Earth and Space Sciences

▪ 1996

Introduction

GEOLOGY AND GEOCHEMISTRY

      In 1995 significant developments took place in the realm of geologic mapping, which provides the foundation for the presentation and comparison of data in the Earth sciences. The most important observational development of the past decade was the appearance of a new map of the topography of the world's ocean floors based in part on formerly classified satellite data. In the late 1980s the U.S. Navy's Geosat satellite measured the heights of the ocean surface with a radar altimeter for the purpose of aiding submarine navigation and missile guidance. The measurements yielded maps of gravity anomalies at sea level that mimic the topography of the ocean floor below. With the declassification of the data between 1990 and 1995, researchers were able to combine the Geosat data with those from the European Space Agency's ERS-1 remote-sensing satellite to produce the new topographic map. David Sandwell of the Scripps Institution of Oceanography, La Jolla, Calif., and Walter Smith of the U.S. National Oceanic and Atmospheric Administration employed a complex modeling algorithm to resolve the topography to a precision 30 times better than that in previous maps. Their map revealed in detail the enormous transform fracture zones that record the history of plate motions over millions of years, new underwater volcanoes and faults, and even structures buried under sediments. (See Oceanography (Earth and Space Sciences ).)

      Improved maps of the continents were promised during the year in a report from Tom Farr of the Jet Propulsion Laboratory, Pasadena, Calif., and seven coauthors. The many scientific applications of high-resolution topographic data have been severely limited by the relatively poor quality of the global digital topographic database for continents. According to the report, a Joint Topographic Science Working Group appointed by NASA and the Italian Space Agency was developing a strategy for improving data quality, the most promising approach being a combination of satellite radar interferometry and laser altimetry. A proposed Global Topographic Mission (TOPAC) would improve the best available global digital coverage by more than two orders of magnitude. The recently developed technique of differential radar interferometry, which was capable of measuring topographic changes of less than a centimetre (0.4 in) that occur rapidly over broad regions, had already been used to map surface changes caused by an earthquake, to show the flow of a glacier, and to detect the deformation of a volcano.

      The promise of a substantially improved understanding of kinematic and dynamic processes that affect regions of continental deformation was offered in a report from M. Burc Oral and six coauthors from the U.S. and Turkey. Slow movements of the crustal plates covering the Earth's surface and their deformation at places where they meet were being measured by the Global Positioning System (GPS), a precise satellite-based navigation and location system developed for U.S. military use. A plate-tectonic theoretical framework for understanding deformation in the eastern Mediterranean area had first been formulated 25 years earlier and was subsequently developed on the basis of the analysis of global oceanic spreading, fault systems, and earthquake slip. The new space-based GPS measurements supported that basic framework—with an important modification. Western, central, and east-central Turkey and the southern Aegean region and Greece were now seen to be moving as a single tectonic plate, whereas the previous interpretation had called for independent Aegean and Turkish plates that were separated by a zone of north-south extension in western Turkey. The new model had considerable geologic implications.

      A 25-year debate about the source and origin of mid-ocean-ridge basalts (MORBs) appeared to have been resolved. The generation and eruption of these lavas at the sites of seafloor spreading, where new crust is being formed, are fundamental processes in the origin of the oceanic crust and the evolution and chemical differentiation of the Earth. According to one hypothesis, MORBs are generated by partial melting of rocks of the Earth's mantle at a depth of about 40 km (25 mi) and are separated from the mantle source at that depth (batch melting). According to the opposing hypothesis, partial melting of the mantle at considerably greater depths generates hotter, magnesium-rich basalt, which precipitates olivine crystals as it ascends and transforms into lavas having the compositions of MORBs.

      During the year Michael Baker and Edward Stolper of the California Institute of Technology, using a novel technique developed independently by Ikuo Kushiro of the University of Tokyo, reported experimental results showing that neither hypothesis was satisfactory. They demonstrated that the first hypothesis is impossible—the lavas must have been formed at greater depths—and that the second hypothesis is inadequate—olivine precipitation alone during uprise of the lava from greater depths could not change its composition to that of MORBs. More complicated processes were indicated, and the new model involved upwelling of mantle beneath mid-ocean ridges accompanied by partial melting through a range of depths, with melts of various compositions separating rapidly almost as soon as they form. The melts rise through the rock matrix, and the different melt fractions become aggregated at several depths en route to the surface. Blending and crystal fractionation occurs in magma chambers beneath the ridge before eruption.

      Bill Collins of the University of Newcastle, Australia, similarly demonstrated that the history of the granitic rocks forming the continents is more complex than many geologists had believed. A classification system based on origin had been in vogue for 20 years, ever since the granitic rocks of the Lachlan fold belt in Australia were identified as consisting of two contrasting chemical groups and, thus, interpreted to be derived from partial melting of two distinct source rocks in the lower crust. The S-type granites had geochemical characteristics indicating derivation from sedimentary rocks, whereas the I-type granites had characteristics indicating derivation from igneous rocks that had been emplaced in the crust from a mantle source. That classification was widely accepted and the principles applied to granitic rocks worldwide.

      Collins pointed out that such a classification led to a paradox: the geochemical differences between S- and I-type granites are not reflected in the composition of their isotopes. Instead, the complete set of S- and I-type granitic rocks shows a continuous range of variation in the isotopes of strontium, neodymium, lead, and oxygen, as if all the rocks of both types had been formed by simple mixing of basalt from the mantle and granite from the crust. Similar arguments had been rejected previously on other geochemical grounds. Collins then showed that his combined field and geochemical data could be explained with a mixing scheme involving three, rather than two, source components. According to Collins, the I-type granites are themselves the products of mixing of mantle-derived basalt with siliceous magma that was formed by partial melting of igneous rocks in the lower crust; subsequent crystallization of the mix produced all the I-type granites. On the other hand, the S-type granites do contain a major sedimentary component, which was identified as Ordovician sediment from mid-crustal levels. The isotopic compositions and the other geochemical characteristics of all the various S-type granites appeared to be explained by the blending of magma derived from the sedimentary source with the magma mix for the I-type granites described above. The new geochemical and petrological interpretations had significance for interpreting the tectonic history of a given region.

