Mysterious Missing Martian Methane Methane would be a fairly remarkable compound to find in the atmosphere of Mars. Methane would have a very short lifetime in the martian atmosphere (10,000 years or less) so there would need to be an active supply of methane from the planet into the atmosphere. On Earth, most methane emissions are associated with biological activity (present or fossilized), so a source of martian methane would be a fascinating thing to find, possibly even indicating life existing somewhere within the planet. It turns out people have been reporting methane in Mars’s atmosphere since the Mariner 7 spacecraft flew past Mars in 1969. Typically methane is measured by spectroscopy; it absorbs light in certain wavelengths and so the spectra of light coming through the atmosphere could tell whether it was there. Several spacecraft and telescopes have reported methane present in the martian atmosphere, particularly in the last decade. There is even a planned spacecraft (not sure of its current status) called the Mars Trace Gas Orbiter originally scheduled for a 2016 launch which would have the main goal of characterizing methane abundances (among other trace gases) in Mars’s atmosphere. However, the measurement of methane has been challenged by some very good quality science in the past as well. Researchers have noted that detections of methane from telescopes are difficult because other compounds, like CO2 and Water, can absorb light at similar frequencies, giving a signature that looks like methane. Those issues are compounded by taking measurements through the Earth’s atmosphere or from spacecraft that may not have enough resolution to distinguish methane from Water. It would be really nice if there was some ground-truth to the methane observation before missions fly to investigate it. The Curiosity rover went to Mars with the tools to look for methane. The remarkable SAM (Sample Analysis on Mars) instrument includes equipment that can do a high-quality measurement of the atmosphere. SAM has equipment that can sample atmospheric gases, concentrate them, and then measure the makeup of those gases by performing spectroscopy with a laser-based system. This really is as cool as it sounds; the part of SAM that can do this is called a tunable laser spectrometer and it is pictured in this photo. Previous measurements have suggested that there could be as much as 45 ppb of methane in Mars’s atmosphere. The methane measurements by Curiosity were published this week in the journal Science Express. They found methane unmeasurable by Curiosity; the upper limit for methane in the gases measured is 1.3 ppb. There’s no way the methane could have just vanished if it was there, and it’s extremely unlikely that there would be methane in some part of Mars’s atmosphere but none in Gale Crater. That leaves one likely possibility; the spectral measurements of methane just weren’t accurate enough and produced a result that isn’t matched on the ground. The lack of methane is disappointing for anyone who hoped that Mars might still somewhere host an active biological community, perhaps deep in its crust where it is warm. Life as we know it on Earth would probably give off something like methane as a byproduct. The fact that methane isn’t there doesn’t guarantee Mars is uninhabited, but it sure points in that direction. -JBB Press release and image credit: jpl.nasa.gov/news/news.php?release=2013-285 On why the spectral details might not have told the full story: sciencedirect/science/article/pii/S001910351000446X Original paper sciencemag.org/content/early/2013/09/18/science.1242902.abstract ExoMars: esa.int/Our_Activities/Space_Science/Tooling_up_ExoMars
Posted on: Sun, 22 Sep 2013 04:02:24 +0000
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