Why do some asteroids contain hydrated minerals?

Some asteroids contain hydrated minerals primarily due to a process known as aqueous alteration.1

Soon after the formation of their parent bodies, some asteroids experienced modest internal heating. In these environments, temperatures were sufficient to allow liquid water to exist, which reacted with the original minerals to transform them into complex mixtures of hydrated minerals. This process, along with thermal metamorphism, was largely completed within the first 50 million years of the solar system’s history.1

The presence of these minerals is often identified through spectral reflectance measurements. For instance, many C-class asteroids exhibit spectral features associated with iron-bearing hydrous silicates, suggesting they are related to carbonaceous chondrite meteorites that underwent similar hydrothermal alteration. While most asteroids of this type are characterized by these features, some M-class asteroids also paradoxically exhibit spectral signatures of hydrated minerals, despite typically being associated with metallic compositions.23