尋找一個(gè)曾經(jīng)藍(lán)天白云，小河流水的火星

Mars Rover Finds Stronger Potential for Life

尋找一個(gè)曾經(jīng)藍(lán)天白云，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(xué)家基本公認(rèn)，無生命的化學(xué)物質(zhì)自行組合形成生命，需要具備三個(gè)前提條件：

■ Standing water and an energy source.

■ 靜態(tài)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(shí)間，漫長(zhǎng)的時(shí)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(huán)境的過程中，“好奇號(hào)”火星探測(cè)車找到了靜態(tài)水、能源和原子電量正好合適的基本元素。據(jù)此，科學(xué)家得出結(jié)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.

但在火星歷史上，所有這些條件都具備的時(shí)間跨度大約是幾百到幾千年，而以生命起源的標(biāo)準(zhǔn)衡量，那實(shí)在不過是轉(zhuǎn)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(xiàn)在有了新發(fā)現(xiàn)。負(fù)責(zé)“好奇號(hào)”火星項(xiàng)目的加州理工科學(xué)家約翰·P·格羅青格(John P. Grotzinger)周一(12月8日——譯注)在新聞發(fā)布會(huì)上宣布，“好奇號(hào)”向著夏普峰歷時(shí)一年的跋涉發(fā)現(xiàn)了有力的新證據(jù)，證明蓋爾環(huán)形山曾有大型湖泊、河流和三角洲，而且這些地貌斷斷續(xù)續(xù)存在的時(shí)間有數(shù)百萬到數(shù)千萬年之間。他說，已知的火星地理特征說明，即使是在地表水干涸的時(shí)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(xué)家團(tuán)隊(duì)還得出結(jié)論，環(huán)繞火星的衛(wèi)星發(fā)現(xiàn)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干涸之后的遺跡。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(qiáng)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(duì)于我們科學(xué)團(tuán)隊(duì)來說，火星作為一個(gè)生命宜居行星，顯得十分誘人，”格羅青格說。“不僅是在蓋爾環(huán)形山，也不僅是有限幾個(gè)地點(diǎn)，而是在不同歷史時(shí)期遍布火星各處。”

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(guó)國(guó)家航空航天局(NASA)科學(xué)副總監(jiān)、前宇航員約翰·M·格倫斯菲爾德(John M. Grunsfeld)也說，“好奇號(hào)”在火星的近28個(gè)月讓科學(xué)家學(xué)到很多東西，知道該怎樣尋找古代生命的線索，以及到哪里去找。“我們不知道火星上是否曾有生命起源，但如果有的話，我們[在今后探索任務(wù)中]發(fā)現(xiàn)它的機(jī)會(huì)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(gè)缺失的環(huán)節(jié)是一直沒有找到有機(jī)化合物——構(gòu)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(diǎn)也可能很快會(huì)有突破。今年春季，“好奇號(hào)”的幾位科學(xué)家報(bào)告發(fā)現(xiàn)了可能是火星上的某些簡(jiǎn)單有機(jī)物。這個(gè)發(fā)現(xiàn)沒有遽下結(jié)論，但航天局已經(jīng)安排了 12月14日在美國(guó)地球物理學(xué)會(huì)(American Geophysical Union)年會(huì)上召開新聞發(fā)布會(huì)，發(fā)布關(guān)于探索火星有機(jī)物的“新消息”。那篇論文的作者之一、戈達(dá)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(fā)現(xiàn)的就是火星有機(jī)物——并沒有改變。在AGU年會(huì)上我們會(huì)進(jìn)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(hào)”沒有搭載生命探測(cè)設(shè)備，主要是因?yàn)榫烤乖撚檬裁礃拥脑O(shè)備，科學(xué)家沒有達(dá)成共識(shí)。1977年“海盜號(hào)”探測(cè)器報(bào)告發(fā)現(xiàn)了生命跡象，基于的是一開始被認(rèn)為是代謝行為的現(xiàn)象。但那次發(fā)現(xiàn)爭(zhēng)議很大，航天局最終做出了否定判斷。因此，科學(xué)家開始用各種工具——包括地質(zhì)學(xué)等地球科學(xué)的工具、有機(jī)和無機(jī)化學(xué)、大氣探測(cè)以及精密攝像機(jī)等等——去判斷蓋爾環(huán)形山和其他類似地點(diǎn)是否曾經(jīng)有生命出現(xiàn)并存活。