      Renewed interest in the once-disdained idea that catastrophic events can cause profound changes to the physical Earth and the course of biological evolution had focused during the past 15 years on the relationship of asteroid or comet impacts and mass extinctions during the past 540 million years. In contrast, Andrew Glikson of Parkes, Australia, considered the effects of such impacts on Precambrian rocks, those older than 540 million years. He pointed out that existing models of the geologic evolution of the Precambrian crust fail to explain the episodic nature of major igneous and rifting events seen in the crustal record and also ignore the tectonic and thermal effects of the large-scale extraterrestrial impacts that came after the heavy asteroid bombardment of the young Earth, which ended about 3.9 billion years ago. Estimates of cratering rates left no doubt that the Earth continued to experience many major extraterrestrial impacts between 3.9 billion and 540 million years ago. The possible correlation between the impact that formed the Chicxulub crater in Mexico's Yucatán Peninsula and the massive outpouring of basalt in India (the Deccan Traps)—both of which occurred about 65 million years ago, when the dinosaurs became extinct—led Glikson to seek connections between giant impacts and Precambrian rifting, igneous activity, and other major geologic events. He summarized the correlations of Precambrian impact events with major thermal and tectonic episodes and also concluded that the geochemical signatures of more recent impacts need to be sought in sedimentary rocks distant from the impact structures. Such signatures might take the form of anomalies in the concentrations of platinum-group elements, similar to the iridium anomaly caused by the Chicxulub impact, which appears globally in sediment marking the 65 million-year-old boundary between the Cretaceous and Tertiary periods. (PETER JOHN WYLLIE)

      This updates the articles dinosaur; Earth (geoid); Earth sciences; geochronology; volcano.

GEOPHYSICS

      The most deadly earthquake of 1995, having a magnitude of 7.2, struck January 17 in the vicinity of Kobe, Japan. Named the Great Hanshin Earthquake, it killed some 6,000 persons and injured more than 30,000. Nearly 200,000 buildings were destroyed or seriously damaged, and more than 300,000 people had to be housed in temporary shelters. Ground effects included liquefaction of the surface in the vicinity of the epicentre and a nine-kilometre surface fracture, with horizontal displacements reaching 1.5 m. (One kilometre is about 0.62 mi; one metre is about 3.3 ft.) Another high-fatality earthquake, having a magnitude of 7.5, occurred May 28 in and around the town of Neftegorsk, Sakhalin Island, in the Sea of Okhotsk off eastern Russia; nearly 2,000 people lost their lives.

      Scientists from Oregon State University mapped a blind thrust fault in Ventura county, Calif. The structure, named the Oak Ridge Fault, was designated as blind because it does not reach the surface but is overlaid by the Santa Susana thrust fault. It is the site of the Jan. 17, 1994, Northridge earthquake, which caused more than 60 deaths and major destruction throughout the stricken area. During the Northridge quake both sides of the Santa Susana Fault were displaced owing to the movement on the fault hidden beneath it. It was postulated that if a fault runs through the mountains, rather than along the edge of a valley, as is the case with the Santa Susana Fault, then it is probable that a blind fault lies beneath it.

      The physical mechanism by which energy is suddenly released in deep-focus earthquakes—i.e., those that occur below about 400 km depth—has long been a puzzle to seismologists. At such depths high temperature and pressure should cause rock under stress to flow smoothly rather than rupture suddenly, as it does in earthquakes near the surface. Recent studies by researchers at the University of California, Santa Cruz, showed that on average the deeper the focus, the more symmetrical the pattern of energy release over time. As recorded on a seismograph, the disturbances caused by a deep-focus earthquake tend to begin abruptly, build to a maximum, and then end relatively quickly and smoothly. The researchers believed that such a pattern is due to the uniformity of the material at the focus but could not determine whether it is the result of a rupture or a geochemical transformation that releases a burst of energy.

      A strong impetus to the search for an acceptable theory for deep-focus earthquakes resulted from the occurrence of the great Bolivian earthquake of June 9, 1994. At magnitude 8.2 it was the largest shock on record to have had a focus more than 600 km below the surface, at the base of the upper mantle. Upon analysis by investigators of the Carnegie Institution of Washington, D.C., and the University of Arizona, the rupture zone was found to be many times too large—it covered a horizontal area 30× 50 km—to fit the currently accepted olivine-spinel transformation theory. According to that explanation, transformation under pressure of the mineral olivine into a more stable mineral, spinel, causes microfissures, which permit an earthquake to occur. Because deep-focus earthquakes generally take place beneath areas of active subduction, where the edge of one of a pair of colliding crustal plates is descending beneath the edge of the other plate, it was thought that such quakes have their origin in subducted crustal slabs that have survived the descent to deep-focus depths. Because the slab supposedly erodes and thins as it descends, however, at 600 km or deeper it would be much thinner than the size of the fracture zone calculated for the Bolivian earthquake. Several studies were under way to test various alternative theories. One speculative idea was that under the extremes of temperature and pressure at depth, some kind of nuclear reaction occurs that releases energy directly, with little or no physical deformation.

      As was happening in other spheres of science, geophysics was benefiting greatly from high technology. Developments in computers and instrumentation were increasing accuracies and resolution manyfold. Two techniques for exploring beneath the Earth's surface recently gained recognition. One, called cross-borehole seismology, was first used by scientists at the French Petroleum Institute in the early 1970s but did not attain wider acceptance until advances in instrumentation made it feasible. Seismic studies on the surface collect data on wavelengths of 20-100 m, while well logs (records made during well drilling) register wavelengths of 0.3-1 m and measure the environment immediately around the borehole. In contrast, cross-borehole seismology covers the range of wavelengths from two to five metres. Instruments are set up in an array, with receivers vertically spaced in one borehole and signal generators placed in surrounding boreholes at distances of 100-300 m. The generated signals are tailored so as not to damage the borehole but still be strong enough for reception. By means of multiple receivers and multistation receiver cables, it is possible to record as many as 25,000 seismograms in a few days. The analysis of the data is quite complex, combining the techniques of medical X-ray computed tomography and more conventional wave-tracing techniques of exploration seismology with enhancement from standard reflection imaging. The dramatic enhancement of rock-structure definition gained by the technique was expected to increase the detection of high-porosity zones and permeability barriers and thus help identify oil reservoirs and their dimensions.

