The asteroid Bennu continues to provide new clues to scientists’ biggest questions about the formation of the early solar system and the origins of life. As part of the ongoing study of pristine samples delivered to Earth by NASA’s OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, and Security-Regolith Explorer) spacecraft, three new papers published Tuesday by the journals Nature Geosciences and Nature Astronomy present remarkable discoveries: sugars essential for biology, a gum-like substance not seen before in astromaterials, and an unexpectedly high abundance of dust produced by supernova explosions.
Sugars essential to life
Scientists led by Yoshihiro Furukawa of Tohoku University in Japan found sugars essential for biology on Earth in the Bennu samples, detailing their findings in the journal Nature Geoscience. The five-carbon sugar ribose and, for the first time in an extraterrestrial sample, six-carbon glucose were found. Although these sugars are not evidence of life, their detection, along with previous detections of amino acids, nucleobases, and carboxylic acids in Bennu samples, show building blocks of biological molecules were widespread throughout the solar system.
For life on Earth, the sugars deoxyribose and ribose are key building blocks of DNA and RNA, respectively. DNA is the primary carrier of genetic information in cells. RNA performs numerous functions, and life as we know it could not exist without it. Ribose in RNA is used in the molecule’s sugar-phosphate “backbone” that connects a string of information-carrying nucleobases.
“All five nucleobases used to construct both DNA and RNA, along with phosphates, have already been found in the Bennu samples brought to Earth by OSIRIS-REx,” said Furukawa. “The new discovery of ribose means that all of the components to form the molecule RNA are present in Bennu.”
The discovery of ribose in asteroid samples is not a complete surprise. Ribose has previously been found in two meteorites recovered on Earth. What is important about the Bennu samples is that researchers did not find deoxyribose. If Bennu is any indication, this means ribose may have been more common than deoxyribose in environments of the early solar system.
Researchers think the presence of ribose and lack of deoxyribose supports the “RNA world” hypothesis, where the first forms of life relied on RNA as the primary molecule to store information and to drive chemical reactions necessary for survival.
“Present day life is based on a complex system organized primarily by three types of functional biopolymers: DNA, RNA, and proteins,” explains Furukawa. “However, early life may have been simpler. RNA is the leading candidate for the first functional biopolymer because it can store genetic information and catalyze many biological reactions.”
The Bennu samples also contained one of the most common forms of “food” (or energy) used by life on Earth, the sugar glucose, which is the first evidence that an important energy source for life as we know it was also present in the early solar system.
Mysterious, ancient ‘gum’
A second paper, in the journal Nature Astronomy led by Scott Sandford at NASA’s Ames Research Center in California’s Silicon Valley and Zack Gainsforth of the University of California, Berkeley, reveals a gum-like material in the Bennu samples never seen before in space rocks – something that could have helped set the stage on Earth for the ingredients of life to emerge. The surprising substance was likely formed in the early days of the solar system, as Bennu’s young parent asteroid warmed.
Once soft and flexible, but since hardened, this ancient “space gum” consists of polymer-like materials extremely rich in nitrogen and oxygen. Such complex molecules could have provided some of the chemical precursors that helped trigger life on Earth, and finding them in the pristine samples from Bennu is important for scientists studying how life began and whether it exists beyond our planet.
Scott SandFord
Astrophysicist, NASA’s Ames Research Center
Bennu’s ancestral asteroid formed from materials in the solar nebula – the rotating cloud of gas and dust that gave rise to the solar system – and contained a variety of minerals and ices. As the asteroid began to warm, due to natural radiation, a compound called carbamate formed through a process involving ammonia and carbon dioxide. Carbamate is water soluble, but it survived long enough to polymerize, reacting with itself and other molecules to form larger and more complex chains impervious to water. This suggests that it formed before the parent body warmed enough to become a watery environment.
“With this strange substance, we’re looking at, quite possibly, one of the earliest alterations of materials that occurred in this rock,” said Sandford. “On this primitive asteroid that formed in the early days of the solar system, we’re looking at events near the beginning of the beginning.”
