Neptune's ring system

While all of the gas giant planets have ring systems, Neptune’s are unique. They consist of a series of concentric rings and partial arcs. Why Neptune’s rings developed differently is a matter of debate among scientists.

Overview

After Uranus’s discovery, scientists noticed that it did not behave as expected. They concluded that the gravitational effects of a yet unknown planet caused discrepancies noted in Uranus’s orbit. In the early 1840s, British astronomer John Adams and French mathematician Urbain Le Verrier independently calculated the unknown planet’s orbit and mass. In 1846, German astronomer Johann Galle discovered Neptune within one or two degrees of its predicted location.

It was not until 1984, however, that scientists at two observatories found the first evidence of a ring system around Neptune. On July 22 of that year, Neptune occulted star SAO 186001. Astronomers use the occultation of stars to look for rings and satellites or to measure a planet’s exact size. The observatories (which were located in Chile about one hundred kilometers apart) recorded a brief occultation lasting just a second. The starlight was reduced by a mere 35 percent when Neptune passed across the line of sight from Earth to the star. Usually, this would be considered evidence of a new satellite. In this case, however, to fit with the data, a satellite would only have been ten to twenty kilometers in diameter. The astronomers concluded instead that they had discovered a ring around Neptune. However, neither observatory noticed any reduction in starlight on the other side of Neptune.

Neptune’s arc-type ring structures remained a mystery until the Voyager 2spacecraft arrived at Neptune in 1989. The first images from Voyager were taken at a distance of about 20.8 million kilometers during Voyager’s approach to Neptune. The photographs show two arcs around the planet. The first arc was measured to be about 48,000 kilometers long and tilted at a forty-five-degree angle with respect to Neptune. The inner arc was measured at about 9,600 kilometers wide. Both arcs were found near recently discovered satellites of the planet. Later measurements have calculated Neptune's rings as ranging from a distance of 41,900 kilometers from the planet's center to 62,900 kilometers. Three of the rings—Le Verrier, Arago, and Adams—are less than one hundred kilometers wide; the other two rings, Galle and Lasell, range in width from two thousand to five thousand kilometers. Voyager 2 located the partial rings early enough for scientists at the spacecraft’s headquarters in California to redirect its camera toward them. Initial photographs show a differing brightness in the rings, which scientists think might have resulted from size, density, or rock variations among the ring particles. Saturn’s F ring has a similar varying brightness.

On August 23, 1989, Voyager 2 transmitted images back to Earth that solved the mystery of what had seemed to be a partial ring system. What had looked like arcs were actually full rings that encircled Neptune but were not completely visible from Earth. The spacecraft photographed the rings in both forward and backward scattered light. Microscopic particles cannot be seen from Earth since they are too small to reflect enough sunlight to be seen at such a distance. However, when they were backlit—imaged with the sun behind them—Voyager 2’s cameras were able to detect them as small dust particulates.

In total, Neptune has five rings and five partial arcs. Each ring is named for an astronomer who made an important discovery about the planet. The innermost ring, named Galle, is inside the orbit of Neptune’s satellite Naiad. This ring is composed mostly of dust, resembling the partial arcs. The Le Verrier ring is next closest to Neptune. It is also the second most prominent of the rings. It is narrow and dusty, located seven hundred kilometers outside the orbit of Despina. The widest ring, Lassell, extends for four thousand kilometers. It is one of the less dusty and more complete rings. Its outer edge, which is significantly brighter than the rest, has been named the Arago ring. The outer and most prominent ring is called Adams. Orbiting Neptune about one thousand kilometers outside the path of Galatea, it is narrow and dim in comparison to the bright rings of Uranus and Saturn. The Adams ring also contains five ring arcs: Courage, Liberté, Égalité (1 and 2), and Fraternité.

After Voyager 2 reached Neptune, astronomers confirmed that the planet had a complete ring system. They still did not know, however, what had caused the arcs of the Adams ring to remain incomplete. In 1991, planetary scientist Carolyn Porco postulated that the arcs were a result of Neptune’s signature satellite Galatea. The small satellite orbits Neptune one thousand kilometers inside the Adams ring. Porco's theory is that Galatea acts as a shepherding moon whose gravitational effects keep the matter in the arcs from forming a complete ring. Analysis of data collected by Voyager 2 showed that the arcs “wiggle,” or shift positions, by up to thirty kilometers. According to Porco, arc distortion occurred at the right speed to be attributed to the small satellite. Critics of this theory point out that ring-arc material would need to have orbits that intersect each other to maintain its overall shape. Porco agreed that this would cause collisions, leading to the inevitable destruction of the arcs themselves. Her model also only partly explains the possible origin of the arcs. It can only determine places where arcs are more likely to develop, not specifically where they already have developed. Because of the lumpiness of the partial rings, Porco and other scientists believe they could once have been a small moon that was destroyed.

