VOYAGER NEPTUNE SCIENCE SUMMARY<\/p>\n
In the summer of 1989, NASA’s Voyager 2 became the
\nfirst spacecraft to observe the planet Neptune, its final
\nplanetary target. Passing about 4,950 kilometers (3,000 miles)
\nabove Neptune’s north pole, Voyager 2 made its closest approach
\nto any planet since leaving Earth 12 years ago. Five hours
\nlater, Voyager 2 passed about 40,000 kilometers (25,000 miles)
\nfrom Neptune’s largest moon, Triton, the last solid body the
\nspacecraft will have an opportunity to study.
\n Neptune is one of the class of planets — all of them
\nbeyond the asteroid belt — known as gas giants; the others in
\nthis class are Jupiter, Saturn and Uranus. These planets are
\nabout 4 to 12 times greater in diameter than Earth. They have no
\nsolid surfaces but possess massive atmospheres that contain
\nsubstantial amounts of hydrogen and helium with traces of other
\ngases.
\n Voyager 2 is one of twin spacecraft launched more than
\na decade ago to explore the outer solar system. Between them,
\nthese spacecraft have explored four giant planets, 48 of their
\nmoons, and their unique systems of rings and magnetic fields.
\n Voyager 1, launched September 5, 1977, visited Jupiter
\nin 1979 and Saturn in 1980. It is now leaving the solar system,
\nrising above the ecliptic plane at an angle of about 35 degrees,
\nat a rate of about 520 million kilometers a year.
\n Voyager 2, launched August 20, 1977, visited Jupiter in<\/p>\n
1979, Saturn in 1981 and Uranus in 1986 before making its closest
\napproach to Neptune on August 25, 1989. Voyager 2 traveled 12
\nyears at an average velocity of 19 kilometers a second (about
\n42,000 miles an hour) to reach Neptune, which is 30 times farther
\nfrom the Sun than Earth is. Voyager observed Neptune almost
\ncontinuously from June to October 1989. Now Voyager 2 is also
\nheaded out of the solar system, diving below the ecliptic plane
\nat an angle of about 48 degrees and a rate of about 470 million
\nkilometers a year.
\n Both spacecraft will continue to study ultraviolet
\nsources among the stars, and their fields and particles detectors
\nwill continue to search for the boundary between the Sun’s
\ninfluence and interstellar space. If all goes well, we will be
\nable to communicate with the two spacecraft for another 25 to 30
\nyears, until their nuclear power sources can no longer supply
\nenough electrical energy to power critical subsystems.<\/p>\n
BACKGROUND
\n Astronomers have studied Neptune since September 23,
\n1846, when Johann Gottfried Galle, of the Berlin Observatory, and
\nLouis d’Arrest, an astronomy student, discovered the eighth
\nplanet on the basis of mathematical predictions by Urbain Jean
\nJoseph Le Verrier. Similar predictions were made independently
\nby John Couch Adams. (Galileo Galilei had seen Neptune during
\nseveral nights of observing Jupiter, in January 1613, but didn’t
\nrealize he was seeing a new planet.) Still, any knowledge and
\nunderstanding of Neptune was limited by the astronomer’s abilityto see the distant object, almost 4.5 billion kilometers (2.8
\nbillion miles) from Earth.
\n Scarcely a month after Galle and d’Arrest first saw
\nNeptune, British astronomer William Lassell spotted a satellite
\norbiting the planet and named it Triton. Triton, almost the size
\nof Earth’s Moon, is the only large satellite in the solar system
\nto circle a planet in a retrograde direction — in a direction
\nopposite to the rotation of the planet. That phenomenon led some
\nastronomers to surmise that Neptune had captured Triton as it
\ntraveled through space several billion years ago.
\n In 1949, astronomer Gerard Kuiper discovered Nereid,
\nthe second of Neptune’s escorts. Nereid is only about 340
\nkilometers (210 miles) in diameter and is so far from Neptune
\nthat it requires 360 days to make one orbit — only five days
\nless than Earth takes to travel once around the Sun.
