POSSIBLE ULTIMATE CAUSE OF THE K-T IMPACT

Orbital calculations are increasingly being used, in conjunction with sedimentary and geochemical data, to analyze environmental evolution on geological timescales. In order to better understand the history of the inner Solar System over hundreds of millions of years, we have carried out several accurate, long-term, numerical simulations of the orbits of the nine major planets using physical models with increasing complexity. In this work, equations of motions are directly integrated by a Stormer-Cowell multi-step scheme, which is optimized for reduced round-off errors. We gradually refined the physical models to include corrections required by general relativity, the finite size of the lunar orbit, and the solar gravitational quadrupole moment. In one case, the Earth-Moon system was resolved as two separate bodies and the results were compared to those based on analytically averaging the lunar orbit. A better analytical averaging model is obtained by means of these comparisons.

The computed orbits are in good agreement with those of previous studies for the past five million years but not for earlier times. Chaos in the motion of the inner planets limits the validity of the computations beyond 50 million years. Nevertheless, we detect a number of significant transitions at earlier times that involve changes in the precession of the orbits of Earth, Mercury and Mars. One in particular, about 65 million years ago, is associated with clearly discernible, macroscopic changes in the evolution of Mercury’s orbit, suggesting the possibility of a dramatic shift in the dynamics of the Solar System at that time. This dynamical transition could have been responsible for significant perturbations within the asteroid belt. We note that it corresponds, approximately, and perhaps fortuitously, to the Cretaceous-Tertiary boundary.