Caltech researchers use the "unnatural selection" of directed evolution to alter a bacterial enzyme

In a novel process that makes the evolution of species look like an engineering design contest, California Institute of Technology scientists have forced a bacterial strain to "evolve" a beta caratenoid enzyme . The evolved enzyme can carry out reactions that normally require other proteins and expensive agents. These reactions are important for making drugs and chemicals.

The enzyme, called a cytochrome P450, is one of a class of enzymes that inserts oxygen atoms into a huge number of compounds, according to Caltech chemical engineering and biochemistry professor Frances Arnold. In an article appearing in the June 17 issue of the journal Nature, Arnold and her team demonstrate an evolutionary process to alter the enzyme and overcome several of the natural limitations that make it inefficient and expensive to use. "The P450s do some great chemistry, but they are complex and ill-behaved," says Arnold, who helped pioneer directed evolution some years ago. "We hope to create stable P450s that need no expensive external cofactors to work. We'd like to pare them down to the absolute minimum and see how well they can do. They may well be much better catalysts without all the fancy machinery."

Nature helps these enzymes along by providing a retinue of protein assistants and complex chemicals that are either impossible or very expensive to reproduce outside a cell. In the Nature paper, the Arnold team reports the laboratory evolution of a P450 that no longer needs any of this help to catalyze its reaction.

Directed evolution has been heralded recently as a means of creating new enzymes and even whole organisms with new or vastly improved characteristics. In contrast to natural evolution, in which the survival of the fittest dictates the direction of change, directed evolution engineers enzymes for specific purposes. These purposes may have nothing to do with what the enzymes do in their natural organisms. For example, the enzymes may be better suited for removing laundry stains, or they may be used to treat diseases.

In directed evolution, the scientists dictate which enzyme characteristics will be selected in each generation, very much the way plant breeders use mutation and selective breeding to create new corn varieties or animal breeders have introduced new varieties of cattle or sheep.

In the case of the P450 enzymes, Arnold and her team wanted an enzyme that would work without any additional help. To do this, they made use of a known feature of the P450s—that hydrogen peroxide could support the reaction in the absence of the helper proteins and cofactor. Their goal was to evolve what Arnold calls this "biochemical oddity" to make it the primary pathway for the enzyme to work.

Arnold is interested in creating enzyme catalysts that could be used to manufacture drugs and chemicals. "The ideal is a rock-stable, very fast, very active enzyme that you could put in a bottle," she says. "Over the next five to 10 years, we're aiming at enzymes that the chemical industry could use." These improved enzymes could also perhaps lead eventually to new technologies that remove toxic wastes in soil or water.

"In this paper, we present the results of two generations, which makes the enzyme 20 times better than it originally was," she says. "We don't know how far we'll be able to go. But we hope that 10 more generations could result in something remarkably good. With other enzymes, for example, we have seen improvements of 500-fold over 10 generations."

Arnold's procedure for directed evolution is to first target an interesting enzyme. In the case of the current paper, the enzyme is from Pseudomonas putida, a bacterial strain that uses the P450 enzyme as a sort of "digestive aide" to eat camphor that is found in soil. The researchers then take the gene that codes for the enzyme and create millions of mutants, which they put back into a bacterial "workhorse" that generates the mutant enzymes. The scientists then screen these mutants for the desired qualities.

The best candidates from that generation can either be "bred" together to obtain further improvements, or the process can be started over again with new mutations.

Arnold says that each generation of the P450 enzyme takes about a week or two. Practically, a total effort in directed generation takes at least several months.

The other authors of the paper are Hyun Joo and Zhanglin Lin, both postdoctoral scholars in chemical engineering at Caltech.

Robert Tindol
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Lack of Energy Makes Life on Europa Unlikely, Caltech Study Concludes

Embargoed for Release at 3 p.m. Thursday, June 3, 1999

PASADENA—Future space travelers to the watery Jovian moon Europa should probably leave their fishing tackle at home. A new study conducted by California Institute of Technology and Jet Propulsion Laboratory scientists shows that the Europan ocean is unlikely to harbor any life form more complex than single-celled organisms—and maybe not even that.

