Abstract
Abstract We use the known surface boulder-size distribution of the C-type rubble pile asteroid Ryugu (NEA 162173) to determine its macroporosity, assuming it is a homogeneous granular aggregate. We show that the volume-frequency distribution of its boulders, cobbles, and pebbles, is well-represented by a lognormal function with σ = 2.4 ± 0.1 and μ = 0.2 ± 0.05. Application of linear-mixture packing theory yields a value for the macroporosity of ϕ = 0.14 ± 0.04. Given its low bulk density of 1.19 gm cm−3, this implies an average density for Ryugu’s rocks of 1.38 ± 0.07 gm cm−3 throughout its volume, consistent with a recent determination for surface boulders based on their thermal properties. This supports the spectrum-based argument that interplanetary dust particles may be the best analog material available on Earth, and it suggests that high-density, well-lithified objects such as chondrules and chondrule-bearing chondrites may be rare on Ryugu. Implications of this result for the origin of chondrules, a long-standing problem in cosmochemistry, are discussed. We propose that chondrules and most chondrites formed together in rare lithification events, which occurred during the accretion of chondritic envelopes to large, differentiated planetesimals at a time when they were still hot from 26Al decay.
Highlights
Asteroids and meteorites can be associated with one another by their reflectance spectra (Greenwood, Burbine & Franchi 2020)
We propose that chondrules and most chondrites formed together in rare lithification events, which occurred during the accretion of chondritic envelopes to large, differentiated planetesimals at a time when they were still hot from 26Al decay
The central question is whether the meteorites found on Earth are representative samples of the primitive material from which the asteroids formed, or are an unrepresentative population that experienced higher than normal lithification, which was critical to their survival? Here we discuss how a simple, but inefficient, local mechanism for chondrule and simultaneous chondrite formation can account for a broad range of observed phenomena that have challenged global models, including complementarity in the composition of chondrules and matrix within chondrites, uniformity of chondrule sizes within meteorite classes, the existence of compound chondrules and cluster chondrites, and chondrule formation in the presence of a weak magnetic field
Summary
Ryugu indicate that such high porosity boulders are very much the rule, not the exception on its surface (Okada et al 2020). Grott et al (2019) have shown that the Ryugu boulder they studied probably does not have the tensile strength to survive passage through the Earth’s atmosphere It may be, that these asteroids are not composed of material already familiar to us from meteorite collections, even though they have similar reflectance spectra. Sears (1998) pointed out decades ago that an extremely rare class of chondrites on Earth, CI’s, which contain no chondrules at all, may be the most common type of chondrite in space Their low density and tensile strength make them much less likely to survive passage through the Earth’s atmosphere and terrestial weathering, should they reach the Earth’s surface. This impacts discussion of the long-standing and vexing problem of the origin of chondrules, which has implications for the structure and origin of asteroids and terrestrial planets, as we discuss
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