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Board of Directors . FROM THE CARL SAGAN BRUCE MURRAY EDITOR President Vice President Director, Laboratory for Planetary Professor of Planetary Studies, Cornel! University Science, California Institute of Technology LOUIS FRIEDMAN Executive Director JOSEPH RYAN O'Melveny & Myers MICHAEL COLLINS Apollo 11 astronaut STEVEN SPIELBERG director and producer THOMAS O. PAINE . former Administrator. NASA; HENRY J. TANNER Chairman, National financial consultant Commission on Space

Board of Advisors

DIANE ACKERMAN JOHN M. LOGSDON poet Bnd author g:;~O:'~f:~~;g~cr;~7:~~~~' t had been like reading a wonderful ager 2 in August 1981. There they dis­ JSAAC ASIMOV author HANS MARK I adventure book, one you never want to covered that the famous rings are actual­ Chancellor, RICHARD BERENDZEN University of Texas System end. At the close of the last chapter, you ly made of thousands of thin, tenuous educator and astrophysicist JAMES MICHENER feel a gentle melancholy because you can ringlets. The images they returned to JACQUES BLAMONT author Chief Scientist. Centre never relive your first experience of Earth also revealed the "spokes" of National d'Etudes Spatia/es. MARVIN MINSKY France Toshiba Professor of Media Arts meeting these characters and sharing charged particles and the "kinks" that and Sciences, Massachusetts RAY BRADBURY Institute of Technology poet and author their story. complicated our understanding of plane­ ARTHUR C. CLARKE PHILIP MORRISON That was how I felt in 1989 at the end tary rings. The Voyagers also investigated author Institute Professor, Massachusetts Itystitute of Technology of Voyager's last encounter. As a member the dense nitrogen atmosphere of Sat­ CORNELIS DE JAGER PAUL NEWMAN Professor of Space Research. of the press, I had participated in the mis­ urn's largest moon, Titan, and discovered The Astronomical Institute at actor Utrecht, the Netherlands JUN NISHIMURA sion only vicariously. Still, for 12 years I that it is a veritable factory for organic FRANK DRAKE Director-General, Institute of Space felt as if I had flown with Voyager, fIrst to compounds. Professor of Astronomy and and Astronautical Sciences, Japan Astrophysics, University of Jupiter and Saturn, then on to Uranus and After Saturn, the Voyagers' paths di­ California at Santa Cruz BERNARD M. OLIVER Chief, SETf Program, LEE A. DUBRIDGE NASNAmes Research Center Neptune. verged: Voyager 1 swung northward, former presidential science advisor SALLY RIDE Through Voyager's robotic eyes, I had heading out of the ecliptic plane in which Director, California Space Institute, JOHN GARDNER University of California at peered into the heart of Jupiter's Great the planets orbit; Voyager 2 entered founder, Common Cause San Diego, and former astronaut Red Spot, I had watched the "spokes" undiscovered territory and became the MARC GARNEAU ROALD Z. SAGDEEV Canadian astronaut former Director, dance around Saturn's elegant rings and I fIrst emissary from Earth to visit Uranus Institute for Space Research, GEORGIY GOLITSYN Academy of Sciences of the USSR Institute of Atmospheric Physics, had marveled at the bizarre terrain of and Neptune. Academy of Sciences of the USSR HARRISON H. SCHMIIT Uranus' moon Miranda. Finally, I had As Voyager 2 flew by those distant THEODORE M. HESBURGH former US Senator, New Mexico, and Apollo 17 astronaut President Emeritus, gazed back at Neptune and its moon Tri­ worlds, everything it saw or sensed was a University of Notre Dame S. ROSS TAYLOR ton knowing this was the last time Voy­ fIrst. In 1986 it reached Uranus, the fIrst SH IRLEY M. HUFSTEDLER Professorial Fellow, Australian educator and jurist Narional University, Canberra ager and I would visit a planet together. planet discovered since antiquity. With a GARRY E. HUNT LEWIS THOMAS space scientist. Chancellor, Memorial Sloan I had shared these moments not only bland face and a bizarre magnetic fIeld, United Kingdom Kettering Cancer Center with members of the Voyager project coal-black rings and battered little moons, SERGEI KAPITSA JAMES VAN ALLEN Institute for Physical Problems, Professor of Physics, team, but with people around the world. Uranus surprised, baffled· and intrigued Academy of Sciences of the USSR University of Iowa The two Voyager spacecraft-with a mut­ the team. And it whetted their appetites The Planetary Report (lSSN 0736-3680) is published six times yearly al ed transmitter, an arthritic scan platform for the last encounter, with the Neptune the editorial offices of The Planetary Society, 65 North Catalina Avenue, Pasadena, CA 91106, (818) 793-5100. It is available to members of The and assorted other ills-had captured the system in August 1989. Planetary Society. Annual dues in the US or Canada are $25 US dollars or $30 Canadian. Dues outside the US or Canada are $35 (US). hearts of the public as perhaps no other As the spacecraft approached its final Editor, CHARLENE M . ANDERSON spacefaring robots ever had. They were target, anticipation of both new fIndings Technical Editor, JAMES D. BURKE Assistant Editor, DONNA ESCANDON STEVENS explorers, and they were survivors. and the mission's completion jumbled the Copy Editor, GLORIA JOYCE Production Editor, MITCHELL BIRD Launched in 1977, Voyagers 1 and 2 feelings of team members and project Art Director, BARBARA S. SMITH had followed the aptly named Pioneer 10 watchers alike. Earth-based observations Viewpoints expressed in columns or editorials are those of t.he authors: and ~~ ad~i~~~:si>a~t~eg~ef~~t ~~~~~~ ~~~:t:'lanetary Society , its officers to Jupiter. In March 1979, Voyager 1 seemed to suggest that the neptunian sys­ I,n Canada. Second Class Mail Registration Number 9567 reached this monarch among planets and tem might encompass phenomena even made the first of the epic string of discov­ stranger than those seen at Jupiter, Saturn COVER: From the edge of our planetary system, the spacecraft Voyager 2 transmitted back to Earth images eries that came to distinguish the mission. and Uranus. of Neptune, its large moon Triton, its strange ring sys­ The spacecraft showed us that, like Sat­ Was Neptune circled by discontinuous tem and its retinue of small satellites. In this montage urn, Jupiter was circled by a ring. Perhaps arcs where complete rings should be? constructed from high-resolution images, the south pole of Triton dominates, with cloud-streaked Neptune in the most memorably, Voyager 1 caught a vol­ Were there lakes of liquid nitrogen lying background. The flyby of Neptune in 1989 -was the last cano in the act of erupting on the moon 10. on Triton's surface? Would Neptune's Voyager planetary encounter. These two doughty space­ craft opened up the outer solar system for scientific Then the spacecraft followed the path magnetosphere conform to the norm, or exploration, and we shall not see their like again. blazed by Pioneer 11 to Saturn, Voyager would it behave as strangely as that of Montage: Alfred McEwen, US Geological Survey, Flagstaff 1 arriving in November 1980 and Voy- (continued on page 24) N EVVS BRIEFS

NASA scientists now say that an ozone hole may be widening over As administrators of a membership organization, The Planetary Society's Directors populated regions of North Ameri­ and staff care about and are influenced by our members' opinions, suggestions and ca, Europe and Asia. NASA's Up­ ideas about the future of the space program and of our Society. We encourage per Atmospheric Research Satellite members to write us and create a dinlogue on topics such as a space station, a lunar has reported high levels of chlorine outpost, the exploration of Mars and the search for extraterrestrinllife. monoxide in the upper atmosphere Send your letters to: Members' Dinlogue, The Planetary Society, 65 N. Catalina over London, Moscow and Amster­ Avenue, Pasadena,CA 91106. dam. Chlorine monoxide is a free ...... •...... • radical that separates itself from chlorofluorocarbons and destroys Louis D. Friedman's statements regarding space station Freedom (SSF) and science in the ozone layer. World Watch and "An Executive Report to Members" in the November/December 1991 "We are no longer dealing with issue of The Planetary Report are not true and are divisive within the space community. The remote, high-latitude areas," says statements I refer to are, "In an extraordinary display of unity, many government advisory NASA's Michael Kurylo. "The data groups and national scientific organizations have stressed that Freedom will do very little show we are dealing with a prob­ either for science or for exploration . .." and "The Officers of The Planetary Society argued lem that extends to populated re­ against building space station Freedom as it is now designed, yet Congress funded it-to gions of the Northern Hemisphere." the detriment of space science and exploration." -from Steve Scauzillo in the It is not true that there was an extraordinary display of unity among government advisory Pasadena Star-News groups and national scientific organizations. On the contrary, the Aerospace Medical Advi­ sory Committee, the Space Science and Applications Advisory Committee (SSAAC), the Life Sciences Advisory SujJCommittee of SSAAC, the Committee on Space Biology and Medicine (Space Studies Board of the National Research Council), the Augustine report, Magellan successfully began its the Stafford report, the American Physiological Society, the Aerospace Medical Associa­ third radar mapping cycle on Jan­ tion, the American Society for Space and Gravitational Biology, the American Academy of uary 24, despite earlier transmitter Otolaryngology-Head and Neck Surgery, and the American Institute of Biological Sciences problems. The spacecraft's primary were all on record that SSF is essential for science and exploration. The science users of transmitter stopped functioning on ' SSF will be materials scientists and life scientists, and human exploration cannot proceed January 4, and engineers had to turn without a space station to solve certain life science problems encountered during long­ to the ailing backup transmitter. duration spaceflight, including physiological, psychological and life support problems. A new operational mode was These statements reflect the author's narrow view of science. Life science is also a sci­ devised for the backup transmitter. ence and, like materials science, will benefit greatly from SSP. To leave the life sciences This will allow it to continue map­ out of science is unwise, if only because it invites the wrath of a science community that ping the planet, but at less than half in terms of numbers is larger than all of the other space science communities combined. the previous rate. Freedom will allow the generation of data that are useful both to the space effort (they -from the Jet Propulsion will enable human exploration) and to medical problems (such as osteoporosis) and other Laboratory problems on Earth. Space scientists should work together rather than oppose one another. Both the life sciences and the physical sciences are important. We need one another. The upcoming repair of the Hubble Space Telescope is an obvious example. Last year Congress slashed funds -FRANCIS J. HADDY, Bethesda, Maryland for many of NASA's planned sci­ ence projects, offering instead $137 We are in complete agreement with Dr. Haddy that life science research in Earth orbit is million for new programs that the important and should be supported. Such research is central to any human exploration of space agency hadn't realized it the solar system. However, space station Freedom won 't be able to support this research needed. One new project was an for many years, and we (and most life scientists) believe that there are much better ways to "advanced liquid dispensing tech­ advance life sciences and human planetary flight than the present SSF plan. nology evaluation" promoted by Several of the groups Dr. Haddy cites are NASA-funded organizations, whereas I was lobbyists for the soft drink industry. referring to the more independent professional science organizations, which banded to­ By the time Congress was through, gether to issue a statement just prior to the final Senate vote. I would also argue that the NASA and space shuttle astronauts Augustine and Stafford reports damned space station Freedom by faint praise. were amazed to learn that austerity The cancellation of the Comet Rendezvous Asteroid Flyby and curtailment of Magellan had doomed the space infrared tele­ (see page 26) are this year's space science casualties of last year's battle over the space scope, the orbiting solar laboratory, station. ~Louis D. Friedman, Executive Director the flight telerobotic server and seven other research projects-but The Planetary Report's NovemberlDecember 1991 articles regarding asteroids, comets not a plan to design a dispensing and meteorites presented the facts that the people of Earth truly have a potentially serious machine that would allow orbiting issue to debate and that some action will eventually be required. It's sad to think that per­ astronauts to choose between Coke haps in order to initiate such global action, it may require the presence of an asteroid with and Pepsi. Earth as its imminent target. -from The Bulletin of the -BILL MERKES, Freeland, Washington Atomic Scientists 3 he year 1989 was a banner one The Pursuit Begins around Neptune. The event was not for scientists enthralled with With the discovery of the uranian mirrored, however, on the other side planetary rings, those decorative rings in 1977 and the stunning but of the planet, so the occulting object T was obviously not a continuous ring. appendages encircling all the gaseous unsettling realization that Saturn was planets in the outer solar system. In not the only ringed planet, planetary Instead, the occultation was attribut­ late August, nearing the end of its astronomers began to search for ed to a previously unknown neptuni­ epic 12-year journey across the solar rings around the other outer planets. an satellite, tentatively called 1981Nl. system, Voyager 2 paid a visit to the Voyager 1 uncovered Jupiter's ring distant blue planet Neptune. in 1979, leaving Neptune the sole Nothing else in the vicinity of Nep­ Very little was known about Nep­ remaining ringless giant planet. tune was observed that year or the tune or its rings before Voyager ar­ The first big chance for astrono­ next. Nevertheless, efforts continued. rived there- so little, in fact, that mers to check out Neptune came in Another stellar occultation by Nep­ some scientists vociferously doubted 1981. In its orbit around the Sun, tune was observed in the summer of the existence of neptunian rings right Neptune passed nearly in front of 1983 at observatories in Australia, up to the bitter end. Not that anyone three stars that year. In one instance, Tasmania and Hawaii-with negative could blame them. Neptune's rings observed from the Catalina Moun­ results. Finally, circum-neptunian ma­ had been elusive, and the story of tains near Tucson, Arizona, the star's terialonly 15 to 20 kilometers (9 to 12 their pursuit illustrates both the joys light was dimmed by an opaque ob­ miles) wide occulted three stars in the and the pitfalls of scientific investi­ ject some 180 kilometers (110 miles) summers of 1984 and 1985---each 4 gation. in diameter, presumably in orbit time, on only one side of the planet. Figure 1 - This is the view of Neptune and its rings as we thought they would look - before Voyager 2 encoun­ tered the planet. Perplexing observations from Earth had led scientists to postulate that Neptune was orbited by dozens of ring arcs, rather than complete rings like those encircling Jupiter, Sat­ urn and Uranus. The space­ craft proved this view wrong: Neptune possesses complete but tenuous rings, with only one punctuated with dense sections that appeared to observers on Earth and to Voyager 2 as the ring arcs. Computer graphic: JPUNASA

