Late Pebble Accretion of Comet 81P/Wild 2 Nucleus: Evidence from a Plagioclase-bearing Chondrule Fragment, Pyxie

Abstract

Comet 81P/Wild 2 is a ∼4.5 km-sized primordial object that almost has not been modified by internal heating by 26Al decay. Its nucleus could have been formed by hierarchical agglomeration or gravitational collapse of pebble swarms concentrated by streaming instability. To shed light on the cometesimal formation mechanism from laboratory sample analysis, we reexamined the 26Al–26Mg isotope systematics of the plagioclase-bearing fragment, Pyxie (from Wild 2 track 81), with significantly improved analytical precision. The revised upper limit of the initial (26Al/27Al)0 of Pyxie is ≤1.5 × 10−6, 2 times smaller than those estimated from other Wild 2 fragments. Assuming homogenous distribution of 26Al in the early solar system, the minimum crystallization age of Pyxie is estimated to be >3.6 Ma after calcium–aluminum-rich inclusions. Additional petrologic examination demonstrated that it is a chondrule fragment formed in disk environments enriched in moderately volatile elements comparable to the Si-rich rim of CR chondrules before accreting by comet Wild 2. The late accretion of the Wild 2 nucleus with most silicates likely from a common source are not favored by the hierarchical agglomeration model that considers early and continuous accretion. Instead, the results are more in line with comet formation by gentle gravitational collapse of pebbles when the 26Al abundance is extremely low (26Al/27Al ≤ 1.5 × 10−6) before gas dispersal.