Aurorae indicate subterranean ocean on Ganymede

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Artist's concept of aurorae and Ganymede (Image: NASA, ESA, and G. Bacon (STScI))

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An ultraviolet light show has provided space scientists with the best evidence yet that the Jovian moon Ganymede has a gigantic ocean beneath its icy surface. Images captured by the Hubble Space Telescope of the aurorae in the moon's tenuous atmosphere provide what NASA calls the best evidence yet for a wet Ganymede where life could exist.

Ganymede is the largest moon in the Solar System and the only moon with a magnetic field. Since the 1970s, it's been suspected that the moon has an ocean beneath the crust of ice that covers its entire surface, but until recently the only support for the hypothesis has been models and speculation.

Now evidence for that ocean comes in the unexpected from of aurorae. Ganymede has an extremely tenuous atmosphere of oxygen and atomic hydrogen that is being constantly stripped away by Jupiter as fast as it's replenished, but there's enough air present to support aurorae like the Aurora Borealis on Earth that glow bright enough to be seen from our planet.

Ganymede Aurorae (Image: NASA, ESA, and J. Saur (University of Cologne, Germany))

According to NASA, a team of scientists led by Joachim Saur of the University of Cologne has been using the Hubble to study Ganymede in the ultraviolet part of the spectrum, which allowed them to capture images of the aurorae. As on Earth, the aurorae of Ganymede are caused by the interaction of charged particles and the moon's magnetic field, which produces a glowing spectacle. Unlike on Earth, the aurorae on Ganymede appear at lower latitudes due to the differences in the local magnetic field.

Direct information on Ganymedes's magnetic field is a bit sketchy. Though the unmanned Galileo probe spent years in the neighborhood, it was orbiting Jupiter itself and the mechanics of visiting the Jovian moons is almost as difficult as getting around the Solar System, so, according to NASA, Galileo made only six 20-minute encounters with Ganymede that were close enough for magnetic measurements.

Ganymede's field is weaker than Earth's and is mostly seen as a flux in Jupiter's much stronger field. It's generated by the moon's molten iron core, which probably remains hot due to tidal effects from the pull of Jupiter and acts as a dynamo. This field interacts with Jupiter's and "rocks" back and forth, which can be seen in the aurorae as it moves with the field.

The math says that this rocking should be about six degrees, but observations by the Hubble show it rocking only two degrees. This means that something is dampening the effect and NASA believes that it could be a salty ocean under the ice crust of Ganymede. Of all the materials that could be altering the magnetic field, liquid water is the most likely candidate and it needs to be salty to conduct electricity, so it can affect the field and reduce the rocking. By calculating the impact of such an ocean, the team was able to determine that, if it exists, it's 95 mi (150 km) beneath the crust of Ganymede, 60 mi (100 km) deep, and holds more water than all of Earth's oceans combined.

The presence of an ocean on Ganymede means that the moon is yet another on a growing list of places in the Solar System where life could exist outside of Earth. The water present is no guarantee of life and the fact the salt isn't sodium chloride, plus tremendous cold and pressure of a 60-mile deep ocean under almost a hundred miles of ice, poses its own problems, but the water does fulfill a necessary condition for life being present.

"This discovery marks a significant milestone, highlighting what only Hubble can accomplish," says John Grunsfeld, associate administrator of NASA’s Science Mission Directorate. "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."

The video below discusses the Ganymedian ocean.

Source: NASA

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