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SPACE Digest                                     Volume 11 : Issue 116

Today's Topics:
       Electronic Journal of the ASA, Vol. I, No. VIII
----------------------------------------------------------------------

Date: 5 Mar 90 03:24:32 GMT
From: zaphod.mps.ohio-state.edu!swrinde!emory!mephisto!eedsp!chara![e-mail]  (Donald J. Barry)
Subject: Electronic Journal of the ASA, Vol. I, No. VIII


                        THE ELECTRONIC JOURNAL OF
                THE ASTRONOMICAL SOCIETY OF THE ATLANTIC

                     Volume 1, Number 8 - March 1990

                       ###########################

                            TABLE OF CONTENTS

                       ###########################

         * ASA Membership/Article Submission Information

         * Stellar Spectroscopy: At the Heart of Astrophysics

             - Interview with Dr. Ingemar Furenlid by Edmund G. Dombrowski

         * Mars 1994 - Andrew J. LePage

         * The Ice Moons of the Jovian Worlds - Craig M. Levin

                       ###########################

                       ASA MEMBERSHIP INFORMATION

        The Electronic Journal of the Astronomical Society of the
    Atlantic (EJASA) is published monthly by the Astronomical Society
    of the Atlantic, Inc.  The ASA is a non-profit organization dedicated
    to the advancement of amateur and professional astronomy and space
    exploration, and to the social and educational needs of its members.

         Membership application is open to all with an interest in
    astronomy and space exploration.  Members receive the ASA Journal
    (hardcopy sent through U.S. Mail), the Astronomical League's REFLECTOR
    magazine, and may additionally purchase discount subscriptions to
    SKY & TELESCOPE, ASTRONOMY, DEEP SKY, and TELESCOPE MAKING magazines.

         For information on membership, contact the Society at:

          Astronomical Society of the Atlantic (ASA)
          c/o Center for High Angular Resolution Astronomy (CHARA)
          Georgia State University (GSU)
          Atlanta, Georgia  30303  U.S.A.
          [e-mail] -or- [e-mail]

         or telephone the Society recording at [phone] to
         leave your address or receive the latest Society news.

         ASA Officers and Council -

         President - Don Barry
         Vice President - Bill Bagnuolo
         Secretary - Ken Poshedly
         Treasurer - Alan Fleming
         Board of Advisors - Bill Hartkopf, Edward Albin, Jim Bitsko
         Council:  Larry Klaes, Michael Wiggs, Max Mirot,
                   Eric Greene, Patti Provost, Paul Pirillo,
                   Becky Long, Jim Bitsko, Julian Crusselle,
                   Toni Douglas

         ARTICLE SUBMISSIONS -

         Article submissions on astronomy and space exploration to
    the EJASA are most welcome.  Please send your on-line articles
    in ASCII format to Larry Klaes, EJASA Editor, at the following
    net addresses:

         [e-mail], or ...!decwrl!wrksys.dec.com!klaes,
    or [e-mail], or [e-mail],
    or [e-mail]
    -or- the Society address [e-mail], [e-mail]

         You may also use the above net addresses for EJASA backissue
    requests, letters to the editor, and ASA membership information.

         Please be certain to include either a network or regular mail
    address where you can be reached, a telephone number, and a brief
    biographical sketch.

         DISCLAIMER -

         Submissions are welcome for consideration.  Articles submitted,
    unless otherwise stated, become the property of the Astronomical
    Society of the Atlantic, and although they will not be used for profit,
    are subject to editing, abridgment, and other changes.  Copying or
    reprinting of the EJASA, in part or in whole, is encouraged, provided
    clear attribution is made to the Astronomical Society of the Atlantic,
    the Electronic Journal, and the author(s).  This Journal is Copyright
    (c) 1990 by the Astronomical Society of the Atlantic.


              STELLAR SPECTROSCOPY: AT THE HEART OF ASTROPHYSICS

       An interview with Dr. Ingemar Furenlid by Edmund G. Dombrowski

        Stellar spectroscopy, in addition to being extremely challenging,
    and at times mentally taxing, has always struck me as being one of the
    most fascinating fields in astrophysics.  My first exposure to spectral
    analysis goes back to high school, where I received an essentially
    inadequate description of a very powerful technique.  In spectroscopy,
    the light from an object is separated into its many colors or
    frequencies, revealing much about the source.  When I became more
    interested in astronomy, my understanding and awareness of the method
    became more polished.  I performed some spectral analysis, perhaps not
    deep in comparison to what the specialists in the field are used to,
    but enough to understand the rigors involved, as well as to appreciate
    the temperament and patience a stellar spectroscopist must have when
    dealing with the thousands of lines that may appear in a spectrum.

