The Planetary Report Volume 15 Issue 4

24 Pages·1995·5.6 MB·English

Table of Volume XV
On the Cover:
Number 4
Neptune hangs on Triton's horizon in this three
Contents
dimensional view created from Voyager images. July/August 1995
While this view is dramatic, it isn't what an
explorer would actually see while flying over
Triton: The relief has been exaggerated about
30 times. In the foreground are Triton's maria
("seas"), with terraces testifying to several
episodes of volcanic flooding- only the floods
were of molten ice, not rock. Voyagers greatest Features 16
legacy is that the spacecraft revealed strange Basics of
worlds such as this to us for the first time. Spaceflight:
4 What's NelN, Voyager: Making Tracks
Image: United States Geological Survey, Flagstaff
The Discoveries There are few landmarks and signposts in
Continue space, so how do people stuck on Earth
Launched nearly two decades ago, the guide a spacecraft to its destination? We
Frown Voyager spacecraft still have much to teach tackle that question with a short discussion
us. Data from the six planetary encounters on determining speed and position, and
The
continue to be mined for more discoveries, navigating in space.
and the spacecraft go on operating, search
Editor 18
ing now for the edge of our solar system. NelNs and
RevielNs
10 Journey to the End NASA was created to expand the bound
A. s we go to press, NASA is fac
of the Dinosaur Era: aries of science into the solar system and
~ng what could be the greatest
A Society Expedition beyond. But the budget cuts now proposed
crisis in its existence. As you'll read in
to Belize for the agency will squeeze planetary sci
World Watch, the agency is threatened Planetary science has laid claim to the entists past the point of simple discomfort.
with budget cuts that could destroy its solution of one of the most perplexing Many of the best are leaving the field, as
vitality. Ifthe budget proposed by the mysteries of the natural world: What killed our faithful colunmist-himself a planetary
United States House of Representatives off the dinosaurs? An extraterrestrial object scientist-reports.
becomes reality, what survives as is now the leading suspect, and here we
19
NASA may be a pale shadow of the report on a Planetary Society expedition Society
agency that took us to the planets. in search of evidence in the investigation. NelNs
Another agency of explorers is in even While others critique the new information
more danger: Some in Congress are technology, the Society is using it: We're
Departl'l1lent s
threatening to eliminate the US Geolog holding an on-line conference with the
ical Survey. Through its branch of astro 3 leaders of our Belize expedition. We're also
geology, the USGS is a major force in Members' forging ahead with our Red Rover, Red
planetary exploration, and its loss Dialogue Rover project, holding a space science
Mining the Moon, a perennially hot topic, workshop with the United Nations and
would be a body blow to those endeav
has again generated debate among our the European Space Agency, preparing to
ors supported by The Planetary Society.
members. Our rover project with the Jason celebrate Galileo's arrival at Jupiter and
Things are worse in Russia. Mars
Foundation and our support for a Pluto publishing another newsletter. Plus, our
Together, the program supported by the
mission have also prompted letters. members have met the Micron challenge!
Society, appears to be in trouble. The
Russian Space Agency just doesn't 20
15 World Questions and
have enough money for all its projects.
Watch AnslNers
This year could mark a downward
NASA is under fire from budget-cutters, Radiation belts surround many of the
turn in planetary exploration. What can which is a not uncommon occurrence, but worlds in our solar system, affecting their
you do to stop it? First, let your govern this time the attack could be devastating. interactions with the pervasive solar wind.
ment know that you support planetary Adding to the trouble is a delay in the We explore those regions and revisit the
exploration. Then reaffirm your support Mars Together program. It is truly a time collision of comet Shoemaker-Levy 9
of the Society. Our voices joined will of crisis in planetary exploration. with Jupiter.
be more effective in the fight to preserve
Earth's ability to explore the planets.
