龙宫母星子在CO$_2$雪线之外由小卵石形成：来自热演化模型的见解
Formation of Ryugu's parent planetesimal beyond the CO$_{2}$ snow line from small pebbles: insights from thermal evolution modeling
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中文总结 AI 辅助
通过热演化模型约束龙宫母星子在CO$_2$雪线外由小卵石形成，发现其吸积年龄早于多数碳质球粒陨石母星子，核心卵石尺寸仅几毫米，解释了样品中无球粒陨石的现象。
中文摘要 AI 辅助
对行星形成环星盘的天文观测表明，星子由0.1毫米至1厘米大小的尘埃聚集体（通常称为“卵石”）形成。当这些卵石在水雪线之外吸积时，所产生的冰质星子会发生水-岩石分异，并形成多孔的卵石堆积核心，其孔隙被液态水饱和。水的循环增强了核心中的热传输，抑制了由放射性核素衰变引起的温度升高。在本研究中，我们通过模拟冰质星子的热演化，并将结果与小行星龙宫返回样品中识别出的水成矿物的沉淀年龄和温度进行比较，来约束龙宫母星子的吸积年龄和组成卵石的大小。我们的数值结果表明，龙宫母星子在钙铝 inclusions（钙铝难熔包体）形成后200万年内吸积。推断出的早期吸积年龄支持了龙宫母星子比大多数含球粒陨石的碳质球粒陨石母星子形成更早的假说，这可能解释了龙宫样品中缺乏球粒陨石的原因。我们还发现，母星子的核心由不超过几毫米大小的卵石组成。这种小卵石尺寸与在CO$_2$雪线之外形成的星子的理论预测一致。
英文摘要
Astronomical observations of planet-forming circumstellar disks indicate that planetesimals form from 0.1-mm- to 1-cm-sized dust aggregates, commonly called ``pebbles.'' When such pebbles accrete beyond the water snow line, the resulting icy planetesimals undergo water--rock differentiation and develop porous pebble-pile cores whose voids are saturated with liquid water. Circulation of this water enhances heat transport in the core, suppressing the temperature rise caused by the decay of radionuclides. In this study, we constrain the accretion age and constituent pebble size of Ryugu's parent planetesimal by modeling the thermal evolution of icy planetesimals and comparing the results with the precipitation ages and temperatures of aqueously formed minerals identified in samples returned from asteroid Ryugu. Our numerical results suggest that Ryugu's parent planetesimal accreted within 2.0 Myr of the formation of calcium--aluminum-rich inclusions. The inferred early accretion age supports the hypothesis that Ryugu's parent planetesimal formed earlier than most chondrule-bearing carbonaceous chondrite parent planetesimals, potentially explaining the absence of chondrules in Ryugu samples. We also found that the core of the parent planetesimal was composed of pebbles no larger than a few millimeters. Such small pebble sizes are consistent with theoretical predictions for planetesimals formed beyond the CO$_{2}$ snow line.
发表机构
- Japan Agency for Marine-Earth Science and Technology（日本海洋地球科学技术局）
- Earth-Life Science Institute, Institute of Science Tokyo（东京科学研究所生命理工学研究院）
- Department of Earth and Planetary Sciences, Hokkaido University（北海道大学地球行星科学系）
- Faculty of Science, Ibaraki University（茨城大学理学部）
- School of Science, Institute of Science Tokyo（东京科学研究所理学院）
- Department of Earth, Atmospheric, and Planetary Sciences, Purdue University（普渡大学地球大气与行星科学系）
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