        I guess I really owe this latter appreciation to one of the most
    intriguing astronomers I have ever met, Dr. Ingemar Furenlid, who is
    a well known specialist in the field of stellar spectroscopy.  Dr.
    Furenlid is presently an associate professor in the Department of
    Physics and Astronomy at Georgia State University.  In addition to
    serving on my thesis committee, Dr. Furenlid has provided me with a
    thorough background of stellar astrophysics through my coursework with
    him as well as through the many topical discussions we have had since
    I arrived at G.S.U. over four years ago.  Since then I have kept a
    keen interest in his ongoing research.  His contributions to the field
    have and continue to be invaluable as spectroscopy itself faces new
    challenges.  These topics, and his recent collaborations, formed the
    basis of an interview which took place on Tuesday, December 13, 1988.

        Dr. Furenlid's background consists mostly of astrophysical work,
    specifically the interpretation of stellar spectra.  This has been his
    mainstay ever since he received his doctorate from the University of
    Stockholm in Sweden.  However, at G.S.U. he has become a member of
    the Center for High Angular Resolution Astronomy (CHARA), not to do
    astrometry (measurement of star positions), but to add spectroscopy
    as a useful complement.  When a spectrum is taken, spectral lines
    (specific colors which show higher or lower levels of brightness)
    are Doppler shifted from their usual places, depending on the velocity
    of the star towards or away from us.  According to Furenlid, these
    radial velocities of stars, specifically those in binary systems,
    must be measured in order to better determine the fundamental quanti-
    ties of the components.

        The National Science Foundation (NSF) supports Dr. Furenlid with
    a grant to measure high precision radial velocities of binaries with
    orbits well determined by the GSU/CHARA group.  According to Furenlid,
    "By adding velocities to the angular orbits determined by speckle
    interferometry, you get the absolute size (in kilometers) of the
    orbits.  And if you have the absolute size and the angular size you
    can get the distance, the luminosity, and the mass of the stars
    involved, in favorable cases with very high accuracy."

        His association with CHARA has also placed him in charge of
    developing the spectroscopic and photometric capabilities of the
    proposed CHARA array.  Dr. Furenlid explained that interferometric
    measurements with the system would require good seeing conditions.
    If the seeing is bad, the array can be switched to spectroscopy and
    photometry, therefore, no time would ever be wasted on the system.
    However, he pointed out that under normal conditions, both radial
    velocities and astrometric data could be measured simultaneously.
    This is of extreme importance, since the spectroscopic data will
    give high precision radial velocity components of binary systems;
    and these velocities are needed to determine a set of accurate
    masses for all kinds of stars that are found in binary systems.
    In Furenlid's opinion, "It is an extremely important fundamental
    project."

        When asked about his current research, Dr. Furenlid pointed out
    that besides the high priority radial velocity work, he and his
    graduate student, Tom Meylan, have been working on astrophysical
    interpretations of stellar spectra for some time.  More specifically,
    they are endeavoring to determine the chemical abundances of solar
    type stars in the solar neighborhood.  The first such star analyzed,
    apart from the Sun (Sol), is Alpha Centauri A, which has yielded some
    very interesting results.  For instance, the strong overabundance of
    copper is very puzzling.  The purpose of this work is to define the
    chemical composition in the solar neighborhood at our distance from
    the galactic center.  According to Furenlid, the chemical composition
    of the Milky Way Galaxy changes with time as supernovae and evolved
    stars "feed processed stellar material into the interstellar medium
    where it forms new stars."  So, we are "mapping out the change with
    time and the change in chemical abundance as you go out through the
    galaxy; and you have a slower and slower enrichment, generally
    speaking, the further away from the center you are." [Editor's Note:
    See the September 1989 EJASA Volume 1, Issue 2 for details on Dr.
    Furenlid and Meylan's work on the star Alpha Centauri A.]