      The second technique, geophysical diffraction tomography, is similarly derived from medical tomography. First developed in the early 1980s, it involves the mathematical combination of many individual signals from a specifically designed array of instruments to produce a three-dimensional image of the region traversed by the signals. As of 1995 it had been used to detect underground tunnels across the demilitarized zone between North Korea and South Korea; to trace the outline of the still unexcavated fossil bones of Seismosaurus, an enormous dinosaur discovered in the southwestern U.S.; and to map the remains of ancient underground settlements in the Negev region of Israel.

      Using data collected by satellites of the Global Positioning System (GPS), researchers from the University of Colorado and Stanford University found that Australia is moving north-northeast with respect to Antarctica at a rate of five to eight centimetres (two to three inches) per year. The detection of that heretofore unknown movement was made possible by means of weekly measurements of the relative positions of points all over Antarctica, Australia, Hawaii, New Zealand, Tahiti, and Tasmania carried out by GPS satellites and disseminated on the Internet. The GPS system was capable of measuring positional variations of less than 2 mm (0.08 in).

      Work carried out on Legs 152 through 158 of the International Ocean Drilling Program (ODP), which studied the crust beneath the world's oceans by means of the coring and extraction of rock samples from below the seafloor, was confined to the Atlantic Ocean. Exploration proceeded from sites on or near the continental shelf southeast of Greenland (Leg 152) to the Mid-Atlantic Ridge south of the Kane Fracture Zone (Leg 153), to a transept across the Ceara Rise in the western equatorial Atlantic (Leg 154), to the Amazon River deep-sea fan (Leg 155), to the deformation front of the North Barbados Ridge (Leg 156), to the Canary Basin (Leg 157), and finally to the Mid-Atlantic Ridge at latitude 26° N (Leg 158). The ODP expeditions collected data relevant to paleoceanography (study of the ocean in past ages), seafloor spreading, and the evolution of the Mid-Atlantic Ridge at those critical sites.

      (RUTLAGE J. BRAZEE)

      This updates the articles earthquake; plate tectonics.

HYDROLOGY

      Floods and drought again played a large role in global hydrology during the year. Although flooding in the U.S. Midwest was less severe than that experienced in 1993, it continued to raise questions about the need for flood-management policy in the major river basins. California pursued its recovery from the multiyear drought of the late 1980s and early '90s with a vengeance as storms and floods hit throughout the state early in the year.

      In northwestern Europe flooding of the Rhine, Main, Meuse, Waal, and other major rivers during January and February was as great as it had ever been in the past 40 years. Valley residents evacuated as rivers rose throughout the subcontinent; the Rhine reached the highest level witnessed since the 18th century. Paradoxically some of the same areas later endured a summer that was among the hottest and driest on record. Flooding also plagued Morocco and Egypt, and North Korea was so badly affected that it requested aid from the UN.

      Drought persisted in the northeastern U.S. and the Caribbean, including Puerto Rico. Scientists speculated that the Caribbean islands were experiencing a Sahel-like dry period that recurred about every 25 years. Desperate farmers in northern Mexico watched their fields wither once again under the onslaught of a third year of drought.

      Water-management efforts around the globe continued to effect large-scale geologic changes and thus to raise concerns about environmental problems. Dam-building projects in India promised to create large amounts of water-storage capacity and hydroelectric power within three years, but opponents objected on environmental and social grounds since the reservoirs would flood many villages and much farmland and inundate thousands of hectares of riverside habitat. In the face of both local and worldwide criticism over population displacement and environmental damage, construction continued on the nearly 2-km (1 1/4-mi)-wide Three Gorges Dam on the Chang Jiang (Yangtze River) in China, which would form a reservoir 600 km (370 mi) long when completed. In Germany a plan to alter the flow of the Danube River with locks in order to move more commercial traffic met with vehement objections from residents all along the river.

      A chronically disappearing lake was caught in the act of reappearing. Lake Merzbacher in the Tien Shan Mountains of Kyrgyzstan, in Central Asia, mysteriously drains and refills on an annual, or sometimes biannual, cycle. Aerial photographic studies in 1995 recorded the lake as it returned. Interest also was focused on another hydrologic mystery in Central Asia, the rise in the level of the Caspian Sea, which has persisted since the late 1970s despite the presence of numerous hydroelectric dams and reservoirs on its inflowing rivers. As the world's largest inland sea encroached on towns and industrial sites along its shores, experts debated various explanations, including changing weather patterns, tectonic activity affecting the seafloor, increased influx from the Volga River, and even an underground shift of water from the shrinking Aral Sea, which lies about 500 km (300 mi) to the east. (N. EARL SPANGENBERG)

      See also Natural (Disasters ).

      This updates the articles hydrosphere; ocean; river.

METEOROLOGY AND CLIMATE

      An abnormally strong and southward-displaced jet stream across the Pacific Ocean, partially fueled by an unprecedentedly prolonged El Niño warming of the eastern tropical Pacific (see Sidebar (METEOROLOGY AND CLIMATE: What's Happening to El Nino?? )), steered strong storms into the western United States that produced excessive precipitation and severe flooding across California in January and again in March. In stark contrast, a relatively mild, dry winter prevailed over the eastern United States, while a severe drought, also influenced by the El Niño, afflicted Hawaii from October 1994 to March 1995.

