NASA確認火星表面存在流動的液態水
Scientists have for the first time confirmed liquid water flowing on the surface of present-day Mars, a finding that will add to speculation that life, if it ever arose there, could persist now.
科學家首次確認在目前的火星表面有液態水的流動，這一發現令人們愈發開始猜測，如果這個星球上曾經出現生物，那麼也許到現在仍有存活。
“This is tremendously exciting,” James L. Green, the director of NASA’s planetary science division, said during a news conference on Monday. “We haven’t been able to answer the question, ‘Does life exist beyond Earth?’ But following the water is a critical element of that. We now have, I think, great opportunities in the right locations on Mars to thoroughly investigate that.”
“這是非常激動人心的發現，”NASA行星科學部主任詹姆斯·L·格林(James L. Green)在週一的新聞發佈會上說。“‘地球以外的地方是否存在生物？’這個問題我們尚無法回答。但對水的追蹤是這種搜尋的一個關鍵。我認爲，我們現在極有可能處在恰當的地點，從而對此進行充分的調查。”
That marks a shift in tone for NASA, where officials have repeatedly played down the notion that the dusty and desolate landscape of Mars could be inhabited today.
這標誌着NASA在立場上發生轉變。此前他們反覆強調，不要太過期待在這片遍地砂礫的荒原上棲居着生物。
But now, John M. Grunsfeld, NASA’s associate administrator for science, talked of sending a spacecraft in the 2020s to one of these regions, perhaps with experiments to directly look for life.
但現在，NASA科學任務指揮署副署長約翰·M·格倫斯菲爾德(John M. Grunsfeld)表示，可能在2020年代派一艘航天器前往其中一個區域，也許會直接以尋找生物爲目的進行一些實驗。
“I can’t imagine that it won’t be a high priority with the scientific community,” he said.
“這勢必是科學界一件頭等大事，”他說。
Although Mars had rivers, lakes and maybe even an ocean a few billion years ago, the modern moisture is modest — small patches of damp soil, not pools of standing water.
火星在幾十億年前有河流、湖泊甚至可能有一片海洋，但到了近代那裏的溼度很低——只有小塊潮溼的土壤，沒有大片的死水。
In a paper published in the journal Nature Geoscience, scientists identified waterlogged molecules — salts of a type known as perchlorates — on the surface in readings from orbit.
在一篇刊載於《自然·地球科學》(Nature Geoscience)的文章中，科學家通過軌道飛行器讀數識別出火星地表有被曾被水浸沒的分子——一種叫做高氯酸鹽的鹽類。
“That’s a direct detection of water in the form of hydration of salts,” said Alfred S. McEwen, a professor of planetary geology at the University of Arizona, the principal investigator of images from a high-resolution camera on NASA’s Mars Reconnaissance Orbiter and one of the authors of the new paper. “There pretty much has to have been liquid water recently present to produce the hydrated salt.”
“這是直接探測到了以水合鹽類形式存在的水，”亞利桑那大學行星地質學教授阿爾弗雷德·S·麥克尤恩(Alfred S. McEwen)說，他是NASA火星勘測軌道飛行器高分辨相機發回圖像的首席調查員，也是這篇論文的作者之一。“這基本上確認了在近期有過液態水，從而產生水合鹽。”
By “recently,” Dr. McEwen said he meant “days, something of that order.”
而麥克尤恩說，所謂“近期”的意思是“幾天內，這樣一個數量級”。
Scientists have long known that large amounts of water remain — but frozen solid in the polar ice caps. There have been fleeting hints of recent liquid water, like fresh-looking gullies, but none have proved convincing.
科學家早已知道在極地冰蓋中有大量固態水。而近期存在液態水的跡象時有閃現，比如看上去較新的沖溝，但都不具說服力。
In 2011, Dr. McEwen and colleagues discovered in photographs from the orbiter dark streaks descending along slopes of craters, canyons and mountains. The streaks lengthened during summer, faded as temperatures cooled, then reappeared the next year.
2011年，麥克尤恩和同事在軌道器發回的圖片中發現，在撞擊坑、峽谷和山巒上有沿斜坡而下的暗色條紋。這些條紋在夏季會變長，隨着氣溫下降而漸漸消失，第二年又會再次出現。
They named the streaks recurring slope linae, or R.S.L.s, and many thousands of them have now been spotted. “It’s really surprisingly extensive,” Dr. McEwen said.
他們將這些條紋命名爲“季節性坡紋”，目前已經找到成千上萬處這樣的坡紋。“數量之多令人驚訝，”麥克尤恩說。
Scientists suspected that water played a critical role in the phenomenon, perhaps similar to the way concrete darkens when wet and returns to its original color when dry.
