In 2004, NASA's Opportunity rover found evidence in Martian soils that water had once flowed across the surface there, buoying¹ hopes that the red planet may once have supported primitive² 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, acidity³ and salinity⁴."

Tosca and his team analyzed⁵ 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 briny⁶.

"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. Knoll⁷, 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 Advancement⁸ 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 initially⁹ 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 robust¹⁰ 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，美国宇航局的“机遇号”探测器在火星土壤中发现有水流过火星表面的迹象，这一发现激发了人类对于这颗红色星球曾有原始生命体存在的希望。

但一项最新研究却给这个梦想泼了盆冷水，还撒了把盐，让人们对这个希望持保留意见。

哈佛大学博士后研究员、研究小组成员尼古拉斯•J•托斯卡说：“液态水是地球上所有物种赖以生存的必需物质，所以我们曾认为水是支持火星上生命体存在的最基本元素。但要真实评估火星的宜居程度，还需考虑火星液态水的特性。在地球上，并不是所有的水都能支持生命存在，水的温度、酸度和盐分在很大程度上决定了陆地生物的生存期限。”

托斯卡及其研究小组对“机遇号”探测到的已有40亿年历史的火星熔岩盐层进行了分析。分析结果表明，曾流过这些古老火星熔岩表面的水含盐量极高。

研究小组的另一位来自哈佛大学的安德鲁•H•诺尔说：“过去我们一直觉得，虽然火星现在的生存环境很恶劣，但在很久以前，火星的生存环境可能与地球很相似。但这一结果充分说明，即使在40亿年前，火星表面的生存环境仍然很恶劣。无论我们将火星的历史追溯到多远，可能都无法知道这颗星球究竟在何时与地球很相似。”

该研究结果于今年2月在波士顿召开的美国科学促进会的年会上公布。

尽管目前地球上某些地方存在一些能在高盐分水域中存活的喜盐生物，但科学家认为，这种生物体很可能是由生存在较纯净水域的生物体进化而来的，这说明生命不可能起源于含盐量极高的水域。

诺尔说，或许高盐分“能够支持我们从未见过的某种生物体的生存，但能在含盐量如此高的环境中存活下来的物种必须具备比地球上生命力最强的喜盐生物更独特的生化特性。”

诺尔和托斯卡说，这一发现并不能排除盐分较低的水曾流过火星的可能。