Rings and Ring Arcs
In the summer of 1989, NASA’s Voyager 2 became the first spacecraft to observe the planet Neptune, its final planetary target. Passing about 4,950 kilometers (3,000 miles) above Neptune’s north pole, Voyager 2 made its closest approach to any planet since leaving Earth 12 years ago. Five hours later, Voyager 2 passed about 40,000 kilometers (25,000 miles) from Neptune’s largest moon, Triton, the last solid body the spacecraft will have an opportunity to study.
Neptune is one of the class of planets – all of them beyond the asteroid belt – known as gas giants; the others in this class are Jupiter, Saturn and Uranus. These planets are about 4 to 12 times greater in diameter than Earth. They have no solid surfaces but possess massive atmospheres that contain substantial amounts of hydrogen and helium with traces of other gases.
Voyager 2 is one of twin spacecraft launched more than a decade ago to explore the outer solar system. Between them, these spacecraft have explored four giant planets, 48 of their moons, and their unique systems of rings and magnetic fields.
Voyager 1, launched September 5, 1977, visited Jupiter in 1979 and Saturn in
Voyager 2, launched August 20, 1977, visited Jupiter in 1979, Saturn in 1981 and Uranus in 1986 before making its closest approach to Neptune on August 25,
Both spacecraft will continue to study ultraviolet sources among the stars, and their fields and particles detectors will continue to search for the boundary between the Sun’s influence and interstellar space. If all goes well, we will be able to communicate with the two spacecraft for another 25 to 30 years, until their nuclear power sources can no longer supply enough electrical energy to power critical subsystems.
Astronomers have studied Neptune since September 23, 1846, when Johann Gottfried Galle, of the Berlin Observatory, and Louis d’Arrest, an astronomy student, discovered the eighth planet on the basis of mathematical predictions by Urbain Jean Joseph Le Verrier. Similar predictions were made independently by John Couch Adams. (Galileo Galilei had seen Neptune during several nights of observing Jupiter, in January 1613, but didn’t realize he was seeing a new planet.) Still, any knowledge and understanding of Neptune was limited by the astronomer’s ability to see the distant object, almost 4.5 billion kilometers (2.8 billion miles) from Earth.
Scarcely a month after Galle and d’Arrest first saw Neptune, British astronomer William Lassell spotted a satellite orbiting the planet and named it Triton. Triton, almost the size of Earth’s Moon, is the only large satellite in the solar system to circle a planet in a retrograde direction – in a direction opposite to the rotation of the planet. That phenomenon led some astronomers to surmise that Neptune had captured Triton as it traveled through space several billion years ago.
In 1949, astronomer Gerard Kuiper discovered Nereid, the second of Neptune’s escorts. Nereid is only about 340 kilometers (210 miles) in diameter and is so far from Neptune that it requires 360 days to make one orbit – only five days less than Earth takes to travel once around the Sun.
In 1981, astronomers leaped at an infrequent opportunity: A star would pass behind Neptune so that observers could measure the starlight and how it changed as it passed through the upper layer of Neptune’s atmosphere. That would provide clues to its structure.
But the star’s light winked off and on before Neptune passed in front of it. Similar measurements were obtained during the mid-1980s. Astronomers concluded that some material (perhaps like that of the rings of Saturn) orbits Neptune, and was responsible for occasional blockage of the star’s light. In each observed event, astronomers saw that the ring or rings did not appear to completely encircle the planet – rather, each appeared to be an arc segment of a ring.
The laws of physics say that, with nothing else acting upon them, rings must orbit a planet at about the same distance from the center all the way around. Ring material, if unrestrained, will tend to disperse uniformly around the planet. In order to have “ring arcs,” scientists thought that some objects – perhaps small satellites – must shepherd the arcs, keeping them in their place by gravity.
Earth-based telescopic observations of Neptune over the last few years showed tantalizing hints of dynamic cloud structures on the distant planet, from which scientists could estimate the speed of winds circling the planet.
