Ancient stardust discovered in meteorite

Tania Firouzabady. 01/15/2021

In September 1969, a meteorite fell near the town of Murchison in Victoria, Australia. Weighing 100 kilograms, this massive rock was initially overlooked as an ordinary meteorite. However, scientists have recently discovered that it contains the oldest material on Earth: 7-billion-year-old stardust.

The dust is composed of presolar grains, which are microscopic dust particles older than our sun. Though the primary molecule found in the grains was silicon carbide, the traces of neon-21 that were also found played a key role in unveiling the age of the dust. Neon-21 is produced when silicon carbide is struck by galactic cosmic rays. These rays are high-energy protons and nuclei that move through space at extremely high speeds. They can be produced by the sun, outside the solar system and even in other galaxies. The larger the neon-21 concentration in the grains, the more time that has passed since it was created.

The Proceedings of the National Academy of Sciences (PNAS) outlines the importance of the interstellar dust: “[interstellar dust] carries a large fraction of the elements heavier than He, including the elements that form terrestrial planets and are essential for life.” Its potential to reveal the habitability of planetary systems makes interstellar dust highly valuable to researchers.

Additionally, the PNAS highlights the methods used to analyze the dust found within the Murchison meteorite. Astronomical spectroscopic observations (based on measuring the spectrum of electromagnetic radiation) are used to determine the composition, structure and size of the dust particles, while a variety of theoretical models are used to find the dust’s lifespan. In this particular instance, scientists approached the challenge by finding the interstellar lifetimes of individual presolar silicon carbide grains.

No presolar grains have ever been discovered in rocks on Earth, making the grains on the meteorite very valuable to researchers. As explained by scientist Philipp Heck, the reason for this can be found in Earth’s geological processes: volcanism and plate tectonics transform any presolar dust that may have been left during Earth’s formation.

To better understand the nature of the stardust, the team measured the size of the grains. The majority of presolar grains are one micron long. In contrast, those found on the meteorite ranged from 2 to 30 microns. Their astounding size led the team to call them ‘boulders.’

Heck and his colleagues analyzed 40 of the boulders by first grinding pieces of the meteorite and then using acid to dissolve the minerals and silicate. This ultimately allowed them to determine which of the grains were acid-resistant. As outlined by the PNAS, “These grains are identified as presolar by their large isotopic anomalies that exclude an origin in the solar system.” The majority of presolar grains are destroyed during thermal metamorphosis and aqueous altercation, processes stimulating their growth in the parent bodies they come from. Hence, they are among the most ancient materials in the solar system and are of great value to scientists.

Once the grains have been isolated, mass spectrometry, a technique used for measuring the mass to charge ratio of ions, is utilized to determine the number of nuclides produced by the grains during reactions with galactic cosmic rays. Nuclides are a specific type of atom or nucleus characterized by the number of protons and neutrons they contain. When they come into contact with high-energy particles, some of them break apart and those that are left accumulate in the presolar grains. The nuclide concentration is proportional to the timespan the grains were exposed to radiation, revealing various properties that indicate how old the dust is.

These techniques ultimately revealed that the grains date to before the formation of our solar system. According to Heck, the grains “formed in an episode of enhanced star formation. There was a time before the start of the solar system when more stars formed than normal.” To clarify, the grains allow scientists to better understand stellar evolution, the ways in which stars change with time, particularly with regards to bursts of star formation.

Another unusual finding was that the stardust particles appeared to be stuck together. This phenomenon can be explained by the dust’s electrical charge. As explained by NASA, “Gravity pulls dust and gas in interstellar clouds together, but because the electrostatic force over short distances is so much stronger than gravity, even a small electrostatic repulsion between dust grains can influence (and possibly prevent) a cloud’s collapse.”

When the Murchison meteorite was discovered in 1969, no one could have imagined the remarkable material it kept hidden. At first glance, it would appear to be an ordinary meteorite. But as revealed by Philippe Heck and his colleagues, the stardust it contains, which is invisible to the naked eye, provides clues to the origins of stars and the history of the solar system. Though the exact age of the dust is unclear, its significance to astronomical research is evident.

Cover Photo: (New Atlas)