Are Impact Craters and Extinction Episodes Periodic? Implications for Planetary Science and Astrobiology.

A review of the results of published spectral analyses of the ages of terrestrial impact craters (58 analyses) and biotic extinction events (35 analyses) reveals that about 60% of the crater trials support a statistically significant cycle averaging ∼29.7 million years (My), and about 67% of the trials of extinction episodes found a significant cycle averaging ∼26.5 My. Cross-wavelet transform analysis of the records of craters and extinctions over the past 260 My shows a mutual ∼26 My cycle and a common phase, suggesting a connection. About 50% of the best-dated impact craters seem to occur in approximately nine pairs or clusters in the past 260 My, apparently carrying the signal of an ∼26- to 30-My cycle. It has been suggested that periodic modulation of impacts and extinctions might be related to periodic comet storms that follow the solar system's oscillations in and out of the galactic mid-plane. Problems arise, however, with regard to the compatibility of such periodic pulses of comet flux with the makeup of the steady-state Near Earth Object (NEO) population, the estimated long-term NEO cratering rates on the terrestrial planets, and the predicted small contribution of Oort Cloud-derived comets to the terrestrial cratering record. Asteroid storms may be possible, but at present there are no accepted mechanisms for creating an ∼30-My period in asteroid breakup events and impacts. Astrobiological implications arise if extra-solar habitable planets suffer similar cyclical or episodic catastrophic bombardment episodes affecting long-term biotic evolution on those planets. Other planetary systems might commonly have comet reservoirs, but they are less likely to contain an asteroid belt in the proper orbital position. Further, frequent impacts of ∼1-km diameter comets and asteroids could affect the establishment and longevity of technological civilizations, including our own.

Is the solar system's galactic motion imprinted in the Phanerozoic climate?

A new δ(18)O Phanerozoic database, based on 24,000 low-Mg calcitic fossil shells, yields a prominent 32 Ma oscillation with a secondary 175 Ma frequency modulation. The periodicities and phases of these oscillations are consistent with parameters postulated for the vertical motion of the solar system across the galactic plane, modulated by the radial epicyclic motion. We propose therefore that the galactic motion left an imprint on the terrestrial climate record. Based on its vertical motion, the effective average galactic density encountered by the solar system is 0.172±0.006stat±0.006sysM∘pc⁻³. This suggests the presence of a disk dark matter component.