Page images

Because Mount Rainier poses a significant hazard to life and property in the surrounding heavily populated areas (particularly in the Seattle/Tacoma metropolitan area), the volcano has been designated as a Decade Volcano as part of the International Decade for Natural Hazard Reduction. The USGS has begun a multidisciplinary assessment of natural hazards associated with the volcano for use in developing effective mitigation actions.

For more information on debris-flow hazards at Mount Rainier, contact Joseph

Walder at:

Telephone; (360) 696-7693



giant debris flows have focused on estimating their frequency and determining the areas likely to be inundated when such flows occur. The largest of these debris flows begin as enormous landslides. Hot, chemical-laden ground water that circulates through a volcano's interior alters the rock to a weak, clayrich form. Such weakened rock may break away from the rest of the volcano, particularly during a volcanic eruption. The landslide that produced the enormous Osceola Mudflow 5,700 years ago removed 0.7 cubic mile of rock from the summit of Mount Rainier. Events of this sort have occurred, on average, about once every 500 to 1,000 years and produced debris flows that reached the lowland around Puget Sound, inundating nowpopulated areas near Tacoma and Seattle.

Giant debris flows also form during volcanic eruptions when hot rock avalanches move down the volcano's flanks and incorporate snow and glacier ice. Debris flows formed in this way occur on average about once every 100 to 500 years at Mount Rainier. Although not all are mobile enough to reach the Puget Sound lowland, they do inundate presently populated areas in river valleys radiating from Mount Rainier.

Along with carrying out scientific studies of potential volcanic debris-flow hazards in the Cascade Range of Washington, Oregon, and California, a key goal of USGS investigators is to prepare hazards assessments that can be used by public officials who are concerned with land-use planning and emergency services. Concise summary reports aimed at nonspecialist audiences, along with maps that delineate types and frequency of debris-flow hazards, are being prepared. Spatial data will be made available in forms readily accessible to users of geographic information systems.

Joseph S. Walder has worked at the Cascades Volcano Observatory

since 1989, focusing on glaciological processes and hazards on volcanoes.

Comet Collision With Jupiter

iscovery of Periodic Comet Shoemaker

Levy 9, which collided with Jupiter in July 1994, was one of the more spectacular results of a detailed, long-term, telescopic sur

vey of the night sky by U.S. Geological Survey (USGS) geologist Eugene M. Shoemaker and two USGS volunteers, Carolyn S. Shoemaker and David H. Levy, at Palomar Observatory in California. The Palomar Asteroid and Comet Survey was begun in 1983 to improve the statistical base for the study of planet-crossing asteroids, particularly Earth crossers, and comets. Gene had long been involved in research concerning impact craters on Earth, the other terrestrial planets, and the satellites of the solar system, but little was known about the populations of the planet-crossing asteroids and comets that produced these craters. He had begun his research on impact and crater formation by working on nuclear craters at the Nevada Test Site, followed by geologic mapping at Meteor Crater in Arizona. In succeeding years, he extended his work to lunar cratering problems and lunar geology in the Ranger, Surveyor, and Apollo missions and then studied cratering on the moons of the outer planets during the Voyager missions. To evaluate the ages of geologic units on these distant bodies, it was necessary to estimate the flux of asteroids and comets in their neighborhoods and the rate at which these cosmic bullets produce craters. An ideal instrument to use in searching for these objects is the Palomar 18-inch Schmidt telescope. This photographic instrument is capable of covering 60 square degrees of sky in a single exposure. The USGS team spends 7 nights a month, 11 months of the year, observing with the 18-inch Schmidt. Over the years, techniques for searching the sky have improved steadily. Four-minute exposures evolved to 6 and then 8 as new, finer grained films were used. At first, on a long winter's night, as many as 96 exposures were taken. In recent years, that pace has slowed to about 60 somewhat longer exposures. The number of planet-crossing asteroids and comets discovered has gradually increased. After each observing run at Palomar, the team returns to Flagstaff, where the films are scanned with a stereomicroscope, and the positions on the sky of objects of interest are measured. The orbits of discovered objects are used to calculate the probability that they will collide with the planets and satellites, and populations and impact cratering rates are estimated from the number of objects discovered per unit area of sky photographed. By the winter of 1993, 29 comets and 44 Earth-approaching asteroids had been discovered in the project; at present, the total

number of newly discovered comets found by the Palomar Asteroid and Comet Survey stands at 33.