In this week's issue of the journal Science, Caltech geobiologist Eric Gaidos and coauthors Kenneth Nealson and Joseph Kirschvink show that nearly all forms of energy used by life on the Earth are unavailable to the organisms that might live beneath Europa's surface ice layer.

According to Gaidos, "One must be careful when doing comparative planetology. It is not a safe assumption to use Earth as an analogy. A liquid-water ocean on Europa does not necessarily mean there is life there."

On Earth, chemical energy is derived either from sunlight by means of photosynthesis or from the oxygen that is a byproduct. This oxygen reaches even the exotic animals inhabiting the super-hot volcanic vents in the deep sea that were discovered 20 years ago.

Even for the organisms living under ice sheets on Earth, the system is not closed. Energy from outside is available for the organisms underneath.

Unlike Earth, Europa is a closed system. The ice layer cannot be penetrated by sunlight and the only available energy in the system comes from within. This study shows that the energy available is very small compared to levels used by organisms on the Earth. It seems very unlikely that multicellular life could survive, and the lack of energy puts constraints on the likelihood of finding even hardy single-celled organisms.

Gaidos uses the analogy of an energy waterfall. "Chemical energy is falling from a high state to a low state just as water falls due to gravity. Life acts as a waterwheel in this process and harnesses the energy. However, without a source of chemical energy, the waterwheel stops."

Kirschvink adds, "Earth has a lot of metabolic energy available for life, but if you shut off the source, you shut off the system."

The study doesn't completely rule out the possibility of life, however. Gaidos says the study "assumes that the life we look for is based on the same energy sources used by life on Earth.

"The study puts limits on what life is possible," says Gaidos. "Complex life is very unlikely, but there are other possible alternatives for simple organisms to acquire the necessary energy."

One such possibility is that the organisms derive the necessary biochemical energy from oxidized iron (rust) that may exist under the ice. Other possibilities may exist, so long as there is a source of energy and life can insert its waterwheel at some point in the system.

"But we are talking about very simple organisms that can live on these energy sources. These are not multicellular creatures," Gaidos says.

Only the future will reveal what scientists might find under the ice of Europa. But we do know that no fish will be biting.

Robert Tindol

New digital sky survey uncovers rare celestial objects

CHICAGO—A large new digital sky survey has been used by astronomers at the California Institute of Technology to discover distant quasars and other rare types of cosmic objects, including mysterious new objects of an unknown nature.

These results are being reported today at the meeting of the American Astronomical Society in Chicago.

The Caltech team, led by S. George Djorgovski, professor of astronomy, made the discoveries in an initial scientific exploration of the Digital Palomar Observatory Sky Survey (DPOSS). The survey, now nearing completion, covers the entire northern sky in three colors, and it is based on a photographic sky atlas (POSS-II) produced at Palomar Observatory.

The final product of the survey is the Palomar-Norris Sky Catalog, which will contain information on over 50 million galaxies and about two billion stars. It will be made available to the general astronomical community, beginning a few months from now.

When complete, DPOSS will contain several terabytes of information (a terabyte is 8 trillion bits, or about the amount of information contained in two million thick books). This is also over a thousand times larger than the amount of information in the entire human genome.

Comparable amounts of data are now being produced by several other digital sky surveys, including the Two-Micron All-Sky Survey (2MASS) in the infrared wavelengths, the forthcoming Sloan Digital Sky Survey (SDSS), which, like DPOSS, will cover the visible light part of the spectrum, and several NASA missions.

Other projects of a similar scope are now under way or are being planned.

"This is the dawn of the new era of information-rich astronomy," says Djorgovski. "This unprecedented amount of astronomical information will enable scientists and students everywhere, without access to large telescopes, to do first-rate observational astronomy."

Surveys like DPOSS can be used to study the universe in a systematic manner—for example, to probe the large-scale structure in the distribution of galaxies in some detail. But they can also be used to discover rare, or even previously unknown types of astronomical objects: the sheer numbers of detected sources make it possible to find objects that are one in a million or even one in a billion—an astronomer's needle in a digital haystack.

