探索：火星上水太鹹 可能無生命

探索：火星上水太鹹 可能無生命
Mars perhaps too salty for life

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In 2004, NASA's Opportunity rover found evidence in Martian soils that water had once flowed across the surface there, buoying hopes that the red planet may once have supported primitive a new study throws some cold water, and a big pinch of salt, on those hopes.

In 2004, NASA's Opportunity rover found evidence in Martian soils that water had once flowed across the surface there, buoying hopes that the red planet may once have supported primitive life.

But a new study throws some cold water, and a big pinch of salt, on those hopes.

"Liquid water is required by all species on Earth and we've assumed that water is the very least that would be necessary for life on Mars," said study team member Nicholas J. Tosca, a Harvard University postdoctoral researcher. "However, to really assess Mars' habitability we need to consider the properties of its water. Not all of Earth's waters are able to support life, and the limits of terrestrial life are sharply defined by water's temperature, acidity and salinity."

Tosca and his team analyzed salt deposits in the 4-billion-year-old Martian rock investigated by Opportunity. The new analysis shows that the water that would have flowed across these ancient Martian rocks may have been exceedingly briny.

"Our sense has been that while Mars is a lousy environment for supporting life today, long ago it might have more closely resembled Earth," said Andrew H. Knoll, also of Harvard and on the study team. "But this result suggests quite strongly that even as long as four billion years ago, the surface of Mars would have been challenging for life. No matter how far back we peer into Mars' history, we may never see a point at which the planet really looked like Earth."

The research was presented in February at the annual meeting of the American Association for the Advancement of Science in Boston.

Halophiles, or organisms that can tolerate high-salinity waters, are known to exist in places on Earth, but they likely evolved from organisms that lived in purer waters, scientists think, making it unlikely that life would actually arise initially in extremely briny waters.

The high salinity, however, "doesn't rule out life forms of a type we've never encountered," Knoll added, "but life that could originate and persist in such a salty setting would require biochemistry distinct from any known among even the most robust halophiles on Earth."

Knoll and Tosca also say the finding doesn't rule out the possibility that less salty waters once flowed on the planet.

2004年，美國宇航局的“機遇號”探測器在火星土壤中發現有水流過火星表面的跡象，這一發現激發了人類對於這顆紅色星球曾有原始生命體存在的希望。

但一項最新研究卻給這個夢想潑了盆冷水，還撒了把鹽，讓人們對這個希望持保留意見。

哈佛大學博士後研究員、研究小組成員尼古拉斯•J•托斯卡說：“液態水是地球上所有物種賴以生存的必需物質，所以我們曾認爲水是支持火星上生命體存在的最基本元素。但要真實評估火星的宜居程度，還需考慮火星液態水的特性。在地球上，並不是所有的水都能支持生命存在，水的溫度、酸度和鹽分在很大程度上決定了陸地生物的生存期限。”

托斯卡及其研究小組對“機遇號”探測到的已有40億年曆史的火星熔岩鹽層進行了分析。分析結果表明，曾流過這些古老火星熔岩表面的水含鹽量極高。

研究小組的另一位來自哈佛大學的安德魯•H•諾爾說：“過去我們一直覺得，雖然火星現在的生存環境很惡劣，但在很久以前，火星的生存環境可能與地球很相似。但這一結果充分說明，即使在40億年前，火星表面的生存環境仍然很惡劣。無論我們將火星的歷史追溯到多遠，可能都無法知道這顆星球究竟在何時與地球很相似。”

該研究結果於今年2月在波士頓召開的美國科學促進會的年會上公佈。

儘管目前地球上某些地方存在一些能在高鹽分水域中存活的喜鹽生物，但科學家認爲，這種生物體很可能是由生存在較純淨水域的生物體進化而來的，這說明生命不可能起源於含鹽量極高的水域。

諾爾說，或許高鹽分“能夠支持我們從未見過的某種生物體的生存，但能在含鹽量如此高的環境中存活下來的物種必須具備比地球上生命力最強的喜鹽生物更獨特的生化特性。”

諾爾和托斯卡說，這一發現並不能排除鹽分較低的水曾流過火星的可能。

habitability: 宜居性

briny: salty（鹹的）