Zhang Zhongtian, Bercovici David
Department of Earth and Planetary Sciences, Yale University, New Haven, CT 06511.
Proc Natl Acad Sci U S A. 2023 Aug 8;120(32):e2221696120. doi: 10.1073/pnas.2221696120. Epub 2023 Jul 31.
Paleomagnetic records of iron meteorites of the IVA group suggest that their parent body (an inward-solidified metal asteroid) possessed an internal magnetic field. The origin of this magnetism is enigmatic because inward solidification typically leads to light element release from the top of the liquid, which depresses convection and dynamo activity. Here, we propose a possible scenario to help resolve this paradox. The formation of a metal asteroid must involve a disruptive, mantle-stripping collision and the reaccretion of metal fragments. We hypothesize that a small portion of metal fragments may have substantially cooled before being reaccreted. These fragments could have formed a cold, rubble-pile inner core, which extracted heat from the liquid layer, leading to solidification and light element expulsion at the inner core boundary to power a dynamo. In the portions of the inward-growing crust that cooled below the remanence acquisition temperature, the magnetic field could be recorded.
IVA 族铁陨石的古地磁记录表明，它们的母体（一个向内凝固的金属小行星）拥有一个内部磁场。这种磁性的起源是个谜，因为向内凝固通常会导致轻元素从液体顶部释放，从而抑制对流和发电机活动。在这里，我们提出一种可能的情况来帮助解决这个悖论。金属小行星的形成必然涉及一次破坏性的、剥离地幔的碰撞以及金属碎片的再积聚。我们假设一小部分金属碎片在被再积聚之前可能已经大幅冷却。这些碎片可能形成了一个寒冷的、碎石堆状的内核，它从液体层中提取热量，导致在内核边界处凝固并排出轻元素，从而为发电机提供动力。在向内生长的地壳中冷却到剩磁获取温度以下的部分，磁场可以被记录下来。