Caltech astronomers did exactly that in their initial scientific verification tests of the DPOSS data. The group used novel techniques to search the data for star-like objects with colors unlike those of the ordinary stars.

Some of these are types of objects they expected to find: for example, very distant quasars, seen at the time when the universe was less than 10 percent of its present age. Such quasars are valuable probes of the early universe and galaxy formation. The Caltech team has so far identified over 70 of them, more than the number found by all other groups in the world combined.

Perhaps even more interesting are surprises, unexpected findings of anomalous objects. The Caltech team has one such object whose nature is still unknown.

"It has a spectrum unlike anything else I have ever seen," says Djorgovski. "We have combed the literature and asked all kinds of experts, but no one can tell us what it is. It is the first one of something new—and a complete mystery to us."

Another discovery is objects that can vary in brightness by a large factor. Since the photographs used in DPOSS are taken at different times with different filters, objects that are much brighter at one time would stand out as having peculiar colors. One such discovery is a starlike object which is associated with an extremely faint galaxy.

When the survey photograph was taken, the object was several hundred times brighter than the galaxy itself, perhaps a hundred times brighter than a supernova explosion. Astronomers speculate that it may have been associated with an undetected gamma-ray burst, but it could also be something even more strange and previously unseen.

Astronomers at Caltech and elsewhere are discussing the concept of the future National (or Global) Virtual Observatory, to be built in cyberspace rather than on some mountaintop. This would be a way to organize and combine many of the large new and forthcoming sky surveys and other astronomical data, to make them accessible over the Web, and to provide novel data-mining tools for their scientific exploration.

Astronomers and computer scientists are now starting collaborations to make this vision a reality. This would be a new way of doing astronomy, with a computer and a rich data archive, rather than with a telescope.

"We are really only beginning to explore the universe in some detail. There must be many wonderful new and unexpected things out there, waiting to be discovered, and large sky surveys are the best way to find them," concludes Djorgovski.

In addition to Djorgovski, the Caltech team includes postdoctoral scholars Stephen Odewahn and Robert Brunner, graduate student Roy Gal, and several Caltech undergraduates. Professor of Physics Tom Prince is also one of the leaders of the effort to create the Virtual Observatory. The work on the DPOSS survey is supported by a grant from the Norris Foundation and by other private donors.

Robert Tindol

Mars Global Surveyor's triumphs follow recovery from ill-fated earlier mission

PASADENA—Arden Albee is often asked how it felt to lose the Mars Observer.

Albee was project scientist of the ill-fated space probe that somehow lost its way in the depths of space in 1993. On its final approach to Mars, scientists lost contact and never heard or saw anything of the spacecraft again. To this day, neither Albee nor anyone else is certain what happened.

"Our suspicion is that, in the process of pressurization, there was a rupture in the lines that set it spinning," says Albee, a professor of geology and planetary science at the California Institute of Technology and longtime collaborator on Jet Propulsion Laboratory planetary missions.

"Mars Observer probably didn't blow up, but we were never able to figure out where it might be because we didn't know if it got captured by Mars," he says. "If it did, then it could be in a Martian orbit; if not, then it's in a sun orbit. But we searched for it in both orbits without luck."

The loss was extremely disappointing to the planetary science community. Had Mars Observer been successful, the spacecraft would have mapped the surface of the Red Planet and sent back a huge amount of data—far more data than had ever been sent back from Mars by all interplanetary probes combined, since the advent of the space program.

Certainly, many scientists would have been in despair after seeing two decades of their life's work evaporate. But Albee in particular and NASA scientists in general are different.

"When we lost Mars Observer, we were so busy putting together the pieces that we didn't really have time to get overly depressed," he says. "For one thing, we weren't immediately sure that the mission was permanently lost. And also, the next launch opportunity was the next fall, so the question was immediately whether we could recover in time to launch another spacecraft during that opportunity.

"There was a fair amount of despair, obviously, but we had a lot of other things to do."

As it turned out, Albee and the hundreds of other scientists, engineers, technicians, and support staff who worked on Observer decided against launching again in 1994. But another window of opportunity opened in December 1996, and this time they were ready with Mars Global Surveyor.

