Vast underground ocean discovered on Jupiter’s largest moon
March 13, 2015
In this artist’s concept, the moon Ganymede (right) orbits the giant planet Jupiter. NASA’s Hubble Space Telescope observed aurorae (lights) on the moon generated by Ganymede’s magnetic fields. A saline ocean under the moon’s icy crust best explains shifting in the auroral belts measured by Hubble. (credit: NASA/ESA)
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.
NASA’s Hubble Space Telescope observed a pair of auroral belts encircling the Jovian moon Ganymede, observed in ultraviolet light and colored blue in this illustration, overlaid on a visible-light image of Ganymede taken by NASA’s Galileo orbiter. The locations of the glowing aurorae are determined by the moon’s magnetic field, and therefore provide a probe of the moon’s interior, where the magnetic field is generated. The amount of rocking of the magnetic field, caused by its interaction with Jupiter’s own immense magnetosphere, provides evidence that the moon has a subsurface ocean of saline water. (credit: NASA, ESA, and J. Saur/University of Cologne, Germany)
“A deep ocean under the icy crust of Ganymede opens up further exciting possibilities for life beyond Earth,”said John Grunsfeld, associate administrator of NASA’s Science Mission Directorate.
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, similar to those visible close to the Earth’s magnetic poles. 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.
This is a sketch of the magnetic field lines around Ganymede, which are generated in the moon’s iron core. Hubble Space Telescope measurements of Ganymede’s aurorae, which follow magnetic field lines, suggest that a subsurface saline ocean also influences the behavior of the moon’s magnetosphere. (credit: NASA, ESA and A. Feild/STScI)
This chart plots the excursion of a pair of auroral belts on Jupiter’s moon Ganymede. Their motion provides insight into the moon’s interior. Ganymede has a magnetic field produced by an iron core. Because Ganymede is close to Jupiter, it is also embedded in Jupiter’s own magnetic field. When Jupiter’s magnetic field changes, the aurorae on Ganymede also change, “rocking” back and forth. This amount of rocking is inhibited if the moon has a subsurface ocean. 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. (credit: NASA, ESA and A. Feild/STScI)
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.
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)
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.
Abstract of The search for a subsurface ocean in Ganymede with Hubble Space Telescope observations of its auroral ovals
We present a new approach to search for a subsurface ocean within Ganymede through observations and modeling of the dynamics of its auroral ovals. The locations of the auroral ovals oscillate due to Jupiter’s time-varying magnetospheric field seen in the rest frame of Ganymede. If an electrically conductive ocean is present, the external time-varying magnetic field is reduced due to induction within the ocean and the oscillation amplitude of the ovals decreases. Hubble Space Telescope (HST) observations show that the locations of the ovals oscillate on average by 2.0° ±1.3°. Our model calculations predict a significantly stronger oscillation by 5.8° ± 1.3° without ocean compared to 2.2°±1.3° if an ocean is present. Because the ocean and the no-ocean hypotheses cannot be separated by simple visual inspection of individual HST images, we apply a statistical analysis including a Monte Carlo test to also address the uncertainty caused by the patchiness of observed emissions. The observations require a minimum electrical conductivity of 0.09 S/m for an ocean assumed to be located between 150 km and 250 km depth or alternatively a maximum depth of the top of the ocean at 330 km. Our analysis implies that Ganymede’s dynamo possesses an outstandingly low quadrupole-to-dipole moment ratio. The new technique applied here is suited to probe the interior of other planetary bodies by monitoring their auroral response to time-varying magnetic fields.