      As spring progressed, the displaced jet stream pushed strong storms into the Midwest, bringing precipitation more than twice normal to many areas between mid-April and mid-June. Water levels along the middle and upper Mississippi River, the lower and middle Missouri, and their tributaries approached but did not exceed those reached during the 1993 floods. In contrast, the aforementioned atmospheric pattern kept much of the Atlantic Seaboard unusually dry, and during the summer subnormal rainfall persisted across the Northeast and Middle Atlantic states. In July a short-lived but intense heat wave enveloped the central and eastern U.S., accounting for nearly 1,000 heat-related deaths from the High Plains to the Atlantic Seaboard, including more than 700 in the Chicago area alone.

      One of the most active Atlantic hurricane seasons in history, featuring 17 storms of at least tropical-storm strength through mid-October, abetted wetness across parts of the Caribbean islands, Florida, and the southern U.S. Allison, the first June hurricane in 10 years, tracked through western Florida and the south Atlantic states. In August remnants of Tropical Storm Dean inundated southeastern Texas and parts of the Great Plains, while Hurricane Erin pushed through The Bahamas before striking Florida twice, once along the central Atlantic coast and again along the western Panhandle. Subsequently, Hurricane Felix buffeted Bermuda with strong winds and heavy rain and then stalled in the western Atlantic, which resulted in prolonged high winds, rough surf, and beach erosion along the U.S. East Coast. In late August and September Hurricanes Iris, Luis, and Marilyn all battered parts of the eastern Caribbean islands. The latter two storms hit the northeastern Leeward Islands head on, causing widespread damage. All three storms stayed away from the eastern U.S., but the coastline again took a prolonged beating from rough surf and very high tides. In October yet another hurricane, Opal, struck the western Florida Panhandle with winds gusting to 232 km/h (144 mph). Opal's remnants spawned locally heavy rains and tornadoes in the East but brought much-needed rainfall to the Northeast. The storm also took 10 lives on Mexico's Yucatán Peninsula, which then was hit by Hurricane Roxanne a week later.

      During January and February heavy rains caused localized flooding and crop damage in south-central Brazil. In contrast, almost eight months of exceptionally dry weather were reported across east-central Brazil. April brought beneficial rains to those regions, but heavy rains farther south soaked northeastern Argentina and produced brief but severe flash flooding near Buenos Aires.

      Between 100 and 250 mm (4 and 10 in) of precipitation fell on saturated ground across much of central and western Europe during the last two weeks of January, pushing several rivers to levels rivaling those observed during the December 1993 "Flood of the Century." In June hot, dry weather enveloped the British Isles, eastern Europe, and western Asia. The conditions expanded across most of Europe and northwestern Africa through July and August and were particularly extreme in the British Isles. Dryness dominated many areas of southern Africa in late January and February, and above-normal temperatures further stressed crops. Late-March rains finally brought relief to most locations, although heavy rains evaded Zambia and northern Zimbabwe, where soil-moisture shortages persisted. The African Sahel wet season (May-September) was rather uneventful, with most areas receiving near-normal rains.

      A heat wave overspread Pakistan and northern India during June. Temperatures reached 50° C (122° F) at some locations, causing hundreds of deaths. By month's end, however, monsoonal showers had begun advancing through the region, and torrential rains fell on many locations throughout July and early August, causing sporadic river flooding. For the Indian subcontinent as a whole, the summer of 1995 was the seventh wettest since 1934.

      Conditions varied markedly with time and location across the Far East. Between mid-April and mid-July, heavy rains doused parts of northern Hunan and Jiangxi provinces in China, leading to severe flooding that claimed more than 1,000 lives. In addition, heavy rains during an 11-week period that ended in early September spawned severe flooding across lower northeastern China and North Korea. Beginning in April unusually wet weather also dominated southeastern China (through August) and western Japan (into late July), punctuated by Typhoon Faye, which lashed southern South Korea and western Japan in mid-July. From late July through early October, eight tropical storms or typhoons pummeled parts of the Philippines, Taiwan, southern China, and northern Vietnam. By contrast, much of central and east-central China endured abnormally dry summer conditions. Summer dryness also plagued south-central and eastern Japan before Typhoon Oscar soaked the region, including Tokyo, in September.

      After a rather dry start to Australia's 1994-95 wet season, the year commenced with subnormal January rains along the northwestern and eastern coastlines, but at least twice the normal January rain pelted areas from central Queensland southward through eastern Tasmania, resulting in localized flooding. Farther west, Cyclone Bobby brought rare heavy rains and locally severe flash flooding to much of Western Australia. Subnormal precipitation during March and April adversely affected agriculture in Queensland, but widespread beneficial rains fell on the eastern half of Australia during May. (ELBERT W. FRIDAY, JR.)

      See also Natural (Disasters ).

      This updates the articles atmosphere (atmosphere, evolution of); climate.

OCEANOGRAPHY

      One of the most important themes in oceanography in 1995 was exploration. Some of it was conducted in the traditional mode, from ships, but much was done from Earth-orbiting satellites. Remarkably, satellite radar measurements were able to tell scientists not only about the motion of the ocean's surface waters but also about the shape of the underlying seafloor. Radar measurements of the distance from the satellite to the sea surface provided a picture of the shape of the Earth that was accurate to a few centimetres once the effects of waves and tides had been removed. (A centimetre is about 0.4 in.) Such determinations were possible because the solid material beneath the seafloor gravitationally attracts the water above it in a way that mirrors seafloor topography. For example, the sea surface near a seamount is a few metres farther from the Earth's centre than is the sea surface far from the seamount, and the sea surface over a submarine trench is a few metres closer than is the sea surface far from the trench. (A metre is about 3.3 ft.) Satellite radar easily measures such differences in sea level and thus, in principle, can map the seafloor.

      The U.S. Navy had made such global satellite radar measurements in the late 1980s, but the data only gradually became available to researchers. In 1995 the last of the data were released and combined with similar radar measurements from other satellites to form a global database. The most exciting result was a map of the global seafloor. Because much of the seafloor previously had been only sparsely surveyed, the new map revealed many new features. The large-scale features of the seafloor continued to be understandable in terms of the theory of plate tectonics, according to which the global seafloor is divided into about a dozen plates of crust that move rigidly away from mid-ocean ridges toward regions of subduction (where one plate is plunging beneath another), such as deep-ocean trenches, or sometimes directly collide with one another. Nevertheless, the new map showed features suggesting that the plates are not entirely rigid but, rather, are compressed or pulled apart as they approach different subduction regions. Because the gravitational attraction of seafloor material depends on how heavy it is, such satellite maps of the seafloor also contained information about the density and temperature of the material underlying the seafloor and thus should aid in understanding of the global distribution of mineral resources on the seafloor. (See Geology and Geochemistry (Earth and Space Sciences ).)

