High-temperature water-rock interactions and hydrothermal environments in the chondrite-like core of Enceladus.

Yasuhito Sekine,Motoko Yoshizaki,Takazo Shibuya,Hsiang-Wen Hsu,Katsuhiko Suzuki,Megumi Mori,Tatsu Kuwatani,Sin-Iti Sirono,Frank Postberg,Yuka Masaki,Shogo Tachibana,Peng K Hong

doi:10.1038/ncomms9604

Abstract

It has been suggested that Saturn's moon Enceladus possesses a subsurface ocean. The recent discovery of silica nanoparticles derived from Enceladus shows the presence of ongoing hydrothermal reactions in the interior. Here, we report results from detailed laboratory experiments to constrain the reaction conditions. To sustain the formation of silica nanoparticles, the composition of Enceladus' core needs to be similar to that of carbonaceous chondrites. We show that the presence of hydrothermal reactions would be consistent with NH3- and CO2-rich plume compositions. We suggest that high reaction temperatures (>50 °C) are required to form silica nanoparticles whether Enceladus' ocean is chemically open or closed to the icy crust. Such high temperatures imply either that Enceladus formed shortly after the formation of the solar system or that the current activity was triggered by a recent heating event. Under the required conditions, hydrogen production would proceed efficiently, which could provide chemical energy for chemoautotrophic life.

Highlights

It has been suggested that Saturn’s moon Enceladus possesses a subsurface ocean
Water-rich plumes of vapour and ice particles with sodium salts erupting from warm fractures near the south pole of Saturn’s icy moon Enceladus suggest the presence of a liquid water reservoir in the interior[1,2,3,4]
Products of ongoing hydrothermal reactions would have been transported upwards from an interior ocean located at a depth of B30 km beneath the surface at Enceladus’ south pole[6,7] and would have been ejected into the plume[8]

Summary

Introduction

It has been suggested that Saturn’s moon Enceladus possesses a subsurface ocean. The recent discovery of silica nanoparticles derived from Enceladus shows the presence of ongoing hydrothermal reactions in the interior. Supported by the results of hydrothermal experiments, it is indicated that these particles originated from nanosilica colloids that formed when silica saturation was reached upon cooling of hydrothermal fluids[5] The presence of these particles provides tight constraints on the particular conditions of the interior ocean; that is, the presence of high-temperature reactions (ZB90 °C), moderate salinity (rB4%), and alkaline seawater (pH 1⁄4 8.5–10.5)[5]. Na þ is a major constituent in Enceladus’ alkaline ocean[1,11,12], as Cassini has detected sodium salts, such as NaHCO3 and NaCl, in the plume’s ice grains[1,2] Another possible difference between hydrothermal reactions on Enceladus and those currently occurring on Earth is the rock composition. We propose thermal evolution scenarios that could support ongoing hydrothermal activity within Enceladus

Methods

Results

Conclusion