Ashes to Dust, Dust to... Life?

Most people associate asteroids with death and devastation rather than life and longevity. Indeed, it was a flaming hunk of space rock that smashed into prehistoric Earth 66 million years ago and caused the mass extinction of the dinosaurs who, at that point, had roamed our planet for 200 million years.

Recent evidence, however, has supported the notion that the bombardment of Earth by astronomical bodies billions of years ago provided the ingredients — water and basic organic compounds — for an early primordial soup. Two articles published at the end of last month in the leading research journals Nature and Nature Astronomy highlighted the presence of these building blocks in dust samples of the asteroid Bennu.

Bennu spans 0.5 kilometres in diameter, approaches Earth every six years, and is thought to have originated from a larger astronomical body that came from the icy outer fringes of the solar system. In 2020, the OSIRIS-REx spacecraft collected a dust sample from Bennu before delivering it back to Earth in 2023 for analysis. The sample has since been found to contain salt minerals, such as magnesium sodium phosphate and sodium carbonate, that are thought to have been left behind in the evaporation of water from Bennu’s parent body and thus forming a brine.

It is the aqueous conditions provided by this salty brine that are thought to have enabled the organic chemistry from which other building blocks of life, such as amino acids and nucleobases, emerged. Of the twenty amino acids which are responsible for the makeup of proteins on Earth, fourteen were found in Bennu. Moreover, all five nucleobases — which together constitute our genetic material of DNA and RNA — were also present within this asteroid. As such, Bennu reveals important information about the types of compounds hidden within the celestial bodies that struck Earth in its early geology.

While some of these chemicals have been found in samples from other asteroids — such as the asteroid Ryugu, from which a sample was delivered by the Japanese probe Hayabusa2 in 2020 — the combinations present in Bennu have never been observed. Like Ryugu, Bennu also has a composition not dissimilar from a group of rare meteorites called CI chondrites, which are particularly challenging to sample and analyse due to their extremely fragile makeup. However, this recent analysis represents a considerable advancement, as the Bennu sample collected was maintained in pristine condition throughout its return to Earth and recovery thanks to the protective environment provided by the OSIRIS-Rex probe, which prevented contamination of the sample within Earth’s atmosphere.

One of the senior scientists at NASA who led the analysis of the Bennu sample, Daniel Glavin, has highlighted that these are only the initial findings from the characterisation of the asteroid and that the examination of more samples is currently ongoing. Further work will aim to expand the evidence already collected and will include additional detailed analysis of the Bennu samples, searching for extra organic compounds like sugars. Future missions will also collect material from other astronomical bodies, such as the asteroid Apophis, which OSIRIS-REx will visit and harvest samples from in 2029.

Named after an ancient Egyptian deity closely associated with the Sun and rebirth, renewal, and creation, Bennu clearly symbolises a major step in re-tracing the origin of life back to its beginnings.

Image by Wikipedia Commons