Reduced solar quadrupole moment compensates for lack of asteroids in long-term solar system integrations

Abstract

State-of-the-art long-term solar system integrations include several second order effects such as the Sun's quadrupole moment J2 and a contribution from asteroids (plus the Moon and general relativity). We recently showed that including 10 asteroids and a reduced J2 in our astronomical solutions provides the best match with geologic data to -58 Myr. However, the rationale for the reduced J2 remained ambiguous and may suggest that parameters for long-term integrations compatible with geologic observations are not fully compatible with our knowledge of the current solar system (specifically J2). Here we show that a reduced J2 compensates for a diminished asteroid population in long-term solar system integrations, which may appear surprising. We present an analysis and offer a mechanism for the long-term compensating effects of J2 and asteroid mass in the solar system (not planetary systems in general). Our analysis suggests that "differential effects" on specific secular frequencies involved in resonant terms (i.e., (g4-g3) and (s4-s3)), are critical in the long term, rather than short-term effects on the orbital elements of individual planetary orbits across the board. Also, our results indicate that if long-term intergrations including the full asteroid population were computationally feasible, a J2 value (within errors) compatible with our current knowledge of the solar system could be used. Attempts to improve the long-term accuracy of astronomical solutions by, e.g., tinkering with initial conditions using current/future astronomical observations are futile unless asteroid deficiencies in the solar system model are addressed.