        This same method also allows one to sort how individual stars are
    influenced by the heavy element enrichment of the galaxy.  According
    to Furenlid, when you analyze the high precision spectrum of a star by
    simulating its atmosphere on a computer, it appears that every star is
    individual in its chemical abundance.  Therefore, it is possible to
    determine how a star has gotten its unique signature of abundances and
    to pinpoint the probable processes that formed it.  Research of this
    kind has broader implications yet to be seen.

        Another interest of Dr. Furenlid for years has been his work on BW
    Vulpecula, a hot pulsating Beta Cephei star.  The fascination of this
    star, and similar stars, is that the mechanisms behind their pulsations
    are unknown.  Interpretation of the spectra is used by the astronomer
    to search for clues explaining the physics of these stars.  Dr. Furenlid
    is part of an active International Campaign directed towards obtaining
    high precision data of selected Beta Cepheid stars with twenty-four hour
    coverage in hopes of piecing together a more complete astrophysical
    picture.

        When asked about the type of data he deals with, Furenlid
    explained that there are probably 40-50,000 lines seen in a typical
    solar type spectrum of very high resolution.  In order to get a high
    precision measurement of a particular feature, one must observe each
    piece of the spectrum for a long time to achieve a high signal to
    noise ratio, that is, to obtain mostly pure signal and less random
    fluctuations.

        "If you measure," Furenlid adds, "a sufficient number of
    absorption lines of different elements and then apply to those
    measurements a stellar model atmosphere, you will find that there is
    only one temperature and pressure that can generate this particular
    set of lines.  The strength of a line depends on the chemical
    abundance of the element, the temperature, the pressure, and the
    microturbulent motions in the stellar atmosphere."  If you have a
    properly selected set of stellar absorption lines, there is only
    one unique solution for these parameters.  This is the basis of
    determining the chemical composition of stars:  One observed
    spectrum is matched to one computed model spectrum.

        A variance of the analysis, which is applied by most spectro-
    scopists, is to determine the difference between the chemical
    abundance of the Sun and that of a star under investigation.  This
    difference is more precise than an absolute determination of a
    star's abundance and gives higher precision for determining other
    astrophysically important quantities of a star's atmosphere and
    interior.

        Besides his work with CHARA and his ongoing spectral analysis of
    solar type stars, Dr. Furenlid is collaborating with astronomers from
    Mexico, which has led to the design and construction of a new
    spectroscopic system.  During his ten-year stay at Kitt Peak National
    Observatory (KPNO) in Tucson, Arizona, Dr. Furenlid had made many
    connections to astronomers from all over the world.  As an outcome of
    this, a few years ago he was contacted by Dr. Poveda of the National
    University in Mexico City asking Furenlid if he was interested in
    consulting with them on a new spectrograph for the observatory they
    were finishing at Cananea.  Dr. Furenlid then got in touch with the
    group building the observatory and spectrograph, the Instituto Nacional
    de Astrofisica, Optica, y Electronica, located in Tonantzintla, and
    ended up playing a major role in the development of the new system.

        The Cananea observatory itself is located in a favorable area for
    observing, just south of the Arizona border between Nogales and Bisbee,
    about 240 kilometers (150 miles) southeast of Tucson.  From the
    observatory, one can see Kitt Peak, so it is located in a fairly good
    region noted for good seeing.

        The spectrograph was first tested at San Diego State University
    during October of 1989.  It will be installed in Cananea during the
    spring of 1990.  Designed as the main instrument at the observatory,
    it is very versatile.  It can adapt to measure both high-dispersion
    spectra of brighter objects, and also low-dispersion spectra of dimmer
    ones.  For more precise results, an optical fiber will connect the
    telescope to the spectrograph.  The result is that image drift or poor
    guiding will not generate uncertainty in the observed spectrum.
    Describing the fiber system, Furenlid states, "Instead of a spectral
    slit where the stellar image is moving around due to seeing, the slit,
    in this case, is a number of individual optical fibers, and they are
    in an absolutely stable, fixed configuration.  So, we get a very
    precise spectrum on the detector."  The detector itself is a CCD
    cooled to about -110 degrees Celsius (-230 degrees Fahrenheit).

        During installation at Cananea, the spectroscope will be mounted
    on the telescope pier inside a box with heavy thermal insulation to
    improve its stability.  After adjustments, this box will be sealed
    as much as possible, and most access will be conducted by remote
    electronic controls during use.  No effort will be spared in
    stabilizing the system, since it will hopefully be used for years
    to come.  As far as Dr. Furenlid is concerned, he will begin using
    the Cananea system this summer.

