月球上火山活动何时停止？曾经的岩浆活动如何维持？月球内部有多少水？10月19日，中国科学院发布了嫦娥五号月球科研样品最新研究成果，对上述问题进行了解答。

 

An analysis of moon rocks brought back to Earth by China's Chang'e 5 mission suggests the samples are a new type of lunar basalt, different from those collected during previous Apollo and Luna missions.

研究发现，嫦娥五号月球样品为一类新的月球玄武岩，不同于美国阿波罗和苏联月球系列采样任务返回的月球样品。

 

月球直到20亿年前仍存在岩浆活动

 

Previous radioisotope dating of lunar samples brought back by the Apollo and Luna missions suggested that most lunar volcanic¹ activities ceased by around 2.9 billion or 2.8 billion years ago. Those samples were collected within 30 degrees of the moon's equator and could not represent the large lunar surface.

以往对美国、苏联的月球样本的放射性同位素定年研究显示，月球的岩浆活动至少持续到大约28亿至29亿年前。这些样品都是在月球赤道附近30度的范围内采集的，不能代表整个月球表面的情况。

 

After studying the isotopes² of the rocks brought back by Chang'e 5, scientists confirmed that the basalts came from the same eruption³ lava⁴, which occurred around 2 billion years ago, delaying the end of the Moon's volcanic activity by 800 million to 900 million years.

研究人员对嫦娥五号带回的样品进行同位素研究后证实，这些玄武岩来自于约20亿年前的岩浆活动，比以往月球样品限定的岩浆活动停止时间晚了约8亿—9亿年。

 

月球冷却慢的两种解释都被排除

 

为什么月球冷却如此之慢，目前科学界存在两种可能的解释：岩浆源区中富含放射性元素以提供热源（a lunar mantle⁵ rich in radioactive elements to provide a heat source），或富含水以降低月幔熔点（a lunar mantle rich in water to lower the melting point）。

 

Scientists discovered previously⁶ that the volcanically⁷ active region on the moon was rich in some specific heat-producing elements－potassium, rare earths and phosphorus. The elements were believed to be associated with the near side's volcanism by providing a heat source.

研究人员此前发现，月球上的火山活跃区域富含钾、稀土元素、磷的生热元素。一般认为，这些元素能够产生热源，与月球正面的火山活动相关。

 

But researchers found that the high content of those elements in the samples collected by Chang'e 5 did not originate in the moon's mantle, where lava would ordinarily have been formed. The previous studies used remote sensing techniques without returned samples, meaning the deeper composition could not be detected.

但是研究人员发现，嫦娥五号月球样品富集的这些元素并不是月幔熔岩形成时产生的。之前的研究采用遥感技术勘测，没有返回样本，月幔深层次的成分无法勘测。

 

这一结果否定了初始岩浆熔融热源来自放射性生热元素的主流假说，揭示了月球晚期岩浆活动过程。

 

对于月球岩浆是否富含水，科研团队也进行了分析。

 

The recent analysis of the water composition shows that the deep lunar mantle of the sampling area was much drier 2 billion years ago than that of older regions with volcanism.

此次关于水含量的研究显示，采样区所在的20亿年前的月幔源区比更早时期的火山活动区域更干燥。

 

这一发现也排除了月幔富水（the rich content of water in the lunar mantle）而具有低熔点，导致该区域岩浆活动持续时间异常延长的猜想。

 

专家表示，新的研究成果为未来月球的探测和研究开辟了新的方向。

 

中科院院士、地质地球所研究员李献华表示：

 

The above results raise new questions on the study of the thermal⁸ evolution of the moon.

上述研究成果对月球热演化的研究提出了新问题。

 

It is still unclear why the moon cooled down so slowly, and it will require a new theoretical framework and evolutionary⁹ model to provide new directions for future lunar exploration and research.

月球冷却如此之慢的原因并不清楚，需要全新的理论框架和演化模型，对未来的月球探测和研究提出了新的方向。

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