Jupiter Growth as an Essential Factor for the Formation of the Planetary System
Abstract
The duration of Jupiter's and Saturn's formation via two-phase growth is revised. For this scenario, the total formation time has been estimated to be as long as 10^8 years or more because of the long initial phase implied by the model of orderly growth (Safronov and Ruskol, 1982). The initial phase associated with the formation of planetary cores from condensed material may take a shorter time, up to 10^7 years, if this process is treated like the accumulation of the terrestrial planets, using the model of the runaway growth of the major body (Pollack et al., 1996). The growth can also be accelerated owing to the increased surface density of the solid material in the feeding zones of Jupiter and Saturn on account of water-ice condensation in these zones with the entrainment of most of the volatiles from the zone of the terrestrial planets into the inner part of Jupiter's zone and the turbulent diffusion of a smaller portion of volatiles into Saturn's zone. An approximation formula for the growth of the planet's mass at the second phase (caused by gas accretion onto the core) is derived. According to this formula, the duration of this phase turns out to be of the order of 10^4 yr, which is close to an estimate obtained by numerical simulation (Magni and Coradini, 1998). Hence, the entire cycle of the growth of two giant planets in the protoplanetary disk almost does not exceed 10^7 yr, which is consistent with estimates for the time of existence of the circumplanetary disks of T Tauri type stars with masses close to the solar mass. The role of growing Jupiter in the dynamics and accumulation of asteroid-belt bodies is examined. The in situ formation of large asteroids (Ceres and others) requires limitations on the variance of the feeding-body velocities, as well as a sufficiently high surface density of matter at the stage of the asteroids' growth. In the period when the growth of asteroids changes for their fragmentation, the velocity variance should be in the range 0.51km/s. This makes it possible to explain the formation of the Trojan asteroids in inelastic collisions accompanied by the partial fragmentation of the main-belt asteroids that have found their way into the neighborhood of the triangular libration points L_4 and L_5 in Jupiter's orbit, as well as the formation of the outer irregular satellites of Jupiter by the capture of colliding asteroids. Similar conditions are required for the formation of old asteroid families (for example, the Koronis family) through the disruption of the parent asteroids with comparatively low fly-apart velocities of the fragments. The idea that the subsequent growth of the velocity variance in the asteroid belt is associated with the effect of perturbations caused by large bodies that grew in Jupiter's zone simultaneously with the main nucleus (Safronov, 1979) is developed. In the time of the growth of the main body (Jupiter's core), smaller bodies gain chaotic velocities in excess of 3 km/s, which permit them to pierce the asteroid belt, sweeping out from it the bodies they encounter and increasing the velocity variance of the asteroids remaining in the belt.
- Publication:
-
Solar System Research
- Pub Date:
- 1998
- Bibcode:
- 1998SoSyR..32..255R