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  3. On earth, most of the methane in the atmosphere has been belched by living o

On earth, most of the methane in the atmosphere has been belched by living o

游客2023-12-28  9
问题     On earth, most of the methane in the atmosphere has been belched by living organisms, so finding the gas on Mars would J 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.
    Previous observations, from orbit and by telescopes on Earth, suggested Mars might spot 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, 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 0.65 ppbn in the summer, with spikes up to 7.0 ppbn.
    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.
    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. 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.
    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, it 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.
    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.
    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 5 km is beyond its range. Moreover, when ExoMars Trace Gas Orbiter flew over Gale, a dust storm obscured NOMAD’s view of anything within 30 km of the surface. 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.
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答案     地球大气中大部分的甲烷是由生物释放出来的，所以在火星上发现甲烷气体对于寻找外星生命的科学家来说无疑是个好消息。然而，12月12日，在华盛顿特区举行的美国地球物理联盟(AGU)年度会议上发布了一则令人沮丧的新闻。环绕火星飞行两年之久的欧洲航天器火星微量气体任务卫星 (ExoMars Trace Gas Orbiter)发现的初步结果表明火星大气中并不存在甲烷。
    而此前通过轨道和地球望远镜的观测结果表明，火星上可能有甲烷存在。该发现得到了美国火星探测器好奇号(Curiosity)的数据支持。它围绕着名为盖尔(Gale)的环形山行驶了6年，不仅探测到了甲烷，还记录了其浓度的季节性变化，从冬季仅0.25 ppbn(十亿分之一)上升到夏季的0. 65 ppbn，其峰值可达到7.0 ppbn。
    这种周期变化模式引起了地球上研究人员的兴趣。一般来说，火星上的甲烷有两种可能的来源。一是来自外太空，那里富含碳的分子，其中一些很可能会分解成甲烷，不断地到达大小各异的流星上；二是来自火星地表之下。
    两种来源的甲烷最终会混入大气中，但是，如果甲烷来自地下，那么它将更集中于其源头附近，并且有可能出现季节性变化。这可能是一个地质或地质化学过程，并受到夏季相对温暖的气候影响。那会是有趣的发现。它也有可能是生物活动的产物，有些虫子能够释放出甲烷，在夏季的几个月中，它们会活跃过来。要想解释好奇号观测到的季节性现象，它必须幸运地着陆在甲烷渗出的区域，但这种幸运的事情确实发生了。
    不管甲烷从何而来，根据地球上的实验，火星大气中的任何甲烷分子都将存在几个世纪。为了发现更广泛的甲烷迹象，研究人员设计了搭载在火星微量气体任务卫星上工作的掩日与天底点光谱仪(NOMAD)，但它没能在火星大气中发现丝毫甲烷气体。由于该光谱仪的敏感度是好奇号上甲烷探测器的20倍，这对于科学家来说是个坏消息：
    然而，从属美国国家航空和宇宙航行局(NASA)的加州艾姆斯研究中心(Ames Research Centre)的研究员凯文.扎恩勒(Kevin Zahnle)对这一发现并不感到惊讶，扎恩勒博士一直主张好奇号关于火星存在甲烷的报告是错误的。
    尽管如此，乐观主义者不会改变他们的观点。他们指出，光谱仪只能探测到火星大气上层，海拔低于5千米的大气层不在其探测范围之内。此外，当火星微量气体任务卫星，飞过盖尔环形山时，一场沙尘暴挡住其视线，导致它不能探测到距离地面30千米以内的任何东西，但是光谱仪的发现似乎确实可以说明，即使火星大气中存在甲烷，它也很稀薄，并且仅存在于大气低层，但就目前而言，双方都不愿让步。
解析     1.第1段第二句anything but为固定搭配，意思是“绝对不”，故结合本文语境，anything but happy可译为“令人沮丧的”。
    2.第1段第三句thumbs-down为合成词，意思是 “拒绝、反对或谴责的例子或姿态”，但此处不需要直接译为“反对、不同意……观点”，据本文语境，此处是说明卫星并没有在火星大气中发现甲烷的这个结果。只需要译出结论“火星大气中并不存在甲烷”便可。
    3.第3段第三句break down为周定搭配，意思是“把某物分成更小的部分”，故可译作“分解”。
    4.第4段lucky breaks为固定搭配，意为“一个幸运且意外的转折”。结合本文语境，此处是说好奇号幸运地着陆在甲烷渗出的区域，故可译作“幸运的事情”。
    5.第6段第二句artefacts原义为 “(尤指有历史或文化价值的)手工艺品”，但在此指的是好奇号关于火星存在甲烷的报告存在问题，故译为“错误的”更为合适。
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