East and West Antarctica once began separating but then stopped, new research shows

PASADENA—Earth was well on its way to having two Antarcticas long ago, but a tectonic separation between the eastern and western portions of the continent suddenly stopped after 17 million years of spreading, researchers say.

In the March 9 issue of Nature, lead author Steve Cande of the Scripps Institution of Oceanography, Joann Stock of Caltech, and their colleagues in Australia and Japan report that the rift between East and West Antarctica began about 43 million years ago, then ended 17 million years later, after the seafloor had spread about 180 kilometers. The researchers discovered the motion after making several cruises over a period of years in the waters off the Antarctic coast and after gathering data on the seafloor itself.

"The two pieces of Antarctica pulled apart and then stopped," says Stock, a professor of geology and geophysics at Caltech. "If it had kept on going, there would eventually have been two Antarcticas."

The primary scientific value of the study is that it answers some nagging questions about the "missing" motion in the Antarctic region. For a variety of reasons, geophysicists have had a hard time getting a handle on the precise directions and amounts of motion there, and how the motion fits into the grand scheme of global plate tectonics.

"It's like a jigsaw puzzle," Stock says. "You have to know how one piece moved relative to the other pieces to understand how it all fits together.

"A lot of the tectonic plate history for western North America, for example, depends on what happened in Antarctica. You wouldn't think so, but that's the way plate tectonic movements work."

The key to the new results was the researchers' discovery of an underwater feature off Cape Adare that they have named the Adare Trough. This trough is about 230 kilometers long and runs roughly northwest-southeast near the 170th meridian. The sharp break in the direction of the magnetic lines on either side of the trough allows the researchers to infer the ancient relative motions of the plates, and the age and shape of the trough and seafloor around it indicates the period when the spreading occurred.

Seafloor spreading in the area accounts for the "missing" motion in the plate circuit linking the Australia, Antarctic, and Pacific plates, the researchers also found. Too, the 180-kilometer-wide zone of extension is most likely related to the uplift that has occurred in the Transantarctic Mountains to the west, and explains other geological features that have hitherto been puzzling.

And finally, the new results could shed new light on global issues such as the motion between hotspots in the Pacific and Indo-Atlantic oceans.

In addition to Cande and Stock, the other authors are Dietmar Müller of the University of Sidney and Takemi Ishihara of the Geological Survey of Japan.

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Robert Tindol
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Physicists create atom-cavity microscope, track single atoms bound in orbit with single photons

PASADENA—In a promising development with applications to science at the single-atom level, physicists have constructed an "atom-cavity microscope" that tracks the motion of individual atoms.

California Institute of Technology physics professor H. Jeff Kimble, his Caltech colleagues, and collaborators from New Zealand report in the February 25 issue of Science that they have succeeded in monitoring the motion of individual cesium atoms bound in orbit by single photons inside a high-quality optical resonator. The atom is trapped in orbit by a weak light field, and the same light field can be used to observe the atom's motion within the cavity.

This advance is an important development toward the eventual realization of quantum technologies, which would enable quantum computation and communication.

The stage for this microscopic dance is the optical cavity, a pair of highly reflective mirrors that face each other only 10 microns (0.0004 inches) apart. The mirrors are so reflective that a photon, the fundamental quantum of light, enters the cavity and bounces back and forth between the mirrors hundreds of thousands of times before it escapes again through one of the mirrors.

In this way a single photon confined in the cavity builds up an electric field strong enough to influence the motion of an atom and even to bind the atom in orbit within the cavity.

Collaborating theorists A. Scott Parkins and Andrew Doherty in New Zealand first recognized the potential of this trapping technique, in which the atom and the cavity share a quantum of excitation.

"I like to think of it as an atom-cavity molecule," says Christina Hood, a Caltech graduate student and primary author of the paper. "In a molecule, two atoms give up their own electron orbits, their separate identities, to share electrons and form something qualitatively different. In the same sense, in our experiment the atom and the cavity field are bound together strongly by sharing a series of single photons."

