Cassini Finds a Sea Under the Ice of Enceladus

Gravity measurements show a body of water as big as Lake Superior on Saturn’s tiny moon.

Color Enceladus.jpg
Cassini view of Enceladus.

Using gravitational data from the Cassini spacecraft in orbit around Saturn, a research group led by Luciano Iess from the Sapienza University of Rome has found strong evidence of a regional sea under the icy crust of the small moon Enceladus.

The new gravity analysis—based on slight changes in the spacecraft’s orbital velocity as it passed Enceladus—led Iess and his colleagues to the conclusion that a regional sea exists at the moon’s south pole, under 30 to 40 kilometers of ice. The sea might extend nearly halfway to the equator.

Scientists have long suspected the existence of an ocean on Enceladus, which is only 310 miles across, or about the size of Great Britain. Cassini had previously detected plumes of water ice and gas being ejected from fractures in the moon’s south polar region. The measured high heat flow and plume activity had strongly suggested a differentiated structure for the moon, with an icy crust overlying liquid water. A global ocean can’t be ruled out by this new analysis, but the gravity measurements, along with modeled heat fluxes and the observed topography of Enceladus’ surface, make it less likely than a regional sea.

Should we expect to find life in this hidden sea? That is a definite possibility, and the detection of water vapor, nitrogen, carbon dioxide, and methane within the plume has raised our hopes. On the other hand, Enceladus may just be a giant geyser with tidal forces squeezing out liquids, and no life at all.

Certainly, the presence of water in contact with a rocky mantle—as we think is the case on Jupiter’s moon Europa—would enhance the chances for life considerably. On Earth we see hydrothermal vent systems that provide nutrients and heat for oases of life in the deep ocean. Life is more likely where water comes in contact with rock, instead of being sandwiched between ice layers, as we theorize for Jupiter’s moon Ganymede.

According to Jonathan Lunine of Cornell University, a co-author of the new Cassini study, the liquid water is likely in contact with rocks, for reasons having to do with the type of ice on Enceladus. That theory is supported by Cassini’s detection of salt content in the ejected water plumes. Even if it is, however, life still needs metals for catalysis, and metals are likely very hard to find on Enceladus based on its overall low density. That’s why, even though this is an exciting result, I still consider Europa a more life-friendly place than Enceladus.

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