小惑星から採取の試料に「生命をつくる基」 研究

【パリＡＦＰ＝時事】小惑星ベンヌから採取した試料に「生命をつくる基」となる物質が含まれていたとする二つの研究論文が２９日、発表された。地球上の生命がどのように始まったのかという長年の疑問に光を当てる発見だ。（写真は米ワシントンの自然史博物館で初めて一般公開された地球近傍小惑星「ベンヌ」の試料サンプル）
　「生命をつくる基」の物質は、米航空宇宙局（ＮＡＳＡ）の無人探査機「オシリス・レックス」が地球近傍小惑星「ベンヌ」から２０２０年に採取した１２０グラムの試料の中に含まれていた。
　ベンヌから採取されたサンプルは２３年、地上約３億キロの距離から投下カプセルで回収された。これまでの分析で、試料には水や炭素が含まれていたことがすでに判明している。
　ベンヌは約６５００万年前に、約４５億年前の母天体の破片から形成されたと考えられている。さらなる研究で、ベンヌの母天体にはかつて液体の水があり、それらの液体の水が蒸発することで塩と鉱物の「塩辛いスープ」が残ったことが示唆されている。
　論文の共著者でスミソニアン国立自然史博物館の隕石（いんせき）学の専門家であるティム・マッコイ氏は、「われわれは、生命への道の次のステップを発見した」と、博物館が発表のプレスリリースで述べた。
　博物館によると、鉱物の中には、これまで地球外から採取した試料では見られなかった化合物も含まれていたという。
　研究論文の一つは、分析の結果、試料は「地球外起源」であることを強く示唆しており、地球上の生命の起源が宇宙空間からもたらされたという説を裏付ける可能性があるとしている。【翻訳編集ＡＦＰＢＢＮｅｗｓ】
〔ＡＦＰ＝時事〕（2025/01/30-19:33）

Pristine samples of the asteroid Bennu transported to Earth contain the basic building blocks for life, shedding new light on the perennial question of how life began on our planet.
The revelation, in two studies published Wednesday, is the result of work on just 120 grams of material -- about the weight of a banana -- collected from Bennu by NASA's OSIRIS-REx spacecraft in 2020.
The samples from Bennu, then around 300 million kilometres (186 million miles) from Earth, were returned in a capsule that OSIRIS-REx dropped off during a pass-by in 2023.
Initial analysis had already revealed evidence of high-carbon content and water.
But the new research found that evaporated water on Bennu's parent asteroid left behind the raw ingredients of life, said Tim McCoy, curator of meteorites at the Smithsonian's National Museum of Natural History and co-lead author of one of the studies.
We have discovered that next step on a pathway to life, he said in a press release issued by the museum.
Bennu appears to have formed around 65 million years ago from the debris of a parent asteroid dating back some 4.5 billion years.
The findings suggest Bennu's parent was once home to pockets of liquid water. When these evaporated, they left behind a briny broth of salts and minerals.
Some of the minerals include compounds that have never been seen in samples from outer space, the museum said.
And analysis of the samples strongly suggests a non-terrestrial origin, adds one of the studies.
That could lend support to the theory that life on Earth was seeded from outer space.
The samples give unprecedented insight into the processes that drove the formation of the Solar System, according to Yasuhito Sekine, a professor at the Institute of Science Tokyo.
This discovery was only possible by analysing samples that were collected directly from the asteroid then carefully preserved back on Earth, he added.
The salts would otherwise have rapidly absorbed moisture in the Earth's humid atmosphere.
- 'Huge progress' -
The researchers believe similar salty brines may exist on other extraterrestrial bodies, including the dwarf planet Ceres and Saturn's moon Enceladus, as well as other asteroids.
They plan to reexamine specimens already on Earth for traces of compounds that previous research might have missed.
Even though asteroid Bennu has no life, the question is could other icy bodies harbour life? said Nick Timms, an associate professor at Curtin University's School of Earth and Planetary Sciences also involved in the research.
Much about life's origin remains unclear despite the secrets revealed from Bennu, McCoy cautioned.
We now know we have the basic building blocks to move along this pathway towards life, but we don't know how far along that pathway this environment could allow things to progress, he said.
Still, Sara Russell, co-lead author with McCoy and a cosmic mineralogist at the museum, said the research had made huge progress in understanding how asteroids like Bennu evolved, and how they may have helped make the Earth habitable.
OSIRIS-REx wasn't the first probe to rendezvous with an asteroid and bring back samples for study -- Japan succeeded in the feat twice, returning celestial dust in 2010 and 2020.
In addition to scientific insights, better understanding of Bennu's composition could prove useful if humanity ever needs to steer it away.
Space agencies are constantly monitoring asteroids over potential impact risks.
A recently discovered asteroid dubbed 2024 YR4, estimated to be between 40 and 100 metres (130 and 330 feet) wide, has a 1.2 percent chance of impacting Earth on December 22, 2032, the European Space Agency said Wednesday.
That is only considered a level-3 risk on the 10-level Torino Impact Hazard Scale, the ESA said, adding that an asteroid's chances of impact often drop after additional observations.