Albee was named project scientist of Global Surveyor, and out of the spare parts of the ill-fated Observer mission came the beginnings of a more compact, more cost-effective mission that has already brought back a tremendous amount of scientific data. In fact, by the time the mission is completed in a couple of years, Global Surveyor will fulfill Observer's original promise of returning more planetary data than all the other missions to date combined.

The Mars Global Surveyor mission, like the highly visible Pathfinder mission of 1997, is an embodiment of the new NASA mantra "faster, better, cheaper." About a ton in weight and the size of an office desk, the Global Surveyor orbiter is bigger than the little Pathfinder rover that so captured the public's imagination on Independence Day 1997. But Global Surveyor is designed to send back vastly more data and perform considerably more science over a much longer period of time.

Very soon after the spacecraft went into Martian orbit, Global Surveyor captured, for the first time, the start of a major dust storm on Mars and followed it through its development and demise.

This and other early accomplishments came at a time when Global Surveyor personnel were undoubtedly feeling a nauseating sense of déjà vu. Early on, the spacecraft developed a glitch when it first began tightening up its orbit.

Global Surveyor, to be "faster, better, cheaper," had been set on a course that took it initially into a huge sweeping elliptical orbit of Mars. On its near approach in each orbit, the probe was to dip into the upper atmosphere of Mars in a maneuver known as aerobraking, which would effectively slow the probe down and eventually place it into a near-circular orbit. But a solar-panel damper failed early in the mission, and damage to the solar panel forced the team to slow down the rate of the aerobraking, and the actual mapping mission only got under way this year.

A couple of other annoying glitches have come and gone, including an antenna stoppage a few weeks ago, and a burn that didn't work quite right. But all in all, Global Surveyor has already turned out to be a very good consolation for those who would have benefited from the data returned by Observer.

The Mars Global Surveyor team announced early in the mission that the on-board magnetometer shows Mars to have a more complex magnetic field than once thought. But results published earlier this year showed that Mars once had a magnetic field even stronger than that of Earth.

This is important because many scientists think a strong magnetic field may be crucial for the evolution of life on a planet, because without it the surface is constantly bathed in strong radiation that would kill most life forms that we know of. Moreover, atmospheric constituents like oxygen and water tend to be "sputtered" away by the solar wind and cosmic rays.

The magnetometer has also recently shown the presence of magnetic stripes that show similarities to stripes that occur on the ocean floor on Earth. It is tempting to conclude that their origin was similar, but this is not yet certain.

The mapping mission is working so well that scientists not only have new knowledge about the surface of Mars, but also vastly improved meteorological understanding. The instruments are monitoring water-ice clouds, carbon dioxide clouds, and dust storms—all highly variable and localized atmospheric phenomena.

Further, the knowledge obtained about Martian weather could perhaps even provide new insights on the nature of weather on Earth.

"At this point, we basically know the topography of Mars, in a sense, better than we know that of the continents of Earth," Albee says.

The basic shape is that of a triaxial ellipsoid with the topography imposed upon it.

Whatever the remaining two years hold for Global Surveyor, Albee says this is probably his last space mission. A Caltech professor since 1959, Albee was originally a geologist, but got involved in space exploration while investigating the lunar samples returned by the Apollo missions of the late 1960s and early 1970s.

Albee got more and more involved in planetary science as time went on. He was involved in the Viking mission to Mars toward its end in the mid-1970s, and in 1978 was named chief scientist at the Jet Propulsion Laboratory. He served until 1984, when he became Caltech's dean of graduate studies—a position he holds to this day, in addition to his ongoing work as a JPL project scientist and a Caltech professor.

"I think I'll let the new generation go back to Mars," he says, adding that the future promise of breakthroughs such as extraterrestrial sample returns will provide strong motivation for his successors.

As for his unflappable demeanor during the Mars Observer loss, Albee sums it all up in a single sentence:

"It's called optimism."

Robert Tindol

Aeronautics researchers generate cracks that move as fast as the speed of sound, and resemble certain earthquake shear ruptures

PASADENA-When a brittle material breaks, the resulting cracks tend to spread quite rapidly. Anyone who has inadvertently subjected a favorite vase to "floor stress" can attest to this.

