Alien Oceans

Kevin Peter Hand

Princeton Univ., $27.95

Life as we know it requires liquid water. That’s why astronomers get excited when a planet is found in a star’s “habitable zone,” the Goldilocks region in which a planet is neither too close nor too far from its star to have liquid surface water.

In Alien Oceans, NASA scientist Kevin Peter Hand argues that the notion of the habitable zone should be expanded. At least six moons in the outer solar system are likely to have oceans of water beneath icy facades. For these orbs, the sun isn’t the heat source that keeps oceans liquid. Instead, the decay of radioactive elements inside a moon’s rocky core might keep things warm. Another possibility is tidal flexing: If a moon follows an elongated orbit around a planet, ever-changing tides would create friction within the core and release heat. Similar conditions could allow for ice-covered oceans on planetary bodies in other solar systems too.

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Chapter by chapter, Hand lays out the evidence for the existence of distant oceans in our solar system. Take Europa, Jupiter’s fourth-largest moon. As far back as the early 1970s, spectroscopic images from an Earth-based telescope hinted at the presence of water ice. During the late 1990s and early 2000s, repeated flybys of Europa by the Galileo spacecraft helped reveal how the moon’s mass is distributed in layers; some layers are denser than others, hinting at the presence of water. Data from Galileo’s magnetometer, which detected changing magnetic fields caused by the flow of charged particles, bolstered the idea that the moon hosts a salty ocean.

Similar observations point to subsurface oceans on two other moons of Jupiter, Callisto and Ganymede; Enceladus and Titan, which circle Saturn; and Neptune’s moon Triton.

Liquid water alone is not enough to make these moons habitable. Hand discusses other conditions needed for life, including chemical substances that would have to be present for organisms to build bodies and have metabolisms. He also describes plans to further explore these places. Missions could include landers that carry robotic probes that would melt or drill their way through ice and then release a mini-sub to explore the hidden ocean (SN: 5/17/14, p. 20).

Alien Oceans offers a historical look — as well as a peek into the future — at one of the most exciting aspects of space exploration. With the technology at hand, we could determine whether there’s life beyond Earth.

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