A moon of Jupiter may conceal under its icy
surface an ocean with more water than all of that
on Earth’s surface, new findings suggest.
Scientists used NASA’s Hubble Space Telescope
to investigate Ganymede, the Solar System’s
largest moon—about one-and-a-half times wider
than ours.
Ganymede is also the only moon with its own
magnetic field. The magnetic field causes
aurorae, which are ribbons of glowing, hot
electrified gas, in areas circling the north and
south poles.
But Ganymede is also embedded in Jupiter’s
magnetic field. When Jupiter’s magnetic field
changes, the aurorae on Ganymede also change,
“rocking” back and forth, scientists explain.
The thinking behind the study is that if a
saltwater ocean were there, Jupiter’s magnetic
field would create a secondary magnetic field in
that ocean. This secondary field would counter
Jupiter’s own—and as a result, suppress the
rocking of the aurorae.
This suppression indeed happens, and so strongly
that it reduces the rocking of the aurorae by
some two-thirds, the investigators calculated. As
a result, they estimate the ocean is 60 miles (100
kilometers) thick — 10 times deeper than Earth’s
oceans — and is buried under a 95-mile (150-
kilometer) crust of mostly ice.
Scientists led by Joachim Saur of the University
of Cologne in Germany came up with the idea of
using Hubble to learn more about the inside of
the moon.
“I was always brainstorming how we could use a
telescope in other ways,” said Saur. “Is there a
way you could use a telescope to look inside a
planetary body? Then I thought, the aurorae!
Because aurorae are controlled by the magnetic
field, if you observe the aurorae in an appropriate
way, you learn something about the magnetic
field. If you know the magnetic field, then you
know something about the moon’s interior.”
The findings were published online in the Journal
of Geophysical Research: Space Physics on March
12.