Brief Follow-up on Recent Studies of Theia's Accretion

TL;DR

This paper compares two studies on Theia's composition using simulations, finds that differences in simulation parameters affect isotopic similarity predictions, and concludes that the likelihood of Theia having Earth-like isotopic composition is about 5-8%.

Contribution

It unifies the results of two previous studies, clarifies how simulation choices influence isotopic similarity predictions, and refines the estimated probability of Theia's Earth-like composition.

Findings

Kaib & Cowan's analysis suggests a 5-8% chance of Theia having similar isotopic composition to Earth.

Differences in simulation parameters significantly affect isotopic similarity outcomes.

The probability of Theia's Earth-like isotopic composition remains low despite uncertainties.

Abstract

Kaib & Cowan (2015) recently used terrestrial planet formation simulations to conclude that the moon-forming impactor (Theia) had only a ~5% or less chance of having the same oxygen isotope composition as Earth, while Mastrobuono-Battisti et al. (2015) used seemingly similar simulations and methods to arrive at a higher value of ~20% or more. Here we derive the results of both papers from a single set of simulations. Compared to Kaib & Cowan (2015), the analysis of Mastrobuono-Battisti et al. (2015) systematically yields more massive Theia analogs and imposes flatter isotopic gradients across the original protoplanetary disk. Both of these effects diminish isotopic differences between Earth and Theia analogs. While it is notoriously difficult to produce systems resembling our actual terrestrial planets, the analysis of Kaib & Cowan (2015) more often selects and analyzes Earth and Mars analogs at orbital locations near the real planets. Given this, we conclude that the greater isotopic differences between Earth and Theia found in Kaib & Cowan (2015) better reflect the predictions of terrestrial planet formation models. Finally, although simulation uncertainties and a terrestrial contribution to Moon formation enhance the fraction of Theia analogs consistent with the canonical giant impact hypothesis, this fraction still remains in the 5-8% range.