Sand Transport on Titan: A Sticky Problem

Abstract

Saturn's moon Titan has the only solid surface in the solar system today, apart from our own, on which it rains. As any child on a beach knows well, the presence of even small amounts of dampness can drastically change the mechanical properties of sand by providing cohesion between the grains. 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.Ironically, it was the expected presence of moisture (in frigid Titan's case, liquid methane) that might immobilize sand that led this author to speculate before Cassini (Lorenz et al., 1995) that sand dunes would not be found. That prediction was spectacularly inaccurate, since more than 10% of Titan's surface proved to be draped with giant dark sand dunes, hundreds of km long and in some places over 100 m high. This welcome surprise arose from Titan's climate, which segregates the methane moisture into polar lakes and seas, dessicating the equatorial regions.This drying of low latitudes is a feature that was quickly found in Global Circulation Models (GCMs) of the climate (and in part results from Titan's slow 16-day rotation). Much subsequent work focused on attempting to relate the dune morphology (linear, e.g., Radebaugh et al., 2010) and orientation (indicating sand migration toward the east, e.g., Lorenz and Radebaugh, 2009) to the wind patterns predicted in these models.After some initial confusion, in that the average winds at low latitudes should be toward the west, the big-picture agreement of models with the pattern has been satisfying. In fact, the dominant near-surface flows are seasonally alternating north-south winds-such bidirectional flows in abundant sands give the linear morphology. The wrinkle is that during the stormy equinox seasons (around 2009, 2024, and 2039) occasional storms pull down the prograde eastwards flow from higher altitudes, and so the winds that are strong enough to move sand are biased in that direction, even though the usual (gentler) winds are biased westwards. While the details differ between models (e.g., Charnay et al., 2015;Lucas et al., 2014;Tokano, 2010) the general picture seems secure. Since 100 m dunes take millennia to grow and change, the details of the dune pattern may also carry some memory of recent Croll-Milankovich climate cycles (e.g., Ewing et al., 2015;McDonald et al., 2016).In part stimulated by the Titan discoveries, models of dune formation, migration, and growth have been studied extensively in the last decade. In particular, it has been recognized that limited sand supply (which may just be a manifestation of higher cohesion limiting transport) can lead to a dune alignment (Courrech du Pont et al., 2014) that is quite different from the traditional one for a...

Detection and characterization of wind-blown charged sand grains on Titan with the DraGMet/EFIELD experiment on Dragonfly

“…There is an optimum particle size for which the threshold wind speed for wind-driven transport is minimum. To assess this optimum size, it is key to understand the ability of Titan's particles to charge (Lorenz, 2022). In particular, triboelectric charging of sand grains could create strong inter-particle forces requiring higher wind speeds for saltation and suspension (Lorenz, 2014).…”

Gentle Perseverance Lifts the Veil on Martian Dust

“…Recent simulations of the atmospheric dynamics in the almost unknown Venusian planetary boundary layer (Lefèvre, 2022) suggest that convective plumes that can loft dust may be common in daytime. At Titan, the Dragonfly rotorcraft lander will perform experiments using its rotor downwash to interrogate the sand‐ and dust‐moving thresholds (Lorenz, 2022). But it is at Mars where understanding the dust threshold is perhaps most critical: this parameter—perhaps rather lower than had previously been thought—opens the way to the interpretation of natural surface darkening such as dust devil tracks and impact blast waves (Daubar et al., 2016).…”