The
Rings And "Ring Arcs"
As
Voyager 2 approached Neptune, scientists had been working
on theories of how partial rings, or "ring arcs," could
exist. Most settled for the concept of shepherding satellites
that "herd" ring particles between them, keeping the particles
from either escaping to space or falling into the planet's
atmosphere. This theory had explained some new phenomena
observed in the rings of Jupiter, Saturn and Uranus.
When
Voyager 2 was close enough, its cameras photographed three
bright patches that looked like ring arcs. But closer approach,
higher resolution and more computer enhancement of the images
showed that the rings do, in fact, go all the way around
the planet.
Neptune's rings (Click on the image for a larger view) |
The
rings are so diffuse, and the material in them so fine,
that Earthbound astronomers simply hadn't been able to detect
the full rings. (Based on Voyager's findings, one Earth-
based observation of the ring arcs is now attributed to
the passage of a small satellite through the ring area.)
Late
in the encounter, the scientists were able to sort out the
number of rings and a preliminary nomenclature:
- The
"Main Ring" (officially known as 1989N1R, following the
IAU convention) orbits Neptune about 38,100 kilometers
(23,700 miles) above the cloud tops. The main ring contains
three separate regions where the material is brighter
and denser, and explains most of the sightings or "ring
arcs." Several Voyager photographs show what appear to
be clumps embedded in the rings. Scientists are not sure
what causes the material to clump.
- The
"Inner Ring" (1989N2R) -- about 28,400 kilometers (17,700
miles) above the cloud tops.
- An
"Inside Diffuse Ring" (1989N3R) -- a complete ring located
about 17,100 kilometers (10,600 miles) from Neptune's
cloud tops. Some scientists suspect that this ring may
extend all the way down to Neptune's cloud tops.
- An
area called "the Plateau," a broad, diffuse sheet of fine
material just outside the so-called "Inner Ring."
The
material varies considerably in size from ring to ring.
The largest proportion of fine material -- approximately
the size of smoke particles, is in the Plateau. All other
rings contain a greater proportion of larger material.
Both
Voyagers have now completed all of the planetary encounters
on their itinerary, but both still have work to do. Voyager
1 is heading out of the solar system, climbing above the
ecliptic plane in which the planets orbit the Sun. Voyager
2 is also outbound, traveling below that plane. Both are
searching for the heliopause, a boundary that marks the
end of the solar wind and the beginning of interstellar
space. Assuming both spacecraft remain healthy, flight controllers
expect to be able to operate the spacecraft for another
20 years, investigating magnetic fields and particles in
interplanetary and interstellar space, and observing ultraviolet
sources among the stars.
