单选题   Para. 1 ①On 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. ②Sadly, news announced on December 12th, at the annual meeting of the American Geophysical Union (AGU), in Washington, DC, was anything but happy. ③Preliminary 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.
    Para. 2 ①Previous observations, from orbit and by telescopes on Earth, suggested Mars might spot traces of the gas. ②These were backed up by data from Curiosity, an American Mars rover. ③In 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.
    Para. 3 ①That cyclical pattern has intrigued researchers back on Earth. ②Broadly speaking, there are two possible sources for Martian methane. ③One is outer space, whence carbon-rich molecules, some of which are likely to break down into methane, arrive constantly on meteors of various sizes. ④The other is from under the planet's surface.
    Para. 4 ①Methane from both sources will mix eventually into the atmosphere. ②But if the gas is coming from underground, it will be more concentrated near its source, and might well appear on a seasonal basis. ③The process could be a geological or geochemical one that is encouraged by the relative warmth of summer. ④That would be interesting. ⑤Or it could be biological, with methane-generating bugs waking up during the summer months. ⑥For 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. ⑦But such lucky breaks do happen.
    Para. 5 ①Regardless 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. ②It 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. ③And, it has failed to find the slightest hint of methane in the Martian atmosphere. ④Since NOMAD is 20 times more sensitive than the methane detector on board Curiosity, this is bad news.
    Para. 6 ①But they do not surprise Kevin Zahnle of the Ames Research Centre, in California, a laboratory belonging to NASA, America's space agency. ②Dr Zahnle has long argued that Curiosity's reports of Martian methane are artefacts.
    Para. 7 ①The optimists will not be deflected, though. ②They note that NOMAD Can probe only the upper part of Mars's atmosphere. ③Air with an altitude of less than 5 km is beyond its range. ④ Moreover, when ExoMars Trace Gas Orbiter flew over Gale, a dust storm obscured NOMAD's view of anything within 30 km of the surface. ⑤What 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. ⑥But for now, neither side is willing to give way.