Europa's Hidden Ocean: The Third Piece of the Puzzle

The gravitational field of Europa is the third piece of evidence that suggests the existence of that moon's inner ocean. Other evidence includes surface features and the behavior of Jupiter's magnetic field.

In recent years, more scientists have begun to routinely acknowledge the existence of Europa's ocean. For example, in the anthology "Planetary Systems and the Origins of Life" (Pudritz, Higgs, Stone, p. 286), Richard Greenberg writes: "This Chapter reviews what is known about the physical character of Europa's potential biosphere, specifically the 100-km-deep liquid water ocean, and the thin layer of ice that lies above it. The global ocean just below the ice..."

Editor Linda M. Howe of Earthfiles.com writes: "Prof. Greenberg did work with the Galileo spacecraft team when Galileo orbited Jupiter starting in 1995 for eight years ... That's when he concluded that all the cracked patterns on Europa could be explained if you had a combination of tides and liquid water below the ice. And what could produce bigger tides than massive Jupiter? - its gravitational force pulling and stretching Europa continually - not only causing its ice to crack and break up, but the friction of all that pushing and pulling would heat up the water of Europa, keeping it liquid - in fact, the largest known liquid water ocean in our solar system with twice as much liquid water as in all of Earth's oceans combined."

In discussing when it became known that Europa had an subsurface ocean, Greenberg says they knew around the time of the Voyager spacecraft of 1979 that there was twice as much water in Europa than on Earth. They drew the inference based on the gravitational field of the Jovian moon which suggested low density matter like water rather than rock. The images from Galileo confirmed these inferences.

So it's really three pieces of evidence that together strongly point to an ocean within Europa. First, the gravitational field suggests a liquid interior rather than rock. Such a liquid interior answers to tidal forces, which explains the second piece of evidence, the surface features. Those surface features suggest that they are being changed by subsurface tidal pressure. The behavior of Jupiter's magnetosphere, the third piece of evidence, strongly indicates a conductive material inside the moon, and seawater is much more conductive than ice.

Therefore it is known with confidence that Europa has an ocean within, though the water has never been directly observed. It's a little more complicated than driving to the beach and looking at the ocean!

Sources:

NASA Press Release, August 25, 2000

Planetary Systems and the Origins of Life (Pudritz, Higgs, Stone)

www.earthfiles.com, Edited by Linda M. Howe, comments by Dr. R. Greenberg