This is an artist’s concept of the moon Ganymede as it orbits the giant planet Jupiter. NASA’s Hubble Space Telescope observed aurorae on the moon that are controlled by Ganymede’s magnetic fields. Hubble measured slight shifts in the auroral belts due to the influence of Jupiter’s own immense magnetic field, enabling scientists to probe the moon’s interior. As on Earth, Ganymede’s aurorae are produced by energetic charged particles causing gases to fluoresce. Illustration credit: NASA, ESA, and G. Bacon (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.”
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 behaviour of the moon’s magnetosphere. Image credit: NASA, ESA, and A. Feild (STScI)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.
Compass and image scale for Ganymede and observations of its aurorae. Image credit: NASA, ESA, and Z. Levay (STScI)“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.
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. Image credit: NASA, ESA, and A. Feild (STScI)Scientists estimate the ocean is 60 miles (100 kilometres) thick — 10 times deeper than Earth’s oceans — and is buried under a 95-mile (150-kilometre) 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 were published online in the Journal of Geophysical Research: Space Physics on March 12th.
Learn more about this fascinating discovery in the following recording of a live Hubble Hangout discussion between astronomers about Ganymede:
Despite more than seven weeks having passed since opposition, the Solar System’s largest planet Jupiter is still big and bright in the UK evening sky of May, highest in the south around 10pm BST. Find out about the phenomena of Jupiter and its moons that you can see from the British Isles for the remainder of the month, starting with a transit of Jupiter’s Great Red Spot on 19 May.
NASA has selected nine science instruments for a future mission to Jupiter’s moon Europa in which a spacecraft would make multiple close flybys of the icy world — thought to contain a global subsurface ocean — to investigate whether the mysterious moon could harbour conditions suitable for life.