- Charlene MAnderson
"~
The Planetary Report (ISSN 0736-3680) is published bimonthly at the editorial offices of The Planetary Society, ~5 North Catalina Avenue,
Pasadena, CA 91106-2301, 818-793-5100. It is available to members ofThe Planel.ary SC?clety. Ann!-lal dues In the US or Canada are
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Editor, CHARLENE M. ANDERSON Technical Editor, JAMES D. BURKE THE-PL.A~T1RY SOCIETY
Assistant Editor, DONNA ESCANDON STEVENS Copy Editor. GLORIA JOYCE -e-
Production Editor, MICHAEL HAGGERTY Art Director, BARBARA S. SMITH o .~ cP -B- 0
Viewpoints expressed in columns or editorials are tho~e of the authors and do not necessarily represent positions of The Planetary Society,
its officers or advisors. © 1995 by The Planetary Society
Members'
Board of Directors
CARL SAGAN
President
Director, Laboratory for Planetary Studies,
Dialogue Cornell University
BRUCE MURRAY
Vice President
Professor of Planetary Science,
California fnstitute of Technology
LOUIS FRIEDMAN
Executive Director
NORMAN R. AUGUSTINE
PreSident,
Lockheed Martrn Corporation
JOSEPH RYAN
Executive Vice President and
General Counsel, Marriott International
STEVEN SPIELBERG
director and producer
Jason views on the Friends ofP luto and over the quarter of a million miles
LAUREL L. WILKENING
Recently I had the pleasure of Charon concept. I would also like that separate us. Chanceflor, University of California, Irvine
attending an interactive session to point out that in 1991 we pub The Moon is a pristine wilder Board of Advisors
sponsored by the Jason Foundation lished a letter by Keith in Members' ness, to be sure. By the very fact DIANpEo AetC aKnEdR aMutAhoNr
and The Planetary Society at a PIN Dialogue in which he suggested a of its low gravity, it will never be BUZZ ALDRIN
Apollo 11 astronaut
(Primary Interactive Network site) Pluto/Charon mission using a US able to hold an atmosphere for
RICHARD BERENDZEN
near my home. (See the May/June built spacecraft and a Russian any length of time, so terraforming educator and astrophysicist
1995 issue of The Planetary launch vehicle. it will never be a viable option. Chief ScJiAenCtiQstU, ECSe nBtrLeA NMaOtioNnaTl
Report.) Not only was the presen - Louis D. Friedman, Therefore, humans must always d'Etudes Spatia/es, France
RAY BRADBURY
tation interesting and informative, Executive Director live underground or in small sur poet and author
but the student displays of data on face establishments. Even with ARTHUR C. CLARKE
author
Hawaii, volcanism and plate tecton Moon Mining these and the mining efforts to
CORNELIS DE JAGER
ics were well done and indicated a I read with interest the letters in support them, on the large scale Professor of Space Research,
The Astronomical Institute at Utrecht,
lively interest in scientific subjects. the March/April 1995 issue of The the "magnificent desolation" will the Netherlands
FRANK DRAKE
The final three minutes of the Planetary Report. While I agree prevail for all time. Professor of Astronomy and Astrophysics,
University of California, Santa Cruz
presentation were devoted to the with the spirit, if not the entire -JEFFREY E. HOYLE,
JOHN GARDNER
Mars Rover. A 14-year-old student content, of Marge Currie's letter, San Jose, California founder, Common Cause
got the chance to drive the rover, I take issue with that of D. Downs. MARC GARNEAU
Canadian astronaut
and she did very well. I want to While the Moon belongs to no I was disappointed at D. Downs' GEORGIY GOLITSYN
lnstitute of Atmospheric Physics,
take this opportunity to thank the one in particular, it belongs to the letter in the March/April Members' Russian Academy of Sciences
Society for enabling me to take part. human race in general, by virtue Dialogue. If the Moon's resources THEODORE M. HESBURGH
President Emeritus,
It certainly was time well spent. of the fact that Earth is its primary. belong to no one, then like all University of Notre Dame
-EDWARD J. WARD, Hicksville, If Downs is concerned that we other resources throughout history SHIRLEY M. HUFSTEDLER
educator and jurist
New York will turn the Moon into a slag they will eventually belong to GARRY E. HUNT
space scientist, United Kingdom
heap, I say that it is already a slag whoever puts them to use or
SERGEI KAPITSA
Pluto"s Pals heap by its very nature. It has no seizes them by force. The rain {flsutute for Physical Problems,
Russian Academy of Sciences
Viva Louis Friedman for his leg ecology or fragile environment. forests are being burned down for
GEN. DONALD J. KUTYNA (RET.)
