11. Uranus and Neptune
Rings of Uranus and Neptune
Narrow, widely spaced rings around Uranus
Astronomers have had a history of happy accidents concerning Uranus, starting with William Herschelís (1738-1822) serendipitous discovery of the planet in 1781. Another lucky incident occurred on 10 March 1977, when the planet was scheduled to pass in front of a faint star. By observing such a stellar occultation, astronomers hoped to determine properties of the planetís atmosphere, and to accurately establish its size from the duration of the starís disappearance behind it.
Because of uncertainties in the predicted time of the star's disappearance, one telescope was set into action about 45 minutes early. Soon after the recording began, the starlight abruptly dimmed but then it almost immediately returned to normal, producing a brief dip in the recorded signal. At first, the dip was attributed to a wisp of cloud on Earth or to an unexpected change in the telescope's orientation. But the star blinked on and off several times before and after the planet covered it. Moreover, each dip on one side of Uranus was matched by another one on the other side, at the same distance from the planet. The symmetrical, brief dips indicated that Uranus is surrounded by a family of narrow rings that blocked out the starís light but could not be seen directly from the Earth.
During the next few years, observations of more than 200 stellar occultations by Uranus revealed the details of nine narrow rings. In order of increasing distance from Uranus, the rings are named 6, 5, 4, a, &beta, &?,?, d, and e, following the differing notation of the discoverers. From the brief duration of the dips of blocked starlight, astronomers concluded that all but one of the individual rings could be no wider than 10 kilometers. The relatively long time between the dips indicated that the threadlike rings are separated by hundreds of kilometers of nearly empty space. These skeletal, web-like rings are unlike any seen before, all very narrow and widely spaced from each other.
When Voyager 2 arrived at Uranus in 1986, nearly a decade after the discovery of its narrow rings, instruments on the spacecraft confirmed all the known rings, and added at least two. They found the ? ring, a narrow strand between the d and &epsilon: rings, and another one interior to ring 6. The spacecraft also discovered at least 10 small moons that are located just outside the ring system.
The particles in the main narrow rings of Uranus are both dark and large. They range between a softball and an automobile in size, or between 0.1 and a few meters across. And they contain very few smaller particles in the millimeter to centimeter, or 0.001 to 0.01 meter, range, with surprisingly small amounts of micron-sized dust about 10-6 meters across.
Broad sheets of dust were nevertheless detected in the wide gaps between the rings. When Voyager 2 entered Uranusís shadow and looked back at the rings, about 100 very diffuse, nearly transparent bands of microscopic dust were seen with sunlight streaming past them. The dust is lit up when the Sun shines through the rings, in the same way that grime on a car's windshield becomes visible when struck by the lights of an oncoming car.
The irregular orientation and shapes of the Uranian rings are attributed to small moons that lie just outside them. The repeated gravitational tugs of two of them, Cordelia and Ophelia, pull the epsilon ring into its oval shape and restrain its edges. These tiny moons flank the ring, controlling its shape in much the same way that the shepherd satellites, Pandora and Prometheus, constrain Saturnís F ring. Nearby moons probably sharpen the edges of the other rings, keeping them from spreading out as the result of particle collisions, but many of the expected moons have not been found. They may have been too dark or too tiny for Voyagerís cameras to record.
Neptuneís sparse thin rings and arcs
After the discovery of the rings of Uranus by watching a distant star pass behind the planet, astronomers hoped to repeat the achievement by observing stellar occultations by Neptune, but the results were inconclusive. Sometimes the starlight would remain unchanged before and after the planet directly occulted the star. At other times the star would blink on and off, but always on just one side of the planet. Because the brief dimming of starlight was not symmetrical about the planet, and not all stellar occultations produced a blinking signal, the hypothetical rings became shortened, in the minds of the astronomers, to ring-arcs that only reached part way around the planet. Chance might then dictate which astronomers would detect the obscuration.
Voyager 2 clarified the problem. Neptune's ring-arcs turned out not to be isolated segments, but rather three thicker portions of one very thin ring. The ring is narrow and continuous, stretching all the way round the planet just like any well-behaved ring. Its material is generally spread so thinly that it does not noticeably dim a starís light. The ring is only dense enough to hide a star in three arc-like concentrations, subsequently named Libertť, Egalitť and Fraternitť after the French revolutionary slogan. It was these high-density clumps that had been detected from Earth, blocking starlight and giving the impression of disconnected arcs. The rest of the ring couldnít be seen from Earth because it is so transparent, and hence below the threshold of detectability.
Formation and evolution of the rings of Uranus and Neptune
It is now thought that all the planetary rings are younger than the age of the solar system, so they cannot be permanent features dating back to its origin. And the present rings are now viewed as a passing stage in an ongoing process of creation and loss.
The austere rings that now circle Uranus and Neptune may have had a violent and chaotic past, arising from catastrophic collisions of moons or when one larger satellite moved inward by tidal interaction with the planet until it was close enough to be ripped into pieces. The inner small moons and larger particles in the rings were then probably gradually broken up by collisions into smaller ones. And all the ring particles we see today will eventually be eroded away by meteoritic bombardment, ground into fine dust by particle collisions, or displaced by gravitational interaction with neighboring satellites.
Thus an entire ring system will eventually be turned into dust. And because all the dust is dragged into the planet's atmosphere or ejected from the system, the rings will inevitably decay and disappear over astronomical times.
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Copyright 2010, Professor Kenneth R. Lang, Tufts University