The Building Blocks for Life Found in Asteroid Bennu Samples

The study of asteroid samples is a highly lucrative area of research and one of the best ways to determine how the Solar System came to be. Given that asteroids are leftover material from the formation of the Solar System, they are likely to contain vital clues about how several key processes took place. This includes how water, organic molecules, and the building blocks of life were distributed throughout the Solar System billions of years ago. For this reason, space agencies have attached a high importance to the retrieval of asteroid samples that are returned to Earth for analysis.

This includes NASA’s Origins, Spectral Interpretation, Resource Identification, and Security–Regolith Explorer (OSIRIS-REx) mission. This spacecraft rendezvoused with asteroid (101955) Bennu on December 3rd, 2018, returning 121.6 grams of material (the largest sample ever) to Earth by September 2023. A recent analysis by scientists from NASA’s Goddard Space Flight Center revealed molecules key to life on Earth, including all five nitrogen bases – molecules required for building DNA and RNA. These findings support the theory that asteroids could have delivered the building blocks of life to Earth in the distant past.

The research was led by Daniel P. Glavin and Jason P. Dworkin, two senior scientists with the Solar System Exploration Division (SSED) at NASA Goddard. They were joined by multiple colleagues from the SSED, the Goddard Center for Research and Exploration in Space Science and Technology (CRESST), the Astromaterials Research and Exploration Science Division (ARES) at the NASA Johnson Space Center, and multiple universities and institutes. Their findings were presented in papers that appeared in Nature and Nature Astronomy.

A poster depicting all the compounds discovered in the OSIRIS-REx sample. ©NASA

Their results represent the first in-depth analyses of the minerals and molecules in the Bennu samples. Among the most compelling detections (reported in the Nature Astronomy paper) were 14 of the 20 amino acids life on Earth uses to make up protein cells. They also detected five nucleobases vital to DNA and RNA, which most complex lifeforms on Earth use to store and transmit genetic instructions, including how to arrange amino acids into proteins. As Associate Administrator Nicky Fox of the Science Mission Directorate at NASA Headquarters explained in a NASA press release:

“NASA’s OSIRIS-REx mission already is rewriting the textbook on what we understand about the beginnings of our solar system. Asteroids provide a time capsule into our home planet’s history, and Bennu’s samples are pivotal in our understanding of what ingredients in our solar system existed before life started on Earth.”

The teams also reported exceptionally high abundances of ammonia in the Bennu samples and formaldehyde. Ammonia is an important component in biology since it can react with formaldehyde to form complex molecules like amino acids. These building blocks have previously been detected in other rocky bodies, including meteorites retrieved on Earth. However, the way OSIRIS-REx found them in pristine condition on an asteroid supports the theory that objects that formed far from the Sun could have delivered the raw material for life throughout the Solar System. Said Glavin:

“The clues we’re looking for are so minuscule and so easily destroyed or altered from exposure to Earth’s environment. That’s why some of these new discoveries would not be possible without a sample-return mission, meticulous contamination-control measures, and careful curation and storage of this precious material from Bennu.”

Illustration of the asteroid Bennu. Credit: NASA Jet Propulsion Laboratory

Glavin and Dworkin’s team analyzed the Bennu samples for hints of compounds related to life on Earth. Meanwhile, Tim McCoy and Sara Russell, the curator of meteorites at the Smithsonian’s National Museum of Natural History in Washington and a cosmic mineralogist at the Natural History Museum in London (respectively), looked for evidence of where these molecules formed. As they reported in the study appearing in Nature, they discovered hints that they came from an ancient prebiotic environment.

These included traces of 11 minerals ranging from calcite to halite and sylvite, compounds that form from salts dissolved in water that become solid crystals (brines) once the water dissolves. Evidence of similar brines have been detected on Ceres, Saturn’s moon Enceladus, and other bodies in the Solar System. While scientists have also detected brines in meteorites that fell to Earth, they have never seen a complete set created by an evaporation process that could have lasted thousands of years or more. Moreover, some minerals found in Bennu have never been detected in other extraterrestrial samples.

Another analysis was carried out by members of the OSIRIS-REx sample analysis team, including researchers from the Japan Agency for Marine-Earth Science and Technology (JAMSTEC), Hokkaido University, Keio University, Kyushu University, and Tohoku University. Together, they analyzed a 17.75 mg sample using high-resolution mass spectrometry for organic molecules with a ring structure containing carbon and nitrogen (N-heterocycles). This revealed a concentration of N-heterocycles 5-10 times higher than that reported from the sample taken from Ryugu (~5 nmol/g) by the Hayabusa2 mission.

In addition to the five nitrogenous bases, their analysis showed evidence of the purines xanthine, hypoxanthine, and nicotinic acid (vitamin B3). “In previous research, uracil and nicotinic acid were detected in the samples from asteroid Ryugu, but the other four nucleobases were absent,” said team member Dr. Toshiki Koga of JAMSTEC. “The difference in abundance and complexity of N-heterocycles between Bennu and Ryugu could reflect the differences in the environment to which these asteroids have been exposed in space.”