work in lobbying NASA since Downs, as a member of the farmland by people who have no former Commander, US Space Command
1993 to accept a joint Russian Society, should know better than wish to see their children starve Director,J SOpHacNe MPo.l iLcyO GInSstDituOteN,
George Washington University
United States Pluto probe, using anyone that mining the Moon for and who are receiving little con
HANS MARK
each side's strengths-a Russian terrestrial benefit will never be an sideration from the "planetary The Universify of Texas at Austin
Proton arid a lightweight US space economical proposition due to community." JAMES MICHENER
author
probe. The Planetary Society transportation costs, engineering If we were to go after the Moon
MARVIN MINSKY
deserves our congratulations for difficulty, and so forth. If the with everything we've got, from MediaTo Asrhtsib aa nPdr oSfceiessnocer so,f .
Massaehusetts Institute of Technology
seeing this through to the Gore Moon were made of solid gold and mining equipment to nuclear
PHILIP MORRISON
Chernomyrdin text. (See the studded with diamonds, it would warheads, it would look exactly Institute Professor,
Massachusetts Institute of Technology
September/October 1994 issue not make sense to mine the ore and like-the Moon. The difference
PAUL NEWMAN
of The Planetary Report.) bring it back, since even the lowest would only be whether its vast actor
The Pluto/Charon probe is still grade terrestrial ore would be more material, territorial and gravitation formeJr UDNire NctIoSrH GIMenUeRraAl,
Institute of Space and
under threat from financial squeeze economical to process. Rather, al resources would go to feed those Astronautical SCience, Japan
on both the US and Russian sides. when we do mine the Moon, the starving children (and save the BERNARD M. OLIVER
Sen/or Technical Advisor,
Planetary Society members need material harvested will be used rain forests) or continue to sit, SETI lnstitute
to organize something like the there to build a new society and untouched and useless. I find little SALLY RIDE
Director, California Space Institute,
Mars Underground-maybe the construct the infrastructure that poetic inspiration in moonlight University of Caanldif ofronrima,e Sr aans trDoinegaou,t
Friends of Pluto and Charon. We will ultimately set us on the path to when it shines on poverty, suffer ROALD Z. SAGDEEV
former Director,
need to petition NASA to give this the planets. ing and environmental destruction. Institute for Space Research,
Russian Academy of Sciences
mission priority over less time Downs spoke of destroying the -E. WEBER JONES,
HARRISON H. SCHMITT
constrained space science projects. "poetic inspiration for humankind," Cincinnati, Ohio former US Senator, New Mexico,
and Apollo 17 astronaut
-KEITH GOTTSCHALK, as if these activities would be visi S. ROSS TAYLOR
Professorial Fellow,
Claremont, South Africa ble to the unaided eyes of the poetic Please send your letters to Members' Australian Nalional University, Canberra
dreamers. Aside from possible Dialogue, The Planetary Society, 65 North JAMES VAN ALLEN
Professor of Physics, University of Iowa
This is a good idea, and I invite minor changes in albedo, there Catalina Ave., Pasadena, CA 9110 6-2301
Society members to share their would be no visible manifestation or e-mail [email protected]. 3
__
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by Ellis D. Miner
Nearly six years have elapsed since Voyager 2 beyond the heliosphere. Perhaps just as interesting and
flew by Neptune, and we will soon celebrate the important are discoveries being made from continued
15th anniversary of Voyager 1's final planetary analysis of data taken during Voyager's six giant-planet
encounter (Saturn). Yet it is far more than nostalgia that flybys (two at Jupiter, two at Saturn and one each at
keeps these hardy space robots on our minds. Uranus and Neptune). When Galileo begins its detailed
This most successful of all planetary missions is still atmospheric probe and orbital mission this December, its
very much alive. NASA's Deep Space Network-a data will begin to supplant Voyager's as the premier source
system of three tracking complexes, one near Canberra, of jovian information. The arrival of Cassini at Saturn in
Australia; one near Madrid, Spain; and one at Goldstone, 2004 will cause a similar evolution for Saturn data. There
California- continues to receive transmissions from are no plans now to send orbiters to Uranus or Neptune,
Voyager 1 at the rate of about 120 hours of science and so, for these distant neighbors, Voyager data are likely to
engineering data per week. Voyager 2, which shares the remain the defmitive data set for the foreseeable future.
