尋找一個曾經藍天白雲 小河流水的火星

For lifeless chemical compounds to organize themselves into something alive, scientists generally agree, three sets of things must be present:

科學家基本公認，無生命的化學物質自行組合形成生命，需要具備三個前提條件：

■ Standing water and an energy source.

■ 靜態水和能量來源。

■ Five basic elements: carbon, oxygen, hydrogen, phosphorus and nitrogen.

■ 五種基本元素：碳、氧、氫、磷和氮。

■ And time, lots of time.

■ 還有時間，漫長的時間。

In its search for environments where life might have started on Mars, the Curiosity rover has found the standing water, the energy and the key elements with the right atomic charges. As a result, scientists have concluded that at least some of the planet must have been habitable long ago.

在尋找火星上適宜生命的環境的過程中，“好奇號”火星探測車找到了靜態水、能源和原子電量正好合適的基本元素。據此，科學家得出結論，火星上至少有些地方在很久以前是適宜生命存在的。

But the period when all conditions were right was counted in hundreds to thousands of years, a very small opening by origin-of-life standards.

但在火星曆史上，所有這些條件都具備的時間跨度大約是幾百到幾千年，而以生命起源的標準衡量，那實在不過是轉瞬之間。

That has now changed. John P. Grotzinger of Caltech, the project scientist for the mission, reported at a news conference on Monday that the rover’s yearlong trek to Mount Sharp provided strong new evidence that Gale Crater had large lakes, rivers and deltas, on and off, for millions to tens of millions of years. The geology shows that even when the surface water dried up, plenty of water would have remained underground, he said.

不過，現在有了新發現。負責“好奇號”火星項目的加州理工科學家約翰·P·格羅青格(John P. Grotzinger)週一（12月8日——譯註）在新聞發佈會上宣佈，“好奇號”向着夏普峯歷時一年的跋涉發現了有力的新證據，證明蓋爾環形山曾有大型湖泊、河流和三角洲，而且這些地貌斷斷續續存在的時間有數百萬到數千萬年之間。他說，已知的火星地理特徵說明，即使是在地表水乾涸的時候，也有地下水存在。

Moreover, the team concluded, numerous deltalike and lakelike formations detected by orbiting satellites are almost certainly the dried remains of substantial ancient lakes and deltas. None of this proves that life existed on the planet, but the case for an early Mars that was ripe and ready for life has grown stronger.

此外，科學家團隊還得出結論，環繞火星的衛星發現大量類三角洲或類湖泊地貌，幾乎可以肯定就是古代湖泊和三角洲乾涸之後的遺蹟。這些均不能證明火星上曾有生命，但早期火星適宜生命存在的可能性極大地增強了。

“As a science team, Mars is looking very attractive to us as a habitable planet,” Dr. Grotzinger said in an interview. “Not just sections of Gale Crater and not just a handful of locations, but at different times around the globe.”

“對於我們科學團隊來說，火星作爲一個生命宜居行星，顯得十分誘人，”格羅青格說。“不僅是在蓋爾環形山，也不僅是有限幾個地點，而是在不同歷史時期遍佈火星各處。”

And John M. Grunsfeld, a former astronaut who is NASA’s associate administrator for science, said that after almost 28 months on Mars, Curiosity has given scientists insight into how and where to look for clues of ancient life. “We don’t know if life ever started on Mars, but if it did, we now have a better chance of discovering it” on future missions, he said.

美國國家航空航天局(NASA)科學副總監、前宇航員約翰·M·格倫斯菲爾德(John M. Grunsfeld)也說，“好奇號”在火星的近28個月讓科學家學到很多東西，知道該怎樣尋找古代生命的線索，以及到哪裏去找。“我們不知道火星上是否曾有生命起源，但如果有的話，我們[在今後探索任務中]發現它的機會更大了，”他說。

Another missing piece of the story has been the inability to detect organic compounds — the carbon-based building blocks of life.

探索生命起源的過程中另一個缺失的環節是一直沒有找到有機化合物——構成生命的碳基元件。

That too may soon change. Last spring, several Curiosity team members reported the detection of some simple organics that appeared to be Martian. The findings were not definitive, but NASA has scheduled a news conference Dec. 14 at the annual meeting of the American Geophysical Union with “new information” about the search for organics. “Our original interpretation — that there was a good chance the organics we were seeing are Martian — hasn’t changed,” said Daniel P. Glavin of the Goddard Spaceflight Center, an author of the earlier paper. “This interpretation will be expanded on at A.G.U.”

