The problem of the origin of Kirkwood gaps in the asteroid belt and conditions in the early Solar system

Abstract

The orbits of asteroids from the MPC catalogue of January 21, 2022 with absolute magnitudes H < 16 m, in the 3:1, 5:2 and 2:1 mean motion resonances (MMRs) with Jupiter were selected. The number of the orbits in the 2:1 MMR is dozens of times greater than in two other resonances. The are fragments of parent bodies of neighbour asteroid families, in particular the Themis family, in the 2:1 MMR. The sizes of almost all bodies in resonant orbits do not exceed 10 km. It is shown that in the past, there were bodies of much larger sizes in these tree gaps. In particular, more than two billion years ago there were several tens of bodies with sizes of more than 50 km in the 2:1 gap, including those with sizes of more than 100 km. Numerical calculations were performed to evaluate the evolution of the selected resonant orbits over hundreds of thousands years. Perturbations from all eight major planets and the relativistic effects of orbital perihelion displacement were taken into account. For all orbits in the 3:1 and 5:2 MMRs an increase in the orbit eccentricities, which are sufficient for the bodies to approach Mars, was confirmed. It can explain the exit of asteroids from these two gaps. In the 2:1 MMR, a sufficient increase in the orbit eccentricities was not detected. Such increase in orbit eccentricities in this resonance may occur due to the action of non-gravitational effects (NGEs). The action of the Yarkovsky effect may explain exit of an asteroid with a size of 5 km from the 2:1 MMR over a period about 1 billion of years or more. It takes tens of billions years for large bodies that were once there, to exit from the 2:1 gap, under the action of the Yarkovsky effect of today power. To form the gap in the 2:1 resonance at the very beginning, the physical conditions for bodies in the asteroid belt had to be significantly different from the today ones. Most likely, the NGEs were influencing a much stronger on bodies in the asteroid belt in the early Solar system as compared with the today NGEs. In particular, the radiation of the Sun in the early Solar system could be much more intense, in comparison with the today one.