Cometary impacts, molecular clouds, and the motion of the Sun perpendicular to the galactic plane

Raup and Sepkoski1have presented evidence from marine fossils for a 26-Myr periodicity in the occurrence of mass extinctions. Using the same data Rampino and Stothers2obtained a different period, 30±1 Myr, which agrees with the 33±3-Myr half-period for the vertical oscillation of the Solar System about the plane of the galaxy3. To explain this agreement they suggest2that encounters with molecular clouds perturb the Sun's family of comets, causing many to enter the inner Solar System where one or more collide with the Earth; the cloud encounter rate is modulated at twice the oscillation frequency, because the number density of clouds peaks at the galactic plane at the midpoint of the solar oscillation crossed by the Solar System twice per period. Notwithstanding an apparent objection to this that the most recent extinctions are not in phase with the solar oscillation4, their model, given its stochastic nature, can accommodate a few events with large phase discrepancies. The degree of modulation is crucial: it depends on the scale height of the population of molecular clouds relative to the amplitude of the solar motion and tends to zero if this ratio is large and encounters are entirely random. Here we present data from CO surveys of molecular clouds both within and beyond the solar circle, which permit explicit calculation of the strength of the modulation. The cloud layer near the Sun is too extended and, as a consequence, the modulation of cloud encounters is too weak for a statistically significant period to be extracted from the nine extinctions analysed by Rampino and Stothers.