這一點也可能很快會有突破。今年春季，“好奇號”的幾位科學家報告發現了可能是火星上的某些簡單有機物。這個發現沒有遽下結論，但航天局已經安排了12月14日在美國地球物理學會(American Geophysical Union)年會上召開新聞發佈會，發佈關於探索火星有機物的“新消息”。那篇論文的作者之一、戈達德航天中心(Goddard Spaceflight Center)的丹尼爾·P·格萊文(Daniel P. Glavin)說：“我們最初的解讀——很可能我們發現的就是火星有機物——並沒有改變。在AGU年會上我們會進一步介紹。”

Curiosity does not carry life-detection instruments, in large part because there is no consensus on what such an instrument might be. A finding of life based on what at first appeared to be metabolic activity, detected during the Viking missions of 1977, was so controversial that NASA ultimately rejected it. So scientists have been using a variety of tools — from geology and other earth sciences, organic and mineral chemistry, atmospheric measurements and sophisticated cameras — to determine whether life could have arisen and survived in Gale Crater and other locations with similar characteristics.

“好奇號”沒有搭載生命探測設備，主要是因爲究竟該用什麼樣的設備，科學家沒有達成共識。1977年“海盜號”探測器報告發現了生命跡象，基於的是一開始被認爲是代謝行爲的現象。但那次發現爭議很大，航天局最終做出了否定判斷。因此，科學家開始用各種工具——包括地質學等地球科學的工具、有機和無機化學、大氣探測以及精密攝像機等等——去判斷蓋爾環形山和其他類似地點是否曾經有生命出現並存活。

Another member of the Curiosity team, Roger Summons of M.I.T., says that findings from that rover and previous missions suggest that early Mars may have been quite similar to early Earth.

“好奇號”團隊的另一名成員、麻省理工的羅傑·薩曼斯(Roger Summons)說，“好奇號”和歷史上其他的火星探索顯示，早期火星或許和早期地球十分相近。

For the first billion years, he said, both planets had stable environments that could support life for substantial periods, and both still share the same chemistry and processes for altering rocks. There is a general scientific consensus that life began on Earth some 3.8 billion years ago, and Dr. Summons said it was clear that the same could have happened on Mars. Or as Dr. Grunsfeld put it, “What I get excited about is imagining a Mars 3.5 to 4 billion years ago, a planet with a thick atmosphere, maybe a blue sky with puffy clouds and mountains and lakes and rivers.”

他說，最初的10億年，兩個星球都有穩定的環境，允許生命在相當長的時間裏存在。直到今天，地球和火星仍然有相同的化學構成以及促使岩石發生轉變的地質過程。科學界基本同意，生命在地球上開始於大約38億年前。薩曼斯稱，很明顯同樣的事情可能也發生在火星上。或者像格倫斯菲爾德說的那樣：“讓我激動的是想像一個35到40億年前的火星，一個包裹在厚厚的大氣層中的星球，可能有藍天白雲，有山脈、湖泊和河流。”

Many similarities disappeared after Mars, a much smaller planet, lost much of its protective atmosphere by the end of its first billion years. So searching for possible Martian life involves digging deep below the surface or detecting microbial remains billions of years old. Identifying ancient microbial life has proved extremely difficult and controversial on Earth, and the challenge on Mars is considerably greater.

在它基本度過第一個10億年的時候，火星這顆小得多的行星喪失了大部分保護它的大氣層，於是很多相似性也消失了。因此，搜尋火星生命就需要深深地挖掘地表下岩層，或者探測幾十億年前的微生物遺蹟。在地球上辨識古代微生物極其困難，而且也難令所有人信服；在火星上，這種挑戰就更大了。

For that reason, scientists have long called for a mission to bring rock and soil samples back to Earth for sophisticated analysis. The Mars mission scheduled for 2020 would begin the effort by experimenting with methods to select, lift and store promising samples.

因此，科學家一直呼籲把火星岩石和土壤樣本帶回地球做深入分析。2020年開始的火星計劃將開始嘗試挑選、提取和儲存有希望的樣本。

But there are no Mars samples now — except those that arrive as long-traveling meteorites — so astrobiologists have to conduct their search for life using other methods and teasing out hidden evidence.

但是目前還沒有火星樣本——除了長途飛行來到地球的隕石——因此，宇宙生物學家只能用其他方法進行尋找生命的工作，梳理出隱藏的線索。

The search for water on Mars, for instance, goes back decades and many missions. But scientists were never certain that the carved canyons and deltas were results of water running long ago, or perhaps lava or frozen carbon dioxide. Because of Curiosity, there is now a wide consensus that early Mars had much water.

比如，尋找火星水的工作在幾十年前的火星計劃中就開始了。但科學家一直不能肯定，那些大峽谷和三角洲究竟是古代河流沖刷形成的，還是熔岩或凍結的二氧化碳造成的。多虧了“好奇號”，現在科學家普遍同意，早期火星上有很多水。

This conclusion has been difficult to square with climate models, which point to a colder early Mars with a thin atmosphere that could not have supported large bodies of standing water, or rivers that ran for millions of years. But faced with mounting evidence of longstanding water and consequently warmer conditions, the climate scientists have gravitated toward two interwoven explanations — both with implications for early life.

