Many planetary materials exhibit distinct isotopic compositions. For example, the isotopic compositions of oxygen differ between the terrestrial planets: Mercury, Venus, Earth and Mars. These differences indicate that each planet formed from a separate isotopic reservoir, and possibly different regions, in the protoplanetary disc. Numerous attempts have been made to reconstruct the distribution of isotopic compositions in the protoplanetary disc using the current compositions of the planets. However, recent dynamic modeling of the migration of giant planets during the formation of the solar system suggests early conditions that would have favored rapid mixing of materials in the protoplanetary disk and, therefore, would not support early isotopic differences.
A recent study by Jingyi Mah and Ramon Brasser of the Tokyo Institute of Technology modeled the protoplanetary disk using available isotope data to understand how terrestrial planets formed. Mah and Brasser combined the dynamic modeling of the formation of planets with the isotopic stresses of meteorites and obtained results consistent with the isotopic differences observed between the planets being due to an original isotopic gradient in the protoplanetary disk. This contrasts with the dynamic models of planetary formation which require efficient mixing and result in almost identical isotopic compositions for all planets. Mah and Brasser argue for a mass depleted internal disk where terrestrial planets accumulate from locally sourced material. The authors argue that the region of the protoplanetary disc where the telluric planets developed probably did not experience sufficient mixing to completely homogenize an early isotopic gradient. Instead, Mah and Brasser’s results indicate that every accreting planet is built using locally derived materials and is not compatible with a complex mixture of solar system-wide materials. READ MORE