Not so lonely sea in the sky

A great hole in Pluto

IS THE solar system about to get another ocean? So far, besides Earth, six bodies are known or suspected to harbour oceans. These are Europa, Callisto and Ganymede (all moons of Jupiter), Enceladus and Titan (both moons of Saturn) and Triton (a moon of Neptune). The latest candidate is Pluto, the most famous inhabitant of the Kuiper belt, a girdle of asteroids that orbit the sun beyond Neptune.

Pluto’s claim to an ocean, argued this week in two papers published in Nature, is based on data collected in 2015 by New Horizons, a robotic spacecraft that zoomed past it in July of that year. The ocean in question, if it exists, is beneath Pluto’s surface. That makes it unlike Earth’s ocean, but like those of the other six bodies. To human sensibilities that is, perhaps, a funny sort of ocean. But add it to the other six and it is Earth’s surface ocean that looks anomalous, rather than Pluto’s buried one.

The argument for a Plutonic ocean—advanced by teams led by Francis Nimmo of the University of California, Santa Cruz, and James Keane, of…Continue reading

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