\n In 1981, astronomers leaped at an infrequent
\nopportunity: A star would pass behind Neptune so that observers
\ncould measure the starlight and how it changed as it passed
\nthrough the upper layer of Neptune’s atmosphere. That would
\nprovide clues to its structure.
\n But the star’s light winked off and on before Neptune
\npassed in front of it. Similar measurements were obtained during
\nthe mid-1980s. Astronomers concluded that some material (perhaps
\nlike that of the rings of Saturn) orbits Neptune, and was
\nresponsible for occasional blockage of the star’s light. In each
\nobserved event, astronomers saw that the ring or rings did not
\nappear to completely encircle the planet — rather, each appeared<\/p>\n
to be an arc segment of a ring.
\n The laws of physics say that, with nothing else acting
\nupon them, rings must orbit a planet at about the same distance
\nfrom the center all the way around. Ring material, if
\nunrestrained, will tend to disperse uniformly around the planet.
\nIn order to have “ring arcs,” scientists thought that some
\nobjects — perhaps small satellites — must shepherd the arcs,
\nkeeping them in their place by gravity.
\n Earth-based telescopic observations of Neptune over the
\nlast few years showed tantalizing hints of dynamic cloud
\nstructures on the distant planet, from which scientists could
\nestimate the speed of winds circling the planet.
\n Against that background, Voyager’s scientists prepared
\nfor the first encounter of Neptune, perhaps the only close-up
\nlook at Neptune in the lifetime of many of us. What they found
\nwill force scholars to rewrite the astronomy textbooks, and
\nscientists to adjust their views of the solar system’s other
\ngiant planets.<\/p>\n
NEPTUNE
\n Nearly 4.5 billion kilometers (3 billion miles) from
\nthe Sun, Neptune orbits the Sun once in 165 years, and therefore
\nhas made not quite a full circle around the Sun since it was
\ndiscovered. With an equatorial diameter of 49,528 kilometers
\n(30,775 miles), Neptune is the smallest of our solar system’s
\ngas giants. Even so, its volume could hold nearly 60 Earths.
\nNeptune is the densest of the four giant planets, about 64 percent heavier than if it were composed entirely of water.
\n The most obvious feature of the planet in Voyager
\npictures is its blue color, the result of methane in the
\natmosphere. Methane preferentially absorbs the longer
\nwavelengths of sunlight (those near the red end of the spectrum).
\nWhat are left to be reflected are colors at the blue end of the
\nspectrum.
\n While methane is not the only constituent in Neptune’s
\natmosphere, it is one of the most important. Methane cycles
\nthrough the atmosphere like this:
\n * Solar ultraviolet radiation destroys methane high in
\nNeptune’s atmosphere by converting it to hydrocarbons such as
\nethane, acetylene and haze particles of more complex polymers.
\n * The haze particles sink to the cold lower
\nstratosphere, where they freeze and become ice particles.
\n * The hydrocarbon ice particles gently fall into the
\nwarmer troposphere, where they evaporate back into gases.
\n * The hydrocarbon gases mix deeper into the atmosphere
\nwhere the temperature and pressure are higher, mix with hydrogen
\ngas and regenerate methane.
\n * Buoyant, convective methane clouds then rise great
\ndistances to the base of the stratosphere or higher, returning
\nmethane vapor to the stratosphere.
\n Throughout the process there is no net loss of methane
\nin Neptune’s atmosphere.
\n Neptune is a dynamic planet, even though it receives
\nonly 3 percent as much sunlight as Jupiter does. Several large,<\/p>\n
dark spots are reminiscent of Jupiter’s hurricane-like storms.
\nThe largest spot is big enough for Earth to fit neatly inside it.
\nDesignated the “Great Dark Spot” by its discoverers, the feature
\nappears to be an anticyclone similar to Jupiter’s Great Red Spot.
\nNeptune’s Great Dark Spot is comparable in size, relative to the
\nplanet, and at the same latitude (the Great Dark Spot is at 22
\ndegrees south latitude) as Jupiter’s Great Red Spot. However,
\nNeptune’s Great Dark Spot is far more variable in size and shape
\nthan its Jupiter counterpart. Bright, wispy “cirrus-type” clouds
\noverlaying the Great Dark Spot at its southern and northeastern
\nboundaries may be analogous to lenticular clouds that form over
\nmountains on Earth.
