The

VolulUe XVIII NUlUber 1 On the Cover: Volume XVIII Subtle blue-g ray clouds hover over the dusty rose Martian Table of Number 1 surface near Valles Marineris. a series of interconnected east-west canyons spanning roug hly 4.000 kilometers Contents January/February 1998 (2. 500 miles). This is the first color image from the Mars Orbiter Camera aboard Mars Global Surveyor. taken last October 3 about 11 minutes after the spacecraft passed close to the planet for the th irteenth ti me. It was during Features th ese in itial passes with in the Martian atmosphere that Mars Global Surveyor team members noticed slight The Sun Sets on Mars Pathfinder movement in an unlatched solar panel. causing mission 4 officials to change the pace of aerobraking. and delaying After the drama of the Mars Pathfinder landing and the exultation of its successes the mapping portion of the mission on e year. must come the sorrow of its demise. This was an engineering test mission to prove the effec­ tiveness of new technologies. The science accomplished was a bonus. So even though the Image: Courtesy of Malin Space Science Systems and JPUNASA lander and the Sojourner rover left some questions unanswered and some stones unturned, it demonstrated that airbags are an inexpensive and lightweight landing system, that micro­ rovers are good little explorers, that solar-charged batteries can power spacecraft systems, and that "faster, cheaper, better" works in the real world(s). So let's not mourn the end of this remarkable mission. Let's celebrate the new trail to the planets blazed by this true pathfinder. Frol11l 6 The Shiva Hypothesis: The hnpacts, Mass Extinctions, and the Galaxy Every once in a while, we like to venture away from reporting mission plans and results to Editor theoretical analyses by scientists trying to fit together pieces of really big puzzles. The rocks of Earth give incomplete evidence of what has caused the pulses of extinction and evolution that mark the passage of geologic time. But by combining discoveries from astronomy, n The Planetary Report, we try to balance planetary science, paleontology, and geology, some scientists have begun to piece together I our reporting on new missions of dis­ a possible story about what has driven changes in life on Earth. covery with the theoretical work that tries to make sense of it all. With the Planetary 1 2 Ice, Water, and Fire: Society's emphasis on exploration, we some­ The Galileo Europa Mission times give more weight to the discovery Calileo has now completed its original mission of exploration in the Jovian system. Although side of the scale. But I think the testing of the mission can be credited with many discoveries, the most intriguing story has been the hypotheses and building of theories can be possibility of oceans- and maybe a habitat for life- beneath the crust of Europa. Even with just as exciting. An old professor of mine a crippled high-gain antenna and recalcitrant tape recorder, the spacecraft remains capable of used to call this work "geomythology." tackling another mission- this time zeroing in on Europa's ice, Jupiter's water, and Io 's fire. And in these stories of how we, the world, and the universe came to be, I find great enjoyment and satisfaction. Departl11lents In tillS issue, we present a hypothesis-in­ process that might help explain one of the 3 Melnbers' Dialogue great mysteries of science: why the advance of life has followed a jerky course of 16 Basics of Spaceflight explosive evolution and massive extinction. Traditionally, scientists have looked for 18 Nevvs & Revievvs answers on Earth. But as we have learned more about the solar system and galaxy in 19 World Watch which we live, as well as our planet, new Questions and Ansvvers suspects have presented themselves. 20 Mike Rampino and his colleagues devel­ 22 Society Nevvs oped the Shiva hypothesis in one attempt to solve the mystery. I hope you will find the same excitement I did in reading the story ofthis hypothesis' genesis. It still Contact Us has obstacles to overcome before reaching the lofty height of scientific consensus. Mailing Address: The Planetary Society, 65 North Catalina Avenue, Pasade na, CA 91106-2301 But it's great fun watching its progress. General Calls: 626-793 -51 00 Sales Cal ls Only : 626 -793-1675 - Charlene 1\1. Anderson E-mail: tps@m ars.planetary. org World Wi de We b Home Pag e: http ://planetary. org

T!a~~~~~~6AR~~~~J~~~~1~~266~7~~!~~g~.blli~~eadv~:~g~~htl6 ~~~b~~~o~\atlh~f~I~~~~~~: ~~~~:~~~YA~~~:jYd~~S~~~~:~tsri~~e ~v;~ue, (US dollars): in Canada, $35 (Canadian dollars). Dues in other countries are $40 (US dollars). Printed in USA. Third-class postage at Pasadena, California, and at an additional mailing office. Canada Post Agreement Number 87424. A Publication of Editor. CHARLE NE M. ANDERSON Technical Editor, JAMES D. BURKE Assistant Editor, DONNA ESCANDON STEVENS Copy Editor. KARL STULL THE-PL.A~T1RY SOCIETY Production Editor, WILLIAM MCGOVERN Art Director, BARBARA S. SMITH Editoria. l Assista.nt. JENNIFER VAUGHN o .~ -B-- ¢ -B- 0 Viewpoints expressed in columns or editorials are those of the authors and do not necessarily represent positions of th e Planetary Society, its officers, or advisors. © 1998 by the Planetary SOCiety o Co-founder CARL SAGAN Members' 1934-1996 Board of Directors BRUCE MURRAY Dialogue President Professor of Planetary Science and Geology, california Institute of TechnOlogy LAUREL L. WILKENING Vice President Chancellor, UniverSity of California, Irvine

LOUIS D. FRIEDMAN Executi ve Director NORMAN R AU GUSTIN E Chairman and CEQ, Lockheed Martin Corporation

ANN DRUYAN Go Metric! However, your suggestion about make it nearly impossible to author and producer DONALD J. KUTYNA In the July/August 1997 issue of using footnotes for conversions enact them. The public can and former Commander, US Space COmmand The Planetary Report, we see is a breath offresh air. we 'u test should get involved with the edu­ JOSEPH RYAN Executive Vice President and numerous references to "degrees it in-house to determine whether cation of our children. Contact General Counsel, Marriott International ROALD Z. SAGDEEV Fahrenheit" (for example, page 7, footnotes create undue problems your local school administrator or former Director, Institute for Space Research, "1,000 degrees Fahrenheit" and in magazine layout. Ifyour idea district office and tell them that Russian Academy of Sciences STEVEN SPIELBERG "2,250 degrees Fahrenheit"). works, look for a change in 1998. you wish to volunteer your time director and producer This is a very unscientific way in -Karl Stull, to share your hobby or career. KATHRYN D. SULLIVAN President and CEO, which to report the temperature. Copy Editor They will listen. Ohio's Center of Science and Industry and former astronaut Furthermore, whether it was in -NYLA RANKIN, NEIL TYSON Director, Hayden Planetarium, degrees Fahrenheit or degrees Take It to the Brawley, California American Museum of Natural History Celsius, lay persons would prob­ Schools Advisory Council ably not actively relate to it (it Mr. Byers' suggestion of bringing The Planetary Society is develop­ JOHN M. LOGSDON Chairman would, to them, just be "damned the $30.00 educational packages ing a "planetary exploration slide Director, Space Policy Institute, hot"); however, readers with a to the schools, instead of waiting show" for volunteers to present George Washington University DIANE ACKERMAN deeper interest would, almost for them to come to us, is good at school assemblies. This pre­ poet and aulllOf certainly, prefer the SI version (see Members' Dialogue in the sentation is designed to appeal BUZZ ALDRIN Apollo 11 astronaut [Systeme International is the sys­ September/October 1997 issue). to students ranging in age Fom RICHARD BERENDZEN tem of measures used in science, However, I believe I have gone early elementary through high educator and astrophysicist ,lAC DUES BLAMONT based on the kilogram, second, one better and perhaps others can school. For more details, contact Chief SCientist, meter, ampere, kelvin, mole, and join me. Two years ago, I took my me at Society headquarters. Centre National d'Etudes Spa tiales, France RAY BRADBURY candela]. So, why not just quote love for space science to school - Linda Hyder, poet and author degrees Celsius and see what - my children's elementary Manager ofProgram Development DAVID BRIN author happens? school. I put together a simple ARTHUR C. CLARKE While still on the subject of but exciting "lecture" using the HUlnans in Space author CORNELIS OE JAGER units, one frequently sees such beautiful pictures I received when I'd like to comment on Norman Professor of Space Research, The Astronomicallnstitule al Utrecht, references as "10,000 kilometers I joined the Planetary Society. H. Horowitz's letter in the July/ the Netherlands (6,000 miles)." These double Each print ofthe planets has inter­ August 1997 column. From a FRANK DRAKE President, SET{ Institute; Professor of Astronomy and Astrophysics, references interrupt the flow of esting facts on the back. I include "step further back" point of view, University of California, Santa Cruz reading the article. Why not just information from The Planetary Mr. Horowitz is failing to take STEPHEN JAY GOULD Alexander Agassiz Professor of lo%gy, stick with SI (accompanied with Report that would be understand­ into account the effect of good Harvard University footnotes for conversion back to able for children. When I explain public relations on the flow of SHIRLEY M. HUFSTEDLER educator and jurist imperiaI)? This would make for the distance to Andromeda on money for space exploration GARRY E. HUNT better readability and would accom­ the chalkboard with all those O's, from our tax-paying citizens. space scient/st, United Kingdom SERGEI KAPITSA modate those who might want the the kids gasp! I have also found If a human mission to Mars Institute for Physical Problems, information in the old terms. pictures in National Geographic (expensive, poor science that it Russian Academy of Sciences HANS MARK - D. T. BATH, of the moons of Saturn and Jupiter, is) has enough of a purse-string­ University of Texas at Austin Peterborough, Ontario, Canada Venus' valleys and awesome loosening effect that twice as MARVIN MINSKY Toshiba Professor of Media Arts and Sciences, atmosphere, and various galaxies. many "small, cheap, fast" mis­ Massachusetts lns/itute of Technology PHILIP MORRISON Every year or so we wrestle with My lectures are free and are sions can then be sent out, it Institute Professor, the issue ofmetric to English usually 40-50 minutes long with would be worth the time, money, Massachusetts Institute of Tecflno/ogy STORY MUSGRAVE conversions, raising the same 10 minutes of question and answer and effort. astronaut points you mention, and we end time. I have done about 25 lectures I'm for it. PAU L NEWMAN actor up keeping the conversions for in two years. The teachers enjoy - HELEN FREELAND, JUN NISH IMURA former Director General, Institute of Space and this one reason: so non-scientific the break, the students love it, and Schenectady, New York As/ronaulica/ Science, Japan readers will know they are part the rewards are tremendous. I am LYNDA OBST ofour audience, right along with not a teacher but a homemaker Please send yo ur letters to producer ADRIANA OCAMPO the scientifically aware and the with a love of science. I have also Members ' Dialogue, p/anetal}' SCientist, Jet Propulsion Laboratory OONNA L. SHIRLEY space science professionals who given my lectures to several Girl The Planetary Society, Manager, Mars Program, Jet Propulsion Laboratory also read The Planetary Report. Scout troops. 65 North Catalina Ave , S. ROSS TAYLOR Emeritus Professor, Broadening support for SET! The schools are in dire need of Pasadena, CA 91106-2301 Australian National University, Canberra and planetary exploration is a specialized programs but lack of or e-mail:

