Questions: Late Heavy Bombardment and Planetary Migration

Question 1

The Nice model explains the Late Heavy Bombardment as the gradual clearing of leftover debris from the initial planetary formation phase. Which of the following best describes what the model actually proposes?

• A. The LHB was indeed gradual debris clearing — the Nice model refines the timing but not the mechanism
• B. The LHB was triggered by Jupiter and Saturn crossing an orbital resonance, destabilizing the outer solar system and flinging planetesimals inward
• C. The LHB was caused by a rogue star passing through the solar system and gravitationally perturbing asteroid orbits
• D. The LHB was triggered by Mars forming late and disturbing the asteroid belt through repeated close passes

Answer: (B)

The Nice model proposes that the LHB was NOT gradual debris clearing but a dynamically triggered event. When Jupiter and Saturn crossed the 2:1 mean-motion resonance, it destabilized the entire outer solar system, flinging Uranus and Neptune into the planetesimal disk and scattering trillions of impactors inward. This explains why there was a quiet period (~400 million years) between the end of planetary formation and the bombardment spike — the LHB was not a continuation but a distinct second event.

Question 2

Beyond the obvious destructive impact record, the Late Heavy Bombardment may have contributed constructively to Earth's habitability. Which of the following best describes this constructive role?

• A. The impacts melted and homogenized Earth's mantle, allowing plate tectonics to begin
• B. The bombardment sterilized the early Earth, giving simple chemistry time to produce the first life
• C. Cometary and asteroidal impactors delivered volatile compounds including water and organic molecules to the inner planets
• D. The heat from impacts drove volcanic activity that built up the early atmosphere

Answer: (C)

The LHB impactors — both icy comets from the outer disk and rocky asteroids from the disturbed asteroid belt — delivered significant quantities of volatile compounds: water, CO₂, nitrogen, and organic molecules. Some models attribute a substantial fraction of Earth's ocean water to this period. The bombardment was destructive to early life (option B gets the effect backwards — it repeatedly sterilized environments rather than protecting them), and the end of the bombardment ~3.8 Gya roughly coincides with the first isotopic evidence for persistent life.

Question 3

The timing gap between the end of planetary formation (~4.5 Gya) and the Late Heavy Bombardment (~3.8–4.1 Gya) is one of the key puzzles the Nice model addresses.

• T. True
• F. False

Answer: True

This is exactly the central puzzle. If bombardment were simply leftover debris from planet formation, it should have tapered off continuously from ~4.5 Gya onward. Instead, lunar samples show a distinct spike in impact rates ~3.8–4.1 Gya — some 400 million years after the planets assembled. The Nice model explains this gap through a period of slowly evolving orbital resonances that eventually crossed a critical threshold, triggering sudden instability rather than a gradual decline.

Question 4

The Late Heavy Bombardment was simply the continuation of the debris-clearing phase from planetary formation, not a distinct cataclysmic event.

• T. True
• F. False

Answer: False

The LHB was a distinct second pulse, not gradual debris clearing. Apollo lunar samples show a spike in impact ages concentrated between 3.8 and 4.1 Gya, with a quieter period before it — inconsistent with steady exponential decline from 4.5 Gya. The Nice model's mechanism (resonance crossing and outer-planet migration) explains why impactors were suddenly injected from the outer planetesimal disk rather than being leftover material from initial formation. The distinction between gradual decline and a dynamically triggered spike is central to the LHB hypothesis.

Question 5

Why does the Nice model predict that the giant planets would trigger a bombardment hundreds of millions of years after planetary formation, rather than immediately?

Model answer: The Nice model proposes that the giant planets began in a compact configuration and slowly evolved toward a resonance crossing through gradual gravitational interactions with the outer planetesimal disk. The slow drift of orbital periods eventually brought Jupiter and Saturn to the 2:1 mean-motion resonance — a process taking hundreds of millions of years. Only at that resonance crossing did the instability become dramatic enough to scatter Uranus and Neptune outward into the disk, which then flung trillions of icy bodies inward. The delay is the time it took orbital drift to reach the critical resonance threshold.

The key is that the resonance crossing requires orbits to evolve slowly first. Gravitational interactions with the planetesimal disk gradually shift orbital periods. Only when the ratio of orbital periods reached the critical value (2:1 for Jupiter-Saturn) did the resulting resonant perturbations become large enough to trigger the cascade. This is why the LHB is a spike rather than a continuous feature — it represents the moment the system crossed a dynamical threshold.