e-Pulsar - Apr

e-Pulsar Apr - May 98

EUROPA: The Icy Moon

By Mike Carroll & Friends

Pia01178 Ice, slush, crystals, bands, plates, and… water???

Europa is probably the most enigmatic moon in the solar system. Orbiting some 671,000 kilometers from Jupiter, the 3,100 km diameter moon zips around the great planet in just over three and a half days.

When two Voyager spacecraft flew by Europa in 1979, they glimpsed a unique worldlet with mysterious brown fractures across its smooth surface. In some cases, these brown lines had stripes down the middle, earning the nickname "highways". These so-called triple bands had an unknown origin, and the baffled scientists hoped more detailed views from the Galileo orbiter would clear up the mystery. Galileo has given us spectacular images of Europa and the other Galilean satellites. During its extended "Galileo Europa Mission" (GEM), the craft promises to show us even more. But the triple bands are more complex than originally thought. Theories abound, but Ron Greeley and other planetologists at JPL believe Europa’s triple bands may form in this way: (images by Mike Carroll)

a) Bands begin as thin cracks in the surface. (Figure 1 on the right. Note the "floating iceberg" formations on the horizon at left.)

b) Liquid water seeps --or explosively escapes --to the surface and flows outward, building up linear ridges on either side of the fracture. (Figure 2)

c) As the ridge becomes more massive, the ice crust cannot support its weight (despite Europa’s gravity being around half of the Moon’s) and more parallel fractures form. These cracks, in turn, go through the same process until several parallel ridges rise from the surface. (Figure 3. Note the dome forming in the background, perhaps indicative of volcanism on the ocean floor under the ice.)

d) The ice crust cannot support any taller structures; the fully formed triple-band then begins to sink back into the surface, ultimately leaving a smooth, striped plain. (Figure 4. Note the darkened spots along the crack.)

Those dark spots and the brown bands mentioned above have created a great deal of speculation. What is causing all the color variations? Could it be geysering or cryo-volcanism of liquid from below? Dale Darby wondered if radiation from Jupiter would do something to the ice. Could cometary and meteoritic impact deposit some compounds that melting and/or sublimation could leave behind as stains? Bill Hartmann speculates that "primordial ice out there has sooty carbonaceous dust in it that colors it black. On Europa, most present ice probably erupted as liquid and the black stuff sank out. In general on the Galilean satellites, the black stuff is less volatile than the ice. Therefore MICROMETEORITE BOMBARDMENT will preferentially tend to sublimate the ice and leave the black stuff behind. So I have speculated that this is why the surface layers, especially on Callisto and Ganymede which have more primitive ice, tend to get darker. Europa, though, may be protected from this mechanism to some degree because it has pretty clean ice at the surface."

Perhaps the most intriguing thing about Europa is what’s underneath. Models suggest a deep water ocean (up to 60 km) with active thermal vents on the ocean floor. Could these vents be host to an alien ecosystem whose organic material is staining the cracks on the surface ice? GEM will get spectral data of the bands in detail, so we may have more clues soon. Europa is a beautiful, frozen, and possibly violent world. All in all, it's a great place to paint!