But exactly how fast a crack can move has been a subject of debate for some time. To understand how fast is fast, one has to know how quickly stress waves spread. In a solid material, waves spread with two speeds-the slower shear waves move at the shear wave speed, and the faster pressure waves move at the pressure wave speed, also commonly known as the speed of sound in the material.

The researchers who specialize in the topic have thought that cracks move at speeds substantially slower than either of these wave speeds in the material (usually less than 20 percent of the speed of sound).

But new work from California Institute of Technology researchers shows that a certain type of crack can exceed the shear wave speed through the material, creating a sort of "sonic boom," and can almost reach sound speed.

According to Ares Rosakis, professor of aeronautics and applied mechanics, such a crack (called a shear crack) can be generated under controlled circumstances and photographed with ultrahigh-speed equipment capable of taking two million photographs in a second. The pictures show a crack with angled shock waves (Mach cones) that closely resemble photographs of a supersonic bullet breaking the sound barrier.

Click here to view or download the high speed photographs showing the propegation of shear cracks (QuickTime format, 2MB)

This result has practical applications, Rosakis says, because there is reason to believe that certain earthquakes can arise from similar shear breakages. A better understanding of the way in which these cracks get rolling could help seismologists with their models of earthquakes along shear faults, Rosakis believes.

To test the idea, Rosakis and his graduate students Omprakash Samudrala and Demirkan Coker bonded two sheets of a clear polyester material called "Homalite." They introduced a notch along the bond line, and rammed the bottom sheet from the side with a steel projectile moving only at 25 meters per second (56 miles per hour). High-speed photographs of the event showed a shear crack starting from the notch tip and accelerating almost up to the sound speed in the material.

The Rosakis team used this particular setup because a fault zone itself is the weak link between two planes. So if the shearing force along a fault plane indeed sets up shear cracks, then the act of breaking two weakly bonded Homalite plates by sliding them apart would be very similar dynamically.

The high-speed photographs indeed show that shear cracks propagate along the bond line at about 2,200 meters per second (5,000 miles per hour). Further, the Mach cones were clearly visible because the cracks were outrunning material shear wave speed, just as a bullet from a high-powered rifle outruns the speed of sound in air. In fact, these shear cracks are faster than speeding bullets and also faster than the fastest supersonic jet planes!

"I believe processes like that have to do with certain events that may happen in the earth's crust. They may also be very relevant to the way that layered solids or composites, involving bonds, fail when subjected to impact loading," says Rosakis. "This is the first lab demonstration that shows these phenomena are possible."

The work is being published this week in the May 21 issue of the journal Science. Samudrala and Coker, both Caltech graduate students in aeronautics, are the other authors of the paper.

This work has been sponsored by the Office of Naval Research and the National Science Foundation.

Robert Tindol

A unique class of neurons in humans and apes that may participate in cognition, volition, and self-awareness discovered by researchers

PASADENA-Clusters of large neurons found exclusively in the brains of humans and other primates closely related to humans may provide these species with enhanced capacities for solving hard problems, as well as for self-control and self-awareness.

In the April 27 issue of Proceedings of the National Academy of Science, neurobiologists Patrick Hof from Mount Sinai and John Allman from Caltech and their colleagues have found an unusual type of neuron, that is likely to be a recent evolutionary acquisition.

The neurons in question are spindle-shaped cells, which are almost large enough to be seen with the naked eye. Their location in the brain is in the frontal lobe near the corpus callosum, which connects the two halves of the brain.

Allman, the Hixon Professor of Psychobiology and professor of biology; Hof; and their team studied 28 different species of primates and found the spindle neurons only in humans and very closely related apes. The concentration of spindle neurons was greatest in humans, somewhat less in chimpanzees, still less in gorillas, and rare in orangutans.

According to Allman, "This declining concentration matches the degree of relatedness of these apes to humans." There were no spindle cells in gibbons, which are small apes, or in of any of the other 22 species of monkey or prosimian primates they examined. The spindle cells were also absent in 20 nonprimate species examined including various marsupials, bats, carnivores and whales.