How do the scientists actually "see" what is going on inside the tiny optical system? The Caltech group and others had already used similar cavities to sense single atoms whizzing through the cavity. To do this, they illuminate one mirror of the cavity and measure the light escaping from the opposite mirror. "The cavity is a resonator for light, like a half-filled soda bottle is for sound," says Theresa Lynn, a Caltech graduate student and coauthor of the paper. "What we do is similar to holding a tuning fork up to the bottle and listening to hear it resonate. You'll only hear a ring if the right amount of water is in the bottle."

In this case, amazingly, it's a single atom that plays the role of the water in the bottle, dramatically altering the resonance properties of the cavity by its presence or absence. By measuring the amount of light emerging from the cavity, the researchers can tell whether an atom is in the cavity or not.

The major step of the current work is that now they can determine precisely where the atom is located within the light field, creating "movies" of atomic motion in the space between the cavity mirrors. Examples of these movies can be viewed at the group's web site:

http://www.its.caltech.edu/~qoptics/atomorbits/

The movies show atoms trapped in the cavity as they orbit in a plane parallel to the cavity mirrors. The atoms have orbital periods of about 150 microseconds and are typically confined to within about 20 microns of the cavity's center axis.

The Kimble team was able to measure the atomic position to within about 2 microns in measurement times of about 10 microseconds. Continuous position measurements at this level of accuracy and speed allowed them to capture the orbital motions of the atoms.

"The interaction of the atom with the cavity field gives us advantages in two distinct ways," says Kimble. "On the one hand, it provides forces sufficient to trap the atom within the cavity at the level of single photons. On the other hand and more importantly, the strong interaction enables us to sense atomic motion in a fashion that has not been possible before," he says.

Both aspects are important to physicists who probe the limits of our ability to observe and to control the microscopic world, in which the rules and regulations are set by quantum mechanics. According to one basic rule of quantum mechanics, the Heisenberg uncertainty principle, any measurement performed on a system inherently disturbs the future evolution of that system. The principle presents a challenge to physicists who strive to control or "servo" individual quantum systems for use in quantum computation and other quantum technologies.

In collaboration with Caltech assistant professor Hideo Mabuchi, the Caltech team is pursuing extensions of the current research to implement real-time quantum feedback to control atomic motion within the cavity. The operating principles for such "quantum servos" are a topic of contemporary theoretical investigation at Caltech being pursued by Mabuchi, Doherty, and their colleagues.

The cavity as a powerful sensing tool by itself also presents possibilities outside the quantum realm. The same techniques that produce movies of orbiting atoms could be adapted to more general settings, such as to "watch" the dynamics of molecules engaged in chemical and biochemical reactions. Mabuchi is pursuing an independent effort along these lines to monitor single molecules engaged in important biological processes such as conformationally gated electron transfer.

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Robert Tindol
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Astrobiologists should look for both water and energy sources when searching for life on other worlds, researcher says

PASADENA—When planetary scientists first saw evidence of a water ocean beneath the frozen surface of Europa, everyone immediately began pondering the likelihood that the Jovian moon could harbor advanced life forms—perhaps even fishlike creatures.

But last summer a group of planetary scientists from the California Institute of Technology and Jet Propulsion Laboratory threw water on the theory—so to speak—when they took a novel approach and concluded that advanced life forms were not likely.

"Water is a good place to look for life, but is only one ingredient for life," says Kenneth Nealson, an astrobiologist who holds joint appointments at Caltech and JPL, and who was a coauthor of the 1999 paper on Europa.

"You also need energy and, probably, organic carbon."

Nealson and his colleagues Eric Gaidos and Joseph Kirschvink (both of Caltech) wrote in the controversial 1999 Science paper that life on Earth is not necessarily the best analogy for life on another world. In other words, astrobiologists should be prepared to use chemistry and physics to analyze the possibilities for extraterrestrial life, rather than merely assuming life will exist wherever there is water.

Specifically, the authors showed that nearly all forms of energy used by life on Earth would be unavailable to the organisms that might live beneath Europa's surface ice layer. This did not preclude primitive unicellular organisms, but boded poorly for anyone hoping to someday see Europan creatures with gills and backbones.

"There is a trap in the thinking, because on Earth, virtually everywhere you find water you also find life," Nealson says. "And conversely, on Earth, about the only thing you can associate with lifelessness is the lack of water.

"But on another planet, just because you find water doesn't mean you're necessarily going to find life there."

