How Astronomers Found the Hidden Global Ocean on Saturn's Moon Enceladus

To astronomers, a massive global ocean looks like just the slightest wobble—a thin deviation from an anticipated orbit. Such an irregularity can't be accounted for in models of Saturn's moon Enceladus as a completely frozen sphere. The moon's icy exterior clearly isn't connected directly to its solid rock guts. In between must be liquid water, and a lot of it.

This is the conclusion announced by NASA this month via a paper accepted for publication in the journal Icarus. It comes courtesy of measurements taken by the agency's Cassini spacecraft, which has been zipping around Saturn and its moons since its 2004 arrival in the system.

The presence of water on Enceladus is in itself no surprise—the moon announces this feature via regular blasts or vents of liquid water from a depression located near its south pole. The quantity of water is, however, unexpected.

"The geologically diverse Saturnian satellite Enceladus vigorously vents liquid water and vapor from fractures within a south polar depression and thus must have a liquid reservoir or active melting," the Cassini team writes. "However, the extent and location of any subsurface liquid region is not directly observable." But it can be inferred.

Over seven years the spacecraft has collected measurements of the moon's surface at various control points, allowing astronomers to compile a model of its precise rotational state. What they found is a significant "libration," which is a small amount (0.120° from normal) of oscillatory motion relative to Saturn itself—a slight wobble, in other words.

"This libration is too large for a rigid connection between shell and core, thus we conclude Enceladus has a global, subsurface ocean," the astronomers write.

Scientists have been speculating about Enceladus and its oceans or would-be oceans for some time now. For one thing, evidence indicates that the water is able to remain unfrozen thanks to some "anomalous energy output," which has been theorized to be the effect of tidal heating. As outside gravitational forces act on a body, causing it to warp and flex, friction and, thus, heat results. That heat is enough to keep a far-flung moon warm in a cold-ass region of the solar system.

Initially it was thought that this liquid water was confined to the south pole regions of the moon, taking the shape of a sort of contact lens. Research published in 2011 offered the suggestion of a global ocean, however, as evidenced by the geography of fractures located around the moon's southern polar reaches. These cracks, wrote planetary geologists D. Alex Patthoff and Simon A. Kattenhorn, were likely formed, "in a temporally varying global stress field related to nonsynchronous rotation of a floating ice shell above a global liquid ocean."

Note that Enceladus global ocean is at least the third of its kind among outer planetary satellites within our solar system, joining the esteemed ranks of Jupiter's Europa and Saturn's Titan. All offer increasingly tantalizing glimpses of life-harboring conditions.

It so happens that Cassini is due for its closest ever pass by on October 28. It will dive deeply into those polar water jets, zipping by at just 30 miles above Enceladus' surface.