The cells in question are found in an area of the brain already linked to psychiatric diseases. According to Allman, "In brain imaging studies of depressed patients, there is less neuronal activity in the region and the volume of the area is smaller. The activity of the area is increased in obsessive compulsive patients."

The activity of the area has been shown to increase with the difficulty of the cognitive task being performed. This suggests that the area enhances the capacity to do hard thinking. Activity is also increased when a subject withholds a response or focuses its attention, suggesting the area is involved in self-control.

Furthermore, the spindle neurons themselves are especially vulnerable to degeneration in Alzheimer's disease, which is characterized by diminished self-awareness. From this Allman suggests, "Part of the neuronal susceptibility that occurs in the brain in the course of age-related dementing illnesses may have appeared only recently during primate evolution."

Robert Tindol

Caltech biologists reveal structure of protein responsible for weight loss in cancer and AIDS patients

PASADENA-Caltech biologists have determined the three-dimensional structure of a protein that causes wasting in cancer and AIDS patients. The discovery could lead to new strategies for controlling weight loss in patients with devastating illnesses-and conversely, perhaps new strategies for fighting obesity.

The protein is commonly known as ZAG and is found in most bodily fluids. But researchers have been aware for some time that the protein is particularly abundant in patients who have cancer.

More recently, researchers have discovered that the protein is involved in the wasting syndrome known as cachexia, which is associated with both cancer and AIDS.

"This protein has something to do with fat metabolism," says Pamela Bjorkman, a professor of biology at Caltech and associate investigator of the Howard Hughes Medical Institute. Bjorkman and her team recently published a paper in the journal Science showing ZAG's structure.

One of the most noteworthy features of the structure is the resemblance between ZAG and a family of proteins known as class I major histocompatibility complex molecules, or MHC.

"MHC proteins have a large groove that binds a peptide derived from a pathogen," says Bjorkman, explaining that their new picture of the ZAG crystal shows an unexpected blob in the ZAG counterpart of the MHC peptide binding grove.

"It's not a peptide, but some organic molecule," she says. "We suspect that it is involved in the function of ZAG. If this compound is involved in breaking down lipids, then maybe you could design a drug that replaces it and interfere with lipid breakdown."

According to Bjorkman, other research shows that tumor cells themselves seem to stimulate the body to overproduce ZAG somehow, which in turn leads to the breakdown of body fat.

Thus, people suffering from cachexia don't lose body weight because they don't eat, but because the fat in their bodies is ultimately destroyed by an interaction involving ZAG.

An intervention to stop the wasting, then, might be to disrupt the overexpression of ZAG, and this might be accomplished with monoclonal antibodies or small molecules that bind to ZAG, she says.

The research appeared in the March 19 issue of Science, and was also the subject of an article in HHMI news, published by the Howard Hughes Medical Institute.

The other authors of the paper are Luis Sanchez and Arthur Chirino, both senior research fellows in Bjorkman's lab.

Robert Tindol

Caltech Question of the Month: What causes the auroral lights?

Submitted by Catherine E. Wendt, Pasadena, California.

Answered by Paul Wennberg, associate professor of atmospheric chemistry and environmental engineering science, Caltech.

Ions and electrons produced in the sun's atmosphere form the "solar wind." This stream of charged particles interacts with Earth's magnetic field and impacts the atmosphere in the region called the aurora oval. In the northern hemisphere, the collisions produce a wonderful light show called the aurora borealis. In the southern hemisphere the phenomenon is known as the aurora australis. The light, best observed near the fall or spring equinox, is produced when atmospheric gases that have been excited by these collisions, relax back to their normal states in a process known as fluorescence.

The color and altitude of the aurora tell us which atmospheric gases are being excited. Below 100 kilometers in altitude, nitrogen is responsible for blue and red auroral light. Between 100 and 200 kilometers, green light is produced by oxygen atoms, while above 200 kilometers, red light from oxygen dominates the auroral light.

Much more information, and beautiful photos, can be found at "The Aurora Page,"


Caltech observes brightest gamma-ray burst so far

PASADENA-An extraordinarily bright cosmic gamma-ray flash turns out to be the most energetic one measured so far, according to a team of astronomers from the California Institute of Technology.

