Salts trapping mechanisms within ice in frozen saline lakes in Mongolia: Implications for salt-containing icy crust of Solar System bodies.

Geologically-active icy bodies in the Solar System, such as Europa, Enceladus, and Ceres, possess (or possessed) subsurface oceans under the icy crusts. These icy crusts may contain salt through trapping of oceanic water at the interface between the crust and ocean. Salt materials in the icy crusts can affect the rheology, surface material evolution, and oceanic compositions. However, no quantitative investigations on the mechanisms of trapping of oceanic water have been conducted.

Here, we conduct a field survey for frozen saline lakes, the Orog and Olgoy lakes, in south-central Mongolia in February 2019. Both of the lake depths are ~1 m. The Orog lake was covered with ice with thickness of ~0.9 m; whereas, the Olgoy lake was completely frozen. Drilled ice samples were collected at both of the lakes at a few sites from the surface to bottom for every 0.1 m. We also collected bottom lake water at the Orog lake. The ice samples were freeze-dried and, then, analyzed with X-ray diffraction (XRD) and X-ray absorption fine structure analysis (XAFS). The collected lake water was analyzed with an inductive coupled plasma optical emission spectrometer (ICP-OES) and an ion chromatograph.

The ice samples contain 0.01 to 0.1 wt% of NaCl and Na₂SO₄. The total salt contents increase with depth, although the contents vary 3-4 times at different drilling sites. Since the collected bottom lake water is unsaturated with these salts, the salts in the samples are trapped lake water. The volume fraction of trapped lake water in ice is estimated to be up to ~5%. This fraction is several times larger than prediction by the sea ice formation model that considers trapping of seawater in pores of ice grains. This indicates that lake water would be trapped effectively within cracks in ice in addition to pores of ice grains. Such effective trapping of oceanic water may also occur in icy crusts of Solar System ice bodies. Viscosity of the ice decreases in one order of magnitude when volume fraction of liquid exceeds 5%. This result implies that effective salts trapping processes, e.g., cracks, would change rheology of the lowermost part of icy crust.