        In addition to his intended research with the new system, Dr.
    Furenlid will also be a member of a telescope allocation committee,
    and a scientific advisory committee.  The telescope allocation
    committee will welcome anybody with a viable scientific project within
    the constraints imposed by the Mexican Institute.  So, like any other
    private institution, users must qualify and abide by the rules.  As
    far as G.S.U. and Dr. Furenlid are concerned, however, there is a
    written contract with his Mexican collaborators giving Furenlid a
    minimum of fifty nights a year.  With a modern, fast detector and
    spectrograph of this power, fifty nights a year is an ample amount for
    both short and long term projects.  The main projects that Furenlid
    will be doing will be in collaboration with scientists at the
    Institute as well as the University of Mexico.

        When the system is up and running, it is expected to be extremely
    productive.  Dr. Furenlid plans to continue his existing programs and
    even expand them when the time is right.  With commitments to Cananea
    and the CHARA array, he will certainly be involved with new technology
    that will be at the forefront of astrophysics.

        About the Author -

        Edmund G. Dombrowski is pursuing advanced study in astronomy
    under Hal McAlister at the Center for High Angular Resolution
    Astronomy of Georgia State University.  His professional interests
    include speckle interferometry, photometry, and the Hyades distance
    scale.


                MARS 1994

               By Andrew J. LePage

        For decades the track record for the Soviet Union's program of
    unmanned exploration of the planet Mars has been less than impres-
    sive.  Several early spacecraft never reached an Earth parking orbit.
    Three others are known to have never made it beyond Earth orbit.
    Three more probes failed enroute to Mars.  One craft failed in its
    attempt to orbit the Red Planet.  Three orbiters, while successful
    in accomplishing their main tasks, returned less information than
    hoped.  Four landing attempts either crashed, missed the planet
    entirely, or ceased functioning soon after touchdown.  Most
    recently, PHOBOS 2 failed before it was able to complete its
    rendezvous mission with the Martian moon Phobos.

        The Soviets have proven that they are capable of better things
    in the field of planetary exploration, however.  Ten unmanned probes
    have successfully landed on the hellish world of Venus.  Four other
    spacecraft were placed into orbit around the cloud-shrouded planet.
    Two of these orbiters made high-resolution radar maps of Venus'
    northern hemisphere.  Their data on the planet's surface will not
    be surpassed until the United States' MAGELLAN spacecraft goes into
    Venusian orbit in August of 1990.  Two more Soviet probes placed
    balloons in the Venusian atmosphere, while the main buses of these
    spacecraft flew on to study Comet Halley.  After their impressive
    string of accomplishments with the VENERA and VEGA programs, the
    Soviets felt they had the technology, confidence, and international
    savvy to attempt their first new missions to Mars since 1974.

        Starting in the early 1980s, the Soviets slowly developed a
    ambitious program to explore the Red Planet.  A totally new, modular
    spacecraft bus was designed to replace the twenty-year-old second
    generation planetary bus that the Soviets used in their VENERA
    program.  The two PHOBOS spacecraft launched in the summer of 1988
    towards Mars were the first phase in the Soviet's new Mars initiative,
    becoming the first to make use of this new spacecraft design.
    Unfortunately, they were also the first to uncover a number of its
    design deficiencies - the result of hurried planning and lack of
    communication between the spacecraft manufacturer and the mission
    scientists.

        The Soviets have already stated that the next missions to use
    this third generation planetary bus will have upgraded computers,
    an omnidirectional antenna for receiving emergency commands, an
    autonomous attitude recovery capability, and improved backup
    batteries installed.  These improvements, along with much better
    overall mission planning, should prevent the failures experienced
    by the PHOBOS 1 and 2 spacecraft.

        In late 1989, the Soviet government approved a three hundred
    million ruble (about 450 million dollar) program called MARS 1994.
    The goal of the MARS 1994 mission is to place two spacecraft in orbit
    around Mars which will deploy landers and balloons to make direct
    measurements of the Martian surface.  The orbiters will employ their
    own instruments to make remote observations from space.  Like the VEGA
    and PHOBOS missions, the Soviets intend to make this a cooperative
    scientific program which will involve France, many Eastern European
    nations, and the United States.  While most of the spacecraft's
    instruments and many of the mission details remain to be determined,
    a fairly clear picture of the MARS 1994 mission is emerging, thanks
    in part to the new "openess" of the Soviets.

