THE SOLAR NEBULA, SECULAR RESONANCES, GAS DRAG, AND THE ASTEROID BELT

We assume that the asteroids were initially distributed uniformly from Mars to Jupiter. Orbits between the 3:2 resonance with Jupiter (0.763 a(J), where a(J) is Jupiter's semimajor axis) and Jupiter were chaoti c because of overlapping mean motion resonances and were rapidly remov ed by close encounters with Jupiter. However, in numerical simulations extending as long as 10(8) years, we have been unsuccessful in removi ng asteroids between 0.630 and 0.763 a(J) (the 2:1 resonance and the 3 :2 resonance with Jupiter), when Jupiter and Saturn were the only pert urbers. We now suggest that a combination of the secular resonance bet ween the apsidal rotations of Jupiter and the asteroids, moved outward by the gravitational potential of the primitive solar nebula and the gas drag of that nebula, removed those asteroids. A minimum mass solar nebula will move one of the secular resonances outward from its prese nt location at 0.40 a(J) to the 3:2 resonance. The sweeping secular re sonance pumped up the eccentricities of the asteroids causing many out er belt asteroids to be ejected by close encounters with Jupiter. The high eccentricities also increased the relative motion of the gas and the asteroids, greatly enhancing the efficiency of the drag, which cau sed other asteroids to spiral into the inner Solar System. That nebula , removed on a time scale of 10(4)-10(5) years, cleared the asteroid b elt exterior to the 2:1 resonance and increased the eccentricities of the remaining asteroids to an average value of about 0.15. We found th at a much longer time scale mould have removed all the asteroids, whil e a much shorter time scale would not have increased the initial low e ccentricities. (C) 1997 Academic Press.