More precisely, how does one go about measuring the small amount of water vapor in what is almost a vacuum by terrestrial standards?
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1Infrared measurements will do nicely, see e.g. "Water vapor in the atmosphere of Mart: From Pathfinder to Mars Polar Lander" D.V. Titot et al.. One can find plenty of wavelengths at which water vapor is strongly absorbing: http://en.wikipedia.org/wiki/Electromagnetic_absorption_by_water. For this one can use the sun and an IR photometer. A local measurement can be done with a mass spectrometer like on the Viking probes and the Curiosity rover. – CuriousOne Jun 10 '15 at 17:39
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I would have thought there would be problems with a mass spectrometer given the masses of N2 and H2O are so close – Jun 10 '15 at 20:39
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Molecular nitrogen has a molecular weight of approx. 28u (214), water of approx. 18u (16+21). Even the most simple mass spectrometer can keep those apart, but you do have a point, there are cases in mass spectrometry where only a very high resolution instrument can distinguish different molecules of the same mass. – CuriousOne Jun 10 '15 at 21:47
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Sorry - my dyslexia kicking in - HO2! – Jun 11 '15 at 08:08