The winter of 1993 was a difficult one at Palomar. Stormy weather occurred throughout the world, and Palomar was not exempt. In January and February, very few films were taken because of rain and clouds. March proved no exception and, after the first clear night, promised to be almost a “wipeout.” The Shoemakers, Levy, and a visiting astronomer from France, Philippe Bendjoya, were more than a little discouraged the second night when a storm approached after observations had barely started. Because films are taken in sets of four fields that are then repeated to give pairs suitable for stereoscopic examination, a set must be completed for anything to be discovered. Two sets were down, and a third was started as clouds began to cover the sky. Persistence paid off when the observers shot through the “holes” in the clouds and finished the last half of the third set. To take that set, they had used marginal film, which was partially light struck as a result of the film storage box having been opened accidentally in Flagstaff. Had the film not been damaged, it would not have been “wasted” on a dubious sky. Two days later in late afternoon, after scanning all the other films, Carolyn started looking at the pairs from the damaged film. While scanning a field with Jupiter on it, she came across a most unusual-looking comet, which was bar shaped and had a coma, tail, and wings, unlike the usual round comet, which has a coma and tail. The team telephoned Jim Scotti, who was observing on the Spacewatch

USGS volunteer Carolyn Shoemaker

discovered her first comet in the fall of Time sequence of plume rising above the limb of Jupiter from the impact of fragment G, 1983. With her discovery of five more the largest fragment of Comet Shoemaker-Levy 9. The impact occurred on July 18, 1994, comets in 1984, she established a new at 7:28 UT; minutes later, the first hint of the fireball of superheated gases appeared record for the rate of comet discovery above the shadow of Jupiter (shown as the dark band separating it from the limb of the from a ground-based telescope. In 1987, planet). It rose to a height of almost 3,000 kilometers before it collapsed into a thin

stratospheric pancake. The top image was taken at 7:33 UT in the methane band, the second at 7:38 UT in the red, the third at 7:41 UT in the green, the fourth at 7:44 UT in the blue, and the last at 7:51 UT in the violet.

retired USGS geologist Henry Holt joined the project as a volunteer. In the following years, he discovered numerous asteroids, including one that passed extremely close to Earth. In 1989, author and amateur astronomer David Levy became a volunteer in the project. The comet project would not have been able to continue without the time and effort of these dedicated Volunteers for Science.


Jupiter as photographed by the Hubble Space Telescope's Planetary Camera. Eight impact sites are visible. From left to right are the E/F complex (barely visible on the edge of the planet), the star-shaped H site, the impact sites for tiny N, Q1, small Q2, and R, and, on the far right limb, the D/G complex. The D/G complex also shows extended haze at the edge of the planet. The features evolved rapidly on time scales of days. The smallest features in this image are less than 200 kilometers across. This image is a color composite from three filters at 9530, 5550, and 4100 angstroms.

Two views of the impact zone on Jupiter of fragment C of Comet Shoemaker-Levy 9. The image on the left was made in green light with the Planetary Camera channel of the Wide Field Planetary Camera 2 (WFPC2). The image on the right is the same field taken through the WFPC2 methane filter. Data for the images were obtained in the early morning hours of July 18, 1994. The impact site is visible as a complex pattern of circles seen in the lower left of the partial planet image. The small, dark feature to the left of the pattern of circles is the impact site of fragment D. The dark, sharp ring at the site of the fragment Gimpact is 80 percent of the size of the Earth. The comet broke up into 21 fragments during a close passage by Jupiter in July 1992. Fragment G was one of the brightest and likely the largest of the 21 fragments. The remaining fragments continued to impact Jupiter through July 22, 1994. Scientists estimate that the combined energy from all the impacts approached the equivalent of 40 million megatons of TNT. Jupiter was approximately 477 million miles from Earth when the image was taken.


telescope at Kitt Peak in Arizona. By that time, snow was beginning to fall on Palomar, but Scotti was not yet clouded out. With his more powerful telescope, he was able to confirm the comet and said that he could see five separate comets lined up in a bar with wings of dust. This comet was 1993e, to become known as Periodic Comet Shoemaker-Levy 9, for it was the ninth periodic (orbit less than 200 years) comet discovered by the team of the Shoemakers and Levy.

