Why is the moon's far side so weird? China's lunar sample-return mission may have figured it out

为什么月球的背面如此怪异？中国的月球采样返回任务可能已经弄明白了

The origin of the strange differences between the near and far sides of the moon are a step closer to being solved, thanks to new findings from China's Chang'e 6 mission that returned samples from the South Pole-Aitken (SPA) basin.

由于中国嫦娥六号任务从南极-艾特肯（SPA）盆地返回样本的新发现，月球近端和远端之间奇怪差异的起源离解决又近了一步。

The near side of the moon is familiar to us as the only side that we can see from Earth. Dark regions called maria are vast lava plains filling lowland impact basins, and give us the pattern of the "man in the moon." Yet the far side of the moon, which can only be seen by orbiting spacecraft, has barely any maria and is covered by craggy, cratered and ancient highlands. That's not the only difference between the two hemispheres; the thickness of the moon's crust is thinner on the near side, volcanic activity appears to have occurred at different points in time, and the mantle beneath the far side seems heavily depleted in certain elements compared to the near side.

我们熟悉月球的近侧，因为它是我们从地球上唯一能看到的一面。被称为玛丽亚的黑暗区域是广阔的熔岩平原，充满了低地撞击盆地，给了我们“月球上的人”的模式。然而，只有轨道航天器才能看到的月球背面几乎没有任何玛丽亚，被崎岖、坑坑洼洼和古老的高地覆盖。这并不是两个半球之间的唯一区别；月球近侧地壳的厚度较薄，火山活动似乎发生在不同的时间点，与近侧相比，远侧下方的地幔似乎严重缺乏某些元素。

However, while we have many samples from the lunar near side, particularly those brought back to Earth by the Apollo missions, the Soviet Luna missions and China's own Chang'e 5, we had nothing from the far side with which to test theories.

然而，虽然我们有许多来自月球近侧的样本，特别是那些由阿波罗任务、苏联月球任务和中国自己的嫦娥五号带回地球的样本，但我们没有来自月球远侧的任何东西来测试理论。

Then, in June 2024, China's Chang'e 6 mission landed in the SPA basin and brought back samples totaling 1,935.3 grams (68.27 ounces).

随后，2024年6月，中国嫦娥六号任务在温泉盆地着陆，带回了总计1935.3克（68.27盎司）的样本。

The SPA basin is the largest impact site on the moon, spanning 2,500 kilometers (1,600 miles) and extends from the lunar south pole and onto the far side of the moon. It's also the oldest known impact feature on the moon, with an age of 4.25 billion years. What impact — pardon the pun — could the sheer violence of the SPA basin's formation have had on lunar geology and the thermal evolution of the moon's interior? Could it have caused the dichotomy between the moon's two hemispheres?

斯帕盆地是月球上最大的撞击点，跨度2500公里（1600英里），从月球南极延伸到月球背面。这也是月球上已知最古老的撞击特征，有42.5亿年的历史。原谅我的双关语，温泉盆地形成的纯粹暴力会对月球地质学和月球内部的热演化产生什么影响？它会导致月球两个半球之间的二分吗？

Chang'e 6's samples are the first from the lunar far side, and have offered a unique opportunity to test models that could potentially explain the difference between the moon's two hemispheres.

嫦娥六号的样本是第一个来自月球背面的样本，为测试可能解释月球两个半球之间差异的模型提供了一个独特的机会。

Consequently, following analysis of the samples, researchers led by a team from the Chinese Academy of Sciences have announced four major discoveries.

因此，在对样本进行分析后，由中国科学院团队领导的研究人员宣布了四项重大发现。

The first is that the samples contain volcanic rocks called basalts that date to prolonged volcanic outbursts on the lunar far side in two distinct phases, one around 4.2 billion years ago and another 2.8 billion years ago.

首先，这些样本含有被称为玄武岩的火山岩，可以追溯到月球背面两个不同阶段的长期火山爆发，一个大约在42亿年前，另一个在28亿年前。

"We propose that the 4.2-billion-year-old basalt was associated with the formation of the SPA basin because it is a high-aluminum basalt, requiring the incorporation of crustal plagioclase into its source," Wei Yang, a professor at the Institute of Geology and Geophysics at the Chinese Academy of Sciences, told Space.com in an email interview. Meanwhile, "the 2.8-billion-year-old basalts originated from the deep mantle, the product of the early stage crystallization of the lunar magma ocean."

