Abstract
The study of planetary oceanography is a new and exciting field of research. While humanity’s formal scientific studies of Earth’s ocean began nearly 150 years ago with the launch of the Challenger Expedition (Thomson et al., 1873), the study of oceans beyond Earth commenced only in this millennium. The first confirmation of an extensive saltwater ocean anywhere beyond Earth came relatively late within the lifetime of NASA’s Galileo mission (1989–2003; Kivelson et al., 2000), but continuing exploration has now revealed compelling evidence for large-volume watery oceans on five ice-covered moons of our outer solar system (Figure1), with as many as 10–20 candidate moons and dwarf planets also under consideration (Hendrix et al., 2019). Of the five confirmed ocean worlds (Jupiter’s moons Europa, Callisto, and Ganymede; Saturn’s moons Enceladus and Titan), three have oceans so deep that a high-pressure form of ice develops deep within the ocean, beneath the liquid water but overlying any rocky interior (Nimmo and Papallardo, 2016). As a consequence, the watery ocean is trapped within an “ice sandwich.” By contrast, the other two confirmed ocean worlds (Europa and Enceladus) have oceans that are in direct contact with a rocky interior.