One of the enduring mysteries of evolution is the demise of the dinosaurs. They disappeared in what, geologically speaking, was the wink of an eye -- leading some scientists to wonder whether they were victims of a cosmic catastrophe.
Now a research team at the University of California has evidence that such a catastrophe may indeed have occurred 65 million years ago, when half of the genera of earthly life, including dinosaurs, suddenly vanished from the fossil record. They have found an anomalous jump in the concentration of the rare element iridium in rocks of that era -- an anomaly that may be the signature of a nearby supernova star explosion.
This is the first ever suggestive evidence to support scientists' speculations that events far beyond the solar system can have a direct impact on Earth's environment.
Two decades ago, Soviet astrophysicist Iosef Shklovsky linked the dinosaurs' extinction with a supernova. Subsequently, other experts have speculated on the possible effect such an explosion would have on Earth.
A supernova explosion marks the end of many stars. It is a gigantic outburst of radiation (mostly X-rays and gamma rays) and of material debris that expands outward into space. Enveloping Earth, such debris and radiation would probably affect life indirectly by inducing climatic or other environmental changes.
In 1974, for example, M.A. Ruderman of Columbia University estimated that pulses of energetic X-rays and enhanced bombardment by coxmic-ray particles could temporarily destroy most of Earth's ozone shield. This is a layer high in the stratosphere that is rich in ozone, a form of oxygen that screens out much of the sun's harmful ultraviolet (UV) rays.
Ruderman concluded that temporary loss of the ozone could be devastating for many life forms. He estimated that supernovae close enough to seriously disrupt the ozone layer (i.e., within 50 light-years of the solar system) probably would occur every 100 million years or so. Thus Earth's life would have been subjected to intense UV on several occasions with inknown, but probably important, evolutionary effects.
Other scientists have suggested that loss of ozone could upset climate and that such climatic change also would affect earthly life.
Three British scientists point out that Earth would likely be enveloped in the expanding shell of debris from a nearby supernova. The shell from an explosion, say, 30 light-years away, would reach the solar system system in 2,000 years and might envelop Earth for several centuries.
Following up such suggestions, G.C. Reid and J.R. McAfee of the U.S. National Aeronautics and Space Administration and P.J. Crutzen of the National Center for Atmospheric Research have made a detailed study of the possible atmospheric consequences if a supernova exploded nearby.
They reported in the journal Nature last October that the passage of Earth through the cloud of debris "would be likely to cause harsh environmental conditions for the biosphere, including high levels of damaging UV radiation, cooler global temperatures and generally dry conditions. . .and reduced photosynthetic activity." They cautiously add that "whether or not these severe conditions would lead to major extinctions of animal populations is a matter of speculation."
However, it was noted in last December's issue of New Scientist, "speculation is a legitimate part of the scientific method, so long as exaggerated claims are not made on the basis of mere circumstantial evidence."
That is the perspective in which Walter Alvarez, Luis Alvarez, Frank Asaro and Helen V. Michel of the University of California at Berkeley offer their evidence of a possible supernova explosion at the time the dinosaurs disappeared.
The rocks they analyzed came from Gubbio, Italy, from a formation with one of the most complete geological records from the period when the dinosaurs faded out. They show a 25-fold increase in the concentration of iridium that lasted for 20,000 to 30,000 years. The sudden enrichment could have been due to radiation from a supernova. This would have had to explode close by, probably within a tenth of a light-year.
A star explosion that close could indeed be assumed to have had major environmental effects on Earth. Any direct evidence that it happened at the time of the dinosaurs' extinction would greatly strengthen the notion that supernovae have had a role in earthly evolution.
Thus the California scientists are being reserved about drawing conclusions. If a supernova were responsible for the iridium enhancement, then the deposit also should show the presence of a distinctive form of plutonium -- plutonium 244. If this is not found, then a supernova may not have occurred after all.
So while they are looking for the plutonium, the most that Walter Alvarez will say of the iridium-enhancing event is that "it looks more extraterrestrial than terrestrial."
Meanwhile, this idea has underscored the conviction of a number of scientists that the evolution of life in the solar system is not isolated from the larger universe.