Mar 10, 2005
Before spacecraft sent back images of Jupiter’s moons, planetary scientists expected to find rocky bodies covered with craters like our own Moon. The story of moon formation from collisions of planetary “leftovers” was still intact, and astronomers expected to find a pristine record of ancient impacts on the surfaces of the Jovian satellites.
When Voyagers 1 and 2 reached Jupiter’s realm and began to capture images of its moon Europa, astronomers were surprised to detect the spectral signature of water ice. But nothing prepared them for the weirdness of Europa’s surface. The first Voyager images revealed a smooth whitish sphere crisscrossed with lines like a well-used skating rink. Science writers described it as “a surface that looked as if it had been clawed by a tiger with talons several kilometers wide.” They called the surface “enigmatic” and “difficult to interpret in terms of mechanism” (above left).
In May 1997, Fred Hoyle and Chandra Wickramasinghe wrote a letter about the channels to the scientific journal, Nature: “[The channels] have an almost uncanny persistence. They cross over each other, maintaining their identities over distances...very large compared to their individual widths.” After favoring the analogy of a system of ropes, the authors asked the obvious question, “How did the ropes come to be laid in the complex pattern in which we now see them?”
The letter was not published, but Hoyle and Wickramasinghe had asked a valid and crucial question. The rope-like and “paintbrush” features carved on the surface of Europa are characteristic of filamentary plasma discharges. See the following “Pictures of the Day”—
February 16, 2005, Electric Scars on Venus
February 25, 2005, Electric Scars on Enceladus
March 7, 2005 Electric Arcs in Planetary Science
Features in the “chaos” regions indicate that in the past Europa has been subjected to heating and to titanic sliding forces. The subsurface was melted and the grooved surface was broken into a jigsaw puzzle of angular and jumbled ice rafts. That the rafts did not melt suggests that the disturbances were short-lived (above right). Here the lines and clusters of lines often mismatch those on adjacent pieces. Using the unique pattern of grooves in their surfaces as a key, many rafts can be traced to previous positions.
But some lines plow across several pieces, unaffected by the underlying ridges, grooves and edges. Other lines and clusters of lines are sliced cleanly by lines running in different directions, but the former lines are not offset on the far side of the slice. If the lines were cracks from fracturing of the surface, later fractures that crossed earlier ones would cause the earlier ones to be offset.
The larger channels on Europa travel thousands of kilometers along great circles without being diverted by the terrain. They maintain astonishingly consistent depths and widths over these immense distances. Whatever mechanism formed them must explain these fundamental observations. Repeated tidal cracking and compression of ice is too chaotic a process to explain them, but the Hoyle/Wickramasinghe analogy of ropes stretched between two points is remarkably apt. First impressions are often astute. They evoke insights before the beliefs about what is possible or impossible narrow perception.
Another early description of the channels called them “sinuous rille-like features” because they resembled the sinuous rilles on the Moon. But the analogy was not pursued because rilles have traditionally been attributed to collapsed lava tubes.
Some channels are up to 70 km wide and run over 3,000 km, or 30 percent of the distance around Europa. Yet they maintain parallel sides with a constant width and cross-section over almost the entire length. Tidal forces on tiny Europa, acting on the moon’s irregular surface relief, could not maintain a uniform displacement over such a distance.
Only an electrical arc is known to create all the features actually observed. That Europa has “a surface that looked as if it had been clawed by a tiger with talons several kilometers wide” is a perfectly apt description of the effect of surface lightning as it tears great furrows for hundreds or thousands of kilometers, throwing material to either side to create levees. The surface electric currents would have induced short-lived subsurface electromagnetic induction heating as well, thus accounting for the ice rafts.
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