Where Does Titan Sand Come From: Insight From Mechanical Properties of Titan Sand Candidates

Abstract

Extensive equatorial linear dunes exist on Titan, but the origin of the sand, which appears to be organic, is unknown. We used nanoindentation to study the mechanical properties of a few Titan sand candidates, several natural sands on Earth, and common materials used in the Titan Wind Tunnel, to understand the mobility of Titan sand. We measured the elastic modulus (E), hardness (H), and fracture toughness (Kc) of these materials. Tholin's elastic modulus (10.4 ± 0.5 GPa) and hardness (0.53 ± 0.03 GPa) are both an order of magnitude smaller than silicate sand, and it is also smaller than the mechanically weak white gypsum sand. With a magnitude smaller fracture toughness (Kc = 0.036 ± 0.007 MPa·m1/2), tholin is also much more brittle than silicate sand. This indicates that Titan sand should be derived close to the equatorial regions where the current dunes are located, because tholin is too soft and brittle to be transported for long distances.

Conditions for aeolian transport in the Solar System

“…Venusian sands are likely sourced by non-aeolian mechanisms, and transport has negligible hysteresis akin to water on Earth 7 . On Titan, more work is needed to understand the aggregation and attrition of Tholins, but our results support locally-sourced and low-density Tholin dune sands 4,34 . Finally, on Mars we suggest that sand grains observed by Curiosity actively produce dust through attrition 35 , and require stronger winds to move than GCMs predict 17 -perhaps through katabatics 36 or strong instability in the boundary layer 16 .…”

“…from Evans & Goetze (1979) M54 , Yu et al (2018) 4 and Courtney (2005) M55 , respectively, where H is the hardness measured with a nano-indentor of angle β I , E 0 (T 0 ) is the elastic modulus measured at temperature T 0 and T m is the melting temperature. We note that it would be ideal to use a theory on chipping of geologic materials over this approach, but current theories require measurements of fracture toughness that have not been taken for material and environments applicable to this study 27,M56 .…”

“…Researchers are challenged to explain how Mars' thin atmosphere produces sufficiently strong winds to maintain active dunes 17 . And there is no consensus on the sediment composition of dunes on Titan 4 or Pluto 3 , where candidate materials are either too weak or too heavy for sustained saltation. These debates suggest that our theoretical understanding of threshold and collision dynamics of wind-blown sediments are incomplete.…”

Spectral Science Priorities for an Infrared Spectrometer with Interstellar Probe

“…To supplement these observations, advances in modeling capabilities can further assist in IR chemistry and spectroscopy. Laboratory work for low temperature-low pressure studies have also improved to better understand the origin of ice grain condensation and irradiated products, such as tholins and other complex by-products, on planetary or cometary surfaces (McDonald et al 1996;Yu et al 2018).…”

Single particle triboelectrification of Titan sand analogs

“…On Titan, the dune particles are interpreted to be made of mainly organics or organic coated ices (e.g., McCord et al, 2006;Soderblom et al, 2007;Clark et al, 2010). There are several possible candidates that could make up Titan's sand particles (see e.g., Barnes et al, 2015;Yu et al, 2018), including complex tholin-like organics, evaporites (dissolved part of the complex organics in Titan's liquid hydrocarbon lakes and seas), and water ice that makes up the bulk of Titan's crust.…”