In the search for extraterrestrial life, liquid water has long been regarded as crucial, and subsurface oceans on moons and other rocky bodies in the outer solar system have emerged as prime search sites. This is a key reason that scientists show interest in underground oceans such as the one inferred to exist on Pluto by the New Horizons mission. Oceans can exist beneath the surfaces of cold celestial bodies due to internal heating from radioactive decay, as well as tidal stretching and squeezing. Radioactive decay of heavy elements like uranium releases energy, heating planetary interiors. Tidal stretching and squeezing is caused by the motion of nearby massive celestial bodies, resulting in frictional heating.

Amirhossein Bagheri (Institute of Geophysics in Zürich, Switzerland) and colleagues investigated the histories of the subsurface oceans of Pluto and its moon Charon using a computer model they developed. They examined the balance of energy between internal heating and cooling due to radiation into space and were able to determine how the internal temperatures of Pluto, Charon, and the depths of their subsurface oceans varied over time. Model parameters include viscosity of ice, radius of the core, initial ocean thickness, and percentage of ammonia in the underground oceans. Bagheri and colleagues predict that Pluto still has a subsurface ocean with a thickness between 25 and 95 miles (40 and 150 kilometers), while Charon’s ocean completely froze sometime in the last 500 million years. For Pluto, more viscous ice, a smaller core, and more ammonia would result in a thicker subsurface ocean today, while for Charon, only the smaller core substantially affected the time its subsurface ocean froze. Missions incorporating seismometers, such as the future Europa and Titan Dragonfly, will be better equipped to investigate subsurface oceans on solar system bodies. READ MORE