On earth, most of the methane in the atmosphere has been belched by living organisms, so finding the gas on Mars would be happy news for seekers after extraterrestrial life. Sadly, news announced on December 12th, at the annual meeting of the American Geophysical Union (AGU), in Washington, DC, was anything but happy. Preliminary results from ExoMars Trace Gas Orbiter, a European craft that has been circling Mars for the past two years, give a thumbs-down to the idea that there is methane in its atmosphere.
地球大氣中的大部分甲烷源于生物體，因此在火星上發(fā)現(xiàn)甲烷，于尋找外星生命的人來說是個好消息。令人傷心的是，12月12日在華盛頓舉辦的美國地球物理聯(lián)盟年會的消息十分掃興。歐洲飛行器“外火星追蹤氣體軌道器”過去兩年一直繞火星運(yùn)行，據(jù)其初步結(jié)果，火星大氣層中沒有甲烷。

Previous observations, from orbit and by telescopes on Earth, suggested Mars might sport traces of the gas. These were backed up by data from Curiosity, an American Mars rover. In its six years crawling around a crater called Gale (pictured above) Curiosity has both detected methane and recorded seasonal ups and downs of the stuff that cycle from a modest 0.25 parts per billion during the winter to 0.65ppbn in the summer, with spikes up to 7.0ppbn.
之前來自于近火星軌道和地球望遠(yuǎn)鏡的觀察表明也許火星上存在著運(yùn)動的氣體軌跡。該發(fā)現(xiàn)由好奇號，一艘美國的火星探測車發(fā)來的數(shù)據(jù)所證實(shí)。在一處名為Gale的隕石坑（見圖）中巡游了六年之后，好奇號不僅探測到了甲烷，還記錄了這種物質(zhì)的季節(jié)性漲落，從冬季的0.25億分之二到夏季的6.5億分之二，峰值高達(dá)70億分之二。

That cyclical pattern has intrigued researchers back on Earth. Broadly speaking, there are two possible sources for Martian methane. One is outer space, whence carbon-rich molecules, some of which are likely to break down into methane, arrive constantly on meteors of various sizes. The other is from under the planet’s surface.
這種循環(huán)模式讓地球上的研究人員十分著迷。大體上說，有兩種可能的甲烷來源。其一是來自外部空間，一些富含碳元素的分子分解成甲烷，隨著大小不一，不斷撞擊火星的流星到達(dá)。其二則是來自于該行星的地表之下。

Methane from both sources will mix eventually into the atmosphere. But if the gas is coming from underground, it will be more concentrated near its source, and might well appear on a seasonal basis. The process could be a geological or geochemical one that is encouraged by the relative warmth of summer. That would be interesting. Or it could be biological, with methane-generating bugs waking up during the summer months. That would be headline-grabbing. For either to be the explanation of the seasonality observed by Curiosity, the rover would have to have had the luck to land in an area of such methane seeps. But such lucky breaks do happen.
來自于兩種源頭的甲烷最終會混合進(jìn)入大氣。但如果是來自于地下的氣體，它在氣源附近會更加密集，或?qū)⒈憩F(xiàn)出明顯的季節(jié)性。這一過程可能是一種被相對暖和的夏季所促成的地質(zhì)或地質(zhì)化學(xué)現(xiàn)象。那真的太有趣了。也有可能是生物的原因，比如來自于在夏季蘇醒的產(chǎn)甲烷細(xì)菌。要是這樣就能上頭條了。但是，如果想要對好奇號的發(fā)現(xiàn)做出解釋，探測車就必須幸運(yùn)的降落在甲烷滲透區(qū)。但是，這種幸運(yùn)的事真的發(fā)生了。

Regardless of their source, any methane molecules in Mars’s atmosphere would, on the basis of experiments on Earth, be expected to hang around for centuries. It was to find signs of this more widespread material that a spectroscopic instrument called NOMAD (Nadir and Occultation for MArs Discovery), which is on board ExoMars Trace Gas Orbiter, was designed. And, as Ann Carine Vandaele of the Belgian Institute for Space Aeronomy told the AGU, NOMAD has failed to find the slightest hint of methane in the Martian atmosphere. Since NOMAD is 20 times more sensitive than the methane detector on board Curiosity, this is bad news.
不管氣體的源頭是什么，根據(jù)以往經(jīng)驗(yàn)來說，任何處于火星大氣中的甲烷分子都會和地球上的一樣，獨(dú)自存在了很多個世紀(jì)。為了應(yīng)對這種在大氣層中的探索，一個叫做NOMAD的光學(xué)設(shè)備被設(shè)計出來，裝在火星氣體追蹤軌道探測器上。但是，比利時太空高層大氣物理與化學(xué)研究所的A告訴AUG，NOMAD沒有在火星大氣層中發(fā)現(xiàn)哪怕一絲的甲烷，考慮到NOMAD上那個比起好奇號靈敏20倍的甲烷探測器，這真的是一個壞消息。

The findings are still under review, pending publication in a journal, and are therefore preliminary. But they do not surprise Kevin Zahnle of the Ames Research Centre, in California, a laboratory belonging to NASA, America’s space agency. Dr Zahnle has long argued that Curiosity’s reports of Martian methane are artefacts. They are only marginally higher than control readings, and those readings indicate the rover itself may be a source of methane (though nobody knows how), which further complicates taking accurate readings.
這些發(fā)現(xiàn)仍在審查中，有待發(fā)表在期刊上，因此是初步的。但這并不讓加州艾姆斯研究中心的凱文·扎恩勒感到驚訝，該中心是美國宇航局下屬的一個實(shí)驗(yàn)室。Zahnle博士一直認(rèn)為好奇號關(guān)于火星甲烷的報告是人工創(chuàng)造的。這些讀數(shù)僅略高于對照讀數(shù)，而這些讀數(shù)表明，“漫游者”本身可能是甲烷的一個來源(盡管沒人知道是如何做到的)，這進(jìn)一步增加了獲取準(zhǔn)確讀數(shù)的難度。

The optimists will not be deflected, though. They note that NOMAD can probe only the upper part of Mars’s atmosphere. Air with an altitude of less than 5km is beyond its range. Moreover, when ExoMars Trace Gas Orbiter flew over Gale, a dust storm obscured NOMAD’s view of anything within 30km of the surface. Such optimists thus argue that the idea of methane seeping from the crater cannot be discounted. What NOMAD does seem to show is that, if methane exists at all in Mars’s air, it is rare and confined to low levels of the atmosphere. But for now, neither side is willing to give way.
然而，樂觀主義者不會改變立場。他們指出，游牧民只能探測火星大氣層的上部。海拔低于5公里的空氣不在其射程之內(nèi)。此外，當(dāng)外火星的微量氣體軌道飛行器飛越大風(fēng)時，一場沙塵暴遮蔽了Noman在距離地面30公里內(nèi)看到的任何東西。因此，這些樂觀主義者認(rèn)為，甲烷從火山口滲出的觀點(diǎn)是不容忽視的。NOMAD所做的似乎表明，如果火星空氣中存在甲烷，它是非常罕見的，而且僅限于低水平的大氣中。但就目前而言，雙方都不愿讓步。