What's the cause of this gap in this simulation of the Nice model?
Answers
The Nice model has undergone a number of modifications since its initial publication as the understanding of the formation of the Solar System has advanced and significant differences between its predictions and observations have been identified. Hydrodynamical models of the early Solar System indicate that the orbits of the giant planet converge resulting in their capture into resonances. During the slow approach of Jupiter and Saturn to the 2:1 resonance, Mars can be captured in a secular resonance, exciting its eccentricity to a level that destabilizes the inner Solar System. The eccentricities of the other terrestrial planets can also be excited beyond current levels by sweeping secular resonances after the instability. The orbital distribution of the asteroid belt is also left with an excess of high inclination objects due to secular resonances exciting inclinations and removing low inclination objects. Other differences between predictions and observations included the capture of few irregular satellites by Jupiter, the vaporization of the ice from Saturn's inner moons, a shortage of high inclination objects captured in the Kuiper belt, and the recent discovery of D-type asteroids in the inner asteroid belt.
The first modifications to the Nice model were the initial positions of the giant planets. Investigations of the behavior of planets orbiting in a gas disk using hydrodynamical models reveal that the giant planets would migrate toward the Sun. If the migration continued it would have resulted in Jupiter orbiting close to the Sun like recently discovered exoplanets known as hot Jupiters. Saturn's capture in a resonance with Jupiter prevents this, however, and the later capture of the other planets results in a quadruple resonant configuration with Jupiter and Saturn in their 3:2 resonance. A late instability beginning from this configuration is possible if the outer disk contains Pluto-massed objects. The gravitational stirring of the outer planetesimal disk by these Pluto-massed objects increases their eccentricities and also results in the inward migration of the giant planets. The quadruple resonance of the giant planets is broken when secular resonances are crossed during the inward migration. A late instability similar to the original Nice model then follows. Unlike the original Nice model the timing of this instability is not sensitive to the distance between the outer planet and the planetesimal disk. The combination of resonant planetary orbits and the late instability triggered by these long distant interactions has been referred to as the Nice 2 model.
The second modification was the requirement that one of the ice giants encounters Jupiter, causing its semi-major axis to jump. In this jumping-Jupiter scenario an ice giant encounters Saturn and is scattered inward onto a Jupiter-crossing orbit, causing Saturn's orbit to expand; then encounters Jupiter and is scattered outward, causing Jupiter's orbit to shrink. This results in a step-wise separation of Jupiter's and Saturn's orbits instead of a smooth divergent migration. The step-wise separation of the orbits of Jupiter and Saturn avoids the slow sweeping of secular resonances across the inner solar System that resulted in the excitation of the eccentricities of the terrestrial planets and an asteroid belt with an excessive ratio of high- to low-inclination objects. The encounters between the ice giant and Jupiter in this model allow Jupiter to acquire its own irregular satellites. Jupiter trojans are also captured following these encounters when Jupiter's semi-major axis jumps and, if the ice giant passes through one of the libration points scattering trojans, one population is depleted relative to the other. The faster traverse of the secular resonances across the asteroid belt limits the loss of asteroids from its core. Most of the rocky impactors of the Late Heavy Bombardment instead originate from an inner extension that is disrupted when the giant planets reach their current positions, with a remnant remaining as the Hungaria asteroids.Some D-type asteroids are embedded in inner asteroid belt, within 2.5 AU, during encounters with the ice giant when it is crossing the asteroid belt.