In 1999, a group of scientists argued that Galatea could not be the sole influence causing Neptune’s arcs. They studied data collected in 1998 using the Hubble Space Telescope. The largest change in position was of the Liberté arc, which was displaced 1.9 degrees compared to the Égalité arc. The scientists argued that this finding was not a result of varying particle size, given Voyager data that support size conformity within the arc. Their findings, however, did not rule out the shepherding moon theory that relies on the effects of two satellites. Porco and her colleague Fathi Namouni published a paper in 2002 again arguing in favor of their Galatea shepherding-moon model. They believed that Galatea’s elliptical orbit kept the arc particles from having many collisions. By refining the mathematical model of Neptune’s ring system, Porco and Namouni showed that Galatea might still help answer the arc mystery. More precise data concerning Galatea’s orbit and the partial rings were needed before scientists will be able to determine what really causes the arcs around Neptune.

In 2002 and 2003, a group of scientists led by Imke de Pater photographed Neptune’s outer rings. Studying the images, they discovered that all of Neptune’s arcs appeared to be fading away. The Liberté arc showed the most deterioration when compared with the Voyager data. If the dissipation continues, the Liberté arc will be gone within less than one hundred years. Their observation showed that whatever is holding the ring arcs together is not regenerating them fast enough to sustain them.

Knowledge Gained

Several stellar occultations with Neptune were observed, but only five showed evidence of a ring or arc system. Scientists working at the European Southern Observatory and the Chilean Cerro Tololo Observatory both witnessed the same brief occultation of star SAO 186001. The two teams noticed that the planet itself did not block the star, because the starlight diminished by only about 35 percent. Neither group was able to locate any reduction of light on the opposite side of Neptune. These data led to the conclusion that Neptune has only a partial ring or arc system.

The Voyager program consisted of two spacecraft focused on studying the outer planets of our solar system. Launched in 1977, Voyager 2 reached Neptune twelve years later. The spacecraft had two video cameras, infrared and ultraviolet spectrometers, and other instruments. It was not until Voyager 2 started its approach toward Neptune that the first photographs of the rings and arcs were taken. Scientists learned that Neptune, in fact, does have a mostly complete ring system, like the rest of the gas giants, but that the dust particles that compose the majority of the rings are too small to be detected by Earth-based telescopes. Full rings can be seen only when backlit by the sun—a view scientists can get only with the use of space probes such as Voyager 2.

Scientists have more recently been able to use the Hubble Space Telescope’s Near Infrared Camera and Multi-Object Spectrometer (NICMOS) to study Neptune’s ring system. The camera was powerful enough to detect ring arcs during two different occultations in 1998. One of the difficulties in studying them from Earth is the visual proximity of the rings to Neptune itself. NICMOS solved this problem by using a special filter that blocks wavelengths at which methane, a main component of Neptune’s atmosphere, reflects light.

Starting around the year 2000, astronomers were able to use Earth-based telescopes to study Neptune’s ring system. Imke de Pater and his colleagues viewed Neptune’s ring arcs using the ten-meter Keck Telescope in Hawaii. Advances in image resolution and light-gathering power made this possible. However, the Keck can still detect only the brighter Adams ring; the others remain too faint for observation. When the Keck data were analyzed, scientists found evidence that the ring arcs were fading away, with the Liberté arc showing the most damage. If the arcs continue to degrade at the rate they have since 1989, within less than a hundred years, they will have disappeared. In 2022, the James T. Webb telescope produced the clearest pictures of Neptune’s rings yet taken, offering scientists the opportunity to truly study them.

Context

All of the gas giants in our solar system have ring systems. They all share similarities and have their differences as well. Neptune’s system contains five full rings and five arcs. With the James T. Webb Telescope, technology advanced enough to detect the faint dust particles that compose the rings, and no longer only the brightest, outermost ring, the Adams ring was able to be photographed from Earth.

Learning more about the ring systems promises to help scientists better understand the origins and evolution of the solar system itself. Scientists are also still debating what is preventing Neptune’s arcs from forming complete rings, though they believe the gravitational effects of Galatea, a moon, may play a role. This main theory is based on the idea of shepherding moons that gravitationally trap the particles in arcs. Further advancements must be made before more can be learned about Neptune’s ring system, and sending another spacecraft with more advanced instruments may be the best way to do so.

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