Mars has been teasing us for a long, long time. It has canals! announced Italian astronomer Giovanni Schiaparelli in 1877. No, it doesn’t, responded NASA‘s Mariner 4 spacecraft when it flew by the planet in 1965. But there might still be microbial life, we all insisted. Not that we can detect, answered the twin Viking spacecraft when they landed on the surface in 1976.
It’s gone back and forth like that ever since, with Earth-based telescopes as well as NASA’s flock of Mars orbiters and rovers detecting topographic formations and surface chemistry definitively proving that Mars was once a warm and very waterlogged place. What’s more, deposits of water persist still, locked in polar ice and in underground deposits, some of which melt and percolate to the surface in spring. None of the scooping and sampling and scanning by the spacecraft has produced any proof of biology, but there have been tantalizing hints of biological byproducts—in the form of methane.
The most common of all of the solar system’s hydrocarbons, methane is present in high concentrations on Earth, and up to 95% of it is produced biologically—by either living or dead and decaying organisms. Over the past decade or so telescopes and spacecraft alike have similarly spotted signs of methane all over Mars. There are readings from the Canada-France-Hawii Telescope on the summit of Mauna Kea Mountain on Hawaii’s big island showing global Martian methane averages of up to 13 parts per billion (ppb). There are readings from the European Space Agency’s Mars Express spacecraft more or less paralleling those findings and also reporting seasonal spikes of up to 45 ppb in the north polar region. The Mars Global Surveyor satellite pushed that maximum to 60 ppb. Other observations suggest that at least some of this methane exists in the form of great atmospheric plumes.
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Now, the best, most precise and most persuasive findings of all have come back from the Mars Curiosity rover, which landed on the Red Planet in 2012, and the answer, according to a paper just published in Science is: a big thumbs-down. Never mind the global averages and the giant plumes, the best guess on the total concentration of methane on Mars is now a puny 1.3 ppb—and that’s the upper limit. As for how statistically confident the investigators are of this conclusion? At least 95%.
“Our result sets an upper limit that is [about] six times lower than the recent measurements,” the authors of the paper wrote. This, they said, “greatly reduces the probability” of significant methane-producing microbial activity.
What makes the results reliable—if keenly disappointing—is the mere fact that the equipment aboard Curiosity is so bloody good. The atmospheric analysis was conducted by an instrument called the Tunable Laser Spectrometer (TLS), which uses two different kinds of laser detectors to do its work. One looks for the spectral lines of methane alone; the other can detect methane, water, carbon dioxide and various isotopes of carbon and oxygen. Neither one saw methane in anywhere near the concentrations the more remote sensors did. And that remoteness is the key to their earlier readings’ lower reliability: an on-site instrument, the investigators write, “offers unambiguous identification of methane.” And, alas, of its unambiguous absence as well.
Had heavy methane concentrations been found, it would by no means have sealed the deal for microbial life. Methane can be produced by geological activity, such as geysers or volcanoes; and it can also be carried in aboard comets or meteorites. Those explanations would have fit nicely with the idea atmospheric plumes, but even those temporary concentrations have been called into question by the new findings. Methane concentrations as high as 60 ppb should have persisted in the atmosphere longer than they have, which suggests that they were not there in the first place.
The good news for Mars-ophiles is that the new findings do not remotely seal the deal against life either. The TLS sampled only the lowest three feet (1 m) of the atmospheric column and methane could exist higher up, though the experimenters believe that their understanding of how the gas moves, disperses and breaks down suggests that the bottom of the column is representative of the rest. What’s more, life still may be living and metabolizing deeply underground; indeed, on a dry, all-but airless surface bombarded by cosmic rays, the safer, wetter subsurface would be the smartest place to hunker down. What’s more, even state-of-the-art instruments are not flawless instruments. The TLS could always be wrong—or at least missing something.
For now, however, the planet that has made a habit of raising, lowering and confounding our expectations has let us down again. If history is any indicator, we have a right to be disappointed—but not to give up hope.
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