wondered how narrow arcs could be prevented from spreading. After all, it would take only about three years for a clump of ring material 15 kilometers (about 9 miles) wide to form a 360- degree ring. This happens because material on the inside of the clump orbits faster than, and eventually catches up with, material on the out­ side, completing a circuit around the planet. Maybe the arcs weren't very old. But in the belief that they were, several mechanisms to confine them were proposed. One of these mechanisms suggest­ ed that arcs could be confined in both length and width by the combined action of perturbations arising from a set of gravitational resonances pro­ duced by a single, nearby satellite. One kind of satellite resonance would keep material confined in length, while the other would help keep it The Birth of the Ring Arc Theory confirmed at more than one telescope. confined in width. This theory re­ So was born the idea that Neptune did From this collection of successes quired that the satellite be about 200 not have rings as we had come to and failures emerged the widely ac­ kilometers (120 miles) in diameter know them, but instead had very nar­ cepted view that the vicinity of Nep­ and that its orbit be inclined to that of row, tenuous and incomplete rings, tune was populated by perhaps dozens the arcs. Voyager would surely be ca­ called ring arcs. These features were of ring arcs, ranging from 100 to pable of detecting such satellites if nothing at all like the massive rings 1,000 kilometers (about 60 to 600 they were there. of Saturn or the narrow but substan- miles) in length, distributed in the .. tial and complete rings of Uranus. equatorial plane of the planet as Voyager at Work This idea became so pervasive that ~hown in Figure 1. By analogy with Planning for Voyager's encounter tlie original 1981 satellite detection the uranian rings, the material com­ with Neptune began shortly after the was, in some circles, discounted as prising the Neptune ring arcs was spacecraft left the Uranus system in being improbable and was attributed thought to be exceedingly dark, with a 1986. Narrow ring widths, inherently to an optically thick, wider-than­ reflectivity no brighter than that of dark material and the low sunlight average ring arc. charcoal. And most, if not all, of the levels at Neptune all called for very By August 1989, more than 50 arcs would be very narrow. long camera exposure times, longer stellar occultations by Neptune had than those Voyager investigators had been observed. Yet only a handful Where Can a Ring Arc Come From? used in the Uranus encounter. New produced believable sightings of ring The possibility of a new variety of algorithms were programmed into the material, and only three of these were ring feature intrigued theorists, who spacecraft's computer to stabilize the 5 scan platform carrying the cameras. The camera-commanding software was rewritten to allow for extremely Figure 2 - Backlighted by the Sun (and with long exposures, in some cases as long the bright planet as 30 minutes. With Voyager 2 read­ blocked out), the thin ied for its encounter with Neptune, neptunian rings be­ come visible. The three the countdown began. most prominent rings Sixty-four days before its closest ap­ are named for the men proach to Neptune, Voyager found who shared the discov­ ery of Neptu{le in 1846: Proteus, 400 kilometers (about 250 John Couch Adams and miles) in diameter. Larger than the pre­ Urbain Jean Joseph viously known satellite Nereid, Proteus Leverrier independently calculated the planet's orbits Neptune at a distance of 118,000 position and Johann kilometers (about 70,000 miles). But Galle found it. A more no ring arcs were seen. diaphanous ring, N4R, appears between Lever­ By four weeks before closest ap­ rier and Adams, and proach, three more moons-Larissa, another discovery of Despina and Galatea-had all been Voyager 2, N5R, is not uncovered. (Larissa turned out to be quite visible here be­ tween N4R and Adams. 1981Nl, discovered from Earth.) Still, Image: JPUNASA no ring arcs were found. Then, on August 11, 1989, when Voyager was only two weeks and 20 million kilometers (about 12 million miles) away, and closing in at a swift 17 kilometers ( 11 miles) per second, it detected the very arcs that had been so difficult to snare from Earth. They looked, of course, nothing like what tation by one of the arcs, confirming by the Sun (Figure 2). The arcs are ac­ we had imagined- not small bits of results obtained in similar ground­ tually part of the diffuse and narrow ring scattered here and there, but sev­ based experiments. Adams ring, 10 times less substantial eral arcs, like beads strung out on a than the arcs themselves, orbiting at a faint and narrow string. Sifting Through the Data distance of 62,932 kilometers (about The following two and a half The two and a half years that have 39,000 miles) from Neptune. Another weeks saw the unveiling of the entire passed since Voyager's encounter with ring, Leverrier, sits 53,200 kilometers neptunian ring and inner satellite sys- Neptune have brought new insights (about 33,000 miles) from Neptune and appears equally nar­ row and diaphanous. Each narrow ring is attended by a satellite-Adams by Galatea and Leverrier by Despina-orbiting within 1,000 kilometers (about in the Rings 600 miles) of it. Galatea, 1(j0 kilometers (about 100 miles) in di­ hen one child pushes another in a playground swing, steadily increasing the height of ameter, is itself co-orbit­ Wthe ride, we observe resonance. The pushes are timed so as to add energy. The word ing with what appears to resonance itself refers to sound (singing in the shower or the squeal of loudspeakers in a be very tenuous and per­ maladjusted public-address system), but the concept is applied to any periodic phenome­ haps discontinuous ring non that can be driven by correctly timed pulses. In the accompanying article the pulses material, which is so faint are provided by the mutual gravity of ring particles and small satellites orbiting near each it is barely visible in Voy­ other. The resonances caused by the satellites could perturb the ring particles' orbits and ager images. Extending confine these tiny bodies in the clumps seen from Earth as ring arcs. -James D. Burke out from Leverrier is a plateau of material, about 4,000 kilometers (2,500 miles) wide, bounded by a narrow feature that may tem. During that time, the Voyager and discoveries, gleaned from the be a separate ring. (Both of these fea­ cameras acquired about 800 images analysis and interpretation of these tures have yet to receive official of the rings and discovered two more precious bits of information. What names. They are provisionally called satellites, Thalassa and Naiad. The have we learned? 1989N4R and 1989N5R.) Finally, Voyager photopolarimeter and ultra­ Neptune is surrounded by several there is the Galle ring, 1,700 kilome­ violet spectrometer experiments both continuous and very dusty rings, ters (about 1,100 miles) wide. At 6 successfully observed a stellar occul- which show up best when backlighted 42,000 kilometers (26,000 miles) or- field of view. But officially named- Liberte, Egalite this light can be and Fraternite-there are actually overwhelming in five distinct arcs, all contained within the minutes-long a 40-degree region (Figure 4). Four exposures required of the arcs are less than 4,400 kilo­ to see Neptune's meters (about 2,700 miles) long; the rings. For this rea­ fifth and trailing one is 10,500 kilo­ son, it is difficult to meters (about 6,500 miles) long. say with certainty All of Neptune's dusty rings bright­ whether any en up when backlighted by the Sun, extended ring ma­ in the same way that the dust on a car terial exists very windshield becomes very much more close to Neptune. apparent when one drives toward the Consequently, the Sun. But the arcs brighten up most of possibility of a all, indicating a dust content larger continuous, diffuse than in the other rings. sheet of dust ex­ Estimates of the production rate of tending from inside dust by micrometeoroids striking the Leverrier all the rings of Neptune are a hundred, per­ way down to the haps a thousand, times too small to planet cannot be explain the abundance of arc dust ob­ discounted. served. The only viable alternative is that dust is created when larger-than­ Jewels average particles in the arcs, moving in the Crown at relative velocities of several meters The arcs are unde­ per second, collide with one another. niably the precious These velocities would be about a jewels in the deli­ thousand times greater than the colli­ bital radius, it is the innermost of cate rings that crown the globe of sional velocities between the particles Neptune's rings. The Galle ring, Neptune, and most of the attention in Saturn's rings. Galatea's companion ring, 1989N4R devoted to the rings during Voyager's and 1989N5R are about 100 times encounter was lavished on them. Closing In on an Explanation less optically thick than Adams or Voyager' s highest-resolution ring im­ Certainly the most intriguing aspect of Leverrier. ages were all targeted to the arcs af­ the arcs is the mechanism by which Light from the planet falling onto ter previous images had been used to they are maintained, and many lines the camera aperture, scattering within determine their orbits. One arc was of evidence suggest that Galatea may the optics and ultimately falling onto even captured as Voyager plunged be single-handedly responsible for the the detector is normally too faint to through the ring plane (Figure 3). Al­ arcs' appearance. The satellite is large see unless the planet is in the camera's though only three features have been enough and close enough to the arcs

Figure 6 - Are there large objects lurking within the neptunian ring arcs? Images such as this give a tantalizing hint of their possible existence. The upper hatched region in the inset shows the position of a group of four to six clumps; the lower hatched region shows two clumps. The "r" indicates a reference mark in the camera; the "b" indicates a blemish. 7 Image: JPLlNASA. Diagram: Carolyn Porco, University of Arizona Figure 3 (left) - As Voyager plunged through the ring plane, its wide-angle camera captured the entire ring system in a compressed view. One of the arcs in the Adams ring is clearly visible.

Figure 4 (below) - With image processing, the three named arcs in Neptune's rings were revealed to be five arcs, seen here on the right as the red and yel/ow segments in Adams, the outside ring. The first four arcs are less than 4,400 kilometers long; the fifth and trailing arc is 10,500 kilometers long. Images: Carolyn Porco, University of Arizona I I I \ I Figure 5 - The interactions \ I between the small moon \ I Galatea and the ring parti­ \ I / cles not only maintain the \ / arcs' pOSitions but produce '\ , an interesting scalloped "- GALATEA ~ wave pattern in Neptune's "- /' rings. If you were to track " /' the arcs as they orbited the ------planet, it would appear as if a wave were traveling , - , through the arcs at '...... ,I .... _, Galatea's orbital speed. \_/ r , ... Illustration: Ron Miller '- " ~, -:,'----r--':-", "",­ \_~ \.. / \ ~ ,

to provide the two types of gravita­ Something Different Earth, the rings of Saturn resided in an tional resonances required for single­ In all, Voyager 2 discovered six new icy body not unlike the saturnian satellite arc confinement. Its orbit is satellites around Neptune. Four of satellite Mimas (Figure 7). Even more inclined to the arcs' orbit by a small them-Galatea, Despina, Thalassa compelling a thought is this: A time but adequate 0.02 degree. and Naiad-fall well within the ring may come in the distant future when Moreover, all the arcs, with one ex­ region and are substantial bodies, the inner satellites of Neptune, pulver­ ception, are less than 4,400 kilometers ranging from 50 to 180 kilometers ized by cometary collisions or tom long, the longitudinal distance over (about 30 to 110 which Galatea's confining action miles) in diame- would be expected to operate. Finally, . ter. This ar­ the ring particles within the arcs are rangement is on distinctly noncircular orbits, ar­ dramatically dif­ Figure 7 - The ranged around Neptune in such a way saturnian satellite ferent from Mimas exhibits a that every 8.57 degrees or 9,415 kilo­ those observed very large crater, meters (5,850 miles) along the arcs in the other out­ Herschel, created they find each other at the same or­ in an impact that er planetary sys­ came very close bital phase. That is, if one were to tems. There, on­ to smashing this study the arcs as they orbit Neptune, ly the very moon to bits. Oth­ it would appear as if a scalloped wave er, less fortunate smallest satel­ satellites might pattern with a 9,415-kilometer wave­ lites, about 20 have suffered the length were traveling through the arcs kilometers (12 ultimate blow and at Galatea's orbital speed (Figure 5), been transformed miles) in diame­ into the rings of causing the arcs to bow in and out by ter, orbit at the Saturn. Someday 30 kilometers (about 19 miles)-a outer limits of the same fate may convincing manifestation of Galatea's each planet's befall a neptunian satellite, creating gravitational perturbations. rings. If we rings to rival Interparticle collisions occurring at compare the to­ those of Saturn. several meters per second within the tal mass within Image: JPUNASA arcs are a natural consequence of Saturn's rings Galatea's perturbations, and they pro­ with that con­ vide a cogent explanation for the large tained within the arc dust content. But collisions are al­ confines of Nep­ so the bane of the arcs' existence, as tune's rings, we they can eventually disrupt arc con­ would find that finement. Maybe big bodies, several Saturn wins, but kilometers in size, within the arcs can only by about a factor of 5. Yet almost asunder by neptunian tides, spread help reduce the destabilizing effects of all the saturnian mass is contained in their remnants far and wide and form collisions while providing the source rings, whereas almost all the neptuni­ a glorious array of rings to rival those for arc dust. an mass is contained in satellites. of Saturn. Do big bodies exist within the Current wisdom says that rings arcs? Perhaps. Several Voyager im­ were not born alongside their parent Carolyn Porco is an associate profes­ ages revealed brighter-than-average planets, but instead are the result of sor in the Department of Planetary Sci­ knots or clumps within the arcs, sepa­ the catastrophic breakup of a plane­ enceslLunar and Planetary Laboratory rated by about 550 to 1,100 kilome­ tary satellite. They are also believed to of the University ofArizona in Tucson. ters, or 340 to 680 miles (Figure 6, be temporary fixtures, lasting perhaps She was a member of the Voyager page 7). Though it is unclear if these no longer than 100 million years. If imaging team, and is now the leader features are the manifestations of big this is true, then it is possible that long of the imaging teamfor the Cassini bodies, it is a provocative thought. ago, when dinosaurs still roamed mission back toSatum and its rings. 9 The Magnetosphere of