DSN's southern hemisphere coverage with Galileo, never Voyager's voice has been heard in many places. Theo
theless provides about 90 hours of data per week. It is retical analyses built around boundary conditions provid
anticipated that these data collection rates will continue ed by the Voyager data continue to provide unprecedented
well past the turn of the century. insights into the nature of the Jupiter, Saturn, Uranus and
Some of the new data are intriguing in their implica Neptune systems. Any theories that predict results incon
tions for the nature of the magnetic bubble blown by the sistent with the Voyager observations are either discarded
4 Sun (the heliosphere) and of the interstellar medium or revised. Ground-based and Earth-orbital observations
Voyager 2 completed its
reconnaissance of the
giant-planet systems in
August 1989 with its flyby
of Neptune and Triton.
It thus accomplished the
most ambitious planetary
mission yet flown from
Earth. As they now follow
paths leading out of our
solar system, the two
Voyagerspacecrafl
continue to explore and
to relay data back to
their home world.
computer graphic courtesy of
Charles Kohlhase, JPLlNASA
of the gas giants and their systems, although of poorer items of information that have been discovered within
resolution than Voyager observations, nevertheless are the past five years.
stimulated by and benefit greatly from the understand
ing provided by Voyager data. Studies of the satellites Edgi"gToward the Heliopause
of the giant planets by the Hubble Space Telescope and An early 1995 mission status report from NASA's Jet
ground-based instruments are good examples of this Propulsion Laboratory said that the Voyager spacecraft
aspect of Voyager influence. "are using their ultraviolet spectrometers to map the
Finally, the great success of the Voyager mission has heliosphere and study the incoming interstellar wind
positively influenced the scientific objectives and obser [a stream of electrons and protons blowing in from
vation designs of the Galileo and Cassini missions. nearby hot stars]. The cosmic ray detectors are seeing
While some of their objectives are general enough to the energy spectra of interstellar cosmic rays in the outer
have been generated before the Voyager encounters heliosphere. The magnetometer sensors are still measur
of Jupiter and Saturn, the majority are based either on ing the strength and direction of the solar magnetic field.
questions raised or on questions left unanswered by the The plasma detectors looking back at the Sun record the
Voyager observations. solar wind parameters. The low-energy charged particle
I have chosen to concentrate on only two aspects of experiment studies the energy spectra of particles
Voyager's legacy here- new data on the heliosphere coming from the Sun. The plasma wave instrument is
and new conclusions drawn from Voyager's giant studying the incoming signals from the direction of the
planet flybys. I will also limit the discussion to those heliosphere. " 5
Right: Four spacecraft are now searching for the edge of our
solar system, a boundary called the heliopause where the
solar wind blowing from the Sun gives way to the stream
of protons and electrons blowing in from the stars. The
plasma wave instruments on the Voyager spacecraft
have detected intense low-frequency radio emissions
that may signal the spacecraft's approach to the
heliopause. The fastest moving craft, Voyager 1,
may reach this boundary early in the coming
century. Chart: JPUNASA
Above: The Galilean moon 10 has proven to be the most volcanically active
body in our solar system. Since its 1979 discovery, 10 has become a prime
target for theoreticians and observational astronomers. Pele, the first dis
covered and largest ionian volcano, sits in the middle of the horseshoe
shaped feature in this image. The feature has been formed by material
erupted from the caldera in the center. The moon's colors, ranging
from white and yellow to orange, red and black, are all derived
from compounds of sulfur erupted from beneath the surface.
