Cassini ‘senses’ hidden ocean beneath Saturn’s moon Enceladus

Possible interior of Saturn’s moon Enceladus based on a gravity investigation by NASA’s Cassini spacecraft and NASA’s Deep Space Network in 2014. Gravity measurements suggest an ice outer shell and a low density, rocky core with a water ocean sandwiched in between at high southern latitudes. Jets of water vapor blast from cracks near the moon’s south pole. Credit: NASA/ JPL-Caltech

Long suspected as the source of the icy geysers on Saturn’s moon Enceladus, Cassini now has now uncovered evidence of an underground water ocean about 6 miles (10 km) deep, beneath the moon’s 19 to 25 miles (30 to 40 kilometers) thick crust of ice.

The ocean is likely restricted to the moon’s south polar region but given the moon’s 310 miles (500 km) diameter, that’s a potentially vast bathtub favorable for microbial life.

Enceladus is an inner, icy moon of Saturn 310 miles wide and shines as brightly as a fresh snowfall. The little moon reflects more light than any object in the solar system. Its surface has few craters and appears to have been reworked by heating. Credit: NASA

Earlier studies of the plumes or geysers blasting from the south polar region of Enceladus (en-SELL-uh-duss) by Cassini revealed most water ice particles with a small amounts amounts of methane, salts and even hydrocarbons such as propane, ethane and acetylene.

Geysers spray water ice, salts and organic compounds from fissures near the moon’s south pole nicknamed ‘tiger stripes’. Credit: NASA

To infer the presence of an ocean under miles of crust on a moon nearly 900 million miles from Earth, scientists made use of the Doppler Effect. Just to refresh, we experience the Doppler Effect every time an ambulance or fire truck goes by. As the vehicle approaches, the sound waves its horn gives off become more compressed and rise in pitch. When the truck passes and moves into the distance, the sound waves spread out and the pitch drops.

The same principal applies to light waves and radio waves. When Cassini flies past Enceladus, which it’s done now 19 times, it changes speed slightly and continuously depending upon the subtle variations in the moon’s gravity field caused by surface irregularities like a tall mountain or changes in density beneath the crust caused water in place of solid rock.

An animation illustrating how the Doppler effect causes a car engine or siren to sound higher in pitch when it is approaching than when it is receding. Sound waves bunch up on the left in the direction of the car’s motion to make a higher pitch and stretch apart on the right to make a lower pitch. Credit: Charly Whisky / Wikipedia

“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,” said  Sami Asmar of NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, Calif., a coauthor of the paper. “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.”

Cassini and the Deep Space Network can detect changes in velocity as small as just under one foot an hour. With this precision, the flyby data yielded evidence of a zone inside the southern end of the moon with higher density than other portions of the interior.

Because Enceladus is made largely of ice, it’s surmised that the higher density comes from liquid water which is 7% denser than ice. While a large, subsurface ocean is implicated, there’s no certainty it’s behind the moon’s vaporous plumage. Let’s just say it’s a real possibility.

Closeup of Bagdad Sulcus, one of the ‘tiger stripes’ or fractures where the geysers originate on Enceladus. The picture shows a patch 5 miles (8 km) wide. Credit: NASA

Since the inside of Enceladus has the right stuff for life, astronomers believe the findings broaden our idea of places in which life might thrive.

“Their discovery expanded our view of the ‘habitable zone’ within our solar system and in planetary systems of other stars,” said Linda Spilker, Cassini’s project scientist at JPL. “This new validation that an ocean of water underlies the jets furthers understanding about this intriguing environment.”

3 Responses

  1. Edward M. Boll

    Ocean of water ice or some other liquid? I have not taken a look at Saturn in a while. I am interested in Comet Jacques. It seems that this comet should already be visible in large binoculars. If not now, soon.

    1. Profile photo of astrobob

      Hi Edward,
      That would be oceans of water as described in the caption in the first image. I’ve tried to see Jacques but still too low to show through the haze, and now the moon’s returned. It should be relatively easy to see in Puppis after full moon this month.

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