jimdamental goal of the Society. funds, time, manpower, etcetera [email protected]·org. 3

. THE PLANETARY REPORT JANUARY/FEBRUARY 1998 Mars Pathfinder images reveal signatures of an early water-rich environment in the Ares Vallis region. Light-colored dunes, composed of sand-sized particles, and sand-sculpted rocks, along with possibly water-worn rock conglomerates, suggest liquid water flowed through the valley for as long as a millennium. "Water, of course, is the very ingredient that is necessary to he rover Sojourner's camera shows dunes not visibLe from support life," said Mars Path­ Tthe Mars Pathfinder Lander. These dunes, Less than a meter high, are severaL meters wide. Scientists specuLate that the finder Project Sdentist Matt dunes were created by winds blowing right to left (northeast to Golombek, "and that leads to southwest). Sediment such as this on Earth wouLd be made of the $64,000 question: 'Are we sand-sized grains, and the presence of sand wouLd pointto a alone in the universe? Did life history of flowing water. However, scientists are stiLL debating ever develop on Mars? If so, whether these particles can accurateLy be caLLed sand. On the what happened to it?'" horizon, the Twin Peaks are about 1 kiLometer (0.6 miLe) away on the right, and on the Left at a distance of 2.2 kiLometers (1.4 miles) is Big Crater. By the time the batteries ran out last September, the Mars Pathfinder mission had collected more than 16,000 images from ew, more detaiLed images from the rover show pebbLe-Like the lander, more than 500 Nstructures, rounded surfaces, and possibLe congLomerate rocks, from the rover, and nearly 8.5 which on Earth can form through a variety of geoLogic processes that million individual measure­ meLd together fragments ofvarying origins. ments of wind, pressure, and But what forces carved the distinctive surfaces ofthese rocks? temperature. The Alpha Proton Seeking an expLanation for how pebbLes may have been etched from X-ray Spectrometer returned Martian rocks, geoLogists consider a number of possibLe mechanisms. The eroding power of water in waves, floods, or glaciers couLd have 20 studies of rocks and features done thejob, but similar formations couLd also be produced by impacts, on the Martian surface. by Lava flows or pyroclastic eruptions, by concretions formed in sedi­ For a complete archive of mentary rocks, or by any combination of these processes. Mars Pathfinder and Mars Global Two close-up images taken by the rover Sojourner reveaL textures Surveyor images, visit New and structures not visibLe in Lander images. Scientists have identified Views of Mars on the Planetary these rocks as conglomerates because their surfaces have rounded Sodety's World Wide Web page protrusions. The rock Cabbage Patch (right, top) clearly shows these protrusions on the surface. The smaLL rock at bottom right shows that at http://planetary.org. weathering has etched out pebbLes to produce sockets. Two extracted - Jennifer Vaughn, pebbLes (about 0.5 centimeters across, or 0.2 inch) are visibLe at the 4 Editorial Assistant Lower center of the image.

THE PLANETARY REPORT Ma:cs Glabal Survegac Mapping the search continues for clues to P\ an aqueous past on Mars, team Mission Delayed scientists say that today winds have the most significant effect on the planet's QneYear surface. Based on current understanding of how very fine sediment is deposited around t='""arly last November, as Mars Global Surveyor rocks, the map below depicts sediment k performed a series of aero braking passes tails at the Sagan Memorial Station. through the Martian atmosphere, a wobble in an The arrows represent the direction of unlatched solar panel convinced mission offidals the wind, blowing from the northeast to change plans for the spacecraft's route to a low, toward the southwest. The Viking orbiters imaged similar patterns of light and drcular orbit. dark streaks of sediment in and around The spacecraft's two 11-foot sotar panels were Martian craters. designed to remain nearly motionless during aero­ braking. One ofthe panels, which did not properly he rover's eye view of the rock Chimp deploy and latch after Launch, showed slight move­ T(bottom image) shows fine surface ment as the spacecraft approached the Martian textures as well as wind tails, oriented surface, fordng the Mars Global Surveyor team to from lower right to upper left, with small halt aero braking for nearly two weeks while they pebbles in the foreground. Streamlined assessed the problem. Aerobraking is a technique tails, composed of particles deposited that allows a spacecraft to lower its orbit by using by wind on the leeward side, appear the drag produced by atmosphere (for more on aero­ around many rocks in the region. braking, see the Marchj April 1994 Planetary Report). All; mages: J PL/NASA After careful study, the team concluded that aerobraking at a slower rate would not jeopardize the vulnerable panel. "This [slower rate of aero­ braking] changes Mars Global Surveyor's final mapping orbit," said mission manager GLenn E. Cunningham, "but it should not have a signifi­ cant impact on the ability of Global Surveyor to accomplish the mission sdence objectives." Trajectory adjustments will yield an orbit that is an exact mirror image of the original mapping orbit, with data taken along a south to north track rather than north to south. The new mapping orbit will take an additional year to achieve, with the spacecraft arriving at finaL position in mid-January 1999 and beginning to map two months Later. During that extra year, Mars Global Surveyor will be in an elliptical orbit that at its peri apsis, or closest point to Mars, wiLL skim much closer to the surface than the finaL mapping orbit. The spacecraft's full suite of instruments will come on during this opportunity to study the pLanet up-close. "We expect to gain some spectacular new data during this time," said Cunningham. "The spacecraft's orbit will still be elliptical during this period, but at periapsis the surface resolution will be much greater, and the Lighting angles will be excellent." Keep your eye on The Planetary Report and check the Mars Global Surveyor Web site for updates as new images and data become available: http://mars.jpl.gov/mgs. -JV 5

JANUARY/FEBRUARY 199(1 hiva, the world-destroying Hindu deity, symbolizes the rhythm of the universe. When he takes the form Shiva is the Hindu god of destruction and rebirth. When he stops dancing, Sof a graceful dancer, the result is cosmic destruction. this universe will end, and another will be born to start the dance again. But the destruction has another side; it is always followed by Photograph courtesy of the Norton Simon Foundation, Pasadena, California new creations, and new worlds emerge from the ashes of the old. This Hindu imagery of cyclic destluction and renewal prompted the naming ofthe Shiva hypothesis, a proposed explanation for a possible 30-million-year cycle of catastro­ Gubbio, Italy triggered an upheaval in the geological sci­ phe and renewal observed in the geologic record. ences. Walter and Luis Alvarez and their colleagues at the In the history of life on Earth, mass extinctions are University of Califomia at Berkeley hypothesized that the followed by bursts of evolution, as surviving species fill unusual traces of iridium were from dust created by the col­ life-niches emptied by global catastrophe. For several lision of a huge asteroid or comet with the Earth 65 million extinction events, there is now evidence that the instigating years ago~at the precise time when the dinosaurs and some catastrophe was an asteroid or comet impact. Could periodic 75 percent oflife on the planet disappeared (see Figure 1). impacts be responsible for the recurrence of mass extinctions? Investigations of this proposal culminated in further re­ Some scientists are convinced that impacts have been a markable discoveries: a giant, 65-million-year-old, buried driving force in evolution on Earth, and studies indicate impact crater in the Yucatan; widespread evidence in the that impact-related mass extinctions occur at intervals of Gulf of Mexico and Caribbean of a massive tsunami at the about 30 million years. A few researchers suggest that the same time; and rock deposits from storms of debris blasted 30-million-year cycle may result from comet showers, out of the crater. The Planetary Society sent two expeditions pulled in toward the Sun by the regular motion of the solar to Belize in Central America for a first-hand look at these system through the plane of our galaxy. Among scientists, deposits (see the July/August 1995 and July/August 1996 the Shiva hypothesis remains controversial, but if it is cor­ issues of The Planetary Report). rect, our world evolves to the rhythm of extratelTestrial The Chicxulub crater on the Yucatan Peninsula, with a bombardment. diameter of more than 200 kilometers (125 miles) and a precise radiometric date of 65 million years, is the "smoking From qubbio to Chicxulub gun" evidence for the disastrous impact at the KIT boundary. The discovery of anomalous concentrations of the rare That event brought about the abrupt extinction of all large 6 element iridiwn in a thin clay layer at a rock outcrop near land animals (most notably the dinosaurs), many plants,