Nealson says that a very likely place to look for life forms is any place where there is an energy gradient of some sort. Some potential energy gradients that might be available on Europa might arise from the gravitational and magnetic fields of Jupiter, which would almost certainly grind things around inside the moon and result in a heat source.

But when Nealson and his colleagues last year analyzed the closed system beneath Europa, they concluded that this source of energy alone was probably insufficient for multicellular life to survive. Also, they concluded that the redox energy (or available chemical energy) of the moon would also be inadequate for complex life of the kind we are familiar with on Earth.

"Still, I think Europa is a great place to look for very simple organisms," Nealson says today.

Another salubrious way to look for life is to look carefully at any place there is a water cycle, however small. If any of the other Jovian moons, such as Ganymede or Callisto, have a hydrological cycle in which moisture precipitates and runs underground, is heated by an internal source, and ultimately is returned to the surface, then the planet or moon would have the potential for energy gradients, energy flow, and geochemical cycling. All of these may be key to the existence of global life.

And the water cycle could be entirely subterranean and could even be a very limited, closed loop, Nealson says. For example, Mars may still have frozen subterranean waters that are occasionally melted by the planet's internal heat, but never result in water vapor actually surfacing. In such a case, there could be bacterial life that has lived in a closed loop beneath the Martian surface for billions of years.

"There's certainly no present-day atmospheric water cycle on Mars—no rain, no aquifers to collect the rainfall, no recycling," he says. "So if there's life on Mars, it has a hard time existing, and we'd have a hard time finding it without drilling."

While a drilling excavation to Mars is still a few decades in the future, Nealson hopes that one of the orbiters to Mars will soon include a deep-sounding radar instrument. Such an instrument can detect either liquid or frozen water beneath the surface.

The Mars orbiter scheduled for launch in 2003 by the European Space Agency (in conjunction with scientists from JPL) is scheduled to have deep-sounding radar for the detection of subsurface liquid water. A similar device will eventually be sent to Europa.

Perhaps later, the search could be extended to other Jovian moons, as well as the moons of Saturn and even Uranus.

"The moons of Jupiter have changed the way I feel about life in the solar system," Nealson says. "Each of the four large moons has different properties, different energy flows, different likelihoods of water.

"It's important to keep an open mind," he says.

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Robert Tindol
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Snowball Earth episode 2.4 billion years ago was hard on life, but good for modern industrial economy, research shows

PASADENA-For the primitive organisms unlucky enough to be around 2.4 billion years ago, the first global freeze was a real wipeout, likely the worst in the history of life on Earth. Few of the organisms escaped extinction, and those that did were forced into an evolutionary bottleneck that altered the diversity of life for eons.

But 2.4 billion years later, an unlikely winner has emerged from that first planetary deep-freeze, and it's none other than us modern industrial humans. New research from the California Institute of Technology reveals that the world's largest deposit of manganese (a component of steel) was formed by the cascade of chemical reactions caused when the planet got so cold that even the equators were icy-a condition now known as "Snowball Earth."

In a special issue of the Proceedings of the National Academy of Sciences on global climatic change published February 14, Caltech geobiology professor Joe Kirschvink and his team show that the huge Kalahari Manganese Field in southern Africa was a consequence of a long Snowball Earth episode. Kirschvink, who originated the Snowball Earth concept more than a decade ago, says the new study explains how the drastic climatic changes in a Snowball Earth episode can alter the course of biological evolution, and can also account for a huge economic resource.

According to Kirschvink and his team, the planet froze over for tens of millions of years, but eventually thawed when a greenhouse-induced effect kicked in. This warming episode led to the deposit of iron formations and carbonates, providing nutrients to the blue-green algae that were waiting in the wings for a good feeding.

The algae bloom during the melting period resulted in an oxygen spike, which in turn led to a "rusting" of the iron and manganese. This caused the manganese to be laid down in a huge 45-meter-thick deposit in the Kalahari to await future human mining and metallurgy. Today, about 80 percent of the entire world's known manganese reserves are found in that one field, and it is a major economic resource for the Republic of South Africa.

