It’s been suspected for nearly a decade that Saturn’s 315-mile-wide moon Enceladus harbors a hidden ocean beneath its frozen crust, thanks to observations by the Cassini spacecraft of icy plumes spraying from its southern pole, and now scientists have even more evidence supporting its existence: Doppler measurements of the moon’s gravity taken during Cassini’s flybys show variations indicative of a subsurface southern sea as deep as the Pacific’s Mariana Trench!

From a NASA press release:

“The way we deduce gravity variations is a concept in physics called the Doppler Effect, the same principle used with a speed-measuring radar gun,” said Sami Asmar of NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, Calif., a coauthor of the paper. “As the spacecraft flies by Enceladus, its velocity is perturbed by an amount that depends on variations in the gravity field that we’re trying to measure. We see the change in velocity as a change in radio frequency, received at our ground stations here all the way across the solar system.”

The gravity measurements suggest a large, possibly regional, ocean about 6 miles (10 kilometers) deep, beneath an ice shell about 19 to 25 miles (30 to 40 kilometers) thick.

“The Cassini gravity measurements show a negative gravity anomaly at the south pole that however is not as large as expected from the deep depression detected by the onboard camera. Hence the conclusion that there must be a denser material at depth that compensates the missing mass: very likely liquid water, which is seven percent denser than ice. The magnitude of the anomaly gave us the size of the water reservoir.”

– Luciano Iess, Sapienza University of Rome, lead author

The subsurface ocean evidence supports the inclusion of Enceladus among the most likely places in our solar system to host microbial life. Before Cassini reached Saturn in July 2004, no version of that short list included this icy moon, barely 300 miles (500 kilometers) in diameter.

“This then provides one possible story to explain why water is gushing out of these fractures we see at the south pole,” said David Stevenson of the California Institute of Technology, Pasadena, one of the paper’s co-authors.

The findings were published in the April 4 edition of Science.

The data were collected by Cassini and received at NASA’s Deep Space Network in Goldstone, California — a site which I had the opportunity to visit on April 2, 2014, as part of a NASA Social event honoring the facility’s 50th anniversary.

Read more in the NASA press release here and in an article on Nature.com here.