DECEMBER 22ND 2018-JANUARY 4TH 2019
The latest results suggest no methane on Mars. Biology is thus less likely
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.
在地球上，大气层中大部分甲烷是由活着的生物释放出来的，因此，在火星上发现甲烷对寻找地外生物的人来说是个好消息。遗憾的是，12月12号，在华盛顿举办的美国地球物理联盟的年会上，新闻公布了这一令人失望的消息。气体轨道卫星ExoMars的初步探测结果对火星大气层存在甲烷的说法表示否定，过去两年，这颗欧洲飞行器一直在环绕火星飞行。
1 | methane ['mɛθˌeɪn] |
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.
此前的轨道观测和地球上望远镜的观测表明，火星上可能存在这种气体的踪迹。这些得到了美国火星探测车“好奇号”的数据支持。在“好奇号”环绕盖尔陨石坑探测的六年，不仅探测到了甲烷，还记录了这种物质季节性的涨落，从冬天的十亿分之0.25到冬天的十亿分之0.65，峰值为每十亿分之7.0。
1 | observation [ˌɑːbzər'veɪʃn] |
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.
这种循环模式引起了地球上研究人员的兴趣。一般来说，火星上有两种可能的甲烷来源。第一个是外层空间，在那里有富含碳的分子，其中一些可能分解为甲烷，持续地到达各种大小的流星上。另一个来源是地壳下。
1 | cyclical ['saɪklɪkl] |
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.
两种来源的甲烷最后都会混合进入大气层。但是，如果甲烷气体来自地下，那么它将更集中于其来源附近，并可能季节性地出现。这个过程可能是一个地质的或则地质化学的过程，由相对温暖的夏天造成的。那就有趣了。也有可能是生物原因，在夏季，产生甲烷气体的甲虫会苏醒过来。这会是一个大新闻。要想解释好奇号观测到的季节性现象，漫步者必须有降落到甲烷渗漏区的幸运。但这种幸运真的发生了。
1 | basis ['beɪsɪs] |
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.
不管甲烷的来源是什么，根据地球上的实验，火星大气层中的甲烷分子将会存在几百年。为了找到这种分布更广的物质的迹象，科学家设计了一种名为NOMAD的分光仪器，该仪器被安装在ExoMars微量气体探测卫星上。而且，正如比利时的航空探测研究所的Ann Carine告诉AGU的那样，NOMAD没有在火星大气层中发现一点甲烷的迹象。由于NOMAD的灵敏度是好奇号上甲烷探测器的20倍，所以这是个坏消息。
1 | spectroscopic [ˌspektrə'skɒpɪk] |
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，有待发表到期刊上，因此是初步的。但这些并不让加州Ames研究中心的Kevin Zahnle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甲烷的报告是人造的。这些数据仅比对照数据高一点，并且这些数据表明，探测器本身可能就是甲烷的来源（尽管没人知道是如何发生的）这进一步加剧了获取准确数据的难度。
1 | pending ['pendɪŋ] |
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. (616 words)
然而，乐观主义者并不会改变立场，他们指出，NOMAD只能探测火星大气的上层部分。海拔低于5千米的空气并不在其探测范围。此外，当轨道气体卫星ExoMars飞跃Gale陨石坑的时候，一场沙尘暴遮挡了NOMAD离地30千米内能看到的所有东西。因此，乐观主义者认为甲烷从火山口喷出的观点是不容忽视的。就NOMAD所发现的表明，如果火星上的空气中存在甲烷，它是非常罕见的，并且仅限于近地的空气中。就目前而言，双方都不愿意让步。
1 | deflect [dɪ'flekt] |