Read out Jupiter-Trabant Europa: In Search of Life. What is hidden under the ice sheet of Europe? The space probe Galileo explores after the successful completion of their primary mission now specifically the enigmatic Jupiter's moon. Even renowned scientists hold the idea of ​​simple organisms on the celestial body, which is five times farther from the sun than the earth, not excluded. "If Europe owns an ocean, chances are better that there is still life there than for Mars, " planet scientist Eugene Shoemaker of the Lowell Observatory in Flagstaff, Arizona, was convinced shortly before his deadly accident last year.

In recent months, the evidence has further condensed that under the ice sheet of Jupiter's moon could be a vast sea. Although the definitive proof is still lacking, many researchers are confident. There were already scientific congresses where oceanographers and planetary scientists discussed together about possible life "No one has ever operated oceanography of another world. It's an interesting cultural experiment, "says John R. Delaney, a marine scientist from the University of Washington.

The focal point of this new "Galileo Europa Mission" (GEM) is the smallest of the four Jupiter moons. Between December 16, 1997, when Galileo raced past Europe only 200 kilometers away, and February 1, 1999, the moon was approached a total of eight times. The program then includes four close passages to Jupiter's ultimate Great Callisto and finally two close encounters with the volcanic Io. At the heart of GEM is first of all a closer look at Europe's surface. Meanwhile, the experts are convinced that the 100 to 150 kilometers thick crust of Jupiter's moon is not completely frozen - or it was once. There are numerous indications for this:

- All of Europe is covered by 100 to 300 meters high, up to 3 kilometers wide and sometimes more than 1000 kilometers long lines, which the researchers call "Triple Bands" because they appear like three parallel stripes. They are fracture zones formed by tensions in Europe's ice sheet. Ice splashing penetrated through the cracks to the surface, which is interspersed with rocky impurities and therefore appears dark. display

- On Europe's surface there are numerous elliptical pits, bulges and dark spots that are 7 to 15 kilometers wide and often just a few kilometers apart. The Galileo team suspects that it is "Diapire". These structures could arise when bubbles of warm ice rise from the deep and push up or even break up the surface.

- The unusual shape of the Pwyll crater, which was probably only 10 to 100 million years ago, suggests that warm ice filled after the impact of the crater basin and raised the 600 -meter-high central mountain over the height of the crater rim.

- Europe's core has been slowed down by the tidal forces of Jupiter so far that he always shows the giant planet the same side. The surface of the moon, however, seems to be disconnected from this "bounded rotation". The direction, number and age differences of the fracture lines on the surface suggest that the ice crust moves a little faster than the core - for one revolution, of course, it takes tens of thousands, if not millions of years. The crust is therefore not firmly connected to the core and mantle. The lubricant is water or warm ice.

- Europe has a magnetic field that the Moon can not produce itself, but Jupiter owes. According to model calculations by Fritz Neubauer (University of Cologne) and David Stevenson (California Institute of Technology), Europe is to disturb the magnetic field of the giant planet in a way that can be explained well by the assumption of a salty ocean. The salt acts as an electrolyte. In it, Jupiter's magnetic field can induce eddy currents, which in turn generate a magnetic field superimposed on Jupiter's magnetic field. The Jupiter's moon Callisto also has an induced magnetic field. There may also be an ocean under its surface.

- The most convincing evidence for an ocean in Europe was found by a group of scientists headed by Michael H. Carr (US Geological Survey, Menlo Park, California), who also owns Gerhard Neukum from the Institute for Planetary Science of the German Aerospace Center in Berlin, The researchers discovered a mosaic-like compound between two huge, intersecting triple bands, which seems to consist of a confused conglomeration of individual fragments. "The blocks resemble ice floes, as observed in polar regions on Earth, when the pack ice floats during the spring thaw, " explains Ronald Greeley. "Their shape and location suggests that they were floating as a thin layer of ice on water or ice mud and broken by movements in the crust. The older terrain probably slipped away. "

- Also discovered 100 kilometers long plates, which are mutually shifted and twisted. There is darker material between them, once melted.

But the scientists have even more ambitious plans: The Jet Propulsion Laboratory (JPL) is currently considering the Ice Clipper project. In the process, the probe is to shoot a projectile at the moon and in the meantime fly over it at a height of 50 kilometers, catching some of the thrown up pieces of debris in order to analyze them on board and possibly bring a part deep frozen to earth.

Of course, it would be most effective to land on the moon right away. "A seismometer could tell us a lot about the interior, " says Henry M. Harris, who works on the concept of a European Orbiter-Lander mission at the JPL. "We could hear ice masses shift and break, and maybe even listen to ocean waves."

=== Rüdiger Vaas

© science.de

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