科學家推測水在這種現象中扮演了關鍵角色，也許類似於水泥遇水後顏色變暗，乾燥後又恢復原來的顏色。
But that was just an educated guess.
但那只是基於知識的猜測。
Lujendra Ojha, a graduate student at the Georgia Institute of Technology, turned to another instrument on the orbiter that identifies types of molecules by which colors of light they absorb. But this instrument, a spectrometer, is not as sharp as the camera, making it hard to zoom in on readings from the narrow streaks, a few yards across at most.
佐治亞理工學院(Georgia Institute of Technology)研究生魯基恩德拉·歐加(Lujendra Ojha)換用了軌道器上的另一種儀器，根據分子吸收什麼顏色的光來分辨分子的類型。但這種叫做光譜儀的儀器沒有相機那麼高的清晰度，很難將寬度只有幾米的窄流放大至可以提供數據的程度。
“We had to come up with new techniques and novel ways to do analysis of the chemical signature,” said Mr. Ojha, the lead author of the Nature Geoscience article.
“我們不得不拿出新的技術和手段來分析其化學特徵，”作爲《自然·地球科學》論文第一作者的歐加說。
The researchers were able to identify the telltale sign of a hydrated salt at four locations. In addition, the signs of the salt disappeared when the streaks faded. “It’s very definitive there is some sort of liquid water,” Mr. Ojha said.
研究人員在四個地點發現了能說明水合鹽存在的跡象。此外，當條紋消失時，鹽的跡象也會消失。“可以非常確定那裏存在某種液態水，”歐加說。
The perchlorate salts lower the freezing temperature, and the water remains liquid. The average temperature of Mars is about minus 70 degrees Fahrenheit, but summer days near the Equator can reach an almost balmy 70.
高氯酸鹽會降低冰點，讓水保持液態。火星的平均溫度大約是零下70華氏度（合零下56攝氏度），但夏天在赤道附近的溫度可以達到溫暖的70華氏度。
Many mysteries remain. For one, scientists do not know where the water is coming from.
疑惑依然存在。比如科學家不知道水是從哪裏來的。
“There are two basic origins for the water: from above or from below,” Dr. McEwen said. The perchlorates could be acting like a sponge, absorbing moisture out of the air, but measurements indicate very low humidity on Mars — only enough for 10 microns, or about 1/2,500th of an inch, of rain across the planet if all of the wetness were wrung out of the air.
“水的來源基本上就是兩個，從上面來，或從下面來，”麥克尤恩說。高氯酸鹽能起到海綿的作用，吸收空氣裏的水分，但勘測顯示火星上的溼度很低——假設空氣中所有的溼度都被擠出來，那麼整個星球的降水僅爲10微米。
That idea cannot be ruled out if the lower part of the atmosphere turns out more humid than currently thought.
如果大氣層的近地部分實際上比目前估計的更潮溼，那麼這種水從上面來的可能性是不能排除的。
“We have very poor measurements of relative humidity near the surface,” Dr. McEwen said.
“我們對接近地表的相對溼度測量效果很糟，”麥克尤恩說。
The other possibility is underground aquifers, frozen during winter, melting during summer and seeping to the surface.
其他的可能性包括地下含水層，在冬季結冰，到夏季融化，滲入地表。
Liquid water is considered one of the essential ingredients for life, and its presence raises the question of whether Mars, which appears so dry and barren, could possess niches of habitability for microbial Martians.
液態水被認爲是生命的必要成分，液態水的存在讓人心生疑問，看上去幹燥而貧瘠的火星，會不會存在微生物版“火星人”的棲息地。
Christopher P. McKay, an astrobiologist at NASA’s Ames Research Center in Mountain View, Calif., does not think the recurring slope linae are a very promising place to look. For the water to be liquid, it must be so salty that nothing could live there, he said. “The short answer for habitability is it means nothing,” he said.
在加州山景市NASA艾姆斯研究中心(Ames Research Center)工作的天體生物學家克里斯多弗·P·麥凱(Christopher P. McKay)認爲，季節性坡紋並非很有前景的方向。他說，水要保持液態，鹽度就要足夠高，那麼就沒有什麼東西能存活。“在適居性方面，簡單說就是毫無意義，”他說。
He pointed to Don Juan Pond in Antarctica, which remains liquid year round in subzero temperatures because of high concentrations of calcium chloride salt. “You fly over it, and it looks like a beautiful swimming pool,” Dr. McKay said. “But the water has got nothing.”
他提到了南極洲的唐胡安池(Don Juan Pond)，由於含有高濃度的氯化鈣，那裏的水可以在零下的氣溫下全年保持液態。“你從天上看，它就像個美麗的游泳池，”麥凱說。“但那水裏什麼也沒有。”
Earthly life adapts to many hostile environments, but Don Juan Pond is lifeless.
