Tryptophan, the essential amino acid behind the Thanksgiving myth that eating turkey can make you sleepy, has been found to exist on Bennu, a small asteroid that swings by our planet about every six years.
The discovery comes from an unprecedented sample collected by NASA’s OSIRIS-REx mission, which landed a spacecraft on the asteroid in 2020, picked up 4.3 ounces (121.6 grams) of rock and dust, and safely returned the cache to Earth in 2023.
The study of Bennu is important because its composition mirrors that of the early solar system, giving scientists a glimpse into the beginnings of life. Previous research on Bennu samples had already found 14 of the 20 amino acids that come from all living organisms on Earth, as well as all five biological nucleobases — the components that make up the genetic code in DNA and RNA.
The researchers also previously found amino acids in samples from another asteroid, Ryugu, which the Japan Aerospace Exploration Agency collected in 2019, as well as in several meteorites that fell to Earth. This growing body of evidence suggests that asteroids may have delivered essential ingredients of life to our planet early on, according to experts.
Now, a new analysis of Bennu samples has confidently, though not yet conclusively, identified tryptophan, increasing the total protein-building amino acids in the asteroid to 15 out of 20.
A vial containing part of the sample from asteroid Bennu is held by Jason Dworkin, project scientist for NASA’s OSIRIS-REx mission, in 2023. – James Tralee/NASA
“The discovery of tryptophan in the asteroid Bennu is a big deal, because tryptophan is one of the most complex amino acids, and it has never been seen in any meteorite or space sample before,” said José Aponte, an astrochemist at the Astrobiology Analytical Laboratory at NASA’s Goddard Space Flight Center. He co-authored a study on the findings that was published Monday in the journal PNAS.
The presence of tryptophan in an asteroid supports the idea that the recipe for life may not have started only on Earth, Aponte added in an email: “Seeing it form naturally in space tells us that these ingredients were already being made in the early Solar System. This would have made it easier for life to start.”
Jigsaw pieces
Bennu, whose name refers to an ancient Egyptian deity associated with the sun, creation and rebirth, stretches about a third of a mile wide. The space rock likely represents a breakup from a much larger asteroid sometime between 2 billion and 700 million years ago. It probably formed in the main asteroid belt between Mars and Jupiter, and its chemical composition reflects the beginning of the solar system, dating back about 4.5 billion years, according to NASA.
The asteroid has been orbiting close to Earth for about 1.75 million years. The data showed that it could hit our planet in the year 2182, potentially leading to a “global winter.” Scientists currently estimate that the odds of an impact are 1 in 2,700, or 0.037% chance.
Originally, the material that Bennu is made of came from supernovas, explosions of old stars that occurred long before the formation of the solar system. The extreme heat of the explosions acted as a forge, cooking the elements found in the asteroid, which then absorbed more heat from the impact that formed Bennu, as well as radiation from the sun, and continued to change the elements inside. Bennu was also found to contain ammonia, a chemical that can help form molecules such as amino acids, as well as different types of minerals, which present many of the ingredients needed to create the building blocks of life – but not life itself.
A container that holds rock and dust from the Bennu asteroid. – Erika Blumenfeld and Joseph Aebersold/NASA
“They’re like jigsaw pieces that haven’t been assembled yet,” said Angel Mojarro, a postdoctoral researcher and organic geochemist in the Astrobiology Analytical Laboratory at NASA’s Goddard Space Flight Center and first author of the new study. “What it’s telling us is that many, many of the building blocks of life can be produced naturally within asteroids or comets, and the discovery of tryptophan expands the alphabet of amino acids that are produced in space and could have been delivered to Earth.”
A total of 33 amino acids were previously found on Bennu, but only 14 of them are used by living organisms on Earth to build proteins. Tryptophan would join the latter group; it also belongs to a category of amino acids that scientists call essential, because the human body cannot produce them and they must be acquired through the diet.
Mojarro added that more tests are needed to corroborate the presence of tryptophan in the Bennu sample analyzed for the study, which weighed only 50 milligrams. However, given the pristine condition of the Bennu samples, it is likely that the find is not the result of terrestrial contamination, according to George Cody, a staff scientist at the Carnegie Institution for Science in Washington, DC. Cody was not involved in the study but worked on Bennu samples.
“I believe these molecules are legitimately derived from the Bennu asteroid,” Cody wrote in an email.
These images, taken by the PolyCam camera of the OSIRIS-REx spacecraft in 2018, show four views of the asteroid Bennu along with a global mosaic. – NASA/Goddard/University of Arizona
By collecting the sample from the asteroid itself, the researchers did not have to deal with atmospheric entry damage, which changes the chemistry of asteroids that land on Earth, making Bennu a much more reliable “time capsule” of the composition of the early solar system.
“Because OSIRIS-REx returned these pristine samples, we are finally seeing the fragile salts, minerals and organics that meteorites lose upon entry,” said Dante Lauretta, professor of planetary science and cosmochemistry at the University of Arizona, Tucson, who is also a coauthor of the new study.
Bennu’s parent body was a rich geological world with multiple liquid systems operating in different places and at different times, each one conducting its own chemistry,” added Lauretta. “Bennu preserves a collection of distinct chemical systems and together they show that small bodies were dynamic and organic-rich systems long before life appeared on Earth.”
molecular ‘fossils’
This paper adds to scientists’ understanding of what molecules necessary for life can be found in extraterrestrial materials, Cody said. If the natural chemistry that occurred at the beginning of our solar system produces the same molecules that life uses today, he added, then there must be a connection between them.
The United Launch Alliance Atlas V rocket that took the OSIRIS-REx spacecraft into space, at Cape Canaveral Air Force Station in Florida on September 8, 2016. – Kim Shiflett/NASA
The late Harold Morowitz, a pioneer of studies on the origin of life, believed that the molecules that constitute the core of living organisms could be molecular “fossils” from the beginning of the solar system, and the identification of tryptophan and other amino acids that build proteins in Bennu samples adds weight to that idea, Cody explained.
The discovery of tryptophan in Bennu further expands the remarkable diversity of compounds we now know can come from space, said Kate Freeman, Evan Pugh University Professor at Penn State University, in an email. “Asteroids were Earth’s early grocery delivery service, providing a wealth of molecules to our prebiotic world,” added Freeman, who was not involved in the study.
The new research also highlights how important sample return missions are, according to Sara Russell, professor of planetary sciences and leader of the Planetary Materials Group at the Natural History Museum in London, who did not participate in the work. Although scientists have thousands of rocks from space available in laboratories in the form of meteorites, they also need virgin and uncontaminated material brought to Earth by space missions in order to get the full picture, she said.
“The discovery of tryptophan in particular is surprising,” Russell added, “as we don’t see it in meteorites, perhaps because it doesn’t survive the fall from the Earth’s atmosphere and the impact on Earth.”
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