"The burst appeared to be more luminous than the whole rest of the universe, and that would be very hard to explain by most current theories,"said Caltech professor of astronomy and planetary science Shrinivas Kulkarni, one of the principal investigators on the team.

"It was ten times more luminous than the brightest burst seen so far, and that was quite unexpected."

"If the gamma rays were emitted equally in all directions, their energy would correspond to ten thousand times the energy emitted by our sun over its entire lifetime so far, which is about 5 billion years," said Caltech professor of astronomy S. George Djorgovski, another of the principal investigators on the team. "Yet the burst lasted only a few tens of seconds."

Gamma-ray bursts are mysterious flashes of high-energy radiation that appear from random directions in space and typically last a few seconds. They were first discovered by U.S. military Vela satellites in the 1960s. Since then, over a hundred theories of their origins have been proposed, but the causes of gamma-ray bursts remain unknown. Some theorists believe that the bursts originate during the formation of black holes.

NASA's Compton Gamma-Ray Observatory satellite has detected several thousand bursts so far. The chief difficulty in studying these puzzling flashes is in locating them precisely enough and quickly enough to follow up with ground-based telescopes.

A breakthrough in this field was made in early 1997 by the Italian/Dutch satellite BeppoSAX, which can locate the bursts with a sufficient accuracy. A team of Caltech astronomers was then able to establish that the bursts originate in the very distant universe. Since then, about a dozen bursts have been studied in detail by astronomers using ground-based telescopes.

The bursts may last only a few seconds in gamma rays, but leave more long-lived but rapidly fading afterglows in X-rays, visible light, and radio waves, which can be studied further.

This burst, called GRB 990123, was discovered by the BeppoSAX satellite on January 23. It was the brightest burst seen so far by this satellite, and one of the brightest ever seen by NASA's Compton Gamma-Ray Observatory.

Within three hours of the burst, members of the Caltech team, including senior postdoctoral scholar in astronomy Stephen Odewahn and graduate students Joshua Bloom and Roy Gal, used Palomar Observatory's 60-inch telescope to discover a rapidly fading visible-light afterglow associated with the burst.

"This adventure began at 5 a.m. with a wake-up call from our Italian friends alerting us about their burst detection," said Bloom, "But it was certainly worth it. We got to watch a remarkable fireworks show!"

A comparison of images obtained at Palomar Observatory. The image on the top is from the Palomar Observatory's digital sky survey (DPOSS). The image on the bottom is the discovery image obtained by S. C. Odewahn and J. S. Bloom.

Following the Caltech team's announcement, several hours later a team of astronomers known as the ROTSE collaboration, led by Professor Carl Akerloff of the University of Michigan, reported that the visible light counterpart of the burst was also seen in the images taken with a small, robotic telescope operated by their team, starting only 22 seconds after the burst. This was the first time that such rapid measurement of a burst afterglow was made, and its extreme brightness was unexpected.

Meanwhile, a new radio source, coincident with the visible-light afterglow discovered at Palomar, was found at the National Radio Astronomy Observatory's Very Large Array radio telescope, near Socorro, New Mexico, by Dale Frail and Kulkarni.

Such a radio flash was predicted by Dr. Re'em Sari, a theorist at Caltech, and Dr. Tsvi Piran (now at Columbia University), and it provides an important input for theories of gamma-ray bursts.

At the prompting of the Caltech team, a group of astronomers led by Professor Garth Illingworth of the University of California at Santa Cruz, used the W. M. Keck Observatory's 10-meter Keck-II telescope at Mauna Kea, Hawaii, to obtain a spectrum of the burst afterglow.

A distance to the burst was determined from its spectrum, and the burst was found to be about 9 billion light-years from Earth.

The Keck measurement of the distance was crucial. "We were stunned," said Djorgovski. "This was much further than we expected, and together with the observed brightness of the burst it implied an incredible luminosity.

"The peak brightness of the visible light afterglow alone would be millions of times greater than the luminosity of an entire galaxy, and thousands of times brighter than the most luminous quasars known."