The Snowball Earth's cascade of climatic chemical reactions also probably forced the living organisms of the time to mutate in such a way that they were protected from the excess oxygen. Because free radicals can cause DNA damage, the organisms adapted an enzyme known as the superoxide dismutase to compensate.

Kirschvink points out that the enzyme and its evolutionary history are well known to biologists, but that a global climate change apparently has never been suggested as a cause of the enzyme's diversification.

"To our knowledge, this is the first biochemical evidence for this adaptation," says Kirschvink, adding that the data shows that the adaptation can be traced back to the Snowball Earth episode 2.4 billion years ago.

Kirschvink, his former doctoral student Dave Evans (now at the University of Western Australia in Perth), and Nicolas J. Beukes of Rand Afrikaans University proposed the Snowball Earth episode in a 1997 paper in Nature. Their evidence for the freeze of 2.4 billion years ago was based on their finding evidence of glacial deposits in a place in southern Africa that in ancient times was within 11 degrees of the equator, according to magnetic samples also gathered there.

The other authors of the PNAS paper are Eric Gaidos of the Jet Propulsion Laboratory, who also holds an appointment in geobiology at Caltech; L. Elizabeth Bertani and Rachel E. Steinberger, both of the Division of Biology at Caltech; and Nicholas J. Beukes and Jans Gutzmer, both of Rand Afrikaans University in Johannesburg.

The work was supported by the NASA National Astrobiology Institute.

A detailed article on the Snowball Earth phenomenon was published in the January 2000 issue of Scientific American.

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Robert Tindol
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Thunderstorms found to be an energy source for Jupiter's Great Red Spot

PASADENA-Using data from the Galileo spacecraft currently in orbit around Jupiter, scientists have discovered that thunderstorms beneath the upper cloud cover are supplying energy to the planet's colorful large-scale weather patterns-including the 300-year-old Great Red Spot.

In two articles in the February 10 issue of the British journal Nature and an article in the current issue of the journal Icarus, Caltech planetary science professor Andrew Ingersoll and his colleagues from Cornell, NASA, and UCLA write that lightning storms on the giant planet are clearly associated with the eddies that supply energy to the large-scale weather patterns.

Their conclusion is possible because Galileo can provide daytime photos of the cloud structure when lightning is not visible, and nighttime photos of the same area a couple of hours later clearly showing the lightning.

"You don't usually see the thunderstorms or the lightning strikes because the ammonia clouds in the upper atmosphere obscure them," says Ingersoll.

"But when Galileo passes over the night side, you can see bright flashes that let you infer the depth and the intensity of the lightning bolts."

Especially fortuitous are the Jovian nights when there is a bit of moonshine from one of the large moons such as Io, says Ingersoll. When there is no moonshine, the Galileo images show small blobs of glow from the lightning flashes, but nothing else. But when the upper cloud covers are illuminated at night by moonshine, the pictures show both the glow from the lightning some 100 kilometers below as well as eddies being roiled by the turbulence of the thunderclouds.

The association of the eddies with lightning is especially noteworthy in the new papers, Ingersoll says. Planetary scientists have known for some years that Jupiter had lightning; and in fact they have known since the Voyager flyby that the zonal jets and long-lived storms are kept alive by soaking up the energy of smaller eddies. But they did not know until now that the eddies themselves were fed by thunderstorms below.

"The lightning indicates that there's water down there, because nothing else can condense at a depth of 80 or 100 kilometers," he says. "So we can use lightning as a beacon that points to the place where there are rapidly falling raindrops and rapidly rising air columns-a source of energy for the eddies.

"The eddies, in turn, get pulled apart by shear flow and give up their energy to these large-scale features. So ultimately, the Great Red Spot gets its energy and stays alive by eating these eddies."

Adding credence to the interpretation is the fact that the anticyclonic rotation (clockwise in the northern hemisphere and counterclockwise in the southern) of the eddies is consistent with the outflow from a convective thunderstorm. Their poleward drift is consistent with anticyclones being sucked into Jupiter's powerful westward jets.

Ingersoll is lead author of the Nature paper that interprets the new Galileo data. The other authors are Peter Gierasch and Don Banfield of Cornell University; and Ashwin Vasavada of UCLA. (Banfield and Vasavada are Ingersoll's former doctoral students at Caltech).

