Sota Arakawa

Collisional bouncing limits the growth of dust aggregates in protoplanetary disks, but its dependence on aggregate size, collision velocity, and filling factor remains poorly understood. Here we develop a semianalytic model for the sticking probability of colliding dust aggregates. We divide each aggregate collision into two phases: a compression phase and a separation phase. The compression phase is described with an elastoplastic contact model, which determines the maximum contact radius and repulsive energy after compression. The separation phase is treated as fracture of a stochastic network of interparticle bonds, whose fracture energy is evaluated using weakest-link statistics. The model naturally predicts that larger aggregates bounce more readily because larger contact regions are more likely to contain weak bonds. Comparison with distinct element method simulations shows that the model reproduces the simulated sticking–bouncing boundary. Furthermore, applying the calibrated model to moderately porous aggregates inferred from Atacama Large Millimeter/submillimeter Array observations of protoplanetary disks, we find that the predicted bouncing barrier passes through the observationally inferred size–velocity range. Thus, our semianalytic model provides a useful framework for predicting the collisional evolution of protoplanetary dust aggregates.

Most carbonaceous chondrite (CC) meteorites contain abundant chondrules, millimeter-scale spherical inclusions of previously molten material. Ivuna-type carbonaceous chondrites (CIs) do not contain chondrules, nor do samples of the carbonaceous asteroid Ryugu. CIs and Ryugu share isotopic properties distinct from those of other CCs, implying that their parent bodies formed at a different time or place than those of other CCs. We applied manganese-chromium dating and oxygen isotope thermometry to Ryugu and Ivuna samples. The results indicate that the parent bodies of Ryugu and Ivuna formed less than 2 million years after the beginning of Solar System formation—before the majority of chondrules in CCs and the parent bodies of other CCs formed. This implies that Ryugu and CIs are remnants of an earlier generation of carbonaceous planetesimals.

Chondrules are thought to have formed during transient flash-heating events in dust-enriched regions of the solar protoplanetary disk. Although laboratory studies have characterized the oxygen isotopic compositions of chondritic materials, quantitative interpretations based on simulations of disk formation and evolution remain limited. Here, we perform one-dimensional simulations of disk formation and evolution by solving a diffusion–advection equation with mass infall from the parental cloud core. We compute the temporal evolution of oxygen isotopic compositions using an experimentally derived isotope-exchange model. We examine how the oxygen isotopic signatures of the disk depend on the radial distribution of infalling material and the composition of the parental cloud core. We find that the oxygen isotopic compositions of carbonaceous-chondrite chondrules can be reproduced if either (i) the radial extent of mass infall onto the disk is moderate (∼10 au), or (ii) it is large (>10 au) and the parental cloud core was ice-depleted and/or experienced weaker CO self-shielding than is commonly assumed. In contrast, if ordinary-chondrite chondrules formed inside the snow line under background temperatures of ≲500 K, their oxygen isotopic compositions may be difficult to explain within the present disk-evolution model, because oxygen isotopic exchange between silicates and vapor species proceeds efficiently only in the inner disk at T ≳ 500–600 K.

NASA’s OSIRIS-REx mission returned material from the carbonaceous asteroid Bennu, which is dominated by aqueous-alteration products but preserves rare high-temperature components. Here we present an integrated petrographic, oxygen-isotopic and ²⁶Al–²⁶Mg chronological characterization of refractory inclusions in Bennu samples. All identified inclusions are small (<~100 µm) and commonly preserve mineralogical evidence of in situ aqueous alteration, indicating incorporation into Bennu’s parent body prior to hydration. Their mineralogical characteristics and oxygen-isotopic and ²⁶Al–²⁶Mg systematics demonstrate formation during the earliest stage of Solar System evolution and genetic affinity with refractory inclusions in asteroid Ryugu samples and chondritic meteorites. The shared refractory-inclusion signatures across diverse chondritic materials are consistent with widespread outward transport and mixing of early-formed inner-disk solids. In Bennu samples analyzed thus far, large (>submillimeter) refractory inclusions are absent, in contrast to most carbonaceous chondrites. This size distribution implies that Bennu’s constituents accreted in the outer Solar System beyond the pressure bump associated with proto-Jupiter, which effectively filtered distinct dust populations.

Compression-driven jamming of two-dimensional porous aggregates of cohesive, frictionless disks is investigated. The jamming threshold ϕ _J depends on the initial structure, and for ϕ > ϕ _J the pressure scales as P ∼ ( ϕ − ϕ _J ) ² .

Mechanical properties of small bodies such as dust, pebbles, and planetesimals are key to better understanding the formation of planetary systems against their collisional destruction. We present a simple mathematical formulation to determine the static tensile strength, elastic modulus, fracture energy, fracture toughness, and sound velocity for solid particles of any composition, size, structure, and porosity. This formulation is based on the Johnson–Kendall–Roberts theory of contact mechanics, the Griffith theory of fracture mechanics, and the Weibull theory of flaw statistics, in which both aggregate particles and monolithic particles are treated as agglomerates of small constituent grains and molecules, respectively. We show that our formulae are consistent with the results of laboratory experiments and molecular dynamics simulations for porous and nonporous particles of silica or ice, irrespective of the sintering process and the internal heterogeneity. This provides us with a great opportunity to infer the internal structure of asteroids and comets from observational data on their tensile strengths.

The tensile strength of solid particles is the key parameter that controls not only the formation of planetesimals in protoplanetary disks but also outburst and splitting of comets and rotational bursting of asteroids in planetary systems. It is, therefore, highly desirable to find a mathematical model that properly provides the tensile strength of porous and nonporous solid particles as functions of their known material and structural properties. We demonstrate the applicability of a rigorous theory for the tensile strength of particle assemblies to both porous and nonporous solid particles, by treating the particles as solid media with cross-linked networks of molecules. The practicability of the analytical function for the tensile strength of solid particles provided in this paper is proven to a large extent by the reproduction of laboratory experiments and molecular dynamics simulations of water ice, silica, and carbon specimens, irrespective of their crystallinities. While there is still room for improvement, specifically, on the choice of material-dependent parameters, we may conclude that our formulation is a major step forward in the inference of small-bodies’ material and structural properties from observational data on their tensile strengths.