11:45 AM - 12:00 PM
[PPS03-11] Effects of refractory condensates piling up outside the Jovian gap in the protosolar disk on the isotopic dichotomy of the solar system
Keywords:Solar system, Meteorite , Isotopic dichotomy
Recent isotopic analyses of meteorites and returned samples of asteroids have revealed an isotopic dichotomy broadly classifying them into non-carbonaceous (NC) and carbonaceous (CC) groups (Warren, 2011). This dichotomy implies that a heterogeneous distribution of presolar grains was preserved in the primordial solar nebula, thereby imparting distinct stellar nucleosynthetic signatures to the forming bodies (Nittler et al., 2018). The evolution of these isotopic variations over time may reflect dynamic processes such as dust evolution and material transport in the solar nebula. Thus, understanding this isotopic dichotomy is crucial for constraining the formation conditions and accretion histories of planetesimals during solar system formation.
One leading hypothesis for the isotopic dichotomy proposes that proto-Jupiter formed early in the solar nebula, creating a gap that inhibited mixing between NC and CC materials (e.g., Kruijer et al., 2017). However, the ε54Cr values of NC meteorites approach those of CC meteorites as the accretion age of their parent bodies after CAI formation increases (Sugiura & Fujiya, 2014), suggesting that some degree of NC-CC mixing occurred. To investigate this possibility, Homma et al. (2024) simulated the evolution of the size distribution and ε54Cr isotopic anomaly of dust aggregates due to mutual collisions and radial transport in the solar nebula with an early-formed Jupiter. Their simulations suggest that small collisional fragments of the aggregates leaked to NC region across the Jovian gap, explaining the observed correlation between ε54Cr values and the accretion ages of NC meteorites.
However, the model of Homma et al. (2024) does not yet explain the Cr-Ti isotopic dichotomy, as it does not account for the carriers of the 50Ti isotopic anomaly. In this study, we extend their work by incorporating calcium- and aluminum-rich inclusions (CAIs) as 50Ti carriers (Trinquier et al., 2009). We hypothesize that variations in the abundance of these refractory materials across the Jovian gap led to the Cr-Ti isotopic dichotomy (e.g., Hellmann et al. 2023). Our model calculates the radial transport of CAIs and "dust," the latter corresponding to the matrix in chondrites. We assume that dust grains underwent coagulation and fragmentation, whereas CAIs, due to their coarse-grained nature, did not. The CAIs are assumed to have a size distribution similar to those found in CV and CK meteorites (Charnoz et al., 2015). We calculate the mixing ratios of these solids and the resulting ε54Cr-ε50Ti values of the mixtures inside and outside the Jovian gap, neglecting the removal of CAIs and dust from the NC and CC reservoirs due to planetesimal formation.
Our simulations show that the Cr-Ti isotopic compositions inside and outside the gap can partially reproduce the trends observed in NC and CC meteorites, respectively, when the dust grains are assumed to be poorly sticky, with a fragmentation threshold velocity of 1 m/s. Low dust stickiness is crucial because it keeps dust grains small rather than forming large aggregates. Poor stickiness leads to fragmentation, preserving fine particles that can more easily cross the proto-Jupiter gap and promote material mixing. This mixing leads to increased homogenization of isotopic signatures, causing NC material to gradually align with CC values, which explains why later-accreting parent bodies show NC meteorites with ε54Cr values approaching those of CC meteorites.
Our model does not yet reproduce the decreasing trend of ε50Ti with increasing ε54Cr. We speculate that this trend resulted from the combined effects of planetesimal formation and the differential settling of dust and CAIs. Assuming that dust grains are poorly sticky, CAIs are larger, settle more significantly toward the midplane, and are more likely to be incorporated into planetesimals than dust aggregates.
One leading hypothesis for the isotopic dichotomy proposes that proto-Jupiter formed early in the solar nebula, creating a gap that inhibited mixing between NC and CC materials (e.g., Kruijer et al., 2017). However, the ε54Cr values of NC meteorites approach those of CC meteorites as the accretion age of their parent bodies after CAI formation increases (Sugiura & Fujiya, 2014), suggesting that some degree of NC-CC mixing occurred. To investigate this possibility, Homma et al. (2024) simulated the evolution of the size distribution and ε54Cr isotopic anomaly of dust aggregates due to mutual collisions and radial transport in the solar nebula with an early-formed Jupiter. Their simulations suggest that small collisional fragments of the aggregates leaked to NC region across the Jovian gap, explaining the observed correlation between ε54Cr values and the accretion ages of NC meteorites.
However, the model of Homma et al. (2024) does not yet explain the Cr-Ti isotopic dichotomy, as it does not account for the carriers of the 50Ti isotopic anomaly. In this study, we extend their work by incorporating calcium- and aluminum-rich inclusions (CAIs) as 50Ti carriers (Trinquier et al., 2009). We hypothesize that variations in the abundance of these refractory materials across the Jovian gap led to the Cr-Ti isotopic dichotomy (e.g., Hellmann et al. 2023). Our model calculates the radial transport of CAIs and "dust," the latter corresponding to the matrix in chondrites. We assume that dust grains underwent coagulation and fragmentation, whereas CAIs, due to their coarse-grained nature, did not. The CAIs are assumed to have a size distribution similar to those found in CV and CK meteorites (Charnoz et al., 2015). We calculate the mixing ratios of these solids and the resulting ε54Cr-ε50Ti values of the mixtures inside and outside the Jovian gap, neglecting the removal of CAIs and dust from the NC and CC reservoirs due to planetesimal formation.
Our simulations show that the Cr-Ti isotopic compositions inside and outside the gap can partially reproduce the trends observed in NC and CC meteorites, respectively, when the dust grains are assumed to be poorly sticky, with a fragmentation threshold velocity of 1 m/s. Low dust stickiness is crucial because it keeps dust grains small rather than forming large aggregates. Poor stickiness leads to fragmentation, preserving fine particles that can more easily cross the proto-Jupiter gap and promote material mixing. This mixing leads to increased homogenization of isotopic signatures, causing NC material to gradually align with CC values, which explains why later-accreting parent bodies show NC meteorites with ε54Cr values approaching those of CC meteorites.
Our model does not yet reproduce the decreasing trend of ε50Ti with increasing ε54Cr. We speculate that this trend resulted from the combined effects of planetesimal formation and the differential settling of dust and CAIs. Assuming that dust grains are poorly sticky, CAIs are larger, settle more significantly toward the midplane, and are more likely to be incorporated into planetesimals than dust aggregates.