      The sea surface is not exactly where one would expect to find it solely on the basis of knowledge of the way that seafloor material distorts the Earth's gravity field. The discrepancy is small, generally a few tens of centimetres or less, but it can be determined by a comparison of satellite radar measurements of sea-surface shape with the shape calculated from the very best estimates of the Earth's total gravity field. The difference directly reflects the motion of the water in the upper ocean. For example, because of the rapidly flowing Gulf Stream, the sea surface along the U.S. east coast is about a metre closer to the centre of the Earth than that in the Sargasso Sea. During the year researchers continued to study the circulation of the oceans, using satellite measurements made for the joint U.S.-French Topex/Poseidon project. Launched in 1992, the Topex/Poseidon satellite made radar measurements of sea level along the same geographic track once every 10 days and thus provided a unique view of fluctuations in upper-ocean flow over months, seasons, and years. It could resolve variations in sea level ranging from waves that traverse the tropical ocean over a period of months to sea-level differences between different years associated with the anomalous tropical Pacific Ocean warming known as El Niño. (See Sidebar (METEOROLOGY AND CLIMATE: What's Happening to El Nino?? ).) Researchers were also using the satellite to look directly for the slow sea-level rise associated with hypothesized ongoing global warming.

      Despite the strides in satellite oceanography, more traditional measurements made from ships were needed in order to understand the deep flow of the ocean. The World Ocean Circulation Experiment (WOCE), which began in 1990, was a multinational study of ocean circulation. Many different kinds of measurements were made as part of WOCE, but the central field program around which they were organized was a series of hydrographic transects by ship that traversed the major ocean basins. The central measurements made on each transect were of the temperature and salinity of the water from the top to the bottom of the ocean; they were supplemented by measurements of nutrients and dissolved gases as well as by underwater acoustic profiles of currents below the ship. At the very end of 1994, WOCE researchers began a series of research cruises in the Indian Ocean that continued through 1995. The goals of that work were to learn how deep waters flow into the Indian Ocean from around Antarctica and how they rise and then return southward at shallower depths, to learn how the Indian Ocean contributes to the global transport of heat, and to provide a background picture of the deep flow underlying the surface circulation that was being studied by satellite radar and other techniques. (MYRL C. HENDERSHOTT)

      This updates the articles ocean; hydrosphere.

ASTRONOMY

       astronomyThe year 1995 presented astronomers with another set of exciting discoveries. As insights into cometary dynamics and gas-planet atmospheric physics continued to emerge from the spectacular crash of Comet Shoemaker-Levy 9 into the planet Jupiter in 1994, a new comet was detected that could turn out to be even more spectacular. Perhaps the biggest newsmaker in astronomy was the announcement of the discovery of a planet outside the solar system orbiting a star much like the Sun. Other noteworthy reports ranged from the discovery of new satellites of the planet Saturn to a better understanding of the nature of intergalactic matter at the most distant reaches of the universe. (For information on eclipses and other standard astronomical events due to take place in 1996, see Table (astronomy).)

Solar System.

      Saturn is best known for the beautiful rings encircling the planet. Just beyond the main ring system lies the so-called F ring, a wispy band of material sometimes described as braided or clumped. In 1980 and 1981, as the two Voyager spacecraft flew by Saturn, they discovered two moons, later dubbed Prometheus and Pandora, which appear to "shepherd" material into the clumps observed in the F ring. The glare of sunlight reflected off the rings usually makes direct observation from Earth of Saturn's many small moons difficult. Every 14 to 16 years, however, the rings appear edge-on as seen from Earth, and in 1995 astronomers had their first opportunity to use the Earth-orbiting Hubble Space Telescope (HST) to observe the Saturnian environment free of ring glare. Amanda Bosh of the Lowell Observatory, Flagstaff, Ariz., and Andrew Rivkin of the University of Arizona reported finding two, and perhaps as many as four, new satellites of Saturn. Later it was determined that one of the objects was indeed a previously unknown moon. Designated 1995 S4, it is no more than 70 km (45 mi) across and lies just outside the F ring. On the other hand, the other objects were thought to be the previously seen moons Pan, Atlas, or Prometheus. The confusion may have arisen as a result of the complex dynamics between the moons and the rocky debris of the rings, leading to unforeseen motions of the moons. At year's end astronomers counted at least 19 moons around Saturn, though there may well be more. Unfortunately, the next good opportunity to search for such moons from Earth, when the rings will be edge-on and Saturn will be far enough from the Sun's glare, will not occur until the year 2038.

      Between July 16 and 22, 1994, 21 fragments of Comet Shoemaker-Levy 9 collided with the giant gas planet Jupiter. Months later Earth-based infrared telescopes continued to detect dark markings on Jupiter at the planetary latitudes of the impact sites. A new estimate placed the size of the original comet, before it had been tidally fragmented by Jupiter's gravity, at about 2 km (1.2 mi) in diameter. Whether the resulting markings on Jupiter arose from the original cometary material or from compounds synthesized in the impact explosions was still hotly debated.

      Just as public interest in comets began to wane, a new comet was reported that, according to some predictions, could become the brightest since the so-called Great Comet of 1811. Discovered on July 22 by two amateur astronomers, Alan Hale and Thomas Bopp, Comet Hale-Bopp was first spotted at a distance of about seven times that of the Earth from the Sun, beyond the orbit of Jupiter and farther out than any other comet detected to date by amateurs. Given its distance and brightness, astronomers estimated it to be about 5-10 times the size of Halley's Comet, which is roughly 15 km in diameter. When it made its closest approach to the Sun in early 1997, it could be the brightest object in the night sky other than the Moon and Venus, and its tail could stretch as much as a third of the way across the sky.