Right: In the years since Voyager 2 completed its Neptune flyby,
scientists have continued to process, analyze and, in this case,
"stretch" the data collected by the spacecraft. This stereographic
projection of the moon Triton shows the south polar cap as a
human eye would never see it. A bright fringe around the cap
becomes very apparent in this stretch; it's probably composed
of fresh nitrogen frost or snow. The rays pointing out from
the fringe are frozen nitrogen blown out by winds from the
south. Images: United States Geological Survey, Flagstaff
As indicated in this status report, six of Voyager's Measurements of radio bursts in the 2,000-to 3,000-
11 investigations are continuing to collect data about hertz frequency range are of particular significance in
the structure of the heliosphere. Each of the six provides distance estimates. In May and September 1991, Voy
a different piece of the puzzle. When one assembles that agers J and 2 and Pioneers 10 and 11 (launched about
puzzle, the message is clear: Both spacecraft are edging five years earlier but now at comparable distances from
ever closer to the outer boundary of the Sun's magnetic the Sun) detected massive solar bursts. Beginning in
influence. Some data even lead to estimates of where 1992 and continuing until the present, Voyager's plasma
the boundaries are. wave instruments saw enhanced (stronger and more
Two successive boundaries are relevant. The fITst, called spread out in frequency) radio emissions. We now be
the termination shock, marks the distance from the Sun lieve that interaction of the solar material with incoming
where the electrons and protons streaming outward in the interstellar material at the heliopause generates these
solar wind slow abruptly from supersonic to subsonic emissions. By combining estimates of the outward
speeds. The second, called the heliopause, occurs where velocity of the solar wind and the retuming speed-of
the outward flow of the solar wind is stopped altogether. light radio bursts with the time delays involved, one
The heliopause also defines the outermost extent of the can estimate the distance to the heliopause.
Sun's magnetic field. Beyond the heliopause, the particle The distances to the telmination shock and the helio
environment is that of interstellar space. Inside the helio pause are not fixed, but vary both with time and with
pause, it is primarily the Sun that controls the environment. direction in the sky. Voyager 1 and Pioneer 10 are each
Below: During the 1986 encounter, Uranus'moon Miranda stole the show with its strangely
contorted face. In this image, two of its puzzling coronae are seen (top right and bottom).
To explain such features, scientists once suggested that the moon had been blasted
apart by impacts and reassembled by gravity. Now they believe a more mundane
explanation of melting ice and upwelling may account for Miranda's appearance.
Image: United States Geological Survey, Flagstaff
between 60 and 65 astronomical units (l AU equals
149,597,900 kilometers, or about 93 million miles)
from the Sun. However, Voyager 1 and Pioneer 10
are traveling in roughly opposite directions in the
sky. Pioneer 10 sees no evidence of the approach
ing boundaries, whereas such evidence is plentiful
on Voyager 1. Even Voyager 2, at a distance of
about 48 AU, appears closer to the heliopause than
Pioneer 10.
The heliosphere is compressed and smaller in the
direction of the Sun's motion relative to nearby hot
stars. That direction is in the constellation Hercules
near its boundary with the constellation Lyra. Both
Voyager spacecraft are leaving the solar system in
that same general direction; Pioneer 10 may be trav
eling down the "tail" of the Sun's magnetic field.
Solar sunspot activity waxes and wanes on an
II-year cycle. When the solar activity is high, the
heliosphere expands; when solar activity is low, it than a million amperes of current flow in this flux tube.
shrinks. Estimates based on Voyager measurements place
the heliopause distance (in the Voyager 1 direction) at The Saturnian System
between 110 and 160 AU; termination shock distances Saturn was Voyager l's last planetary encounter; it flew
vary from about 70 to 115 AU. Voyager 1 will reach through the system in November 1980. Voyager 2 made
70 AU in 1998 and 110 AU in 2009, but will not reach its flyby in August 1981. Saturn's tantalizing moon Titan,
160 AU until 2023. Using the declining output of their with its thick, nitrogen-rich atmosphere, was a primary
nuclear power sources, the two spacecraft will continue target for investigation, but the spectacular ring system
to transmit useful scientific data until 2015 to 2020. stole the show. Since Voyager, we have discovered that
Voyager data also suggest that the termination shock all the giant outer planets possess rings. Once distin
and heliopause boundaries may not be very sharp. At dis guished from the other planets because of its rings, Saturn
tances beyond the orbit of Neptune (30 AU), electrically has become the paradigm for the study of ringed planets.