by S.M. Krimigis

efore Voyager 2's encounter with familiar territory-the magnetic field relatively empty of matter. Explorer 1, Neptune in 1989, most scientists and magnetosphere of Earth. the first American satellite, discovered Bexpected that Neptune would have Earth's magnetic field is roughly that this space is actually full of elec­ a magnetic field and an associated mag­ aligned with the planet's axis ofrota­ trons and ions that have been boosted netosphere. All the giant planets visited tion, and it rotates with Earth once to tremendously high energies. so far had displayed fascinating electro­ every 24 hours. The atmosphere as we These particles are generally concen­ magnetic features-for example, know it extends only to a few tens of trated into two intense radiation belts, Jupiter's magnetosphere is the largest kilometers altitude. By about 100 kilo­ the VanAllen belts (named after their object within our solar system, and meters (60 miles), much of it has be­ discoverer, James A. Van Allen, an Uranus' magnetosphere is one of the come ionized by solar radiation-that advisor to The Planetary Society). strangest yet encountered (see the is, the neutral atmospheric atoms have Trapped in stable orbits within Earth's NovemberlDecember 1986 Planetary become positively or negatively magnetic field, the particles have such Report). Voyager's discovery of a mag­ charged by losing or gaining one or high energies that they are able to pen­ netosphere was therefore no surprise. more electrons. etrate an inch of steel, thus posing radi­ However, its nature has raised many Before the late 1950s and the onset ation hazards to the health of astronauts questions. To understand Voyager' s of the space age, the region beyond who might be orbiting for extended pe­ findings, we first need to look at more about 100 kilometers was thought to be riods. Astronauts on the space shuttle Earth's Magnetosphere

Figure 1 - Earth's magnetosphere is formed by the interac­ tion of the solar wind with the planet's intrin­ sic magnetic field. The solar wind compress­ es the magnetosphere on the dayside while stretching it out on the opposite side of the planet. Various plasma populations, as well as concentrations of charged particles such as the Van Allen radia­ tion belts, are pro- . duced by processes not yet fully under­ stood by scientists. Illustration:, Michael Carroll

10 Figure 2 - Here we compare the relation­ ships between the magnetic axes and rotation axes of the planets having sub­ stantial magneto­ spheres. The angle between the two axes is greatest for Uranus. Scientists once thought that this tilt was due to the orientation of Uranus' rotation ax­ is, which makes the planet appear to be spinning on its side. The large tilt of Nep­ tune's magnetic axis, however, suggests there is probablyan­ other explanation. Illustration: S.A. Smith

and cosmonauts on Mir orbit Earth searching for its boundary, which they Such "magnetic storms" disrupt within the region of safe flight below are not expected to reach until the next electrical power distribution systems­ 400 kilometers (150 miles). The men century. in 1989, for example, the entire prov­ who flew to the Moon passed through ince of Quebec was without power for this region quickly, so their exposure Oscillating Electrons almost nine hours. The amount of ener­ to the radiation was limited. The aurora borealis, or northern lights, gy deposited over the auroral zone by In addition to the Van Allen belts, seen almost daily by people who live at the particles trapped in the magneto­ Earth's magnetosphere (so called be­ high northern latitudes, such as northern sphere can at times exceed 1,000 bil­ cause it is dominated by Earth's mag­ Canada or Alaska, is a vivid manifesta­ lion watts, which is comparable to all netic field) is filled with electrons and tion of the mag- ions of lower energies. These extend to netosphere-as an altitude of 80,000 kilometers (about is the aurora aus­ 50,000 miles) on the side of Earth fac­ tralis, or southern ing the Sun (the dayside) and at least lights, of south­ Naming the Phenomenon 6 mil\ion kilometers (about 3.7 million ern latitudes. miles) on the side of Earth away from These trapped s is often the case in science, a phenomenon gets a name the Sun (the nightside). particles cause Abefore it is fully understood, and sometimes that name As Figure 1 shows, the envelope of the aurora effect hangs on. The magnetosphere of a planet is certainly not this magnetosphere is asymmetric, be­ as they oscillate spherical, as shown in the accompanying illustrations. But the cause of the solar wind, a flow of plas­ and plunge deep term is still useful, conveying as it does the idea of a huge re­ ma that is continuously emitted by the into the atmo­ gion beyond the troposphere, stratosphere and ionosphere that Sun at about a million miles per hour. sphere, where closely enfold our spherical Earth-a region where electro­ (A plasma is a gas consisting of equal they excite neu­ magnetic effects dominate the motions of the thin but highly numbers of positively and negatively tral atoms into energetic plasma that surrounds us. -James D. Burke charged particles.) This plasma wind radiating energy, "blows back" Earth's magnetic field in­ much as the elec- to a long "tail" while compressing the trons hitting a field on the dayside. This shapes the television screen cause colors to appear the electricity produced in the world. magnetosphere into a huge magnetic and images to form. In addition, when­ Another aspect of magnetospheric cavity filled with large numbers of ever large sunspots appear on the sur­ activity, not readily detected from the trapped particles. (If we could see the face of the Sun and give rise to huge ground, is the production of radio tail, it would be as impressive as any of explosions called flares, the aurora waves. These waves are generated by the cometary tails we see.) The solar fluctuates wildly and intensifies, as it electrons oscillating in Earth's magnet­ wind blows out as far as Neptune and did in 1989, 1990 and 1991. During ic field, much as electrons oscillating beyond. The Pioneer 10 and 11 and those years, the aurora borealis was in a radio station's antenna generate the Voyager 1 and 2 spacecraft are now seen as far south as Key West, Florida. waves we pick up on our radio sets at 11

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Neptune's Magnetosphere

Figure 3 - Neptune's magnetosphere might have appeared some­ thing like this at the time Voyager 2 entered it through the south magnetic polar region. Among its prominent features are the radia­ tion belt confined by the orbit of Triton, the compressed field on the dayside and the magnetic tail streaming out a great distance from the planet. Illustration courtesy of S.M. Krimigis

home. Theyare continuously broadcast the planet were thought to be due to the was an important measurement, for it into space with the power of about 1 peculiar orientation of its rotation axis. enabled the atmospheric scientists to billion watts; by comparison, radio sta­ At Neptune, however, Voyager 2 interpret the motions of the atmospheric tions on Earth typically radiate about found that the magnetic field was tilted features observed by Voyager's imag­ 50,000 watts. some 47 degrees to the rotation axis, ing system. even though Neptune's spin axis is not It took Voyager well over three days Surprising Measurements tilted as drastically as is Uranus'. This to traverse the magnetosphere of Nep­ Many of the phenomena we see in was not what we expected. Further­ tune. Putting together the measurements Earth's magnetosphere also exist in the more, the axis of the magnetic field was taken in the plasma, plasma wave and magnetospheres of the outer planets­ displaced by over half a planetary radius energetic particle experiments, we pro­ Jupiter, Saturn, Uranus and Neptune. from the planet's center, so that the sur­ duced the conceptual model shown in Voyager's measurements at these plan­ face strength of the neptunian magnetic Figure 3. ets, however, surprised us, both in the field varies quite a bit depending on the The principal features of the magne­ nature and orientation of the magnetic longitude. Given this complexity, an tosphere are illustrated in the figure, fields and in the extent and variety of ordinary magnetic compass would be with the red part of the figure indicating phenomena observed. Figure 2 (page useless on Neptune. The equivalent the radiation belt-equivalent to the Van 11) compares Voyager's findings with situation on Earth would have the com­ Allen belts at Earth. These belts are what we know of Earth's magnetic pass pointing to, say, New York instead generally confined inside the orbit of field. of the north pole. Triton, Neptune's largest moon, in a An important characteristic of doughnut-shaped region with Neptune planetary magnetic fields is the angle Traversing Neptune's at its center. There is also a cloud of between the rotation axis of each planet Magnetosphere hydrogen surrounding Neptune along and the magnetic axis; this angle is When Voyager's antennas picked up Triton's orbit, again in the form of a approximately 11.7 degrees at Earth, radio waves about five days before doughnut. 9.6 degrees at Jupiter, but 0 degrees closest approach to the planet, we knew The magnetosphere is of course dis­ at Saturn and nearly 60 degrees at we had detected Neptune's magneto­ torted by the pressure of the solar wind, Uranus. Note that the orientation of sphere. Later examination of the data as we have learned to expect from Uranus' rotation axis is quite different revealed that these radio "bursts" were Earth's and all the other magneto­ from that of the other planets, lying present as early as 30 days before, and spheres investigated by Voyager. very close to the ecliptic plane (the at least 22 days after, closest approach. Neptune's magnetosphere extends plane of Earth's orbit about the Sun). We used the regularity of the radio approximately 870,000 kilometers Thus, the large tilt in Uranus' magnetic emissions to deduce the planetary rota­ (about 540,000 miles) on the dayside 12 axis and its offset from the center of tion period of 16.11 ± 0.05 hours. This and at least 4.6 million kilometers (about 2.9 million miles) on the night­ moons residing within this belt, as well the orbit of Neptune, and, as of this side. as several neptunian rings. These inner writing, at least 8.3 billion kilometers Voyager found that the ionized gas moons and rings are very dark, and we (5.2 billion miles) from the Sun. trapped within Neptune's magnetic field think we know why. High-speed pro­ Thus the next challenge for both consists of two components. One part, tons striking the surface-thought to be Voyager 1 and Voyager 2 is to cross the mostly protons and nitrogen, has tem­ methane (CH4) ice-drive off much of boundary between the magnetosphere peratures ranging from about 100,000 to the hydrogen, leaving carbon behind. of the Sun (called the heliosphere), 200,000 degrees Celsius (about 200,000 My colleagues L.J. Lanzerotti and A.F. and the interstellar medium, which is to 400,000 degrees Fahrenheit); the Cheng estimate that it would take any­ the space dominated by nearby suns. other, mostly protons, has temperatures where from 10,000 to about I million A hypothetical solar magnetosphere, of over a billion degrees-the hottest years for pure methane ice to assume a encompassing all the planets, is shown plasma temperatures measured in any very dark appearance. So the moons and in Figure 4. Also shown are the Pioneer of the planetary magnetospheres en­ the rings could have been created within and Voyager spacecraft heading toward countered by Voyager. Since this plas­ the last million years, just as well as at the boundary that separates our solar ma is exceedingly dilute (a few ions per the beginning of the solar system. system from the interstellar medium. cubic foot), the heat content is minus­ Current estimates are that the Voyager cule and therefore did not heat the The Next Frontier spacecraft will not cross this frontier spacecraft to any measurable extent. Now that we have investigated all but until well after the year 2000. We have Some of this gas finds its way to the one (Pluto) of the planets in the solar to remember, however, that the Voy­ upper atmosphere of the planet, produc­ system, we can state with certainty that ager journey has been one of surprises, ing an aurora, just as is the case for magnetospheres are a ubiquitous feature and we may yet learn that our under­ Earth and the other planets with sizable of rotating planets and even stellar ob­ standing of the Sun's magnetosphere is magnetospheres. The aurora on the dark jects, as inferred from ground-based ob­ rather incomplete. Only the future will side of Neptune is relatively weak, emit­ servations of such astrophysical objects tell us the answer. ting only about 10 million watts of pow­ as pulsars and even entire galaxies. er, compared to emissions at Earth that The obvious question is whether the S.M. Krimigis is head of the Space range to at least 100 billion watts. Sun itself, being a rotating, magnetized Department at the Johns Hopkins Uni­ The presence of the very hot plasma star, possesses a magnetosphere enve­ versity Applied Physics Laboratory, has profound consequences for material lope in its own right. The answer is and Principal Investigator of the Low embedded within the radiation belt. clearly yes, since we have seen that the Energy Charged Particle experiment Voyager's cameras discovered six small magnetized solar wind extends beyond on Voyagers I and 2.