Gierasch is lead author of the other Nature paper, which announces the discovery of moist convection on Jupiter. The other authors are Ingersoll; Banfield; Vasavada; Shawn Ewald of Caltech; Paul Helfenstein and Amy Simon-Miller, both of Cornell; and Herb Breneman and David Senske, both of NASA's Jet Propulsion Laboratory (JPL).

The authors of the Icarus paper are Ingersoll; Vasavada; Senske; Breneman; William Borucki of NASA Ames Research Center; Blane Little and Clifford Anger, both of ITRES Research in Calgary, Alberta; and the Galileo SSI Team.

The Galileo spacecraft has been orbiting Jupiter and its moons for the past four years, and the mission has begun an additional one-year extension.

JPL, a division of Caltech, manages the Galileo mission for NASA's Office of Space Science, Washington, D.C.

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Robert Tindol
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Caltech Professor Yaser Abu-Mostafa Awarded Kuwait State Award

PASADENA-The California Institute of Technology's Yaser Abu-Mostafa, professor of electrical engineering and computer science, received the Kuwait State Award in Applied Science on November 29.

The $50,000 award includes a gold medal, and recognizes original and fundamental research in a designated area of applied science. This year's area was information science and technology. Abu-Mostafa's work on neural networks, learning from hints, and computational finance was cited as the pioneering contribution that merited the award.

Abu-Mostafa is the youngest person to receive this award since its establishment in 1979. The awards ceremony was televised live in a number of countries. A reception by the Emir of Kuwait at the Royal Palace followed.

Abu-Mostafa received a BSc from Cairo University in 1979, an MSEE from the Georgia Institute of Technology in 1981, and a PhD from Caltech in 1983. At Caltech he won the Clauser Prize for the most original doctoral thesis. He has been teaching at Caltech since 1983, and was recognized in 1996 with the Richard P. Feynman Award for Excellence in Teaching.

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Sue Pitts McHugh
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New physics research shows how twisted plasmas can suddenly generate unstable magnetic waves

PASADENA-Plasma physicists have long wondered why the geometric shape, or topology, of magnetic fields immersed in plasma sometimes changes very suddenly, when according to the laws of magnetohydrodynamics the magnetic topology should change only very slowly or not at all.

For example, according to magnetohydrodynamics, the arched twisted shapes of the beautiful prominences protruding from the sun's surface should stay unchanged for years. Yet it is observed that after a quiescent period of days or weeks, a solar prominence can erupt in a matter of minutes, change its shape drastically, and produce a burst of high-energy particles.

The answer is likely in the details, and new research from California Institute of Technology applied physics professor Paul Bellan provides a possible explanation for why seemingly steady magnetized plasma configurations can suddenly become unstable and change their overall shape. The research appears in the December 6 issue of Physical Review Letters.

"The basic prediction of magnetohydrodynamics has been that magnetic fields embedded in plasma tend to be frozen or glued to the plasma because plasma is a very good electrical conductor," says Bellan. "Thus, if the magnetic field moves, the plasma moves in such a way that magnetic field lines cannot slide across the plasma.

"This means that the topology is not supposed to change unless the plasma is a less-than-perfect conductor. Yet, in reality, many situations are observed where the magnetic field suddenly becomes unglued, even though the plasma is essentially a perfect conductor."

For plasmas immersed in strong magnetic fields, electric currents tend to flow along the magnetic field lines, which act like wires guiding the current. The field-aligned current creates its own magnetic field, and, when added to the original magnetic field, results in a twisted or helical magnetic field.

If two such helical magnetic fields happen to be beside each other, like two barber poles lying side by side, there is a sudden change in magnetic field angle at the interface, just like the sudden change in stripe direction in going from one barber pole to its neighbor.

"People have worried for many years about what happens in such a situation," Bellan says. "The answer seems to be that the sharp change in magnetic field angle as one moves from one twisted configuration to its similarly twisted neighbor forces a large sheetlike electric current in the interface between the two configurations.

"This current sheet consists of electrons moving along the magnetic field. The more abrupt the jump in magnetic field angle, the thinner the current sheet and the faster the electrons move."