neutral particles from interstellar space are electrically Following the discovery of ring arcs (partial rings)
charged by interaction with sunlight and become a part around Neptune in the mid-1980s, searches began for
of the solar wind. This process slows the solar wind per such arcs in the other ring systems. Similar arcs were
ceptibly and increases its density. It is the increase in found in the Encke gap of Saturn's A ring. Because the
density that results in "smearing" of the boundaries. same arcs were identified in both Voyager 1 and Voyager
Another interesting discovery is the observation of dust 2 images, these unusual ring features must be stable over
impacts with the spacecraft every few thousand seconds. a period of at least nine months. Detailed study of images
These imply a density of about one dust particle per cubic of the edges of the Encke gap and spiral density waves
kilometer in the outer solar system. Plasma wave scien near those edges led to a prediction that they were caused
tists will continue to monitor these dust impacts. The two by a small moonlet at a determinable position. Other
spacecraft progressively depart farther and farther from images were then searched, resulting in the discovery of
the ecliptic plane in which the planets orbit the Sun. The tiny Pan in the Encke gap. Pan's diameter is estimated to
distribution of dust density with respect to the ecliptic be 15 to 25 kilometers (9 to 16 miles). With the discovery
plane may help determine the source of the dust. of Pan, Saturn's known satellite count is now 18.
Careful study of Voyager and ground-based images
The Jovian System of Saturn's extensive E ring has led to the conclusion
Voyager 1 encountered Jupiter in March 1979; Voyager 2 that E-ring particles have a very narrow size range of
followed only a few months later, in July. Among their 1 plus or minus 0.3 micrometers in radius. This is an
more famous discoveries were a ring around the planet important conclusion for the Cassini Saturn orbiter,
and the erupting volcanoes ofIo, the innermost Galilean which must pass through the E ring more than a hundred
satellite. Since then 10 has been recognized as the most times during its four-year orbital tour. Particles of such
volcanically active body in the solar system, and research small size present no hazard to the spacecraft.
since the encounters has understandably focused on that Saturn's E ring is known to be densest near the orbit of
unusual moon. the moon Enceladus. This has led to the speculation that
Shortly after the Jupiter encounters of Voyagers 1 and the tiny ice or dust particles that make up the E ring may
2, the infrared interferometer spectrometer (IRIS) team originate from "water volcanoes" on the moon. Analysis
published evidence that one of the dark "lava lakes" on ofthe Voyager imaging data and theoretical considera
10 was warmer than its surroundings, perhaps by as much tions now suggest another possibility. Dust particles in
as a few hundred degrees Kelvin (about 500 to 1,000 the E ring collide with eight of Saturn's satellites: Mimas,
Fahrenheit degrees). A more extensive analysis by the Ence1adus, Tethys, Telesto, Calypso, Dione, Helene and
team shows 11 more hot spots identified from IRIS data. Rhea. Some of these impacts are of high enough velocity
Over the years, ground-based measurements ofIo that many new dust particles are launched into Saturn
occasionally have indicated that 10 is brighter than normal orbit. The theory also accounts for the density peak near
for about 10 minutes following its exit from Jupiter's Enceladus. The E ring may thus be self-sustaining.
shadow. One interpretation of this phenomenon is that Ring systems of the four major planets appear to be
sulfur dioxide (S02) condenses as a thin layer of ice on much younger than their planets, according to Voyager
the surface during the cooling caused by the shadow of data and theoretical considerations. While the planets
Jupiter. Recent analysis of images of the dark face ofIo may have had ring systems since shortly after their for
(illuminated by reflected sunlight from Jupiter) has shown mation, the present ring systems must be regenerated by
that it is bluer than the sunlit side. This is what would be ongoing processes. The primary source of ring material
expected if the surface were coated with sulfur dioxide is likely the collisional breakup of small moons, which
frost. The findings therefore provide evidence for con themselves may be the product of the breakup of larger
densing sulfur dioxide frost on the non-sunlit face ofIo. moons.
