Forget Mars, life on Moons - Jupiter, Saturn?

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.

“This discovery marks a significant milestone, highlighting what only Hubble can accomplish,” said John Grunsfeld, associate administrator of Nasa’s Science Mission Directorate at Nasa Headquarters, Washington.
“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.

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.

And Saturn, too

Nasa’s Cassini spacecraft has provided scientists the first clear evidence that Saturn’s moon Enceladus exhibits signs of present-day hydrothermal activity which may resemble that seen in the deep oceans on Earth.

The implications of such activity on a world other than our planet open up unprecedented scientific possibilities.

“These findings add to the possibility that Enceladus, which contains a subsurface ocean and displays remarkable geologic activity, could contain environments suitable for living organisms,” said John Grunsfeld astronaut and associate administrator of Nasa’s Science Mission Directorate in Washington.

“The locations in our solar system where extreme environments occur in which life might exist may bring us closer to answering the question: are we alone in the Universe.”

Hydrothermal activity occurs when seawater infiltrates and reacts with a rocky crust and emerges as a heated, mineral-laden solution, a natural occurrence in Earth’s oceans.

Ganymede auroral belts

Nasa Hubble Space Telescope images of Ganymede's auroral belts (colored blue in this illustration) are overlaid on a Galileo orbiter image of the moon.
The amount of rocking of the moon's magnetic field suggests that the moon has a subsurface saltwater ocean.

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 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.

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 the Earth's atmosphere, which blocks most ultraviolet light.

Saturn science

According to two science papers, the results are the first clear indications an icy moon may have similar ongoing active processes.

"It's very exciting that we can use these tiny grains of rock, spewed into space by geysers, to tell us about conditions on -- and beneath -- the ocean floor of an icy moon," said the paper’s lead author Sean Hsu, a postdoctoral researcher at the University of Colorado at Boulder.

On Earth, the most common way to form silica grains of this size is hydrothermal activity under a specific range of conditions; namely, when slightly alkaline and salty water that is super-saturated with silica undergoes a big drop in temperature.

"We methodically searched for alternate explanations for the nanosilica grains, but every new result pointed to a single, most likely origin," said co-author Frank Postberg, a Cassini CDA team scientist at Heidelberg University in Germany.

The second paper, recently published in Geophysical Research Letters, suggests hydrothermal activity as one of two likely sources of methane in the plume of gas and ice particles that erupts from the south polar region of Enceladus.

The finding is the result of extensive modeling to address why methane, as previously sampled by Cassini, is curiously abundant in the plume.

Cassini first revealed active geological processes on Enceladus in 2005 with evidence of an icy spray issuing from the moon's south polar region and higher-than-expected temperatures in the icy surface there.

With its powerful suite of complementary science instruments, the mission soon revealed a towering plume of water ice and vapor, salts and organic materials that issues from relatively warm fractures on the wrinkled surface.

Gravity science results published in 2014 strongly suggested the presence of a 6-mile- (10-kilometer-) deep ocean beneath an ice shell about 19 to 25 miles (30 to 40 kilometers) thick.
 

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