A JUPITER ORBIT--LUNAR ORBIT RESONANCE MODEL WHICH MAY RELATE TO NEOPROTEROZOIC EVENTS ON EARTH AND MOON

The Neoproterozoic appears to be a critical time in the history of the planet. There are two major horizons of glacial deposits in the more complete sequences and each of the glacial sequences is capped by a significant thickness of laminated carbonates. These features have been interpreted by many investigators as recording alternating "icehouse" and "greenhouse" episodes in a low paleolatitude setting. Many of the glacial-carbonate sequences are several hundred meters thick and are thought to be rift basin fills. Another noteworthy feature of Neoproterozoic sequences is the association of tidally influenced sediments including significant thichnesses of tidal rhythmites.

The Neoproterozoic also appears to be a critical time in the history of the lunar orbit. Ross and Schubert (1989, JGR, 94, p.9553) did extensive calculations on a "standard" model for the evolution of a mainly circular orbit in the equatorial zone of the planet. They found that under normal conditions of rock and ocean tidal energy dissipation, the lunar orbit at 1.0 to 0.6 Ga ago was at about 51 earth radii and that it expanded to about 52 earth radii when the calculation was extended to the present time. They suggested that something happened between 1.0 and 0.6 Ga ago to cause significantly higher energy dissipation rates in the oceans.

Peale and Cassen (1978, Icarus, 36, p.245) identified an orbital resonance state between jupiter's orbit and the lunar orbit when the lunar orbit is at 53.4 earth radii. They state that if the resonance is stable, then there could be "profound" effects on both the earth and moon. We have done some 4-body (sun, earth, moon, jupiter) numerical simulations (4th-order Runge-Kutta integration procedure) on the effects of this resonance. In the short (100-year) runs that we have done on both circular and elliptical lunar orbits with semimajor axes between 50.0 and 53.4 earth radii, we have found a notable increase in orbital eccentricity.

In general, a geologically short-lived forced-eccentricity episode could explain the abundance of tidally influenced sediments of that time. Enhanced solid rock tides on earth could have an influence on the development of continental rift zones and enhanced lunar rock tides could cause lunar heating leading to minor lunar magmatism.