The Murchison meteorite is, by every available measure, one of the more scientifically valuable single rocks ever to fall on the planet. The fireball that delivered it streaked across the sky above the Goulburn Valley in southeastern Australia on a Sunday morning in September 1969, broke apart at high altitude, and scattered fragments across a roughly 35-square-kilometre area of farmland. One piece punched through the roof of a hay shed. The local dairy farmers who recovered most of the fragments over the following weeks sold the bulk of them to museums and universities, with the largest single collection now held by the Field Museum of Natural History in Chicago. Approximately 80 kilograms of the original 100-kilogram fall is currently held in scientific collections around the world. Over the subsequent five decades, Murchison has been examined more intensively than almost any other meteorite — yielding more than 70 distinct amino acids (including some that do not occur in any known terrestrial biology), sugars including ribose (the backbone of RNA), and several of the nucleobases that make up the building blocks of DNA. The rock has been at the centre of multiple revisions to scientific understanding of how organic chemistry distributes itself through the early solar system.
According to NBC News’s coverage of the 2020 Heck et al. paper on presolar grains in Murchison, the team published its dating results in the Proceedings of the National Academy of Sciences in January 2020. The analysis used a technique called cosmic-ray exposure dating, which Heck described with an unusually accessible analogy: cosmic rays — high-energy particles that constantly stream through the galaxy — interact with solid matter and produce new elements at a roughly constant rate. The longer a particle of matter has been drifting through space exposed to cosmic rays, the more of these new elements accumulate inside it. “I compare this with putting out a bucket in a rainstorm,” Heck told reporters at the time. “Assuming the rainfall is constant, the amount of water that accumulates in the bucket tells you how long it was exposed.” Applying this method to 40 large silicon-carbide grains extracted from the Murchison fragments, the Heck team was able to estimate how long each individual grain had been drifting through interstellar space before being incorporated into the asteroid that eventually fell to Earth.
What presolar grains actually are
The grains the Heck team dated are called presolar grains — solid particles that formed in the atmospheres of dying stars before the solar system existed, drifted through the interstellar medium for hundreds of millions or billions of years, and were then incorporated into the molecular cloud from which the Sun and planets eventually condensed. As reported in Sci.News’s coverage of the same paper, the typical formation environment for the silicon-carbide grains examined by the Heck team was the slowly-expanding atmosphere of an asymptotic giant branch (AGB) star — a red giant in the late stages of its evolution, shedding its outer layers into space at a rate of roughly one solar mass every hundred thousand years. The carbon, silicon, and other elements in the stellar atmosphere condense as the gas cools and expands, forming microscopic dust grains that are then carried outward by the stellar wind into the surrounding interstellar medium.
Most of these grains, once released into space, are eventually destroyed by some combination of supernova shock waves, radiation, and chemical reprocessing within the molecular clouds where new stars form. A small fraction survive intact for billions of years, drifting through the galaxy as part of the general inventory of interstellar dust. When a new solar system begins to form by gravitational collapse of a dense region of interstellar gas, this dust is incorporated into the early protoplanetary disc — and the heat and chemistry of solar system formation typically destroys or substantially alters most of it. The grains that survive intact through the entire formation process do so because they are extraordinarily durable (silicon carbide is one of the hardest materials in nature) and because they are quickly incorporated into the deep interiors of primitive asteroids that never undergo significant thermal processing. Murchison’s parent body was one such asteroid. The grains inside the Murchison fragments are, in a literal sense, time capsules from before the solar system existed.
The age distribution
The headline figure from the Heck team’s analysis — that the oldest grains are approximately 7 billion years old, predating the solar system by approximately 2.5 billion years — captures the extreme of the distribution rather than the average. Per ScienceDaily’s coverage of the Field Museum announcement, the full age distribution of the 40 grains was substantially more informative than the single extreme number suggests. Approximately 60 percent of the grains formed between 4.6 and 4.9 billion years ago — in the few hundred million years immediately preceding the formation of the solar system. Approximately 10 percent formed more than 5.6 billion years ago. A small number formed at the extreme end, around 7 billion years ago. Every grain examined was, by the cosmic-ray exposure dating, older than the 4.6-billion-year-old Sun. This is what made them presolar.
The distribution itself carries scientific weight beyond the individual grain ages. The clustering of a substantial fraction of grains in a narrow age band suggests that an unusual number of stars in the Milky Way galaxy formed, lived their full life cycles, and died during a specific epoch several billion years before the Sun ignited — a “burst” of star formation that produced an unusually large population of dying stars whose outflows seeded the surrounding interstellar medium with the dust that would later contribute to the formation of the solar system. The Heck team’s interpretation, drawing on the age distribution, is that this burst occurred approximately 7 billion years ago. The implication is that the Sun and the planets formed, several billion years later, from a galactic environment that had been substantially enriched with material from this earlier generation of stars.
Why no comparable material exists on Earth
As reported by Malay Mail’s coverage of the age distribution, the broader reason presolar grains can only be found in meteorites — never in any rock of terrestrial origin — is that every piece of solid material that contributed to Earth’s formation has subsequently been heated, melted, recrystallised, or otherwise reprocessed by the geological activity of the planet itself. Plate tectonics has churned the entire crust through the mantle and back at least once. Volcanism has melted and re-erupted vast quantities of material. Even the oldest rocks on Earth, found in places like the Acasta Gneiss Complex in northern Canada, are at most 4.0 to 4.2 billion years old — substantially younger than the planet itself, and far younger than the presolar grains preserved inside primitive meteorites.
The Murchison meteorite, in this respect, occupies a peculiar epistemological position. It is the closest available physical sample of the early solar system, but it is also a sample of the pre-solar-system galaxy that produced the early solar system. The grains inside it are older than anything that can be found by digging downward through the rocks of Earth, by any depth or any drilling technology. They are, in essence, a sample of the chemistry of the Milky Way at a moment several billion years before any of the familiar features of the current solar system existed. The fragments collected by Australian dairy farmers in 1969 and sent to museums in Chicago and elsewhere have turned out to contain, embedded in their carbonaceous matrix, a small but real sample of stardust from stars that finished burning before the Sun began.
