[Paper Review] Impact-induced differentiation in icy bodies

W. B. Tonks, E. Pierazzo+1 more|arXiv (Cornell University)|Jul 5, 2016

Astro and Planetary Science36 references3 citations

TL;DR

This paper proposes that large high-speed impacts can trigger whole-body differentiation in icy planetary bodies by generating a transient melt zone that enables gravitational segregation of dense material. Using impact melting models and Monte Carlo simulations, it shows that differentiation becomes certain for proto-satellites at Europa-mass or larger, and explains the differing internal structures of Callisto and Ganymede via differing impact probabilities from heliocentric particles.

ABSTRACT

By the time icy objects grow to about the mass of Europa and silicate bodies grow to approximately lunar mass, a large high-speed impact can generate an intact melt region that allows the dense material to gravitationally segregate, forming a large density anomaly. If this anomaly generates sufficient differential stress, it rapidly segregates to the object's center, triggering whole body differentiation. We used an impact melting model based on the Hugoniot equations, the linear shock-particle velocity relationship, and the empirical relationship between shock pressure and distance coupled with a Monte Carlo simulation of the late accretion process to determine conditions under which large impacts trigger differentiation in icy bodies. In a gas-free environment, impacts of projectiles in the satellite's accretion zone have a small probability of triggering differentiation in surviving proto-satellites as small as Triton. The probability increases to 100% by the time surviving proto-satellites grow to the mass of Europa. The impact of heliocentric particles captured by the central planet also effectively triggers differentiation in the icy satellites. If the largest projectiles in the heliocentric distribution are comparable to the mass of the growing satellites, they trigger differentiation in all surviving proto-satellites by the time they grow to the mass of Callisto. However, Callisto and Ganymede have different probabilities of impact-induced differentiation if they captured a small fraction (0.01 and 0.5%) of the particles they accreted from a distribution of small (largest particles ~10% of the satellite's mass) heliocentric objects. Impact induced differentiation explains the apparent difference between these two objects in a plausible way.

Motivation & Objective

• To investigate whether large impacts can trigger whole-body differentiation in icy planetary bodies during their accretion phase.
• To determine the conditions under which impact-generated melt zones lead to gravitational segregation of dense materials.
• To assess the probability of impact-induced differentiation across a range of icy satellite masses, including Triton, Europa, Callisto, and Ganymede.
• To explain the observed differences in internal structure between Callisto and Ganymede through impact history and particle accretion probabilities.
• To model the role of heliocentric impactors in triggering differentiation in icy satellites, particularly in gas-free environments.

Proposed method

• Utilized an impact melting model based on the Hugoniot equations to estimate shock-induced melt volumes.
• Applied the linear shock-particle velocity relationship to link impact velocity to shock pressure.
• Incorporated an empirical shock pressure–distance relationship to model melt zone extent.
• Conducted Monte Carlo simulations of the late accretion process to sample impactor populations and frequencies.
• Simulated impact outcomes for icy bodies ranging from Triton to Europa and Callisto masses.
• Assessed the probability of generating sufficient differential stress to drive rapid core formation via gravitational segregation.

Experimental results

Research questions

• RQ1 What mass threshold of icy bodies triggers impact-induced differentiation with high probability?
• RQ2 How do impactors from the heliocentric population affect the differentiation likelihood of icy satellites?
• RQ3 Why do Callisto and Ganymede show different internal structures despite similar sizes?
• RQ4 What is the role of transient melt zones in enabling gravitational segregation of dense material?
• RQ5 How does the accretion of small heliocentric particles influence the probability of impact-induced differentiation?

Key findings

• Differentiation via impact-induced melting becomes certain for surviving proto-satellites at Europa-mass or larger, with a 100% probability by that mass.
• In a gas-free environment, impacts from projectiles in the accretion zone can trigger differentiation in proto-satellites as small as Triton, though with low probability initially.
• Heliocentric impactors with masses comparable to the growing satellite trigger differentiation in all surviving proto-satellites by the time they reach Callisto mass.
• The difference in internal structure between Callisto and Ganymede is plausibly explained by their differing probabilities of impact-induced differentiation, depending on the fraction of small heliocentric particles accreted (0.01% vs. 0.5%).
• Impact-induced differentiation provides a viable mechanism for explaining the observed density anomalies and core formation in icy satellites without requiring internal heating from long-lived radioisotopes.
• The model predicts that transient melt zones from large impacts can generate sufficient differential stress to drive rapid gravitational segregation, leading to whole-body differentiation.