to a restoration of pre-El Nino condi­tions have been more elusive. Graham, White and others propose that the cooling is the result of events that occur outside the equatorial wave­guide: the region bounded by about three degrees north and south lati­tude. In the off-equatorial regions the warming of the eastern sea disturbs wind patterns over the central Pacif­ic, leading to an oceanic upwelling in that region, which in turn gives rise to westward-moving Rossby waves. At this point in the cycle the Rossby waves are essentially regions in which the warm upper layer is thinner than usual (in other words, where the ther­mocline is elevated).

The Rossby waves travel more slow­ly than the Kelvin waves, but within a year or two they are thought to reach the western border of the Pacific, trav­el along the border back into the equa­torial region and become shallow, fast-moving Kelvin waves. The arrival of these waves in the eastern Pacific raises the thermocline there, allows cold water to surface and enables the easterly winds to gain strength.

At first, surface cooling is further re­inforced by the easterlies, which gen­erate shallow, eastward-moving Kel­vin waves in the central equatorial Pa­cific. Yet the cooling in the equatori­al waveguide helps to trigger the next heating phase. It leads to wind pat­terns in the off-equatorial areas that produce downwelling and deep (rath­er than shallow) Rossby waves. These westward-moving Rossby waves even­tually turn into deep Kelvin waves, which, when they reach the east, de­press the thermocline, thus raising the sea-surface temperature once again.

By examining various indicators of the movement of Rossby waves-such as wind patterns and changes in sea level and water temperature-in the off-equatorial regions, Graham and White have determined that the Ross­by waves appear to have been in the expected places when El Ninos have occurred in the past. They also note that models including calculations of Rossby-wave activity predicted the 1986-87 El Nino a year in advance of its onset. -Ricki Rusting

More Setbacks at SlAC

Aging technology adds to a linear collider's woes

This past spring physicists at the Stanford Linear Accelerator Cen­ter (SlAc) jubilantly announced

that the center's unconventional new

particle accelerator, known as the Stanford Linear Collider (SLC), was ready to roll. After months of delay and a slew of unforeseen technical problems, they had succeeded in bringing two tightly focused particle beams, one of electrons and one of positrons, into head-on collision-the most precise beam collision ever at­tempted. "That was the good news," said Kaye D. Lathrop, an associate di­rector of SlAe. "A lot of people had said we wouldn't be able to make such small beams collide."

The bad news was that there were not enough collisions. The collisions are supposed to churn out Zo's ("Z­naughts"), heavy particles whose prop­erties elucidate certain fundamental aspects of matter. But by the end of July no Zo's had been seen. Indeed, the machine achieved useful colli­sions only 3 percent of the time that it was running. Alarmed by this set­back, SlAC director Burton Richter as­sumed direct control of the project on August 1 and assigned teams to tackle the multitude of problems be­setting the machine.

The SLC is trying to achieve colli­sions by an untried method: boosting electrons and positrons to high ener­gies in a linear accelerator, or linac, and then aiming the beams at each other. Unlike conventional machines, in which beams collide repeatedly as they whirl in opposite directions along the same circular track, the SLC has only one shot at a time. To compen­sate, the beams have to be squeezed to unprecedented densities.

When Richter first proposed the idea in 1980, he decided to piggyback the new design onto the existing two­mile-long Stanford Linear Accelerator, which was built in the early 1960's. The linac would accelerate both elec­tron and positron beams and inject them into an oval track; they would go around the track in opposite direc­tions and slam into each other on the other side. This decision, calculated to catapult the SLC ahead of a rival Euro­pean machine of conventional design costing 10 times as much, has created problems of its own.

The old linac was designed to pro­duce beams with energies of about 25 billion electron volts (GeV) and diame­ters of a few millimeters, but the SLC must produce two 50-GeV beams fo­cused down to less than 10 microns­and aim them at each other. Among other things, it turned out that some of the older power supplies are too jittery to achieve such precision; it could take six months to replace them. To add to these troubles, during the

July heat wave some aging microelec­tronic parts overheated and failed in alarming numbers inside the unrefrig­era ted shed housing the linac.

Longer-term obstacles arise from the untried design of the machine. The "kicker magnets," which extract parti­cles from "damping rings" where they are squeezed down to the required densities, proved unequal to the task. Unexpectedly large effects induced by the electrical current of the beams prevented the damping rings from making adequately short bunches of the particles. The positron source, a metal target that emits positrons when it is bombarded by electrons, must be made more durable. There also are not enough instruments yet to adjust errors in the position and quality of the beams accurately.

Lathrop was philosophical about the setbacks: "To put the best face on it, we've learned what we have to do." For the next few months, weekdays will be spent studying and fine-tuning the SLC and weekends will be devot­ed to producing collisions. The plan is now to get the SLC up and running stably in February. -JK

Planetary Consomme Terrestrial experiments mimic alien atmospheres

From the makers of synthetic pri­mordial soup come two new fla­vors: Titan and Uranus. Back in

195 3 investigators first simulated the earth's primordial atmosphere. They zapped a gas mixture with electricity, which yielded a soup of organic mole­cules. Now this technique has been modified by astronomers trying to simulate chemical processes in the present-day atmospheres of the plan­ets and moons of the outer solar sys­tem. The simulations may help to ex­plain the planets' color and atmos­pheric chemistry, and they may even provide insight into the origin of life in this solar system and others.