A mosaic image of asteroid Bennu, composed of 12 PolyCam images collected by the OSIRIS-REx spacecraft from a range of 24 kilometers. Credit: NASA/Goddard/University of Arizona

While these findings have provided compelling evidence of where the building blocks of life on Earth came from, several unanswered questions remain. For starters, amino acids can be created in “mirror-image” versions, similar to how complex lifeforms have a left and right side – hands, feet, brains, lungs, heat chambers, etc. While life on Earth almost exclusively exhibits the left variety, the Bennu samples contain an equal mixture of both. This could mean amino acids started in equal mixtures on Earth billions of years ago but made a left turn along the way.

This is not unlike theories regarding matter and antimatter in the early Universe and how “normal” matter came to be predominant. In any case, these findings are a key piece in the ongoing study of how and where life may have emerged in the Solar System. “OSIRIS-REx has been a highly successful mission,” said Dworkin. “Data from OSIRIS-REx adds major brushstrokes to a picture of a solar system teeming with the potential for life. Why we, so far, only see life on Earth and not elsewhere, that’s the truly tantalizing question.”

Further Reading: NASA, Hokkaido University, Nature Astronomy

2 Replies to “The Building Blocks for Life Found in Asteroid Bennu Samples”

  1. It is nice to have an unbiased sample, and see that it joins the growing evidence for ease of organics production in nature.

    Speaking of its absence of natural chiral bias:
    “Glavin is most perplexed by the discovery of an equal mixture of left-handed and right-handed amino acids on Bennu. He, like many scientists, had thought that organic molecules from primordial asteroids would have had the same left-handed dominance as those from life on Earth. Now, researchers have to go back to the drawing board to understand how life might have been seeded on Earth. “I felt a little bit disappointed at first, like it invalidated 20 years of my research,” Glavin says. “But this is why we explore — to learn new things.”” [“Asteroid fragments upend theory of how life on Earth bloomed”, Nature]

    This is likely seen from a classical position of the minority of biochemists within astrobiology, which has not heeded the same 20 years of biology research into evolution of the split between biology and geology. That research established its 4.3 billion year dating that long ago [c.f. TimeTree], but recent cross bracing methods have robustly validated it as well as informed on the organics producing environment of deep ocean hydrothermal vents. The evidence show a half alive exothermal metabolism that imports left- and righthanded organics alike, with a frozen in chiral filter in its genetic machinery producing proteins (70 % preference in tRNA coupling to amino acids, another 70 % in rRNA chaining them) which a modern cell wouldn’t need.

    The latest advance is of seeing some of the pre-LUCA evolution of the genetic code, which show that life did not pick imported organics indiscriminately from source abundance ratios as e.g. biochemists would want us to think but from ease of import and metabolism. The Bennu finds support that find:

    “The order in which the amino acids were added to the genetic code was previously inferred from consensus among forty metrics. Many of these reflect abiotic abundance on ancient Earth. However, the abundances that matter are those within primitive cells that already had sophisticated RNA and perhaps peptide metabolism. Here, we directly infer the order of recruitment from the relative ancestral amino acid frequencies of ancient protein sequences. Small size predicts ancient amino acid enrichment better than the previous consensus metric does. We place metal-binding and sulfur-containing amino acids earlier than previously thought, highlighting the importance of metal-dependent catalysis and sulfur metabolism to ancient life. Understanding early life has implications for our search for life elsewhere in the universe.” [“Order of amino acid recruitment into the genetic code resolved by last universal common ancestor’s protein domains”, Wehbi et al, PNAS, 2024]

  2. By the way, the pre-LUCA code evolution paper suggests Enceladus is a good site to search for life:

    “To explain the different enrichments of pre-LUCA versus LUCA sequences, as well as the surprising conservation of some sites prior to the emergence of the aaRSs that distinguish the relevant amino acids, we propose that some pre-LUCA sequences are older than the current genetic code, perhaps even tracing back to a peptide world at the dawn of precellular life (7). Stepwise construction of the current code and competition among ancient codes could have occurred simultaneously (78, 79). Ancient codes might also have used noncanonical amino acids, such as norvaline and norleucine (80) which can be recognized by LeuRS (81, 82). Along with having different genetic codes, we speculate that pre-LUCA and LUCA might have existed in different geochemical settings. For instance, if pre-LUCA ancestors inhabited alkaline hydrothermal vents, where abiotically produced aromatic amino acids have been found (75), this would explain their enrichment in pre-LUCA relative to LUCA. We note that abiotic synthesis of aromatic amino acids might be possible in the water–rock interface of Enceladus’s subsurface ocean, which is speculated to be analogous to terrestrial alkaline hydrothermal vents (83).”

    [There is more evidence for and data on early code evolution in the paper.]

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