Figure 4 - The shape of the Sun's magneto­ sphere, the heliosphere, may resemble those of the planets investigated by Voyager. ,Four space­ craft, Voyagers 1 and2 and Pioneers 10 and 11, are now heading out of our solar system. S0me­ time in the next century, one of them will cross the heliopause and enter interstellar space. Illustration: JPUNASA

13 Voyoger's Fina.le

by R.H. Brown

t has been more than two years since cayed to its present circular shape. As and crust are some 350 kilometers I first saw Triton close up. Although the orbit decayed, the moon probably (about 200 miles) thick combined. II had studied that enigmatic moon suffered many catastrophic heating The surface layer of volatile ices (prin­ through a telescope for over a decade events that melted it and destroyed cipally frozen nitrogen and methane) before Voyager 2 encountered it, in my most of the surface evidence of its is probably no more than 1 kilometer research I have since thought of little origins. Its present six-day orbit is in­ (0.6 mile) thick. else. Triton is one of the most amaz­ clined by about 160 degrees relative ing, perplexing and beautiful objects to Neptune's equator and precesses Cold on the Outside, Warm Within in our solar system. Tucked away in a (wobbling like a spinning top), so that The large amount of rocky material in strangely inclined and backward orbit the pole of the orbit completes a cycle Triton's interior produces a substantial around Neptune, Triton provided a in about 690 years. amount of heat from the slow decay finale to the Voyager mission that The combination of Triton's preces­ of naturally occurring radioactive ele­ could scarcely have been imagined. sion, Neptune's 164-year orbit and the ments in the rock. Although Triton's Triton represents a new class of inclination of Neptune's equator (about internal heat source is not as large as satellite, unlike anything seen in any 29 degrees) results in a very complex that of 10, the most geologically active of the previous Voyager encounters. Its seasonal cycle. In fact, Triton's south- satellite we know of, it is quite large ern hemisphere is presently relative to the heat Triton receives from nearing solstice in one of the the Sun. This is due partly to the fact longest and warmest sum­ that Triton is about 4.5 billion kilome­ Figure 1 -In compo­ mers it has seen in the last ters (3 billion miles) from the Sun, Sition, Triton is about thousand years or so. and it reflects 80 to 90 percent of the 70 percent rock and organic materials and sunlight that falls on its surface. As a 30 percent water ice. AWorld of Rock and Ice result, at minus 235 degrees Celsius The rocky core is The Voyager measurements (minus 391 degrees Fahrenheit), Triton about 1,000 kilometers in diameter, while its of Triton's mass and radius, is the coldest object we know of in the water-ice mantle and combined with our knowl­ solar system, aside from comets be­ crust are some 350 edge of the behavior of yond the orbit of Pluto. kilometers thick. A Even though Triton is unbelievably thin veneer of volatile common solar system mate­ ices, mostly nitrogen rials at high pressures, tell us cold, its strong internal heat source and methane, is prob­ that Triton is quite dense, as makes its surface slightly warmer­ ably no more than 1 icy moons go. At a density about 0.5 to 1.5 degrees Celsius higher kilometer thick. of 2.06 grams per cubic cen­ -than it would be otherwise. Although Illustration: Ron Miller timeter, over twice that of this may not seem like much, Triton is water ice, Triton must be the only known satellite in the solar composed predominantly of system besides 10 whose intemal heat retrograde (east to west), inclined orbit rocky and organic materials. In fact, its source has a measurable effect on its suggests that it is a moon-sized body bulk composition is probably a mix of surface temperature. that got too close to Neptune. It may about 70 percent rock and organics and In fact, because the predominantly have been captured by Neptune's grav­ 30 percent water ice by mass (see Fig­ nitrogen atmosphere is in equilibrium ity after being slowed down by drag ure 1), with only minor amounts of oth­ with the extensive deposits of nitrogen from the forming planet'stenuous er materials. It has the highest density ice on its surface, and because the pres­ outer atmosphere. In contrast to other of all the known moons in the outer so­ sure of a gas evaporating from a solid satellites in the outer solar system, lar system, except for the jovian satel­ is very sensitive to temperature, the which probably formed from material lites 10 and Europa. warming of the ice from internal heat in orbit around their parent planets, The predominance of rock in Triton raises Triton's atmospheric pressure by Triton may be representative of the suggests that its internal structure is 50 to 150 percent! kinds of objects that coalesced in large differentiated-that is, its components Triton's high reflectivity is due pri­ numbers to form Neptune. have settled into layers. Its rocky core marily to the large amounts of frozen After Triton's capture, its initially is about 1,000 kilometers (600 miles) methane (CH4) and nitrogen (N2) on its 14 large and eccentric orbit slowly de- in diameter, and its water-ice mantle surface. In fact, in many places on this Figure 2 - Triton displays a variety of geologic features not seen anywhere else in the solar system. A vast polar cap, made of nitrogen and methane ices, dominates its appearance. Dark streaks on the bright cap are evidence of geyser-like eruptions. Triton's mid­ latitudes are crisscrossed by fault scarps hundreds of kilometers long. There is the strange cantaloupe terrain, perhaps formed by ammonia-water lavas vaporizing the surface ices. Vast, cryogenic lava lakes cover hundreds of kilometers. Image: US Geological Survey, Flagstaff

moon the reflectivities of such deposits sunlight and condense in those areas billion years ago. are greater than 90 percent-as bright where the Sun is very low in the sky. There are vast, frozen lava lakes as or brighter than freshly fallen snow The greatest amounts of condensation over 100 kilometers (60 miles) in di­ on Earth. are taking place in areas near the north ameter that still show evidence of the Frozen methane and nitrogen are pole that are now in total darkness and volcanic vents that gave rise to them. not the only components of Triton's will remain so for about 40 years. Although these lakes are volcanic in upper surface layers, however. Tele­ Because Triton's equatorial regions origin, and volcanism is a common scopic observations made at Mauna receive, on average, about twice as process in the solar system, the lavas of Kea Observatory have shown evi­ much sunlight as its polar regions, over these extinct tritonian volcanoes are dence for frozen carbon monoxide time the nitrogen and methane ices are fascinating, for these are cryogenic

(CO) and carbon dioxide (C02) , In a being systematically driven from the lavas-lavas that flow at temperatures way similar to the way sunlight pro­ equator to the poles. There they remain, where materials like water and carbon duces brownish smog from hydrocar­ forming vast ice caps (see Figure 2). dioxide gas are frozen solid. bons in the air above many of Earth's These ice caps creep and flow in The material likely to have erupted cities, the slightly pinkish tinge we much the same way as glaciers do here from these ancient volcanoes is a see in Voyager color images suggests on Earth. Only seasonal frost and ice thick, viscous solution of ordinary wa­ that the action of sunlight on methane deposits exist in Triton's equatorial re­ ter and ammonia (although at much and carbon monoxide has produced gions, similar to the cycles of snowfall higher ammonia concentrations than small quantities of smoglike organic and melting that occur over much of found in household cleaning fluids). material. Earth's temperate latitudes. The main This material remains very viscous The organic material thought to exist difference is that winters on Triton last even at temperatures as low as minus · on Triton is probably similar to organic over 80 years, and a warm day is 390 150 degrees Fahrenheit, far below the material on comets and dark asteroids. degrees below zero Fahrenheit! freezing point of ordinary water. Such There probably are many other com­ Even though Triton's south polar ice is the stuff of some tritonian volca­ pounds on Triton, particularly simple cap dominates its appearance in the noes. hydrocarbons, that are currently un­ Voyager images, on closer inspection Also in Triton's northern mid-lati­ known simply because they exist in one can find a fascinating array of tudes are large fault scarps that are quantities that are too small to detect geologic forms. North of the vast south hundreds of kilometers long marking using ground-based telescopes. polar cap lie the darker, redder regions the boundaries of vast, icy tectonic of Triton's northern mid-latitudes. plates. Although there doesn't seem to . Creeping Ice and Cryogenic Lava Here the primordial coating of ni­ be much movement of these tectonic Sunlight on Triton, though weak, also trogen and methane ices has probably plates now, in the last billion years or drives a very complicated cycle of evaporated away, leaving a substrate so Triton's surface may have seen evaporation and recondensation of of water ice and a slag of the organic many large-scale tectonic events, or surface ices. Just as ordinary dry ice . materials thought to be responsible tritonquakes. These quakes would lit­ (frozen carbon dioxide) changes di­ for Triton's overall pinkish color. erally have tom the upper crust of Tri­ rectly from a solid to a gas (a process Here also, in the hard-as-rock water­ ton, leaving the complex series of called sublimation), nitrogen and ice crust, is evidence of the catastroph­ intersecting ridges we see today. methane ices on Triton's surface evap­ ic and large-scale geologic activity The northern mid-latitudes also har­ orate from those areas that are in full that reworked Triton's face about a bor what the Voyager scientists dubbed 15 Figure 3- Dubbed "West from the gas vent, thus Plume," this darlc leaving a characteristic streak (most streak. " prominent in the bottom view) is But, at a tempera­ thought by some ture of minus 235 de­ scientists to be a grees Celsius, 38 de­ continuously erupting geyser of grees above absolute gas, ice and dust. zero, where even gas­ (The dark streak es like nitrogen are appears to move from image to frozen solid, what image due to the could provide the en­ motion of Voyager ergy to drive geyser­ 2 as it flew by Tri­ ton.) After rising like plumes and thus from an 8-kilome­ produce the streaks? ter-high (5-mile) A clue can be found in column, the cloud the distribution of the trails off to the right over 100 kilo­ streaks and in the lo­ meters (60 miles). cation of geysers Such eruptions caught in the act of may be driven by the Sun shining on erupting when Voy­ the nitrogen-ice ager flew past Triton surface and heat­ (see Figure 3). ing ices below, causing them to The streaks are only expand and burst found on Triton's through the sur­ south polar cap ex­ face. tending from the Images: Laurence Soderblom, us Geolog­ boundary of the cap ical Survey, Flagstaff near the equator south to latitudes of about 45 degrees where the ac- tive plumes are found. the "cantaloupe" terrain because it looks The Sun now stands directly overhead like the skin of a cantaloupe. This terrain at 45 degrees south latitude on Triton, is thought to have formed when "warm" and it is there that the maximum ammonia-water lavas intruded into near­ amount of sunlight is incident on this surface regions, causing surface ices satellite. But, as mentioned earlier, sun­ such as methane and nitrogen to explo­ light evaporates the nitrogen ice, and sively vaporize. These explosions left the gas produced quickly carries off the hundreds of polygonal depressions many excess energy to areas that are receiv­ kilometers in size. ing little or no sunlight. This keeps the surface of the nitrogen ice at a frigid Strange, Dark Streaks minus 235 degrees Celsius, and at this These processes, strange as they may temperature there is not enough gas be, still cannot top what may be the pressure to drive the geysers. most unusual form of volcanism in the solar system. A close look at Triton re­ Geysers in the Greenhouse veals strange, dark streaks on the south So how can temperatures and pressures polar cap. These streaks are reminiscent build up enough on Triton to drive gey­ of the fan-shaped deposits called wind sers? One way to do this is with a kind streaks on Mars, which are produced of greenhouse effect. when winds blow dust past obstacles on Imagine that on Triton's south polar the surface. But Triton's atmosphere is cap there are kilometer-sized stretches too thin and its winds too weak to move of nitrogen ice that are relatively clear, enough to break through weak places in

all but the most implausible kinds of tri­ o similar to glare ice on Earth. In these the ice, the energy stored over a large tonian dust particles, so another explana­ clear areas, sunlight penetrates several area gets explosively released through a tion is called for. meters below the surface, where it is small fissure, and a nitrogen gas geyser The best explanation at present gradually absorbed on the way down, erupts. seems to be that high-pressure nitrogen heating the ice. Because frozen nitrogen This explanation is not the only one gas leaks through fissures in the nitro­ is a very poor conductor of heat, the for"the geysers on Triton, however. gen ice on Triton's surface, picking up temperature several meters below the Some scientists have proposed that the dark particles and carrying them high surface slowly builds up. geysers are driven by heat from Triton's into the atmosphere. There they are car­ If these clear areas are large enough, warm interior. ried downwind by the gentle atmo­ they behave like giant solar collectors, As heat leaking from the interior spheric circulation to settle back to the storing energy in the form of gas pres­ builds up below the nitrogen-ice polar 16 surface tens to hundreds of kilometers sure. When the pressure becomes high cap, it melts the cap at its base, produc- Voyager 's television cameras transmit images to Earth as digitsl data containing information on brightness and color. Using computers, image processors trans/ata the data into pictures. Mapmakers can take the process even further, prodUCing ge0- logic maps of the surfaces of planetary bodies. Here are several types of maps of Triton. E Is a stereographic projection centered on the south pole. A is a projection showing the approx/mats natural color of the moon. Band E are contrast-enhanced to make surface features easier to see. C maximizes color separa­ tion, and D and F show the different geologic units. Ihps: Alfred IIcEIIWHI, us Geologies/ SuIWI)', F/8gBtaff

ing geysers of liquid nitrogen as the that the Voyager spacecraft provided system. NASA is looking into the . pressurized liquid works its way to the us with the answers to a number of im­ possibility of sending a spacecraft to surface and breaks through fissures in portant questions about Triton. But, as orbit Neptune, carrying a much better , the ice cap. Although this is plausible, has been the case with all other plane­ complement of instruments than did it may not explain why the active gey­ tary encounters, it opened up many Voyager and returning data for more sers are where the maximum amount more new questions, some of which than four years. Perhaps then we will of sunlight is falling. Perhaps the gey­ will be debated by planetary scientists solve the vexing problem of Triton's sers are only triggered by sunlight, . for quite some time. Without a return geysers, and, in the process, learn while deriving their main source of to Triton, many of these questions will more about the solar system as a energy from Triton's internal heat. remain unanswered. whole. There is hope, however, that we Some Answers, More Questions may yet return to this cold and beau­ R.H. Brown is a planetary scientist at Whatever the explanation, it is clear tiful moon on the fringes of our solar the Jet Propulsion Laboratory. 17 ·The Oouds and Winds of Neptune

