Water worlds and oceans may be common in the Universe

Abstract

Are oceans of liquid water common in the universe, or can they be maintained only under relatively special conditions? This question is highly relevant to speculations about habitats for extra‐terrestrial life, and yet attempts toward an answer have not yet fully exploited reasonable constraints that can be derived from knowledge of the ocean dynamics. While previously there was only one known example of an ocean, recent observations from the Galileo and Cassini spacecraft have indicated that there are at least several oceans in our solar system, with the most typical example represented by ice‐covered oceans on satellites of large planets. In this case the primary source of heat maintaining these liquid oceans is not the sun and identification of the true heat source and its relationship with the ocean state is a research priority. Here we use dynamical considerations to show that initially liquid oceans may indeed be rather hard to completely freeze, suggesting that the persistence of liquid oceans since even primordial times may be relatively common. As an ocean freezes, not only does the accumulating ice layer reduce the rate of heat loss, but the reduction in the depth of the remaining liquid ocean changes the ocean’s dynamic response to tidal forces. It seems that for a wide range of starting assumptions an ocean attempting to freeze must pass through resonance peaks where the tidal‐flow response greatly increases in amplitude. Dissipation of this tidal‐flow energy is a source of heat that can counter further freezing. Because it appears that a freezing ocean must typically encounter such resonant states that work to reduce further freezing, the persistence of liquid oceans may be a common feature in the universe.