Abundant nanophase iron (np-Fe⁰) is ubiquitously formed on the surface of the Moon and other airless bodies through space weathering processes, and plays a dominant role in transforming the optical properties of the lunar surface. The main sources of np-Fe⁰ are usually considered to be evaporative deposition or ion-reduction processes. Here we show that disproportionation reactions triggered by micrometeorite impacts can be the main contributor to np-Fe⁰ formation. We measured the valence states of iron in the microcraters on a fine-grained Chang’e-5 sample (number CE5C0200YJFM00302) and found the presence of np-Fe⁰ and associated Fe³⁺ in the amorphous mixture of olivine, which can be explained by the disproportionation reaction of Fe²⁺ during microimpacts. The chemical composition of the residual impactor suggested that it was formed by a secondary low-velocity impact on lunar anorthite. As the whole process was dominated by impact events without the contribution from the solar wind, these findings inform us on weathering mechanisms on regions or bodies that do not experience a strong solar wind component, such as permanently shadowed areas or outer Solar System bodies.

丰富的纳米相铁（np-Fe ⁰）通过空间风化过程普遍存在于月球和其他无空气物体的表面，并在改变月球表面的光学特性中起主导作用。np-Fe ⁰的主要来源通常被认为是蒸发沉积或离子还原过程。在这里，我们表明由微陨石撞击引发的歧化反应可能是 np-Fe ⁰形成的主要贡献者。我们在细粒嫦娥五号样品（编号 CE5C0200YJFM00302）上测量了微坑中铁的价态，发现存在 np-Fe ⁰和相关的 Fe ³⁺在橄榄石的无定形混合物中，这可以通过微冲击过程中Fe ²⁺的歧化反应来解释。残留撞击物的化学成分表明它是由对月球钙长石的二次低速撞击形成的。由于整个过程由没有太阳风贡献的撞击事件主导，这些发现为我们提供了关于没有经历强烈太阳风成分的区域或天体的风化机制，例如永久阴影区域或太阳系外天体。