Using an infrared microscope, Sandford’s team selected unusual, carbon-rich grains containing abundant nitrogen and oxygen. They then began what Sandford calls “blacksmithing at the molecular level,” using the Molecular Foundry at Lawrence Berkeley National Laboratory (Berkeley Lab) in Berkeley, California. Applying ultra-thin layers of platinum, they reinforced a particle, welded on a tungsten needle to lift the tiny grain, and shaved the fragment down using a focused beam of charged particles.
When the particle was a thousand times thinner than a human hair, they analyzed its composition via electron microscopy at the Molecular Foundry and X-ray spectroscopy at Berkeley Lab’s Advanced Light Source. The ALS’s high spatial resolution and sensitive X-ray beams enabled unprecedented chemical analysis.
“We knew we had something remarkable the instant the images started to appear on the monitor,” said Gainsforth. “It was like nothing we had ever seen, and for months we were consumed by data and theories as we attempted to understand just what it was and how it could have come into existence.”
The team conducted a slew of experiments to examine the material’s characteristics. As the details emerged, the evidence suggested the strange substance had been deposited in layers on grains of ice and minerals present in the asteroid.
It was also flexible – a pliable material, similar to used gum or even a soft plastic. Indeed, during their work with the samples, researchers noticed the strange material was bendy and dimpled when pressure was applied. The stuff was translucent, and exposure to radiation made it brittle, like a lawn chair left too many seasons in the sun.
“Looking at its chemical makeup, we see the same kinds of chemical groups that occur in polyurethane on Earth,” said Sandford, “making this material from Bennu something akin to a ‘space plastic.’”
The ancient asteroid stuff isn’t simply polyurethane, though, which is an orderly polymer. This one has more “random, hodgepodge connections and a composition of elements that differs from particle to particle,” said Sandford. But the comparison underscores the surprising nature of the organic material discovered in NASA’s asteroid samples, and the research team aims to study more of it.
By pursuing clues about what went on long ago, deep inside an asteroid, scientists can better understand the young solar system – revealing the precursors to and ingredients of life it already contained, and how far those raw materials may have been scattered, thanks to asteroids much like Bennu.
Abundant supernova dust
Another paper in the journal Nature Astronomy, led by Ann Nguyen of NASA’s Johnson Space Center in Houston, analyzed presolar grains – dust from stars predating our solar system – found in two different rock types in the Bennu samples to learn more about where its parent body formed and how it was altered by geologic processes. It is believed that presolar dust was generally well-mixed as our solar system formed. The samples had six-times the amount of supernova dust than any other studied astromaterial, suggesting the asteroid’s parent body formed in a region of the protoplanetary disk enriched in the dust of dying stars.
The study also reveals that, while Bennu’s parent asteroid experienced extensive alteration by fluids, there are still pockets of less-altered materials within the samples that offer insights into its origin.
“These fragments retain a higher abundance of organic matter and presolar silicate grains, which are known to be easily destroyed by aqueous alteration in asteroids,” said Nguyen. “Their preservation in the Bennu samples was a surprise and illustrates that some material escaped alteration in the parent body. Our study reveals the diversity of presolar materials that the parent accreted as it was forming.”
NASA’s Goddard Space Flight Center provided overall mission management, systems engineering, and the safety and mission assurance for OSIRIS-REx. Dante Lauretta of the University of Arizona, Tucson, is the principal investigator. The university leads the science team and the mission’s science observation planning and data processing. Lockheed Martin Space in Littleton, Colorado, built the spacecraft and provided flight operations. Goddard and KinetX Aerospace were responsible for navigating the OSIRIS-REx spacecraft. Curation for OSIRIS-REx takes place at NASA’s Johnson Space Center in Houston. International partnerships on this mission include the OSIRIS-REx Laser Altimeter instrument from CSA (Canadian Space Agency) and asteroid sample science collaboration with JAXA’s (Japan Aerospace Exploration Agency’s) Hayabusa2 mission. OSIRIS-REx is the third mission in NASA’s New Frontiers Program, managed by NASA’s Marshall Space Flight Center in Huntsville, Alabama, for the agency’s Science Mission Directorate in Washington.
For more information on the OSIRIS-REx mission, visit:
https://www.nasa.gov/osiris-rex
Karen Fox / Molly Wasser
Headquarters, Washington
202-285-5155 / 240-419-1732
karen.c.fox@nasa.gov / molly.l.wasser@nasa.gov