“我们认为，42亿年前的玄武岩与SPA盆地的形成有关，因为它是一种高铝玄武岩，需要将地壳斜长石纳入其来源，”该研究所教授杨伟说。中国科学院地质与地球物理研究所在电子邮件采访中告诉Space.com。同时，“28亿年前的玄武岩起源于地幔深处，是月球岩浆海洋早期结晶的产物。”

The evolution of this lunar magma ocean that formed the moon's mantle is central to the next discovery, which is that geochemical analysis of the basalt samples points to a source in the lunar mantle deep below ground that is heavily depleted in particular elements such as thorium. It is unknown whether this depleted mantle is found only beneath the SPA basin, or whether it is more extensive across the moon.

形成月球地幔的月球岩浆海洋的演化是下一个发现的核心，即玄武岩样本的地球化学分析指出，月球地幔深处的一个来源，该来源严重耗尽了钍等特定元素。目前还不知道这种枯竭的地幔是只在温泉盆地下面发现的，还是在整个月球上更广泛。

"To be honest, we don't know," said Yang.

“说实话，我们不知道，”杨说。

One possibility is that the moon has had this depleted mantle since birth, in which case both the near and far sides should share this composition. The other possibility is that it was produced after the lunar magma ocean formed and began to crystallize.

一种可能性是，月球自诞生以来就有这种耗尽的地幔，在这种情况下，近侧和远侧都应该共享这种成分。另一种可能是在月球岩浆海洋形成并开始结晶后产生的。

"Personally, I am more inclined to believe the latter, given that such a massive impact has the potential to affect the mantle down to a depth of 250 kilometers [155 miles]," said Yang. "If it is only present in the SPA basin, then it must have formed as a result of the SPA impact. To figure this out, we need to collect more samples from the moon's far side, particularly from areas outside the SPA."

“就我个人而言，我更倾向于相信后者，因为如此巨大的撞击有可能影响250公里（155英里）深度的地幔，”杨说。“如果它只存在于SPA盆地，那么它一定是SPA撞击的结果。为了弄清楚这一点，我们需要从月球背面收集更多样本，特别是SPA以外的区域。”

The third discovery is of something we wouldn't expect to find on the moon: water. However, we are talking parts-per-million here — the Apollo samples were considered "bone dry," and the far side mantle seems to be even drier than that.

第三个发现是我们没想到会在月球上发现的东西：水。然而，我们在这里谈论的是百万分之一——阿波罗的样本被认为是“非常干燥的”，而远处的地幔似乎比这更干燥。

"The water content of this mantle is lower than those of the mantle sources of all the basalts from the near side," said Yang.

“这个地幔的含水量低于所有近侧玄武岩的地幔源的含水量，”杨说。

The final discovery relates to the moon's magnetic field. Earth's natural satellite currently doesn't have a global magnetic field, and traces of magnetism remain only in a handful of anomalous patches called lunar swirls. However, in the distant past it did have a global magnetic field. The Chang'e 6 samples retain a record of it, and show that the magnetic field, after decaying for a time, rebounded in strength about 2.8 billion years ago. This indicates that the moon's internal dynamo fluctuated, possibly episodically, rather than just experiencing a slow but gradual decline.

最后的发现与月球磁场有关。地球的天然卫星目前没有全球磁场，磁性的痕迹只保留在少数被称为月球漩涡的异常斑块中。然而，在遥远的过去，它确实有一个全球磁场。嫦娥六号的样本保留了它的记录，并显示磁场在衰减一段时间后，在大约28亿年前反弹。这表明月球的内部发电机波动，可能是间歇性的，而不仅仅是经历缓慢但逐渐的下降。

The timing coincides with the second phase of volcanism on the far-side.

这个时间与另一边火山活动的第二阶段相吻合。

"The magnetic field rebounded 2.8 billion years ago, which suggests that the interior of the moon still contained a lot of energy," said Yang. "Perhaps convection and the upward flow of hot material existed in the lunar mantle at that time."

“磁场在28亿年前反弹，这表明月球内部仍然包含大量能量，”杨说。“也许当时月球地幔中存在对流和热物质的向上流动。”

Not only could this upward flow have triggered volcanic eruptions, it could have been enough to vaporize much of the water in the mantle, drying it out.

这种向上的流动不仅可能引发火山爆发，还可能足以蒸发地幔中的大部分水，使其变干。

If the creation of the SPA basin is the cause of much of this, then it has repercussions that go far beyond the moon. Other giant impact features are seen on other bodies, particularly on Mercury and Mars. We may have underestimated the role that these giant impacts played on the evolution of the planets' interiors.

如果温泉盆地的形成是造成这种情况的原因，那么它的影响远远超出了月球。在其他天体上也可以看到其他巨大的撞击特征，尤其是水星和火星。我们可能低估了这些巨大的撞击对行星内部演化的作用。

The Chang'e 6 results were presented in four papers (1, 2, 3, 4) that were published recently in the journal Nature.

嫦娥六号的结果发表在最近发表在《自然》杂志上的四篇论文（1、2、3、4）中。