THE PLANETARY REPORT JANUARY/FEBRUARY 1998 and most marine invertebrates, including more than 95 percent of surface-dwelling ocean plankton. The KJT impact spread a layer of ejecta fallout world­ wide. This layer is marked by the anomalous quantities of iridium first seen at Gubbio and also by other trace metals, mineral grains showing evidence of severe shock, glassy tektites and microtektites, the mineral stishovite (an extremely high-pressure form of silicon dioxide), and soot, which seems to have come from the burning of a significant portion of the biosphere. The catastrophic effects of a Chicxulub-scale impact by an asteroid or comet, with a diameter of 10 kilome­ ters (6 miles), would have included an "impact winter" caused by global dispersal of massive amounts of dust. There would have been six months of darkness and cold, with a sulfuric acid haze that may have lasted years, further cooling the climate. Soon after the blast, reactions in the impact fireball would have formed acid rain, and hot ejecta re-entering the atmosphere would have set the wildfires that left the soot in the telltale layer at the KJT boundary. Changes in the world's oceans were important in the transition from the Cretaceous. Jan Smit of the Free University of the Netherlands has studied a very detailed section of the boundary in Spain that has a resolution of a few thousand years, showing that the sudden extinction of calcium carbonate-producing plankton coincides with the iridium-rich fallout layer. Moreover, an abrupt decrease in the carbon-isotopic composition in sediments at the boundary suggests a drastic reduction in the productivity of the surface oceans, an effect called the "Strangelove Ocean" (after a cult movie from the 1960s that climaxes in global annihilation by nuclear war). The Strangelove Ocean persisted for about 500,000 years. Plant communities on land were decimated, and full recovery took about 1.5 million years. Although the impact winter may have been too brief to leave its mark in the sedimentary record, the oxygen­ (continued on page 8)

Figure 1: In a gorge near the Italian town of Gubbio, Cretaceous The Chicxulub impact crater lies buried under sediments in the Yucatan Peninsula. limestones (white) are overlain by darker Tertiary limestones. A thin In this radar image, you can see surface traces that betray the crater beneath. layer of clay containing anomalous amounts of iridium and shocked A faint blue "X" in the center right shows where a road crosses the crater floor minerals marks the infamous KIT boundary, the geologic moment of boundary. The coastline is marked by a ring of cenotes, or fresh water springs, the extinction of the dinosaurs. Phofo: Michael Rampino associated with the crater beneath. Image: JPLlNASA 7

THE PLANETARY REPORT JANUARY/FEBRUARY 1998 Mass Extinctions in 540 Million Years (continued from page 7) Correlating Craters and Extinctions 100 o isotope ratio ofthe The impact cratering record on the terrestrial planets can be 'u'" t o carbonate shells used to test whether there is a general relationship between " 80 t c. * * * CfJ" * (providing an esti­ impacts and extinctions. To start with, work by Gene Shoe­ "c: 60 . ~ mate of the temper­ maker of the US Geological Survey and colleagues, based 2 ature of the ocean on observations ofEat1h-crossing asteroids and comets and "0 40 surface) shows a crater counts on the Moon and other planets, has led to the "'" significant change conclusion that bodies large enough to make a Chicxulub­ C'" 20 "~ at the boundary, sized crater collide with Earth every 100 million years or 0.." equivalent to a 1 0 0 100 200 300 400 500 warming of 8 to Millions of Years Ago 10 degrees Celsius Crater Size and Magnitude (14 to 18 degrees of Resulting Extinction Event Figure 2: Five major mass extinctions and about 20 minor Fahrenheit) for sev­ 100 mass extinctions have occurred over the past 540 million eral thousand years. years, as indicated in this chart showing extinctions of This warming may 1:> .... marine species (data from Sepkoski). Symbols above c: ... --- extinction peaks indicate associated evidence of large, .'"x 80 MANICOJJA'GAN have been caused w well-dated impact craters (f), diagnostic evidence of Q) by increases in "0 / .. impact (*), or documented but minor iridium spikes (0). Note: Large peaks prior to 450 million years ago most atmospheric carbon ~ I 60 I likely reflect incompleteness of the fossil record. .'!''" dioxide resulting PUZEcH- W TUNKI .... --- Q) "c. ... -- from the great per­ I ., Vl 40 I ., turbation of the 0 " - I e " C pfGAI global cycling of Q) I "Q; 20 , carbon, or the warming may have been caused by reduced 0.. / I / cloud cover related to the extinction of plankton. The I '/ dimethyl sulfide gas normally emitted by plankton is a a ~~ major source of condensation nuclei for the formation of 50 100 150 200 250 300 Crater Diam eter (kilometers) clouds over the oceans.

Figure 3: This theoretical "kill curve" relates impact-crater sizes Cycles of Catastrophe to mass extinctions, assuming a cause-and-effect relationship. Jack Sepkoski of the University of Chicago has compiled Dashed lines indicate upper and lower bounds of error (after data showing extinction rates of marine organisms over the Raup). Sizes of the largest weI/-dated craters (dots) are plotted against the magnitude of concurrent extinctions. last 540 million years (Figure 2). In this context, the KIT boundary can be seen as one large peak in a record of puls­ es above a generally low "background" rate of extinction. Major extinctions also occurred at the end of the Ordovician period (about 438 million years ago), in the Late Devonian so (such an impactor would have a diameter of about 10 period (365 million years ago), at the end of the Pennian kilometers, or 6 miles, the rule of thumb being that the period (250 million years ago), and at the end of the Trias­ crater diameter is about 20 times that of the impactor). sic period (203 million years ago). The greatest extinction About five or six such impacts should have occurred over in the record, separating the Permian and Triassic periods, the last 600 million years, which agrees with the record of invol ved the disappearance of about 96 percent of marine five major mass extinctions in that period oftime (Figure 2). species, along with most vertebrates (amphibians and rep­ Collisions with somewhat smaller bodies- with a diameter tiles), plants, and insects, according to studies by Dave of 5 to 6 kilometers (3 to 4 miles) and only one-tenth the Raup, also of the University of Chicago. Sepkoski's curve power on impact- should have occurred every 20 to 30 shows about 20 lesser, but still significant, extinction events million years, and these could account for the roughly 20 in which between 20 and 50 percent of then-existing species minor mass extinctions seen in the fossil record. of marine life disappeared. Studies ofthe atmospheric, climatic, and ecological effects The causes of mass extinction have been intensely debat­ of a large impact have made it clear that such events repre­ ed by the geologic community. Geologic thinking has long sent the greatest known perturbations of the biosphere. been biased against catastrophic change, and although Thus, we might expect that the record of mass extinctions many geologists will now admit that the end-Cretaceous should mirror the record of large impacts. mass extinction was probably related to a cosmic impact, Using the average waiting times between mass extinctions the consensus is that mass extinctions are generally caused and between impacts of various sizes, Raup has produced by a complex interaction of normal terrestrial factors, a quantitative "kill curve" that relates the severity of an including changes in sea level, climatic fluctuations, and extinction to the size of impact craters. To further test this competition among species. Some geologists would say innovative idea, we can plot the largest, well-dated impact that the causes of mass extinction are so complex that we craters against the mass-extinction events that seem to coin­ will never unravel them. However, if the KIT extinctions cide with these impacts (Figure 3). The results not only resulted from the after-effects of a great impact, as now support a general impact-extinction relationship but also seems likely, then a powerful hypothesis is available to suggest a cutoff- that is, craters with a diameter less than explain mass extinctions in general- a hypothesis that about 60 kilometers ( 40 miles), created by objects 2 to 3 8 can be tested in a number of ways. kilometers in diameter (1 to 2 miles), should not produce

THE PLANETARY REPORT JANUARY/FEBRUARY 1998 ------~~------

extinction pulses that would show up in Sepkoski's data. a global "silent spring." Eventually fern-like plants In fact, there are a number of craters with a diameter of 40 recolonized the environment, and then there was a resur­ to 60 kilometers (25 to 40 miles) that are not associated gence to climax-vegetation. However, the recovery of plant with mass extinctions, which makes sense according to diversity took more than 4 million years. This pattern of the curve. disturbance is similar to that at the KIT boundary, where the disappearance of plant life is marked by a brief barren The Biggest Extinction Ever interval, followed by a time when only ferns, and in some Another way of testing a general impact theory of mass ex­ areas fungi, prevailed. tinctions is to study in detail the geologic record of other Is there any evidence of impact at the Permian/Triassic mass extinctions, comparing these with the record at the boundary? Two Permian/Triassic layers somewhat en­ KIT boundary. Several mass extinctions were accompanied hanced in iridium have been found at several sites (Figure by Strangelove Ocean effects. At the end of the Permian, 4). Their iridium values are considerably weaker than those there were abrupt decreases in carbon-isotope ratios, as at the KIT boundary, which could indicate impact by icy shown in work by William T. Holser ofthe University of comets, which are probably less rich in iridium than stony Oregon and others, indicating a great mass mortality and asteroids. lowered productivity in the oceans, similar to that seen at The evidence of impact involvement in the Permian/ the KIT boundary (Figure 4). Analysis of these data by Triassic extinction, the most drastic die-off in the geologic Andre Adler ano me at New York University suggests that record, is tantalizing, and geologists continue to search for other signs of impacts. Preliminary evidence of quartz grains with possible shock features has been identified in Australia and Antarctica by Greg Retallack from the Carbon Isotope Ratios (right) and Iridium University of Oregon and colleagues. These results, if at a Permian/Triassic Site confirmed, could establish a link between impacts and the greatest mass extinction of all time. Late Devonian Comet Shower? The Late Devonian mass extinction (about 365 million 100 years ago) is marked by an abrupt negative shift in carbon