If the jump in magnetic field angle is big enough, the electrons stream at a speed equal to the propagation velocity of a well-known magnetic plasma wave called the Alfven wave. When the electrons move in sync with the wave, they resonantly interact with the wave and cause it to grow unstably, carrying energy away from the current sheet and so altering the current sheet.

"This is interesting because it can rapidly change the magnetic field topology," he says. "So if you push together these two innocent-looking twisted structures, you create an unstable gain mechanism that suddenly generates magnetic waves and so causes a quick, violent change of the magnetic field."

In certain situations these waves could also accelerate ions to high energy, a behavior often observed to be associated with sudden changes in magnetic field geometry, but not well understood.

Bellan's model could provide improved understanding of such diverse phenomena as solar prominence eruptions; the aurora borealis and aurora australis; and plasma behavior in spheromaks, laboratory devices in which magnetic vortices suddenly break off like smoke rings from specially designed magnetized plasma sources.

In short, the new research is a microscopic theory that explains certain puzzling plasma phenomena observed at macroscopic levels.

"My theory is a stab at explaining this," Bellan says. "It's not the end answer, but certainly a new twist."

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Robert Tindol
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Caltech scientists develop new cell sorter

PASADENA-Researchers at the California Institute of Technology have developed a device for sorting individual living cells. This device will provide huge cost benefits for scientists and technologists in clinical medicine as well as in biological and materials research.

According to Stephen Quake, an applied physicist at Caltech and a member of the team that developed the cell sorter, the new device will be significantly cheaper than those now on the market. Cell sorters currently available cost hundreds of thousands of dollars.

"Hopefully, this will make it easier for everyone to have a cell sorter on their bench," says Quake.

Researchers use cell sorters to collect for further study individual cells and single-celled organisms with certain characteristics. The process of looking at and identifying cells one at a time in a rapid fashion is known as "flow cytometry."

In the November issue of the journal Nature Biotechnology, the Caltech team describes their success in separating individual E. coli bacteria with the new device. Not only were the researchers able to sort out the bacteria that were producing a green fluorescent protein, but they were also able to recover sorted bacteria that were still alive and viable."Bacteria are hard to do with conventional cell sorters because they're so small," Quake says. "Up to the present, we've done E. coli, but we think you could do pretty much anything that can be fluorescently labeled."Mycometrix, a Silicon Valley-based company, has licensed the cell sorter and anticipates that it will cost about $25,000, according to company CEO Gajus Worthington.

Optical image showing bead sorting in action. A red bead is being sorted to the collection channel.

"We believe the cell sorter technology can also rapidly identify pathogens in food, water, and human tissue samples," Worthington says.The cell sorter consists of two components-a rubber chip and a reader. The clear chip, which is about an inch square, has three wells linked by a T-shaped channel network. A quantity of biological material is placed in the bottom well, and the individual cells of the material move up the channel by capillary action.

When the cell reaches the junction of the T, the reader shines a laser that causes thecell to fluoresce, thereby providing information on its individual characteristics. The fluorescence is collected by a microscope objective and converted to an electrical signal by a sensitive detector. A computer can then choose whether to direct the cell by means of an electrical current into a waste well to the left or a collection well to the right. The cells that accumulate in the collection well are removed with a pipette for further study.The advantages of the Caltech team's sorter are that the chip is cheap and disposable, and that the system does not produce an aerosol of cells, as do other sorters. The fact that the system is self-contained makes the Caltech sorter particularly suitable for hazardous biological materials.

Optical micrographs of the microFACS device. The device shown has channels that are 100 micrometers wide at the wells, narrowing to 3 micrometers at the sorting junction. The channel depth is 4 micrometers, and the wells are 2 mm in diameter.

In addition to Quake, the others involved in the work are Anne Y. Fu, a graduate student in chemistry who is lead author of the Nature Biotechnology paper; Frances H. Arnold, a professor of chemical engineering and biochemistry; Axel Scherer, a professor of electrical engineering, applied physics, and physics; and Charles Spence, a graduate student in applied physics.

The work was supported in part by the U.S. Army Research Office and the National Science Foundation. In addition to the sorter, Mycometrix has also licensed Quake's DNA sizing technology, a replacement for gel electrophoresis.

