The frozen crust of Ganymede, the largest moon of Jupiter, is repeatedly cracked and torn by sudden movements in the 600-mile-thick mantle of water that lies between the crust and the moon's rocky core, data from the Voyager space probe indicate.
Jupiter's outermost moon, Callisto, may undergo an even more bizarre geologic process. Meteorites crashing into its frozen crust send out violent shock waves in concentric patterns like tidal waves of ice, or "frozen tsunami," as the U.S. Geological Survey's Dr. Laurence A. Soderblom described them today.
Tsunami is the term scientists use for what are commonly called tidal waves -- waves touched off by undersea earthquakes.
Both processes are so strongly suggested in photographs returned today by the Voyager spacecreaft that scientists had no other explanation for what they saw in the pictures.
"This picture of Ganymede is the first time we see on another planet the offset motions and transverse motions caused at a planetary surface by moving faults," Soderblom said at the Jet Propulsion Laboratory, from which Voyager is directed.
"I'm almost willing to bet we'll learn more about fault movement and plate tectonies in studying the pictures of Ganymede than we could looking at pictures of the Earth," he said.
The photograph of Ganymede reveals brightly streaked fractures that are so numerous in its surface that they look like aerial photographs of the countless freeways that criss-cross Los Angels County. The fractures are as wide across as 20 miles and as long as 500 miles. The fractures frequently transverse each other, like the tracks of skiers that cross and come together along slopes covered with fresh snow.
Soderblom called the fractures the result of "water quakes" caused by the abrupt motions of a subsurface ocean that scientists believe makes up the mantle just below the crust of Ganymede. Cornell University's Dr. Carl Sagan thinks that Ganymede's mantle of liquid water runs as deep as 600 miles.
Sagan said that even a small core of radioactive rock would be not enough to maintain a liquid water mantle. Soderblom said that heat from the core could move through the subsurface ocean in ways to suddenly move the ocean around below the surface and cause fractures in the surface.
"We know from Ganymede's density that it has to be about one-half water." Soderblom said. "Knowing that, it isn't hard to speculate that the movement of subsurface water is the reason for all these surface fractures we're seeing in the picture."
Voyager's photograph of Callisto, thought to be made up more of water than is Ganymede, shows a circular basin rimmed by concentric rings that extend out as far as the photograph reaches. Soderblom said the rings are like nothing seen by humans anywhere else in the solar system, including the Earth's moon or the planets Mercury and Mars.
"We think we see what happened after a large meteorite struck Callisto." Soderblom said. "Like ripples on a pond after it's hit by a rock, these rings were left in the surface when the subsurface ice was displaced by the force of the meteorite impact. What you get is liteeally a frozen tsunami, a tidal wave of ice forcing its way in a very harmonic fashion along below the surface."
A third photograph returned today by Voyager as it sped away from Jupiter toward a 1980 enocounter with Saturn shows for the first time a tiny moon of Jupiter called Amalthea, the innermost of the planet's 13 known moon and one which had never been seen before by humans except as a tiny point of light in the biggest telescopes on Earth.
The picture of Amalthea shows a moon shaped and colored like a potato, no more than 180 miles long and 100 miles high. Two bright features and two craters appear on the surface.
"It doesn't look like much, but, after all, Amalthea has never been seen from Earth as anything but a light," said Dr. Bradford Smith of the University of Arizona. "And I would doubt that even one astronomer out of a hundred has even seen it before as a point of light."
As Voyager flew on today, its flight directors were concerned that radiation damage near to Jupiter Momday may have caused its timing devices to go awry.
The clocks that turn Voyagor instruments on and off were running fast by 40 seconds, meaning that the shutter in the spacecraft's two cameras occasionally was opening when it should have been closed and meaning tht photographs were being taken a little sooner than they should have been.
"There's a smear and double image effect in a few pictures," Voyager project manager Robert Parks said. "We're analyzing what we think went wrong so we can make some fixes to the system. These are not serious problems, just nuisances."