      Comets made more news in 1995 when scientists led by Anita L. Cochran of the University of Texas, using the HST, discovered 30 objects lying in a region beyond the orbits of the outermost planets Pluto and Neptune. In the past few years, searches with ground-based telescopes had revealed about 20 such trans-Neptunian objects. The newly discovered bodies appeared to be members of the Kuiper Belt, a ring or shell of objects at the outer reaches of the solar system, which is thought to be the source of most comets. The objects detected by the HST were thought to be about 20 km in diameter, compared with the estimated 200-km diameters of the previously detected trans-Neptunian objects. On the basis of the size of the region surveyed by the HST, astronomers calculated that the Kuiper Belt may hold as many as 100 million objects. According to current thinking, occasional passing stars gravitationally perturb the Kuiper Belt objects, kicking some into the inner solar system and nearer the Sun, where they become visible as comets when their ices and gases evaporate.

Stars.

      The announced detection of a planet orbiting a Sun-like star, if confirmed, may well turn out to be the most exciting astronomical discovery of 1995. Michael Mayor and Didier Queloz of the Geneva Observatory announced the discovery of an object having roughly the mass of Jupiter in orbit around the solar-type star 51 Pegasi, which lies only about 42 light-years from the Sun. Their claim was based on a year and a half of precise observations of the star's velocity. A periodic variation detected in the velocity was interpreted as being due to the gravitational tug of an unseen companion orbiting 51 Pegasi. Although certain unknowns prevented the astronomers from calculating a mass for the companion, they were able to determine a minimum value—about one-half the mass of Jupiter. The unseen object orbits 51 Pegasi with a period of 4.2 days at a distance of only 1/20 the Earth-Sun distance; i.e., the planet must lie inside the hot corona of its star. If the detected velocity variations in 51 Pegasi indeed are due to a companion, the observations raise a number of questions. How could a planet have formed so near to its parent star? Is it gaseous (like Jupiter) or rocky (like Mercury)? Is it really small enough to be a planet, or is it a more massive object such as a brown dwarf, a stellar object too small to produce energy by nuclear reactions?

      Other reports of objects around stars were made during the year. Interpreting near-infrared images and spectra, Shrinivas Kulkarni and collaborators at the California Institute of Technology announced their detection of an object about 20 times the mass of Jupiter orbiting the tiny star GL 229, which lies about 30 light-years from the Sun. The observed infrared spectrum indicated the presence of methane, a molecule unlikely to exist in the atmosphere of a normal star. Though the dividing line between a planet and a brown dwarf was unclear, the companion object to GL 229 is either a massive planet or arguably the best case yet for a brown dwarf.

      Since 1991 evidence had been accumulating that a pulsar (a rapidly spinning neutron star) designated PSR B1257+12 was orbited by at least two planets. Continuing observations of the system in 1995 revealed at least three planets having masses that ranged from a few percent of that of Earth to about 3.4 Earth masses. The three planets orbit the pulsar at distances between 19% and 47% of the Earth-Sun distance. Intriguingly, the ratio of the orbital radii follows precisely the same relation, called Bode's law, as do most of the planets in the solar system.

      Another promising candidate for a brown dwarf was discovered in the Pleiades star cluster, a comparatively young (100 million-year-old) star-forming region lying about 400 light-years from the Sun. From observations with a ground-based telescope in the Canary Islands and other instruments, astronomers concluded that the object, dubbed Teide 1, probably has a mass about 20 times that of Jupiter, although a somewhat higher value could not be ruled out.

      During the year research continued on two remarkable objects lying within the Milky Way Galaxy and exhibiting energetic outbursts. One, called GRS 1915+105, is in a class of objects known as X-ray novas. They produce an X-ray outburst, which then fades away, somewhat akin to the much more energetic outbursts observed in active galaxies and quasars. Also like quasars, the GRS 1915+105 outburst was followed by the ejection of two radio-emitting blobs that were observed to be moving transverse to the line of sight from Earth. Six months of observations indicated that the blobs were moving at 92% of the speed of light.

      Another transient X-ray source, called GRO J1655-40, which lies some 10,000 light-years from the Sun in the constellation Scorpius, was first detected by the Earth-orbiting Compton Gamma Ray Observatory in 1994. Subsequent radio observations with the Very Long Baseline Array in New Mexico revealed ejected material racing away from the central object with the highest rate of angular motion found to date for any object outside the solar system. Although that rate, combined with the distance to the source, yields an apparent speed for the ejected material that is 50% greater than the speed of light, the observations can be understood as arising from motion at less than the speed of light but in a direction nearly along the line of sight to Earth.

Galaxies and Cosmology.

      One of the most remarkable predictions of Einstein's general theory of relativity is that gravity bends light. That effect was first demonstrated during a total solar eclipse in 1919, when the positions of stars near the Sun were observed to be slightly shifted from their usual positions—an effect due to the pull of the Sun's gravity as the stars' light passed close to the Sun. In the 1930s Einstein predicted that a mass distribution could act as a gravitational "lens," not only bending light but also distorting images of objects lying beyond the gravitating mass. In 1995 the HST recorded one of the most spectacular examples of a gravitationally lensed astronomical system. An image of the relatively close galaxy cluster Abell 2218 showed a collection of spiral and elliptical galaxies, along with about 120 filamentary arcs. The arcs are light from galaxies lying much farther away than Abell 2218. Theoretical analysis of the shape and distribution of the arcs suggested that they are images of galaxies formed at a time when the universe was only about one-fourth its present age, only a few billion years after its beginning.

      The presence of the same lensing effect appeared to have misled astronomers four years earlier into claiming that they had detected the brightest object in the universe. The galaxy, called FSC 10214+4724, had been estimated to be about 100 trillion times more luminous than the Sun, or 1,000 times brighter than the entire Milky Way. However, in two independent studies that made use of the giant W.M. Keck Telescope in Hawaii and the HST, astronomers found that a galaxy in the foreground acts as a gravitational lens to increase the apparent brightness of FSC 10214+4724. Their observations, made in the near-infrared, suggested that the galaxy is only about as bright as other giant elliptical galaxies that lie relatively close to the Milky Way.