世間的生物可以適應許多惡劣的環境，但唐胡安池中沒有生命。
Others are not so certain. David E. Stillman, a scientist at the Southwest Research Institute’s space studies department in Boulder, Colo., said water for the streaks might be different in different regions. In some, they form only during the warmest times, suggesting that those waters might not be too salty for microbes.
但有的人不像他這麼肯定。科羅拉多州博爾德市的西南研究院( Southwest Research Institute)太空研究部科學家戴維·E·斯蒂爾曼(David E. Stillman)說，在不同地區造成條紋的水可能是不同的。有的地方只在溫度最高的時候出現，表明那些水可能鹽度沒那麼高，微生物可以存活。
“If it was too salty, they would be flowing year round,” Dr. Stillman said. “We might be in that Goldilocks zone.”
“如果鹽度太高，它們全年都會是流動的，”斯蒂爾曼說。“我們可能處在那個適居帶上。”
Even though recurring slope linae appear to be some of the most intriguing features on Mars, NASA has no plans to get a close-up look anytime soon.
季節性坡紋看來是火星上最吸引人的特徵之一，但NASA不打算在近期對其進行近距離的觀察。
They are treated as special regions that NASA’s current robotic explorers are barred from because the rovers were not thoroughly sterilized, and NASA worries that they might be carrying microbial hitchhikers from Earth that could contaminate Mars.
它們被當做特殊地區，NASA禁止當前的探測機器人進入，因爲這些探測器沒進行過徹底消毒，NASA擔心它們離開地球時攜帶着的微生物可能會污染火星。
Of the spacecraft NASA has sent to Mars, only the two Viking landers in 1976 were baked to temperatures hot enough to kill Earth microbes. NASA’s next Mars rover, scheduled to launch in 2020, will be no cleaner. Sterilizing spacecraft, which requires electronics and systems that can withstand the heat of baking, adds to the cost and complicates the design.
在NASA向火星發射的航天器中，只有1976年發射的兩艘“維京號”(Viking)着陸器接受過高溫烘烤，溫度足以殺死地球上的微生物。NASA的下一輛火車探測車定於2020年發射，將更加清潔。給航天器消毒要求電子器件和系統能經受住酷熱，這增加了成本，也讓設計變得複雜。
In selecting the landing site for the 2020 rover, the space agency is ruling out places that might be habitable, including those with recurring slope linae.
在爲2020年的探測車選擇着陸位置時，NASA排除了可能適合居住的地方，包括有季節性坡紋的地方。
That prohibition may continue even though two candidate streaks have been identified on the mountain in Gale Crater that NASA’s Curiosity rover is now exploring, a mile or two from its planned path.
儘管已在NASA“好奇號”探測車正在探測的蓋爾隕石坑(Gale Crater)的那座山上發現了兩處疑似條紋，前述禁令可能依然有效。那裏距離“好奇號”的原定路線一兩英里。
NASA and the Curiosity team could decide to approach the streaks without driving onto them, or to simply observe from a distance. The rover is still probably a couple of years away from them.
NASA和“好奇號”團隊可能會決定接近，但不接觸那兩處條紋，或者只是從遠處觀察。探測車可能依然還要過幾年，纔會達到條紋所在地。
NASA officials did not reject the possibility of a detour, although they said it would require analysis and debate.
NASA的官員沒有否認繞道的可能性，但他們表示此事還需要分析和討論。
In an interview after the news conference, Dr. Green of NASA said that if the streaks in Gale Crater turned out to be recurring slope linae, the space agency would consider how great a contamination threat Curiosity, irradiated by ultraviolet light for several years, might pose to a potential Martian habitat.
在新聞發佈會結束後接受採訪時，來自NASA的格林表示，如果蓋爾隕石坑的條紋結果是季節性坡紋，NASA會考慮多年接受紫外線照射的“好奇號”，可能會給潛在的火星居住環境造成多大程度的污染威脅。
“If we can go within 20 meters, we can zap it with a laser,” Dr. Green said, referring to an instrument that identifies material inside a rock by the colors of light it emits as it is vaporized. “Then we can learn much more about the details what’s in those R.S.L.s. If we can get closer and actually scoop it up, that would be even better.”
“如果去到20米以內的距離，我們就可以用激光去打它，”格林說。他在這裏提到了一種儀器，可以根據石頭被氣化時發出的光的顏色來判斷裏面是什麼物質。“然後，我們會更瞭解細節，知道那些季節性坡紋內部是什麼。如果能再靠近一些，實地去挖掘一下，效果還要更好。”
