Ice Shell Structure and Composition of Ocean Worlds: Insights from
Accreted Ice on Earth
Abstract
Accreted ice retains and preserves traces of the ocean from which it
formed. In this work we study two classes of accreted ice found on
Earth—frazil ice, which forms through crystallization within a
supercooled water column, and congelation ice, which forms through
directional freezing at an existing interface—and discuss where each
might be found in the ice shells of ocean worlds. We focus our study on
terrestrial ice formed in low temperature gradient environments (e.g.,
beneath ice shelves), consistent with conditions expected at the
ice-ocean interfaces of Europa and Enceladus, and highlight the
juxtaposition of compositional trends in relation to ice formed in
higher temperature gradient environments (e.g., at the ocean surface).
Observations from Antarctic sub-ice-shelf congelation and marine ice
show that the purity of frazil ice can be nearly two orders of magnitude
higher than congelation ice formed in the same low temperature gradient
environment (~0.1% vs. ~10% of the
ocean salinity). In addition, where congelation ice can maintain a
planar ice-water interface on a microstructural scale, the efficiency of
salt rejection is enhanced (~1% of the ocean salinity)
and lattice soluble impurities such as chloride are preferentially
incorporated. We conclude that an ice shell which forms by gradual
thickening as its interior cools would be composed of congelation ice,
whereas frazil ice will accumulate where the ice shell thins on local
(rifts and basal fractures) or regional (latitudinal gradients) scales
through the operation of an “ice pump”.