A Hidden Ocean Might Exist Under Ganymedes Icy Surface

When you think about the Jupiter system, you think of a gas giant planet. It has major storms whirling around in the upper atmosphere. Deep inside, its a tiny rocky world surrounded by layers of liquid metallic hydrogen. It also has strong magnetic and gravitational fields that could be obstacles for any kind human exploration. In other words, an alien place.   Jupiter just doesnt seem like the kind of place that would also have tiny water-rich worlds orbiting around it.   Yet, for at least two decades, astronomers have suspected that the tiny moon Europa had subsurface oceans. They also think that  Ganymede has at least one (or more) oceans as well.  Now, they have strong evidence for a deep saline ocean there. If it turns out to be real, this salty subsurface sea could have more than all the water on Earths surface. Discovering Hidden Oceans How do astronomers know about this ocean? The latest findings were made using the Hubble Space Telescope to study Ganymede. It has an icy crust and a rocky core. What lies between that crust and core have intrigued astronomers for a long time. This is the only moon in the entire solar system that is known to have its own magnetic field. Its also the largest moon in the solar system. Ganymede also has an ionosphere, which is lit up by magnetic storms called aurorae. These are mainly detectable in ultraviolet light.   Because aurorae are controlled by the moons magnetic field (plus the action of Jupiters field), astronomers came up with a way to use the motions of the field to look deep inside Ganymede. (Earth also has aurorae, called informally the northern and southern lights).   Ganymede orbits its parent planet embedded in Jupiters magnetic field. As Jupiters magnetic field changes, the Ganymedean aurora also rock back and forth. By watching the rocking motion of the aurorae, astronomers were able to figure out that theres a large amount of salt water beneath the crust of the moon.The saline-rich water suppresses some of the influence that Jupiters magnetic field has on Ganymede, and that is reflected in the motion of the aurorae.   Based on Hubble data and other observations, scientists estimate the ocean is 60 miles (100 kilometers) deep. Thats about ten times deeper than Earths oceans. It lies under an icy crust thats about 85 miles thick (150 kilometers). Beginning in the 1970s, planetary scientists suspected the moon might have a magnetic field, but they didnt have a good way to confirm its existence. They finally got information about it when the  Galileo spacecraft took brief snapshot measurements of  the magnetic field in 20-minute intervals. Its observations were too  brief to distinctly catch the cyclical rocking of the oceans secondary  magnetic field. The new observations could only be  accomplished with a space telescope high above Earths atmosphere, which  blocks most ultraviolet light. The Hubble  Space Telescope Imaging Spectrograph, which is sensitive to ultraviolet light given off by the auroral activity on Ganymede, studied the aurorae in great detail.    Ganymede was discovered in 1610 by astronomer Galileo Galilei. He spotted it in January of that year, along with three other moons: Io, Europa, and Callisto. Ganymede was first imaged up-close by the Voyager 1 spacecraft in 1979, followed by a visit from Voyager 2 later that year.  Since that time, it has been studied by the Galileo and New Horizons missions, as well as Hubble Space Telescope and many ground-based observatories.The search for water on worlds such as Ganymede is part of a larger exploration of worlds in the solar system that could be hospitable to life.   There are now several worlds, besides Earth, that could (or are confirmed) to have water: Europa, Mars, and Enceladus (orbiting Saturn). In addition, the dwarf planet Ceres is thought to have a subsurface ocean.