The handsome sphere has less smog and is more trans­ face of Neptune is parent than that of Uranus, and we can distinguished by see farther down into it, encountering several cloud and more methane molecules along our line wind features. Most prominent is of sight. The latter possibility seems to the Great Dark be the best choice. Let's consider the Spot, in the center evidence for this. of this image, with its associated white clouds. To Atmospherics, Quick and Easy the lower left is the The conditions in a planetary atmo­ bright white fea­ ture that, in early sphere depend on how much of the image sequences, incident sunlight is being absorbed. If appeared to zip we carefully measure how much sun­ around the planet at great speed; light the planet is reflecting, we can de­ hence its name of termine how much has been absorbed. "Scooter. " Just be­ Because of Neptune's remote position low that is a dark storm smaller than in the solar system, the small amount of the Great Dark absorbed energy leads to temperatures Spot, called D2. of about minus 220 degrees Celsius Image: JPUNASA (about minus 365 degrees Fahrenheit), and we must observe the planet in the far infrared to look for the excess heat flow from these cold clouds. When we obtain this information, we can compute the ratio of heat lost to heat absorbed. If this ratio is greater than 1, the planet must have an internal source of heat. Early heat measurements were made by Reta Beebe from Earth-based telescopes. Voyager's imaging and infrared teams refined these measurements. The results indi­ efore Voyager, we knew Uranus ages of the planet across more than 4 cated that Uranus has a very small and Neptune only as faintly bluish billion kilometers (2.5 billion miles), internal heat source, whe1;"eas Neptune Bobjects orbiting in the outer reaches we were astounded by the variability radiates more than 2.7 times as much of our solar ·system. Only one space­ and beauty of the white clouds we saw energy as it absorbs from the Sun. At craft, Voyager 2, has ever been sent to against the deep blue of the planet. Af­ first it seems that these two frigid plan­ explore them, and we pinned many of ter Voyager's disappointing encounter ets with similar masses must differ our hopes of understanding them on this with smog-shrouded Uranus, these greatly, but if we consider that the at­ one much-traveled robot. spectacular images were a welcome mosphere serves as an insulating blan­ In January 1986, Voyager 2 encoun­ relief. But why should these two giant ket through which the heat escaping tered Uranus, and through the space­ planets look so different? from the interior must pass, we return craft's cameras that planet's atmospheric The beautiful blue color of Neptune's to the idea that Neptune's atmosphere features were, to say the least, disap­ atmosphere can be explained by the must be more transparent than that of pointing. Its bland, blue face was shroud­ manner in which an atmosphere reflects Uranus. ed by smog and revealed detail only sunlight. In this frigid atmosphere, the The rate of heat loss from an atmo­ when we computer-enhanced the im­ clouds lie at lower depths; above them, sphere is governed by how opaque the ages. Even then, there was little to see. there are traces of methane gas, which atmosphere is to infrared light. If, as the As Voyager moved on toward Neptune, absorbs red light, leaving mostly blue infrared light emerges from the warmer we were a little apprehensive that this and green light to be reflected back to­ interior, it is strongly absorbed by the world, nearly identical to Uranus in ward Earth. One might conclude from atmosphere, the atmosphere will insu­ mass, would also disappoint us. this that Neptune's atmosphere has late the inner regions and reduce the So, in August 1989, as Voyager 2 more methane than that of Uranus. But outward heat flow. Small particles­ 18 approached Neptune and returned im- it is also possible that Neptune's atmo- such as the smog obscuring Uranus' atmosphere-absorb infrared light need to track some of the distinct of the planet core, was only 16.1 hours, more efficiently. The deeper blue of clouds that move with the winds. But somewhat less than pre-encounter esti­ Neptune's atmosphere indicates there inspection of the Voyager images in the mates. We then realized that the Scoot­ are fewer bright particles suspended in weeks before nearest encounter showed er was misnamed. This white cloud and this atmosphere to scatter the light white, filmy streaks, and only four well­ D2 were not scooting eastward at high back to us. . defined features on Neptune. speeds. Instead, the Great Dark Spot When solarultraviolet light shines on The Great Dark Spot, with its associ­ was moving westward at more than

methane gas (CH4), the molecules can ated white clouds, was the first measur­ 300 meters per second (about 670 break apart and hydrogen atoms can able feature. Spanning a region from 8 miles per hour). escape. Through a chemical pathway to 28 degrees south latitude, this cloud The only way that we can explain involving larger hydrocarbon molecules system rotated about the planet in 18.3 this is that, even though we found few such as ethane (CZH6) and acetylene hours. As the spacecraft approached the cloud tracers, there are strong local (CzHz), small particles of smog are planet and we continued our search for east-to-west winds. Thus, unlike Jupiter formed. These particles are even more wind tracers---clouds we could use to and Saturn, which have strong eastward efficient in sweeping up large molecules. gauge the wind-a smaller bright spot winds near the equator, Neptune is truly Because Uranus is closer to the Sun, appeared near 42 degrees south latitude. the "Planet of the Westward Wind." the ultraviolet light hitting Uranus is This triangular cloud rotated faster than two to three times more intense than it the Great Dark Spot, passing it every ADisappearing Act is at Neptune, so smog forms more eight days. Because of this behavior Ongoing efforts to derive wind speeds slowly in Neptune's atmosphere. But, our imaging team dubbed the cloud from the highly detailed Voyager 2 im­ since smog can accumulate over time, "Scooter." ages have revealed that the small white there must be some short-term mecha­ A third bright region occurred at 71 clouds change within minutes. Even at nism cleaning Neptune's atmosphere. degrees south latitude. But as we at­ these unbelievably frigid temperatures, tempted to measure its rate of motion the clouds are condensing and evapo­ Scrubbing Neptune's Skies about the planet, we found that it varied rating before our eyes. This makes If gases circulate up from below, ices drastically in brightness from day to them poor markers for defining the will condense onto the smog particles, day. This was a clue that, in this frigid winds. Instead, they are sensitive to increasing their mass so that they sink atmosphere, we were dealing with local variations of pressure and temper­ into the warmer regions below the clouds that could change very quickly. ature. Slight changes in these quantities clouds. There the ices rt}elt, in the pro­ The fourth feature, near 55 degrees will cause the clouds to appear and cess removing smog from the upper at­ south latitude, looked like a dark eye disappear. mosphere. If there is less smog, then with a white pupil that varied in inten­ Although these rapid changes the upper region of the atmosphere will sity or size. This cloud moved at a rate seemed strange at first, we see similar be colder and the heat flow from the similar to that of the Scooter. Since behavior in Earth's atmosphere when lower regions will be greater. A small this was the second dark spot, we called air is forced to rise and flow over sur­ outward heat flow will increase up­ it D2. face features such as mountains. As the welling from below the opaque cloud The radio science team announced air flows over a mountain, slight varia­ deck and provide a mechanism for that the period of rotation of the radio tions in the humidity cause small-scale smog removal. signal, presumably the rate of rotation (continued on page 21) This feedback process can clean Neptune's atmosphere in a manner similar to the way that the water cycle scrubs Earth's atmosphere. The evi­ dence that Neptune's atmosphere has more vertical mixing than that of Uranus not only explains Neptune's deficiency of smog relative to Uranus, Bright streaks of white clouds are but also suggests that individual storm a distinctive fea­ centers, submerged in the deeper cloud ture of Neptune's layers, could be associated with the ma­ atmosphere. This image shows the jor cloud features that make this planet clouds to the so much more interesting to view. east of the Great Dark Spot and, toward the bot­ What Voyager Found tom, the cloud Along with Neptune's bright, white structures near . clouds, we saw distinct cloud bands en­ the south polar circling the planet. This indicates that region. strong winds may be drawing the indi­ Image: JPUNASA vidual douds out into east-west streaks that coalesce to give the belted appear­ ance. The variations in the ways the cloud bands reflect light may be due to variable amounts of white ice in the upper regions of the atmosphere. To demonstrate that this is the case, we 19 turn to lower depths around the edge of the spot. The red col­ oration must be due to an unidentified trace constituent in the upwelling gases that absorbs almost all the ultraviolet and blue light. stronomers have been observing the Great Red Spot of The color scheme is different at Neptune. Because methane AJupiter since the 17th century, watching it brighten and gas absorbs red light more than it absorbs blue, if bits of white fade, grow and shrink. When Voyager 2 discovered Neptune's ice and smog were not present in Neptune's atmosphere to Great Dark Spot, the question immediately arose, Are these scatter the sunlight back to us, the planet would appear even two great storms the same sort of thing? darker blue. Therefore the deep blue of the center of Neptune's This question is not easy to answer. With the Great Red Great Dark Spot indicates that this is a storm center where Spot, small ammonia-ice clouds allow us to trace the winds that slightly warmer smog-free gases are rising, expanding and surround and constrain the spot. Westward winds are deflected sweeping out the overlying smog and ices. around the equatorward side, and eastward winds flow around Even though this explanation seems logical, Neptune's at­ the poleward side of the spot. These opposing winds force the mosphere.is not as accommodating as Jupiter's in providing us edge of the spot to rotate about its center at about 150 meters with markers to map the winds around the spot. By tracking the (500 feet) per second. small white clouds we conclude that the local westward winds The Great Red Spot's east-west speed has been recorded increase from 30 degrees south latitude toward the equator, but for more than 150 years. During that time it has drifted west­ the extent of this change is not enough to constrain the feature ward at speeds of 1 to 5 meters (about 3 to 17 feet) per second into a smoothly spinning oval. Instead the Great Dark Spot ap­ relative to the planet's core. This variable speed indicates that it pears to roll with a counterclockwise motion-looking, in time­ is a large storm that speeds up and slows down in response to lapse sequences, a bit like a crawling garden slug. its atmospheric surroundings. L.M. Polvani and J. Wisdom from the Massachusetts Insti­ Voyager infrared measurements revealed a cold region high tute of Technology and E. De Jong and A.P. Ingersoll from the above the Great Red Spot. This is believed to be caused by up­ California Institute of Technology have proposed a model for welling and expansive COOling of the central region of the spot. how the Great Dark Spot varies with time. Their model de­ This would drive a circulation where colder material would re- scribes the feature as an eddy, with an upward central flow and a motion like that of an oscillating trout-fishing lure as it is pulled through a stream. Thus, we believe that both the Great Red Spot and the Great Dark Spot are caused by local upwellings of the atmosphere. The ways in which they differ from each other are determined by how each responds to the local environment. -RB

