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."

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