Grand tack hypothesis

In planetary astronomy, the grand tack hypothesis proposes that after its formation at 3.5 AU, Jupiter migrated inward to 1.5 AU, before reversing course due to capturing Saturn in an orbital resonance, eventually halting near its current orbit at 5.2 AU. The reversal of Jupiter's migration is likened to the path of a sailboat changing directions (tacking) as it travels against the wind.

The planetesimal disk is truncated at 1.0 AU by Jupiter's migration, limiting the material available to form Mars. Jupiter twice crosses the asteroid belt, scattering asteroids outward then inward. The resulting asteroid belt has a small mass, a wide range of inclinations and eccentricities, and a population originating from both inside and outside Jupiter's original orbit. Debris produced by collisions among planetesimals swept ahead of Jupiter may have driven an early generation of planets into the Sun.

In the grand tack hypothesis Jupiter underwent a two-phase migration after its formation, migrating inward to 1.5 AU before reversing course and migrating outward. Jupiter's formation took place near the ice line, at roughly 3.5 AU. After clearing a gap in the gas disk Jupiter underwent type II migration, moving slowly toward the Sun with the gas disk. If uninterrupted, this migration would have left Jupiter in a close orbit around the Sun like recently discovered hot Jupiters in other planetary systems. Saturn also migrated toward the Sun, but being smaller it migrated faster, undergoing either type I migration or runaway migration. Saturn converged on Jupiter and was captured in a 2:3 mean-motion resonance with Jupiter during this migration. An overlapping gap in the gas disk then formed around Jupiter and Saturn, altering the balance of forces on these planets which are began migrating together. Saturn partially cleared its part of the gap reducing the torque exerted on Jupiter by the outer disk. The net torque on the planets then became positive, with the torques generated by the inner Lindblad resonances exceeding those from the outer disk, and the planets began to migrate outward. The outward migration was able to continue because interactions between the planets allowing gas to stream through the gap. The gas exchanged angular momentum with the planets during its passage, adding to the positive balance of torques; and transferred mass from the outer disk to the inner disk, allowing the planets to migrate outward relative to the disk. The transfer of gas to the inner disk also slowed the reduction of the inner disk's mass relative to the outer disk as it accreted onto the Sun, which otherwise would weaken the inner torque, ending the planets outward migration. In the grand tack hypothesis this process is assumed to have reversed the inward migration of the planets when Jupiter is at 1.5 AU. The outward migration of Jupiter and Saturn continued until they reached a stable equilibrium if the disk was flared, or the gas disk dissipated, and is supposed to end with Jupiter near its current orbit.

The hypothesis can be applied to multiple phenomena in the Solar System.

Jupiter's grand tack resolves the Mars problem by limiting the material available to form Mars. The Mars problem is a conflict between some simulations of the formation of the terrestrial planets, which when begun with planetesimals distributed throughout the inner Solar System, end with a 0.5–1.0 Earth-mass planet in its region, much larger than the actual mass of Mars, 0.107 Earth-mass. Jupiter's inward migration alters this distribution of material, driving planetesimals inward to form a narrow dense band with a mix of materials inside 1.0 AU, and leaving the Mars region largely empty. Planetary embryos quickly form in the narrow band. While most later collide and merge to form the larger terrestrial planets (Venus and Earth), some are scattered outside the band. These scattered embryos, deprived of additional material slowing their growth, form the lower mass terrestrial planets Mars and Mercury.

Jupiter and Saturn drive most asteroids from their initial orbits during their migrations, leaving behind an excited remnant derived from both inside and outside Jupiter's original location. Before Jupiter's migrations the surrounding regions contained asteroids which varied in composition with their distance from the Sun. Rocky asteroids dominated the inner region, while more primitive and icy asteroids dominated the outer region beyond the ice line. As Jupiter and Saturn migrate inward, ~15% of the inner asteroids are scattered outward onto orbits beyond Saturn. After reversing course, Jupiter and Saturn first encounter these objects, scattering about 0.5% of the original population back inward onto stable orbits. Later, as Jupiter and Saturn migrate into the outer region, about 0.5% of the primitive asteroids are scattered onto orbits in the outer asteroid belt. The encounters with Jupiter and Saturn leave many of the captured asteroids with large eccentricities and inclinations. Some of the icy asteroids are also left in orbits crossing the region where the terrestrial planets later formed, allowing water to be delivered to the accreting planets as when the icy asteroids collide with them.

This page was last edited on 19 June 2018, at 20:43 (UTC).
Reference: under CC BY-SA license.

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