Signs of Nemesis: meteors, magnetism.

Signs of Nemesis: Meteors, magnetism

A well-known hypothesis blames impactsof cometes--triggered by Nemesis, the alleged companion star to the sun-- for mass extinctions of biological species that appear to have occurred periodically in the history of the earth. In the attempt to show that such cometary impacts did happen, proponents of this hypothesis are marshaling evidence from other events that might have been the result of an enlarged presence of comets in the inner solar system at the appropriate times.

The latest piece of such evidenceconcerns the ages of the so-called H class of chondritic meteorites. It was presented in San Francisco, at the recent meeting of the American Physical Society and the American Association of Physics Teachers, by Richard A. Muller of the Lawrence Berkeley (Calif.) Laboratory (LBL).

The H class are the chondrites with ahigh iron content. The work by Saul Perlmutter of LBL and Muller indicates a periodicity in the ages of the H chondrites that coincides with the times of mass extinction.

The meteorites are presumed to bepieces broken off asteroids, by the impact of another asteroid or perhaps a comet. It turns out that large numbers of such meteorite liberations bunched up at or near the times of the mass extinctions. The cause of such wholesale meteorite formation, the researchers suggest, could be showers of comets moving through the asteroid belt--the same comets that, hitting the earth, triggered the climate changes necessary for mass extinctions.

The climate changes that would havebeen caused by the comet showers could also have brought about reversals of the earth's magnetic field (SN: 3/29/86, p.197), according to an analysis published by Muller and Donald E. Morris of LBL in the November GEOPHYSICAL RESEARCH LETTERS. There is evidence, although some call it statistically marginal, that the rate of geomagnetic reversals peaked during the mass extinctions. The comet hits would produce debris and smoke from widespread fires, darkening the atmosphere and creating the climatic condition that has become known as nuclear winter.

Besides extinction of species, thiscomet winter would cause increases in the glaciation of land areas near the poles, and consequent sudden drops in sea level. Calculation shows that such changes could be enough to alter the rotation of the earth. The change in rotation speed would cause a shear in the liquid layer that lies between the core and mantle of the earth. The shear would disrupt the rolling convection currents in that liquid that produce the earth's dipole magnetic field, destroying the dipole and leaving magnetic confusion behind. Later, when things stabilized again, the dipole would return, but it might return with reversed polarity. Every comet shower need not produce a magnetic reversal. In particularly warm periods they would probably not cool the earth enough.

Finally there is Nemesis, the ultimatecause of all this. Nemesis would be in such an orbit around the sun that it would periodically disturb the Oort could--the collection of cometary material orbiting the sun at about 100,000 times the earth's distance--and would trigger a comet shower.

Under the direction of Carl Pennypackerof LBL, an automated telescope patrolling the sky for supernovas is also looking for Nemesis. If it is among the 3,000 candidates the fully automated system will search, Muller is confident the program will find it within a few months.