Welcome to the strange world of Saturn's moons: a world where ice acts like rock and perhpas even like lava. It is a world considerably different than even the scientists imagined before last November.
As Voyager 2 brushed past the butterscotch-colored sphere of Saturn, it took pictures of a number of moons that have never been seen clearly before. This complements the efforts of its twin, Voyager 1, which flew by Saturn last November. Together they have provided pictures of the surfaces of seven of these satellites, showing features down to a few miles across.
Jet Propulsion Laboratory (JPL) engineers make flying 1.25 million miles and around unknown planets like Saturn seem routine. Yet early Aug. 26, as Voyager 2 emerged from behind the ringed planet's shadow, there was a reminder that space exploration is an enterprise with an exceedingly small safety margin.
Telemetry from the distant spacecraft showed that the camera platform had frozen facing backward. The camera remained free to move up and down but not right and left. At this writing, the Voyager flight team had no idea whether the problem was mechanical or electrical.
Dust in the gears of Voyager 1's camera platform made it stick temporarily, but the problem was fixed, Voyager officials are hopeful they can fix this problem as well. Even if they cannot, they feel it would still be possible to take a number of important pictures of Uranus in 1986 by controlling the spacecraft's orientations.
Despite this problem, " all the major scientific objectives of the mission appear to have been achieved," says a Voyager projects scientist, Edward C. Stone, of the California Institute of Technology.
Information about the moons already provided by Voyager is expected to give planetary scientists some important clues into the process by which all planets, including Earth, originally formed.
Saturn's moons are a large and motley crew. They range from the planet-size Titan to Hyperion, which looks like nothing so much as a thick hamburger patty floating in the velvet blackness of space. Most of the 17-odd satellites that accompany the ringed planet are actually little more than balls of ice, deep-frozen and mixed with enough rocky material to give them some odd coloraions.
The moons are "unlike any that we have ever seen before," comments Carl Sagan , Cornell University astronomer and host of the public television series "Cosmos."
"Once again we underestimated everything," admits Laurence Soderblom of the US Geological Survey (USGS). The scientists had imagined these satellites would be simple, homogeneous balls of ice with cratered surfaces. Instead, they have proved far more complicated, with large canyons, fracture lines, and enigmatic wispy features on their various surfaces.
The geologists interpret these features as evidence that these worlds are, or have been, internally active. Rift canyons, fault lines, and volcanoes on Earth display our world's internal activity. This is the manifestation of heating caused by the decay of radioactive material deep within our planet. But on the small, Saturnian moons which appear to be mostly or almost entirely ice, the reason for such activity is unclear.
"The complex interplay of internal and external activity that we found was very surprising," says Harold Masursky, also of the USGS. To get ice planets more than 200 degrees F. below zero to flow like lava, they must be heated by about 90 degrees F. It is possible that this degree of heating might come from gravitational tides causing friction between the various satellites. These are similar to the effects that the Moon has on Earth, except that they build up heat by flexing the entire satellite rather than simply sloshing the oceans. Scientists believe that this "gravitational pumping" is the force behind Jupiter's hyperactive moon, Io, which sports the solar system's largest volcanoes.
"We used to think these tidal forces were insignificant. Io proved they can create internal activity," Dr. Soderblom explains.
The internal activity of Saturn's moons appears much more sedate than Jupiter's Io. Nevertheless, Saturn's brightest moon, Enceladus, looks as if it is also quite active. It has very smooth areas that appear to have covered portions of craters.
"There is a good chance that Enceladus is active, continually resurfacing itself," says Soderbloom.
The net result is that these moons, although much smaller and lighter than Jupiter's stable of satellites, let alone the inner planets, look surprisingly familiar. Yet, in a less visible way, these moons are strikingly different.
This difference has to do with their density. Both the planets and the Jovian moons vary in density in a consistent pattern: those closer to the Sun, or Jupiter, are heavier while those farther out are lighter. The reason for this is the temperature differences in the cloud of dust and gas from which the solar and Jovian systems formed. Near the center the temperature was hotter so the heavier material rained out but the lighter, more volatile material did not. Only at the lower temperatures further from the center could the lighter material condense.
Saturn, it appears, formed in a significantly different fashion. There is no evidence that it heated up nearly as much as did jupiter. As a result, the density of its satellites follows a much different trend. Its innermost moons are lighter than those father out. This was predicted by James Pollack, a space agency scientist from Ames Research Center.He believes that rocky material was sucked into Saturn itself from the region nearby. As a result, the inner moon are mostly ice.
Even more important than this, however, solar system envision a period when the dust and gas condensed into billions of asteroid-sized clumps of material dubbed "planetesimals." These were the building blocks from which the planets formed. Up until now, scientists have had to guess the size of these objects. But the variation in the density of the small Saturnian moons suggests that they were formed from only a few planetesmials rather than thousands, says Dr. Soderbloom. As a result, they give the first direct evidence for their size.
Another question about planetesimals has been their variety. Scientists surmise there were probably heavy, asteroid types and light, cometary types, both in a number of varieties. But, like their size, the specific types and relative numbers of each type has been a matter of conjecture. here, too, study of the composition of Saturn's moons may provide some valuable clues, the Voyager scientists believe.
A case in point is the peculiar satellite, Iapetus. This is Saturn's schizophrenic moon. One half of it is as bright as slightly dirty snow. The other half is blacker than asphalt. Since Voyager 1, the scientists have been arguing about the reason for this strange coloration.
One camp has advanced an irreverently named "Foo Foo Dust Theory." This holds that the dark area is thin layer of asteroid which Saturn has captured that orbits beyond Iapetus. This fine dark debris, they argue, drifted into iapetus' orbit and gradually covered its leading hemisphere.
The other camp argues that this darker material is an integral part of the strange moon, perhaps uncovered or pumped to the surface by the internal activity of Iapetus. Voyager 2 photos have shown dark material filling the bottom of craters near the trailing edge of the moon which tend to support this later position.
"There is no reason why an asteroid [containing this dark material] might not have been incorporated into Iapetus along with icy material" and was never mixed thoroughly in, says Dr. Soderblom.
If so, it will be the first direct evidence of the variety of the material from which all the planets were created.