'='(J) (J) isotopes, as also observed at the KIT boundary. The Late 150 ~ Devonian extinction of marine invertebrates apparently f- allowed proliferation of shallow-water algae, which pro­ duced widespread algal mats on the sea floor. Such mats 200 do not develop during normal times because snails com­ monly graze on the algae. The mass extinction of the 250 grazers allowed the algae to grow unimpeded. Z

these major changes took place in less than 50,000 years, and could have been instantaneous. Evidence for sudden warmings and climatic instability at the end of the Permian period is reinforced in the oxygen-isotope record. Many sections ofthe geologic record across the Permian/ Triassic boundary show an almost total disappearance of typical Permian pollen, indicating a dramatic mass extinction of plant life. A thin layer at the boundary contains only Figure 5: The solar system moves up and down, like a one kind of pollen or spore variety-fungal spores. This carousel horse, through the disk-shaped Milky Way galaxy. represents an ecological disaster in which land plants Charts: Redrawn by Barbara S. Smith were devastated and fungi proliferated on the resulting dead organic matter, a situation that might be described as 9

THE PLANETARY REPORT JANUARY/FEBRUARY 1998

I which has been interpreted as a probable tsunami deposit. Calculations suggest that the severe blast and fireball effects of the impact would have killed most creatures on land and in shallow seas within hundreds of, and perhaps a thou­ sand, kilometers of the impact site. There has been much discussion as to whether dinosaurs in western North America were declining in the 10 million years or so prior to the end-Cretaceous impact. Many paleontologists have argued that the disappearance of many species of dinosaurs well before the KJT boundary was good evidence that the impact was not the cause of the dinosaurs' demise. However, the early die-off of those dinsosaurs may have been a regional event, caused by the The last Tyrannosaurus might have l(JOked up to see a comet shower lighting up the sky­ Manson impact. and spelling doom for him and his fellow dinosaurs. Painting: Joe Tucciarone At the time of the Manson impact, nine genera of dinosaurs disappeared in western North America. Soon after, there was a phase of immigration and rapid algae may well have been iridium-enriched, post-impact evolution of other dinosaur species, accompanied by major ocean waters. The case for impact involvement is reinforced changes in mammals and marine reptiles, such as the plesio­ by glassy microtektites near the extinction boundary, re­ saurs. The havoc that followed the blast is written plainly in ported by Philippe Claeys, now with the Humboldt Univer­ the geologic record. Dinosaur bone beds- some indicating sity of Berlin, and others. In addition, the Late Devonian extremely violent conditions and containing entire herds sea floor is marked in a number of localities by chaotic beds of more than 10,000 Hadrosaurids- occur in 74-million­ that may be tsunami deposits, like those accompanying the year-old rocks in Montana, about 1,400 kilometers (900 KJT extinctions. We know of several impact craters with miles) from the Manson impact site. At a similar distance, radiometric ages close to 365 million years, including the the famous "fossil forest" site near Chaco Canyon, New Siljan Ring crater in Sweden (55 kilometers, or 34 miles, Mexico reveals that trees were killed and buried in place wide) and the similarly sized Charlevoix crater in Canada. by sudden events, possibly the blast and fire or possibly Overall, the evidence accumulates for impacts- perhaps a flash floods triggered by the Manson impact. The overlying shower of comets- as a possible cause of the Late Devonian sediments contain a profusion of tiny fossil mammals, extinctions. opportunists, perhaps, that took advantage of the regional Another instance of multiple impacts associated with kill-off of all large predators. mass extinctions may exist at the Jurassic/Cretaceous If this notion of regional extinctions resulting from small boundary- meaning that the Cretaceous world, which end­ impacts is correct, then sub-continental effects of Manson­ ed with a huge impact 65 million years ago, may have been sized impacts should be relatively common in the geologic ushered in by a series of smaller impacts. Three craters have record. been dated close to the Jurassic/Cretaceous boundary, about 142 to 144 million years ago: the Gosses Bluff, Australia Shiva's qalactic Timetable crater (22 kilometersll4 miles wide, about 143 million Analyzing the extinction record over the last 250 million years old), the Morokweng impact structure in South Africa years (the best-dated part ofthe record), Raup and Sepkoski (70 kilometers/40 miles or more wide, 145 million years found an underlying cycle of about 26 million years. My old), and the Mj01nir impact structure in the Barents Sea recent work with Bruce Haggerty at NYU on the improved (40 kilometers125 miles wide, 142 million years old), and better-dated extinction data base shows that a cycle which is associated with an ejecta layer rich in iridium and of about 30 million years persists through the entire 540 shocked quartz grains that can be traced in strata as far million year record. away as 2,500 kilometers (1,600 miles). Raup and Sepkoski' s original findings prompted Richard Stothers of NASA's Goddard Institute for Space Studies Smaller Impacts and Regional Extinctions and me and, independently, Walter Alvarez and Rich Muller The Manson crater in northwest Iowa, attributed to the of the University of California at Berkeley to analyze the impact of a 2-kilometer (I-mile) body about 74 million record of impact craters, and we detected a similar cycle of years ago, is a prime example of the regional environmental about 30 million years in the occurrence of large impacts effects of a relatively small comet or asteroid. The crater on Earth. Furthermore, it seems that clusters of craters of has a diameter of35 kilometers (22 miles). A search for similar ages are closely correlated with the mass extinction Manson ejecta in similar-age marine rocks to the west led to episodes. These results suggest that many impact events, the discovery of shocked minerals in the widespread Crow most likely from showers of comets, have led to periodic 10 Creek bed of the Pierre Shale in southeastern South Dakota, pulses of mass extinction.

THE PLANETARY REPORT JANUARY/FEBRUARY 1998 Geological Thne Scale o think in geologic time is to transform Yyour conception of time as marked by Era Period Epoch VearsAgo minutes, hours, years, and lifetimes. Geologic time begins 4.5 billion years ago, as Earth was Cenozoic Quaternary Recent .. , ...... 10,000 forming. It does not proceed in a measured Pleistocene way, with eras and epochs divided into equal ••• * .. .••• .•.. .•••• .. 1,800,000 parts. Rather, geologists punctuate time by Pliocene marking changes in the dominant life forms 5,000,000 on Earth, as recorded in fossil deposits. Miocene 22,500,000 Nearly 4 billion years of Earth-time lie in Tertiary Oligocene the shadowy Precambrian era, when simple, 37,500,000 single-celled bacteria, plants, and animals Eocene 53,500,000 dominated the planet. Then, just 570 million Paleocene years ago, life changed. Suddenly, complex 65,000,000 fomlS emerged and spread in a phenomenon called the Cambrian explosion. From this Mesozoic Cretaceous ...... 140,000,000 moment on, time is measured by dramatic Jurassic shifts in the fossils that characterize each ...... 200,000,000 Triassic passing age. In other words, geologic time ------230,000,000 is measured by mass extinctions. The Paleozoic is the era of invertebrates Paleozoic Permian ..... 280,000,000 and simple vertebrates, such as fish, amphib­ Carboniferous ians, and primitive reptiles. The Mesozoic ...... 345,000,000 marks the reign of the dinosaurs, an era that Devonian ...... 390,000,000 ended almost instantaneously only 65 million Silurian years ago. Mammals have dominated the ...... 430,000,000 Cenozoic, and not even the most scientifical­ Ordovician ly advanced of that group can say how long ...... 500,000,000 it will be before another mass extinction Cambrian punctuates geologic time. ------570,000,000 - Charlene M. Anderson, Pre-Cambrian Director ofPublications

If the approximately 30 million year periodicity in mass pointing to a general theory relating mass extinctions, and extinctions and impacts is real, it may be related to a known perhaps other geologic events, to the impacts of large comets astronomical cycle- the motion of the solar system up and and asteroids on the Earth. In this view, mass extinctions of down through the plane of the Milky Way galaxy. As the life occur as discrete pulses. Major environmental perturba­ solar system revolves around the galaxy, it also bobs up and tions are shown by changes in carbon isotopes and climatic down, like a horse on a carousel, through the galactic disk. changes by oxygen-isotope shifts. The mass extinctions In this cycle, the Sun and planets pass through the dense re­ are commonly followed by a period of rapid evolution of gion of the disk, packed with stars and clouds of interstellar surviving species, with newcomers filling new niches in the gas and dust, about every 30 million years (Figure 5, page 9). wake of global disaster. Thus with the Chicxulub impact Stothers and I were first to suggest that passage of the solar 65 million years ago, the Mesozoic world, populated by system through the galactic plane could lead to gravitational giant dinosaurs and flying reptiles, gave way to the modern disturbance of some of the trillions of comets that orbit the world of mammals and birds. And thus, like Shiva the Sun in the Oort Cloud, resulting in periodic showers of comets Destroyer, cyclic impacts bring an end to one world, allow­ in the inner solar system. Recent work by astrophysicist ing a new one to begin. John Matese and colleagues at the University of South­ The impact hypothesis of mass extinctions, which began western Louisiana has confirmed that the pull of the com­ with study of a thin clay layer at Gubbio, has grown in bined mass of the material in the galactic disk is sufficient theoretical and observational support, pointing toward an to induce a hail of comets. important unifying concept in the Earth sciences. This This cycle of doom has relevance to the present-day im­ new awareness of impacts as a major force in Earth history pact hazard, as the solar system passed through the galaxy'S promises a revolutionary transformation of our view of plane in the last few million years. By some accounts a the cosmos. barrage of comets recently dislodged from the Oort Cloud should be approaching the planets now. Michael R. Rampino is a Research Consultant at NASA's Though we are far from certain in our understanding of Goddard Institute for Space Studies New York and Profes­ catastrophes on a cosmic scale, exciting findings from sor ofEarth and Environmental Science at New York astronomy, geology, and the history of life seem to be University. 11

