Organic compounds in asteroids have a dark and chilly origin

Carbon-based molecules recovered from an asteroid and a meteorite may have formed in the interstellar medium – the cold, dark region between stars.

These compounds, called polycyclic aromatic hydrocarbons or PAHs, were previously thought to have formed in or near stars.

But an international team of researchers has found that, while some of them have stellar origins, other PAHs formed in cold environments.

The team examined, dust found in the Ryugu asteroid and Murchison meteorite.

The researchers have published their study in Science.

PAHs are organic compounds, so-called because they contain carbon and hydrogen, not because they come from life.

“They are very common compounds, because they’re extremely stable,” co-author Dr Alex Holman, from Curtin University’s Western Australian Organic & Isotope Geochemistry Centre, tells Cosmos.

“So they’re formed from carbon.Mostly on Earth [that’s] by burning of plant material, such as in wildfires. But in they can also be formed from carbon in interstellar objects.”

Holman says that the purpose of this study was to establish whether the PAHs in asteroids were “formed by high temperature processes, such as within stars or in the vicinity of stars, or whether they were formed in low temperatures in interstellar space”.

Holman and fellow Curtin researcher Professor Kliti Grice contributed to the study by burning plants.

Specifically, they looked at PAHs produced by plant combustion experiments: high-temperature formation. This data was used to compare to the PAHs in asteroids.

The researchers compared PAHs by looking at isotopes. Isotopes are atoms of the same element that have slightly different mass. At different temperatures, heavier carbon atoms behave slightly differently to lighter carbon atoms. This affects the way they bond with other atoms.

“The likelihood of them bonding together is dependent on the temperature at which the compounds formed,” says Holman.

Diagram of pahs forming
Schematic of how polycyclic aromatic hydrocarbons (PAHs) can form in different extraterrestrial environments. Credit: Dr S S Zeichner

This showed the team that different PAH molecules in each sample formed at hot and cold temperatures. This means some of them have interstellar origins.

“Select PAHs from Ryugu and Murchison were found to have different characteristics: the smaller ones likely in cold outer space, while bigger ones probably formed in warmer environments, like near a star or inside a celestial body,” says Grice.

“This research gives us valuable insights into how organic compounds form beyond Earth and where they come from in space,” says Holman.

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