        In September of 1994, two six metric ton (13,000-pound) spacecraft
    based on the PHOBOS design will be sent into space on separate PROTON
    launch vehicles.  The two spacecraft will enter highly elliptical
    polar orbits around Mars after an interplanetary voyage of slightly
    less than one year.  After attaining an initial orbit, the vehicles
    will settle into an orbit with an inclination of about one hundred
    degrees and a period of twelve hours.  Their orbits will range from a
    high point of twenty thousand kilometers (12,000 miles) to a low point
    of two hundred to five hundred kilometers (120 to 300 miles) above
    the Martian surface.  While in orbit, the spacecraft will continue
    observations begun six years earlier by PHOBOS 2.  About two hundred
    kilograms (440 pounds) of instruments will be carried, including an
    imaging system with a maximum resolution of about one meter (39.37
    inches) and a collection of other instruments to remotely probe the
    Martian surface properties and composition, observe the atmosphere,
    and make various measurements of the elusive Martian magnetic field.

        Some time after the spacecraft have entered orbit around Mars,
    each will deploy a package towards the surface.  Each package will
    likely contain four spike-like penetrators and a highly novel balloon.
    The landing sites have not been chosen yet, but the selection would
    be based on data obtained by the VIKING orbiters, PHOBOS 2, and the
    MARS OBSERVER (currently scheduled to be launched by the United
    States in 1992), as well as the MARS 1994 orbiters themselves.  It
    is expected that the landing sites will be much more interesting
    than the VIKING landing sites.  Those sites were chosen in 1976
    because they were considered to be "safe" for making a "blind"
    landing.  Unfortunately, "safe" also means "dull".  Areas that
    included interesting features such as volcanoes, channels cut by
    water, canyons, and other exotic terrains are much rougher.  The
    VIKING landers would have most likely been destroyed during a landing
    attempt in one of the places.  Penetrators, on the other hand, are
    very rugged and balloons can fly over most obstacles.  These inherent
    advantages should open up a much wider range of potential landing
    sites that are far more interesting than the two examined to date.

        The design and capabilities of the penetrators has yet to be
    finalized.  Each will weigh a few tens of kilograms and be capable
    of burying themselves into solid rock.  The only experiment selected
    so far has been a seismometer for transmitting information about Mars'
    surface movements for some as yet undefined period of time.  It is
    also likely that simple meteorological measurements such as temperature
    and pressure will be made.  The penetrator design should be announced
    sometime in 1990.

        At this time the balloon package is much better defined and
    numerous tests have already been conducted with it.  The balloon
    package will consist of a twenty-meter (66-foot) tall balloon supplied
    by the French CNES (Centre National d'Etudes Spatiales, the National
    Center for Space Studies), with a four-kilogram (nine-pound) pano-
    ramic camera package hanging below and a four-kilogram (nine-pound)
    instrument-laden "snake" supplied by The Planetary Society dangling
    at its base.  The balloon will be divided into two parts:  A helium
    filled upper portion and a lower portion filled with the ambient
    Martian atmosphere.  At night the balloon will have enough buoyancy
    to hold itself and the camera package above the Martian surface while
    the "snake" lies on the ground.  In this way, the camera package can
    make images of the ground below with a resolution of less than one
    millimeter (0.04 inch), while the instruments in the "snake" make
    measurements of the composition and physical properties of the nearby
    soil and rocks.

        In the morning, the balloon will absorb the Sun's rays and heat
    the cool Martian air inside its lower portion.  The air will begin
    to expand, whereafter the balloon will generate enough additional
    buoyancy to lift the "snake" off the ground.  The balloon will then
    rise at a speed of about one meter per second (39.37 inches per
    second) to an altitude of two to four kilometers (1.2 to 2.4 miles),
    where it will ride on the Martian winds.  The camera package will be
    relaying images of the planet's surface to the Soviet MARS orbiters
    and to a specially installed receiver on the American MARS OBSERVER,
    which should still be functioning after two years in orbit.  It may
    also be possible to use Earth-based radio telescopes to track the
    balloons, as was done with the balloons deployed in the dense
    Venusian atmosphere by the VEGA spacecraft in 1985.  Using VLBI
    (Very Long Baseline Interferometry) techniques, it should be possible
    to determine the balloons' locations and clock the wind speeds in
    various parts of the Martian atmosphere.

