The far side of the moon offers clues to lunar magnetic field mystery

An unusual rock buried on the far side of the moon is providing fresh clues for a longstanding scientific puzzle: the ancient lunar magnetic field.

Analyzing data from lunar probes, a team of researchers found a rock formation that was much denser and more strongly magnetized compared with the surrounding material. The subterranean feature could be an ancient volcanic complex that became magnetized when the magma rose from the moon’s interior and solidified before reaching the surface, according to a new study published September 23 in the journal Science Advances .

The characteristics of the large rock body, measuring about 60 kilometers (37 miles) wide and around 9 kilometers (5.6 miles) deep, suggest that the moon once had a magnetic field when the rock formed about 4.2 billion years ago. The field was likely between one-fifth and one-third the size of Earth’s own magnetic field currently active today, the study authors noted.

Life on Earth as we know it likely wouldn’t be possible without its magnetic field. It creates the magnetosphere, a vast region in space that surrounds our planet. This celestial shield keeps harmful solar wind, cosmic rays and radiation at bay, preventing the atmosphere from being stripped away.

Michael Osadciw/University of Rochester

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Molten metal constantly churning in Earth’s outer core generates electrical currents that sustain the magnetic field, in a process called the dynamo effect . The mechanical principle is similar to a bicycle dynamo, which uses the bike’s rotating wheel to spin magnets inside the generator and produce an electrical current to power lights.

The moon may also have had a core-powered magnetic field in its past, but that activity has long ceased. About 3.2 billion years ago, as the moon’s interior cooled down, the field significantly weakened, according to NASA. Without strong protection, solar radiation ripped apart the moon’s atmosphere.

The ancient magnetic field’s formation and existence, however, has long been the subject of discussion in astronomy. “The debate stretches back to the 1970s, when the Apollo missions first returned lunar rocks that showed signs of having recorded a magnetic field,” said study coauthor Anna Mittelholz, a lecturer at the Swiss Federal Institute of Technology Zurich, or ETH Zurich.

For a long time, Mittelholz added, experts broadly agreed that the moon’s past dynamo was active from roughly 4.25 billion to 3.5 billion years ago. “This is probably still what the majority of the community believes, but in the last decade, new analyses of Apollo samples have found no magnetic signal at all for parts of that same period,” she wrote in an email, “and some researchers have gone as far as questioning whether an early dynamo existed.”

The new study brings evidence for that early dynamo, but without using samples from the Apollo missions. Instead, researchers used data collected from orbit by lunar probes to analyze a region not visible from Earth called Dewar, where they detected the magnetic rock.

Pinning down the history of the moon’s past magnetic field is key to understanding the moon itself, according to study coauthor Adrien Broquet, a researcher at the German Aerospace Center in Berlin.

“Earth and the moon had drastically different geological histories. The moon was smaller and fully molten for a long time. It also had a lot of volcanism early on, and there are many questions we cannot exactly answer if we don’t know how the moon looked like back then,” Broquet said. “Analyzing the magnetic field of the moon is a good way to estimate how the moon looked like early on.”

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The findings could potentially extend beyond the moon, helping scientists study other celestial bodies, Mittelholz added. If the moon “managed to run a dynamo for any length of time, that tells us something about the minimum conditions needed to sustain a dynamo,” she said. “Getting its history right helps calibrate what we think is possible for other small rocky and icy bodies too.”

The researchers used data from NASA probes, including Lunar Prospector , launched in 1998; Kaguya , launched in 2007; and GRAIL, launched in 2011.

One advantage of using orbital data instead of moon rocks is that the samples were pulled out of their geological context more than 50 years ago, according to Mittelholz.

“Their magnetic record can be disturbed quite easily through heating, shock, or even routine handling and storage in a lab,” she said. “So two samples that look similar can end up giving different answers, not necessarily because the moon’s field wasn’t there, but because the evidence itself has had a lot of chances to get scrambled along the way.”

Researchers have used orbital data before to investigate the lunar magnetic field, but the new study takes a different approach.

“Other magnetic studies using satellite data sets generally look at the magnetic signal by itself and try to infer what kind of rock might have produced it,” Mittelholz said. “We instead combined magnetic data with gravity data in one model.”

Gravity measurements described the density of the unearthed rock, while magnetism informed how strongly it’s magnetized, she explained.

“Together they let us pin down an actual buried structure rather than just a signal that could be produced by many magnetized rock configurations,” she said.

The researchers were unable to estimate how long the active dynamo could have existed and said that it’s unclear how the small lunar core could have generated a magnetic field potentially as strong as Earth, as previous studies have suggested. However, the authors of the new study said they believe that their findings shift the debate from whether the moon had a dynamo to how it worked.