The Great Dark Spot: The east-west dimension of the Great Dark Spot is The Second Dark Spot, 02: This feature appears to be a closed variable, averaging about 16,000 kilometers, or 10,000 miles (the diameter eddy or disturbance that is forced to roll in the prevailing winds. It of Earth is about 12,700 kilometers, or 7,900 miles). Surrounded by wispy, is almond shaped, with a bright central area. There are no readily white clouds, it appears to be a giant eddy with clouds flowing upward visible cloud markers around its perimeter. "we could determine in the inner regions. Carefully taken measurements of high-resolution im­ whether the bright core is rotating in a clockwise or counterclock­ ages revealed an eastward wind along the spot's southem edge moving wise sense, it would not only permit us to infer the sense of the at about 30 meters (100 feet) per second relative to the spot. As the en­ winds to the north and south of the feature but would also help croaching wind encounters the spot, the spot's intemal motions deflect establish whether the central region is rising from lower depths. the air equatorward and upward. The air expands slightly and cools; But the central clouds refuse to be analyzed. Instead, structures white condensations of methane or ethane ice form. in the core change rapidly, feathering out in the east-west direc­ the interaction of this wind with the counterclockwise motion of the tion as though they too are responding to an overrunning wind. upwelling spot appears to generate the downstream wave pattern that produces the trailing clouds we see to the east of the spot. Images: JPLlNASA The Bright Polar Feature: There is an elongated white streak of clouds located at 71 degrees south latitude (at bottom in this image). Surprisingly at these frigid temperatures, this large feature varies drastically even within a day. But at high resolution we see evidence that, like the other large cloud systems, this may be a buried upwelling. Within days of closest approach, Voyager revealed small, wedge-shaped clouds moving through the large, white cloudy region at speeds of 150 meters (500 feet) per second. This indicates that the winds are sweeping material over the disturbance. Traveling at this speed, a region at this latitude, containing more than average humidity, would encircle the planet and return and brighten at the same spot within three days. This agrees in general with the variations in brightness that were observed at this latitude. The second dark spot, 02, appears to the west and north of the polar feature in this image. Image: JPUNASA

condensations. Thus, here on Earth, if had moved into a region of increasing that the extent of upward circulation we are looking at a mountain scene wind speed. D2 moved back and forth driven by active convective cells may from a distance, we might see just a between 51 and 55 degrees, with the vary greatly with time. The atmo­ cloud over the mountain. If we fly over speed of its eastward motion decreas­ spheres may even enter periods where it in a plane and look down, we would ing as it moved toward the equator and there is little upward motion. Could it see small regions where condensations increasing as it drifted poleward. This be that we caught Uranus in a quies­ form as the air flows over this obstacle. behavior indicates that once storm cent phase and Neptune displaying ac­ Beyond the mountain the air descends centers are generated in Neptune's at­ tive convection? Are these atmo­ and warms, and the small clouds mosphere they drift north and south, spheres not as different as our first evaporate. responding to the local prevailing glance would lead us to believe? If there are upwellings in Neptune's wind in a manner similar to storms on New developments of ground-based atmosphere, the atmosphere would rise Earth. However, because the condi­ cameras sensitive to infrared light will as it flows over these regions and white tions on Neptune are such that the at­ allow us to measure variations in ice would form. Conversely, if there mosphere should become denser and methane absorption that will indicate are regions where the atmosphere is finally behave as a liquid, there is no changes in the smog levels, and careful descending, the ices and smog particles solid, irregular surface. Therefore computer processing of Hubble Space would melt and the amount of methane neptunian storms will not encounter Telescope images will yield some infor­ gas along our line of sight would in­ the surface resistance or friction that mation on the fate of the Great Dark crease as we looked more deeply into hurricanes do when they drift inland Spot during the next decade. But the the atmosphere. This would make the and dissipate. Thus neptunian storms limitations imposed by remote sensing, atmosphere appear darker blue. should live much longer and die less and the fascination generated by our Keeping this in mind, examine the violent deaths. first look at these two distant, sister images of the Great Dark Spot, Scoot­ During the interval from 1974 to planets will certainly motivate us to er, D2 and the bright polar feature 1984, Dale Cruikshank at Hawaii and return with orbiting missions in the shown 'on these pages. Michael Belton at the National Optical coming decades. Astronomy Observatories in Arizona Variability Over the Long Term observed variations in the brightness of Reta Beebe specializes in the detailed During the period spanned by the Voy­ the whole disk of the planet, indicating analysis of cloud motions in the atmo­ ager 2 observations, the Great Dark that the amount of white cloud struc­ spheres of the outer planets. She is a Spot drifted equatorward at a rate of ture varies with time. This and the professor of astronomy at New Mexico 0.11 degree per day, and its westward sharp contrast between the appearance State University at Las Cruces, and was velocity increased, indicating that it of Uranus and that of Neptune suggest a member of the Voyager imaging team. 21 Neptune's Small Satellites

by Peter Thomas

mall satellites are not the most lite in its system, Nereid. But at Jupiter, kilometers (0.8 mile) on Proteus. glamorous members of the outer Saturn and Uranus we had discovered Luck was a factor in being able even Splanet systems. This class of ob­ well over a dozen small satellites, and to attempt this image, as the combina­ jects is loosely defined as satellites of we expected to find more at Neptune. tion of the dark satellite and low light irregular shape and less than 230 kilo­ (11900 that at Earth) meant that expo­ meters (about 140 miles) in radius. Finding New Satellites sures had to be several seconds long. Compared to the majestic planets them­ While Voyager was still weeks and If the camera could not track its target selves, they are mere bits of debris. tens of millions of kilometers from during exposure, the image would be They do not have the delicate beauty Neptune, project team members pored smeared. While Voyager had several of ring systems. They lack the unique over its images, searching for new ways to perform "image motion personalities of the larger satellites. satellites. Nearly two months before compensation," the limited computer Yet they add distinctive features to the encounter, Steve Synnott of the memory did not permit the team to these systems. Without studying the navigation team found the first, Proteus send the spacecraft additional com­ small satellites, we cannot fully grasp (l989Nl), shown in Figure 1. Images mands to accomplish this. By chance, the natures of the outer planet systems. obtained over the succeeding weeks the spacecraft was scheduled to be Small satellites occupy many niches helped Bill Owen and Robin Vaughn turning for a radio science experiment, in the orbital space around planets, of the navigation team to measure Pro­ and the rate of motion was very close much as do asteroids and comets orbit­ teus' orbit accurately enough to allow to that desired. As can be seen from ing the Sun. Some of these satellites or­ the team to target the high-resolution Figure 1, the targeting and the motion bit close to planets, often in association camera on the satellite near the time compensation were excellent. with rings. Some are in orbits essential­ of encounter. During this period other ly identical to those of large satellites, new satellites appeared in the space­ Characteristics of the but ahead of or behind them by about craft's images, and by encounter five Small Satellites one-sixth of an orbit. Others revolve far more had been located-Naiad, Tha­ The small inner satellites are very dark, from planets in inclined (tilted) and lassa, Despina, Galatea and Larissa neutral in color and irregularly shaped. even retrograde (backward) orbits. (see Figure 2 and table). They reflect only about 6 percent of the Before Voyager 2 reached Neptune in To get the high-resolution images sunlight falling upon them, giving them 1989, we knew of only one small satel- of these new worlds, the team had to albedos (a measure of reflectivity) sim­ know the posi­ ilar to those of some other satellites, tion of each satel­ such as Uranus' Puck, Saturn's Phoebe, lite relative to the and Mars' Phobos and Deimos. Such a spacecraft, within low albedo is hard to appreciate; it is a few hundred approximately that of coal. We do not Figure 1 - This is the kilometers. They know the composition of these dark best image Voyager 2 had to work with surfaces or of the bulk of the satellites. took of Proteus. Its images taken Although we expect to find ices, very dark surface and the speed of the many days earli­ especially water ice, in objects at Nep­ spacecraft combined er, when the res­ tune's distance from the Sun (Neptune's to limit the detail the olution was still large satellite, Triton, apparently has a cameras could pick out. The satellite is extremely low. large fraction of water ice), there really · only about 400 kilo- As was usual is no way at present to determine the · meters across (from with the Voyager proportions of ice and rock in these top to bottom in this mission, this tar­ · image). A 150-kilome­ satellites. Ice can be made very dark if ter crater is visible in geting challenge mixed with just a small amount of the upper right, with was met perfect­ dark, opaque, fine-grained material. smaller craters super­ ly, and the pic­ imposed on it. Neptune's satellites do not show bright tures included the spots, such as those we see on Phoebe, Image: JPUNASA best resolution of which might be ice exposed by impacts any small satel­ through a dark crust. The particles that lite by Voyager, make up Neptune's rings are as dark as revealing details its small satellites-which suggests a 22 as small as 1.3 common heritage. NEREID

Figure 2 - The positions of the satellites of Neptune. The outer­ most satellites, Triton and Nereid, probably formed elsewhere in the outer solar system and were cap­ tured by Neptune's gravity. The NEPTUNE-. - inner satellites most likely formed in orbit. They are intimatelyasso­ ciated with the rings, which are interspersed among their orbits. Illustration: Ron Miller ~ NAIAD THALASSA DESPINA GALATEA LARISSA PROTEUS TRITON

Neptune's outermost small themselves-Saturn's small satellite, Nereid, is nearly three NEPTUNE'S SMALL SATELLITES satellites are bright, probably times as reflective as the inner icy bodies, whereas Jupiter's Orbital Radius Orbital Period Satellite Radius ones. Its albedo of about 15 to (thousands of kilometers) (hours) (kilometers) inner small moons may be 20 percent suggests that it is rocky objects covered with NEREID 5,513.4 8,643.1 170±25 made of ice with a modest mix­ sulfurous material ejected ture of other materials. Its orbit PROTEUS 117.6 26.9 208±8 from 10. The small satellites and its albedo clearly differen­ LARISSA 73.6 13.3 96±7 of Uranus and Neptune are ap­ tiate it from the inner small proximately similar in sizes and satellites, and predominant the­ GALATEA 62.0 10.3 79±12 indistinguishable in albedos. ory is that Nereid was once DESPINA 52.5 8.0 74±10 One of the great discoveries wandering through the outer of Voyager at Saturn was that solar system, until it was cap­ THALASSA 50.0 7.5 40±8 small satellites act as shepherds tured by Neptune's gravity. NAIAD 48.0 7.1 29±6 to ring particles, confining We expect most small satel­ them to narrow pathways lites to be irregularly shaped around the planet. At Neptune, because they are molded by so far only Galatea has been impacts that fracture and fragment This bombardment would also generate associated with the confinement of a them, removing and rearranging signifi­ thick layers of fragmental debris, as on ring. Carolyn Porco of the University of cant portions of their bodies. Because Earth's Moon. While some of this mate­ Arizona found that it should confine the they are small, their gravities are not rial may have landed as thick ejecta arcs in the Adams ring (see pages 4-9) strong enough to smooth out the result­ blankets near craters, the low gravity to a particular distance from Neptune. ing irregularities in their shapes. Nor are of these small satellites makes it likely The ring arcs, which appeared to be sep­ they large enough to generate enough that some material was thrown off into arate objects to Earth-based observers, internal heat to trigger volcanism or the orbit around Neptune and then swept in Voyager's images turned out to be slow creep of material, which could also up again by the satellite. On Proteus, denser portions of complete rings. The smooth them out. surface gravity would be about lO cen­ positions of those clumps within the We know that collisions are common timeters per second squared (l percent ring might also be explained by the in the neptunian system, for we have that on Earth). On Naiad, it would be gravitational effects of Galatea. But we seen the cratered evidence. The two only about l.5 centimeters per second have not yet found a pair of satellites best views of Proteus (one of which is squared, and an object would take shepherding the inner and outer edges shown in Figure 1) show craters up to about 15 seconds to fall the height of a of a ring, as we've seen in the saturnian 150 kilometers (about 90 miles) across. person, who would feel as if he or she and uranian systems. Triton also has some impact craters, weighed less than I pound. We have had only a glimpse of the even though its internal activity has small satellites of the outer planets, and resurfaced much of its area. Comnarison to Other we have much more to learn about What are the conditions on the sur­ Sma I Satellite Systems them. They are clearly related to the faces of these objects? From their Voyager has shown that all four of the ring systems, but convincing theories on albedos we may be fairly sure that giant planets-Jupiter, Saturn, Uranus the origin and evolution of ring/satellite their temperatures are about minus 223 and Neptune-have systems of small systems remain to be formulated. Orbit­ . degrees Celsius, or minus 370 degrees satellites orbiting near and within ring ing spacecraft such as Calileo, now on .. Fahrenheit. (Brighter Triton, reflecting systems. The satellites closer to the its way to Jupiter, the planned Cassini much more of the feeble sunlight, is planets tend to be smaller; the largest of mission to Saturn and the suggested even colder.) At these temperatures, the small satellites orbit just outside the Neptune orbiter may show us how water ice, carbonaceous materials, synchronous orbit distance, the position small satellites relate to the rings, to and most other materials familiar to us where a satellite's orbital period match­ the larger satellites, and to other small in solid or liquid form are extremely es the rotation period of the planet. objects such as comets and asteroids. strong solids and would best be de­ The range of sizes of objects that are scn'bed as rocks. called small is substantial. Proteus, for Peter Thomas works at the Rand Some of the impacts that formed the example, has a volume about 350 Corporation and studies surfaces craters we see on Proteus were large times that of Naiad. There are signifi­ and interiors of planetary satellites, enough to have fractured the satellite. cant differences among the systems as well as eolian features on Mars. 23 From the Editor (continuedfrom page 2)

