基于重力场、地形和磁场联合分析的月壳磁异常起源研究

Magnetic field data acquired from both lunar surface and orbit show that the Moon's crust possesses many magnetic anomalies, and paleomagnetic data of returned lunar samples suggest that a dynamo could have existed for over a billion years. Regardless, the origin of these crustal anomalies on the Moon remains enigmatic, with someone proposing a thermal remanent magnetic origin concerning an early existed lunar core dynamo, and others suggesting a shock remanent magnetic origin related to impact processes. No clear correlation between vast majority of lunar magnetic anomalies and known geologic structure indicate that the sources of lunar magnetic anomalies likely lie below the surface, but it is not known whether these source are related to basin ejecta, basin impact-melt sheets, ancient lava flows, magmatic intrusion, or magnetized highland materials. ..Here, we propose to study this question by using the magnetic field maps that newly derived from both the Kaguya and Lunar Prospector missions, gravity data derived from GRAIL mission and topography data derived from the laser altimeter that onboard the Lunar Reconnaissance Orbiter mission. Given that the primary magnetic carrier in lunar rocks is metallic iron, which is considerably denser than the silicate minerals that make up the crust, it might be expected that the magnetic field would correlate with gravity. If the magnetic anomalies were formed by impact processes, it might be expected that there would also be a correlation between surface topography and magnetic field strength. By testing whether such correlations exist in the anomaly region and further studying the density, magnetization, and depth of the source materials using the Poisson’s ratio between the magnetic field and gravity, it would be possible to constrain the origin of lunar magnetic anomaly, which would provide crucial information on the evolution of the Moon.

现今月球不存在全球性的偶极磁场，但月球的空间磁测和月面磁测结果发现月壳存在磁异常，同时基于返回月岩样本的古月磁分析认为月球上曾存在着偶极磁场，然而月壳磁异常的起源仍旧未知。一方面月壳中磁载体的来源未知，另一方面磁化月壳的外磁场有可能是月球液核产生偶极磁场，也有可能是撞击过程中产生的瞬时磁场。月球磁异常的起源与月球的形成与演化过程密切相关，是月球科学中一个非常重要的问题。本项目中我们计划利用最新的月壳磁场、地形和重力场数据，对月壳磁异常起源问题进行研究。此前对于月壳中磁载体来源的判断多依赖于月表的地质结构、地貌信息等，本项目中我们将首次通过月壳磁场与重力场／地形之间系统性的相关性研究，对月壳中磁载体的来源进行判断，同时根据重力场与磁场间的泊松关系对磁载体的磁化方向、密度、磁化强度等特性进行研究，从而揭示月壳磁异常的起源，期望能为月球偶极磁场、月球的演化过程等相关问题提供约束。

月球现今不存在内禀性的偶极磁场，但月面磁测与空间磁测数据均表明月壳存在非常强的剩余磁场，也称为磁异常。月壳磁异常的起源与月球的形成演化过程密切相关，一直以来都是月球科学中的一个重要问题。本项目采用高精度的月球磁场、重力场和地形数据，对月球壳磁场的性质进行研究，以回答月壳磁异常起源问题。本项目主要开展了以下研究内容：1）系统性地对月表磁场强度与a)月表地形、b)月表自由空气重力、c)月表布格重力的相关性进行了分析，我们发现月表较弱的磁异常与地形或者重力均不存在显著的相关关系，推断这些磁异常极有可能与月壳是同时形成的，此外我们还发现在月球背面的强磁场区域内，磁场强度与月表地形、月球自由空气重力均呈现正相关，且正相关区域呈现v字型结构，该区域磁异常极有可能与南极艾特肯盆地的撞击有关；2）利用最新的火星磁场数据，对火星表面的磁化深度等进行了研究，发现火星磁化深度存在明显的南北二分性，并推断这个大尺度的结构源于火星早期的Borealis撞击过程，该结果对于火星磁场的形成演化过程以及火星本身的演化过程有重要启示；3）在现有研究内容上进行了拓展，我们也对月球古发电机磁场的产生机制进行了预研，获得了初步的结果。