THE PLANETARY REPORT JANUARY/FEBRUARY 1998

I. + magine yourself exploring worlds of extremes, a realm Galileo was originally scheduled to end its exploration on where the deep cold of space freezes water to brittleness December 7, 1997, but NASA and Congress approved a contin­ I while, nearby, molten rock flows under spewing foun- uation of the mission through the last day of 1999. GEM has tains of sulfur. Hung in space behind you is a brilliant three phases, each with tightly focused objectives: the Europa globe sporting large, colorful clouds caught in centuries-old Campaign ("Ice"), Perijove Reduction/Jupiter Water Study/Io storms, with towering thunderclouds that change within Torus Passages ("Water"), and the 10 Campaign ("Fire"). hours. Galileo has begun the Galileo Europa Mission (GEM), spending two more years at Jupiter studying ice, water, and fire there: the icy crust of Europa, the thunder­ storms of Jupiter, and the fiery volcanoes ofIo. Galileo first arrived at Jupiter on December 7, 1995 after a six-year journey from Earth. The spacecraft swung by the moon 10, then fired its main engine, braking into orbit. Galileo collected precious data from a probe it dropped into the Jovian atmosphere, and for the two years of its prime mission- II orbits arOlmd the solar system's biggest planet- Galileo revealed fascinating details of Jupiter and its moons. For example: while Jupiter's composition is mostly the primordial mix of hydrogen and helium, there is also water. Water circulates in the top layers of Jupiter's clouds, causing thunderstorm-like activity in some areas and rendering other areas dramatically dry. Galileo showed us a new Ganymede, the first moon in the solar system known to have its own magnetic field. From Galileo we learned that intensely cratered Callisto is coated with a layer of fine dust in some places. We've seen that lo's surface has continued changing since the Voyagers saw it in 1979. And scientists have found evidence that an ocean existed in recent geologic history­ 12 and may still exist-under Europa's crnst of ice.

PLANETARY REPORT EVR.9PA CAmPAicn evidence of churning water below. They'll count craters, For the first eight orbits of GEM, spanning more than a year, which helps date the youth of the moon's smooth surface Galileo will search for further evidence of an ocean beneath (the fewer the craters, the more youthful the surface). the icy surface of the intriguing moon Europa and try to Galileo will peek at Europa's layered interior by measuring determine if that ocean still sloshes today. Scientists will the effects of the moon's gravity, looking for variations scan the surface for spewing ice volcanoes and other direct in the thickness of its ice shell and possible indications of the depth of the subsurface ocean. A flowing, salty ocean could generate a magnetic field, so scientists will try to determine if the magnetic signals nearest Europa are gener­ ated within. Galileo will get detailed images and atmospheric data from around the Europan globe, including the polar regions, Europa (right with closest-approach distances ranging from 200 to 3,600 half of image) kilometers (120 to 2,240 miles). With resolution three times sports raft-like better than in the prime mission, some planned images will formations of ice the size of show details as small as 6 meters (the size of a truck!). cities on Earth. We'll use stereo imagery to determine heights for Europa's In this image the rafts appear relatively flat surface features. We'll map the distribution to have broken and composition of Europa's ice with resolution as fine as and floated 10 kilometers (6 miles), keeping an eye out for contaminants, apart. The San Francisco Bay which scientists believe may result from comet or meteorite image shown at bombardment or may come from within Europa itself. the left provides a good compar­ ison for scale PEERjnc Down AT iupiTE~ and resolution "Perijove Reduction" isn't some kind offad diet or a way in the Europa image. to shrink the national debt- it's what we need to do to get the spacecraft in an orbit close enough to Jupiter for flybys Image: JPUNASA ofIo. For six months in mid-1999, in four successive orbits, Galileo will use the gravitational pull of Callisto, along 13

JANUARYIFEBRUARY 1998 Above: The region around a dark "hot spot" on Jupiter appears in natural color on top and false color below. The atmosphere within a hot spot descends and dries out, then rises nearby to form billowing thunderstorm-like clouds. Left: Jupiter's Great Red Spot has been a study in sameness for nearly four centuries. But images from Galileo show how gigantic cloud features associated with the ancient storm system evolve in mere hours (insets).

with thruster bums for fine tuning, to halve the spacecraft's as 100 meters. But much ofIo's forbidding environment closest distance to Jupiter (called "perijove"). remains a mystery, and there will be much to learn from Peering at Jupiter from the closest distances since Arrival the final two orbits in GEM, which will feature flyovers Day, Galileo will study details of wind and storm patterns, from 500, then 300 kilometers (310, then 190 miles). including the billowing thunderstorms that grow to heights Are scientists trying to keep us all in suspense, waiting several times those we have on Earth. Water circulates until the last part of the mission to glimpse fiery 10 with vertically in Jupiter's top layers, leaving large areas drier breathtaking detail (resolving features as small as 6 meters)? than the Sahara and others drenched like Earth's tropics. The suspense will indeed be high as Galileo flies right over Mapping this distribution of water and understanding its Pillan Patera's plume of frozen sulfur. But the reason for role in Jupiter's weather can help us understand Earth's waiting until the end of the mission to explore 10 is that it faster-paced weather changes. minimizes changes in perijove, leaving more time and Once each orbit, during its passage from "ice" to "fire," resources for science studies. It also lessens the exposure Galileo will shoot through the 10 torus, a donut-shaped of the spacecraft to Jupiter's intense radiation, which cloud of charged particles that ring the orbit of 10. Galileo grows in intensity as you get closer to the gas giant­ will measure the density of the sulfur that streams from lo's strong enough in the vicinity of 10 to kill a human being. volcanoes and will also measure sodium and potassium, Galileo has been exposed to varying levels of radiation which get "sand-blasted" offthe Ionian surface by particles during its orbits of Jupiter. We expect the spacecraft will swept along in Jupiter's rotating magnetic field. continue operating through the intense exposure of the 10 campaign. However, radiation will likely pepper the cam­ io CAmPAicn era's light detector with blinding hits to many pixels and The closest Galilean moon to Jupiter, 10 is also the most may cause the computer's bits to flip in random ways. In active body in the solar system, sizzling with dozens of that event, Galileo will put itself into "safe" mode until sulfur and silicate volcanoes. The volcanoes are driven by further commands come from Earth. (It's hard to think with 14 tides that flex the otherwise solid surface of 10 as much your bits flipped!)

THE PLANETARY REPORT JANUARYIFEBRUARY 1998 Right: The yellow-green glow around 10 comes from the scattering of light by a moon-shrouding cloud of sodium atoms. In the near-right inset, Prometheus' volcanic plume casts a reddish shadow across lo's surface. In the far-right inset, Pillan Patera spits gas and dust, which appear blue against the black of space. Images: JPUNASA

Galileo should have ample power from its radioisotope the seven taken during the prime mission. For the remainder thermoelectric generators for the spacecraft and its instru­ of each orbit, we'll gather only minimal data on Jupiter's ments, and there should be plenty of propellant for the magnetic environment, using that time for playback of thrusters. One item of potential concern is Galileo's tape stored data. We are having to do without many mission recorder, which has already surpassed its design limit for specialists, who have moved on to other jobs. However, stops and restarts. If it fails beyond repair, we'll load the some experts may be brought back as a tiger team should onboard computer with a program that allows the instru­ any serious problems arise. After GEM, Galileo will no ments to take and transmit data in real time. Using this longer return science data but will keep slicing through the strategy, Galileo will be able to handle only a limited intense radiation near Io's orbit, reporting on spacecraft amount of data, significantly reducing the mission's scope. health until it is silenced by radiation damage. Weare keeping our fingers crossed. During the GEM mission of ice, water, and fire, Galileo will point the way for new investigations of these Jovian CETTinc mo~ F~m LESS worlds of extremes, possibly confirming that an ocean In keeping with NASA's vision oflower-cost space explo­ currently exists on Europa and locating areas where the ice ration, GEM's design takes advantage of an already orbit­ is thinnest. This important step will guide future Europa ing spacecraft to perform a tightly focused, lower-cost, missions- orbiting and potentially ice-boring missions­ higher-risk mission. To stay below a cost of $15 million that will look into a key question for the 21st century: is per year, we have trimmed spacecraft and ground opera­ there life on Europa? tions to a minimum. Engineering and science teams have You can follow Galileo on the World Wide Web at automated and streamlined wherever possible so that a http://www.jpl.nasa.gov/galileo. barebones staff-just 20 percent of the personnel from the prime mission-can operate GEM. Leslie L. Lowes, Member of the Technical Staff at the Jet For each orbit's closest-approach to Jupiter or a target Propulsion Laboratory, is Lead Outreach Coordinator for moon, Galileo will take only two days of data rather than the Galileo mission. 15

