Titan's Dunes Research Articles

The Role of Seasonal Sediment Transport and Sintering in Shaping Titan's Landscapes: A Hypothesis.

Titan is a sedimentary world, with lakes, rivers, canyons, fans, dissected plateaux, and sand dunes. Sediments on Saturn's moon are thought to largely consist of mechanically weak organic grains, prone to rapid abrasion into dust. Yet, Titan's equatorial dunes have likely been active for 10s–100s kyr. Sustaining Titan's dunes over geologic timescales requires a mechanism that produces sand‐sized particles at equatorial latitudes. We explore the hypothesis that a combination of abrasion, when grains are transported by winds or methane rivers, and sintering, when they are at rest, could produce sand grains that maintain an equilibrium size. Our model demonstrates that seasonal sediment transport may produce sand under Titan's surface conditions and could explain the latitudinal zonation of Titan's landscapes. Our findings support the hypothesis of global, source‐to‐sink sedimentary pathways on Titan, driven by seasons, and mediated by episodic abrasion and sintering of organic sand by rivers and winds.

Titan’s Prevailing Circulation Might Drive Highly Intermittent, Yet Significant Sediment Transport

AbstractTitan, the largest moon of Saturn, is characterized by gigantic linear dunes and an active dust cycle. Much like on Earth, these aeolian processes are caused by the wind‐driven saltation of surface grains. It is still unclear, however, how saltation on Titan can occur despite the typically weak surface winds and the potentially cohesive surface grains. Here, we explore the hypothesis that saltation on Titan may be sustained at lower wind speeds than previously thought, primarily through granular splash rather than aerodynamic lifting of surface grains. We propose a saltation mass flux parameterization for Titan and use it to quantify sediment transport with a general circulation model. The results suggest that Titan's prevailing circulation can generate highly intermittent yet significant saltation, with mass fluxes of the order of 104 kg m−1 year−1, and that Titan dunes may be formed primarily by fine grains, approximately 0.1 mm in size.

Texture and Composition of Titan's Equatorial Sand Seas Inferred From Cassini SAR Data: Implications for Aeolian Transport and Dune Morphodynamics

AbstractThe texture, composition, and morphology of dunes observed in the equatorial regions of Titan may reflect present and/or past climatic conditions. Determining the physio‐chemical properties and the morphodynamics of Titan's dunes is therefore essential to understanding of the climatic and geological history of the largest moon of Saturn. We quantitatively derived average surface properties of dune and interdune areas (texture and composition) from modeling of the microwave backscattered signal and Monte Carlo inversion of the despeckled Cassini/Synthetic Aperture Radar data over Titan's three largest sand seas: Belet, Shangri‐La, and Fensal. We present the first analysis of the backscatter functions extracted from despeckled Synthetic Aperture Radar images that cover such a large range in incidence angles, including data from the beginning of the Cassini mission up to its Grand Finale. We show that dunes and interdunes have significantly different physical properties. Dunes are found to be more microwave absorbent than interdunes. Additionally, potential secondary bedforms, such as ripples and avalanches, may have been detected, providing potential evidence for currently active dunes and sediment transport. Our modeling shows that the interdunes have multiscale roughnesses with higher dielectric constants than the dunes which have a low dielectric constant consistent with organic sand. The radar brightness of the interdunes can be explained by the presence of a shallow layer of significantly larger organic grains, possibly nonmobilized by the winds. Together, our findings suggest that Titan's sand seas evolve under the current multidirectional wind regimes with dunes that elongate with their crests aligned in the residual drift direction.

Topographic Effects on Titan’s Dune-forming Winds

The Cassini mission made an unexpected discovery when it found evidence of linear dune fields on Titan’s surface. The orientation of the dunes and their interaction with topography allow scientists to estimate the dominant wind direction on the surface of Titan. There is some consensus in the community that the dune-forming winds must be net westerly, however, there is an active debate about the dune-forming wind regime. This debate has been guided by several studies of Earth dune fields considered analogous to the Titan dunes including those in Namibia, the Sahara, the Serengeti, and China. Complicating this active debate about the surface wind regime is the fact that global circulation models (GCMs) have historically not been able to reproduce westerly surface winds in the tropics. Here we use the Titan Community Atmosphere Model (CAM) to quantify the impact of topography and an added torque on Titan’s dune-forming winds. Dunes tend to form at higher elevations on Titan, and adding topography to the model alters the near-surface wind directions, making them more westerly and consistent with the dune orientations. The addition of topography and added torque create a wind regime that is consistent with linear dunes in areas of stabilized sediment.

Raised Rims Around Titan's Sharp‐Edged Depressions

AbstractWe perform a systematic survey of Titan's sharp‐edged depressions (SEDs) using a combination of Cassini synthetic aperture radar images and delay‐Doppler processed altimetry. We identify a characteristic bright‐dark pattern around 172 SEDs that correlates with elevated features only resolved with the highest resolution topographic data. We find that ∼75% of Titan's SEDs have elevated (∼100 m) rim features, with a preference toward larger SEDs. The differences in the distribution of SED rim presence between the north and south are slight, but the data weakly suggest that northern SEDs may be more likely to have a rim. The pattern we identify in synthetic aperture radar images is similar to quasi‐specular increases in backscatter seen on Titan's dunes. We show that this bright‐dark pattern occurs due to elevated local slopes oriented perpendicular to the radar. In general, rims occur on most of Titan's SEDs, an observation that will aid in developing and testing SED formation models.

