Marvelous moons


Is something living under the curious surface of Jupiter's moon, Europa? All images courtesy of NASA.
  Europa -- orbiting incubator?
Look at the average moon, and you see a battered, bruised face. Look at Europa, Jupiter's fourth-largest moon, and you'll see the unworried image of youth. jumpin' jupiter it looks like a skee ballSure, Europa has some lines -- but scarcely any of those unsightly craters or disfiguring bumps that scar most moons.

Even when Europa's surface shapes are strengthened by shadows, it's still strangely smooth for a solar system satellite. Compare Europa to Mars, which has 450 craters larger than 100 kilometers in diameter.

This is not because the Jovian moon is young -- but because it's resilient. Blast Europa with an asteroid, and it will absorb the shock without forming much of a crater. (It's like whacking a hockey puck with a slap shot. Sure, you might make a mark on the puck, but the puck bounces right back.)

Pucks are resilient because they're made of rubber. Europa seems to be resilient because its smooth surface may rest on a giant subterranean ocean.

Did somebody say "ocean"?
Yes. We've known for years that the sixth-largest moon in our solar system is coated with frozen water (its surface temperature is -260 Fahrenheit). But some astonishingly detailed photos taken by the satellite Galileo in December, 1997, have renewed a debate about something even more important than Europa's comely complexion.

Does this frozen orb contains the largest ocean in the solar system? And if so, could life exist beneath this deeply frozen surface?

Before you condemn this as just a ploy to attract research money, consider how Galileo's goodies support the ocean hypothesis:


  pwyll crater

  A stereo image of an abnormally shallow impact crater called Pwyll (don't ask The Why Files to pronounce that!) shows Europa's resilient nature. Notice how the big impact failed to depress the crater much below the surface


  Huge blocks that look like ... blocks of ice floating on slushy ice on Earth.

  if you say so


  I see it too

  Wedge-shaped structures that resemble junctions between Earth's crustal plates at the ocean floor. "It's a series of very long ridges and trenches, much as we see at spreading centers on the sea floor," says Brown University planetary geologist James Head, a long-time student of Europa.


If the ocean hypothesis is correct, Europa is built in layers. Tan = frozen skin (mainly water); Blue = "ocean" Brown = mantle Gray = core
Courtesy NASA

  Since we already know that this moon is coated with frozen water and is not dense enough to be pure rock, the photos support the notion that Europa also contains water inside.

Too cold for swimming?
Make that liquid water. Despite the frigid conditions in Jupiter's neighborhood, Europa's insides may be warmed by the interaction between Jupiter's immense gravity and the gravity of Europa and other moons. (Similar forces from our moon's gravity -- which is microscopic compared to Jupiter's -- don't heat Earth much, but they are strong enough to move our oceans and make the tides.)

just a big ole ice ballHead says the ocean may be as deep as 100 kilometers. He suspects that the upper layer could be slushy, but down lower the water may be liquid, probably containing a lot of salt.

The ocean explains many Earthlike features on Europa. Just as Earth's continental plates float slowly on the softer mantle supporting it, Europa's icy exterior may float on a hidden sea.

And those surface cracks probably occur when slushy ice or water wells up from below and shoulders the ice aside. Head says the upwards movement causes those ridges that parallel the cracks.

Could Europa's ocean be alive?


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