The Macroporosity of Rubble Pile Asteroid Ryugu and Implications for the Origin of Chondrules

Abstract We use the known surface boulder-size distribution of C-type rubble pile asteroid Ryugu (NEA 162173) to determine its macroporosity, assuming it is a homogeneous granular aggregate. show that volume-frequency boulders, cobbles, and pebbles, well-represented by lognormal function with σ = 2.4 ± 0.1 μ 0.2 0.05. Application linear-mixture packing theory yields value for macroporosity ϕ 0.14 0.04. Given low bulk density 1.19 gm cm −3 , this implies an average Ryugu’s rocks 1.38 0.07 throughout volume, consistent recent determination boulders based on their thermal properties. This supports spectrum-based argument interplanetary dust particles may be best analog material available Earth, suggests high-density, well-lithified objects such as chondrules chondrule-bearing chondrites rare Ryugu. Implications result origin chondrules, long-standing problem in cosmochemistry, are discussed. propose most formed together lithification events, which occurred during accretion chondritic envelopes large, differentiated planetesimals at time when they were still hot from 26 Al decay.