Comet Shoemaker-Levy 9 held three surprises for its discoverers and the rest of the world: (1) it was completely disrupted, its 21 visible fragments lined up like “a string of pearls,” as David Jewitt noted in Hawaii; (2) it was captured in orbit about Jupiter; and (3) it was to impact Jupiter a little more than a year after its discovery. Never before had a comet been seen so completely disrupted as this one. It had been broken up by tidal forces during a passage very near Jupiter on July 7, 1992. In addition, never before had any comet been seen in orbit about a planet, although two comets had been found shortly after escape from temporary orbit about Jupiter. More important, never before had any asteroid or comet been predicted to impact a planet. Not only the team but also scientists from many different fields and much of the lay world were elated at the possibility of observing and learning from this collision.

More important, never before had any asteroid or comet been predicted to impact a planet.

The “Great Crash of 1994” commenced on July 16 and ended on July 22 as large fragments of Shoemaker-Levy impacted Jupiter about every 7 to 9 hours. The size of the original comet and the sizes of the individual fragments were not known with any degree of certainty because of telescopic limitations. A major assist came from the Hubble Space Telescope, which was trained on the comet during July 1993 in hopes of being able to determine the sizes of the nuclei. Preliminary estimates of the sizes of the brightest nuclei were made, but uncertainties remained because of the dust cloud surrounding each

one. Further observations were made with the space telescope after its repair in December, and improved images of the train of nuclei and of individual nuclei were taken up to a few hours before impact. Nevertheless, the exact sizes remained uncertain. Several scientific teams interested in the dynamics of impact modeled the impact of various-sized fragments on the basis of a variety of assumptions. One of those groups involving Gene and USGS geologist David Roddy worked with Paul Hassig of the Titan Corporation to carry out numerical modeling of the explosions. They obtained a prediction for a plume produced by a 1-kilometer body having the density of ice.

Time was reserved on telescopes worldwide so that the impacts could be observed at different longitudes depending on impact times. Jupiter was observed ahead of time to provide a basis for recognizing possible changes in the Jovian atmosphere. Amateur astronomers were prepared to look for any new features large enough to be detected by small telescopes. Never before had so many eyes been trained on one planet in the same week.

Because the comet fragments were to impact on the far side of Jupiter just beyond the limb, it was uncertain whether any explosion forming a fireball plume rising above the limb would be seen. Such an event was thought to require at least a 1-kilometer fragment breaking up fairly high in the atmosphere and releasing more energy than the world's total arsenal of nuclear weapons. The plume from fragment A exceeded expectations, and impacts of the brighter, larger nuclei produced even larger plumes, some rising nearly 3,000 kilometers above the visible cloudtops. To everyone's surprise, each plume produced a huge cloud of dark particles that was readily detectable against the bright face of Jupiter. Throughout the week, as more collisions occurred and Jupiter rotated to reveal the impact sites, the Hubble Space Telescope imaged the events along with other major telescopes, including the South Pole Infrared Explorer, which, weather permitting, could observe Jupiter continuously. Reports and images flowed worldwide over the Internet computer network; by July 27, about 2 million images had been taken off the Internet by private viewers. ShoemakerLevy 9 had become everyone's comet.

The comet collision occurred at a time in the world's history when instrumentation and communication were capable of recording and extracting a huge amount of

[merged small][ocr errors]

information. Moreover, the impacts occurred when Jupiter was high in the sky and had not disappeared in the Sun's glare. In addition, although it was on the far side of Jupiter, the impact longitude was close to the limb, where the fireballs could be seen to rise above the atmosphere. Since that week in July, astronomers and other scientists have been studying the results of that once-in-a-millennium event. Those interested in the chemistry of the comet as well as the chemistry of Jupiter's atmosphere have been hard at work. A surge in radio emissions during the impacts and a brightening of the auroras at the poles are being studied. Computer modeling of the impact dynamics continues. Winds in Jupiter's atmosphere are spreading out and merging the dark material from the various plumes, and the evolution of the dark clouds is being monitored closely. The long-term

history of this dark material is especially relevant to predicting the effects of comet impact on Earth. The dark marks in the Jovian atmosphere have persisted well into October. Meanwhile, the USGS team continued the search at Palomar and discovered three more comets. For Gene and Carolyn Shoemaker, Periodic Comet Shoemaker-Levy 9 was the fulfillment of Gene's dream to observe an impact and Carolyn's to discover the comet of the century. The project at Palomar drew to a close at the end of 1994, but new work with other telescopes will begin in Flagstaff, Ariz. Much remains to be learned about asteroids and comets, cratering in our solar system, and the Earth's future.

Carolyn Shoemaker is a USGS volunteer who discovered her first comet in the fall of 1983.

« PreviousContinue »