IDP-like Asteroids Formed Later than 5 Myr After Ca–Al-rich Inclusions

Abstract

The parent bodies of ordinary chondrites, carbonaceous CM chondrites, and interplanetary dust particles (IDPs) represent most of the mass of the solar system s small (D<250 km) bodies. The times of formation of the ordinary and carbonaceous CM chondrite parent bodies have previously been pinpointed, respectively, to ≈2 and 3-4 million years after calcium-aluminum-rich inclusions (CAIs). However, the timing of the formation of IDP parent bodies such as P-and D-type main-belt asteroids and Jupiter Trojans has not been tightly constrained. Here, we show that they formed later than 5-6 million years after CAIs. We use models of their thermal and structural evolution to show that their anhydrous surface composition would otherwise have been lost due to melting and ice-rock differentiation driven by heating from the short-lived radionuclide 26 Al. This suggests that IDP-like volatile-rich small bodies may have formed after the gas of the protoplanetary disk dissipated and thus later than the massive cores of the giant planets. It also confirms an intuitive increase in formation times with increased heliocentric distance, and suggests that there may have been a gap in time between the formation of carbonaceous chondrite (chondrule-rich) and IDP (chondrule-poor) parent bodies.