      A major prediction of the big-bang model of cosmology, which hypothesizes that the universe began with a hot explosion, is that most of the helium observed today was synthesized from hydrogen in nuclear reactions occurring during the universe's fiery first few minutes. In 1995 the spectroscopic signature of helium filling intergalactic space was seen in data from the Astro 2 Observatory carried aloft on the U.S. space shuttle Endeavour in March. Using the space observatory's Hopkins Ultraviolet Telescope, Arthur F. Davidson and collaborators of Johns Hopkins University, Baltimore, Md., reported finding absorption lines characteristic of helium in the spectrum of the quasar HS 1700+64, which lies about 10 billion light-years from the Sun. Detection of intergalactic matter had eluded scientists for more than three decades. Analysis of the observations suggested that intergalactic hydrogen and helium constitute more matter than had been detected in all the visible stars and galaxies seen to date. The exact amount of intergalactic gas was uncertain, however, since it was not clear whether it resides in clumps as intergalactic clouds or as diffuse matter uniformly filling intergalactic space. In either case, the amount of gaseous matter detected, while significant, does not contribute enough mass to the universe to slow its expansion to a halt in the future and then cause it to collapse. (See Physics (Mathematics and Physical Sciences ).) (KENNETH BRECHER)

      This updates the articles Cosmos; galaxy (Milky Way Galaxy); astronomy; solar system; star; telescope.

SPACE EXPLORATION

      Space station practice missions dominated space news during 1995 as the United States and Russia prepared to start building an international space station that could cost a total of $100 billion through the year 2012. By contrast, unmanned exploration took a turn for the smaller and cheaper as the U.S. initiated a low-cost program for planetary exploration. Meanwhile, NASA faced a drastic downsizing on May 19 when Administrator Daniel Goldin announced a cut of 3,560 civil service jobs and up to 25,300 contractor jobs—30% of the NASA-based workforce—by the year 2000. Goldin also revealed that space shuttle operations would be turned over to a single private contractor.

Manned Spaceflight.

       Manned Spaceflights, 1995(For information on manned spaceflights in 1995, see Table (Manned Spaceflights, 1995).)

      The highlight of the year was the docking of the U.S. space shuttle Atlantis with Russia's space station Mir. The rendezvous came almost 20 years after the Apollo-Soyuz Test Project, the first docking of manned spacecraft from two separate countries. Before the 1995 docking, a practice rendezvous was flown by the space shuttle Discovery in February to demonstrate the shuttle orbiter's ability to approach and maneuver safely around Mir. Despite a leaky thruster that might have damaged Mir's solar arrays, rendezvous occurred on schedule on February 6 at an altitude of 392 km (245 mi). Discovery came within 11.3 m (37 ft) of Mir at speeds as low as 0.03 m (0.1 ft) per second. Discovery then moved into a separate orbit for another week of operations, including a space walk by mission specialists who tested new gloves on the space suits.

      Following the Discovery rendezvous, U.S. astronaut Norman Thagard rode the Soyuz TM-21 on March 14 with commander Vladimir Dezhurov and flight engineer Gennady Strekalov to rendezvous with and board Mir. Thagard stayed aboard for three months to start developing U.S. expertise in long-term space operations, including biomedical experiments.

      The shuttle docking with Mir was achieved by Atlantis on June 29 and continued until July 4. The mission included the exchange of crew members as Thagard (who set a U.S. space record of 115 days), Strekalov, and Dezhurov returned to Earth aboard Atlantis. They were replaced by Anatoly Solovyev and Nikolay Budarin, who rode up on Atlantis. The docking was made possible with a special module similar to the one that was to be used to link shuttles with the international space station when it was completed.

      Endeavour carried the Astro-2 cluster of three telescopes to observe the heavens in ultraviolet light. Information on the mission, on March 2-18, was available in real time via the Internet, and more than 350,000 requests were logged during its three-day availability.

      The launch of Discovery in July had been delayed for several weeks to repair damage by woodpeckers. This odd "space first" happened when northern flicker woodpeckers mistook the shuttle's external tank's reddish-coloured foam insulation for rotting wood and bored numerous holes in the foam. Once in orbit, the crew launched the last Tracking and Data Relay Satellite to replace the one that was lost when Challenger was destroyed in 1986. During the mission NASA started trial operations with its new Consolidated Control Center, which used advanced computer workstations in place of the familiar 1960s-era digital television displays.

      After a delay of more than a month because of defective booster nozzles and a generator malfunction, Endeavour was launched on September 7. Attempts to operate the specialized Wake Shield Facility (WSF) satellite were frustrated. After being launched from the shuttle, the WSF flew for several orbits to process special electronics materials in the ultrahard vacuum created in its own wake, although it was pointed in the wrong direction for part of the flight. Two days after launching the satellite, the shuttle crew retrieved it and found that it had shut down automatically. Two crew members walked in space to test tools and techniques for assembling a space station. The crew also launched the Spartan 201 solar observatory.

      On one of the longest missions of the year, from October 20 to November 5, Columbia carried the U.S. Microgravity Laboratory-2. Experiments included the growth of crystals and other materials and the first use by astronauts of the Geophysical Fluid Flow Facility, which was designed to simulate the flow of the atmosphere of Jupiter. During a flight on November 12-20, astronauts aboard Atlantis attached to Mir a docking module for use by future shuttle missions.