It has long been known that 10 influences dekametric Continued analysis of Voyager data now confirms that
(having wavelengths of about 10 meters) radio emissions there is a 99 percent probability that the amount of argon
from Jupiter. Studies of changes in the Jupiter aurora in Titan's atmosphere is less than 6 percent. This has
(northern lights) now show that those changes also are important implications for the design of the Huygens
influenced by the position ofIo in its orbit. In both cases, probe's descent to Titan's surface in 2004. The probe,
the connection is probably through the 10 flux tube built by the European Space Agency, will be carried to
(a flow of electrons between 10 and Jupiter). More Saturn as a part of NASA's Cassini mission.
----------------------------------------
--
Within Saturn's bright, icy rings is a dark division-usual/y cal/ed the Encke
gap after Johann Franz Encke-that is relatively free of ring particles. If you
look closely at this series of images, you can see what has been sweeping
this gap clean for centuries: a small moon named Pan. Voyager 2 took
these images in 1981, but Pan was not discovered for 10 more years,
after painstaking investigations to determine what caused the gap.
Mark Showalter predicted where a moonlet should be, and when
he checked back through the Voyager images, there it was.
Images courtesy of Mark Showaller
the time of encounter, this beautiful blue world
was marked in its southern hemisphere with a
giant storm, called the Great Dark Spot because
of its resemblance to the famous Great Red Spot
on Jupiter. The jovian spot has been around for
at least 300 years-as long as people have been
able to see it. Neptune's spot seemed remark
ably similar to Jupiter's, so some assumed it was
a long-lived feature. But in June of 1994 the Hub
ble Space Telescope looked for it, and it was
gone. Then, a few months later, a nearly identical
spot appeared in the northern hemisphere.
Neptune is an extraordinarily dynamic planet
that continues to surprise us. Its largest moon,
Triton, provided surprises of its own during en
Just beyond Titan is Saturn's moon Hyperion. Initial counter. Ice geysers erupt through the surface of this
studies indicated that Hyperion's rotation was completely icy moon in one of the strangest forms of volcanism yet
chaotic, changing in direction and speed each time the seen in our solar system.
moon passed Titan. More recent analyses point to a Triton's internal heat source is larger, relative to heat
long-term regularity, with a rotation period near 13 days. ing from the Sun, than that of any solar system satellite
except 10. It has been suggested that some of this heat
The Uranian System may arise from a solid-state greenhouse effect, in which
Uranus' bland, blue face was a bit ofa disappointment sunlight penetrates through relatively clear nitrogen ice
for scientists during Voyager 2's January 1986 encounter and heats the subsurface. Because the nitrogen ice is
with the planet. Its thin, dark ring system could not com opaque to the resulting infrared radiation, the heat cannot
pete with the glimmering disk around Saturn. But one escape, and heat buildup occurs.
member of the uranian system captured the attention of Triton's surface temperature has been refined to 34.5
scientists and public alike: Miranda, the moon with some degrees Kelvin (minus 398 degrees Fahrenheit), even
of the most bizarre terrain yet seen in our solar system. colder than previously thought. The warmest detected
Miranda's coronae (nearly rectangular areas of multi temperatures are found on the nightside of Triton, north .
concentric "racetrack" features at three locations on the of its equator. The derived temperature of38 degrees
surface) are continuing to offer a theoretical challenge Kelvin (minus 391 degrees Fahrenheit) provides the
to scientists. Initially, many thought that Miranda had first evidence for an area of Triton's surface that is
been shattered, and that the present satellite of Uranus devoid of nitrogen ice.
was reassembled gravitationally. It now seems more Neptune's moons have also been officially named
likely that the coronae were formed from partial local by the IAU; they are, in order from the planet, Naiad,
ized melting and upwelling of partly melted ices, rather Thalassa, Despina, Galatea, Larissa, Proteus, Triton
than from the total disruption of Miranda. and Nereid.
The International Astronomical Union's Nomencla So, even though the Voyager spacecraft completed
ture Commission has officially named the newly dis their last planetary encounter in 1989, they continue
covered satellites of Uranus. In order from the planet, to surprise and delight us, even while teaching us more
the moons are Cordelia, Ophelia, Bianca, Cressida, about the solar system in which we live. And if their
Desdemona, Juliet, Portia, Rosalind, Belinda, Puck, power sources and other subsystems hold out, we may
Miranda, Ariel, Umbriel, Titania and Oberon. have 25 more years of this most extraordinary mission
of discovery.
