Water and the ingredients of life

Some meteorites carry water locked in minerals and a rich chemistry of carbon compounds, including amino acids and nucleobases. Samples returned from asteroids have confirmed it.

Water in rock

Carbonaceous chondrites come from asteroids that formed with ice. Heat from radioactive decay melted the ice, and the water reacted with the rock to make clay minerals, carbonates and magnetite: a process called aqueous alteration. In CI and CM chondrites, water bound in these minerals makes up roughly 10 to 20% of the mass. The ratio of deuterium to ordinary hydrogen in that water is similar to Earth's oceans, supporting the idea that such asteroids supplied a share of Earth's water.

Carbon chemistry

The Murchison meteorite, which fell in Australia in 1969, contains tens of thousands of distinct carbon compounds, including more than 70 amino acids, many of which life does not use. Sugars, including ribose, and all five nucleobases used in DNA and RNA have since been found in carbonaceous meteorites. Isotope ratios and the presence of non-biological molecules show these compounds formed in space, not from contamination.

Confirmation from asteroids

Japan's Hayabusa2 returned about 5.4 g from asteroid Ryugu in 2020, and NASA's OSIRIS-REx returned about 122 g from asteroid Bennu in 2023. Both samples are rich in carbon and clay minerals, closely resembling CI chondrites. Bennu's sample includes amino acids, all five nucleobases and salts left when brines evaporated, all collected without exposure to Earth's surface. None of this is life, but it shows that the chemical building blocks were widespread and were very probably delivered to the young Earth.

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