Only Moon with its Own Magnetic Field
"100-Kilometer-Deep Ocean a Hotspot for Life"
In a video uploaded to YouTube on August 3rd (below), engineers from the
Russian space agency, Roscosmos, proposed an orbiter and lander mission to
Ganymede. The video suggests a launch could come in the next decade. Although
the commentary is in Russian, the video appears to suggest that Ganymede may be
as good a candidate or better for life than Europa.
In March of 2015, NASA's Hubble Space Telescope revealed 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, recently retired assistant
administrator of NASA's Science Mission Directorate at NASA Headquarters.
"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, shown above. 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.
Space Telescope Science Institute (STScI)
Image Credit: NASA/ESA and USGS Astrogeology Science Center/Wheaton/NASA/JPL-Caltech
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