(ORDO NEWS) — More ingredients for life found in meteorites.
Space rocks that have fallen to Earth over the past century contain five bases that store information in DNA and RNA, scientists report April 26 in the journal Nature Communications.
These “nucleobases” – adenine, guanine, cytosine, thymine and uracil – in combination with sugars and phosphates make up the genetic code of all life on Earth.
Whether these basic ingredients for life came from outer space or formed in the warm soup of Earth’s chemistry is still unknown (SN: 9/24/20). But according to the researchers, the discovery adds to the evidence that life’s precursors originally came from outer space.
Scientists have found pieces of adenine, guanine and other organic compounds in meteorites since the 1960s (SN: 8/10/11, SN: 12/4/20). Researchers have also seen hints of uracil, but cytosine and thymine have remained elusive so far.
“We have completed the set of all the bases found in DNA, RNA and life on Earth, and they are present in meteorites,” says astrochemist Daniel Glavin of NASA‘s Goddard Space Flight Center in Greenbelt, Maryland.
A few years ago, geochemist Yasuhiro Oba of the University of Hokkaido in Sapporo, Japan, and colleagues developed a technique to accurately extract and separate various chemical compounds in liquid meteorite dust and then analyze them.
“Our detection method has orders of magnitude higher sensitivity than that used in previous studies,” says Ohba. Three years ago, researchers used the same method to detect ribose, a sugar essential for life, in three meteorites (SN: 11/22/19).
In the new study, Oba and his colleagues joined forces with NASA astrochemists to analyze one of the three meteorites and three more in search of another type of essential ingredient for life, the nucleobase.
The researchers believe their gentler extraction method, which uses cold water instead of regular acid, keeps the compounds intact. “We found that this extraction method is very convenient for these fragile nucleobases,” says Glavin. “It’s more like cold brewing than hot tea.”
Using this method, Glavin, Both and their colleagues measured the abundance of bases and other life-related compounds in four meteorite samples that fell decades ago in Australia, Kentucky, and British Columbia.
In all four samples, the team detected and measured adenine, guanine, cytosine, uracil, thymine, several compounds associated with these bases, and several amino acids.
Using the same methodology, the team also measured the chemical abundance in soil collected from the Australian site and then compared the meteorite’s readings to those of the soil. For some of the compounds found, meteorite rates were higher than those found in the surrounding soil, suggesting that these compounds made their way to Earth in these rocks.
But for other compounds found, including cytosine and uracil, the content in soil is 20 times higher than in meteorites. This could indicate terrestrial pollution, says space chemist Michael Callahan of Boise State University in Idaho.
“I think the researchers positively identified these compounds,” Callahan says. But “they haven’t presented enough convincing data to convince me that they are really extraterrestrial.” Callahan had previously worked for NASA and collaborated with Glavin and others on measuring organic materials in meteorites.
But Glavin and his colleagues point to a few specific chemicals that have been found to support the interplanetary origin hypothesis. In the new analysis, the researchers measured more than a dozen other life-related compounds, including nucleobase isomers, Glavin says.
Isomers have the same chemical formulas as their associated bases, but their ingredients are organized differently. The team found some of these isomers in meteorites, but not in soil. “If there was contamination from the soil, we should have seen these isomers in the soil as well. But we didn’t,” he says.
Going directly to the source of such meteorites – pristine asteroids – may shed light on this issue. Both and colleagues are already using their method to extract pieces from the surface of the asteroid Ryugu, which Japan’s Hayabusa2 mission will bring to Earth in late 2020 (SN: 12/7/20). NASA’s OSIRIS-REx mission is due to return in September 2023 with similar samples from asteroid Bennu (SN: 1/15/19).
“We’re very excited about the stories these materials can tell,” says Glavin.
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