Return-path: X-Andrew-Authenticated-as: 7997;andrew.cmu.edu;Ted Anderson Received: from beak.andrew.cmu.edu via trymail for +dist+/afs/andrew.cmu.edu/usr11/tm2b/space/space.dl@andrew.cmu.edu (->+dist+/afs/andrew.cmu.edu/usr11/tm2b/space/space.dl) (->ota+space.digests) ID ; Mon, 5 Mar 90 02:40:05 -0500 (EST) Message-ID: <8ZwVSqm00VcJECo05R@andrew.cmu.edu> Reply-To: space+@Andrew.CMU.EDU From: space-request+@Andrew.CMU.EDU To: space+@Andrew.CMU.EDU Date: Mon, 5 Mar 90 02:39:35 -0500 (EST) Subject: SPACE Digest V11 #116 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!don@tut.cis.ohio-state.edu (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. asa%chara@gatech.edu -or- asa@chara.uucp or telephone the Society recording at (404) 264-0451 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: klaes@wrksys.dec.com, or ...!decwrl!wrksys.dec.com!klaes, or klaes%wrksys.dec@decwrl.dec.com, or klaes@wrksys.enet.dec.com, or klaes%wrksys.enet.dec.com@uunet.uu.net -or- the Society address asa%chara@gatech.edu, asa@chara.uucp 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!moonman@a.cs.uiuc.edu ARPA: cepu!bradley!bucc2!moonman@seas.ucla.edu BITNET: moonman@cc2.bradley.edu THE ELECTRONIC JOURNAL OF THE ASTRONOMICAL SOCIETY OF THE ATLANTIC March 1990 - Vol. 1, No. 8 Copyright (c) 1990 - ASA -- Donald J. Barry (404) 651-2932 | don%chara@gatech.edu Center for High Angular Resolution Astronomy | President, Astronomical Georgia State University, Atlanta, GA 30303 | Society of the Atlantic ------------------------------ End of SPACE Digest V11 #116 *******************