火星上的液态水去了哪里?一项新理论提供了新的线索

Mars once had water flowing over the surface, but it disappeared billions of years ago. A new study suggests that in addition to being lost to space, much of the water could have been trapped in minerals in the planet's crust.

火星表面曾经有过水，但数十亿年前就消失了。一项新的研究表明，除了消失在太空之外，大部分水可能被困在这个星球地壳的矿物质中。

Oceans’ worth of ancient water may have been locked up in minerals in Mars's crust, increasing estimates for the total amount of that once flowed on the red planet.

具有海洋价值的古代水可能被锁在火星地壳的矿物质中，这使得研究人员对曾经在这颗红色星球上流动的水总量的估计增加了。

BY ROBIN GEORGE ANDREWS

作者罗宾·乔治·安德鲁斯

Today, Mars is a frigid desert. But dried up deltas and riverbanks reveal that water once flowed over the plant’s surface. Where did it all go? Scientists have been trying to answer this question for decades, hoping to understand how Mars became an arid wasteland while its neighbor, Earth, kept hold of its water and became a biological paradise.

目前，火星是一个寒冷的沙漠。但这个星球上干涸的三角洲和河岸表明这个星球表面曾有水的痕迹。但是水都到哪里去了呢? 几十年来，科学家们一直试图回答这个问题，希望了解在其邻居地球能保持水源，成为生物天堂的情况下，火星是如何变成贫瘠的荒地的。

Now, by plugging observations of the red planet into new models, a team of geologists and atmospheric scientists has come up with a new picture of Mars’s past: Much of the planet’s ancient water could have been trapped within minerals in the crust, where it remains to this day.

最近，一个由地质学家和大气科学家组成的团队通过将对红色星球的观测数据插入到新的模型中的方法，计算得出了火星过去的新图像: 地球上许多古老的水可能被封存在地壳的矿物质中，而且这些水直到今天仍存在那里。

Prior research suggested that most of Mars’s water escaped into space as its atmosphere was stripped away by the sun’s radiation. But this new study, published today in the journal Science and virtually presented at this year’s Lunar and Planetary Science Conference, concludes that Mars’s water experienced both an atmospheric exodus and a geologic entrapment.

先前的研究表明，由于火星的大气层被太阳辐射剥离，火星上的大部分水都逃到了太空中。但最近发表在《科学》(Science)杂志上的这项新研究得出结论称，火星上的水实际上经历了大气流失和地质圈藏两种方式。这项研究还在今年的月球和行星科学会议上进行了虚拟展示。

Depending on how much water you start with, the new model estimates that anywhere between 30 and 99 percent of it was incorporated into minerals in the planet’s crust, while the remaining fraction escaped into space. It’s a big range, and both processes likely played a role, so “somewhere in there, the reality lies,” says Briony Horgan, a planetary scientist at Purdue University who wasn’t involved with the new study.

根据最初的水含量多少，新的模型估计，30%到99%的水都融入了地壳的矿物质中，而其余部分则逃逸到了太空中。这是一个很大的范围，而且这两个过程可能都起了作用，所以“现实就存在于其中的某个地方，”普渡大学的行星科学家布里奥妮·霍根说，不过她没有参与这项新研究。
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If the new model is accurate, then the story of the planet’s adolescence needs a rewrite. All of the water thought to be trapped in the Martian crust today means that the planet had far more surface water in its youth than previous models had estimated—and that early epoch may have been even more amiable to microbial life than previously thought.

如果新的模型是准确的，那么地球处于青春期的理论就需要推翻重写。现如今所有被认为是被困在火星地壳中的水意味着这个星球在其年轻时期的地表水比以前的模型所估计的要多得多，而且也意味着早期时代的火星可能比以前认为的更适合微生物生命。
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“This paper allows for the possibility of a once-blue Mars, even for a short period of time,” says Paul Byrne, a planetary scientist at North Carolina State University who wasn’t involved with the new study.

