The Evolution Of The Solar Nebula

The origin of the solar system is one of the most important questions in science. The standard theory of the origin of the solar system in the modern era is the nebula hypothesis. Therefore, to understand the history of the solar system, an extensive work needs to be done on the numerical simulation of the evolution of the solar nebula.The current knowledge suggests that the viscosity in the solar nebula is not uniform and calculations of the nebula evolution with constantαmay miss useful information about the history of the solar system. It is pointed out that the angular momentum transport (AMT) mechanism can be the magnetohydrodynamic (MHD) turbulence driven by the magnetorotational instability (MRI). Considering the effect of ohmic diffusion on the MRI in the solar nebula, it is pointed out that in the outer region of the nebula, the surface density is low enough for cosmic rays to penetrate and the ionization is high enough that the MRI can survive. The MRI can also survive in the inner region due to thermal ionization. But the MRI cannot survive in the intermediate region between these two regions, and the viscosity drops significantly. The AMT is nonuniform and cannot be described with a uniformα.It is suggested that nebula models of uniform viscosity do not match the planet mass distribution. Based on a constantαviscosity, Hartmann et al. obtained a similarity solution and showed that surface density decreases outward with radius. Jin calculated heavy element masses of the planets with the solution. Comparing these masses with the observed masses of the terrestrial planets and heavy element masses of Jovian planets given by current planet model, Jin showed that constantαviscosity cannot account for the planet mass distribution.In the standard star formation theory, stars are thought to form from molecular cloud cores by gravitational collapse. Systems of star+disk are formed from the collapse. The systems go through infall, viscous, and clearing stages. In most of previous work of the solar nebula evolution, the emphasis has been on the viscous stage. In this work, we investigate the nebula evolution during both infall and viscous stages by including the mass influx onto the nebula from the gravitational collapse.In this work, we investigate the formation, structure, and evolution of the solar nebula by including nonuniform viscosity and the mass influx from the gravitational collapse of the molecular cloud core. The calculations are done by using currently accepted viscosity, which is nonuniform, and probable mass influx from star formation theory. In the calculation of the viscosity, we include the effect of magnetorotational instability. The radial distributions of the surface density and other physical quantities of the nebula are significantly different from nebula models with constantαviscosity and the models which do not include the mass influx.The main results of the numerical calculations are summarized as the following.(1) In previous models, an artificial initial condition is assumed and then an isolated nebula adjusts to a self-similar solution. The mass of the nebula decreases due to the accretion onto the protostar. In order to understand the nebula evolution, we have to take into account the initial conditions. Unlike previous calculations, in the present calculation, the nebula starts to form from the inner boundary and then expands outward due to viscous expansion. (2) In models with uniformα, the surface density is a monotonic function of the heliocentric distance. In our model, there are minimums (the TM and SM) near 1.5 AU due to viscosity difference between the inner region and the intermediate region. (3) Apart from the TM movement and the gradual expansion, the general shape ofΣis sustained before the infall stops because the mass supply from the collapse offsets mass loss accreted onto the protosun and provides mass needed for the nebula expansion. This feature is strikingly different from the evolution of an isolated nebula where the nebula mass andΣdecrease with time. (4) Whenωis high, the nebula becomes gravitationally unstable in some durations. When the nebula is unstable,Σdistribution takes the one with uniformαcaused by the instability. When it is stable,Σdistribution takes the one with nonuniformα. (5) If at one time, the surface density has a distribution like current planet mass, our numerical calculations demonstrate that the inflow time of the gas is shorter than the slow capture time for Uranus and Neptune and the inflow time is longer than the slow capture time for Jupiter and Saturn. The gas already flows inward to the Jupiter–Saturn region before Uranus and Neptune finish slow capture. Hence, Uranus and Neptune did not reach the rapid accretion, but Jupiter and Saturn went through the rapid accretion. Therefore, our calculations confirm Jin's early suggestion that this explains the differences in mass and gas content among Jovian planets. (6) We find that the surface density in the nebula is not monotonic and that there is a minimum in the Mars region. This naturally fits the planet mass distribution, especially the Mars drop. We suggest that the existence of this minimum leads to the low mass of Mars in the following three ways. (a) The low surface density of the Mars region gives a low mass supply, and (b) it gives a low rate of planetesimal formation from dust. (c) The low surface density in the Mars region preferentially makes Mars a leftover protoplanet without gaining much mass during chaotic growth, the last stage of planet formation.