The results were announced in 2009: researchers had been watching Mars for several years using Earth-based telescopes, and they'd seen something unusual in its atmosphere. In time with the red planet's seasons, large amounts of methane appeared in its atmosphere. Since this chemical shouldn't last for long in the Martian atmosphere, the observation gave researchers an obvious challenge: come up with some mechanism that could be producing a seasonal release of newly created methane.

Although our original headline suggested a choice between geology or life on Mars, researchers have since proposed eight different processes that might account for the seasonal plumes, although none of them is without its issues. In addition, there was a ninth option: the researchers behind the original findings were misinterpreting their data, and there was far less methane around than their work suggested.

Now, a new study is out that may split the difference. Although it can't account for the full amount of methane suggested by the first paper, it proposes a source of methane that should produce the sorts of seasonal increases seen in the earlier study: a combination of carbon-rich meteors and exposure to UV light.

The work relied on the Murchison meteorite, which struck Australia in 1969. It belongs to a class of bodies called carbonaceous chondrites, which, as their name implies, are carbon rich, and contain a lot of complex organic compounds. Any methane it held would have been lost during its trip through the Earth's atmosphere, but the chemicals remaining in the meteor could potentially break down into methane under the right conditions.

The authors of the new paper, based in labs throughout Europe, have found some conditions that liberate methane from the meteor, though they had to grind up some pieces of it to do so. The key ingredient turned out to be UV light. When exposed to UV, the meteor fragments rapidly released methane. The reaction would eventually tail off, but several activities—shaking the fragments, raising the temperature, and lowering the pressure—could all boost the production of methane again. An intact fragment also worked, suggesting that size isn't a limiting factor.

What's this got to do with Mars? To begin with, Mars' atmosphere is too thin to support significant amounts of ozone, so UV light penetrates directly to the surface of the planet. Mars also receives a lot of material via meteorites each year, with estimates ranging in the thousands of metric tons all the way up to 60,000 metric tons. In addition, the dust storms that scour the planet's surface would do a good job of providing the equivalent of shaking up the meteors, which provided a boost to their methane release.

Then there's the change in temperatures. The Martian summer would lead to a large boost in production by heating any material near the surface. In these experiments, heating enabled UV light to produce more methane.

So, in many ways, these experiments indicate that the combination of UV light and meteor fragments could account for many of the more compelling features of the Mars observations. They do, however, fall short in two ways. One is that they can't explain why the methane in the atmosphere appears to be concentrated over specific regions of the martian surface. The second issue is that the estimates of the total tonnage of meteorites hitting the planet's surface don't seem to bring enough carbon in to fully account for the methane seen in the atmosphere. These data can only fully account for Mars' methane if the more recent estimates of its volume are right and the original report was wrong.

So, depending on how future observations play out, the new study might account for the presence of methane on Mars without the need for any life on the red planet. But that's not all the study says about life, as it has some significant implications for our ability to search for it.

Our solar system has an average ratio of the two common carbon isotopes, but life tends to change that, since biochemical reactions slightly favor the use of lighter isotopes. We can use that to identify when a sample has been in contact with living creatures, since it will show a strong bias away from 13C and towards the lighter 12C. Unfortunately, the UV-driven reactions in these meteorites show a very similar bias, resulting in a carbon isotope signal that overlaps with those we normally consider diagnostic of life. So, if we solely rely on carbon isotopes as a test for life, there's a chance we can end up badly misled.

The situation isn't hopeless, as the authors found that the isotope ratios of hydrogen clearly indicated that the source of methane was extraterrestrial. But the findings provide a very clear caution that we'll have to look very carefully at any samples we think might contain signs of life.

Nature, 2012. DOI: 10.1038/nature11203 (About DOIs).