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Mutations in the mitochondrial DNA of cells dramatically increase with aging, Caltech study shows

PASADENA-Certain effects of aging could be caused by mutations in the DNA molecules of the energy-producing engines of cells known as mitochondria, according to new research from the California Institute of Technology and the University of Milan.

The study, published in the October 22 issue of the journal "Science", describes the results of skin-cell biopsies of about 30 individuals in a variety of age groups. The study concludes that damage to mitochondrial DNA dramatically increases around the age of 65.

"It's not a magic number, but we see a clear trend," says Giuseppe Attardi, who is Grace C. Steele Professor of Molecular Biology at Caltech and leader of the team authoring the paper.

Attardi and his colleagues focused their efforts on the small structures in cells known as mitochondria. Every cell can have tens to hundreds of these structures, which play an important metabolic role in the energy production that allows the cell to do its work.

Each of the mitochondria has about 10 to 20 molecules of DNA, which means that a single cell can have hundreds or thousands of mitochondrial DNA molecules.

But mitochondrial DNA is known to be susceptible to mutations over the course of a lifetime. These mutations can be due to oxidative damage, some enzyme malfunction, or even the cell's own efforts to repair itself. But prior to the new study, molecular biologists had difficulty in detecting aging-related mutations.

Over a period of about five years, Attardi and his colleagues developed a technique for detecting aging-related mutations in the main control region of mitochondrial DNA. This provided a very reliable method for determining the percentage of mitochondrial DNA molecules in a cell that had actually undergone mutations.

With this technique, they then studied tissue samples provided by the National Institutes of Health (NIH) and the University of Milan from skin biopsies. These biopsies came from individuals ranging from a 20-week-old fetus to a 101-year-old subject, which allowed the researchers to determine the prevalence of mutations in different age groups.

The results showed virtually no aging-related mutations for any of the subjects under the age of 65. But a dozen or so individuals above the age of 65 showed a dramatic increase in mutations. And not only did the rate of mutations sharply increase with age, but individuals also showed a sharp increase in mutations if they passed the age of 65 between biopsies.

Overall, the researchers found that up to 50 percent of the mitochondrial DNA molecules had been mutated in subjects 65 or over.

Attardi says future study will be needed to ascertain the precise effects of the mutations and the relationship to the known characteristics of aging. In addition, the researchers would like to know how the original mutation "amplifies," or is established in thousands of other molecules.

Also, the precise mechanism of the mutations is not known at this time. And finally, the study was done only on skin cells, although Attardi says the effect may possibly be seen in other cells of the human body.

In addition to Attardi, the other authors are Yuichi Michikawa, a senior research fellow in biology at Caltech; and Franca Mazzucchelli, Nereo Bresolin, and Guglielmo Scarlato, all of the University of Milan.

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Robert Tindol
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Largest Explosions in the Universe May Come from the Death of Massive Stars

PASADENA-Cosmic gamma-ray bursts, the brightest known explosions in the universe, may come from the fiery deaths of very massive stars in supernova explosions, a team of astronomers said today.

In a paper to appear today in the international journal Nature, the international team led by the California Institute of Technology presents evidence that the gamma-ray burst of March 26, 1998 (GRB 980326) is apparently associated with a supernova explosion.

This would then indicate that some gamma-ray bursts are associated with the formation of black holes during the fiery deaths of very massive stars. If true, this would be some of the first direct evidence for what produces gamma-ray bursts.

As a consequence, the team suggests that a burst of gamma rays are seen when one of the jets from the supernova's central black hole is pointed directly toward Earth. Gamma-ray bursts are brilliant flashes of high-energy radiation that occur at seemingly random times and from random places in the sky.

While these objects have been known since 1967, it was only recently demonstrated that these bursts originate from galaxies in the very distant universe and are by far the most brilliant bursts in the universe. This breakthrough was made possible due to the launch of the Italian-Dutch satellite BeppoSAX in 1996, which for the first time pinpointed the location of the bursts with a sufficient accuracy to enable their detailed studies with ground-based telescopes such as the W. M. Keck Telescope.

Despite the strides, scientists were still left wondering what produces these spectacular explosions. Various theories of their possible origins are still vigorously debated.

