Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA 02138, USA.
Department of Earth Sciences, Utrecht University, Utrecht 3584 CS, Netherlands.
Sci Adv. 2023 May 24;9(21):eade9071. doi: 10.1126/sciadv.ade9071.
The 4.1-billion-year-old meteorite Allan Hills 84001 (ALH 84001) may preserve a magnetic record of the extinct martian dynamo. However, previous paleomagnetic studies have reported heterogeneous, nonunidirectional magnetization in the meteorite at submillimeter scales, calling into question whether it records a dynamo field. We use the quantum diamond microscope to analyze igneous Fe-sulfides in ALH 84001 that may carry remanence as old as 4.1 billion years (Ga). We find that individual, 100-μm-scale ferromagnetic mineral assemblages are strongly magnetized in two nearly antipodal directions. This suggests that the meteorite recorded strong fields following impact heating at 4.1 to 3.95 Ga, after which at least one further impact heterogeneously remagnetized the meteorite in a nearly antipodal local field. These observations are most simply explained by a reversing martian dynamo that was active until 3.9 Ga, thereby implying a late cessation for the martian dynamo and potentially documenting reversing behavior in a nonterrestrial planetary dynamo.
这块 41 亿年的陨石 Allan Hills 84001（ALH 84001）可能保存了已灭绝的火星磁场发电机的磁场记录。然而，之前的古地磁研究报告称，在亚毫米尺度上陨石的磁化方向存在不均匀、非单一的现象，这对其是否记录了磁场发电机产生的磁场提出了质疑。我们使用量子钻石显微镜分析了可能具有 41 亿年（Ga）之久剩磁的 ALH 84001 中的火成 Fe-硫化物。我们发现，单个 100 微米尺度的铁磁性矿物组合在两个几乎相对的方向上被强烈磁化。这表明，在 41 亿至 39.5 亿年前的撞击加热之后，陨石记录了强磁场，之后至少有一次进一步的撞击以近乎相反的局部磁场使陨石重新磁化。这些观测结果最合理的解释是火星磁场发电机在 39 亿年前仍处于活跃状态，这意味着火星磁场发电机的活动结束较晚，并可能记录了非地行星磁场发电机的反转行为。