Exploring morphology, layering and formation history of linear terrestrial dunes from radar observations: Implications for Titan

Understanding the morphology and internal layering of large aeolian dune fields through radar observations can provide unique insights into the climatic and geophysical conditions that led to their formation. In this study, we perform a large-scale characterization of the morphology and internal layering of linear dunes in hyper-arid areas on Earth, through utilizing multiple complementary radar datasets (SIR-C imaging, SRTM interferometry-derived elevations and radar sounding or Ground Penetrating Radar (GPR)). Linear dune fields in the Egyptian desert are of special interest, due to their significance as planetary analogs to dunes on Mars and Saturn's largest moon, Titan. Satellite radar imagery and elevation data of the region show significant variance in the geomorphology of different dune fields in Egypt. In addition, GPR probing of the first few meters suggests different inner settings in the layering of dunes of different ages in eastern and western Egypt, reflecting different paleo-climatic regimes that led to their formation. Furthermore, our radiometric analysis suggests that dunes with different inner layering arrangement also exhibit different radar backscatter returns as a function of their heights. For relatively younger dunes with a homogeneous inner structure, like the ones in eastern Egypt in the Qattaniya dune field, we observe that sigma0 does not change as a function of the dune height. For relatively older dunes in western Egypt like the Great Sand Sea (Northern (Siwa) and Southern dune fields), we observed a linear correlation between sigma0 and the dune height. Thus, surface properties of dunes like morphology and backscatter variation with height are related to inner characteristics like arrangement of internal layering, relative ages and can be used to infer their depositional history.Linear dunes discovered in the equatorial regions of Titan by the Cassini-Huygens mission are morphologically very similar to these linear dune fields in the Egyptian Sahara. Hence, assessing the variability of morphology and radar backscatter properties of Titan's dunes as a function of their heights can help constrain the ambiguities associated with their internal structure and formation history and provide insights into Titan's paleo-wind regimes.

Methane storms as a driver of Titan’s dune orientation

Titan's equatorial regions are covered by eastward propagating linear dunes. This direction is opposite to mean surface winds simulated by Global Climate Models (GCMs), which are oriented westward at these latitudes, similar to trade winds on Earth. Different hypotheses have been proposed to address this apparent contradiction, involving Saturn's gravitational tides, large scale topography or wind statistics, but none of them can explain a global eastward dune propagation in the equatorial band. Here we analyse the impact of equinoctial tropical methane storms developing in the superrotating atmosphere (i.e. the eastward winds at high altitude) on Titan's dune orientation. Using mesoscale simulations of convective methane clouds with a GCM wind profile featuring superrotation, we show that Titan's storms should produce fast eastward gust fronts above the surface. Such gusts dominate the aeolian transport, allowing dunes to extend eastward. This analysis therefore suggests a coupling between superrotation, tropical methane storms and dune formation on Titan. Furthermore, together with GCM predictions and analogies to some terrestrial dune fields, this work provides a general framework explaining several major features of Titan's dunes: linear shape, eastward propagation and poleward divergence, and implies an equatorial origin of Titan's dune sand.

Growth mechanisms and dune orientation on Titan

AbstractDune fields on Titan cover more than 17% of the moon's surface, constituting the largest known surface reservoir of organics. Their confinement to the equatorial belt, shape, and eastward direction of propagation offer crucial information regarding both the wind regime and sediment supply. Herein, we present a comprehensive analysis of Titan's dune orientations using automated detection techniques on nonlocal denoised radar images. By coupling a new dune growth mechanism with wind fields generated by climate modeling, we find that Titan's dunes grow by sediment transport on a nonmobile substratum. To be fully consistent with both the local crestline orientations and the eastward propagation of Titan's dunes, the sediment should be predominantly transported by strong eastward winds, most likely generated by equinoctial storms or occasional fast westerly gusts. Additionally, convergence of the meridional transport predicted in models can explain why Titan's dunes are confined within ±30° latitudes, where sediment fluxes converge.

Global pattern of Titan's dunes: Radar survey from the Cassini prime mission

We present a map of the orientation and extent of Titan's sand dunes derived from the complete radar imaging dataset from the Cassini prime mission. The 16,000 dune segments we have mapped cover ∼8% of the Titan's surface (suggesting a total coverage of ∼20%), are confined within 30° of the equator, and show local and regional deviations of dune orientation of up to about 40° from due Eastwards. There is no obvious global longitudinal pattern, although some divergence with latitude is apparent. The most striking pattern is one of collimation by and divergence around bright and/or high terrain. Obstacles 100–300 m high obstruct dunes when the local slope is 1/50 or steeper, while slopes 1/200 or shallower cause dunes to thin out as they climb, or to deviate around the obstacles.

The drying of Titan's dunes: Titan's methane hydrology and its impact on atmospheric circulation

We explore the effect of a finite reservoir of methane on Titan's atmospheric circulation, precipitation patterns, and surface methane content. We develop a soil model that accounts for the methane cycle in the surface‐atmosphere system, and we implement this surface model in a two‐dimensional model of the Titan's atmosphere. Seasonal oscillations in latitude of the large‐scale circulation accomplish net drying of the low‐latitude surface by diverging methane vapor from low latitudes to higher latitudes. Simulations with an initially deep methane reservoir indicate this mechanism is able to dry ∼1.75 meters of liquid methane per Titan year from the low‐latitude surface. The existence of low‐latitude desert morphologies suggests that the system has had sufficient time to completely remove the surface methane by this mechanism. We then varied the reservoir size, focusing on initial depths of 30 meters of liquid methane or less and compared the results to available observations. The climate system has an abrupt transition to a warmer state with less precipitation and nearly global surface drying near the level at which the atmosphere can store the majority of the methane reservoir as vapor or around 6.5 meters of equivalent liquid methane for our particular choice of parameters. A comparison of our model results with Huygens' observations suggests Titan's climate mimics a state in which most of the methane inventory with direct access to the atmosphere (i.e., excluding underground sources) is stored in the atmosphere.

Titan's Dunes

Titan's Dunes