Against that background, Voyager’s scientists prepared for the first encounter of Neptune, perhaps the only close-up look at Neptune in the lifetime of many of us. What they found will force scholars to rewrite the astronomy textbooks, and scientists to adjust their views of the solar system’s other giant planets.
Nearly 4.5 billion kilometers (3 billion miles) from the Sun, Neptune orbits the Sun once in 165 years, and therefore has made not quite a full circle around the Sun since it was discovered. With an equatorial diameter of 49,528 kilometers (30,775 miles), Neptune is the smallest of our solar system’s gas giants. Even so, its volume could hold nearly 60 Earths. Neptune is the densest of the four giant planets, about 64 percent heavier than if it were composed entirely of water.
The most obvious feature of the planet in Voyager pictures is its blue color, the result of methane in the atmosphere. Methane preferentially absorbs the longer wavelengths of sunlight (those near the red end of the spectrum). What are left to be reflected are colors at the blue end of the spectrum.
While methane is not the only constituent in Neptune’s atmosphere, it is one of the most important. Methane cycles through the atmosphere like this:
Throughout the process there is no net loss of methane in Neptune’s atmosphere.
Neptune is a dynamic planet, even though it receives only 3 percent as much sunlight as Jupiter does. Several large, dark spots are reminiscent of Jupiter’s hurricane-like storms. The largest spot is big enough for Earth to fit neatly inside it. Designated the “Great Dark Spot” by its discoverers, the feature appears to be an anticyclone similar to Jupiter’s Great Red Spot. Neptune’s Great Dark Spot is comparable in size, relative to the planet, and at the same latitude (the Great Dark Spot is at 22 degrees south latitude) as Jupiter’s Great Red Spot. However, Neptune’s Great Dark Spot is far more variable in size and shape than its Jupiter counterpart. Bright, wispy “cirrus-type” clouds overlaying the Great Dark Spot at its southern and northeastern boundaries may be analogous to lenticular clouds that form over mountains on Earth.
At about 42 degrees south, a bright, irregularly shaped, eastward-moving cloud circles much faster than does the Great Dark Spot, “scooting” around Neptune in about 16 hours. This “scooter” may be a cloud plume rising between cloud decks.
Another spot, designated “D2” by Voyager’s scientists, is located far to the south of the Great Dark Spot, at 55 degrees south. It is almond-shaped, with a bright central core, and moves eastward around the planet in about 16 hours.
Voyager also measured heat radiated by Neptune’s atmosphere. The atmosphere above the clouds is hotter near the equator, cooler in the mid-latitudes and warm again at the south pole. Temperatures in the stratosphere were measured to be 750 kelvins (900 degrees F), while at the 100 millibar pressure level, they were measured to be 55 K (-360 degrees F). Heat appears to be caused, at least in part, by convection in the atmosphere that results in compressional heating: Gases rise in the mid-latitudes where they cool, then drift toward the equator and the pole, where they sink and are warmed.
Long, bright clouds, reminiscent of cirrus clouds on Earth, can be seen high in Neptune’s atmosphere. They appear to form above most of the methane, and consequently are not blue.
At northern low latitudes (27 degrees north), Voyager captured images of cloud streaks casting their shadows on cloud decks estimated to be about 50 to 100 kilometers (30 to 60 miles) below. The widths of these cloud streaks range from 50 to 200 kilometers (30 to 125 miles), and the widths of the shadows range from 30 to 50 kilometers (20 to 30 miles). Cloud streaks were also seen in the southern polar regions (71 degrees south), where the cloud heights were about 50 kilometers (30 miles).
Most of the winds on Neptune blow in a westward direction, which is retrograde, or opposite to the rotation of the planet. Near the Great Dark Spot, there are retrograde winds blowing up to 1500 miles an hour – the strongest winds measured on any planet, including windy Saturn.
The character of Neptune’s magnetic field is important because it helps scientists understand what goes on deep in the planet’s interior.
