金星に生命体は存在し得ない　大気の水分濃度低く　論文

【パリＡＦＰ＝時事】金星の大気に含まれる水分濃度を測定した結果、金星を厚く覆っている雲の主成分である硫酸の微粒子の中では、地球で確認できるような生命体は存在し得ないとする論文が２９日、英科学誌ネイチャー・アストロノミーに掲載された。（写真は資料写真）
　金星に生命体が存在する可能性をめぐっては、２０２０年の研究論文で、地球では生命体から発生するガス「ホスフィン（リン化水素）」の痕跡を金星の大気から検出したことが発表され、その期待が高まっていた。
　英クイーンズ大学ベルファストが主導する研究チームは、２０２０年の研究論文に触発されてこの理論を違う角度から検証し、金星の大気中に生命体が存在し得るだけの水分が存在するのかを調べた。
　同大学の微生物学者、ジョン・ホールズワース氏は、陸生真菌が相対湿度５８．５％でも生存可能なことを２０１７年に発見している。これは生命活動が確認された中で最も乾燥した環境と確認された。
　このたび会見を行ったホールズワース氏は、地球上で最も乾燥に耐性がある微生物が金星でも活動し得ることを立証しようと試みたと話し、その上で、金星の大気に含まれるわずかな水分で活動できるような微生物が存在しないことを指摘した。金星の大気中の水分は相対湿度０．４％に相当する。
　同氏は、０．４％という相対湿度は、地球で微生物の生存が確認された最低の相対湿度を大きく下回り、その差は１００倍以上にもなると説明。「生命が活動するために必要な湿度とは、桁違いの差がある」と続けた。
　研究チームは金星の水分濃度を計算するため、１９７０年代後半から１９８０年代前半にかけて米国と旧ソ連が金星に送った７機の探査機と１機の軌道周回機による既存の測定値を使用した。
　米航空宇宙局（ＮＡＳＡ）の惑星科学者で、論文の共同執筆者のクリス・マッケイ氏は、結論の根拠としたデータが入手可能な範囲に限定された直接的な観測結果であることから研究結果は不完全であると指摘し、「このまま研究を続けても、結果が変わるとは考えにくい」と記者団に述べた。
　２０３０年前後には、あと三つの金星探査計画が予定されており、マッケイ氏は、今回の研究で使用された測定値を確認できるはずだとしている。
　さらに、数十億年前の金星に生命体が存在していたかどうかについては、探査の一つで解明される可能性があると述べた。【翻訳編集ＡＦＰＢＢＮｅｗｓ】
〔ＡＦＰ＝時事〕（2021/07/01-12:17）

A study measuring water concentration in Venus's atmosphere concluded Monday that life as we know it is not possible among the sulphuric acid droplets that make up the planet's famously cloudy skies.
The search for life on our nearest neighbour has so far proved fruitless, although a 2020 paper rekindled hopes for Venus when it claimed to have detected phosphine gas -- known to be produced by bacteria on Earth -- in the planet's clouds.
The authors have since called their own findings into question.
But the claim inspired scientists led by Queen's University Belfast to test the theory from a different angle: whether there is enough water in Venus's atmosphere to make life possible.
In 2017, microbiologist John Hallsworth discovered a terrestrial fungus that can survive at 58.5 percent relative humidity -- the driest conditions at which biological activity has ever been measured.
We bent over backwards to argue that the most extreme, tolerant microbes on Earth could potentially have activity on Venus, said Hallsworth at a press conference.
But he said nothing could cope with the miniscule amount of water in the planet's atmosphere, which is equivalent to a relative humidity of 0.4 percent.
It's more than 100 times too low. It's almost at the bottom of the scale, at an unbridgeable distance from what life requires to be active.
- Jupiter 'more optimistic' -
To calculate the concentration of water, scientists used existing measurements from seven US and Soviet probes and one orbiter mission sent to Venus in the late 1970s and early 1980s.
Chris McKay, a NASA planetary scientist and co-author of the research published in Nature Astronomy, noted that the conclusions of the study were based on the limited direct observations available, and therefore incomplete.
It's hard to imagine that the results will change as we do further exploration, McKay told reporters.
The team also analysed measurements taken from probes that visited other planets -- and discovered potentially the right amount of water activity to support life in the clouds of Jupiter.
The results were much more optimistic, said McKay.
There is at least a layer in the clouds of Jupiter where the water requirements are met.
He emphasised, however, that it is much easier to rule out life in Venus's atmosphere than to prove life is possible in Jupiter's clouds.
To show that that layer is habitable we would have to go through all the requirements for life and show that they're all met, he said, adding that determining things like ultraviolet exposure and energy sources would require further exploration.
- 'Search for life' -
Three more Venus missions are planned for sometime around 2030 and McKay feels certain they will confirm the measurements used for the study.
He also said that one mission could shed light on a question not addressed by current research: whether life could have existed on Venus several billion years ago.
There could've been a time when Venus was earth-like, McKay said.
One of the missions will fly through the atmosphere and measure trace gases... which will tell us a lot about Venus's evolutionary history and will start to address questions like how much atmosphere did Venus have, where did it go, what happened?
And the study's authors hope their method of determining water activity will be applied to planets beyond our solar system -- especially with the upcoming launch of the James Webb Space Telescope (JWST) later this year.
The JWST will be able to determine atmospheric profiles of temperature, pressure and water abundance in exoplanet atmospheres, the study concludes.
These will allow assessments of water activity in their atmospheres using our approach.