Mar. 3, 1998
PROVIDENCE, R.I. -- The most detailed images ever taken of the Jupiter moon Europa show more evidence for slush beneath the bright moon's icy surface, say planetary scientists from Brown University and NASA who have analyzed data recently transmitted from the Galileo spacecraft.
Slightly smaller than Earth's moon but many times brighter, Europa's icy surface has intrigued scientists ever since the Voyager spacecraft missions flew through the Jupiter system in 1979. At -260° F, the moon's surface temperature could deep-freeze an ocean over several million years, but some scientists are beginning to think that warmth from a tidal tug of war with Jupiter and neighboring moons could be keeping large parts of Europa's ocean liquid.
The latest images released today were taken in December 1997 by the Galileo spacecraft and just received on Earth. The new images provide three key pieces of evidence showing that Europa may be slushy just beneath the icy crust and possibly even warmer at greater depths. The evidence includes a strangely shallow impact crater, chunky textured surfaces like icebergs, and gaps where new icy crust seems to have formed between continent-sized plates of ice.
Some of the new images focus on the shallow center of the impact crater known as Pwyll. Impact rays and debris scattered over a large part of the moon show that a meteorite slammed into Europa relatively recently, about 10-100 million years ago. The darker debris around the crater suggests the impact excavated deeply buried material. But the crater's shallow basin and high set of mountain peaks may mean that subsurface ice was warm enough to collapse and fill in the deep hole, says Brown graduate student Geoffrey Collins, a member of the Galileo research team.
A subsurface ocean warm enough to be slushy also may explain the origins of an area littered with fractured and rotated blocks of crust the size of several city blocks, called "chaos" terrain. The new images show rough and swirly material between the fractured chunks, which may have been suspended in slush that froze at the very low surface temperatures, says Robert Pappalardo, a postdoctoral research scientist at Brown and a member of the Galileo research team.
On a larger scale, large plates of ice seem to be sliding over a warm interior on Europa, much like Earth's continental plates move around on our planet's partly molten interior.
The new images of Europa show that the darker wedge-shaped gaps between the plates of ice have many similarities to new crust formed at mid-ocean ridges on the Earth's sea floor, says Brown graduate student Louise Prockter, a member of the Galileo research team 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. The new crust welling up between the separating plates on Europa was likely initially slushy ice or possibly liquid water that has frozen and fractured, Prockter says.
"Together, the evidence supports 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 and a group leader of the Galileo research team.
"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," Head says. "Europa, like Mars and the Saturn moon Titan, is a laboratory for the study of conditions that might have led to the formation of life in the solar system."
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