This remarkable light flash contained only a small fraction of the total burst energy in the gamma rays. Caltech astronomers note that even more energy was likely emitted in forms that are difficult to observe, such as gravitational waves or neutrinos, elusive particles that can penetrate the entire planet Earth without stopping.

As the burst's afterglow faded, the Caltech team discovered a faint galaxy adjacent to it in the sky, in infrared images obtained with the W. M. Keck Observatory's 10-meter Keck-I telescope at Mauna Kea.

This is almost certainly the galaxy in which the burst originated. The galaxy is about as faint as an ordinary 100-watt lightbulb would be if seen from a distance of half a million miles, about twice the distance to the moon.

Subsequently, following a proposal by the Caltech team and others, the Hubble Space Telescope obtained visible-light images of this galaxy and the burst's afterglow. The analysis of these images by the Caltech team indicates that the galaxy is not unusual in its properties, compared to other normal galaxies at comparable distances from Earth.

A detailed follow-up study of the burst's afterglow by the Caltech team revealed a change in its brightness that could be interpreted as a sign of a jet of energy, moving close to the speed of light, and pointing nearly toward Earth.

"This was the first time that such behavior was seen in a gamma-ray burst," emphasized Kulkarni, "and it may help explain in part its enormous apparent brightness."

Scientists are still debating whether such a powerful beaming of energy occurs in gamma-ray bursts.

The team's findings appear in the April 1 issue of the scientific journal Nature, and in a forthcoming issue of the Astrophysical Journal Letters.

In addition to Kulkarni, Djorgovski, Odewahn, Sari, Bloom, and Gal, the Caltech team also includes Professors Fiona Harrison and Gerry Neugebauer, Drs. Chris Koresko and Lee Armus, and several others.


Robert Tindol

Earth's water probably didn't come from comets, Caltech researchers say

PASADENA—A new Caltech study of comet Hale-Bopp suggests that comets did not give Earth its water, buttressing other recent studies but contrary to the longstanding belief of many planetary scientists.

In the March 18 issue of Nature, cosmochemist Geoff Blake and his team show that Hale-Bopp contains sizable amounts of "heavy water," which contains a heavier isotope of hydrogen called deuterium.

Thus, if Hale-Bopp is a typical comet, and if comets indeed gave Earth its water supply billions of years ago, then the oceans should have roughly the same amount of deuterium as comets. In fact, the oceans have significantly less.

"An important question has been whether comets provided most of the water in Earth's oceans," says Blake, professor of cosmochemistry and planetary science at Caltech. "From the lunar cratering record, we know that, shortly after they were made, both the moon and Earth were bombarded by large numbers of asteroids or comets.

"Did one or the other dominate?"

The answer lies in the Blake team's measurement of a form of heavy water called HDO, which can be measured both in Earth's oceans using mass spectrometers and in comets with Caltech's Owens Valley Radio Observatory (OVRO) Millimeter Array. Just as radio waves go through clouds, millimeter waves easily penetrate the coma of a comet.

This is where cosmochemists can get a view of the makings of the comet billions of years ago, before the sun had even coalesced from an interstellar cloud. In fact, the millimeter-wave study of deuterium in water and in organic molecules in the jets emitted from the surface of the nucleus shows that Hale-Bopp is composed of 15 to 40 percent primordial material that existed before the sun formed.

The jets are quite small in extent, so the image clarity provided by the OVRO Millimeter Array was crucial in the current study. "Hale-Bopp came along at just the right time for our work," Blake says. "We didn't have all six telescopes in the array when Halley's comet passed by, and Hyakutake was a very small comet. Hale-Bopp was quite large, and so it was the first comet that could be imaged at high spatial and spectral resolution at millimeter wavelengths."

One other question that the current study indirectly addresses is the possibility that comets supplied Earth with the organic materials that contributed to the origin of life. While the study does not resolve the issue, neither does it eliminate the possibility.

Also involved in the Nature study are Charlie Qi, a graduate student in planetary science at Caltech; Michiel Hogerheijde of the UC Berkeley department of astronomy; Mark Gurwell of the Harvard-Smithsonian Center for Astrophysics, and Duane Muhleman, professor emeritus of planetary science at Caltech.

Robert Tindol


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