\n At about 42 degrees south, a bright, irregularly
\nshaped, eastward-moving cloud circles much faster than does the
\nGreat Dark Spot, “scooting” around Neptune in about 16 hours.
\nThis “scooter” may be a cloud plume rising between cloud decks.
\n Another spot, designated “D2” by Voyager’s scientists,
\nis located far to the south of the Great Dark Spot, at 55 degrees
\nsouth. It is almond-shaped, with a bright central core, and
\nmoves eastward around the planet in about 16 hours.
\n Voyager also measured heat radiated by Neptune’s
\natmosphere. The atmosphere above the clouds is hotter near the
\nequator, cooler in the mid-latitudes and warm again at the south
\npole. Temperatures in the stratosphere were measured to be 750
\nkelvins (900 degrees F), while at the 100 millibar pressure
\nlevel, they were measured to be 55 K (-360 degrees F). Heat
\nappears to be caused, at least in part, by convection in theatmosphere that results in compressional heating: Gases rise in
\nthe mid-latitudes where they cool, then drift toward the equator
\nand the pole, where they sink and are warmed.
\n Long, bright clouds, reminiscent of cirrus clouds on
\nEarth, can be seen high in Neptune’s atmosphere. They appear to
\nform above most of the methane, and consequently are not blue.
\n At northern low latitudes (27 degrees north), Voyager
\ncaptured images of cloud streaks casting their shadows on cloud
\ndecks estimated to be about 50 to 100 kilometers (30 to 60 miles)
\nbelow. The widths of these cloud streaks range from 50 to 200
\nkilometers (30 to 125 miles), and the widths of the shadows range
\nfrom 30 to 50 kilometers (20 to 30 miles). Cloud streaks were
\nalso seen in the southern polar regions (71 degrees south), where
\nthe cloud heights were about 50 kilometers (30 miles).
\n Most of the winds on Neptune blow in a westward
\ndirection, which is retrograde, or opposite to the rotation of
\nthe planet. Near the Great Dark Spot, there are retrograde winds
\nblowing up to 1500 miles an hour — the strongest winds measured
\non any planet, including windy Saturn.<\/p>\n
THE MAGNETIC ENVIRONMENT
\n The character of Neptune’s magnetic field is important
\nbecause it helps scientists understand what goes on deep in the
\nplanet’s interior.
\n To have a magnetic field, scientists believe, a planet
\nmust fulfill these conditions:<\/p>\n
* There must be a region within the planet that is
\nliquid;
\n * The region must also be electrically conducting;
\n * There must be an energy source that sets the region
\nin motion and then keeps it moving.
\n Neptune’s magnetic field is tilted 47 degrees from the
\nplanet’s rotation axis, and is offset at least 0.55 radii (about
\n13,500 kilometers or 8,500 miles) from the physical center. The
\ndynamo electric currents produced within the planet, therefore,
\nmust be relatively closer to the surface than for Earth, Jupiter
\nor Saturn. The field strength at the surface varies, depending
\non which hemisphere is being measured, from a maximum of more
\nthan 1 gauss in the southern hemisphere to a minimum of less than
\n0.1 gauss in the northern. (Earth’s equatorial magnetic field at
\nthe surface is 0.32 gauss.) Because of its unusual orientation
\nand the tilt of the planet’s rotation axis, Neptune’s magnetic
\nfield goes through dramatic changes as the planet rotates in the
\nsolar wind.
\n Voyager’s first indication of the Neptunian magnetic
\nfield was the detection of periodic radio emissions from the
\nplanet. The spacecraft crossed the bow shock, the outer edge of
\nthe field that stands ahead of the planet like a shield in the
\nsolar wind, as a shock wave stands out before a supersonic
\nairplane, at 7:38 a.m. August 24, and shortly thereafter entered
\nthe planet’s magnetosphere. Voyager 2 remained within the
\nmagnetosphere for about 38 hours, or slightly more than two
\nplanetary rotations, before passing once again into the solarwind.
