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We are Made of Starstuff

Notes from the Mini Museum: Extraterrestrial Amino Acids Specimen

Each year nearly 40,000,000 kilograms (88.1 million pounds) of meteoritic material rains down on the Earth from outer space. Less than 1% of these falls holds traces of organic compounds, and within this tiny subset scientists sometimes come across even rarer material: amino acids.

The story of these extraterrestrial amino acids begins at 10:58 AM on September 28th, 1969. A bright fireball appeared in the sky near the small, riverside town of Murchison, Australia. Under tremendous stress, the bolide separated into three main pieces, spreading fragments across 13 square kilometers (5 sq.mi.), including one lump which crashed through a barn roof and landed in a pile of hay.

As astronomical as the odds might be for this soft landing, the Murchison meteorite would turn out to be literally one of the rarest of all meteorite finds: a remnant formed at the very birth of the solar system, which also happened to carry the building blocks of life.

The oldest of these meteorites, known as carbonaceous chondrites, date to the formation of the solar system. Recent studies suggest that the amino acids found in some carbonaceous chondrites may have come from the pre-solar nebula.

This type of meteorite is distinguished by calcium–aluminium-rich inclusions (CAI), minerals that are among the first solids to condense in the high temperature gases of a young, protoplanetary disk. In addition to CAIs, Murchison also carries a fantastic array of more than 70 different amino acids, including 8 of the 20 proteinogenic amino acids used to build proteins encoded in our DNA as well as all life here on Earth.

Update January, 14th 2020: Even after decades of study, the Murchison meteorite continues to surprise science with new discoveries. The latest is that materials in this meteorite are likely far, far older than our own solar system. Billions of years older, in fact.

In "Lifetimes of interstellar dust from cosmic ray exposure ages of presolar silicon carbide", scientists at the Field Museum in Chicago, studied cosmic ray exposure of silicon carbide grains extracted from samples of the Murchison meteorite. In 12 samples, they discovered strong evidence that these grains originated in stars formed roughly 7,000,000,000 years ago and were parts of aggregates travelling through the Interstellar Medium. Whether any of these particular grains are in a single specimen in the Fourth Edition is hard to say but it is incredible to think about!

"Interstellar dust is an important component of our galaxy. It influences star formation as well as the thermal and chemical evolution of the galaxy. Although dust only presents ∼1% of the mass in the interstellar medium (ISM), it carries a large fraction of the elements heavier than Helium, including the elements that form terrestrial planets and are essential for life. Thus, interstellar dust is a key ingredient of stars and habitable planetary systems, making increased knowledge about its composition and lifecycle desirable."

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