Constraints on the Formation Age of Cometary Material from the NASA Stardust Mission

We measured the ²⁶Al-²⁶Mg isotope systematics of a approximately 5-micrometer refractory particle, Coki, returned from comet 81P/Wild 2 in order to relate the time scales of formation of cometary inclusions to their meteoritic counterparts. The data show no evidence of radiogenic ²⁶Mg and define an upper limit to the abundance of ²⁶Al at the time of particle formation: ²⁶Al/²⁷Al < 1 x 10⁻⁵. The absence of ²⁶Al indicates that Coki formed >1.7 million years after the oldest solids in the solar system, calcium- and aluminum-rich inclusions (CAIs). The data suggest that high-temperature inner solar system material formed, was subsequently transferred to the Kuiper Belt, and was incorporated into comets several million years after CAI formation.