\n Because Neptune’s magnetic field is so highly tilted,
\nand the timing of the encounter was such that the south magnetic
\npole was very nearly pointed at the Sun, Voyager 2 flew into the
\nsouthern cusp of the magnetosphere, providing scientists a unique
\nopportunity to observe this region of a gigantic magnetic field.
\n Magnetospheric scientists compared Neptune’s field with
\nthat of Uranus, which is tilted 59 degrees from the rotation
\naxis, with a center that is offset by 0.3 Uranus radii. After
\nVoyager 2 passed Uranus in January 1986, some scientists thought
\nthey might have seen the planet as its magnetic field was
\nreversing direction. Others found it difficult to believe such a
\ncoincidence just happened as Voyager passed through the
\nneighborhood. Scientists have no definite answers yet, but think
\nthat the tilt may be characteristic of flows in the interiors of
\nboth Uranus and Neptune and unrelated to either the high tilt of
\nUranus’ rotation axis or possible field reversals at either
\nplanet.
\n Neptune’s magnetic field polarity is the same as those
\nof Jupiter and Saturn, and opposite to that of Earth.
\n Neptune’s magnetic field provided another clue to the
\nplanet’s structure and behavior. Observers on Earth hadn’t been
\nable to determine the length of a Neptunian day. Cloud motions
\nare a poor indicator of the rotation of the bulk of the planet,
\nsince they are affected by strong winds and vary substantially
\nwith latitude. The best telescopic estimate was a rotation
\nperiod of approximately 18 hours. The best indicator of the<\/p>\n
internal rotation period of the planet is periodic radio waves
\ngenerated by the magnetic field. Voyager’s planetary radio
\nastronomy instrument measured these periodic radio waves, and
\ndetermined that the rotation rate of the interior of Neptune is
\n16 hours, 7 minutes.
\n Voyager detected auroras, similar to the northern and
\nsouthern lights on Earth, in Neptune’s atmosphere. The auroras
\non Earth occur when energetic particles strike the atmosphere as
\nthey spiral down the magnetic field lines. But because of
\nNeptune’s complex magnetic field, the auroras are extremely
\ncomplicated processes that occur over wide regions of the planet,
\nnot just near the planet’s magnetic poles. The auroral power on
\nNeptune is weak, estimated at about 50 million watts, compared to
\n100 billion watts on Earth.<\/p>\n
TRITON
\n The largest of Neptune’s eight known satellites, Triton
\nis different from all other icy satellites Voyager has studied.
\nAbout three-quarters the size of Earth’s Moon, Triton circles
\nNeptune in a tilted, circular, retrograde orbit (opposite to the
\ndirection of the planet’s rotation), completing an orbit in 5.875
\ndays at an average distance of 330,000 kilometers (205,000 miles)
\nabove Neptune’s cloud tops. Triton shows evidence of a
\nremarkable geologic history, and Voyager 2 images show active
\ngeyser-like eruptions spewing invisible nitrogen gas and dark
\ndust particles several kilometers into space.
\n Triton has a diameter of about 2,705 kilometers (1,680
\nmiles) and a mean density of about 2.066 grams per cubic
\ncentimeter (the density of water is 1.0 gram per cubic
\ncentimeter). This means Triton contains more rock in its
\ninterior than the icy satellites of Saturn and Uranus do.
\n The relatively high density and the retrograde orbit
\noffer strong evidence that Triton did not originate near Neptune,
\nbut is a captured object. If that is the case, tidal heating
\ncould have melted Triton in its originally eccentric orbit, and
\nthe satellite might even have been liquid for as long as one
\nbillion years after its capture by Neptune.
\n While scientists are unsure of the details of Triton’s
\nhistory, icy volcanism is undoubtedly an important ingredient.
\n To understand what is happening on Triton, one must
\nask, “How cold is cold? How soft is soft? How young is young?”
\nWater ice, whose melting point is 0 degrees Celsius (32 degrees
\nFahrenheit), deforms more easily and rapidly on Earth than rock
\ndoes, but becomes almost as rigid as rock at the extremely low
\ntemperatures found on Triton, more than 4.5 billion kilometers
\nfrom the Sun. Most of the geologic structures on Triton’s
\nsurface are likely formed of water ice, because nitrogen and
\nmethane ice are too soft to support much of their own weight.