To have a magnetic field, scientists believe, a planet must fulfill these conditions:
Neptune’s magnetic field is tilted 47 degrees from the planet’s rotation axis, and is offset at least 0.55 radii (about 13,500 kilometers or 8,500 miles) from the physical center. The dynamo electric currents produced within the planet, therefore, must be relatively closer to the surface than for Earth, Jupiter or Saturn. The field strength at the surface varies, depending on which hemisphere is being measured, from a maximum of more than 1 gauss in the southern hemisphere to a minimum of less than 0.1 gauss in the northern. (Earth’s equatorial magnetic field at the surface is 0.32 gauss.) Because of its unusual orientation and the tilt of the planet’s rotation axis, Neptune’s magnetic field goes through dramatic changes as the planet rotates in the solar wind.
Voyager’s first indication of the Neptunian magnetic field was the detection of periodic radio emissions from the planet. The spacecraft crossed the bow shock, the outer edge of the field that stands ahead of the planet like a shield in the solar wind, as a shock wave stands out before a supersonic airplane, at 7:38 a.m. August 24, and shortly thereafter entered the planet’s magnetosphere. Voyager 2 remained within the magnetosphere for about 38 hours, or slightly more than two planetary rotations, before passing once again into the solarwind.
Because Neptune’s magnetic field is so highly tilted, and the timing of the encounter was such that the south magnetic pole was very nearly pointed at the Sun, Voyager 2 flew into the southern cusp of the magnetosphere, providing scientists a unique opportunity to observe this region of a gigantic magnetic field.
Magnetospheric scientists compared Neptune’s field with that of Uranus, which is tilted 59 degrees from the rotation axis, with a center that is offset by 0.3 Uranus radii. After Voyager 2 passed Uranus in January 1986, some scientists thought they might have seen the planet as its magnetic field was reversing direction. Others found it difficult to believe such a coincidence just happened as Voyager passed through the neighborhood. Scientists have no definite answers yet, but think that the tilt may be characteristic of flows in the interiors of both Uranus and Neptune and unrelated to either the high tilt of Uranus’ rotation axis or possible field reversals at either planet.
Neptune’s magnetic field polarity is the same as those of Jupiter and Saturn, and opposite to that of Earth.
Neptune’s magnetic field provided another clue to the planet’s structure and behavior. Observers on Earth hadn’t been able to determine the length of a Neptunian day. Cloud motions are a poor indicator of the rotation of the bulk of the planet, since they are affected by strong winds and vary substantially with latitude. The best telescopic estimate was a rotation period of approximately 18 hours. The best indicator of the internal rotation period of the planet is periodic radio waves generated by the magnetic field. Voyager’s planetary radio astronomy instrument measured these periodic radio waves, and determined that the rotation rate of the interior of Neptune is 16 hours, 7 minutes.
Voyager detected auroras, similar to the northern and southern lights on Earth, in Neptune’s atmosphere. The auroras on Earth occur when energetic particles strike the atmosphere as they spiral down the magnetic field lines. But because of Neptune’s complex magnetic field, the auroras are extremely complicated processes that occur over wide regions of the planet, not just near the planet’s magnetic poles. The auroral power on Neptune is weak, estimated at about 50 million watts, compared to 100 billion watts on Earth.
The largest of Neptune’s eight known satellites, Triton is different from all other icy satellites Voyager has studied. About three-quarters the size of Earth’s Moon, Triton circles Neptune in a tilted, circular, retrograde orbit (opposite to the direction of the planet’s rotation), completing an orbit in 5.875 days at an average distance of 330,000 kilometers (205,000 miles) above Neptune’s cloud tops. Triton shows evidence of a remarkable geologic history, and Voyager 2 images show active geyser-like eruptions spewing invisible nitrogen gas and dark dust particles several kilometers into space.
Triton has a diameter of about 2,705 kilometers (1,680 miles) and a mean density of about 2.066 grams per cubic centimeter (the density of water is 1.0 gram per cubic centimeter). This means Triton contains more rock in its interior than the icy satellites of Saturn and Uranus do.