The Neptunian System
Neptune was the last encounter for Voyager 2 before it Ellis Miner is science manager for the Cassini mission
headed off in search of the edge of the solar system. At to Saturn at the Jet Propulsion Laboratory in Pasadena. 9
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.cllcll sRid chRn'lC is '1t""""Rl, ,4i.t". SRt"IMin R'It"ccd, at least 10 kilometers (6 miles) in diameter
that struck Earth 65 million years ago. Dust
qculu'lic titnC f"'fillcd thcit" nccd.
and debris from such a large impact, they
l$"t SUtnC CRSCS "'ct"c iwcuct"cnt,
suggested, would have enveloped the plan
~"ddcn chRn'lcs tnut"C camcnt, et and blocked out the Sun, shutting down
eWRnt budies ft"utn thc hCRucns did thc dccd. photosynthesis and cooling the surface dra
matically. The Alvarez team proposed that
this catastrophic change to the biosphere
brought about what is known as the K/T.
mass extinction.
4 s living organisms, we have a vested interest ql'ound ~ct"o ~ound
in the evolution of life and its intricacies. The The Alvarez theory met with much skepticism, and for
9
geologic record contains evidence of mass many people only the discovery of a huge crater would
extinctions, where large numbers of species that flour- supply the necessary proof. Ironically, such a discovery
ished for millions of years abruptly disappeared. Such was made in Mexico by a team of petroleum geophysi
extinctions pose a riddle whose cists led by Glen Penfield and Antonio Camargo at about
solution concerns us all. the same time that the Alvarez theory was presented.
Among the five major mass However, few heard ofthis discovery, and it took 10
~xtinctions that are recorded in years to rediscover the crater now believed to be the
the stratigraphic layers of our protagonist in this melodrama.
planet, the one that took place The crater, over 200 kilometers (120 miles) in diame
65 million years ago at the K/T ter, is located in the northwestern Yucatan peninsula in
boundary-the boundary between Mexico and has been named Chicxulub, "the devil's
the Cretaceous period and the tail," in the local Mayan dialect. (Chicxulub is also
Tertiary period that followed the name of a small town near the center of the crater.)
it- is perhaps the most relevant Overwhelming evidence has been gathered that points to
to hufuans. Chicxulub as ground zero of this catastrophe: Shocked
Over 50 percent of all living quartz (fractured crystals indicative of high-velocity
species became extinct at that impact), tektites (spherical glass droplets known to form
time. The dinosaurs, which had during impact), impact- .
reigned for over 150 million melted rock hundreds
years, disappeared, and mam- of meters thick that has
Walter Alvarez,
one of the dis· mals, which were represented by been radiometrically
coverers of the mere rodents at the time, assumed the evolutionary path to dated to 65 million years
iridium anomaly
in the boundary dominance. It is a challenge to explain the sudden disap ago, and tsunami wave
between the pearance of the dinosaurs. Certainly something significant deposits- all these have
Cretaceous and
happened that provided the opportunity for the emergence been found in or around
Tertiary periods,
was a welcome of mammals and eventually Homo sapiens. As we shall the crater.
participant in see, the phrase "Thank your lucky stars" has a very spe
this Planetary
Society expedi cial meaning when it comes to our own evolution. A ~ccn"rio
tion. Here he The first clue to the cause ofthe demise of the dino tOl' e~tinction
examines the saurs was found in Gubbio, Italy, in 1980 by a team of But the question is, did
rocks marking
the KIT bound scientists led by Luis and Walter Alvarez. Team members this impact produce the
ary in Belize. found a large concentration of iridium, an element rare on global catastrophe that
Photo: Lu Coffing Earth but relatively common in comets and asteroids, in caused the mass extinc
a layer of clay that marked the K/T boundary. They pro tion? Current theory
posed that such an unusually high concentration of iridi paints this picture:
10 um could only have come from an extraterrestrial object Immediately after the