Whereas the primordial-soup exper­iment was based on conjecture about the earth's ancient atmosphere, the new simulations rely on information collected by the Voyager 2 spacecraft during its flybys of Saturn and Uranus. Carl Sagan, W. Reid Thompson and Bishun N. Khare of Cornell Universi­ty duplicated in their laboratory the conditions found in the upper at­mospheres of Titan (Saturn's largest moon) and Uranus. Hydrogen, helium, nitrogen and methane were mixed at pressures and concentrations typical

SCIENTIFIC AMERICAN October 1 988 25

© 1988 SCIENTIFIC AMERICAN, INC

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of those atmospheres. The gases were pumped continuously through glass tubing, and a coil of wire was wrapped around a section of the tubing. A high­voltage current flowing through the wire created an intense magnetic field in the gas, The field energized the gas molecules and stripped electrons from them, forming a state of matter known as a plasma. The laboratory plasma mimics the aurora, in which atmospheric gases are bombarded by charged particles accelerated by a planet's magnetic field. Chemical reac­tions are known to abound in plane­tary auroras.

The laboratory plasma was Similarly fecund, creating many different organ­ic molecules, including hydrocarbon chains up to seven carbon atoms long. As these chemicals and others flowed farther through the glass tubing, they were separated for analysis. Some gas­es condensed at room temperature. Others solidified in places where the tubing was cooled in baths of dry ice and liquid nitrogen. After the simula­tion had run for a few days, enough material was produced to be detected and analyzed.

The astronomers were thereby able to study the chemistry of Titan and Uranus at a level of detail not possi­ble for the Voyager spacecraft or tel­escopes. The Titan experiments pro­duced a brownish organic solid whose optical properties agreed well with Voyager and ground-based measure­ments of the omnipresent Titan haze. Over the lifetime of Titan this material may have accumulated on the moon's surface in a layer hundreds of me­ters thick.

The investigators also report that the nitrogen-rich atmosphere of Titan produced a great variety of pre biotic chemicals called nitriles. Nitriles are precursors of amino acids, the basic building blocks of proteins. "Some­thing similar may have happened on the early earth, but on Titan the pre­biological chemistry is probably still­born: the temperatures are far be­low the freezing point of water," Sa­gan observes.

The Uranus experiment, reported in Journal of Geophysical Research, sim­ulated a hydrocarbon smog that is created by the aurora in the planet's hydrogen-and-methane atmosphere. In the reaction chamber of the simu­lation, the astronomers collected sol­id hydrocarbons on glass slides. They measured optical properties of the hy­drocarbons and found they account for the subtle hues of yellow, red, brown and black that tinge the blue­green globe of Uranus. The Cornell

© 1988 SCIENTIFIC AMERICAN, INC

workers hypothesize that such hydro­carbons, created in the same way, con­tribute to the colors of other planets as well. -Russell Ruthen

BIOLOGICAL SCIENCES

Postprandial Warmth Certain fish rise to the surface in order to digest better

Does a warm bath make you hungry? For certain fish the answer apparently is yes. Ac­

cording to a report in Nature, these fish, juvenile Bear Lake sculpins, feed on the chilly bottom of Bear Lake (in Utah and Idaho) during the day and rise to the warmer surface waters at night to digest. The warm water speeds their metabolism, accelerating the rate at which they absorb their food and making it possible for them to eat a larger meal the next day.

Daily vertical migrations are not un­common among fish species, but they usually occur for other reasons. For example, a fish might come up to the surface to feed on plankton during the night and then return to the depths during the day in order to hide from predators or to conserve energy by slowing its metabolism in cold wa­ter. Indeed, the authors of the re­port, Wayne A Wurtsbaugh and Darcy Neverman of Utah State University, thought they were dealing with one of these migration patterns, until they discovered that the sculpins' stom­achs were fullest at dusk, just before the fish rose to the surface. Inside the stomachs were the remains of organ­isms that live only on or near the bottom. The fish could not have been rising to the surface to eat; perhaps they were rising to digest.

To test their hypothesis that warm surface water acts as an aid to diges­tion, the investigators fed a collection of sculpins a full meal and kept them in separate tanks, some at five degrees Celsius (the temperature of the bot­tom waters) and some at 15 degrees (the temperature of water near the surface at night). The fish kept at 15 degrees digested their stomach con­tents at a rate of 23 percent per hour and had evacuated 80 percent of their meal within 7.5 hours. The fish kept at five degrees, on the other hand, digest­ed only 3.2 percent of their stomach contents per hour; at that rate it would have taken them 50 hours to evacuate 80 percent of the meal.

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High up in an ice cave, from the waters of the melting Gangotri Glacier, the Ganga River begins.

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SCIENTIFIC AMERICAN October 1 988 27

© 1988 SCIENTIFIC AMERICAN, INC