Scientists said on Monday they have detected in the harshly acidic clouds of Venus a gas called phosphine that indicates microbes may inhabit Earth’s inhospitable neighbor, a tantalizing sign of potential life beyond Earth.
The researchers did not discover actual life forms, but noted that on Earth phosphine is produced by bacteria thriving in oxygen-starved environments. The international scientific team first spotted the phosphine using the James Clerk Maxwell Telescope in Hawaii and confirmed it using the Atacama Large Millimeter/submillimeter Array (ALMA) radio telescope in Chile.
“I was very surprised — stunned, in fact,” said astronomer Jane Greaves of Cardiff University in Wales, lead author of the research published in the journal Nature Astronomy.
Photo: NASA via Reuters 照片:美國太空總署/路透
The existence of extraterrestrial life long has been one of the paramount questions of science. Scientists have used probes and telescopes to seek “biosignatures” — indirect signs of life — on other planets and moons in our solar system and beyond. “With what we currently know of Venus, the most plausible explanation for phosphine, as fantastical as it might sound, is life,” said Massachusetts Institute of Technology molecular astrophysicist and study co-author Clara Sousa-Silva.
“I should emphasize that life, as an explanation for our discovery, should be, as always, the last resort,” Sousa-Silva added. “This is important because, if it is phosphine, and if it is life, it means that we are not alone. It also means that life itself must be very common, and there must be many other inhabited planets throughout our galaxy.”
Venus has not been the focus of the search for life elsewhere in the solar system, with Mars and other worlds getting more attention. Phosphine — a phosphorus atom with three hydrogen atoms attached — is highly toxic to people. Earth-based telescopes like those used in this research help scientists study the chemistry and other characteristics of celestial objects.
Phosphine was seen at 20 parts-per-billion in the Venusian atmosphere, a trace concentration. Greaves said the researchers examined potential non-biological sources such as volcanism, meteorites, lightning and various types of chemical reactions, but none appeared viable. The research continues to either confirm the presence of life or find an alternative explanation.
Venus is Earth’s closest planetary neighbor. Similar in structure but slightly smaller than Earth, it is the second planet from the sun. Venus is wrapped in a thick, toxic atmosphere that traps in heat. Surface temperatures reach a scorching 471°C, hot enough to melt lead.
“I can only speculate on what life might survive on Venus, if indeed it is there. No life would be able to survive on the surface of Venus, because it is completely inhospitable, even for biochemistries completely different from ours,” Sousa-Silva said. “But a long time ago, Venus could have had life on its surface, before a runaway greenhouse effect left the majority of the planet completely uninhabitable.”
Some scientists have suspected that the Venusian high clouds, with mild temperatures around 30°C, could harbor aerial microbes that could endure extreme acidity. These clouds are around 90 percent sulphuric acid. Earth microbes could not survive that acidity. “If it’s microorganisms, they would have access to some sunlight and water, and maybe live in liquid droplets to stop themselves dehydrating, but they would need some unknown mechanism to protect against corrosion by acid,” Greaves said.
On Earth, microorganisms in “anaerobic” environments — ecosystems that do not rely on oxygen — produce phosphine. These include sewage plants, swamps, rice fields, marshlands, lake sediments and the excrements and intestinal tracts of many animals. Phosphine also arises non-biologically in certain industrial settings. To produce phosphine, Earth bacteria take up phosphate from minerals or biological material and add hydrogen.
NASA Administrator Jim Bridenstine called the new findings “the most significant development yet in building the case for life off Earth.” “It’s time to prioritize Venus,” Bridenstine wrote on Twitter. While previous robotic spacecraft have visited Venus, a new probe may be needed to confirm life. “Fortunately, Venus is right next door,” Sousa-Silva said. “So we can literally go and check.”
