Do meteorites contain water?
Yes, many do. Most of it is not liquid but locked inside minerals that formed in water, and carbonaceous chondrites can hold over a tenth of their weight in this bound water.
The word water might suggest droplets sloshing around inside a rock, but in meteorites it is almost always hidden in the crystal structure of minerals. On several asteroids, early in Solar System history, ice melted, probably heated by radioactive decay, and liquid water seeped through the rock. It reacted with the original minerals to form hydrated minerals, chiefly clay-like phyllosilicates, together with carbonates and other minerals that can only form in water.
The wettest meteorites are the carbonaceous chondrites. The CI group, such as Orgueil, which fell in France in 1864, and the CM group, including Murchison and the Winchcombe meteorite that fell in England in 2021, hold a large fraction of their weight as water bound in minerals; in CI chondrites it can approach a fifth. Studies of Winchcombe found that the hydrogen in its water has an isotopic make-up similar to that of Earth's oceans, supporting the idea that asteroids of this type may have helped supply our planet's water. The relative contributions of different sources are still debated.
Remarkably, actual liquid water has been seen too. Tiny purple-blue crystals of halite, ordinary rock salt, in two ordinary chondrites that fell in 1998, Monahans in Texas and Zag in Morocco, contain microscopic bubbles of salty brine trapped since the early Solar System.
Even meteorites that look bone-dry can carry traces of water in minerals such as apatite, and Martian meteorites contain minerals altered by water on Mars. The water-bearing minerals in carbonaceous chondrites are fragile and react quickly with Earth's air, which is one reason scientists value freshly fallen stones collected quickly and stored carefully.