Uranus? Were there clouds and storms to mark its atmo­ With Pioneers 10 and 11, they are heading out of our spheric activity, or would it appear as bland as Uranus? solar system. All four spacecraft are searching for the he­ Were there any satellites too small to see from Earth, wait­ liopause, the boundary where the solar wind of ionized ing to be discovered? particles from our Sun gives way to the wind from the Voyager 2 answered all these questions, taught us that in stars. Who will be first to reach it? some cases we'd been asking the wrong questions, and All these spacecraft also carry messages from the people demonstrated once again that the denizens of the solar sys­ of Earth to whoever might someday encounter one of our tem are stranger than we can imagine. robotic emissaries. The Pioneers carry plaques and the In this issue of The Planetary Report, we've asked some Voyagers golden records that commemorate the planet of the leading Voyager scientists to share their recent find­ from which they came. Perhaps the Voyager spacecraft ings with Society members. These articles represent the will someday encounter something even more wonderful: state of our knowledge so far, but we still have not learned an alien civilization in the depths of interstellar space. all the lessons Voyager has to teach us about Neptune, and So the story of Voyager is not yet over. At the last press scientists are still studying its data. This issue is probably briefing of the last encounter, Project Scientist Ed Stone, the last special one we will publish about the Voyager who had stayed with Voyager throughout its epic journey, planetary encounters, but new findings are bound to spoke for everyone when he quoted T.S. Eliot: emerge from the continuing research and analysis. "Not farewell, but fare forward, voyager?' And the Voyagers have not yet completed their mission. - Charlene M. Anderson ------

applications for the following: In the years since, Gene tails for the test program. • New Millennium Awards for helped the Society again and -Louis D. Friedman, Execu­ high-school students, with up again by speaking at our tive Director From August 29 to September to $5,000 to be awarded. events and attending our fund­ 5, The Planetary Society will • College Fellowships, with up raising dinners. Occasionally, host 20 high-school students to five $1,000 prizes available. we were able to reciprocate by and their mentors from around • The Mars Institute Contest, offering technical advice as In June The Planetary Society the world. The students will open to all high-school and he developed Star Trek: The and the Irvine School District be the winners of the H. Dud­ college students, which offers Next Generation. will cosponsor a six-week ley Wright International a $500 prize plus a trip to a We will miss our friend. "Planetary Academy" for stu­ "Together to Mars" contest. conference about Mars. -Charlene M. Anderson, dents 12 to 18, to be held at Student-and-mentor pairs To receive an application or Director of Publications the Discovery Museum of will be arriving at Dulles In­ more information, write to Santa Ana, California. For ternational Airport through­ The Planetary Society, Atten­ more information, contact out the day on Saturday, Au­ tion Scholarship Department. Mike Slage at The Planetary gust 29. We need the help of The deadline for completed Society. -SL Washington-area volunteers entries is June 30. -Carlos 1. Planetary Society members to pick up our visitors at the Populus, Contest Coordinator have just received three more airport and to take them to dividends from their invest­ the Alexandria, Virginia, ho­ ment in our Asteroid Discov­ tel that will lodge the group. ery Project. Eleanor "Glo" The yearly audit conducted There, a reception will be Helin and her team in the by the firm of Stanislawski held for our volunteers and A melancholy note. A few Planet-Crossing Asteroid and Company has determined international guests. days after Gene Roddenber­ Survey (PCAS) discovered The Planetary Society's 1991 This is a wonderful oppor­ ry's death, we received his last the first comet of 1992, as financial statement to be in tunity to meet young, creative donation to The Planetary So­ well as two more asteroids. conformity with generally ac­ space science enthusiasts from ciety, earmarked for Project On the night of January 9, cepted accounting principles. many nations. If you are able META, our radio search for Glo and her colleagues Jeff Copies of our financial state­ to join us in welcoming these extraterrestrial intelligence. Alu and Ken Lawrence were ment, which includes a report visitors to Washington, please Gene had been a friend to searching with the 0.46-meter on member donations re­ write to Mike Slage at The the Society since its founding. (l8-inch) Schmidt telescope stricted to special use, are Planetary Society. -Susan In 1981, as we were beginning at Palomar Mountain Obser­ available on request. ~Lu Lendroth, Manager of Events to build up our membership, vatory. On one of their film Coffing, Financial Manager and Communications he broke a long-standing rule plates, Glo and Jeff found not to let his relationship with the comet now named comet Star Trek fans be used for any Helin-Alu (1992a) after its • -, - '~ ./ = purpose not related to that discoverers. , .';' ,)' J ::' ::1 . • I ; show. Gene signed an open On their next observing The Planetary Society is plan­ letter to all Star Trek fans rec­ run, the PCAS team discov­ ning an exciting tour of Flori­ ommending that they join and ered the asteroid now desig­ da to celebrate the September support The Planetary Society. nated 1992BC, whose orbit launch of Mars Observer (see In our early days, the Soci­ crosses that of Earth, and the J anu,ary IFebruary 1992 ety was sometimes criticized 1992BS, which follows an Planetary Report for further for being more interested in orbit inside that of Earth. details). For a launch tour launching robots into space -CMA brochure, write to me at The than in sending humans to ex­ Planetary Society. -Cindy plore the planets. In The Plan­ lalife, Manager of Member­ etary Report (April/May 1981), ship Services Gene rose to our defense, strongly stating his belief that We have set the dates of May The Planetary Society filled 21 to 28 for our tests of the the role of a broad-based or­ Russian Mars Rover in Death The 1992 Planetary Society ganization representing all Valley. Rover team members scholarship competitions have pathways toward our goals who have contributed to the begun! We are now accepting in space. project will soon be sent de- 25 by Louis D. Friedman

WASHINGTON, DC-In February, sion, but it would have to act quickly; technology for future exploration was President George Bush sent his pro­ NASA and JPL are implementing the cut by 12 percent. posed budget for fiscal year 1993 to the cancellation. Cassini is funded for the The President's proposed budget will United States Congress. It was not a moment, but its budget requirements for now wend its way through the congres­ good budget for the US planetary ex­ fiscal year 1994 may be a problem. (See sional committee process. It will be ploration program. the March/April 1987 and January/ considered, modified, debated and ulti­ The Comet Rendezvous Asteroid Fly­ February 1989 Planetary Reports for mately passed by three sets of commit­ by (CRAF) mission was canceled out­ mission descriptions.) tees: on budget, authorization and ap­ right. Its companion mission, Cassini to • Life Sciences: This item received an propriations. the saturnian system, barely survived almost 20 percent increase for 1993. Can anything be done to influence and is still not out of the woods. This includes NASA' s project in the Congress' action on the NASA budget? The Magellan mission, which has Search for Extraterrestrial Intelligence, Is it worthwhile to write to members of been mapping Venus so brilliantly, did which will become operational late this Congress? Public support for CRAF, not receive funds to continue operations year . Cassini, Magellan and other exploration beyond April 1993. • Magellan: The spacecraft has com­ missions can make a difference-and These actions are deplorable and con­ pleted its initial radar mapping mission. may be essential to save US planetary stitute a major blow to US planetary ex­ By April 1993, it should have finished exploration. ploration. The Planetary Society is mapping Venus' gravitational field. Sci­ Special interest groups, such as the strongly protesting them. entists had hoped to adjust its orbit to aerospace industry and professional sci­ Overall, NASA's budget rose 4.5 per­ get even more detailed radar data, but entific organizations, do influence the cent to just under $15 billion. The allo­ no funding was provided for this. (See NASA budget. But the public deserves a cation for "Space Exploration" was in­ the N ovemberiDecember 1990 and voice in the budget, and it is up to us in creased by 7 percent, but this is May/June 1991 Planetary Reports.) The Planetary Society to provide the misleading. Several programs are • Space Exploration Initiative: grass-roots support for planetary explo­ grouped under this heading, and of these NASA Associate Administrator ration that Congress needs to see. only the space station received an in­ Michael Griffin has placed a new em­ Senator Wyche Fowler, a staunch crease-of 11 percent. Planetary explo­ phasis on robotic precursor missions for supporter of planetary exploration, re­ ration was cut by 9 percent. this program to send humans back to cently told me that he has received lit­ Here is a little background on items in the Moon and on to Mars. The good tle public support for his work on the the proposed NASA budget: news in this budget is that $29 million Appropriations Committee. He says • CRAF/Cassini: CRAF was eliminat­ has been provided for two small and as­ that members of Congress assume that ed from the budget. It is possible for yet-undefined lunar missions. But mis­ the public does not care about planetary Congress to restore funding for the rnis- sion studies were cut by 40 percent and missions. You can prove that wrong. Let Congress know that you want the Unit­ ed States to keep sending spacecraft to Monitoring the Russian Mars Program explore other worlds. The most important action you can veryone involved with the formerly Soviet space program is asking the same take is to contact your own representa­ Equestions: Will the republics continue the program? Will they launch Mars '94 tives and senators and let them know and '96? For Planetary Society members, the question is more specific: Will they fly what your views are. the Mars Balloon and the Rover? Your calls and letters can also be Even though I am in almost constant contact with our Russian colleagues, I cannot directed to these key individuals: answer these questions. But I am hopeful. The people with whom we work are still at • Rep. Leon E. Panetta, House Budget . their jobs and meeting their schedules. Yet this good news cannot override the uncer­ Committee tainty about all formerly Soviet institutions and the rampant inflation that is eating • Rep. George Brown, House Commit­ . away at even fully funded programs. tee on Science, Space and Technology We have just received extremely good news from France, a partner in the Mars • Rep. Bob Traxler, Subcommittee on Balloon project. The French Space Agency, CNES, has established a hard currency VA, HUD and Independent Agencies, reserve of several million dollars to protect the Mars '94 and '96 projects. House Appropriations Committee All the spacefaring nations are having budget problems, but the fact remains that All can be reached at the US House only Russia still has missions planned to land on Mars in this decade. (Ironically, of Representatives, Washington, DC, their program continues while the US cancels CRAP and stops funding for 20515. Magellan.) This could change. We will watch the situation closely and inform our • Sen. Jim Sasser, Senate Budget members immediately of change. -LDF Committee . • Sen. Ernest F. Hollings, Senate Committee Richard Truly announced his resigna­ Committee on Commerce, Science All can be reached at the US Sen­ tion, apparently at President Bush's and Transportation ate, Washington, DC, 20510. request. • Sen. Barbara Mikulski, Subcom­ mittee on V A, HUD and Independent WASHINGTON, DC-As we were Louis D. Friedman is the Executive Agencies, Senate Appropriations going to press, NASA Administrator Director of The Planetary Society.

As indicated in World Watch, the pro­ dezvous Asteroid Flyby (CRAF) mission many would-be explorers dreamed of posed federal budget has provoked a would be the first time that an approved sailing centuries ago, but those we re­ crisis in US planetary science and ex­ planetary flight project would be axed so member are the ones who actually made ploration. Clark R. Chapman, our faith­ far along in its development. NASA is the daring voyages. And the nations that ful News & Reviews columnist, has actually closing it down before Congress prospered from exploration were those written this opinion piece reflecting the has spoken. The clues asteroids and that committed to long-term investment views of many involved in NASA's plan­ comets possess about planetary creation in far-flung lands. etary program. We invite other opinions would remain hidden from a generation On Columbus' 500th anniversary, "re­ and reactions. of space scientists. There is a chilling alists" inside the Washington Beltway message to young scientists and engi­ say there is no hope of resurrecting neers who might contemplate devoting CRAF; we're asked to think small; and their careers to a space mission: We can we're not supposed to anger administra­ Crisis in pull the rug out from under you! tion officials who are fixated on the Planetary • Turning off the Magellan spacecraft, space station. But scientists and other which is again healthy and performing folk who favor exploring the solar sys­ Science: brilliantly in its Venus-mapping task, tem-first by robotic spacecraft, then by Push Forward means that no matter how successful a astronauts-must cry out in alarm at this mission is, long-term plans can be termi­ waste of scientific and engineering tal­ or Fall Sack? nated and taxpayer investments wasted ent. Over $450 million has been invested without warning. A tiny fraction of the in CRAFICassini so far ($50 million to by Clark R. Chapman $750 million already invested in Magel­ $100 million on CRAF alone), which lan was sought for operations in FY would be totally wasted if nothing is 1993, but Bush said no. NASA may launched. Yet even as NASA cancels its t is 1992, the quincentennial of withhold FY 1992 funds and stretch sole mission to comets and asteroids, I Columbus sailing the ocean blue and them out to delay turning Magellan off, Bush has proposed a new asteroid mis­ "exploration" is the theme for this Inter­ but research concerning our sister planet sion to be done as part of "Star Wars"! national Space Year. A third of a century would then suffer immediately. Such vacillation is not what placed into the Space Age, with the Cold War Even more threatening, the president's Columbus' name in our history books. behind us, we should be poised for a re­ budget book outlines the logic for termi­ We should be moving out into space, newed push toward the planets, our mod­ nating the Mariner Mark II spacecraft not canceling and turning off excellent ern cosmic ocean. program, NASA's modular, phased ap­ missions we've already invested in. Why Instead President Bush's fiscal year proach to exploring the small bodies and are CRAF and Cassini threatened after 1993 budget would devastate planetary _outer planets, which had been developed our scientists and engineers have brought exploration. Many planetary scientists in the 1980s to respond to cost con­ them this far along? It is partly because believe Bush would kill off planetary straints. Evidently, Bush's advisors tried the space station is gobbling up NASA's science to make room, within NASA's to kill the Cassini Saturn mission along limited funds while we ignore any option constrained budget, for a Moon/Mars­ with CRAF until NASA's top brass ob­ of using the Russians' Mir for our life only endeavor oriented around the "Star jected; but the administration has sig­ sciences research. Also "Star Wars" mil­ Wars" technology of mini-spacecraft. naled its intention to kill it next year. itarists, having no more Soviet enemy, Bush wants two new lunar orbiters that This is a shortsighted and stupid way are turning to space as a new playground would do a fraction of what the proposed to run the US space program. Space ex­ for their high-tech bombs, lasers and ag­ Lunar Observer could do, and would be ploration requires consistent long-term ile mini -spacecraft. Planetary scientists operated by the Exploration Office of planning, protected from annual politi­ fear that Vice President Quayle's Na­ NASA (Code X), not the Space Science callbudgetary ups and downs, but Bush tional Space Council and Bush's Office Office (Code S). The goal of Code X is doesn't understand. "Think small, think of Management and Budget may be to further the President's focused goal of less, retreat!" is the message from the heeding such "Star Wars" priorities. . sending human beings to the Moon and White House. Imagine Queen Isabella Maybe that's why they won't spend Mars, much narrower than the broad sci- calling out to Columbus to tum back and more than 3 percent of NASA's growing . entific exploration goals of Code S, just explore the Azores. budget on planetary science and explo­ which address the miracles of the uni­ NASA continues to fight fires, abro­ ration and would thus kill approved verse and the lessons the study of other gate commitments and treat forefront re­ and/or operating missions. worlds has for understanding our own. search as an expendable frill. The US International Space Year is an Ameri­ The Bush budget would mark the and Europe might collaborate on a comet can presidential election year. Though most serious retrenchment ever in Unit­ sample-return mission to replace CRAF, Bush's decisions could be reversed by ed States solar system exploration. Con­ but comet scientists are skeptical: This Congress, it might be especially effec­ gressional . approval would create two year's cancellations are what's real, not tive if other presidential candidates could terrible precedents: fanciful missions slated for the out-years be convinced to formulate decent space • Cancellation of the Comet Ren- on NASA's planning charts. No doubt policies and get a debate going. D 27 .".----_.- ...._. -... _-----_.------_._-