(Reuters)
科學家週一指出,他們在極度酸性的金星雲層中發現一種名為磷化氫的氣體,暗示微生物或許存在於不適人居的鄰近行星。這個徵兆暗示地球外的生命存在,相當吊人胃口。
研究人員並未發現真正的生命形態,但是他們提到,地球上的磷化氫來自缺氧環境中繁衍的細菌。該國際科學團隊最初使用夏威夷的「詹姆士克拉克麥克斯威爾望遠鏡」發現磷化氫的存在,而後再利用智利的「阿塔卡瑪大型毫米及次毫米波陣列」無線電波望遠鏡證實此發現。
該研究的主要作者、威爾斯卡地夫大學的天文學家珍‧葛瑞夫茲表示:「我感到非常驚訝──其實,我很震驚。」這篇研究日前刊登於《自然天文學》期刊。
地球外生命存在與否一直都是科學界的重大問題之一。多年來,科學家藉由太空探測船與望遠鏡,在太陽系內外的其他星球與衛星尋找「生物特徵」,也就是生命的間接跡象。研究共同作者、麻省理工學院的分子天體物理學家克拉拉‧索沙-席瓦說:「就我們目前對金星所知,磷化氫存在最合理的解釋,雖然聽起來很荒唐,代表有生命存在。」
索沙-席瓦補充表示:「我想要強調,生命,作為我們這次發現的一種解釋方式,應該和往常一樣,屬於最後的選項。」她說:「這點非常重要,如果這個氣體確實是磷化氫,如果生命確實存在,這代表我們並不孤單。這也會意味生命一定非常普遍,銀河系中一定還有許多有生命棲息的星球。」
金星一直都不是科學家尋找太陽系何處還有生命存在的主要目標,火星和其他行星獲得較多的關注。磷化氫是三個氫原子附著在一個磷原子所組成,對人類有強烈毒性。至於地球上的太空望遠鏡,也就是這份研究中使用的類型,則用來協助科學家研究天體的化學結構等特徵。
科學家觀察到磷化氫佔金星大氣層的十億分之二十,濃度屬於微量級。葛瑞夫茲指出,研究人員檢驗過其他可能的非生物性來源,例如火山活動、隕石、閃電,以及各類化學反應,但是似乎都不符合。這份研究將會持續,證實金星上有生命,或找到另一種解釋。
金星是最靠近地球的行星夥伴,兩者結構相似,但是金星比地球小,並且是太陽系從內數來的第二顆行星。金星被包裹在濃厚而有毒的大氣層內,高溫無法逸失,地表溫度可以達到炙熱的攝氏四百七十一度,足以把錫融化。
索沙-席瓦指出:「我只能揣測什麼樣的生命形態──如果真的存在──可能在金星上生存。沒有任何生命可以存活在金星表面,那裡完全不適合生命棲息,就算是迥異於我們的生物化學結構也是如此。」她推測:「但是很久以前,金星表面可能有生命存在,一直到失控溫室效應發生,使得行星上大部分地區完全不宜居住。」
有些科學家猜測金星高聳的雲層中──溫度適中,約為攝氏三十度──可能庇護著可以忍受極端酸性的空中微生物。金星的雲層有百分之九十為硫酸,地球微生物無法存活於這種酸度。葛瑞夫茲表示,「如果是微生物,它們或許可以取得一定程度的陽光和水,也許它們生活在液態水滴中,因而不至於脫水,但是它們會需要某種未知機制,來保護自己不被酸腐蝕。」
在地球上,生活在「厭氧」環境中──也就是不仰賴氧氣的生態系統──的微生物會製造磷化氫。這些環境包括污水處理廠、溼地、稻田、沼澤、湖底淤積,以及動物的排泄物和胃腸道。磷化氫也會以非生物性的方式產自特定的工業環境中。製造磷化氫時,地球上的細菌會從礦物或生物性物質中吸收磷酸鹽,再加入氫。
美國太空總署署長吉姆‧布萊登斯坦稱這項發現是「建立地球外生命存在的立論中,目前最重大的發展。」他在推特寫道:「是時候優先關注金星了」。儘管先前曾有無人太空船拜訪過金星,現在也許需要新的探測任務,確認生命存在與否。「好在金星就在隔壁,」索沙-席瓦說:「我們真的可以跑一趟金星,去那邊看看。」
(台北時報章厚明編譯)
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