金星に生命の痕跡か　大気からホスフィン検出

【パリＡＦＰ＝時事】金星の大気から、地球では生命体によって生み出されるガスであるホスフィン（リン化水素）の痕跡を検出したとする研究論文が14日、英科学誌ネイチャー・アストロノミーに掲載された。米航空宇宙局（NASA）のジム・ブライデンスタイン長官は、地球外生命体探査史上「最大」の発見だとしている。（写真は金星を捉えた紫外線写真）
　金星は地球に最も近い惑星だが、日中の気温は鉛が溶けるほど高く、大気はほぼすべてが二酸化炭素で構成されていることから、しばしば地獄のような環境だと表現される。
　論文を発表した研究チームは米ハワイとチリのアタカマ砂漠にある望遠鏡を使い、金星の表面から約６万メートル上空にある雲の上層部を観測し、ホスフィンの痕跡を検出した。
　ホスフィンは地球上では有機物の分解により発生することの多い可燃性ガス。論文の主執筆者であるカーディフ大学宇宙物理学部のジェーン・グリーブズ氏はＡＦＰの取材に対し、ホスフィンが地球以外の岩石惑星で検出されたのは初めてだと説明した。
　研究チームは、ホスフィンの存在が金星上の生命の存在を証明するものでないと強調しているが、灼熱（しゃくねつ）の金星表面を渦巻く雲は強酸性でホスフィンを即座に破壊することから、金星にはホスフィンを生み出しているものが存在することが示されたとも指摘している。
　ホスフィンの生成を理解するため、研究者らはモデル化を繰り返し試みた。そして、金星に「変則的かつ未解明の化学現象」が存在する証拠となったと結論付けた。
　ＮＡＳＡのブライデンスタイン長官はツイッターに「金星に生命？ 嫌気性生物が生み出すホスフィンの発見は、地球外生命体の存在を示す上でこれまでで最大の進展だ。ＮＡＳＡは約１０年前、地球の上空１２万フィート（約３万６６００メートル）の高層大気で微生物を発見した。金星を優先させる時がきた」と投稿した。【翻訳編集ＡＦＰＢＢＮｅｗｓ】
〔ＡＦＰ＝時事〕（2020/09/15-12:36）

The atmosphere of Venus contains a gas that on Earth can be attributed to living organisms, scientists said Monday, a discovery the head of NASA called the most significant development yet in the hunt for extraterrestrial life.
Conditions on our planetary neighbour are often described as hellish with daytime temperatures hot enough to melt lead and an atmosphere comprised almost entirely of carbon dioxide.
However, a team of experts detected traces of phosphine, a flammable gas that on Earth often occurs from the breakdown of organic matter.
They used telescopes in Hawaii and Chile's Atacama Desert to observe Venus' upper cloud deck, around 60 kilometres (45 miles) from the surface.
Writing in Nature Astronomy, the team stressed the presence of phosphine did not prove the presence of life on Venus.
But, as the clouds swirling about its broiling surface are highly acidic and therefore destroy phosphine very quickly, the research did show that something was creating it anew.
The researchers conducted several modelling calculations in a bid to explain the new phosphine production.
They concluded that their research provided evidence for anomalous and unexplained chemistry on Venus.
Lead author Jane Greaves, from Cardiff University's School of Physics and Astronomy told AFP that the presence of phosphine alone was not proof of life on Earth's next door neighbour.
I don't think we can say that -- even if a planet was abundant in phosphorus, it might lack something else important to life -- some other element, or conditions might be too hot, too dry, she said.
Greaves added that it was the first time phosphine had been found on a rocky planet other than Earth.
The breakthrough was hailed by NASA administrator Jim Bridenstine, who tweeted, it's time to prioritize Venus.
Life on Venus? The discovery of phosphine, a byproduct of anaerobic biology, is the most significant development yet in building the case for life off Earth, he wrote.
The bulk of current efforts to look for past extraterrestrial life focus on Mars, which is known to have once contained all the necessary ingredients to support carbon-based organisms.
The US and China recently sent rovers to the Red Planet, while the UAE sent an atmospheric probe.
- 'Exciting' -
Reacting to the study, Alan Duffy, an astronomer from Swinburne University and Lead Scientist of The Royal Institution of Australia, said it while it was tempting to believe that the phosphine was produced by lifeforms, we have to rule out all possible other non-biological means of producing it.
He called the finding one of the most exciting signs of the possible presence of life beyond Earth I have ever seen.
Thomas Zurbuchen, associate administrator of NASA's Science Mission Directorate, which has conducted several fly-bys of Venus, called Monday's research intriguing.
As with an increasing number of planetary bodies, Venus is proving to be an exciting place of discovery, though it had not been a significant part of the search for life, he tweeted.
He added that the planet was the focus of two out of four of NASA's next four candidate missions under its Discovery Program, as well as Europe's proposed EnVision mission, in which NASA is a partner.
Venus, which rotates in the opposite direction to Earth and where a day lasts 243 times longer, is a subject of intense interest among astronomers.
It is so close and of such similar size to our home planet that some experts believe it serves as a warning of the dangers of runaway climate change.
Previous studies have unearthed tantalising clues suggesting Venus has active volcanoes, including signs of recent lava flows.