Questions Answers

Is the martian atmosphere sufficiently only 7.5 millibars. Hence these com­ How do you fly into outer space? dense to burn up infalling material? mon meteors would certainly appear as -Cassandra Besser, age eight, Cal­ Would a person be able to see "shoot­ "shooting stars" to an observer viewing abasas, California ing stars" from the surface? the night sky from the martian surface, -Roderick S. MacDonald, Brox­ unless a martian dust storm was taking The short, simple answer to your ques­ burn, Scotland place at the time! tion is that we fly into outer space by Given that Venus and Titan both going very fast. Think about it this On Earth, most meteors "bum out" at have constant, ubiquitous "cloud" cov­ way: If you throw a ball straight up in­ altitudes of about 70 kilometers (about ers, and all other objects in the solar to the air, it will rise a certain distance, 40 miles). The atmospheric pressure system have either very tenuous atmo­ then fall back. If you throw it harder (in there is approximately 0.1 millibar. spheres, no atmospheres or no solid other words, faster), it will go higher. If (The standard sea level pressure is surfaces, Mars and Earth appear to be you throw it upward fast enough, 1,013 millibars.) This same pressure the only two bodies where meteors can Earth's gravity, which becomes weaker level of 0.1 millibar is reached at an al­ be viewed in this way. as you go higher, will no longer be titude of 40 kilometers (about 25 miles) -TOBIAS OWEN, University of strong enough to pull the ball back. on Mars, where the surface pressure is Hawaii Then the ball escapes into space. To es- I I

-.

Two years ago, Ruben E. Lianza, then with the Argentine Air Force All but 2 of the 10 meteoritic gouges form a corridor about 30 kilome­ and now with L TV Aircraft Group in Dallas, discovered these gigantic ters (20 miles) long and 2 kilometers (about 1 mile) wide running skid marks from the air. This close-up shows the small gouge in the northeast to southwest north of the Rio Cuarto, 560 kilometers (350 photo at right. The arrow points to Captain Lianza, who is standing miles) west of Buenos Aires. The largest depression is 4.5 kilometers 28 inside the gouge. Photographs: Ruben E. Lianza (3 miles) long and 1 kilometer (0.6 mile) wide. The small gouge (see arrow) is shown enlarged at left for scale. cape Earth's gravity, an object has cent. Saturn's rings are unique in FACTI NOS to go about 40,000 kilometers their brightness; the ring particles (25,000 miles) per hour. You and I of Jupiter, Uranus and Neptune are Alexander Wolszczan of Cornell Univ.ersi­ ty's National Astronomy and Ionosphere can't throw anything that fast, but very dark, containing a substantial rockets can and that is why we use amount of the rocky and/or car­ Center and Dale A. Frail of the National Radio Astronomy Observatory in Socorro, them to go into space. bonaceous material that is abun­ New Mexico, have found what they think is If we want to go to the Moon or dant in asteroids and comets. the first definitive evidence of planets orbit­ Mars, we make our rockets go fast The stability of Saturn's rings enough to escape Earth's gravity despite a long list of rapid removal ing a star outside our solar system. The researchers used the Arecibo radio and we make sure that we are processes has become a major area headed in just the right direction to of study. Ironically, sublimation, telescope in Puerto Rico to take precise timing get to the planet we want to reach. which was one of the first process­ measurements from the recently discovered pulsar PSR1257+ 12. They noticed a tiny If our aim is a little off, we fire our es suggested, is probably not im­ "wobble" that could be caused by orbiting rockets again to correct our course. portant because of the very low planet -sized bodies. Suppose that, rather than escap­ evaporation rate of water ice at the The two planets detected so far are 2.8 ing into outer space, you want to frigid temperatures of the outer so­ orbit around and around Earth the lar system. However, several other and 3.4 times the mass of Earth. Their re­ spective distances from the pulsar are 0.47 way the space shuttle does. We processes lead to "short time-scale can't do that by going straight up. problems" (survival times much and 0.36 astronomical unit (the distance be­ tween Earth and the Sun, about 150 million Let's think about throwing the ball ~horter than the age of the solar again. This time throw it as if you system) with all known ring sys­ kilometers), and they move in almost circu­ were tossing it to a friend. If you tems: lar orbits with periods of 98.2 and 66.6 days. -from Nature throw it at a certain speed, it will I. Incessant meteoroid bom­ fly along a curved path for some bardment tosses chips and dust all distance before it hits the ground. around the rings. Most of these If you throw it faster, it will travel merely land elsewhere in the rings, Andrew Lyne of the University of Man­ chester's Nuffield Radio Astronomy Labora­ along a bigger curve and hit the but each impact vaporizes some ground farther away. Remember fraction of ring material and this tories in England now reports that the dis­ covery he announced last July of a planet that Earth is round. If you get might fall into the planet. above the atmosphere and throw 2. The rings may absorb enough orbiting a distant pulsar has turned out to be erroneous. (See the September/October 1991 the ball fast enough, the curved meteoroid mass to "weigh them path it follows becomes as big as down" and cause them to fall into Planetary Report.) "The planet just evaporated," he said, , Earth and it never hits the ground. their planets in 10 million to 100 Then it is in orbit. To do that it has million years. because changes in signals from a distant pulsar -which he and some colleagues to go about 30,000 kilometers 3. Because meteoroid material is (18,000 miles) per hour. You and I dark (remember Halley's Comet), thought were caused by a planet revolving around the pulsar-turned out to be caused can't throw things that fast either the large amount of this being but, once again, rockets can and mixed in with ring material makes by Earth revolving around the Sun instead. However, Alexander Wolszczan said that's why the shuttle uses rockets it very hard to understand how Sat­ Lyne's misfortune "does not change my to get into orbit. urn's bright rings could remain so thinking about what I have found." Other -J.R. FRENCH, JRF Engineering "clean" unless they are less than experts who have examined Wolszczan's Services 100 million years old. data hailed his evidence as the best yet of 4. Gravitational interactions be­ other planets. In a recent Parade article, Carl tween rings and their nearby shep­ -from the Los Angeles Times Sagan wrote that "the bright rings herding satellites cause spiral den­ of Saturn are composed mainly of sity waves much like those seen in frozen water and could be de­ galaxies. These interactions are scribed as made of snowballs or thought to cause the rings and The explosion of a giant meteorite as it ice balls." satellites to evolve away from their grazed Earth less than 10,000 years ago I remember reading that if Sat­ current locations in a similarly gouged a chain of elongated depressions in urn's rings were made mainly of short time. the Argentine pampas, scientists say. ice and were as old as they are Because these processes affect The energy released by the meteorite, said to be, the ice would, by this all ring systems, many scientists which was 150 to 300 meters (500 to 1,000 time, have disappeared by subli­ now believe that rings are trans­ feet) in diameter, appears to have been mation. ient phenomena. They must be re­ equivalent to the explosion of 350 million .-Bolton Davidheiser, La Mira­ created sporadically over the aeons tons of TNT. This is 30 times larger than da, California when moons are destroyed by un­ the energy thought to have been released usually large impacts. The Cassini in 1908 by the explosion of a mysterious The very bright ring particles of orbiter currently being planned by object over Tunguska in Siberia. Saturn are almost pure water ice, NASA and the European Space Laboratory tests show that the object although their slightly orange­ Agency will make measurements approached Earth at an angle of less than brown color shows that they must that allow us to determine the age 7 degrees. Peter H. Schultz of Brown Uni­ contain some other material, such of Saturn's rings and establish how versity, one of the scientists investigating as silicates or primitive organic they were created. the event, said that it exploded as it did so, material. The amount of non-icy - JEFFREY N. CUZZI, NASA gouging the landscape. material may be as small as 1 per- Ames Research Center - from Walter Sullivan in The New York Times 29 Worlds of Wonders

Contact Hugg-A-Planet Earth Lithograph Curl up with Mother Earth, This colorful. 14-inch-diameter pillow of our home A limited-edition lithograph planet is a fun way to brush up on of the art created for the geography, It's great for resting or just cover of Carl Sagan's novel tossing around. Place one on the sofa to about first contact with add a global touch to your decor. 3 Ib, extraterrestri als. The painting #526 " " "" " """""" "" $15,00 shows Earth looming on the outskirts of a distant galaxy, Artist Jon Lomberg has produced only 950 of these prints, each signed and numbered. Must be shipped TNT Air outside the United States. 19" x 24". 21b. #306 ...... , " '" $48,00

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Book, Books, Books The Planetary Society offers a wide range of books on planetary science and space exploration. Here is a selection of just four of them: The Peterson First Guide to the Solar System, by Jay M. Pasachoff, is an in-depth examination of our solar system, suitable for both beginning and knowledgeable observers, In the Stream of Stars represents a joint venture by Russian and American space artists, Edited by William K. Hartmann, Andrei Sokolov, Ron Miller and Vitaly Myagkov, it presents full-color reproductions of nearly 200 paintings from art workshops sponsored by The Planetary Society. The New Solar System. edited by J, Kelly Beatty and Andrew Chaikin, has become a standard reference work on our solar system, Wanderers in Space, by Kenneth R. Lang and Charles A. Whitney, presents the results of spacecraft missions throughout our solar system, Peterson First Guide to the Solar System 218 pages lib. #101 , " , , " " ' " ,$4,50 In the Stream of Stars 183 pages 41b, #124 , " , , ,' " ",$27,00 The New Solar System 326 pages 41b, # 180 , , , , , " , , , ,, $22,50 Wanderers in Space 316 pages 31b, #197, , , , , " , , " ,$24,00 A Planetary Miscellany

Exploring the Universe Note: Although we have fewer On Robot Wings - 1992 Calendar items listed here this month, A Flight Through the Solar System Produced by the creators of Astronomy most of the merchandise Take a trip to the outer reaches of the solar magazine in cooperetion with The Planetary system-without leaving your living room. SOCiety. Each month features a striking full-color that has appeared in previous Produced by the Jet Propulsion Laboratory to photograph or painting. and notes dates issues of The Planetary Report assist in the study of terrains. this collection of six important to the history of space science. is still available, short video clips uses computer-generated Columns of text cover subjects ranging from graphics to simulate low-altitude flight through supernova 1987 A to Isaac Newton' s conception For a complete list of items, three-dimensional landscapes. of universal gravitation. 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A full-disk picture of Neptune features the Great Dark Spot Dive Europa T-Shirt Protect your c opies of The Planetary Report and high white c louds of methane. A mosaic of while keeping them handy for future reference. Triton . Neptune's largest moon. shows the widely Why "Dive Europa"? Th is moon of Jupiter may Our handsome blue-and-gold binder will hold varied terrain. extensive nitrogen ice cap and conceal a vast global ocean beneath its two years' worth of issues. 2 lb. evidence o f active geysers. 1 lb. cracked. icy surface. In the distant future #545 ...... $10.00 #331 ...... $10.00 explorers may use special diving gear to help Two binders (4 lb.) for $18.00! unlock the secrets of this world. a nd even An Explorer's Guide to Mars search for water-based life. Earth From Space 100"10 cotton. Sizes M. L. XL. 1 lb. 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Perfect painting by Jon Lomberg depic ts the galaxy for the beginning stargazer. and popular with from slightly above the spiral arms. with the older Th is 3~' x 35 '. full-color poster outlines human amateur and professional astronomers and redder galactic core in the distance. From achievements in space. depicting the solar worldwide. Comes with a c learly written a limited edition of 950. each print signed and system and some of the spacecraft that have activities handbook. three helpful accessories numbered. Must be shipped TNT Air outside the explored it. Comes with a 3D-page guide. 2 1b. and a one-year warranty. 2 1b . United States. 17' x 24'. 2 lb. #334 ...... $11.00 #548 ...... $45.00 #329 ...... $38.00 0 R D E R F 0 R M

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Space artist Arthur Gilbert lives and works in the United Kingdom. He is a member of the Inter­ national Association for the Astronomical Arts, and his work has appeared in the Smithsonian's National Air and Space Museum and in Space World and Ad Astra magazines.

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