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Key building block of life found on ocean world orbiting Saturn
A key chemical building block of life has been found on Saturn’s moon Enceladus.
Phosphorus was detected in salty ice grains that were released into space by plumes that erupt between the cracks of the moon’s ice shell.
An ocean exists beneath the thick, icy surface of Enceladus, and plumes of material regularly release from geysers at the moon’s south pole.
That material becomes incorporated into Saturn’s outermost E ring.
Scientists used data from NASA’s Cassini mission, which studied Saturn and its moons between 2004 and 2017. The spacecraft flew through the plumes of Enceladus and Saturn’s E ring many times, and Cassini’s Cosmic Dust Analyzer detected minerals and organic compounds necessary for life.
Previously, researchers detected the presence of sodium, potassium, chlorine and carbonate compounds in the ice grains collected and analyzed by Cassini. Now, scientists can add phosphorus to the list. A study detailing the findings was published Wednesday in the journal Nature.
“Phosphorus in the form of phosphates is vital for all life on Earth,” said lead study author Dr. Frank Postberg, a professor of planetary sciences at Freie Universität Berlin, in a statement. “It is essential for the creation of DNA and RNA, cell membranes, and ATP (the universal energy carrier in cells) for example. Life as we know it would simply not exist without phosphates.”
It’s the first time that phosphorus has been discovered in an ocean beyond Earth.
“Previous geochemical models were divided on the question of whether Enceladus’ ocean contains significant quantities of phosphates at all,” Postberg said. “These Cassini measurements leave no doubt that substantial quantities of this essential substance are present in the ocean water.”
A habitable ocean on another world
The data revealed high concentrations of sodium phosphates, or molecules that chemically bind sodium, oxygen, hydrogen and phosphorus together, inside the ice grains.
The collective detection of phosphorus and other organic compounds in Enceladus’ ocean suggests that it could be habitable for life, if it exists on the icy moon, the researchers said.
“By determining such high phosphate concentrations readily available in Enceladus’ ocean, we have now satisfied what is generally considered one of the strictest requirements in establishing whether celestial bodies are habitable,” said study coauthor Dr. Fabian Klenner, postdoctoral researcher at the University of Washington, in a statement.
Although the ocean exists beneath an ice shell, there are indications of hydrothermal environments along the seafloor that keep the ocean at a warmer temperature.
“The important part for habitability is that we found phosphates that have been dissolved in Enceladus’ ocean and, with that, are readily available for the formation of potential life,” Postberg said.
“In most cases, phosphates (on Earth and elsewhere in the solar system) are locked up in rocky minerals, but on Enceladus, they are dissolved in large quantities (as salts) in the ocean.”
The researchers also conducted lab experiments to model Enceladus’ salty ocean, and they determined the phosphate concentrations are at 100 times and possibly up to 1,000 times higher than in Earth’s oceans. That’s because “soda oceans,” or those rich in carbonates and carbon dioxide like the one on Enceladus, can dissolve large amounts of phosphates that are otherwise locked inside rocky minerals, Postberg said.
“High phosphate concentrations are a result of interactions between carbonate-rich liquid water and rocky minerals on Enceladus’ ocean floor and may also occur on a number of other ocean worlds,” said study coauthor Christopher Glein, a planetary scientist and geochemist at Southwest Research Institute in San Antonio, Texas, in a statement. “This key ingredient could be abundant enough to potentially support life in Enceladus’ ocean; this is a stunning discovery for astrobiology.”
Some of the additional ocean world moons around Jupiter and Saturn include Europa, Titan and Ganymede. Future missions to Europa, like the European Space Agency’s Jupiter Icy Moons Explorer and NASA’s Europa Clipper, could determine more about the ingredients within these oceans.
Searching for signs of life
Although the Cassini mission ended by intentionally burning up in Saturn’s atmosphere in 2017, the data collected by its instruments is changing the way scientists understand Enceladus and similar ocean worlds, which may be the best bet for finding life beyond Earth across our solar system.
“This latest discovery of phosphorus in Enceladus’ subsurface ocean has set the stage for what the habitability potential might be for the other icy ocean worlds throughout the solar system,” said Linda Spilker, planetary scientist and Voyager project scientist at NASA’s Jet Propulsion Laboratory in Pasadena, California, in a statement. Spilker, who served as Cassini’s project scientist, was not involved in the study.
“Now that we know so many of the ingredients for life are out there, the question becomes: Is there life beyond Earth, perhaps in our own solar system? I feel that Cassini’s enduring legacy will inspire future missions that might, eventually, answer that very question,” Spilker said.
Although the building blocks of life and conditions for habitability exist on Enceladus, no actual life has been detected yet.
“Having the ingredients is necessary, but they may not be sufficient for an extraterrestrial environment to host life,” Glein said. “Whether life could have originated in Enceladus’ ocean remains an open question.”
“The next step is clear — we need to go back to Enceladus to see if the habitable ocean is actually inhabited,” said study coauthor Dr. Nozair Khawaja, a planetary scientist and postdoctoral researcher at Freie Universität Berlin.
Sending a dedicated mission to Enceladus is a priority for astronomers. Plans are underway to design the Enceladus Orbilander, which would both orbit the moon and land on the surface. If the mission were to launch later this decade, it would likely arrive at the distant moon in the early 2050s.
A spacecraft capable of landing on plume deposits on Enceladus could perform a chemical analysis to further investigate the subsurface ocean, said planetary geochemist Mikhail Zolotov, a research professor at Arizona State University and author of an accompanying News & Views article to the Nature study. Zolotov did not participate in the study.