      The 10-year-old Mir continued to operate, thanks to frequent repairs and the concerted efforts of ground and flight crews. Going into 1995, Mir was crewed by Aleksandr Viktorenko, Yelena Kondakova, and Valery Polyakov. Viktorenko and Kondakova had been launched aboard Soyuz TM-20 on Oct. 3, 1994, along with European Space Agency (ESA) astronaut Ulf Merbold. The craft returned to Earth on March 22 with Viktorenko, Kondakova, and Polyakov after they were replaced by the Soyuz TM-21 crew (Merbold had returned on Soyuz TM-19 on Nov. 4, 1994). Polyakov set the world record for duration in space: 439 days on this mission and a career total of 680 days. Kondakova set a women's record of 174 days. On September 3 ESA astronaut Thomas Reiter was launched along with Sergey Avdeyev and Yury Gidzenko aboard Soyuz TM-22 for a 133-day stay aboard Mir. The TM-21 crew returned to Earth on September 11. Mir was expanded with the addition of the 20-ton Spektr experiment module, launched on May 26. Spektr carried experiment gear plus new solar arrays to extend the space station's operating life.

      With its design settled, work moved ahead quickly on the international space station (the earlier name, Alpha, had been dropped), and flight hardware started taking shape. In Huntsville, Ala., the Boeing Co. completed the main structure for the laboratory module and for the first of two nodes that would join the lab and habitat modules.

Space Probes.

      In mid-July the Galileo spacecraft, nearing the end of its six-year odyssey to Jupiter, released a probe that plunged into the atmosphere of Jupiter in early December 1995 and provided the first direct measurements of the composition and structure of the gas giant planet. After plunging through the planet's atmosphere, the probe jettisoned its heat shield and deployed a parachute for a slower descent through the atmosphere while it measured winds, clouds, and atmospheric conditions. The probe collapsed when it was so close to Jupiter that the outside pressure equaled 100 times that of Earth's atmosphere at sea level.

      The probe's data were received by the Galileo spacecraft for retransmission to Earth. Galileo flew past two of Jupiter's moons, Europa and Io, on December 7, about the same time the probe entered Jupiter's atmosphere and then went into orbit around the planet on December 8. Galileo was scheduled to spend at least a year taking pictures of Jupiter and its moons. However, because the spacecraft's large parabolic antenna resisted all attempts to deploy completely, pictures had to be transmitted through a slower antenna. This reduced by 80% the number of pictures that scientists would receive during the mission.

      Scientists during the year were preparing to launch the Discovery program, designed to achieve one planetary mission a year at a total cost of less than $250 million. Three missions were scheduled, and selection was under way for a fourth. In most cases NASA would allow universities and corporate laboratories to develop the spacecraft with minimum supervision and without reporting through a NASA field centre.

Unmanned Satellites.

      One of the most interesting events of the year was the release of information about satellites that had flown more than 30 years earlier. Following directions from U.S. Pres. Bill Clinton, the CIA on February 24 declassified details of spy satellites it operated from 1960 through 1972 and started releasing some 800,000 photographs. During those years the CIA developed a series of "keyhole" satellites, starting with KH-1, that ultimately could resolve details just a few meters across. Pictures were recorded on film magazines that were returned to the Earth in small reentry capsules. The KH-series satellites were free to roam across the entire Soviet Union, and they returned millions of images of Soviet military and civil installations. Among other revelations, the U.S. discovered that the Soviets had built only 25 ballistic-missile launchpads, about a tenth of what had been estimated by other means.

      Europe and Israel entered the spy satellite business during the year. On July 7 ESA launched Europe's first spy satellite, Helios 1, a joint venture of the French, Spanish, and Italian governments. Germany was expected to participate in the Helios 2 mission. On April 5 Israel launched the Ofeq 3 (Horizon 3) satellite, which was believed to be a forerunner of more sophisticated craft.

      Exploration of the space environment around Earth intensified with the launches of several satellites under the International Solar Terrestrial Physics program. NASA's Wind spacecraft, launched Nov. 1, 1994, was to move into a "halo orbit" between the Sun and Earth by late 1995. On August 2 Russia launched its Interbol 1 to study the structure of Earth's magnetosphere. On December 2 ESA's Solar Heliospheric Observatory was launched into a halo orbit, where it would constantly monitor the Sun.

      In space astronomy, ESA launched its Infrared Observatory (ISO) in November atop an Ariane 4 rocket. ISO carried a telescope and instruments cooled to -270° C (-454° F) to observe the coldest and darkest objects in the universe.

Launch Vehicles.

      The DC-X "Delta Clipper" vertical takeoff and landing rocket flew two tests, June 12 and July 7, for the U.S. Air Force and then was transferred to NASA for refurbishment and further test flights as the DC-XA. Many of the technologies tested in the DC-X program were to be applied to the X-33 project to demonstrate "single stage-to-orbit" launch capability. Three teams, led by Boeing and McDonnell Douglas Corp., Rockwell International, and Lockheed Martin Corp., were to develop competing concepts. Each would take off vertically, but only the Boeing/McDonnell Douglas concept would land vertically; the others would land like an aircraft.

      The X-33 should lead to an unmanned Reusable Launch Vehicle capable of resupplying the space station faster and more cheaply than the shuttle. In a related program, Rockwell International and Orbital Sciences Corp. won a NASA contract to develop the X-34. It was to have a reusable first stage that would boost a satellite and its orbital insertion stages to an altitude of 96 km (60 mi) and then glide back to the airstrip for reuse. The first flight was expected in 1998.

      ESA moved into the final phases of developing its new Ariane 5 launch vehicle. Testing problems with the Vulcain main engine delayed the first launch until February 1996. Japan's H-2 launch vehicle made its third flight on March 18. The payload included the Space Flyer Unit (to be retrieved by the U.S. space shuttle in 1996) and a weather satellite. The Space Flyer carried an infrared telescope plus life sciences and materials sciences experiments.

      Despite a failure in March, Russia started marketing a small satellite launcher based on its SS-26 ballistic missile. China lost a U.S.-built communications satellite, Apstar-2, in a launch accident in January that killed six people on the ground as debris fell from the sky. (DAVE DOOLING)

      See also Business and Industry Review: Aerospace (Business and Industry Review ); Energy (Business and Industry Review ); Mining (Business and Industry Review ); Telecommunications (Business and Industry Review ); The Environment (Environment ); Media and Publishing: Radio (Media and Publishing ); Television (Media and Publishing ); Military Affairs .

      This updates the article space exploration.

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