Cosmic Ray Induced Mass-Independent Oxygen Isotope Exchange: A Novel Mechanism for Producing $^{16}$O depletions in the Early Solar System

TL;DR

This paper proposes a novel cosmic-ray induced isotope exchange mechanism in dust grains that can explain the $^{16}$O depletion in early solar system bodies, offering an alternative to photodissociation models.

Contribution

It introduces a new mechanism of oxygen isotope exchange via cosmic-ray irradiation of dust, explaining $^{16}$O depletion without fine-tuned mixing timescales.

Findings

Cosmic-ray irradiation can cause significant oxygen isotope exchange in silicate dust at low temperatures.

The model accounts for $^{16}$O depletion observed in terrestrial bodies and early solar system materials.

Rapid isotope exchange (10-100 years) can occur during the Sun's T-Tauri phase.

Abstract

A fundamental puzzle of our solar system's formation is understanding why the terrestrial bodies including the planets,comets,and asteroids are depleted in $^{16}$O compared to the Sun. The most favored mechanism,the selective photodissociation of CO gas to produce $^{16}$O depleted water,requires finely tuned mixing timescales to transport $^{16}$O depleted water from the cold outer solar system to exchange isotopically with dust grains to produce the $^{16}$O depleted planetary bodies observed today. Here we show that energetic particle irradiation of SiO$_2$ (and Al$_2$O$_3$) makes them susceptible to anomalous isotope exchange with H$_2$O ice at temperatures as low as 10 K. The observed magnitude of the anomalous isotope exchange (D$^{17}$O) is sufficient to generate the $^{16}$O depletion characteristic of the terrestrial bodies in the solar system. We calculated the cosmic-ray exposure times needed to produce the observed $^{16}$O depletions in silicate (SiO2) dust in the interstellar medium and early solar system and find that radiation damage induced oxygen isotope exchange could have rapidly (~10-100 yrs) depleted dust grains of $^{16}$O during the Sun's T-Tauri phase. Our model explains whythe oldest and most refractory minerals found in the solar system, the anhydrous Calcium with Aluminum Inclusions (CAIs),are generally $^{16}$O enriched compared to chondrules and the bulk terrestrial solids and provides a mechanism for producing $^{16}$O depleted grains very early in the solar system's history. Our findings have broad implications for the distribution of oxygen isotopes in the solar system, the interstellar medium, the formation of the planets and its building blocks as well as the nature of mass-independent isotope effects.