Since Hayabusa2 delivered its material to Earth in 2020, several studies have shown that this extraterrestrial treasure consists of organic molecules that serve as building blocks of life and contains water, which could provide clues to the origin of the substance on Earth. Scientists say this discovery provides a better understanding of how ingredients for life on Earth may have come from space, although they are still working to learn the exact mechanisms.
The Hayabusa2 mission marks the second time humankind has retrieved a piece of an asteroid in space and brought it back to Earth for analysis. NASA’s OSIRIS-REx will bring a sample back to Earth from asteroid Bennu this year.
“There’s a lot of things you can do once you have a good definition of life and a way of making a life from nonlife. It’ll be a really profound thing because you only have one example of life. All life on Earth is connected,” said NASA astrochemist Jason Dworkin, who helped lead analysis of the organic material in the Ryugu sample and is project scientist for NASA’s OSIRIS-REx mission.
To an untrained eye, the sample may not look that different from rock on Earth, said Dworkin. The carbon-rich asteroid “is blacker than what most people see as black,” as dark as the darkest asphalt.
Researchers classified this carbon-containing asteroid as a carbonaceous chondrite (stony meteorite). That material is some of the most common and ancient in the solar system. But the Ryugu chondrite is one of the rarest types to encounter on Earth. Researchers found that the iron composition of Ryugu is similar to that of only five known meteorites out of more than a thousand found on Earth’s surface. These particular rocks are thought to originate from the same region in space, on the outskirts of the solar system.
“These are very primitive meteorites, and they’re among the most rare that we have in our collection,” said the planetary scientist Fred Ciesla, who is not involved in Ryugu research. “The fact that we went to Ryugu [and] were able to get a sample of something that is so otherwise rare here is very exciting because it allows us to study these things in a whole new way.”
Early studies showed that Ryugu contained a similar abundance of elements as our Sun, meaning it probably formed at the same time as our young host star. “This gives you a window on what the solar system was like at formation four and a half billion years ago,” said Dworkin.
Laboratory measurements showed the asteroid sample had more water than scientists initially thought on the basis of Hyabusa2’s remote measurements. NASA scientist Kaitlyn McCain, who published research on the mineral sample, said the presence of water on asteroids such as Ryugu could help explain the presence of some of Earth’s water. Scientists are interested in how Earth got its water, because it is a bit too close to the Sun to have formed its own.
“It seems like Ryugu, and especially because it’s so uncontaminated by the Earth, represents a really unique way to test whether Ryugu, meteorites and asteroids like it, could have served as a real source of water to the Earth,” said McCain. “There are some studies that suggest that it could have. It contains the right oxygen composition to essentially serve as a portion of Earth’s water source.”
The organic cosmochemist Christian Potiszil said what is present in the sample is not pure fresh water to drink but probably is water mixed with various chemicals over a long period that could make it salty or even toxic today. But, he found, the water could have been critical for forming certain molecules. Reactions between the water and the rock over a long period, such as through a process known as aqueous alteration, could have helped form salts and organic material.
“Aqueous alteration is like effectively putting a load of rock in water in a big beaker and leaving it for a very long time, maybe at room temperature, maybe a bit higher,” said Potiszil, an assistant professor at Okayama University in Japan. “Basically, certain reactions happen, and some of these reactions can even favor the generation” of organic compounds, including those critical for life.
Researchers are interested in analyzing the sample for the presence of amino acids, which are present in all living things on Earth. These organic molecules are the building blocks of proteins, which help catalyze reactions, replicate genetic material, transport molecules and provide structural support to our cells.
Dworkin said the Ryugu sample contained “a fairly simple mixture” of nearly two dozen amino acids that were also found on similar meteorites exposed to water. Potiszil, working on a separate team from Dworkin’s, said he and his colleagues found the amino acid dimethylglycine (used within the human body as part of our metabolism), which has not been found on other meteorites, probably because he and his colleagues used a new technique in analyzing the Ryugu sample. Other research has found carbonates, sulfites, nitrogen-containing compounds and magnetite, all of which form in the presence of liquid water.
“If asteroids like Ryugu, or fragments of them, did bombard the early Earth, then understanding their organic matter inventory can help us to understand what kinds of amino acids and other building blocks of life might have been available at the origin of life on Earth,” said Potiszil.
Researchers also found the nucleobase uracil in the sample. Nucleobases are building blocks of RNA, which converts information stored in DNA to proteins. Uracil has been spotted on other meteorites found on Earth, but finding some on an asteroid sample pulled in space rules out any possible contamination.
Hiroshi Naraoka of Kyushu University, who led the soluble organics analysis team, said these sample analyses are important because “prebiotic organic molecules could be transported from the asteroid surface over the solar system.”
And that could be a key for us to understand how life may have started on Earth and elsewhere.
Catalyst for life on Earth?
Many scientists hypothesize that at least some organic compounds and ingredients that initiated life on Earth came from space, but understanding the mechanisms is a bit rocky at the moment. Scientists do not even know which organic molecules are important for the origin of life, nor is there a well-understood mechanism [of] making life from nonlife, said Dworkin.
“We know about when. We don’t know how,” said Dworkin. “The actual chemistry of the early Earth is very hard to tell. It’s all based on models, assumptions, very little direct evidence.”
Aside from the specific chemical mechanisms, Dworkin said that asteroids could have been involved in delivering organic molecules to an early Earth but that Ryugu would not have been involved. Ryugu broke off from a larger asteroid after Earth had formed. But, he said, its parent body was an ancient asteroid that broke up, and bits could have traveled to the inner solar system, with some material landing on Earth. Or perhaps multiple asteroids or comets brought organic molecules to kick-start life on Earth.
“At current, we are unsure if Ryugu has all the building blocks of life necessary, but it could at least provide some of them and if mixed with other asteroidal material, then it should be enough,” said Potiszil.
Others, though, including Ciesla, do not think Earth directly inherited organic molecules from space. For one, he thinks that such organic molecules would have been destroyed in the early days of a hot, volatile Earth. Instead, he sees asteroids such as Ryugu as analogues of “what kinds of environments may have existed on the early Earth and how [organics would] have been produced there.”
In truth, we might never know exactly how life started on Earth, said Dworkin. But analyzing asteroid samples, such as from Ryugu, and Bennu later this year, could help us understand how life might form on a future Earth.
“I think we can find the answer to how life can start, which is a pretty compelling answer,” said Dworkin.