The first tests for methane performed by Curiosity have come back negative. At least during the rover's first weeks on Mars, any methane present in the air above Gale Crater was at levels lower than five parts-per-billion. But the scientists studying the Martian atmosphere haven't come away disappointed. Detailed measurements of the isotopes present in the first samples indicate that the Martian atmosphere was once much thicker, which may help explain the past existence of liquid water on its surface.

The data come from Curiosity's Sample Analysis at Mars, or SAM instrument. Among other items, this contains a laser spectrometer, which can identify the chemicals present in the sample, and a mass spectrometer, which can identify their weights. The latter can also separate individual isotopes of elements. The data has also been cross-correlated; in a few cases (like CO 2 ), the laser spectrometer can pick out the presence different isotopes, and check the ratios it obtains against those from the mass spectrometer. The readings were also tested against gasses trapped in a meteorite that was blasted off the surface of Mars and fell to Earth. So, we have a really good idea that all the instruments are operating well.

And, so far at least, those instruments are not picking up any methane. As of right now, the researchers operating the instrument put the upper limit on its presence at five parts-per-billion. They expect that limit to drop simply by statistically aggregating multiple measurements. That probably rules out much in the way of large, active sources on Mars at this moment, because it only takes about three months for Mars' atmosphere to evenly mix. But past evidence of methane suggests it might be released both locally and seasonally. So, the scientists are being quite cautious, and simply saying they haven't detected any methane within Gale Crater.

The current plan is to keep sampling, and see if anything turns up as the seasons change on the red planet. There is also a way of using the pumps that supply the gas to the instruments to selectively concentrate methane. That should increase the sensitivity by 10-fold.

So, what is present in Mars' atmosphere? About 96 percent of it is carbon dioxide, with two percent each of argon and nitrogen. Oxygen is only present at 0.14 percent, and carbon monoxide at 0.06 percent.

The arrival of winter on Mars freezes out about 35 percent of the atmosphere's CO 2 at one of the poles, even as it's being liberated from the other. Depending on the precise timing, this could radically shift the relative amounts of most of the components of the atmosphere.

The instruments have also provided an indication that Mars' atmosphere has changed dramatically over time. We know what the probable isotope ratio of Mars' atmosphere was, based on measurements on Earth and other bodies. But the ratios no longer look much like that. In fact, the fraction of the heavier isotopes of argon is a full seven times what it is on Earth.

How can something like this happen? Some chemicals could have reacted with the Martian surface, but this isn't an option for argon, which is chemically inert. The only way to explain these isotopic differences is if Mars has been losing its atmosphere, a process that should preferentially result in the loss of lighter isotopes. This, in turn, indicates that Mars' atmosphere was once much thicker.

That's consistent with the extensive evidence of liquid water on Mars' surface (some obtained by Curiosity itself). If the heavier atmosphere was probably mostly CO 2 and water vapor (as you might expect), the enhanced greenhouse warming would help raise the temperature, possibly enough to keep the water liquid.

Deviations between the isotope ratios of different elements may provide some hints of whether some chemicals are being refreshed from the interior of Mars, or are part of a larger chemical cycle. So nailing these isotope ratios down and looking for any long term changes may give us a better idea of whether there are any active processes reshaping the Martian atmosphere beyond the escape of gasses to space.