\n On the other hand, nitrogen and methane, which form a
\nthin veneer on Triton, turn from ice to gas at less than 100
\ndegrees above absolute zero. Most of the geologically recent
\neruptions at those low cryogenic temperatures are due to the
\nnitrogen and methane on Triton. <\/p>\n
Evidence that such eruptions occur was found in Voyager
\nimages of several geyser-like volcanic vents that were apparently
\nspewing nitrogen gas laced with extremely fine, dark particles.
\nThe particles are carried to altitudes of 2 to 8 kilometers (1 to
\n5 miles) and then blown downwind before being deposited on
\nTriton’s surface.
\n An extremely thin atmosphere extends as much as 800
\nkilometers (500 miles) above Triton’s surface. Tiny nitrogen ice
\nparticles may form thin clouds a few kilometers above the
\nsurface. Triton is very bright, reflecting 60 to 95 percent of
\nthe sunlight that strikes it (by comparison, Earth’s Moon
\nreflects 11 percent).
\n The atmospheric pressure at Triton’s surface is about
\n14 microbars, a mere 1\/70,000th the surface pressure on Earth.
\nTemperature at the surface is about 38 kelvins (-391 degrees F),
\nthe coldest surface of any body yet visited in the solar system.
\nAt 800 kilometers (500 miles) above the surface, the temperature
\nis 95 kelvins (-290 degrees F).
\n Despite remarkable differences between Triton and the
\nother icy satellites in the solar system, photographs reveal
\nterrain that is reminiscent of Ariel (a satellite of Uranus),
\nEnceladus (a satellite of Saturn), and Europa, Ganymede and Io
\n(satellites of Jupiter). Even a few reminders of Mars, such as
\npolar caps and wind streaks, can be seen on Triton’s surface.
\n Triton appears to have the same general size, density,
\ntemperature and chemical composition as Pluto (the only outer
\nplanet not yet visited by any spacecraft), and will probably beour best model of Pluto for a long time to come.<\/p>\n
SMALL SATELLITES
\n In addition to the previously known satellites Triton
\nand Nereid, Voyager 2 found six more satellites orbiting Neptune,
\nfor a total of eight known satellites. The new objects have not
\nyet been named, a task for the International Astronomical Union
\n(IAU), but were given temporary designations that tell the year
\nof discovery, the planet they are associated with and the order
\nof discovery; for example, 1989N1 was the first satellite of
\nNeptune found that year. The final new body was designated
\n1989N6.
\n Nereid was discovered in 1948 through Earth-based
\ntelescopes. Little is known about Nereid, which is slightly
\nsmaller than 1989N1. Voyager’s best photos of Nereid were taken
\nfrom about 4.7 million kilometers (2.9 million miles), and show
\nthat its surface reflects about 14 percent of the sunlight that
\nstrikes it, making it somewhat more reflective than Earth’s Moon,
\nand more than twice as reflective as 1989N1. Nereid’s orbit is
\nthe most eccentric in the solar system, ranging from about
\n1,353,600 km (841,100 miles) to 9,623,700 km (5,980,200 mi).
\n * 1989N1, like all six of Neptune’s newly discovered
\nsmall satellites, is one of the darkest objects in the solar
\nsystem — “as dark as soot” is not too strong a description.
\nLike Saturn’s satellite, Phoebe, it reflects only 6 percent of
\nthe sunlight that strikes it. 1989N1 is about 400 kilometers
\n(250 miles) in diameter, larger than Nereid. It wasn’t<\/p>\n
discovered from Earth because it is so close to Neptune that it
\nis lost in the glare of reflected sunlight. It circles Neptune
\nat a distance of about 92,800 kilometers (57,700 miles) above the
\ncloud tops, and completes one orbit in 26 hours, 54 minutes.
\nScientists say it is about as large as a satellite can be without
\nbeing pulled into a spherical shape by its own gravity.
\n * 1989N2 is only about 48,800 kilometers (30,300 miles)
\nfrom Neptune, and circles the planet in 13 hours, 18 minutes.
\nIts diameter is about 190 kilometers (120 miles).