THE PLANETARY REPORT JANUARY/FEBRUARY 1998 by DaV'e Doody

CCass:i:nEi the Venusian atmosphere up to its final moments. Today Cassini continues to fall along its intended solar In its primary mission, Magellan carried out detailed orbit, on target to pass close behind Venus this April in a radar mapping of the planet's surface, and then it went on to spiraling path that will deliver the spacecraft to Saturn in perform the first demonstration of aero braking at a distant July 2004. Cassini is keeping its large, high-gain dish world. Magellan also mapped most of Venus' gravity field antenna facing the Sun to shade the rest of the spacecraft from a low, circular orbit. Many other spacecraft are today during its tenure in the inner solar system. Heaters controlled part of the evening "star," including probes, landers, and by stored settings and commands from Earth are main­ orbiters sent by the US and by the former Soviet Union. taining healthy temperatures on board and are also used to boil off any contaminants that may cling to sensitive science M:o;y,a:gEfr:..s ell Astronomical Unit, the mean Uranus and Neptune, and then it, too, went out of the distance between the Sun and Earth). ecliptic plane but in a southerly direction. Both Voyagers Routine tracking and data processing were suspended on are still healthy today and returning data from interplanetary March 31, 1997 for budget reasons. Power from the space­ space en route to interstellar space. They have identified craft's RTGs (radioisotope thelmoelectric generators) is low-frequency radio emissions from the heliopause, the low and decreasing. boundary between the Sun's outflowing wind and the sparse Because of a similar power decline, Pioneer 11 ' s mission interstellar wind coming from other stars in our region of ended in November 1995. Pioneer 11 could not continue to the galaxy. The Voyagers may survive to report crossing track Earth, which moves back and forth yearly from the the heliopause, estimated to be about 50 AU away from the spacecraft's distant viewpoint, today about 50 AU from the spacecraft. Sun and increasing 2.5 AU each year. Pioneer 11 was the Probably the first resource to fail for the Voyagers will first spacecraft to fly by Saturn and return images from the be sunlight, which their sensors use to locate Earth for radio ringed planet. communications. This far from home, the Sun appears as These tllle Pioneers proved it was possible to penetrate a bright star, but eventually, around the year 2020, it will the asteroid belt, and they paved the way for Voyagers 1 become too faint for the Voyagers to see. Power from the and 2, Galileo, Ulysses, and Cassini. Flybys of Jupiter gave R TGs will diminish about the same time to a level too the Pioneers enough energy to escape the solar system, and low to continue essential systems. Today, however, both rather than fall back to the Sun they will silently drift within Voyagers continue to watch the Sun and a distant star, and our Milky Way galaxy forever. Pioneer 11 will pass one of they continually adjust their attitude for Earth tracking, the stars in the constellation Aquila in about 4 million years. rocking slowly back and forth by firing tiny bursts of hot gas from their rocket thmsters. lf8'~:e'lJa:n When you see Venus, brilliant in the twilight sky, you're CD..'Y..5SS~!!tS seeing a planet that has been visited many times. The Ulysses, a joint project of the European Space Agency remains of NASA's Magellan spacecraft are now mostly (ESA) and NASA to explore the Sun's polar regions, is in gases mixed into Venus' atmosphere. Some of its more excellent health and beginning its second 6.2-year orbit of massive components, such as the attitude-control reaction the Sun. In April 1998 the spacecraft will reach the far point wheels, may lie on the planet's surface. Flown intention­ in its orbit, a Jupiter-like 5.4 AU from the Sun. Then it will 16 ally to its destlllction, Magellan was gathering data about return to high solar latitudes and continue its primary work.

THE PLANETARY REPORT JANUARY/FEBRUARY 1998 orbit the Sun, fly by Mars, and then encounter a comet and an asteroid. DS-l will test a handful of technologies, including a solar-electric propulsion system, known for decades by science-fiction enthusiasts. DS-l's system employs solar panels to generate electricity to power an ion engine. In addition to missions to visit bodies in the solar system and validate technologies under the Discovery and New Millennium programs, NASA is implementing a number of other scientific investigations in space. Be sure to visit the NASA site on the World Wide Web to find out more: http://www.nasa.gov.

;Sa,sjcs fC»:'f ~s.:p:ac-eJiI 'i"Bb:t And, finally, where is Basics of Spaceflight headed? This installment concludes its mn in The Planetary Report. For three years, it's been an honor to touch upon the "how it all works" side of space exploration with you, with readers who share my Four spacecraft are heading out of our solar system and will someday cross own excitement in the use of robotic spacecraft to the heliopause and enter interstellar space. The other operating spacecraft explore our cosmic back yard. are locked in orbit about the Sun or planets and will remain artificial satellites Now the ball is in your court. Let's open a within the solar system. Chart: Dave Doody; redrawn by Barbara S. Smith dialog about these subjects on the Web : what's on your mind? If the World Wide Web isn't available from your home or office, you might find a public The spacecraft will be working those high solar latitudes library nearby that offers connectivity and a little help. I look during the Sun's next period of maximum activity-a fortu­ forward to hearing from you via the Society's Web site: nate opportunity. Ulysses will execute its south polar pass http://planetary.org. from September 2000 to January 2001 and its north polar pass from September to December 2001. Before then, how­ Dave Doody is a member of the Jet Propulsion Laboratory's ever, Ulysses will coordinate its observations with ESA's Advanced Mission Operations Section and is currently work­ SOHO spacecraft, which carries experiments for studying ing on the Cassini mission to Saturn. the Sun's corona and the solar wind.

:m::i,s-c.o ·,u:er:¥ Thanks, Dave, The NASA Discovery program incorporates state-of-the-art and See You Soon technologies into small projects with a fast turnaround. NEAR, the Near-Earth Asteroid Rendezvous, was the first of the ave Doody and his Basics of Spaceflight col­ Discovery missions to be launched. NEAR encountered the D umn are leaving the pages of The Planetary asteroid 253 Mathilde in June 1997 en route to its destination, Report with this issue. But this is not goodbye-with asteroid 433 Eros, which it will orbit. Mars Pathfinder, his knowledge, talents, and generous spirit, Dave has renamed on Mars as the Carl Sagan Memorial Station, was been too valuable to the Planetary Society for us to the second mission in the Discovery series. Each of the Dis­ let him go. He is simply transferring from a fonnal covery missions costs less than a major Hollywood movie, print medium to an informal electronic fomm. including everything from design and fabrication through We are developing an interactive page in the Soci­ launch and 30 days of operation. ety's home page where Dave will be available to an­ The next mission to be launched under the Discovery pro­ swer questions posed by members. With this feature gram will be Lunar Prospector, designed to spend a year in we will explore new territory, so check our Web site polar orbit around the Moon. Lunar Prospector will carry out frequently to monitor our progress. remote sensing ofthe Moon from low orbit using five science In the next issue of The Planetary Report, we instmments, including one that will be able to determine for will introduce a new column called The Stuff of Life. certain whether there is water ice at the Moon's poles. Noted biologists, chemists, astronomers, and plane­ Stardust, the fourth Discovery mission, launches in Febm­ tary scientists will offer short essays on the elements, ary 1999 to gather material from a comet's coma (the "halo" compounds, and processes that make life on Earth of gas and dust surrounding the comet core) and return it to possible and extend their musings to the possibility Earth for analysis. of life on other worlds. .N:e:vv [Nli:•• : en:n 'i .u .... So, we thank Dave for his contributions to The NASA's New Mi llennium program is a series of missions Planetary Report. And we invite our readers to join designed to test new technologies so they can be confidently us as we branch out with new endeavors. used on science missions in the 21st century. The first in this - Charlene M. Anderson, Director ofPublications series is Deep Space One CDS-I), which launches this July to 17