北卡罗莱纳州立大学的行星科学家保罗·伯恩没有参与这项新研究，不过他说:“这篇论文考虑到了曾经的蓝色火星存在的可能性，即使只是很短的一段时间。”

From drenched to desiccated

从湿透变成干燥

A multitude of dried up riverbeds, deltas, lake basins, and inland seas make it clear that Mars once had a lot of water on its surface. It may have even had one or several different oceans in its northern hemisphere, although that’s a matter of intense debate. Today, aside from a possible series of briny underground lakes and aquifers, most of Mars’s water is locked up in the polar caps or in ice buried below the surface.

大量干涸的河床、三角洲、湖泊盆地和内陆海清楚地表明火星表面曾经有大量的水。它的北半球甚至可能有一个或几个不同的海洋，不过这是一个引发激烈的争论的问题。今天，除了可能存在的一系列含盐的地下湖泊和蓄水层外，火星上的大部分水都被封存在极地冰帽或埋藏在地表以下的冰中。
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Hydrogen is still escaping from Mars today, and scientists can measure the rate to work out how much water is being permanently lost. If this rate held steady over the past 4.5 billion years, it would be nowhere near enough to explain the disappearance of so much surface water, says lead author of the new study Eva Linghan Scheller, a doctoral student at Caltech.

时至今日，氢仍在从火星逃逸，科学家们可以通过测量其速度来计算出有多少水在永久流失。加州理工学院的博士生伊娃·林汉·谢勒是这项新研究的主要作者，他说，如果这个速度在过去的45亿年里一直保持稳定的话，这将远远不足以解释为何有这么多地表水从火星消失。

Another clue came courtesy of all the orbiters and rovers examining Mars’s rocks. Over the past two decades, a lot of water-bearing minerals have been discovered, including plenty of clays. At first, only patches were found here and there. But today, “we see evidence for a huge volume of hydrated minerals on the surface,” Horgan says.

另一条线索来自于所有检查火星岩石的轨道飞行器和陆行器。在过去的二十年里，它们发现了大量的包括粘土在内的含水矿物质。起初，只在各处发现了一些小块儿含水矿物质。但最近，“我们看到了地表有大量水合矿物质的证据。”霍根说。

All those extremely old hydrated minerals suggest that, long ago, plenty of water was flowing across the ancient Martian soil—much more than the atmospheric deuterium signal indicated.

所有这些极其古老的水合矿物质表明，很久以前是有大量的水曾流过古老的火星土壤的，而且比大气中的重氢信号所显示的要多得多。

“It’s taken a while to find all the hydrated mineral exposures that we have found, and then to fully acknowledge their importance on a global scale,” says Kirsten Siebach, a planetary scientist at Rice University who wasn’t involved with the work.

赖斯大学的行星科学家柯尔斯滕·西巴赫没有参与这项研究，他说:“我们花了一段时间才找到我们现在发现的所有火星水合矿物，然后才进一步充分认识到它们对火星的重要性。”

Two ways to kill a planet

毁灭一个星球的两种方法
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One problem was that previous models didn’t adequately take into account the crust’s ability to lock up water inside minerals, Scheller says. She and her colleagues decided to make a new model to estimate where Mars’s water went over its entire 4.5-billion-year lifetime.

谢勒说，之前的一个问题是以前的模型没有充分考虑到地壳将水锁在矿物质内部的能力。她和她的同事们决定建立一个新的模型来估计火星上的水在其45亿年的生命周期中都是在哪里流动的。
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The model makes some assumptions, such as how much water Mars had to begin with, how much was delivered later by watery asteroids and icy comets, how much was lost to space over time, and how much volcanic activity deposited more water onto the planet’s surface. Depending on the values of those variables, the team found that Mars could once have had enough surface water to make a global ocean 330 to 4,900 feet deep.

该模型做了一些假设，比如火星一开始有多少水，有多少水后来由含水的小行星和冰冷的彗星输送，有多少水随时间流失到太空，有多少火山活动使更多的水沉积在火星表面等。根据这些变量的值，研究小组发现，火星曾经有足够的地表水，水量足以形成一个330到4900英尺深的全球海洋。

Between 4.1 and 3.7 billion years ago, the amount of surface water decreased significantly as it was soaked up by minerals in the crust and as it escaped into space. None of the hydrated minerals found so far have been younger than three billion years, Scheller says, which implies that Mars has been an arid wasteland for most of its lifetime.

