The surfaces of airless bodies such as lunar and S-type asteroids typically become spectrally redder in visible to near-infrared reflectance with longer exposures to space weathering. However, some carbonaceous asteroids instead become spectrally bluer. Space weathering experiments on carbonaceous meteorites have provided some clues as to the space weathering products that could produce spectral bluing. We applied these experimental results to our Hapke radiative transfer model, with which we modeled spectral data from the OSIRIS-REx mission in order to determine whether these space weathering products—specifically, nanophase and microphase metallic iron, troilite, and magnetite—could explain the globally blue spectrum of the carbonaceous asteroid (101955) Bennu. The model suggests that the surface of Bennu has microphase iron, nanophase magnetite, and nanophase and microphase troilite. Considering previous space weathering experiments together with our spectral modeling of Bennu, we posit that the presence of nanophase magnetite is what causes a carbonaceous asteroid to become spectrally bluer with exposure time. Nanophase magnetite can form on asteroids that have Fe-bearing hydrated minerals (phyllosilicates). On anhydrous carbonaceous asteroids, nanophase iron forms instead of magnetite, leading to spectral reddening. We therefore predict that samples returned by the OSIRIS-REx mission from Bennu will have more nanophase magnetite than nanophase iron with nanophase and microphase sulfides, whereas samples returned by the Hayabusa2 mission from the carbonaceous asteroid (162173) Ryugu, which is spectrally red, will contain nanophase and microphase sulfides as well as more nanophase iron than nanophase magnetite.

无空气天体（例如月球和 S 型小行星）的表面通常在可见光到近红外反射率下变得更红，并且暴露在太空风化中的时间更长。然而，一些碳质小行星反而变得光谱更蓝。碳质陨石的空间风化实验为可能产生光谱蓝化的空间风化产物提供了一些线索。我们将这些实验结果应用于我们的 Hapke 辐射传输模型，我们利用该模型对来自 OSIRIS-REx 任务的光谱数据进行建模，以确定这些空间风化产物——特别是纳米相和微相金属铁、硫钛矿和磁铁矿——是否可以解释碳质小行星 (101955) Bennu 的全球蓝色光谱。模型表明 Bennu 表面有微相铁、纳米相磁铁矿、和纳米相和微相陨硫铁。考虑到之前的空间风化实验以及我们对 Bennu 的光谱建模，我们假设纳米相磁铁矿的存在是导致碳质小行星随着暴露时间变得光谱更蓝的原因。纳米相磁铁矿可以在具有含铁水合矿物（页硅酸盐）的小行星上形成。在无水碳质小行星上，形成纳米相铁而不是磁铁矿，导致光谱变红。因此，我们预测，从 Bennu 的 OSIRIS-REx 任务返回的样品将比具有纳米相和微相硫化物的纳米相铁含有更多的纳米相磁铁矿，而 Hayabusa2 任务从碳质小行星 (162173) Ryugu 返回的样品具有光谱红色，