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.
地球大气中的大部分甲烷源于生物体，因此在火星上发现甲烷，于寻找外星生命的人来说是个好消息。令人伤心的是，12月12日在华盛顿举办的美国地球物理联盟年会的消息十分扫兴。欧洲飞行器“外火星追踪气体轨道器”过去两年一直绕火星运行，据其初步结果，火星大气层中没有甲烷。
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.
之前来自于近火星轨道和地球望远镜的观察表明也许火星上存在着运动的气体轨迹。该发现由好奇号，一艘美国的火星探测车发来的数据所证实。在一处名为Gale的陨石坑（见图）中巡游了六年之后，好奇号不仅探测到了甲烷，还记录了这种物质的季节性涨落，从冬季的0.25亿分之二到夏季的6.5亿分之二，峰值高达70亿分之二。
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.
这种循环模式让地球上的研究人员十分着迷。大体上说，有两种可能的甲烷来源。其一是来自外部空间，一些富含碳元素的分子分解成甲烷，随着大小不一，不断撞击火星的流星到达。其二则是来自于该行星的地表之下。
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.
来自于两种源头的甲烷最终会混合进入大气。但如果是来自于地下的气体，它在气源附近会更加密集，或将表现出明显的季节性。这一过程可能是一种被相对暖和的夏季所促成的地质或地质化学现象。那真的太有趣了。也有可能是生物的原因，比如来自于在夏季苏醒的产甲烷细菌。要是这样就能上头条了。但是，如果想要对好奇号的发现做出解释，探测车就必须幸运的降落在甲烷渗透区。但是，这种幸运的事真的发生了。
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.
不管气体的源头是什么，根据以往经验来说，任何处于火星大气中的甲烷分子都会和地球上的一样，独自存在了很多个世纪。为了应对这种在大气层中的探索，一个叫做NOMAD的光学设备被设计出来，装在火星气体追踪轨道探测器上。但是，比利时太空高层大气物理与化学研究所的A告诉AUG，NOMAD没有在火星大气层中发现哪怕一丝的甲烷，考虑到NOMAD上那个比起好奇号灵敏20倍的甲烷探测器，这真的是一个坏消息。
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.
这些发现仍在审查中，有待发表在期刊上，因此是初步的。但这并不让加州艾姆斯研究中心的凯文·扎恩勒感到惊讶，该中心是美国宇航局下属的一个实验室。Zahnle博士一直认为好奇号关于火星甲烷的报告是人工创造的。这些读数仅略高于对照读数，而这些读数表明，“漫游者”本身可能是甲烷的一个来源(尽管没人知道是如何做到的)，这进一步增加了获取准确读数的难度。
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，乐观主义者不会改变立场。他们指出，游牧民只能探测火星大气层的上部。海拔低于5公里的空气不在其射程之内。此外，当外火星的微量气体轨道飞行器飞越大风时，一场沙尘暴遮蔽了Noman在距离地面30公里内看到的任何东西。因此，这些乐观主义者认为，甲烷从火山口渗出的观点是不容忽视的。NOMAD所做的似乎表明，如果火星空气中存在甲烷，它是非常罕见的，而且仅限于低水平的大气中。但就目前而言，双方都不愿让步。