Aqueous alteration
Aqueous alteration is the chemical change of a meteorite's minerals by liquid water inside its parent asteroid.
Many asteroids formed with ice mixed into their rock. Heat, mainly from the decay of short-lived radioactive aluminium-26, melted that ice, and the liquid water reacted with the surrounding minerals. Olivine, pyroxene, glass and metal were converted into hydrated clay minerals (phyllosilicates), along with carbonates, magnetite, sulfides and other secondary minerals. This process is called aqueous alteration. Dating of carbonates suggests much of it happened within the first few million years of Solar System history.
Aqueous alteration is measured by the petrologic types 2 and 1, with type 1 the most altered. It shows that water was present and active on small bodies very early, and it is linked to the formation of organic compounds such as amino acids. It also matters for understanding where Earth's water came from.
Orgueil, a CI1 chondrite that fell in France in 1864, is almost entirely made of alteration minerals, and Murchison (CM2) shows partial alteration around surviving chondrules. Samples from asteroids Ryugu and Bennu are similarly altered. Such meteorites tend to be dark, soft, crumbly and lightweight, with few or no visible chondrules and little metal.
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