The third of NASA's "Great Observatories," a powerful X-ray telescope, took a big step closer to completion recently with the assembly of its high-resolution mirrors.

The last of four pairs of unique mirrors that form the heart of the Advanced X-ray Astrophysics Facility (AXAF) were aligned and cemented into place at Eastman Kodak's Federal Systems Division in Rochester, New York, in September.

"The extreme sensitivity of the mirrors made the installation a very delicate and painstaking process," said John Humphreys, Project Development Manager at Marshall Space Flight Center. "Successful completion of the process represents a real achievement in the development of the telescope."

The high-resolution mirror assembly was outfitted with additional hardware and a covering in preparation for testing and calibration in a special facility at Marshall that began in mid November.

Unlike the concave, nearly flat mirrors used in optical telescopes, the AXAF mirrors are shallow, almost cylindrical cones. The four pairs of mirrors are nested inside each other. X-rays enter the telescope, graze off the mirrors -- much like a stone skipping across the surface of a pond -- and are focused onto a plane 30 feet behind the front of the mirrors.

The largest of the mirrors is 47.2 inches, the largest ever made. The size and accuracy of the mirrors will make AXAF 100 times more sensitive than previous X-ray telescopes, producing images 10 times sharper.

The observatory is scheduled for a shuttle launch in 1998. In orbit, it will observe energetic X-ray sources such as neutron stars, black holes, debris from exploding stars, quasars, cores of galaxies, and galaxy clusters. Joining the Hubble Telescope and the Compton Gamma Ray Observatory, it will extend the "Great Observatories" exploration into the X-ray spectrum.


Scientists reporting in the November 7 issue of Nature believe they have evidence for life in ancient sediments on Akilia Island in southern West Greenland that are at least 3.85 billion years old.

The scientists, from UC San Diego's Scripps Institution of Oceanography, UCLA's Department of Earth and Space Sciences, the Australian National University, and England's Oxford Brookes University, present evidence that suggests life may have emerged 300 million to 400 million years earlier than previously thought.

"We look in rocks like this for chemical suggestions and isotopic evidence, and we found both," said T. Mark Harrison, professor of geochemistry at UCLA and director of the W. M. Keck Foundation Center for Isotope Geochemistry. "It would be wonderful to see a head and toes, and, while we don't have those, we have found very strong isotopic evidence for ancient life."

". . . in the cases of Earth's most ancient rocks and minerals, we are actually better off relying on this type of isotopic evidence -- chemofossils -- rather than on the shape of life-like objects with which nature has often been deceiving the unwary," said Gustaf Arrhenius, professor of oceanography at UC San Diego and principal investigator for the research project.

The researchers analyzed carbon inclusions in apatite grains to measure the ratio of 12C to 13C using a high-resolution ion-microprobe mass spectrometer. They found that the carbon aggregates in the rock have a ratio of about 100 to 1 of the lighter isotope compared to the heavier. "The light carbon, 12C, is more than 3% more abundant than scientists would expect to find if life were not present, and 3% is, in this case, a very large amount," Arrhenius said.

The ratio is within the range of measurements made on younger rocks that are more certainly associated with biological activity, such as the 3.25 million-year-old Australian chert that contains bacteria-like organisms. Metamorphic alteration of the Akilia formation has left no trace of such organisms in the older rocks.

The inclusion of the carbon in apatite, which is often formed by biotic processes, but can also be formed inorganically, is "suggestive, and not surprising, but does not in itself establish life," according to Arrhenius.

If they are indeed the result of living organisms, these residues suggest that life arose almost simultaneously with the end of the late heavy bombardment of the inner solar system by meteoric debris. They could even represent a biota that began and was subsequently erased by catastrophic impact 3.8 billion years ago, to be followed by life's reemergence several hundred million years later.

"Life is tenacious, and it completely permeates the surface layer of the planet," said Steve Mojzsis of Scripps. "We find life beneath the deepest ocean, on the highest mountain, in the driest desert and the coldest glacier, and deep down in the crustal rocks and sediments. Not knowing what conditions are needed for the emergence of life, it is only possible to speculate about its existence elsewhere in the universe. An important contribution to the solution of this problem could come from exploration of the surface of Mars for traces there of extinct life."


The mystery of where and how high-energy bursts of gamma rays originate has been given a puzzling new twist with the detection of the first sequence of repeated bursts in one region of the sky.

