1997-1998 indexDistributed March 2, 1998
Background on Europa data from the Galileo Mission to Jupiter
At today's press briefing, Brown and NASA scientists will show the most detailed images ever seen of the Jupiter moon Europa. Recently transmitted from the Galileo spacecraft, the images provide three key pieces of evidence supporting the idea that water may lurk beneath Europa's surface. (See also news advisory.)
Water or ice? Liquid or slushy or frozen solid? Ever since the Voyager spacecraft missions flew through the Jupiter system in 1979, planetary scientists have wondered about the layer of ice surrounding the planet. Europa's blindingly bright ice surface makes it one of the brightest objects in our solar system. Recent Galileo spacecraft images have provided evidence that Europa had a liquid ocean underneath the frozen crust sometime in its history, but it is not clear if this ocean still exists. Of the various explanations proposed by scientists, most scenarios of Europa's evolution have the water layer freezing solid earlier in its history. The moon's surface is -260° F, which could freeze an ocean over several million years. But some scientists are beginning to think that the warming caused by a tidal tug of war with Jupiter and neighboring moons could be keeping large parts of the ocean liquid.
New stereo and very high resolution images of Europa just transmitted to Earth from the Galileo Europa Mission fly-by in December 1997 may help support the theory that water or slush may slosh beneath Europa's frozen crust. Detailed enough to see a truck-sized object on the surface, the new images are hundreds of times higher resolution than the best Voyager images and three to 20 times higher than earlier Galileo pictures. The Brown and NASA scientists point to three key pieces of evidence from the detailed images:
"Together, the craters, chaos and wedges support the hypothesis that in Europa's most recent history, liquid or at least partially liquid water existed at shallow depths below the surface of Europa in several different places," says James Head, Brown University professor of geological sciences. "These and other data lend support to the hypothesis that Europa is warm and active today and potentially characterized by a global subsurface water layer or ocean. Europa, like Mars and the Saturn moon Titan, is a laboratory for the study of conditions that might have led to the formation and evolution of life. The combination of interior heat, liquid water, and infall of organic material from comets and meteorites means that Europa has the key ingredients for life, and it represents an exciting environment that is worthy of further detailed exploration."
Rays and debris from the impact that formed Pwyll Crater radiate over a large part of the moon's surface. Galileo took pictures of the impact crater from two perspectives to determine the three-dimensional shape of the crater. Colleagues at the DLR (German Aerospace Research Establishment) converted these images into a colored map showing the depth of the crater and the height of its peaks. Unlike most young, deep impact craters, the floor of Pwyll is at the same level as the exterior, says Brown graduate student Geoffrey Collins. The central peaks of the crater are more than 2,000 feet high - four times higher than the Washington Monument - and higher than the crater rim. This means that this young crater was warm and weak and collapsed during or very shortly after the meteorite impact, in contrast to craters formed in cold, stiff material. Debris that flowed from the violent impact is dark, suggesting excavation of different material from below the surface. All this suggests that water just beneath the surface was warm enough to be slushy in the moon's recent history.
The new images from Galileo help answer some questions about other areas of Europa that are littered with fractured and rotated blocks of crust the size of several city blocks (dubbed chaos terrain). These fractured ice chunks appeared to be either sliding on soft glacier-like ice below the surface or floating like icebergs in a more fluid material. The new images show that the material between the cracked and separated plates of crust is rough and swirly, says Robert Pappalardo, a postdoctoral research scientist at Brown. The pieces are immersed in what appears to be a slush that is now frozen solid. The very low temperatures at the surface of Europa (-260° F) mean that any water exposed at the surface would freeze immediately and might create this kind of texture. The rough chaos terrain, as well as the movement and rotation of the blocks, suggest that the crust was at least partially liquid at shallow depths.
Other images are helping unravel more mysteries. Pieces of the moon's glaringly white crust are separated by wedged-shaped pieces of darker, newer crust, welling up from below, freezing and cracking. The separated pieces of white crust would fit back together like a jig-saw puzzle, suggesting that plate tectonic-like activity might be occurring on Europa to form the wedges. Composed of a set of narrow linear ridges and parallel grooves, the dark wedge has many similarities to new crust formed at mid-ocean ridges on the Earth's sea floor, says Brown graduate student Louise Prockter, who has studied high-resolution sonar images of the Mid-Atlantic Ridge and has visited the Pacific Ocean floor in the research submersible vehicle Alvin. Like Earth, new crust seems to be welling up, separating, and replacing older crust. On Europa, the molten material solidifying on the surface was likely slushy ice or liquid water.
To confirm the existence of such a layer, determine its depth and investigate its nature and global extent, further observations are planned for the Galileo Europa Mission, and other experiments are planned for a Europa Orbiter Mission to be launched in 2003, says Michael J. S. Belton of the National Optical Astronomy Observatory in Tucson, Ariz., and team leader for the solid state imaging system.######