        Once the Sun "sets", the air inside the balloon will cool, causing
    the balloon to slowly sink to the ground.  Once the "snake" comes in
    contact with the surface, the balloon will drop no further and the
    "snake" will be able to make more surface measurements in yet another
    location.  When the Sun "rises" the next morning, the balloon will
    heat up and the process will repeat.  Using this novel method for
    locomotion, the balloon can travel a few hundred kilometers a day,
    studying widely separated locations and obtaining numerous high
    resolution images that will complement the orbiters' images.
    According to current estimates, each balloon should be capable of
    ten such cycles and cover a few thousand kilometers before too much
    helium leaks from the balloon, making it impossible to hold itself
    off the ground at night.

        If this mission proves successful, it should vastly increase our
    knowledge about the planet Mars.  The penetrators scattered over eight
    sites should give new information on the level of geological activity
    on Mars and indications of the planet's internal structure.  The two
    balloons should give us data on the composition of as many as twenty
    widely scattered sites, return highly detailed swaths of images of
    the Martian surface several thousand kilometers long, and yield much
    information on the Martian winds.

        If the quality of the data returned by the Mars orbiters is
    comparable to that briefly returned by PHOBOS 2, the amount of data we
    may receive and what it tells us could be staggering.  When combined
    with the data returned by the penetrators and balloons, scientists
    should have a much better understanding of Mars' surface properties
    and composition on a global scale, a better picture of the atmosphere
    and its motions, and a much more detailed knowledge of the Martian
    water inventory.  All this information should give scientists and
    engineers an excellent foundation to plan future missions to the Red
    Planet, including soil sample return missions and manned expeditions.
    If the Soviets can overcome their legacy of Mars mission failures,
    the MARS 1994 mission stands to make a key contribution to our
    understanding of the Red Planet in the closing years of this century.

        References:

    Blamont, Jacques, "Exploring Mars by Balloon", THE PLANETARY
          REPORT, The Planetary Society, May/June 1987, pages 8-10.

    Friedman, Louis D., "The Mars Balloon", THE PLANETARY REPORT,
          The Planetary Society, September/October 1988, pages 7-11.

    Wilson, Andrew (Editor), INTERAVIA SPACE DIRECTORY 1989-90,
      Jane's Publishing, pages 151-152.

    "Soviet Space Program Strife Threatens Mars Mission Plans",
      AVIATION WEEK & SPACE TECHNOLOGY, May 22, 1989, pages 18-21.

    "Time, Cost Constraints Force Soviets to Alter 1994 Mars Mission",
      AVIATION WEEK & SPACE TECHNOLOGY, August 28, 1989, page 22.

    "A New Soviet Plan for Exploring the Planets", SCIENCE,
          October 13, 1989, pages 211-212.

    "Mars '94 Takes Shape", SPACEFLIGHT, The British Interplanetary
          Society, November 1988, page 417.

        About the Author -

        Andrew J. LePage is a member of the Boston Group for the Study
    of the Soviet Space Program (Krasnaya Orbita).  In addition to his
    interests in astronomical and space related topics, he has been a
    serious observer of the Soviet space program for over a decade.


                     THE ICE MOONS OF THE JOVIAN WORLDS

                 by Craig M. Levin

        In the early years of the Space Age, astronomers knew very little
    about the moons of the Jovian planets.  Though they had data on the
    sizes and the masses of these distant bodies, the scientists could
    only make conjectures on their composition.  Since the recent explora-
    tion of the outer worlds by the VOYAGER spacecraft, planetologists
    have gained a much better concept of the geology of ice and all its
    complexities throughout our solar system.

        Of the four Galilean moons of Jupiter, three fall into the
    category of ice moons:  Callisto, Ganymede, and Europa.  Callisto,
    the outermost of the Galilean moons, appears at first glance to be
    geologically dead.  Its surface is completely covered with craters
    of all sizes.  However, a careful examination of VOYAGER images made
    in 1979 show numerous ridges and faults upon this battered moon,
    signs that Callisto had a very active geological history.