這個結論一度很難在火星氣候模型中說得通。火星氣候模型顯示，早期火星冰冷，大氣稀薄，不支持大量靜態水的存在，不可能有流淌了幾百萬年的河流。但面對越來越多的關於持續存在的水源和由此導致的溫暖環境的證據，氣候學家開始傾向於兩種互相交叉的解釋——兩種解釋都意味着可能有早期生命存在。

The first is that frequent volcanoes and meteorite impacts heated the planet substantially; volcanoes also emit gases known to synthesize into organic compounds. The second is that to explain the substantial water cycle required to keep many Martian lakes filled and rivers flowing, the planet needed a substantial ocean in its northern half. Large swaths of Mars north of its equator are one to three miles lower than the so-called southern highlands, and scientists have proposed that an ocean may have filled and molded the vast depression. Others disagree on several grounds, including that no remnant shoreline has been detected.

第一種解釋是，頻繁的火山活動和隕石撞擊讓星球大幅升溫；火山也會釋放出氣體，能合成有機化合物。第二個解釋是，火星要維持水循環，讓火星湖泊和河流成爲可能，那麼在北半球就應該有一大片海洋。火星赤道以北的大片區域比通稱的南方高地要低一到三英里。科學家提出假設，北半球可能有古代海洋，塑造了廣袤的低地。也有人不同意，基於好幾條理由，包括沒有探測到海岸線的遺蹟。

“We don’t have hard evidence of a northern ocean, but our models require that much water to explain what the geologists have now confirmed,” said Michael A. Mischna of the Jet Propulsion Laboratory, another Curiosity team member. “What Curiosity has done is to bring together atmospheric and climate information with the findings of the geologists and geochemists, and created a broad and consistent story of a very wet early Mars.”

“我們沒有發現北半球海洋的確鑿證據，但我們的模型要求有一大片水體，這樣才能解釋目前地質學家已經證實的結論，”“好奇號”團隊成員、噴氣推進實驗室(Jet Propulsion Laboratory)的邁克爾·A·米什納(Michael A. Mischna)說。“好奇號的工作就是將大氣與氣候信息，和地質學家與地質化學家的發現整合起來，構建一個大跨度、可以自圓其說的理論，描述一個非常溼潤的早期火星。”

While the evidence for water has become increasingly clear, the question of organic compounds is in flux. Such chemicals fall onto Mars all the time in interstellar dust and meteorites, as they do onto Earth. Yet none have been definitively detected.

關於水的證據已經越來越清楚了，但有關有機化合物的疑問還有一大堆。這類物質不斷以星際塵埃和隕石的形式落到火星上，就像落在地球上一樣，但目前還從來沒有被確鑿地發現。

But on this mission, team members knew to look for a salt called perchlorate that has been demonstrated on Earth to destroy or transform organics in the presence of heat. Substantial amounts of perchlorate were found in Gale Crater, suggesting that if early Mars had organic chemicals and they survived eons of radiation bombardment, they are long since gone or they will remain very difficult to detect with current techniques.

不過在這次火星探索中，科學家知道應該尋找一種叫做高氯酸鹽的無機物。在地球上，高氯酸鹽被證明可以在高溫下破壞有機物，或者促使有機物轉化。在蓋爾環形山上發現了大量高氯酸鹽。這意味着，如果早期火星存在有機物，而且它們沒有被持續的宇宙輻射消滅，那麼它們現在也早就不存在了，或者用現有的技術很難發現。

The Sample Analysis at Mars instrument is designed to identify relatively simple organics that burn off as gases in its oven. But it also carries nine cups with a solvent that can alter more complex molecules (like amino acids and nucleic acids) in ways that protect their signature.

火星樣本分析設備是用來識別相對簡單的有機物的。這些有機物將在它的烤爐內作爲氣體燃盡。但它也帶有九個杯子，盛着一種溶劑，可以轉化更復雜的分子（比如氨基酸和核酸）同時保護它們的化學特徵。

This “wet chemistry” has been awaiting the finding of a sample rich in organics. A further problem is that one of its cups leaked, causing enormous headaches and making team leaders wary.

這種“溼化學”技術一直還在等待富含有機物的樣本被發現。另一個問題是設備的其中一隻杯子發生了泄漏，製造了大麻煩，讓團隊負責人不放心。

But Dr. Glavin, a member of the team, hopes the spilled solvent will itself be used to test previously collected Martian samples, making it the first wet-chemistry experiment ever on another planet.

但團隊成員格萊文希望，溢灑的溶劑可以被用來測試之前採集的火星樣本，那將是外星球進行的第一次溼化學試驗。

Clearly, the search for life on Mars — past or present — will be neither straightforward nor swift.

很顯然，尋找火星生命——無論是古代生命還是現今的生命——不會是一帆風順，也不可能馬上成功。