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Moon and Jupiter

June 20, 2015

The worlds of the solar system hold many secrets. The largest moon, for example, conceals an ocean far below its icy crust. It may contain more water than all of Earth’s oceans combined.

Ganymede is the largest moon of Jupiter, the solar system’s largest planet. It’s half again the diameter of our own moon. Unlike the Moon, though, its surface consists mainly of ice.

Ganymede is the only moon that’s known to generate its own magnetic field. More than a decade ago, the Galileo spacecraft detected variations in that field. The most likely cause was the sloshing of an underground fluid — probably a salty ocean.

More evidence of an ocean was reported this year. Scientists used Hubble Space Telescope to study interactions between Ganymede’s magnetic field and the field of nearby Jupiter. Those interactions produce a slight offset in Ganymede’s aurorae, which are like the northern and southern lights here on Earth. The offset is best explained by a salty ocean.

The ocean is probably about 60 miles deep — more than 20 times the average depth of Earth’s oceans. But it’s buried beneath Ganymede’s crust, about a hundred miles below the surface — a watery secret on a giant moon.

Jupiter is in good view tonight. The bright planet is close to the upper right of the Moon at nightfall, with the even brighter planet Venus to the lower right of Jupiter. Through binoculars, Ganymede and Jupiter’s other big moons look like tiny stars near the brilliant planet.


Script by Damond Benningfield, Copyright 2015

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