Artist's Concept of Aurorae and Ganymede
Ganymede Interior
This is an illustration of the interior of Jupiter's largest moon,
Ganymede. It is based on theoretical models, in-situ observations by
NASA's Galileo orbiter, and Hubble Space Telescope observations of the
moon's aurorae, which allows for a probe of the moon's interior. The
cake-layering of the moon shows that ices and a saline ocean dominate
the outer layers. A denser rock mantle lies deeper in the moon, and
finally an iron core beneath that. Credit: NASA, ESA, and A. Feild (STScI) 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 for life as we know it.
"This discovery marks a significant milestone, highlighting what only
Hubble can accomplish," said John Grunsfeld, assistant administrator
of NASA's Science Mission Directorate at NASA Headquarters, Washington,
D.C. "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.
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 6
degrees if the ocean were 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 Earth's atmosphere,
which blocks most ultraviolet light.
The team’s results will be published online in the Journal of Geophysical Research: Space Physics on March 12.
NASA's Hubble Space Telescope is celebrating 25 years of
groundbreaking science on April 24. It has transformed our
understanding of our solar system and beyond, and helped us find our
place among the stars. To join the conversation about 25 years of Hubble
discoveries, use the hashtag #Hubble25.
Contact
Space Telescope Science Institute, Baltimore, Md.
410-338-4488 / 410-338-4514
jenkins@stsci.edu / villard@stsci.edu
Felicia Chou
NASA Headquarters, Washington, D.C.
202-358-0257
felicia.chou@nasa.gov
Joachim Saur
University of Cologne, Cologne, Germany
jsaur@uni-koein.de
Source: HubbleSite
