The OSIRIS-REx spacecraft returned a sample from asteroid Bennu in 2023, and following a year of in-depth analyses in labs across the globe, researchers conclude that these conditions and ingredients may have been common across the solar system, increasing the odds of life forming on other planets and moons.
"These samples from Bennu are an incredible discovery, showing that the building blocks of life were widespread across the early solar system," said Lauretta, Regents Professor of Planetary Science and Cosmochemistry at the U of A Lunar and Planetary Laboratory and a co-author on both papers. "By studying how these ingredients interacted in environments like those on Bennu and in places inferred for the early Earth - such as salty ponds similar to those Darwin once imagined - we can better understand how life might emerge and where to search for it beyond our planet."
Bennu coalesced from a small portion of the leftover rubble resulting from a giant collision of asteroids. Preserved in the vacuum of space since the solar system's formation about 4.5 billion years ago, the samples have provided scientists with unparalleled insights into the conditions of that era.
Based on their findings, presented in two publications in Nature and Nature Astronomy on Jan. 29, the researchers share several theories about the history of Bennu and the solar system.
Bennu's molecular composition suggests the ice and organic compounds in its parent body originated in the extremely cold outermost disk of gas and dust that gave rise to the solar system.
Temperatures in the outer disk could dip to minus 400 degrees Fahrenheit, allowing volatile gases that easily evaporate in warmer conditions to accumulate and freeze - among them water vapor, carbon dioxide, methane and ammonia, which was detected in "exceptionally high" abundances in the Bennu samples, according to the Nature Astronomy paper.
Given the right environment, ammonia can react with formaldehyde, which was also detected in the samples, to form complex molecules such as amino acids - the building blocks of proteins. Fourteen of the 20 amino acids that life on Earth uses to make proteins are found in the Bennu sample. The research team also found all five nucleobases that life on Earth uses to encode structural information in more complex biomolecules like DNA and RNA.
"Besides pointing to the outer solar system origin of abundant ammonia in Bennu's ancestor, our work also supports the idea that objects that formed far from the sun could have been an important source of the raw ingredients for life throughout the solar system," said Danny Glavin, a senior sample scientist at NASA's Goddard Space Flight Center in Greenbelt, Maryland. Glavin, together with colleague Jason Dworkin, led the paper in Nature Astronomy.
With life's ingredients found in Bennu samples, the big question is: How did these building blocks turn into the chains of molecules needed to activate biology?
"You can have all the ingredients for whatever you want to make, but you have to have the environment to make them do something," said Tim McCoy, curator of meteorites at the Smithsonian's National Museum of Natural History in Washington, D.C.
Along with Sara Russell of the Natural History Museum in London, McCoy co-led 66 scientists from four continents in a study of minerals in the Bennu samples. In the Nature paper, they describe evidence of an ancient environment well-suited to kickstart the chemistry of life.
Ranging from calcite to halite and sylvite, scientists identified 11 minerals that comprise a complete set of "evaporites" from a brine, or salt-saturated water. These evaporites form as water containing dissolved salts evaporates over long periods of time, leaving behind the salts as solid crystals.
"We're seeing minerals in Bennu samples that we have never seen before in a meteorite or any extraterrestrial sample," McCoy said.
Finding evaporites indicates that the interior of Bennu's ancestor was warm enough to support liquid water for a substantial amount of time. Liquid water is necessary for life, as it facilitates its essential chemical reactions, while salts can prevent water from freezing. Salts also help concentrate simple molecules, making it easier for them to combine into the complex compounds life depends on.
While several evaporites have been reported from meteorite samples, the Bennu sample represents the first time researchers have seen a complete set preserving an evaporation process that could have lasted thousands of years or more. This process happens in basins of water on Earth, including drying lakes and shallow seas. The researchers deem it possible that on Bennu's ancestor, water could have existed in underground pockets or veins, but not on the surface, as it would have quickly boiled away due to lack of atmospheric pressure.
U of A co-authors on the two publications include Jessica Barnes, Harold Connolly, Dani DellaGiustina, Pierre Haenecour, Dolores Hill, Tom Zega and Zoe Zeszut, all of whom helped with sample analysis taking advantage of the advanced technological resources of U of A's Kuiper-Arizona Laboratory for Astromaterials Analysis. The following graduate students also were part of this work: Maizey Benner, Kana Ishimaru, Nicole Kerrison, Iunn Ong, Beau Prince, Lucas Smith.
Research Report:Abundant ammonia and nitrogen-rich soluble organic matter in samples from asteroid (101955) Bennu
Research Report:An evaporite sequence from ancient brine recorded in Bennu samples
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Kuiper-Arizona Laboratory for Astromaterials Analysis
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