The surface of Mercury is enriched in carbon compared with the other terrestrial bodies. Up to 4 wt% of carbon in the form of graphite has been invoked to explain its mysteriously low surface reflectance. However, the exact abundance and phase of carbon on Mercury were loosely constrained by orbital observations. Here we show that the oldest and darkest colour units on Mercury, comprising low-reflectance materials, consist of two spatially coherent subunits that do and do not have spectral characteristics attributed to graphite enrichment, respectively. We find from spectral modelling that a combination of less than 1 wt% of microcrystalline graphite and similar amounts of metallic iron is adequate for explaining the overall reflectances of various colour units on Mercury. Our results indicate that most carbon on Mercury may occur in forms other than intergrain graphite and that carbon did not entirely drain from the mantle during magma ocean crystallization.

与其他类地天体相比，水星表面的碳含量丰富。高达 4 wt% 的石墨形式的碳被用来解释其神秘的低表面反射率。然而，水星上碳的确切丰度和相位受到轨道观测的松散限制。在这里，我们展示了水星上最古老和最暗的颜色单元，由低反射率材料组成，由两个空间相干的子单元组成，这两个子单元分别具有和不具有归因于石墨富集的光谱特征。我们从光谱模型中发现，少于 1 wt% 的微晶石墨和类似数量的金属铁的组合足以解释水星上各种颜色单位的整体反射率。我们的结果表明，水星上的大多数碳可能以晶间石墨以外的形式存在，并且在岩浆海洋结晶过程中，碳并未完全从地幔中排出。