        Ganymede, the largest known moon in the solar system, is also
    pockmarked with craters like Callisto; but Ganymede also presents us
    with extensive evidence of a once-active interior.  Large patches of
    light-colored ice cover great tracts of this moon.  These patches
    are actually frozen water "lava" flows that flooded the Ganymedian
    lowlands eons ago.

        Europa, somewhat smaller than Earth's moon Luna, is one of the
    few bodies in the solar system that has a virtually uncratered surface.
    This is due to the intense heating caused by the tidal tug-of-war
    between Jupiter, Io, and Europa itself.  This constant source of heat
    generation has kept Europa's icy material (containing substances like
    water, ammonia, and methane) in a molten state, except for a thin,
    solid crust.  Europa's surface is kept in this uncratered state
    because the molten ices occasionally erupt from numerous fissures
    on Europa, which appear as long brown cracks on the Jovian satellite.

        Saturn's own family of major moons offer even more evidence of
    the diversity of ice geology forms.  Dione and Rhea both have streaked
    surfaces, possibly the result of water upwelling from fractures in
    their crusts.  Enceladus shows many prominent ridges, as well as a
    very white and "clean" surface - clear evidence that this little
    world's geological life is definitely far from over.  Craters appear
    to be covered over by new "lava" flows of molten ices.  Enceladus'
    activity may be caused by a tidal link between it and Dione which
    heats the moon.  Tethys and Mimas show many small faults criss-
    crossing each other on their surfaces.  Finally, Iapetus has
    confounded many planetologists by its duo-tone hemispheres:  One
    hemisphere is of the same icy hue as most of the other moons of
    Saturn, yet the other is as dark as charcoal.  There are two
    current explanations for this phenomenon:  Either Iapetus had a
    catastrophic upwelling of carbonaceous ice from its core, or Iapetus
    was struck by a large carbonaceous planetoid or comet, which spread
    the dark debris across that hemisphere.

        Of all the moons in the solar system, Saturn's largest satellite,
    Titan, ranks among the most interesting.  Titan is one of the few
    moons with a significant atmosphere.  Unfortunately for scientists,
    Titan's orange clouds of nitrogenous and hydrocarbon material preclude
    visual observations of its surface.  The moon may contain lakes and
    even oceans of liquid nitrogen.  Confirmation of this concept will
    have to wait for the arrival of the unmanned CASSINI mission in 2002,
    where Titan's hidden surface will be unveiled by both radar imaging
    and the HUYGENS lander probe.

        Uranus' five largest moons also contribute to our knowledge of
    the diversity of ice geology.  None of the Uranian moons are quite
    similar either to each other or any other Jovian ice satellite.
    Oberon, the outermost moon, shows some evidence for crater-filling
    eruptions, but very little for fissure eruptions or faulting.
    Titania has the start of a fault and fissure system on its dark
    surface, yet Umbriel appears to be geologically extinct.  Ariel has
    a complete system of faults and fissures, a sure sign of an active
    geological life; but it is Miranda that is perhaps the most astounding
    moon in the Uranian system.  Despite the satellite's small diameter
    of 546 kilometers (341 miles), Miranda displays almost all types of
    volcanic and tectonic activity.  This includes fissure eruptions of
    both pure and carbonaceous ice, compressional faults, and the well-
    known "circi", which resemble race tracks across the moon's surface.
    About the only type of vulcanism not yet seen on Miranda is the type
    most associated with the word volcano - mountainous vulcanism.

        The last major ice moon of the Jovian worlds examined by VOYAGER
    2 was Neptune's largest satellite, Triton.  Though the probe imaged
    only twenty-five percent of the moon (primarily the southern hemi-
    sphere), at least one type of major surface feature is now known to
    have been formed by internal geological processes which are still
    active.  These are the dark "plumes" covering the southern polar cap.
    Triton now ranks as one of the few worlds in the solar system known to
    be geologically active.  Scientists at the Jet Propulsion Laboratory
    (JPL) in Pasadena, California discovered in several of VOYAGER 2's
    images of Triton a plume of material (most likely carbonaceous ice)
    being ejected from the surface many kilometers into space.  One theory
    on Triton's current activity is that the powerful gravitational pull
    of Neptune is causing tidal stresses on Triton, thus creating the
    eruptions.  This is concurrent with the reason for the extensive
    vulcanism on Jupiter's Galilean moon Io.

