Building blocks from asteroids

Section 3 of 4

Some stones that fall from the sky carry carbon compounds made in space, among them amino acids, the building blocks of proteins.

On 28 September 1969 a meteorite broke up over the town of Murchison in Victoria, Australia, and people picked up more than 100 kg of its pieces. It is a carbonaceous chondrite, a crumbly black stone about 2% carbon, from an asteroid that formed along with the Sun and the planets, 4.6 billion years ago.

In 1970 chemists at NASA's Ames Research Center found amino acids in it, among them glycine and alanine. The hands of the alanine told them where it came from. Alanine comes in two mirror-image hands, and living things on Earth build their proteins from one of them, L-alanine. The Murchison alanine held both hands in about equal amounts, so it had been made by chemistry in the asteroid, without life. Chemists have since found more than 90 amino acids in the meteorite, most of them kinds that living things do not build proteins from, and in 2019 they found ribose, a sugar.

Glycine, C₂H₅NO₂The simplest amino acid. One carbon atom carries both an -NH₂ group and a -COOH group.HOHNCCOHHH
Glycine, C₂H₅NO₂, the simplest amino acid: its side group is a hydrogen atom. It turns up in meteorites, in asteroid samples and in comets.
Adenine, C₅H₅N₅One of the four bases in DNA. A six-membered ring and a five-membered ring sharing one edge, with four nitrogen atoms in the rings and an -NH₂ group.NHNNNNH2
Adenine, C₅H₅N₅, one of the four bases of DNA: two rings of carbon and nitrogen atoms, with an -⁠NH₂ group. The Bennu samples held it, along with the other bases of DNA and RNA.

Samples from two asteroids

A meteorite may lie on the ground for days or years before anyone finds it, so chemists check every sample for traces of life from Earth. To get clean rock, two spacecraft scooped it straight from asteroids and brought it home sealed. Japan's Hayabusa2 returned 5.4 g from asteroid Ryugu in December 2020. NASA's OSIRIS-REx returned 121.6 g from asteroid Bennu in September 2023.

Some of the carbon compounds chemists reported in the asteroid samples
AsteroidWhat they found
Ryuguglycine, alanine and other amino acids, with both hands in about equal amounts; uracil, a base of RNA
Bennu14 of the 20 amino acids in proteins; all five bases of DNA and RNA; ribose, glucose and other sugars; plenty of ammonia

RNA is DNA's close relative. Cells use it to carry copies of a gene's instructions. Its sugar is ribose, and it uses a base called uracil in place of thymine. In the Bennu samples, the amino acids that come in two hands and that life does not use held both hands in about equal amounts, just as in Murchison. Chemistry alone made all these molecules. They show that the early Solar System made many of the molecules life is built from, and that asteroids and meteorites could have carried them to the young Earth.

Quick check

The alanine in the Murchison meteorite held both hands in about equal amounts. What did that tell chemists?