The relatively high density and the retrograde orbit offer strong evidence that Triton did not originate near Neptune, but is a captured object. If that is the case, tidal heating could have melted Triton in its originally eccentric orbit, and the satellite might even have been liquid for as long as one billion years after its capture by Neptune.
While scientists are unsure of the details of Triton’s history, icy volcanism is undoubtedly an important ingredient.
To understand what is happening on Triton, one must ask, “How cold is cold? How soft is soft? How young is young?” Water ice, whose melting point is 0 degrees Celsius (32 degrees Fahrenheit), deforms more easily and rapidly on Earth than rock does, but becomes almost as rigid as rock at the extremely low temperatures found on Triton, more than 4.5 billion kilometers from the Sun. Most of the geologic structures on Triton’s surface are likely formed of water ice, because nitrogen and methane ice are too soft to support much of their own weight.
On the other hand, nitrogen and methane, which form a thin veneer on Triton, turn from ice to gas at less than 100 degrees above absolute zero. Most of the geologically recent eruptions at those low cryogenic temperatures are due to the nitrogen and methane on Triton.
Evidence that such eruptions occur was found in Voyager images of several geyser-like volcanic vents that were apparently spewing nitrogen gas laced with extremely fine, dark particles. The particles are carried to altitudes of 2 to 8 kilometers (1 to 5 miles) and then blown downwind before being deposited on Triton’s surface.
An extremely thin atmosphere extends as much as 800 kilometers (500 miles) above Triton’s surface. Tiny nitrogen ice particles may form thin clouds a few kilometers above the surface. Triton is very bright, reflecting 60 to 95 percent of the sunlight that strikes it (by comparison, Earth’s Moon reflects 11 percent).
The atmospheric pressure at Triton’s surface is about 14 microbars, a mere 1/70,000th the surface pressure on Earth. Temperature at the surface is about 38 kelvins (-391 degrees F), the coldest surface of any body yet visited in the solar system. At 800 kilometers (500 miles) above the surface, the temperature is 95 kelvins (-290 degrees F).
Despite remarkable differences between Triton and the other icy satellites in the solar system, photographs reveal terrain that is reminiscent of Ariel (a satellite of Uranus), Enceladus (a satellite of Saturn), and Europa, Ganymede and Io (satellites of Jupiter). Even a few reminders of Mars, such as polar caps and wind streaks, can be seen on Triton’s surface.
Triton appears to have the same general size, density, temperature and chemical composition as Pluto (the only outer planet not yet visited by any spacecraft), and will probably be our best model of Pluto for a long time to come.
In addition to the previously known satellites Triton and Nereid, Voyager 2 found six more satellites orbiting Neptune, for a total of eight known satellites. The new objects have not yet been named, a task for the International Astronomical Union (IAU), but were given temporary designations that tell the year of discovery, the planet they are associated with and the order of discovery; for example, 1989N1 was the first satellite of Neptune found that year. The final new body was designated 1989N6.
Nereid was discovered in 1948 through Earth-based telescopes. Little is known about Nereid, which is slightly smaller than 1989N1. Voyager’s best photos of Nereid were taken from about 4.7 million kilometers (2.9 million miles), and show that its surface reflects about 14 percent of the sunlight that strikes it, making it somewhat more reflective than Earth’s Moon, and more than twice as reflective as 1989N1. Nereid’s orbit is the most eccentric in the solar system, ranging from about 1,353,600 km (841,100 miles) to 9,623,700 km (5,980,200 mi).
1989N1 and its tiny companions are cratered and irregularly shaped – they are not round – and show no signs of any geologic modifications. All circle the plant in the same direction as Neptune rotates, and remain close to Neptune’s equatorial plane.
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.
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 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 25 to 30 years, investigating magnetic fields and particles in interplanetary and interstellar space, and observing ultraviolet sources among the stars.
The Voyager Project is managed by the Jet Propulsion Laboratory for NASA’s Office of Space Science and Applications.