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NASA’s
Hubble Space Telescope has the best evidence
yet for an underground saltwater ocean on
Ganymede, Jupiter’s largest moon. The
subterranean ocean is thought to have more
water than all the water on Earth's surface.
Identifying liquid water is crucial in the
search for habitable worlds beyond Earth and
for the search of life as we know it.
“This discovery marks a significant
milestone, highlighting what only Hubble can
accomplish,” said John Grunsfeld,
associate administrator of NASA’s Science
Mission Directorate at NASA Headquarters,
Washington. “In its 25 years in orbit,
Hubble has made many scientific discoveries
in our own solar system. A deep ocean under
the icy crust of Ganymede opens up further
exciting possibilities for life beyond
Earth.”
Ganymede is the largest moon in our solar
system and the only moon with its own
magnetic field. The magnetic field causes
aurorae, which are ribbons of glowing, hot
electrified gas, in regions circling the
north and south poles of the moon. Because
Ganymede is close to Jupiter, it is also
embedded in Jupiter’s magnetic field. When
Jupiter’s magnetic field changes, the
aurorae on Ganymede also change,
“rocking” back and forth.
By watching
the rocking motion of the two aurorae,
scientists were able to determine that a
large amount of saltwater exists beneath
Ganymede’s crust affecting its magnetic
field.
Hubble telescope image of Ganymede auroral
belts
NASA Hubble Space Telescope images of
Ganymede's auroral belts (colored blue in
this illustration) are overlaid on a Galileo
orbiter image of the moon. The amount of
rocking of the moon's magnetic field
suggests that the moon has a subsurface
saltwater ocean.
Image Credit: NASA/ESA
A team of 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."
If a saltwater ocean were present,
Jupiter’s magnetic field would create a
secondary magnetic field in the ocean that
would counter Jupiter’s field. This
“magnetic friction” would suppress the
rocking of the aurorae. This ocean fights
Jupiter's magnetic field so strongly that it
reduces the rocking of the aurorae to 2
degrees, instead of the 6 degrees, if the
ocean was not present.
Scientists 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 first suspected an ocean in
Ganymede in the 1970s, based on models of
the large moon. NASA's Galileo mission
measured Ganymede's magnetic field in 2002,
providing the first evidence supporting
those suspicions. The Galileo spacecraft
took brief "snapshot" measurements
of the magnetic field in 20-minute
intervals, but its observations were too
brief to distinctly catch the cyclical
rocking of the ocean’s secondary magnetic
field.
The new observations were done in
ultraviolet light and could only be
accomplished with a space telescope high
above the Earth's atmosphere, which blocks
most ultraviolet light.
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