What is aqueous alteration in meteorites?
Aqueous alteration is the change of minerals by liquid water inside asteroids, which turned olivine, pyroxene and metal into clays, carbonates and other water-bearing minerals.
Many carbonaceous chondrites show clear evidence that liquid water once flowed through their parent asteroids. This process is called aqueous alteration.
The parent bodies of these meteorites probably formed with ice mixed into rock and dust. As radioactive isotopes such as aluminium-26 decayed, they released heat, melting the ice. The resulting water reacted with the original minerals. Olivine, pyroxene, glass and metal were converted into phyllosilicates, or clay minerals, such as serpentine and saponite. Other products include carbonates, magnetite, sulfides, sulfates and phosphates.
The degree of alteration varies. CI chondrites, such as Orgueil, are almost entirely altered, with few original minerals left. CM chondrites, such as Murchison and Winchcombe, are partly altered, often with chondrules still visible. CR chondrites are generally less altered. Scientists use alteration scales to rank these meteorites, and petrologic types 1 and 2 indicate altered meteorites.
Alteration happened early. Dating of carbonates using the manganese–chromium method shows they formed within about 4 to 5 million years after CAIs. The temperatures were usually low, roughly 0 to 150 °C, judged from oxygen isotopes in the carbonates.
Aqueous alteration is important because it shows that liquid water existed in the asteroid belt and affected organic molecules there. It may have helped form amino acids and other compounds. Samples from Ryugu and Bennu show the same kinds of alteration, confirming the processes seen in meteorites.