Titan's prevailing circulation might drive highly intermittent, yet significant sediment transport

Abstract

Titan, the largest moon of Saturn, is characterized by gigantic linear dunes and an active dust cycle. Much like on Earth, these and other aeolian processes are caused by the wind-driven mobilization of surface grains, known as saltation. To date, very little is known about the conditions that allow for the occurrence of saltation on Titan. In fact, Titan saltation may be fundamentally different from Earth saltation given the denser atmosphere, the lower gravity, and the cohesion of its surface grains. Here, we draw on experiments, theory, and modeling to progress towards a comprehensive understanding of saltation on Titan. We find that aerodynamic lifting of surface grains requires strong wind speeds due to the high cohesion of the grains. However, saltation may be sustained through granular splash at wind speeds much smaller than those required to initiate grain motion. This suggests that most saltation transport on Titan is intermittent rather than continuous. We account for these insights by proposing a saltation mass flux parameterization specific for Titan conditions that accounts for transport intermittency, and use it to quantify yearly sediment transport with a general circulation model. The results show that Titan's prevailing atmospheric circulation is capable of generating highly intermittent yet significant saltation, yielding yearly transport rates similar to those on the most active dunes of Mars. Furthermore, we find that accounting for surface topography might be critical to answering open questions related to Titan's landscape evolution, including the formation of linear dunes in opposite direction to the prevailing circulation. Significance statementThe Cassini-Huygens mission has revealed that Titan's landscape evolution and dust cycle may be controlled by the wind-driven transport of surface grains, known as saltation. It is still unclear, however, how saltation can occur on Titan despite the weak winds and the stickiness of the surface grains. Using a combination of experiments, theory, and modeling, we find that, like saltation on Mars, saltation on Titan can be sustained at much lower wind speeds than those required to lift grains from the surface. Accordingly, the prevailing weak winds on Titan may be capable of driving intermittent yet significant saltation, with transport rates comparable to those observed on the most active dunes on Mars.

Modeling Snow Saltation: The Effect of Grain Size and Interparticle Cohesion

“…Further efforts must be made to fully model the effect of bed characteristics on snow saltation. For example, interparticle cohesion is also expected to influence particle ejection velocity during splash and the fluid threshold for the onset of aerodynamic entrainment (Comola et al., 2021). Moreover, the strength of interparticle bonds between grains that did not leave the surface and between those that failed to rebound might not be the same.…”

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

“…Eolian abrasion is then modeled by scaling fluvial abrasion rates to account for the different fluid properties and for typical eolian saltation trajectories on Titan (Kok et al., 2012; Figure 2a; Section 2.3). This approach is justified because, owing to Titan's thick atmosphere, windblown sand transport is expected to be more akin to sand transport by water on Earth than by air on Earth and Mars, with minimal importance of grain splash (Kok et al., 2012) unless sand is highly cohesive (Comola et al., 2022). In the case of cohesive sediments, our scaling relationship for eolian abrasion rates still provides a first‐order estimate of abrasion rate for the saltating grain population, whereas splashed grains would likely experience lower abrasion rates.…”

“…From saltation mechanics, we know that grain sizes within dune fields should be relatively narrowly distributed, and on Titan, are predicted to be around 150–300 μm in diameter (Burr et al., 2015; Kok et al., 2012), although slightly finer or coarser grains may be permitted if dune materials were cohesive (e.g., Comola et al., 2022; Lorenz, 2017). Finer grains are more prone to stabilizing interparticle interactions (possibly including triboelectric charging; Mendéz Harper et al., 2017) that raise the wind speed required to mobilize them; in turn, coarser grains are heavier and thus also harder to move.…”

Sand Transport on Titan: A Sticky Problem

“…The modeling in Comola et al. (2022) disagrees with threshold speeds higher than actually measured in Titan‐like laboratory measurements (Burr et al., 2015). The wind tunnel (a repurposed Venus tunnel, only 20 cm in diameter) in these experiments was not operated at Titan conditions, but at higher pressures (12 vs. 1.5 bar) and temperatures (293K vs. 94), aiming to match the ratio of density to viscosity on Titan.…”

“…A penetrometer instrument on the Huygens probe in 2005 determined that the streambed sediments at the landing site were damp (Atkinson et al., 2010), data initially interpreted (in)famously as indicating mechanical properties similar to crème brûlée (e.g., Lorenz, 2020). Some new work (Comola et al., 2022) explores the implications of possibly enhanced cohesion of the carbon‐rich sands elsewhere on Titan.…”