Icy Moon Tethys Had Ancient Underground Ocean

Richard A. Lovett
for National Geographic News
March 24, 2008
An enormous rift on Saturn's moon Tethys might be evidence that the giant iceball once had an underground ocean, scientists announced at a meeting earlier this month.

While most of this ancient ocean would have frozen solid long ago, a few dregs might still exit.

Tethys, Saturn's fifth largest moon, hasn't drawn much attention from astronomers because unlike the planet's other moons, it seems surprisingly ordinary, the researchers said.

"It was geologically active in the past, but it's not doing anything interesting today," said study co-author Francis Nimmo, a planetary scientist at the University of California, Santa Cruz.

But the 662-mile-wide (1,066-kilometer-wide) moon hasn't always been quiescent. Billions of years ago tectonic forces produced an enormous rift similar to the East African Rift Valley on Earth, Nimmo said.

The rift on Tethys is about 620 miles (1,000 kilometers) long, 62 miles (100 kilometers) wide, and 3 to 6 miles (5 to 10 kilometers) deep.

"[It] cuts across almost half the satellite," he said.

Nimmo and his graduate student, Erinna Chen, believe that the energy required to make such a rift is evidence that Tethys once hosted an ocean.

The pair recently described their findings at the 2008 Lunar and Planetary Science Conference in League City, Texas.

Tidal Forces

Chen and Nimmo began by evaluating earlier work that calculated the amount of energy that must have poured out of Tethys's interior when the rift formed.

"This gave [us] an idea of how much heat it was losing a few billion years ago," Nimmo said. "We asked where that heat was coming from."

Because Tethys is made almost entirely of ice, the energy couldn't have come from radioactive decay of heavy elements in its core, he said.

Instead the heat must have been generated by tidal forces from Saturn's enormous gravity.

Similar tidal heating currently creates volcanoes on Jupiter's moon Io and most likely powers geyser-like ice jets on Saturn's moon Enceladus, Nimmo noted.

(Read "Saturn Moon's Ice Geysers Create 'Cosmic Graffiti'" [February 8, 2007].)

But it's difficult to calculate how the tides produced enough heat on Tethys unless the moon once had an underground ocean, he said.

An ocean would allow enough heating for the moon's crust to flex more strongly than it would if Tethys were solid all the way to its core.

Subsequent freezing would then have widened the rift as the water expanded into ice.

Testing this hypothesis might be difficult, Nimmo said, but it might be possible to find places where water once oozed onto the surface and then froze.

This is thought to have occurred on Jupiter's moon Europa, which is widely believed to still have a subsurface ocean.

If confirmed, Tethys would become the fifth moon in the outer solar system believed to have, or to once have had, an ocean.

"The more places we look, the more oceans we're finding," Nimmo said.

And where there once was water, life may have existed.

Ocean Antifreeze

Depending on how much ammonia Tethys's ocean once contained, some liquid water might still exist below the moon's surface, Nimmo said.

"Ammonia acts as a good antifreeze," he noted. As the ocean freezes, much of the ammonia stays in the liquid water.

"It gets harder and harder to freeze the last little bit because it gets more and more concentrated in ammonia," Nimmo said.

Robert Pappalardo, a planetary scientist at NASA's Jet Propulsion Laboratory, called the find intriguing.

"This makes the exploration of icy satellites and their interiors even more important to understanding possible habitats for life in our solar system," Pappalardo said in an email.

The result has important implications for understanding how common life might be in the universe, he added.

Carolyn Porco, leader of the Saturn-orbiting Cassini spacecraft's imaging team, agreed.

Determining how common oceans and other possible precursors to life might be on icy moons "is one of the most gripping questions planetary scientists are asking today," she said in an email.

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