According to the Grand Tack hypothesis, what caused Jupiter's inward migration to reverse direction and move outward?
AJupiter ran out of gas disk material to interact with and naturally decelerated
BSaturn formed, caught up to Jupiter, entered a mean-motion resonance, and the shared gap reversed the disk torques
CThe growing terrestrial planets exerted enough gravitational pull to halt Jupiter's infall
DJupiter reached the inner edge of the gas disk and had nowhere left to migrate
The reversal happened when Saturn, which formed more slowly, caught up to Jupiter and locked into a 2:3 mean-motion resonance. Two giant planets sharing a gap in the gas disk create a torque balance that reverses the migration direction, pushing both outward together. The other options describe mechanisms that do not apply — terrestrial planets were far too small to halt Jupiter, and the disk's inner edge was not the relevant constraint.
Question 2 Multiple Choice
The Grand Tack hypothesis predicts that Mars has only about one-tenth of Earth's mass. What is the proposed mechanism for this?
AMars formed later than Earth and had less time to accrete material before the disk dissipated
BMars is made of less dense material than Earth, so the same volume of solids produced less mass
CJupiter's inward migration through the Mars-forming region depleted the solid material available to build Mars
DA giant impact in the Mars region ejected most of its mass into the asteroid belt
Jupiter migrating inward to approximately 1.5 AU — roughly where Mars orbits — scattered and cleared the planetesimals in that region, truncating the supply of material available for Mars to grow. Standard formation models without this migration consistently fail to reproduce Mars's small mass, generating a planet much more massive than observed. This is one of the Grand Tack's strongest predictive successes.
Question 3 True / False
The Grand Tack hypothesis predicts that Jupiter's outward migration would scatter C-type (water-rich) asteroids from beyond the snow line inward, mixing them with the inner-belt S-type asteroids.
TTrue
FFalse
Answer: True
This is a key prediction of the Grand Tack: as Jupiter migrated outward, it would scatter volatile-rich C-type material from the outer solar system inward into the asteroid belt, naturally producing the observed compositional gradient between the inner belt (S-types) and outer belt (C-types) while also explaining the presence of water-bearing bodies in the inner solar system.
Question 4 True / False
Because Jupiter's migration reversed, the solar system's terrestrial planets contain roughly the same total mass as the densely packed super-Earth systems commonly found around other stars.
TTrue
FFalse
Answer: False
The Grand Tack actually explains why the inner solar system has relatively little total mass compared to compact super-Earth systems around other stars. Jupiter's early inward incursion cleared out solid material that might otherwise have built much more massive inner planets. The result is that our inner system — with four small rocky planets — is unusually sparse compared to many observed planetary systems.
Question 5 Short Answer
What two lines of evidence from the asteroid belt are specifically explained by Jupiter's inward-then-outward migration in the Grand Tack model?
Think about your answer, then reveal below.
Model answer: First, the asteroid belt is depleted in total mass — Jupiter's passage scattered away much of the primordial solid material. Second, the belt contains two compositionally distinct populations (dry S-type asteroids in the inner belt, water-rich C-type asteroids in the outer belt) whose mixing is naturally explained by Jupiter's outward migration scattering C-type material inward from beyond the snow line while S-type material was left behind.
The Grand Tack ties together two facts about the asteroid belt that are otherwise hard to explain simultaneously: its low total mass and its compositional bimodality. The outward migration phase is responsible for the mixing — without it, C-type bodies should be confined to the outer solar system where they formed, not distributed through the belt alongside S-types.