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(hào)”團(tuán)隊(duì)的另一名成員、麻省理工的羅杰·薩曼斯(Roger Summons)說，“好奇號(hào)”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(gè)星球都有穩(wěn)定的環(huán)境，允許生命在相當(dāng)長(zhǎng)的時(shí)間里存在。直到今天，地球和火星仍然有相同的化學(xué)構(gòu)成以及促使巖石發(fā)生轉(zhuǎn)變的地質(zhì)過程。科學(xué)界基本同意，生命在地球上開始于大約38億年前。薩曼斯稱，很明顯同樣的事情可能也發(fā)生在火星上?；蛘呦窀駛愃狗茽柕抡f的那樣：“讓我激動(dòng)的是想像一個(gè)35到40億年前的火星，一個(gè)包裹在厚厚的大氣層中的星球，可能有藍(lán)天白云，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(gè)10億年的時(shí)候，火星這顆小得多的行星喪失了大部分保護(hù)它的大氣層，于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巖層，或者探測(cè)幾十億年前的微生物遺跡。在地球上辨識(shí)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(zhàn)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(xué)家一直呼吁把火星巖石和土壤樣本帶回地球做深入分析。2020年開始的火星計(jì)劃將開始嘗試挑選、提取和儲(chǔ)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(zhǎng)途飛行來到地球的隕石——因此，宇宙生物學(xué)家只能用其他方法進(jìn)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(jì)劃中就開始了。但科學(xué)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，還是熔巖或凍結(jié)的二氧化碳造成的。多虧了“好奇號(hào)”，現(xiàn)在科學(xué)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(gè)結(jié)論一度很難在火星氣候模型中說得通?；鹦菤夂蚰Ｐ惋@示，早期火星冰冷，大氣稀薄，不支持大量靜態(tài)水的存在，不可能有流淌了幾百萬年的河流。但面對(duì)越來越多的關(guān)于持續(xù)存在的水源和由此導(dǎo)致的溫暖環(huán)境的證據(jù)，氣候?qū)W家開始傾向于兩種互相交叉的解釋——兩種解釋都意味著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(dòng)和隕石撞擊讓星球大幅升溫;火山也會(huì)釋放出氣體，能合成有機(jī)化合物。第二個(gè)解釋是，火星要維持水循環(huán)，讓火星湖泊和河流成為可能，那么在北半球就應(yīng)該有一大片海洋。火星赤道以北的大片區(qū)域比通稱的南方高地要低一到三英里?？茖W(xué)家提出假設(shè)，北半球可能有古代海洋，塑造了廣袤的低地。也有人不同意，基于好幾條理由，包括沒有探測(cè)到海岸線的遺跡。

“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(fā)現(xiàn)北半球海洋的確鑿證據(jù)，但我們的模型要求有一大片水體，這樣才能解釋目前地質(zhì)學(xué)家已經(jīng)證實(shí)的結(jié)論，”“好奇號(hào)”團(tuán)隊(duì)成員、噴氣推進(jìn)實(shí)驗(yàn)室(Jet Propulsion Laboratory)的邁克爾·A·米什納(Michael A. Mischna)說。“好奇號(hào)的工作就是將大氣與氣候信息，和地質(zhì)學(xué)家與地質(zhì)化學(xué)家的發(fā)現(xiàn)整合起來，構(gòu)建一個(gè)大跨度、可以自圓其說的理論，描述一個(gè)非常濕潤(rùn)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(guān)于水的證據(jù)已經(jīng)越來越清楚了，但有關(guān)有機(jī)化合物的疑問還有一大堆。這類物質(zhì)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(fā)現(xiàn)。

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(xué)家知道應(yīng)該尋找一種叫做高氯酸鹽的無機(jī)物。在地球上，高氯酸鹽被證明可以在高溫下破壞有機(jī)物，或者促使有機(jī)物轉(zhuǎn)化。在蓋爾環(huán)形山上發(fā)現(xiàn)了大量高氯酸鹽。這意味著，如果早期火星存在有機(jī)物，而且它們沒有被持續(xù)的宇宙輻射消滅，那么它們現(xiàn)在也早就不存在了，或者用現(xiàn)有的技術(shù)很難發(fā)現(xiàn)。

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(shè)備是用來識(shí)別相對(duì)簡(jiǎn)單的有機(jī)物的。這些有機(jī)物將在它的烤爐內(nèi)作為氣體燃盡。但它也帶有九個(gè)杯子，盛著一種溶劑，可以轉(zhuǎn)化更復(fù)雜的分子(比如氨基酸和核酸)同時(shí)保護(hù)它們的化學(xué)特征。

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“濕化學(xué)”技術(shù)一直還在等待富含有機(jī)物的樣本被發(fā)現(xiàn)。另一個(gè)問題是設(shè)備的其中一只杯子發(fā)生了泄漏，制造了大麻煩，讓團(tuán)隊(duì)負(fù)責(zé)人不放心。