“Those plume deposits could be taken inside a lander, melted, and analyzed for dissolved salts, gases, organic compounds, and possible biosignatures that all will characterize oceanic chemistry and habitability,” Zolotov said.
注释：
Enceladus: n
表示"[希神]恩克拉多斯(反叛众神的巨人); 土卫二"，如：Enceladus may be the source of the material in Saturn's tenuous E ring. 土卫二可能是土星的稀薄的E光环中物质的供给源。
Phosphorus: n
表示" 磷"，如：No animal or plant can exist without phosphorus. 没有一种动物或植物离开磷能够生存。
plume: n
表示" 羽毛；羽饰"，means "feather,especially one used for decorating;ornament of feathers;sth suggesting a feather by its shape"，如：Her hat was decorated with a plume. 她帽子上装饰着羽毛。
geyser: n
表示" 间歇泉；天然热喷泉"，means "a spring that discharges hot water and steam"，如：The tourists marveled at the geyser, which spurted water to a height of ten meters. 间歇泉喷出数十米高的水柱,这景观让游客很惊讶。
potassium: n
表示" 钾"，means "a light soft silver-white metallic element of the alkali metal group; oxidizes rapidly in air and reacts violently with water; is abundant in nature in combined forms occurring in sea water and in carnallite and kainite and sylvite"，如：The components of a potassium cycle vary greatly. 钾素循环的各个环节有很大的变化。
chlorine: n
表示" 氯"，如：All the chlorine atoms have already found partners. 所有的氯原子都已找到了配对的伙伴。
phosphate: n
表示" 磷酸盐；磷肥"，means "a salt of phosphoric acid"，如：We make superphosphate by reacting rock phosphate with sulphuric acid. 我们使磷酸盐岩同硫酸发生化学反应制造过磷酸钙。
membrane: n
表示"薄膜；膜状物"，means "a thin pliable sheet of material"，如：A plastic membrane serves as selective diffusion barrier. 一层塑料薄膜起着选择性渗透屏障的作用。
geochemical: adj
表示" 地球化学的"，如：Comprehensive geochemical surveys are analysed. 广义的地球化学普查需要分析。
hydrothermal: adj
表示"水热作用的；热液的", 如：Mineralized hydrothermal water mainly originated from the atmospheric rainfall. 成矿热液的水主要来自大气降水。
astrobiology： adj
表示" 太空生物学"，means "the branch of biology concerned with the effects of outer space on living organisms and the search for extraterrestrial life"，如：NASA now hosts an Astrobiology Institute. NASA现在主持一个天体生物的学会。
Europa: n
表示" [天]木卫二（木星最亮的四颗行星之一）；[希腊神话] 欧罗巴(腓尼基公主)"，means" the 4th largest of Jupiter's satellites; covered with a smooth shell of frozen water",如：In fact, this gibbous phase shows part of Jupiter's moon Europa.事实上，这颗突起的相位显示了部分木卫二。
Titan: n
表示" [希神]泰坦(一个巨人家族的一位成员);土星的神秘卫星 "，means ""，如：He was a surprisingly unassuming man for such a titan among statesmen. 作为政治家中的巨人，邓小平是一个非常谦逊的人。Methane smog enshrouds Titan, Saturn's mysterious moon. 甲烷所形成的烟雾遮蔽了泰坦，也就是土星的神秘卫星。
Ganymede: n
表示" [希神]Zeus带去为众神司酒的美少年（亦作Ganymedes）；【天】木星的最大卫星；[谑]侍童"，means "(Greek mythology) a Trojan boy who was so beautiful that Zeus carried him away to serve as cupbearer to the gods; the largest of Jupiter's satellites"，如：Indeed, Ganymede is the largest moon in the whole solar system. 的确，格尼梅德星是整个太阳系最大的卫星。
中文简要说明：
多年来，人类始终在宇宙寻找外星生命的迹象，探究生命的起源。最近科学家透过分析太空探测器传回的资料，发现了令人振奋的讯息，在土星的卫星「土卫二」（Enceladus）上找到关键元素「磷」，加上原本已知存在的元素，证实土星具备孕育生命的潜力。
据美国有线新闻网（CNN）14日报导，土卫二的表面冰层底下有一座海洋，海洋中的物质随着南极地带的间歇泉向外喷发，释放到太空中，形成最外围的尘埃环「E环」的一部分。从这些带盐的冰粒中，科学家侦测到磷（phosphorus）的存在。
主持这项研究的德国自由大学行星科学家波斯博格（Frank Postberg）指出，这是首次在地球以外的海洋中发现这种基本元素，「磷是构成DNA和RNA、细胞膜和腺苷三磷酸（细胞能量载体，ATP）的基础。」少了磷，生命根本不会存在。
报导指出，科学家在此之前，已经在土卫二的冰粒中发现碳、氢、氮、氧和硫，现在再加上磷，可谓凑齐了组成生物的6种必要元素。
这项发现还有另一个重点，那就是土卫二冰下海洋的磷浓度，至少是地球海洋的100倍。共同研究主笔、德州西南研究院的行星科学家格莱恩（Christopher Glein）说，「这种关键成分可能够丰富，足以支持土卫二海洋中的生命存在。这是天体生物学上的惊人发现。」
不过虽然土卫二具备组成生命的条件，科学家目前还没有发现任何生命的迹象。他们接下来的优先要务，将是向土卫二发送一台探测器，并在其他外星海洋，例如木星卫星「木卫二」（Europa）进行探索。
美国航天总署（NASA）宇宙飞船「卡西尼号」（Cassini）从2004年开始探测土星，到2017年坠落土星大气层「自毁」，期间获得不少宝贵资料。
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