Temperature dependence of cohesion force of organic aerosols on Titan.

Introduction: Saturn’s largest moon, Titan, has a dense reducing atmosphere, where organic aerosols are formed from CH₄ and N₂ via photochemical reactions [1]. These organic aerosols would eventually deposit on the surface [2] and could affect the formation of organic sediments [3]. Thick organic sediments exist as dunes only at low latitude regions of Titan, but not in the middle latitude regions [4, 5], although both low and middle latitude regions are generally arid [6]. Given no thick organic sediments exist on middle latitude regions where potentially H₂O ice crust exposed [7], organic aerosols at middle latitudes may have been transported spontaneously. Previous studies, however, have considered that saltation of organic aerosols would occur only by strong winds due to CH₄ storms at low latitudes [8], but would not occur by seasonal wind at low and middle latitudes based on the high cohesiveness of Titan tholin measured at room temperature [9, 10]. Cohesion force, however, would be different at Titan’s surface temperature (~93 K) [11] because surface energy of organic materials would have temperature dependence. Nevertheless, temperature dependence of cohesion force and the surface energy of Titan’s organic analog materials have been poorly understood.

Here we report our experimental results of cohesion force measurements of Titan tholin at low temperatures (117–300 K) using an atomic force microscope (AFM). We investigated temperature dependence of the surface energy of Titan tholin. Using obtained results, we discussed saltation threshold wind speed of organic sands on Titan’s surface temperature (~93 K) [11].

Methods: The methodology of the formation of laboratory analog of Titan’s organic aerosols (so-called Titan tholin) was based on the previous studies and described elsewhere [3, 12]. An Au-coated cantilever (SI-DF3-A, Seiko Instruments Inc.) and a Si wafer substrate (~5 × ~5 mm; thickness 0.5 mm; SI-500443, Niraco Inc.) were set in a quartz-glass chamber. Films of Titan tholin were formed on the tip of cantilever and the Si wafer substrate after cold plasma irradiation onto gas mixture of CH₄/N₂ = 10/90 at ~200 Pa.

Cohesive forces of organic materials were measured at temperatures of 117–300 K and under a pressure of 2.0 × 10^-4 Pa using an atomic force microscope (AFM) (SII Nanonavi E-sweep, SII technology Inc.). The force curve measurements were conducted 3–10 times in total at 2–5 different locations on the sample. The morphology of Titan tholin-coated tip of the cantilever was observed using a FE-SEM to estimate contact radius during measurements. Surface energy of Titan tholin was estimated by applying DMT theory. The surface energy  was fitted with a curve using Arrhenius equation.

Results & Discussion: Our results suggest that the cohesion force decreases as the temperature decreases. We have confirmed reproducibility of the experiments before and after measurements at 117 K. Temperature dependence of cohesion force would be derived from 1) decreasing of surface energy of Titan tholin or 2) decreasing of contact radius due to increasing of the elasticity with temperature drop. If the former is the case, the surface energy can be estimated from cohesion forces and constant contact radius. Based on FE-SEM images, the diameter of the Titan tholin-coated tip is estimated to be 83 ± 5 nm, which corresponds to the maximum contact radius.  Using the values of the maximum contact radius, the surface energy of Titan tholin was calculated. Our also results suggest that the surface energy of Titan tholin decreases as temperature decreases, following the Arrhenius equation with an activation energy E_surf = 1760 ± 190 J mol⁻¹ and γ₀= 180 ± 20 mJ m^-2. Similar trend of temperature dependence of surface energy of H₂O ice has been reported in the previous study [14].

Implications for Titan: Our results suggest that given the temperature dependence of surface energy of Titan tholin, cohesiveness of organic particles would be weaker at lower temperature compared to 300 K (e.g., a factor of 5–6 lower at 93 K). As a result, the threshold wind speed u* of saltation of organic materials on Titan can be as low as 0.05 m/s, which is about 1/3 times the previously estimated u* for saltation based on the surface energy of Titan tholin at 300 K [9, 10]. Since the wind speed of tidal winds at middle latitudes would reach to 0.07 m/s during summer [15], our results suggest that organic particles would saltate by the tidal winds. Given that the direction of tidal winds is equatorward [4, 15, 16], organic particles at middle latitudes can be transported toward low latitudes, where large-scale dunes of organic materials exist.

Acknowledgments: This study was financially supported by KAKENHI JSPS (Grant JP24KJ1047) and supported by "Advanced Research Infrastructure for Materials and Nanotechnology in Japan (ARIM)" of the Ministry of Education, Culture, Sports, Science and Technology (MEXT). Proposal Number JPMXP1222AT5000.

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