PAH Molecules From Asteroids Likely Predate the Solar System, Suggest Origin of Organic Compound in Colder Regions of Space

Here on Earth, carbon is the building block of all life forms. This element is present in many compounds, like proteins, sugars, and carbohydrates, which comprise everything from bacteria to plants and animals.

One particular carbon-based molecule, polycyclic aromatic hydrocarbons (PAHs), is abundant both on Earth and in space. Understanding where and how they form is important in understanding the Milky Way galaxy's composition and other space regions.

Abundance and Origin of PAHs

Polycyclic aromatic hydrocarbons are molecules made of adjacent aromatic rings terminated by hydrogen. These compounds made of carbon and hydrogen are common on Earth and in celestial bodies such as asteroids and meteorites.

PAHs contain around 20% of the carbon in the interstellar medium. Astronomers consider them prebiotic building blocks that play a significant role in the formation of life on Earth. While scientists know their abundance in the universe, where PAHs originated remains a mystery.

It was believed that these organic molecules are potentially formed in circumstellar environments at temperatures over 1000 K since this condition can supply energy to trigger chemical activity that creates PAHs. They could also be included as part of carbon-rich dust grains processed by nearby energy sources such as stars.

Finding Clues From Rocky Bodies

In a new study, experts from Curtin University's Western Australian Organic & Isotope Geochemistry Centre (WA-OIGC) suggest that PAHs could have formed in the cold areas of space between stars and not in hot regions near stars, as previously assumed. The findings of their study, "Polycyclic aromatic hydrocarbons in samples of Ryugu formed in the interstellar medium," opens new possibilities for examining life beyond Earth and the chemistry of objects in space.

In this experiment, the research team produced PAHs by carrying out controlled burnings of Australian plants. These were then isotopically compared to PAHs from fragments of the Ryugu asteroid, which were returned in 2020 by a Japanese spacecraft, and the Murchison meteorite, which landed in Australia in 1969. Further analysis was done on the bonds between light and heavy carbon isotopes in the PAHs to reveal the temperature at which they were formed.

Asteroid Ryugu belongs to a class of space rocks that represent the composition of an average solar system. It can provide clues to the environments in which Earth and other planets formed and those of planets around other stellar bodies that resemble our Sun.

The PAHs from the celestial bodies were discovered to have different characteristics. The smaller fragments likely formed in cold outer space, while the bigger ones were created in warmer environments near a star or inside a celestial body.

The new analytical tools also determined that the PAHs isolated from the asteroid contained excesses in clumps of carbon-13. This provides the first quantitative evidence that PAHs could have formed in the cold, low-energy regions of the universe.

Molecular clouds refer to the very cold regions of gas in interstellar space spanning billions of miles. They are known for having frigid temperatures, so there is little energy to generate chemical reactions. Since PAHs are relatively large molecules, their formation within cold molecular clouds must occur through chemical reactions that need minimal thermal energy or are helped along by the absorption of energy from light.