid methane’ table. [ABSTRACT FROM AUTHOR]