Dynamics of ocean worlds is controlled by their rotation

(ORDO NEWS) — The discovery that many of the large moons in the outer solar system may contain significant subsurface oceans of liquid water has been a key advance in planetary science. These moons represent some of the most promising habitats for life outside of Earth, but their hidden nature makes direct study difficult.

These oceans are located at a depth of tens or even hundreds of kilometers, limited from above by a thick ice shell, and from below by a source of geothermal heating.

A key element for understanding their nature is deriving patterns of ocean circulation, since it is the ocean that transports heat, salt and potential biosignatures to the surface, where they can be detected by future space missions.

Although some previous studies modeled the dynamics of subsurface oceans, these calculations relied on parameters that were loosely constrained by observations.

In a new study published in the Journal of Geophysical Research: Planets, Bire et al. take a novel approach by presenting their simulations in terms of a dimensionless number, the natural Rossby number, which is the ratio of buoyancy flux, moon rotation rate, and ocean depth, for which has observational restrictions.

The authors present a series of simulations that explore a wide range of parameters for ocean depth, lunar rotation rate, and driving heat flux.

In the regime of low Rossby numbers, probably suitable for icy moons, the rotation speed of the simulated moon has a strong influence on the dynamics of the subsurface ocean. This is contrary to the currently accepted model.

According to arguments based on the well understood dynamics of a rotating fluid in a spherical shell, ocean circulation is divided into two regions. At higher latitudes, convective plumes propagate from bottom to top parallel to the Moon‘s rotation axis.

But at lower latitudes, water moves around the Moon in a longitudinal direction and interacts less intensively with the ocean floor.

This flow pattern likely reduces the efficiency of transferring geothermal heat from the depths of the Moon through the ocean to the surface. Therefore, equatorial regions are less efficient in transporting heat than polar regions, which has important implications for the thickness of the ice sheet on the surface.

According to the authors, the turbulence created by the global convective process likely resulted in bands of alternating ocean currents, similar to the mechanism that creates the colorful zones and belts found in Jupiter’s atmosphere.

In fact, the general circulation pattern found in the oceans of these outer moons of the solar system may bear a striking resemblance to that of Jupiter’s parent.


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