THE PLANETARY REPORT JANUARY/FEBRUARY 1998 News and

RevielNs by Clark R. Chapman

erican geologists gather amlU­ ing fast food before the afternoon ses­ of getting onto the Martian surface, ally to share their research, hunt sions begin. One Hot Lunch I attended and the geologists' Mars mania was A for jobs, scout out new text­ concerned "dubiofossils" in Mars rocks. palpable as Golombek received a stand­ books, and learn what their colleagues The moderator explained that geologists ing ovation. have been up to. Meetings of the Geo­ knew about real fossils and pseudo­ The planetary sessions, including the logical Society of America (GSA) are fossils (things that look like fossils but Shoemaker/Pathfinder evening, were far larger than meetings of astronomers. definitely aren't). The nano-fossils in organized by the GSA division of plan­ The practicality of geology swells its Martian meteorite ALH84001 are neither: etology. At its business meeting, offi­ ranks. But space science, in the guise their nature is lllcertain, or "dubious." cers hawked Mattei's Sojourner toy to of "planetology," is also represented. But they have provided fertile ground raise funds for the Dwornik student­ Last October, the mountains sun-ound­ for research and controversy since they paper awards. There was a spread of ing Salt Lake City glistened with early were announced in Science magazine assorted candies and cookies so plane­ snow. But down in the Mormon capital, in August 1996. tologists could craft other, sweeter where geologists filled most of the city's It appears that the case for real fossils models of Pathfinder. Arizona State hotels, temperatures hovered near 70 in the meteorite is dissolving- at least University planetologist Ron Greeley degrees. In the many lecture halls and in the minds of most researchers. Even was honored with the G. K. Gilbert ballrooms of the sprawling Salt Palace the proponents, authors of the original Award, the first of which was given to convention center, two dozen simulta­ paper, seem to be retreating to open­ Gene Shoemaker in 1983. neous talks proceeded on subjects as minded unceltainty about the biological In a special GSA symposium, deeply diverse as our planet. origins of the micro-worms whose por­ relevant to planetary studies, a fellow At 170 booths on the main floor, traits have been so widely published. dressed in the full regalia of Archbishop entrepreneurs hawked maps, books, (For a review of Mars meteorite fossil Ussher opened a session of invited talks computers, and geologists' tools-of-the­ studies, see the November/December dealing with time. The archbishop is trade (even shoes!). Official country 1997 Planetary Report.) famous, or infamous, for dating the maps were offered for sale by foreign If the nano-fossil basis for renewed origin ofthe Ealih (and universe) at geological surveys while the geologists interest in Mars is faltering, Pathfinder'S 6,000 years ago (4004 BC, to be exact). who made the maps circulated on the success this past swnmer has more than Talks treated the history of more recent floor, making professional contacts. reinvigorated both scientific and public Earth chronologies, from those of Lord The well-attended planetology ses­ curiosity about the Red Planet. A huge Kelvin to present-day findings, including sions presented results about planetary crowd of geologists, eager for the latest the latest Moon, Mars, and meteorites geology, including data from Galileo and Pathfinder findings, overflowed the results from the new hafnium/tungsten Mars Pathfinder. One session ventured largest auditorium at the Salt Palace radioactive isotopic chronometer. Oth­ into the more speculative realm of the one evening, first paying tribute to the er speakers described continuing prob­ origin and early evolution oflife on Emth memory of Gene Shoemaker; US Geo­ lems in measuring rock ages precisely and other planets. Unlike astronomers, logical Survey planetary geologist Mike enough to resolve raging debates about who usually take a casual approach Can- delivered an enthusiastic, personal the rates of catastrophic change in Earth and scribble on vu-graphs, geologists eulogy that Gene would have loved. history. normally present their work in well­ Centuries after Ussher, and even after designed, artistic-color slide shows. Mars Mania the revolutionary insights of Gene Shoe­ Papers not scheduled for oral delivery Then the personable Pathfinder Project maker on the predominance of catastro­ appear in poster displays of equally Scientist, Matt Golombek, delivered an phism on Emth (whether wrought from professional quality, enhanced by the engaging, lavishly illustrated account the skies or not), geologists m"e still graphics technologies long used by of the first months of Mars exploration caught up in philosophical debates on geologists to print large, colorful maps. by Pathfinder and the little rover, So­ geologic methodologies ... providing journer. It remained unclear to many in strategic ground for attacks by the rising Hot Rocks the audience exactly what totally new forces of neo-creationist Luddites. The GSA treats controversial issues of things we've learned from Pathfinder broad interest at so-called "Hot Lunches." about Mars and about the central ques­ Clark R. Chapman spoke before the GSA Panelists debate each other, then open tion of whether Mars harbors evidence about the chronological implications the floor to discussion from the audience of cun-ent or past life. But Pathfinder of the Galileo spacecraft's pictures of 18 -at least from those not busily munch- clearly did demonstrate the technology craters on Jupiter 's satellites.

THE PLANETARY REPORT JANUARY/FEBRUARY 1998 ------~------~-~------.-

World

lNatch by Louis D. Friedman

Cape Canaveral- On October power system efficiency promise to Paris-The European Space Agency 15, at approximately 5:00 a.ill., a mighty reduce the needed amount of nuclear (ESA) has resurrected their interest in Titan 4/Centaur rocket lifted the Cassini power still further. Mars exploration by initiating a Mars spacecraft and Huygens probe on their Express for launch in 2003. The mis­ way to Saturn. The launch was marked Washington, DC-NASA has sion will fly on a Soyuz rocket, built in by controversy because the spacecraft selected two more missions in the Dis­ Russia and sold by a French-Russian carried a nuclear power source. The covery program of solar system explo­ consortium. nuclear power is required because of ration. Contour, led by Joseph Ververka When the Europeans dropped their Saturn's great distance from the Sun, of Cornell University, is being built by Intermarsnet mission plans, it appeared as was the case with Galileo, Voyager, the Applied Physics Laboratory of Johns that they would not be conducting any and the Pioneers. Hopkins University. This mission will Mars exploration in the next decade. The Planetary Society has always fly by three comets, deepening our know­ Intermarsnet was a US-ESA cooperative been sensitive to environmental issues ledge of these bodies and their origins mission to place several Mars landers in related to planetary exploration. In our and perhaps their role in the distribution a seismology/meteorology network on support for the Cassini launch, we of water in the solar system. the Martian surface. attempted to balance the important The other mission, Genesis, is being Mars Express is an orbiter mission concerns of public safety and environ­ led by Donald Burnett of the California that will carry the three main European mental effects with our philosophical Institute of Technology and built by the experiments lost on the ill-fated Mars commitment to planetary exploration. In Jet Propulsion Laboratory. Genesis will '96. The experiments are HRSC, an 1990, before the Galileo mission began, collect samples of solar wind particles to imaging system; OMEGA, an infrared we looked into the issue of nuclear bring back to Earth. The collection site spectrometer; and a Fourier Planetary power aboard spacecraft and, following will be one of the Lagrangian points of Spectrometer. In addition, a sounding Carl Sagan's lead, supported that launch the Earth-Sun system-points in space radar and plasma-measuring instrument while advocating full disclosure of where gravitational balance will hold will complete the science payload. risks and precautions. an object in position. The mission has about 150 kilograms Today we advocate smaller and more These are the fourth and fifth mis­ (330 pounds) of payload mass available efficient nuclear power systems for sions of the Discovery series; the first for a lander or landers. Several designs future missions, recognizing that outer­ three were the Near-Earth Asteroid and scientific proposals are being stud­ planet and planetary-lander missions Rendezvous (NEAR), Mars Pathfinder, ied in England, the Netherlands, France, will continue to require some sort of and Stardust (which, as mentioned in and Germany. ESA has apparently not nuclear power. last issue's Society News, will carry a made a commitment to funding the lan­ The Galileo mission has been a ter­ microchip bearing the names of all ders, so their status remains uncertain rific success at Jupiter with repeated Planetary Society members as of at this time. flybys of Europa, Ganymede, and 10 and November 1997). The successful launch of the Ariane V the extended mission now underway enables Europe to be a player in other (for details on Galileo's discoveries and Cape Canaveral-With its planetary missions. One possibility under the extended mission, see "Ice, Water, launch re-scheduled from September study is using the Ariane V for launch of and Fire" on page 12). Cassini promises 1997 to January 1998, Lunar Prospec­ the nascent Mars sample-return mission similar rewards at Saturn, and we all tor will observe the Moon from polar planned by NASA for 2005. Benefits await the success of its mission there orbit with a prime goal of determining to the Mars Surveyor program would in 2004 and beyond. whether there is water ice in perma­ include increased payload capability Future outer-planet missions, such as nently shadowed regions of craters at and international participation. a Europa probe and the Pluto fast flyby, the lunar south pole. An earlier mission, will be smaller and will require less Clementine, detected the radar signature Louis D. Friedman is Executive Director plutonium. Technological advances in of water ice there. of the Planetary Society. 19

THE PLANETARY REPORT JANUARY/FEBRUARY 1998 With all the detritus that is constantly with a telescope (there were undoubt­ determined that the tail is not likely streaming away from a comet (in edly more).The comet collided with to have been dust. These observations the form of a coma and a tail) and Jupiter in 1994 and became part of the are consistent with a theory presented bleeding off into space, why doesn't process of planetary accretion. by Fraser Fanale and Jim Salvail, of the the comet's nucleus eventually Certain comets pass behind the Sun University of Hawaii, that heating at shrink down to nothing? and disappear, never to be seen again, the surface results in a fused crust, so -James K. Walker, as in the case of comet C11992 Nl. low-gas comets may close off their Brasilia, Brazil This comet was observed three times vents. According to this model, the in July 1992. It then passed behind crust would insulate the icy core from Maybe it does. But comets meet their the Sun and we should have seen it the Sun's rays and the comet's nucleus demise in other ways as well. We see re-emerge on the other side, but we would then look like an asteroid. evidence of the reduction of comets haven ' t. It seems that this comet may Our understanding of comets' behav­ have just fizzled ior and their pathways to extinction is out. Numerous limited. Maybe some comets do waste comets belonging away to nothing. It is something to to the Sun-grazing watch for. Kreutz family -LUCY McFADDEN, meet their demise University ofMaryland as the Sun's tre­ mendous heat and I really enjoyed William Hartmann's gravitational forces article on the Moon's origin (see the consume them. September/October 1997 issue of The In another Planetary Report). Why did the Moon scenario, comets seem to stop receding from Earth just may lose their tails at the distance where it exactly covers and start looking the Sun during a total solar eclipse? more like asteroids. Or is that just a coincidence? In 1948, comet -Steve Ricco, Wilson-Harrington Deerfield Beach, Florida · was photographed as part of the Palo­ It's just a coincidence. The receding mar Sky Survey. motion of the Moon, predicted from This object was tidal theory and actually measured by "discovered" laser signals bounced off reflectors on again in 1979 the lunar surface, is so slow that the as asteroid 1979 Moon has been at about the same dis­ VA. According tance during all of humanity's brief to measurements recorded history. But, on average, it that I made with is still receding and its distance from Alan Chamberlin, Earth varies slightly during its monthly former graduate orbit. Sometimes the Moon is too far when they split. In almost all cases, student, and colleagues in 1992, there away to cover the Sun and if an eclipse tidal forces produce the splitting. is no evidence of cometary activity in occurs, then we get an annular eclipse, After comet Shoemaker-Levy 9 was 1979 VA anymore. Another student with a ring of light remaining visible captured by Jupiter, its orbit evolved of mine, Yan Fernandez, analyzed around the Moon. to the point where tidal forces split it the color and intensity of the tail on -WILLIAM K. HARTMANN, 20 into 26 pieces that we could observe the 1948 photographic plate and Planetary Science Institute

