UNIVERSITY of Arizona astronomer Carolyn Porco advised reporters not to ``lose any sleep over how many rings there are on Neptune.'' But that's exactly what she and her colleagues on the Voyager imaging team have been doing. They are intrigued with the latest example of one of the basic mysteries of the solar system.
The rings and moons that orbit the giant outer planets are interacting systems. Moons can be a source of ring material. They also act to restrain and channel that material. Without them, it would tend to disperse.
If scientists are to understand the planets, they have to understand the origin, processes, and persistence of these ring/moon systems as well. With the completion of Voyager 2's outer-planet tour, they now have a wealth of images and other data on the full set of these systems to study. And as Dr. Porco and some of her colleagues have explained in press briefings, this is proving to be quite a challenge.
For example, there was speculation that the Neptune imagery showed at least five rings. Porco later warned that it is too early to pin down a definite number. ``It's been very difficult for the ring people to get data out of those images,'' she explained.
As a rough guess, she said, there is evidence right now for three distinct rings located respectively 42,000, 53,000, and 63,000 kilometers from Neptune's center. There also is the possibility of a broad sheet of material that may extend right down to the planet.
She noted that some analysts might distinguish what she called a plateau of material extending from 54,000 to 58,500 kilometers from the planet's center - that is, beginning just outside the middle ring.
The point of such a distinction is to show the variety Neptune's ring system includes. The outermost ring is 40 to 60 percent dust. Yet the plateau material has only 10 percent dust. It contains larger constituents. Also, the outer ring - unlike the inner rings - is quite uneven with distinct clumps of material along its length. An image of one of these clumps shows tiny moonlets moving through it.
These varied features make Neptune's ring and moon system ``interesting,'' Porco said. ``It implies variety. And we like variety,'' she added.
She said that the clumpy outer ring reminds her of the Encke and F rings around Saturn. Here, too, ring irregularities seem related to the presence of small moonlets. Now scientists have another example to study. However, Porco noted that Saturn's F-ring clumps still are not well understood eight years after their discovery. ``So we have a lot of work to do,'' she observed wryly.
There is one other puzzle. Is Neptune's ring/moon system a permanent feature or is it in transition? Is it young and just developing or is it old and breaking up, Porco asked.
Scientists now have good data on all the ring/moon combinations in the solar system. This will allow comparative studies. At the same time, Porco pointed out that Saturn's system - ``the granddaddy of them all'' - offers the widest range of phenomena. She said that is the system on which to concentrate most attention.
And in a ``commercial'' for her favorite proposed new mission, she noted that the Cassini Saturn probe will be the next opportunity to check out scientists' ring theories. That mission would include a craft to orbit Saturn and a probe to be sent into the atmosphere of the moon Titan. It is included as a ``new start'' in the administration's 1990 requested budget with a launch date of 1996. But it is uncertain whether or not Congress will approve it.