        Another Triton surface feature which may also be the result of
    internal geological activity is the polar cap itself, possibly caused
    by volcanic outgassing.  Another is the system of faults in the non-
    capped areas, probably the result of expansion of Triton's core as the
    frozen crust trapped internal heat.  Yet another are the open, unmarked
    areas, apparently caused by ice "lava" flows flooding lowlands and
    freezing.  The final known feature is the "cantaloupe" terrain that
    seems to cover the rest of the moon; this complex of hills and valleys
    could be the result of local vulcanism.  Much of Triton's terrain is
    uncratered, which indicates that the moon was quite geologically
    active in its recent past.

        One thing which the amazing VOYAGER missions to the Jovian planets
    has proven is that each of the worlds these probes have examined are
    quite unique in their own ways.  The "pearls-on-a-string" theory of
    ice moon geology has become a thing of the past.  Another matter left
    for consideration is that most of the Jovian moons were not completely
    mapped; in some cases, less than half of their surfaces were imaged
    by the VOYAGERs.  This situation will change in the next few decades,
    when probes such as GALILEO and CASSINI are sent into orbits around
    Jupiter and Saturn for several years of exploration throughout their
    systems of moons and rings.  Perhaps through these missions we will
    learn much more about these icy worlds and eventually discover how
    our solar system came to be.

        I wish to thank Professor John Kenny, for approving of my written
    work and encouraging me to continue, and Larry Klaes, EJASA Editor,
    for his nearly infinite tolerance and patience through numerous
    rewrites of this article.

        References:

        Baugher, Joseph F., THE SPACE-AGE SOLAR SYSTEM, John Wiley &
          Sons, New York, 1988

        Beatty, J. Kelly, "Welcome to Neptune", SKY & TELESCOPE,
          October 1989

        Berry, Richard, "Neptune Revealed", ASTRONOMY, December 1989

        Berry, Richard, "Triumph at Neptune", ASTRONOMY, November 1989

        Briggs, G. A., and F. W. Taylor, THE CAMBRIDGE PHOTOGRAPHIC ATLAS
          OF THE PLANETS, Cambridge University Press, New York, 1988

        Hartmann, William K., and Ron Miller, THE GRAND TOUR: A
          TRAVELER'S GUIDE TO THE SOLAR SYSTEM, Workman Publishing Co.,
          Inc., New York, 1981

        Kinoshita, June, "Neptune", SCIENTIFIC AMERICAN, November, 1989

        About the Author -

        Craig M. Levin began his involvement in astronomy when, in the
    second grade, he received H. A. Rey's book, FIND THE CONSTELLATIONS,
    as a birthday present.  From Rey's work, Craig was able to find those
    few constellations visible through the bright city lights of Chicago,
    Illinois.  Craig's initial interest in astronomy later flagged,
    though, and remained at a low level for a number of years.  Comet
    Halley pulled Craig out of his freshman high school doldrums in 1985-
    1986, as the tiny ball of ice and rock made its latest appearance
    through the inner regions of the solar system.  That January, Craig
    received his first telescope (he has since gone through four sets,
    including binoculars) and began to get involved in astronomy again.
    By his sophomore year in high school, Craig was a member of Chicago's
    Adler Planetarium and The Planetary Society.  As a junior, Craig had
    his first astronomy article published in the now-defunct Small Scope
    Observers' Association's newsletter, and by his senior year in high
    school was helping to establish the ASTRONOMICAL NEWSLETTER, a now-
    defunct periodical based in Atlanta, Georgia.  At present, Craig
    is a physics major at Bradley University in Peoria, Illinois, who
    intends to turn his first love, planetology, into his profession.

        If you would like to contact Craig over the network, please do
    so at the following net addresses:

        uiucdcs\
            noao>bradley!bucc2!moonman
        cepu/

        INTERNET:  bradley!bucc2![e-mail]
        ARPA:  cepu!bradley!bucc2![e-mail]
        BITNET:  [e-mail]


     THE ELECTRONIC JOURNAL OF THE ASTRONOMICAL SOCIETY OF THE ATLANTIC

                        March 1990 - Vol. 1, No. 8

                         Copyright (c) 1990 - ASA


--
Donald J. Barry       [phone]         | [e-mail]
Center for High Angular Resolution Astronomy | President, Astronomical
Georgia State University, Atlanta, GA 30303  | Society of the Atlantic

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End of SPACE Digest V11 #116
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