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.

科学家首次确认在目前的火星表面有液态水的流动，这一发现令人们愈发开始猜测，如果这个星球上曾经出现生物，那么也许到现在仍有存活。

“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.”

“这是非常激动人心的发现，”NASA行星科学部主任詹姆斯·L·格林(James L. Green)在周一的新闻发布会上说。“‘地球以外的地方是否存在生物？’这个问题我们尚无法回答。但对水的追踪是这种搜寻的一个关键。我认为，我们现在极有可能处在恰当的地点，从而对此进行充分的调查。”

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.

这标志着NASA在立场上发生转变。此前他们反复强调，不要太过期待在这片遍地砂砾的荒原上栖居着生物。

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.

但现在，NASA科学任务指挥署副署长约翰·M·格伦斯菲尔德(John M. Grunsfeld)表示，可能在2020年代派一艘航天器前往其中一个区域，也许会直接以寻找生物为目的进行一些实验。

“I can’t imagine that it won’t be a high priority with the scientific community,” he said.

“这势必是科学界一件头等大事，”他说。

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.

火星在几十亿年前有河流、湖泊甚至可能有一片海洋，但到了近代那里的湿度很低——只有小块潮湿的土壤，没有大片的死水。

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.

在一篇刊载于《自然·地球科学》(Nature Geoscience)的文章中，科学家通过轨道飞行器读数识别出火星地表有被曾被水浸没的分子——一种叫做高氯酸盐的盐类。

“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.”

“这是直接探测到了以水合盐类形式存在的水，”亚利桑那大学行星地质学教授阿尔弗雷德·S·麦克尤恩(Alfred S. McEwen)说，他是NASA火星勘测轨道飞行器高分辨相机发回图像的首席调查员，也是这篇论文的作者之一。“这基本上确认了在近期有过液态水，从而产生水合盐。”

By “recently,” Dr. McEwen said he meant “days, something of that order.”

而麦克尤恩说，所谓“近期”的意思是“几天内，这样一个数量级”。

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.

科学家早已知道在极地冰盖中有大量固态水。而近期存在液态水的迹象时有闪现，比如看上去较新的冲沟，但都不具说服力。

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，麦克尤恩和同事在轨道器发回的图片中发现，在撞击坑、峡谷和山峦上有沿斜坡而下的暗色条纹。这些条纹在夏季会变长，随着气温下降而渐渐消失，第二年又会再次出现。

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.

他们将这些条纹命名为“季节性坡纹”，目前已经找到成千上万处这样的坡纹。“数量之多令人惊讶，”麦克尤恩说。

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.

科学家推测水在这种现象中扮演了关键角色，也许类似于水泥遇水后颜色变暗，干燥后又恢复原来的颜色。

But that was just an educated guess.

但那只是基于知识的猜测。

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.

佐治亚理工学院(Georgia Institute of Technology)研究生鲁基恩德拉·欧加(Lujendra Ojha)换用了轨道器上的另一种仪器，根据分子吸收什么颜色的光来分辨分子的类型。但这种叫做光谱仪的仪器没有相机那么高的清晰度，很难将宽度只有几米的窄流放大至可以提供数据的程度。

“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.

“我们不得不拿出新的技术和手段来分析其化学特征，”作为《自然·地球科学》论文第一作者的欧加说。

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.

研究人员在四个地点发现了能说明水合盐存在的迹象。此外，当条纹消失时，盐的迹象也会消失。“可以非常确定那里存在某种液态水，”欧加说。

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.

高氯酸盐会降低冰点，让水保持液态。火星的平均温度大约是零下70华氏度（合零下56摄氏度），但夏天在赤道附近的温度可以达到温暖的70华氏度。

Many mysteries remain. For one, scientists do not know where the water is coming from.

疑惑依然存在。比如科学家不知道水是从哪里来的。

“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.

