[Paper Review] Last giant impact on the Neptunian system. Constraints on oligarchic masses in the trans-Saturnian region

M. G. Parisi, L. del Valle|LA Referencia (Red Federada de Repositorios Institucionales de Publicaciones Científicas)|Apr 4, 2011

Astro and Planetary Science50 references3 citations

TL;DR

This paper models the final giant impact on proto-Neptune to constrain the mass of the last oligarchic impactor responsible for Neptune's current obliquity and irregular satellite population. Using angular momentum and impulse transfer calculations, it finds that the impactor mass must be ≤4 M⊕ to reproduce Neptune's spin and ≤1.4 M⊕ to preserve the current irregular satellite system, independent of formation location or migration history.

ABSTRACT

Stochastic impacts by large bodies are, at present, the usually accepted mechanisms able to account for the obliquity of the ice giants. We attempt to set constraints on giant impacts as the cause of Neptune's current obliquity in the framework of modern theories. We also use the present orbital properties of the Neptunian irregular satellites (with the exception of Triton) to set constraints on the scenario of giant impacts at the end of Neptune formation. We model the angular momentum transfer to proto-Neptune and the impulse transfer to its irregular satellites by the last stochastic collision (GC) between the protoplanet and an oligarchic mass at the end of Neptune's formation. We obtain that an impactor mass greather than 4 Earth masses is not possible since it cannot reproduce the present rotational properties of the planet, unless the impact parameter of the collision were very small. On the other hand, if the impactor mass was greather than 1.4 Earth masses, the present Neptunian irregular satellites had to be formed or captured after the end of stochastic impacts. The upper bounds on the oligarchic masses (4 Earth masses from the obliquity of Neptune and 1.4 earth masses from the Neptunian irregular satellites) are independent of unknown parameters, such as the mass and distribution of the planetesimals, the location at which Uranus and Neptune were formed, the Solar Nebula initial surface mass density, and the growth regime. If stochastic impacts had occurred, these results should be understood as upper constraints on the oligarchic masses in the trans-Saturnian region at the end of ice planet formation and may be used to set constraints on planetary formation scenarios.

Motivation & Objective

• To determine the mass of the last giant impactor that could have altered Neptune’s obliquity through stochastic collisions.
• To constrain the mass of the impactor that could have preserved or captured the current population of Neptunian irregular satellites.
• To establish mass limits for oligarchic bodies in the trans-Saturnian region at the end of ice giant formation, independent of uncertain initial disk parameters.
• To test whether Neptune formed in situ or between 10–20 AU by evaluating the consistency of impact scenarios with current planetary and satellite properties.
• To provide independent constraints on planetary formation models using rotational and satellite system properties as diagnostics.

Proposed method

• Model the angular momentum transfer from the last giant impact to proto-Neptune using conservation of angular momentum and impact dynamics.
• Apply impulse transfer theory to estimate the effect of the impactor on the orbital properties of irregular satellites.
• Use the observed current spin parameters of Neptune (obliquity, rotation rate) as constraints on the impactor mass and impact parameter.
• Use the current orbital and physical properties of Neptunian irregular satellites as constraints on the impactor mass and timing of the impact.
• Assume a minimum impactor mass of 1 M⊕ and vary it up to 4 M⊕ to determine upper bounds consistent with observations.
• Evaluate the scenario under two formation models: in situ formation and formation between 10–20 AU, with or without migration.

Experimental results

Research questions

• RQ1 What is the maximum mass of an oligarchic impactor that can reproduce Neptune’s current obliquity via a single giant impact?
• RQ2 What mass of impactor would be required to preserve or capture the current population of Neptunian irregular satellites without disrupting their orbits?
• RQ3 How do the constraints on impactor mass depend on Neptune’s formation location (in situ vs. 10–20 AU) or migration history?
• RQ4 Can the observed spin and satellite system of Neptune be explained by stochastic impacts alone, and what are the implications for oligarchic mass in the trans-Saturnian region?
• RQ5 What are the implications of these constraints for planetary formation models, particularly those involving the Nice model or in situ formation?

Key findings

• An impactor mass of mi ≤ 4 M⊕ is required to reproduce Neptune’s current obliquity via a single giant impact, regardless of whether Neptune formed in situ or between 10–20 AU.
• An impactor with mi > 4 M⊕ cannot reproduce Neptune’s current rotational properties unless the impact parameter is very small, making such collisions highly unlikely.
• An impactor mass of mi ≤ 1.4 M⊕ is required to preserve or capture the current population of Neptunian irregular satellites, assuming the last giant impact occurred before their capture.
• If the impactor mass exceeded 1.4 M⊕, the current irregular satellite population must have formed or been captured after the end of stochastic impacts.
• The upper bounds on oligarchic masses (4 M⊕ from obliquity, 1.4 M⊕ from satellites) are independent of unknown parameters such as planetesimal mass, initial nebula density, or formation location.
• These constraints provide robust, model-independent limits on the masses of oligarchic bodies in the trans-Saturnian region at the end of ice giant formation.