[M-IS14] Aqua planetology

[MIS14-02] Field investigations on salt partitioning in frozen closed-basin lakes in Mongolia as
terrestrial analogues of subsurface brine reservoirs on Solar System icy bodies

*Masahiro Yoda^1,2, Yasuhito Sekine^2,3, Keisuke Fukushi³, Takuma Kitajima⁴, Baasansuren Gankhurel⁴, Davaadorj Davaasuren⁵, Tuvshin Gerelmaa⁵, Shuukhaaz Ganbat⁴, Daigo Shoji⁶, Yoshio Takahashi¹ (1.Department of Earth and Planetary Science, The University of Tokyo, 2.Earth-Life Science Institute of Tokyo Tech, 3.Institute of Nature and Environmental Technology, Kanazawa University, 4.Division of Earth and Environmental Sciences, Graduate School of Natural Science and Technology, 5.Department of Geography, The National University of Mongolia, 6.Institute of Space and Astronautical Science, Japan Astronautical Exploration Agency)

Keywords:Freezing brine within icy bodies, Water chemistry of frozen saline lakes, Ice-mineralization

Geologically-active icy bodies, e.g., Europa and Ceres, would have possessed near-
surface brine reservoirs, which may be a source of salts on the surfaces (e.g., Europa’s
chaos terrains and Ceres’ bright spots). Previous studies hypothesized a few processes,
in which an overlying ice layer can capture salinity from a subsurface brine reservoir;
however, few field investigations have been conducted for inferring the salt partitioning
between ice layers and subsurface reservoirs on the icy bodies.
Here, we report results of field surveys to ice-covered, closed-basin saline lakes of
Orog and Olgoy lakes in Mongolia in February 2019 and January 2020. The surface ice
exhibited complex geomorphology with pressurized ridges formed by compressive
force and wet cracks formed by extensional force. We collected the ice samples,
including both of the contraction and wet cracks, as well as underlying lake water
samples at the lakes. The salinity of ices collected from field surveys suggest that wet
cracks could provide salinity to the ice layer and surface. In addition, concentrated
bottom lake water, together with suspended authigenic carbonates, would be captured
within the ice layer during freezing. We evaluate both of the salts partitioning and lake
water chemistry in the frozen lakes using a coupled model of mass balance and low-
temperature aqueous chemistry. Our model results can reproduce both the salinity in ice
and major dissolved species, e.g., Na⁺, Cl^-, and SO₄^2-, in the bottom lake water. To explain
the Mg²⁺ and Ca²⁺ concentrations of in the bottom lake water metastable carbonates of
amorphous magnesium carbonate and monohydrocalcite should be taken into account.
We discuss the implications of the salts partitioning and water chemistry to subsurface
liquid reservoirs of the icy bodies.