Four separate gamma-ray bursts were detected in two groups of two in rapid succession on October 27 and October 29. Astronomers based at Marshall Space Flight Center measured the unique sequence using the Burst and Transient Source Experiment (BATSE) instrument onboard the Compton Gamma Ray Observatory. The repeated bursts are unlike any of the other 1700 gamma-ray bursts observed by BATSE, which have been observed in all regions of the sky.

The BATSE finding is expected to add to the already vigorous debate on the distance to the sources of gamma-ray bursts and their causes -- subjects still unresolved despite nearly 25 years of study.

BATSE usually detects only about one gamma-ray burst a day that lasts from 10 to 30 seconds, and the locations of these events on the sky appear to be randomly distributed. "That's what makes these recent events so unusual," said Charles Meegan, BATSE experiment co-investigator. "They came right after one another, about two days apart, and all from the same part of the sky. Moreover, the last one was much longer than usual, lasting 23 minutes."

The BATSE astronomers cannot yet say for sure whether these events were produced by just one object in space or several, but "it would be unlikely that this actually happened by chance" in four unrelated places, said Marshall Space Sciences Laboratory astrophysicist Valerie Connaughton.

"Some astronomers argue for an explanation that the origin of these bursts is fairly local, just outside our own galaxy," says Gerald Fishman, BATSE principal investigator, who agrees that the recent events are likely related. "But most believe that bursts come from remote parts of the universe, at cosmological distances of a billion light years or more."

Another unsolved mystery is how bursts are created. One theory suggests that bursts do not repeat from the same source because they involve a tremendous explosion that destroys the source in the process. Another possibility is that bursts occur when neutron stars merge, which would not be consistent with repeating bursts. "This discovery of multiple bursts adds fuel to the debate as to the source of the bursts," said Fishman.

The discovery was confirmed by three other gamma-ray burst detectors. Scientists from Goddard Space Flight Center, the University of California at Berkeley, and the Ioffe Institute in Russia participated in the discovery.


Refined calculations and new evidence support a revolutionary suggestion that global-scale geologic events produced the bulk of Earth's oxygen supply, a NASA scientist reported at the Geological Society of America meeting in late October.

Scientists have long believed that oxygen collected in Earth's early atmosphere as a by-product of photosynthesis, in which plants take in carbon dioxide and water to produce organic matter and oxygen. David Des Marais, of Ames Research Center, first suggested in 1992 a relationship between oxygen and plate tectonics, in which plate collisions that built enormous mountain ranges and increased erosion buried huge amounts of organic matter in ocean beds.

"Although photosynthesis did provide an oxygen source strong enough to sustain the amount of existing oxygen, the creation and assembly of large modern-sized continents was responsible for early dramatic increases in oxygen," Des Marais said.

His research correlates oxygen surges in the atmosphere 2.2 to 2.0 billion years ago with changes in the amount of carbon stored in Earth's crust at that time. During that time, several of Earth's "micro" continents crashed together, forming new, stable modern-sized continents. As the continental fragments collided, mountain ranges formed. Their steep slopes produced rapid erosion and sedimentation, key to Des Marais' theory.

Organic matter is normally consumed by bacteria and animals, a process that utilizes oxygen (respiration), producing energy and carbon dioxide and water as by-products. According to Des Marais, when huge amounts of organic matter were buried during tectonic collisions, oxygen was freed to accumulate in Earth's early atmosphere.

"The cycle of photosynthesis (which produces oxygen) and respiration (where oxygen is consumed) is an almost break-even process," Des Marais said. Only when large amounts of organic material are buried in ocean sediments during tectonic upheavals can the amount of oxygen in the atmosphere increase substantially, he added.

A recent independent study concludes that approximately three large continental masses were assembled between 2.5 and 1.9 billion years ago by the collision of smaller land masses. Two of these were assembled between 2.2 and 1.9 billion years ago.

These collisions formed Himalayan-class mountains with high rates of sedimentation in the ocean, burying organic matter.

amounts of free oxygen.

Furthermore, new calculations by Des Marais reveal that the increases in atmospheric oxygen and sulfate (oxidized sulfur) in seawater between 2.2 and 2.0 billion years ago were too large to be explained only by the slow decline in volcanic activity over Earth's history. The decline in volcanism had been previously offered as an alternative to Des Marais' continental evolution hypothesis.

Des Marais' research is supported by the space science division at the Ames Research Center and the Exobiology Program in NASA's Office of Space Science, Headquarters, Washington, DC.