Did Impact-Generated Fields Magnetize the Rocks of Vredefort?

行星科学中一个尚未解决的关键问题是，在陨石和月球岩石中观察到的磁化是由母体的金属核心内部产生的，还是由外部来源产生的。特别是，与月球上的陨石坑相关的大型地壳磁场的发现，导致了大规模撞击可能产生磁场的猜测。随后的实验研究表明，强磁场是由电浆云中的电流产生的。如果我们希望利用地外样本来了解内部产生的磁场和其他天体上金属核的演变，我们必须确定撞击磁化是否是太阳系中的一个常见过程。测试撞击是否会产生磁场的最好方法之一是检查地球上陨石坑的岩石。该小组成员最近假设，南非20.2亿年前的弗里德堡陨石坑的岩石磁化也源于撞击产生的等离子体（Carporzen et al. 2005）。他们发现，这些岩石的磁化强度非常强，在几十公里到不到几厘米的空间尺度上定向不均匀。他们将解决三个重要的未回答的问题：弗里德堡撞击是否产生了磁场？如果是的话，磁场的强度是多少？问题2。任何撞击产生的磁场的特征长度尺度是什么？问题3。承载撞击产生的磁化的主要磁性矿物是什么？这些问题的答案对类地行星的内部结构、体积组成和热历史具有重要意义。研究人员计划在弗里德堡撞击构造的基底岩石和沉积物中钻四个10米深的岩心，以获得可能记录撞击事件的未改变样本。因为这些样品应该没有闪电再磁化，所以PIs可以测试一个持续的批评，即弗里德堡岩石中的磁化不是来自撞击，而是来自最近的雷击。对岩心的分析将在麻省理工学院古地磁实验室进行，使用2G超导岩石磁力计（空间分辨率约为10毫米）和SQUID显微镜（空间分辨率为0.1毫米）。这笔拨款的更广泛影响是，它将支持劳伦特·卡波岑在麻省理工学院的博士后培训，以及麻省理工学院研究生和南非地质学家在南非的实地考察。

A key unresolved question in planetary science is whether the magnetization observed in meteorites and lunar rocks was generated internally in the metallic cores of the parent bodies or was instead produced by an external source. In particular, the discovery of large crustal magnetic fields associated with craters on the Moon has led to the speculation that large impacts could generate magnetic fields. Subsequent experimental studies have shown that strong magnetic fields are produced by electric currents in plasma clouds. If we wish to use extraterrestrial samples to learn about internally generated fields and the evolution of metallic cores on other bodies, it is essential that we determine whether impact-magnetization is a common process in the solar system.One of the best ways to test whether impacts can generate fields is to examine rocks from impact craters on Earth. Members of the team have recently hypothesized that magnetization in the rocks of the 2.02 billion year old Vredefort crater in South Africa also originated from impact-produced plasmas (Carporzen et al. 2005). They found that magnetization in these rocks is extremely intense and heterogeneously oriented on spatial scales ranging from tens of kilometers down to less than a few centimeters. They will address three important unanswered questions:Question 1. Were magnetic fields generated by the Vredefort impact and if so what were their intensities?Question 2. What was the characteristic length scales of any impact-generated magnetic fields?Question 3. What is the dominant magnetic mineral that carries impact-generated magnetization?The answers to these questions have critical implications for the interior structures, bulk compositions, and thermal histories of the terrestrial planets.The investigators plan to drill four 10-m deep cores into the basement rocks and sediments of the Vredefort impact structure in order to obtain unaltered samples that are likely to record the impact event. Because these samples should be free of lightning remagnetization, the PIs can test a persistent criticism that magnetization in Vredefort rocks is not from the impact but rather from recent lightning strikes. Analysis of the drill cores will be conducted in the MIT Paleomagnetism Laboratory using a 2G Superconducting Rock Magnetometer (spatial resolution of ~10 mm) and the SQUID Microscope (spatial resolution of 0.1 mm). The broader impact of this grant is that it would support the postdoctoral training of Laurent Carporzen at MIT as well as fieldwork in South Africa with MIT graduate students and South African geologists.