41亿至37亿年前这段时间内，地表水的数量显著减少，因为它被地壳中的矿物质吸收了，还有的水逃逸到了太空中。谢勒说，到目前为止，发现的含水矿物质的年龄没有一种是小于30亿年的，这意味着火星在其生命的大部分时间里都是一片干旱的荒原。

The new model has its limits, with some key details remaining fuzzy. But it is an important step that “will surely assist many future investigations about the history of water on Mars,” says Geronimo Villanueva, a planetary scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, who wasn’t involved with the new study.

不过这个新模型有其局限性，一些关键细节仍不清楚。但这是重要的一步，这“肯定会对未来有关火星上水的历史的许多调查提供帮助，”美国宇航局戈达德太空飞行中心(位于马里兰州绿带)的行星科学家杰罗默·维兰纽瓦说，他也没有参与这项新研究。

For one thing, it helps address a discrepancy between the amount of water estimated by the deuterium measurements and the myriad water features that have been left on the surface. It wasn’t clear how so many rivers and lakes could emerge from so little water, Siebach says, but this new model offers a solution to that mystery by identifying additional water that could have been present on Mars.

一方面，它有助于帮助解决重氢测量所估计的水量与月球表面留下的无数水特征之间的差异。西巴赫说，目前还不清楚这么多河流和湖泊是如何从这么少的水中形成的，但是这个新的模型通过识别火星上可能存在的额外水，为这个谜题提供了解决方案。

However, the research doesn’t change how much water scientists think is available on Mars today—which isn’t much at all. Astronauts may one day bake hydrated minerals on Mars to unleash their water, Horgan says, but that would be an energy-intensive process.

然而，这项研究并没有改变科学家们现在认为的火星上根本就没有多少水的认知。霍根说，宇航员可能有一天会在火星上烘烤含水矿物质来释放水分，但这将是一个能量密集型的过程。

“What this study does is that it says you have more water to play with early in Mars’s history, and that’s when Mars was most habitable,” Siebach says. Microbes, if they ever existed, may have spread through all that available water, but they would have struggled to survive by the time most of it vanished three billion years ago.

“这项研究表明，在火星早期的历史中，有更多的水可以利用，那时的火星是最适宜居住的时期，” 西巴赫说。如果微生物曾经存在过的话，它们可能会扩散到所有可用的水里去，但在30亿年前大部分水消失的时候，它们会挣扎着生存下去。

The idea that a significant volume of water can vanish into the crust also has implications for other rocky worlds, says Byrne of North Carolina State University.

北卡罗莱纳州立大学的伯恩说，大量的水会消失在地壳中的这一想法也暗示了其他岩石世界的存在。

Water binds to Earth’s minerals, too. But on our planet, plate tectonics recycles these minerals, constantly unleashing their water through volcanic eruptions, Siebach says. By contrast, Mars’s stagnant crust may have doomed the planet to become a bitterly cold desert. Did the same world-changing process happen on Venus? Does water end up locked in the crust of exoplanets far from our solar system?

水也可以与地球上的矿物质结合。西巴赫表示，但是在我们的星球上，板块构造使这些矿物质循环起来了，地壳通过火山爆发不断释放内部的水。相比之下，火星不活跃的地壳可能注定了这个星球会变成一个寒冷的沙漠。同样的世界改变过程在金星上也发生过吗?水最终会被锁在远离我们太阳系的系外行星的地壳中吗?

Scott King, a planetary scientist at Virginia Tech who wasn’t involved with the work, says that the model paves the way for an even richer understanding of how Mars and other rocky worlds evolve through the ages.

弗吉尼亚理工大学的行星科学家斯科特·金没有参与这项工作，但他说，这个模型为更丰富地了解火星和其他岩石世界在各个时代的演化过程铺平了道路。