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.