\n * 1989N3, only 27,700 kilometers (17,200 miles) from
\nNeptune’s clouds, orbits every 8 hours. Its diameter is about
\n150 kilometers (90 miles).
\n * 1989N4 lies 37,200 kilometers (23,100 miles) from
\nNeptune. 1989N4, diameter 180 kilometers (110 miles), completes
\nan orbit in 10 hours, 18 minutes.
\n * 1989N5 appears to be about 80 kilometers (50 miles)
\nin diameter. It orbits Neptune in 7 hours, 30 minutes about
\n25,200 kilometers (15,700 miles) above the cloud tops.
\n * 1989N6, the last satellite discovered, is about 54
\nkilometers (33 miles) in diameter and orbits Neptune about 23,200
\nkilometers (14,400 miles) above the clouds in 7 hours, 6 minutes.
\n 1989N1 and its tiny companions are cratered and
\nirregularly shaped — they are not round — and show no signs of
\nany geologic modifications. All circle the plant in the same
\ndirection as Neptune rotates, and remain close to Neptune’s
\nequatorial plane.<\/p>\n
THE RINGS AND “RING ARCS”
\n As Voyager 2 approached Neptune, scientists had been
\nworking on theories of how partial rings, or “ring arcs,” could
\nexist. Most settled for the concept of shepherding satellites
\nthat “herd” ring particles between them, keeping the particles
\nfrom either escaping to space or falling into the planet’s
\natmosphere. This theory had explained some new phenomena
\nobserved in the rings of Jupiter, Saturn and Uranus.
\n When Voyager 2 was close enough, its cameras
\nphotographed three bright patches that looked like ring arcs.
\nBut closer approach, higher resolution and more computer
\nenhancement of the images showed that the rings do, in fact, go
\nall the way around the planet.
\n The rings are so diffuse, and the material in them so
\nfine, that Earthbound astronomers simply hadn’t been able to
\ndetect the full rings. (Based on Voyager’s findings, one Earth-
\nbased observation of the ring arcs is now attributed to the
\npassage of a small satellite through the ring area.)
\n Late in the encounter, the scientists were able to sort
\nout the number of rings and a preliminary nomenclature:
\n * The “Main Ring” (officially known as 1989N1R,
\nfollowing the IAU convention) orbits Neptune about 38,100
\nkilometers (23,700 miles) above the cloud tops. The main ring
\ncontains three separate regions where the material is brighter
\nand denser, and explains most of the sightings or “ring arcs.”
\nSeveral Voyager photographs show what appear to be clumps <\/p>\n
embedded in the rings. Scientists are not sure what causes the
\nmaterial to clump.
\n * The “Inner Ring” (1989N2R) — about 28,400 kilometers
\n(17,700 miles) above the cloud tops.
\n * An “Inside Diffuse Ring” (1989N3R) — a complete ring
\nlocated about 17,100 kilometers (10,600 miles) from Neptune’s
\ncloud tops. Some scientists suspect that this ring may extend
\nall the way down to Neptune’s cloud tops.
\n * An area called “the Plateau,” a broad, diffuse sheet
\nof fine material just outside the so-called “Inner Ring.”
\n The material varies considerably in size from ring to
\nring. The largest proportion of fine material — approximately
\nthe size of smoke particles, is in the Plateau. All other rings
\ncontain a greater proportion of larger material.
\n Both Voyagers have now completed all of the planetary
\nencounters on their itinerary, but both still have work to do.
\nVoyager 1 is heading out of the solar system, climbing above the
\necliptic plane in which the planets orbit the Sun. Voyager 2 is
\nalso outbound, traveling below that plane. Both are searching
\nfor the heliopause, a boundary that marks the end of the solar
\nwind and the beginning of interstellar space. Assuming both
\nspacecraft remain healthy, flight controllers expect to be able
\nto operate the spacecraft for another 25 to 30 years,
\ninvestigating magnetic fields and particles in interplanetary and
\ninterstellar space, and observing ultraviolet sources among the
\nstars.
\n The Voyager Project is managed by the Jet Propulsion
\nLaboratory for NASA’s Office of Space Science and Applications.
\n#####
\n12-20-89 DB\/AMS<\/p>\n