THE PLANETARY REPORT JANUARY/FEBRUARY 1998 Factinos The image ofa crater on the surface ofEuropa on the cover of the September/October 1997 issue of The Planetary Report is very inter­ A recently discovered object, known as 1996 TL66, threatens to esting, but I don't understand your caption disrupt our well ordered vision of the local universe. The Texas­ describing it as an impact basin. If it was sized body, too big to qualify as a comet but too small to be a prop­ caused by an impact, where is the rim and er planet, appears to swing well out of the boundaries of the known where is the debris? solar system. Its highly elliptical orbit carries it to an area of space -Frank Germann, between the Kuiper Belt and the Oort Cloud that scientists believed West Saint Paul, Minnesota to be empty. Jane Luu and Brian Marsden ofthe Harvard-Smithsonian Center Tyre would be better described as an impact for Astrophysics and colleagues reported their discovery in the June structure than a "basin," since it is actually 5, 1997 issue of Nature. While this is the flIst such object to be quite flat. Tyre bears a striking resemblance to found in this "no man's land," Luu estimates that as many as 6,000 Callanish, a similar-size structure on Europa's others like it may exist. "The point is," says David Jewitt, one of the trailing hemisphere whose origin has also been paper's authors, "we have no idea what is in the outer solar system." debated (see Figure 1, "Puzzling Rings," on - from the Harvard-Smithsonian Center for Astrophysics and the page 12 of the March!April 1997 issue of The Los Angeles Times Planetary Report). Galileo imaged Callanish near the morning terminator (the line dividing the light and dark sides of a planet or moon) Earth's upper atmosphere harbors unexpected amounts of water with enough detail to reveal many smaller, sec­ vapor, according to data from a pair of satellites. This discovery ondary craters radiating from a central smooth could bolster a controversial theory that thousands of house-sized patch nearly 50 kilometers (about 30 miles) in comets are hitting the atmosphere every day. "What we have found diameter. We believe that these small pits and is astonishing. We really don't have answers," says Robert R. crater chains have been produced by debris Conway of the Naval Research Laboratory in Washington, DC. ejected during the impact of an object we Conway and his colleagues made their discovery last August estimate to have been more than a kilometer using an instrument called the Middle Atmosphere High Resolution (0.6 mile) in diameter. Both Tyre and Callanish Spectrograph Investigation (MAHRSI), part of a satellite released are surprisingly different from impact features and later retrieved by the space shuttle. The instrument measures on the Moon and the other icy Galilean satel­ -hydroxyl (OH) ions in the upper stratosphere and overlying meso­ lites because of their low relief and sets of sphere, from 35 to 100 kilometers (about 20 to 60 miles) above Earth. concentric fractures. The amount of hydroxyl is directly related to the atmosphere's The best current hypothesis is that the impacts humidity. penetrated through a thin brittle shell into a The MARRS I data revealed a layer with a surprising abundance ductile interior, such as a subsurface sea. Galileo of OH in the upper mesosphere over the Arctic. This finding corrob­ will get a closer look at Tyre in late March orates MAHRSI measurements taken in 1994. 1998 - shedding light, we hope, on these The discovery also backs up observations made by the Halogen mysterious features. Occultation Experiment aboard a satellite in Earth orbit since 1991. - PAUL GEISSLER, - from R. Monastersky in Science News University ofArizona ,

What was the first asteroid ever discovered? Who was the discoverer? Earth has a tiny companion that orbits the Sun exactly in tune -Carla Wilson, with our planet, report scientists from York University in Ontario, Danville, Arkansas Canada. Calling the 9.6 kilometer (6.0 mile)-diameter rock the "Mona Lisa of asteroids" for the subtle beauty of its sympathetic On January 1, 1801 the Italian monk and vibrations with Earth, Paul Weigert said it was a "tlrrilling discovery," astronomer Giuseppe Piazzi was checking the ' like "fIDding a diamond in your own backyard." stellar positions given in a new star catalog Several pairs of bodies within our solar system orbit in harmonic when he noticed that one "star" was not in the resonances, including several moons of Saturn and Jupiter. But out catalog. When he observed that the object was ofbundreds of near-Earth asteroids, only asteroid 3753 is held cap­ moving from night to night and that it lacked tive by our planet's gravitational pull in a way that keeps it pulsing the diffuse appearance of a comet, Piazzi in a regular rhythmic dance. correctly concluded that he had found a new As this asteroid orbits the Sun, it is at times 1,000 times farther solar system body- the first known asteroid. away from Earth than is the Moon, Weigert reports in the June 12, Originally Piazzi named the object Ceres 1997 issue of Nature. However, it is still considered a "companion" Ferdinandea, honoring King Ferdinand IV of because of the exact one-to-one relationship of its orbit to Earth's. Sicily, but the name was later shortened to The asteroid, scientists say, poses no threat to Earth- at least for Ceres, the Roman goddess of com and harvests. the next 100 million years, after which it could stray from its present - DONALD K. YEOMANS, path and careen unpredictably. Jet Propulsion Laboratory -from K. C. Cole in the Los Angeles Times 21

THE PLANETARY REPORT JANUARY/FEBRUARY 1998 Society NelNs

NASA Takes On the page: http://sd-www.jhuapl.edulNEAR. Foundation; Mallinckrodt Specialty Mars Balloon The NEAR spacecraft will reach Chemical Company; McDonnell The Planetary Society has played a Eros in January 1999. For a mission Douglas; McGraw-Hill, Inc; Microsoft; crucial role in developing the Mars overview, see the September/October Pepsico; Platinum Technology Inc; Balloon concept since 1987. Over the 1997 issue of The Planetary Report. Reader's Digest; Stream International, years, our involvement has broadened -Jennifer Vaughn, Inc; The Times Mirror Foundation; to include student projects as well as Editorial Assistant The United Parcel Service; United collaborations with international ex­ Technologies; US West. perts and the French and Soviet space RenelN on the Web - Lu Coffing, agencies. We have passed the torch on You can renew your Society member­ Financial Manager to NASA. ship on the World Wide Web at our NASA is now developing a small home page: http://planetary.org. Planetary Society Day balloon to fly on Mars, possibly by 2003. Renewing electronically saves time at NSTA A Jet Propulsion Laboratory team will and money. If you live outside the The Planetary Society partners with the work in cooperation with international United States, renew with a credit card National Science Teachers Association balloon experts. The Society has and get the best exchange rate on (NSTA) at their national convention donated its spare balloon from last membership dues. in Las Vegas, April 16 to 19, 1998. year's member-supported test at Utah You can also purchase space-related Saturday, April 18, will be Planetary State University, and we will con~inue merchandise and subscribe to our ac­ Society Day, featuring a great line-up our support as NASA prepares to make claimed special-interest newsletters. of speakers, including Society Presi­ the Mars Balloon fly on the Red Planet. Through our recent partnership with dent Bruce Murray, Society Executive - LoUIS D. Fnedman, Books Now, you can order best-selling Director Louis Friedman, Clark Executive Director books by science and science-fiction Chapman of the Southwest Research authors, including Society advisors Institute, and Linda Hyder, the Society's Track Eros on the Web David Brin, Arthur C. Clarke, and Education Manager. Near-Earth Asteroid Rendezvous (NEAR) Society founder Carl Sagan. Link to Society members can register at the scientist Clark R. Chapman has posted the Books Now site through our home NSTA member rate. For a registration star charts showing the path of asteroid page or point your browser directly to: packet, write to Susan Lendroth at Eros through the night sky; you can http://www.booksnow.com/planetary . Society headquarters, or email your find them on the World Wide Web at: -Michael Haggerty, name and address to: tps.sl@mars. http://boulder.swri.edu/clarkifinderos. Electronic Publications Manager planetary.org. htrnl. - Susan Lendroth, Before checking the charts, you might Invest in a Generous Manager of Communications and Events want to visit the NEAR mission's home Matching Grant The following corporations have Sagan Honored lNith promised matching donations for em­ Planet Walk ployees who donate to the Planetary In November, the Ithaca Sciencenter More NelNs Society. If you are employed by one of unveiled the Sagan Planet Walk-an Mars Underground News: the following companies, inquire into outdoor permanent model of the solar Pathfinder mission ends. NASA their matching grant programs: system. The 1 ,200-meter scale model program for sending humans to Mars. Adobe Systems; ALCO Standard; of the solar system consists of 10 Bioastronomy News: American Ref-Fuel Company; Amer­ standing stones, which stretch from Life, the evolution of intelligence, itech Foundation; ARCO Foundation; the Sun, located on Ithaca Commons, and creating the language of SET!. BankAmerica; Becton Dickinson and to Pluto at the Sciencenter. Each plan­ The NEG News: Company; The Boeing Co. ; California et location is accurately spaced on a The Society announces grant Community Foundation; Caterpillar; scale of one to five billion. The Plane­ recipients in a new program to Cisco Systems; Citicorp; CSGH Systems tary Society sponsored the Saturn encourage NEO detection. Inc; Elser Enterprises; Ericsson Inc; station, located in front of the Ithaca For more information on the Fannie Mae; Gannett Co, Inc; John Public Library. Each station will fea­ Society's special-interest newsletters, Hancock Mutual Life Insurance; Illinois ture high-resolution color images of call (626) 793-5100. Tool Works; Iowa Falls Machine Works; its designated planetary body on a 22 Lam Research Corporation; MacArthur porcelain enamel sign. - SL

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Joshua Vaughan lives in Prescott, Arizona. He is a professional photographer whose work has appeared in Sky & Telescope, Popular Photography, and on v the cover of Astronomy.

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