There are currently two popular models, both suggesting that the bursts originate in a formation of a black hole. In one model, two massive objects such as neutron stars or black holes (both of which may be end-products of previous supernova explosions) coalesce, forming a single massive black hole.

In the second model, such a black hole is produced in a catastrophic collapse of the core of a massive star. In this model, one then expects two sources of light: the "afterglow'' emission from the gamma-ray burst itself and light from the exploding star, a supernova. The afterglow rapidly declines whereas the supernova explosion gains in brightness over a period of a few weeks, and then gradually fades away.

The new study reports on the observations of GRB 980326 carried out at the W. M. Keck Observatory's 10-m telescope located atop Mauna Kea, Hawaii. As in many other cases, a visible light afterglow was found following the burst, which then rapidly faded away. However, the Caltech-led team discovered something never previously observed-a dramatic rebrightening of optical emission at the position of the gamma-ray burst.

Normally, the optical light of a gamma-ray burst vastly outshines its host galaxy for weeks. When the light from the gamma-ray burst fades, the apparent total brightness remains constant: all that remains is the light from the host galaxy.

Shrinivas R. Kulkarni of the Caltech team explains, "A month after GRB 980326, it looked as though the host galaxy was dominating the light." However, the next time the team observed, some eight months after the burst, the "galaxy" was gone.

"Galaxies do not just disappear, so we were astonished," Kulkarni said. "Clearly, what we were seeing is a new source of light brightening one month and then fading away. This is something quite new."

This unexpected rebrightening is now believed to be due to the underlying supernova created in the explosion of the massive star. The team had also obtained spectra of the object at different times, and that provided additional clues.

"The spectrum of the source right after the burst was blue, which is common," said S. George Djorgovski of Caltech. "But after a month it was very red, which was unexpected.

"That alone suggested that we were looking at some different phenomenon happening at the same location, but with a time delay of a few weeks."

Both the rebrightening and the spectrum changes are naturally explained by the presence of a supernova. The intensity of the apparent re-burst matches the peak brightness of a supernova seen in a distant galaxy, and its red spectrum also has the right color.

This represents the most direct evidence to date in favor of the massive supernova model. In this scenario, a black hole is quickly formed in the center of a massive star whose core is unable to support itself against gravity.

When the star explodes, powerful jets from the central black hole emerge along the original axis of rotation, and gamma rays are created by the jets. If the jets are not pointed toward Earth, then we see only a supernova and the effects of the exploding star. But gamma rays as well as the light from the supernova arrive at Earth if the jets are pointing in our direction.

Joshua S. Bloom, a graduate student at Caltech and lead author of the paper said, "This appears to be the smoking gun for the origin of some gamma-ray bursts, a perfect marriage of the two brightest events in the universe. It is wonderful to be a part of such a discovery."

Gamma-ray bursts, since their discovery some 30 years ago, have over 150 theoretical models about their possible origins, but only a handful can come close to describing the true trigger of the bursts.

"It is possible that there are other causes for gamma-ray bursts such as the coalescence of neutron stars," Bloom said. "Undoubtedly, astronomers will focus on unearthing new classes in the years to come."

Early reports of the results created some excitement in the astronomical community. Two other groups, from universities of Amsterdam and Chicago, in view of the work presented by the Caltech team, have reanalyzed the data on some other gamma-ray bursts. They appear to find good evidence for an underlying supernova in another well-studied gamma-ray burst.

"It is encouraging to have had such a resounding reception to an unexpected result," said Kulkarni. "Even some of the initial skeptics seem to be converted by these results."

Other members of the Caltech team are graduate student A. C. Eichelberger; postdoctoral scholars P. Côté, J. P. Blakeslee, and S. C. Odewahn; and Assistant Professor F. A. Harrison.

In addition to the members of the Caltech team, the other coauthors include M. Feroci of the BeppoSAX team; D. A. Frail of the National Radio Observatory; A. V. Filippenko, D. C. Leonard, A. G. Reiss, H. Spinrad, D. Stern, A. Bunker, B. Grossan, S. Perlmutter, and R. A. Knop of the University of California at Berkeley; A. Dey of the National Optical Astronomy Observatory; and I. M. Hook of the European Southern Observatory.

Writer: 
Robert Tindol
Writer: 

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