[M-IS19] Aqua planetology

[MIS19-08] Origin of chemical diversity of Enceladus’ plume particles

*Minori Koga¹, Yasuhito Sekine¹ (1.Tokyo Institute of Technology)

Saturn’s moon Enceladus is one of the most geologically-active icy satellites in the Solar System, having a subsurface ocean and cryovolcanic plumes erupted from the ocean. Recent observations revealed a variety in chemical (salt) compositions of plume particles, which include NaCl-rich, Na-(bi)carbonate-rich, and Na-phosphate-rich particles. These observations are enigmatic considering the plumes originated from the subsurface ocean with one single composition. In this study, we investigate segregation processes of salt compositions of plume particles upon cryovolcanism on Enceladus. We first performed decompression experiments to simulate formation of droplets at the interface with the subsurface ocean. In the decompression experiments, droplets with size up to 100 μm are generated upon bubbling of Enceladus’ seawater analog due to decompression. We find a heterogeneous spatial distribution of salts (NaCl and Na-carbonates) in large-sized, freeze-dried droplets. This happens because Na carbonates first precipitate within a freezing droplet upon an initiation of H₂O ice formation. Then, NaCl would precipitate later in remaining brine pockets in a freezing droplet. To further quantify the conditions for generating the heterogeneous distribution of salts, we performed freezing experiments, in which droplets with different sizes were frozen at various freezing rates. Our results show that spatial heterogeneity of salt appears in frozen droplets with size of 10 μm or greater regardless of freezing rate. We suggest that if droplets formed at the interface with Enceladus’ ocean are ~10 μm or greater, they would have a spatial heterogeneity of salts upon freezing in ascending within fractures of the icy crust. Such ice droplets would, then, collide onto walls of the icy crust at ~100 m/s at the uppermost part of the fractures. Kinetic energy of the collision would exceed the material strength of ice particles with size of 10 μm, possibly leading to catastrophic disruption and formation of small fragments with different salt compositions. We suggest that the step-wise precipitations of salts in freezing droplets and subsequent collision-induced disruption can generate the observed chemical diversity of Enceladus’ plume particles.