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Earlier studies have pointed to different hypotheses about the formation of the lunar magnetic field. A 2025 paper indicated that an impact from a large asteroid could have temporarily strengthened an existing, weaker magnetic field. A February study based on Apollo samples suggested instead that the melting of rocks rich in titanium deep inside the moon temporarily boosted the satellite’s magnetic field, making it flicker over time.

As part of the new study, the researchers also observed something peculiar about lunar swirls, patches of the moon’s surface that are brighter than their surroundings. The swirls are often paired with magnetic anomalies, or regions where the rock is magnetized, the study found.

“The leading hypothesis is that these magnetic anomalies deflect solar wind and protect the surface from getting weathered by it,” said lead study author Xi Yang, a doctoral student in the department of Earth and planetary sciences at ETH Zurich.

While this observation is only preliminary, Yang added, it could mean that in the future, the swirls may help identify areas of the moon’s surface that are more protected from harmful solar wind, which would be valuable for lunar missions.

The new study further supports the idea that the moon was able to generate an intense dynamo early in its history, according to Claire Nichols, an associate professor of geology of planetary processes at England’s University of Oxford. She did not participate in the new study.

However, a lot more work is needed to understand for how long the moon could generate a dynamo, and how much that dynamo’s activity varied, Nichols added in an email.

“Overall, I think this study provides further motivation to keep adding to our observations during upcoming missions such as Artemis and Chang’e — there are still a lot of open questions about the moon we are yet to figure out,” she said, referring to lunar exploration programs from NASA and the China National Space Administration, respectively.

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The new research also adds to the growing body of evidence that the moon once possessed a stronger magnetic field more than 4 billion years ago, said Isaac Narrett, lead author of the unrelated 2025 study on the lunar magnetic field. Narrett, a doctoral student in the department of Earth, atmospheric and planetary sciences at the Massachusetts Institute of Technology, was not involved in the study.

By using data collected by orbiting lunar probes, Narrett added, the research is leaving room for future work while “laying the groundwork for uncovering clues of planetary evolution with other gravity and magnetic datasets of the moon, Mercury and Mars.”

月球背面埋藏着一块不寻常的岩石，为长期以来困扰科学界的一个谜题——远古月球磁场——提供了新的线索。

通过分析月球探测器的数据，一个研究团队发现了一种岩层，其密度远高于周围物质，磁性也更强。根据9月23日发表在《科学进展》（Science Advances）杂志上的一项新研究，这种地下地貌可能是一个古老的火山群，其岩浆从月球内部上升并在到达地表之前凝固，从而被磁化。

这块巨大的岩石体宽约60公里（37英里），深约9公里（5.6英里），其特征表明，这块岩石形成于大约42亿年前，当时月球曾经拥有磁场。研究作者指出，该磁场的强度可能约为目前地球磁场的五分之一到三分之一。

如果没有地球磁场，我们所知的地球生命很可能无法存在。磁场形成了磁层，这是一片环绕地球的广阔太空区域。这层天体屏障阻挡了有害的太阳风、宇宙射线和辐射，防止大气层被剥离。

迈克尔·奥萨德西夫/罗切斯特大学

五亿多年前，地球上出现了奇特而复杂的生物。科学家们现在认为他们知道原因了。

地球外核中不断翻腾的熔融金属产生电流，从而维持磁场，这一过程被称为发电机效应。其机械原理类似于自行车发电机，自行车发电机利用车轮旋转带动发电机内部的磁铁旋转，产生电流为照明设备供电。

月球过去可能也曾拥有由地核驱动的磁场，但这种活动早已停止。据美国宇航局称，大约32亿年前，随着月球内部冷却，磁场显著减弱。失去了强有力的保护，太阳辐射撕裂了月球大气层。

然而，古代磁场的形成和存在一直是天文学界长期讨论的主题。“这场争论可以追溯到20世纪70年代，当时阿波罗计划首次带回了显示出记录磁场迹象的月球岩石，”该研究的合著者、苏黎世联邦理工学院（ETH Zurich）的讲师安娜·米特尔霍尔茨说。

米特尔霍尔茨补充说，长期以来，专家们普遍认为月球过去的发电机效应活跃于大约42.5亿年前至35亿年前。“这可能仍然是大多数专家的观点，但在过去十年中，对阿波罗计划采集的样本进行的新分析发现，在同一时期的部分时间里，根本没有磁信号，”她在电子邮件中写道，“一些研究人员甚至质疑早期发电机效应是否存在。”

这项新研究为早期发电机的存在提供了证据，但并未采用阿波罗计划带回的样本。研究人员利用月球探测器从轨道上收集的数据，分析了地球上无法直接观测到的杜瓦区域，并在那里探测到了磁性岩石。

