Delayed chondrule formation in the outer Solar System

Séminaire IPAG de Martin Bizzarro (University of Copenhagen), jeudi 6 juin 2019 à 11h00, salle Manuel Forestini

Elucidating the timescales of solid formation in the early Solar System is critical for a better understanding of disk processes.

Although short-lived radionuclides such as 26Al can theoretically provide high resolution ages, such decay systems critically depend on the disputed assumption of homogeneous distribution of their parent nuclides. A such, only the decay of U to Pb can provide assumption free ages with sufficiently high resolution to determine the timing of individual meteoritic components formed in the first few Myr of the Solar System’s history.

Using this approach, it has been established that the formation of chondrules, the most abundant chondritic component, started simultaneously with the condensation of the Solar system’s first solids – calcium-aluminum-rich inclusions (CAIs) – and lasted 4 Myr.

However, it is uncertain whether this distribution of chondrule ages is representative of the whole protoplanetary disk.

To address this issue, we report on the U-corrected Pb-Pb ages of two populations of chondrules understood to have formed in spatially isolated disk reservoirs based on their nucleosynthetic 54Cr compositions.

The first population consists of 15 individual objects from the NWA 5697 ordinary chondrite and Allende carbonaceous chondrite that are characterised by 54Cr compositions typical of inner disk material, suggesting that these objects were formed in the accretion region(s) of the terrestrial planets.

The second population comprises 17 individual objects from three CR chondrites and have 54Cr compositions indicating formation beyond the orbits of the gas giants.

Although both populations display a similar age range of 4 Myr after CAIs, their mean ages are distinct. Indeed, the inner disk population records a mean U-corrected Pb-Pb age of 1.00±0.62 Myr whereas the outer disk population displays a mean age of 2.38±0.53 Myr.

To assess the statistical significance of this age offset, we performed a Mann-Whitney U test, which evaluates the null hypothesis that the medians of the two populations are identical.

The results indicate that the null hypothesis can be rejected at the 3-sigma confidence level, establishing that the age difference of 1.4±0.8 Myr between the two populations is statiscally meaningful. We suggest that this age offset between chondrules from spatially isolated disk reservoirs reflects different formation mechanisms.

Whereas the formation of ≤1 Myr inner disk chondrules has been linked to disk accretion processes, such an energy source cannot account for the thermal processing of outer disk solids. In contrast, bow shocks associated with planet migration during the disk lifetime provides a viable energy source for the thermal processing of outer disk solids. The mean outer Solar System chondrule age of 2.38±0.53 Myr may thus record the migration of the gas giant planets.