“水的来源基本上就是两个，从上面来，或从下面来，”麦克尤恩说。高氯酸盐能起到海绵的作用，吸收空气里的水分，但勘测显示火星上的湿度很低——假设空气中所有的湿度都被挤出来，那么整个星球的降水仅为10微米。

That idea cannot be ruled out if the lower part of the atmosphere turns out more humid than currently thought.

如果大气层的近地部分实际上比目前估计的更潮湿，那么这种水从上面来的可能性是不能排除的。

“We have very poor measurements of relative humidity near the surface,” Dr. McEwen said.

“我们对接近地表的相对湿度测量效果很糟，”麦克尤恩说。

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层，在冬季结冰，到夏季融化，渗入地表。

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.

液态水被认为是生命的必要成分，液态水的存在让人心生疑问，看上去干燥而贫瘠的火星，会不会存在微生物版“火星人”的栖息地。

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)工作的天体生物学家克里斯多弗·P·麦凯(Christopher P. McKay)认为，季节性坡纹并非很有前景的方向。他说，水要保持液态，盐度就要足够高，那么就没有什么东西能存活。“在适居性方面，简单说就是毫无意义，”他说。

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.”

他提到了南极洲的唐胡安池(Don Juan Pond)，由于含有高浓度的氯化钙，那里的水可以在零下的气温下全年保持液态。“你从天上看，它就像个美丽的游泳池，”麦凯说。“但那水里什么也没有。”

Earthly life adapts to many hostile environments, but Don Juan Pond is lifeless.

世间的生物可以适应许多恶劣的环境，但唐胡安池中没有生命。

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.

但有的人不像他这么肯定。科罗拉多州博尔德市的西南研究院( Southwest Research Institute)太空研究部科学家戴维·E·斯蒂尔曼(David E. Stillman)说，在不同地区造成条纹的水可能是不同的。有的地方只在温度最高的时候出现，表明那些水可能盐度没那么高，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.”

“如果盐度太高，它们全年都会是流动的，”斯蒂尔曼说。“我们可能处在那个适居带上。”

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.

季节性坡纹看来是火星上最吸引人的特征之一，但NASA不打算在近期对其进行近距离的观察。

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.

它们被当做特殊地区，NASA禁止当前的探测机器人进入，因为这些探测器没进行过彻底消毒，NASA担心它们离开地球时携带着的微生物可能会污染火星。

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向火星发射的航天器中，只有1976年发射的两艘“维京号”(Viking)着陆器接受过高温烘烤，温度足以杀死地球上的微生物。NASA的下一辆火车探测车定于2020年发射，将更加清洁。给航天器消毒要求电子器件和系统能经受住酷热，这增加了成本，也让设计变得复杂。

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.

在为2020年的探测车选择着陆位置时，NASA排除了可能适合居住的地方，包括有季节性坡纹的地方。

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.

尽管已在NASA“好奇号”探测车正在探测的盖尔陨石坑(Gale Crater)的那座山上发现了两处疑似条纹，前述禁令可能依然有效。那里距离“好奇号”的原定路线一两英里。

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“好奇号”团队可能会决定接近，但不接触那两处条纹，或者只是从远处观察。探测车可能依然还要过几年，才会达到条纹所在地。

NASA officials did not reject the possibility of a detour, although they said it would require analysis and debate.

NASA的官员没有否认绕道的可能性，但他们表示此事还需要分析和讨论。

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.

在新闻发布会结束后接受采访时，来自NASA的格林表示，如果盖尔陨石坑的条纹结果是季节性坡纹，NASA会考虑多年接受紫外线照射的“好奇号”，可能会给潜在的火星居住环境造成多大程度的污染威胁。

“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米以内的距离，我们就可以用激光去打它，”格林说。他在这里提到了一种仪器，可以根据石头被气化时发出的光的颜色来判断里面是什么物质。“然后，我们会更了解细节，知道那些季节性坡纹内部是什么。如果能再靠近一些，实地去挖掘一下，效果还要更好。”