该研究的合著者、柏林德国航空航天中心的研究员阿德里安·布罗凯表示，弄清月球过去磁场的历史是了解月球本身的关键。

“地球和月球的地质历史截然不同。月球体积更小，而且在很长一段时间内都处于完全熔融状态。早期的月球火山活动也十分频繁，如果我们不知道月球早期的样子，就无法准确回答许多问题，”布罗凯说。“分析月球磁场是估算月球早期样貌的有效方法。”

Shubhonkar Paramanick/罗切斯特大学

天文学家现在表示，月球正在吸收地球大气层中的分子。

米特尔霍尔茨补充说，这些发现可能不仅限于月球，还能帮助科学家研究其他天体。她说，如果月球“能够维持一段时间的发电机效应，那就告诉我们维持发电机效应所需的最低条件是什么。弄清月球的历史有助于我们校准对其他小型岩质和冰质天体可能存在的现象的认知。”

研究人员使用了来自美国宇航局探测器的数据，包括 1998 年发射的月球勘探者号、2007 年发射的辉夜姬号和 2011 年发射的 GRAIL 号。

米特尔霍尔茨表示，使用轨道数据而不是月球岩石的一个优势是，这些样本是在 50 多年前从其地质背景中取出的。

“它们的磁性记录很容易受到加热、冲击，甚至实验室中的日常处理和储存的影响，”她说。“因此，两个看起来相似的样本最终可能会得出不同的结果，这并不一定是因为月球磁场不存在，而是因为证据本身在保存过程中有很多机会被破坏。”

研究人员以前曾利用轨道数据研究月球磁场，但这项新研究采用了不同的方法。

米特尔霍尔茨说：“其他利用卫星数据集进行的磁学研究通常只关注磁信号本身，并试图推断产生该信号的岩石类型。而我们则将磁数据与重力数据结合在一个模型中。”

她解释说，重力测量描述了出土岩石的密度，而磁性测量则表明了它的磁化强度。

“它们共同帮助我们确定了一个实际的埋藏结构，而不仅仅是许多磁性岩石结构可能产生的信号，”她说。

研究人员无法估算出活跃的月球发电机可能存在了多久，并表示，目前尚不清楚月球的微小内核如何能产生强度堪比地球的磁场，尽管此前的研究表明月球可能存在发电机。然而，这项新研究的作者表示，他们相信他们的发现将争论的焦点从月球是否存在月球发电机转移到了月球发电机的运作机制上。

月球上曾经存在过生命吗？

早期的研究提出了关于月球磁场形成的不同假设。2025年的一篇论文指出，一颗大型小行星的撞击可能暂时增强了原本较弱的月球磁场。而今年2月一项基于阿波罗计划采集样本的研究则提出，月球深处富含钛的岩石熔化暂时增强了月球磁场，使其随时间推移而出现闪烁。

作为这项新研究的一部分，研究人员还观察到月球旋涡状区域的一些奇特现象。这些旋涡状区域是月球表面比周围区域更亮的斑块。研究发现，这些旋涡状区域通常与磁异常（即岩石被磁化的区域）同时出现。

“目前最主要的假设是，这些磁异常会偏转太阳风，保护地表免受太阳风的侵蚀，”该研究的主要作者、苏黎世联邦理工学院地球与行星科学系博士生杨曦说。

杨补充说，虽然这一观察结果只是初步的，但这可能意味着，未来这些旋涡可以帮助识别月球表面哪些区域更能免受有害太阳风的侵袭，这对月球任务来说将很有价值。

据英国牛津大学行星过程地质学副教授克莱尔·尼科尔斯（Claire Nichols）称，这项新研究进一步支持了月球在其早期历史中能够产生强烈发电机效应的观点。她并未参与这项新研究。

尼科尔斯在一封电子邮件中补充说，然而，要了解月球能够产生发电机持续多久，以及该发电机的活动变化有多大，还需要做更多的工作。

“总的来说，我认为这项研究进一步激励我们在即将到来的阿尔忒弥斯和嫦娥等任务中继续增加观测数据——关于月球，我们还有很多未解之谜，”她说道，这里分别指的是美国宇航局和中国国家航天局的月球探测计划。

美国国家航空航天局/戈达德太空飞行中心/亚利桑那州立大学

科学家称，他们已追踪到一颗可能对地球构成威胁的近地小行星的起源，它位于月球背面。

这项新研究进一步证实了月球在40多亿年前曾拥有更强的磁场，这是与此无关的2025年月球磁场研究的主要作者艾萨克·纳雷特（Isaac Narrett）的观点。纳雷特是麻省理工学院地球、大气与行星科学系的博士生，他并未参与这项研究。

纳雷特补充说，通过使用绕月探测器收集的数据，这项研究为未来的工作留下了空间，同时“为利用月球、水星和火星的其他重力和磁场数据集来揭示行星演化的线索奠定了基础”。