Maybe the methane results from some geologic or chemical process we don't yet understand. Or, since much of the methane on Earth is produced by biology, perhaps the faint whiffs of methane point to the existence of present-day life on Mars.

Dorothy Oehler, of NASA's Johnson Space Center in Houston, Texas, says there is another source worth considering. She suggests that some methane could come from the remains of past life on Mars - namely, from the thermal alteration of buried kerogen.

Kerogen is ancient organic crud, the remains of decayed organisms. When buried kerogen is cooked by the Earth's internal heat, it turns into oil and methane.

This source of hydrocarbon can be extremely long-lived - the Earth still has Precambrian kerogen that is slowly but steadily releasing methane into the environment.

Many scientists believe that the Meridiani Plains on Mars is an ancient lake bed, because Meridiani is rich in hematite, an iron oxide mineral that forms in the presence of water.

If Meridiani did harbor a lake or sea, any life forms living in the waters would have accumulated on the sea floor after they died. Over time, layers of decaying organisms would have become buried kerogen, just as they do on Earth.

"Everybody seems to think early Mars was not too different from early Earth, and early Earth certainly had a vast array of microbial organisms," says Oehler.

"I think there's at least a fair chance that life evolved on early Mars. Whether it lasted and is still going on, I'm not even prepared to say, but if it were there in the past, then maybe there are remnants of it buried in the sediments."

Like much of Mars, Meridiani is pocketed with many meteor impact craters. Some of these craters seem to have a halo of lighter material surrounding them. According to Oehler, some of these light-colored rings might be bulls-eyes, targeting zones of buried kerogen.

"Last year, scientists pointed out similarities between bright rings around martian craters at the Meridiani site and bleached regions in Utah - places where red sandstones have whitish rims," says Oehler.

The sandstone in Utah is red because iron oxide coats the quartz grains that make up the rock. The light rings in the Utah rocks are thought to have formed when the iron was dissolved by water and was flushed away. In that case, the light rings were due to the iron oxide being removed from the area.

But in a region studied by Oehler, methane that seeped up to the surface transformed iron oxide into pyrite, changing the color of the rock from red to grey. In that case, the "bleaching effect" was due to the red iron oxide mineral being transformed into another substance.

"On Earth, a lot of areas of red bed bleaching are related to seeping hydrocarbons from oil and gas fields at depth," says Oehler, who worked in the oil industry for more than 20 years. "And oil and gas fields are created from the thermal alteration of kerogen."

The thermal alteration of kerogen on Earth is mostly due to geothermal heating. As the kerogen gets buried many kilometers below the surface, the Earth's interior heat turns the kerogen into methane.

Mars likely has some interior heating as well, but because the kerogen is probably not buried that deeply, and because Mars has been cooler than Earth for much of its history, Oehler believes a more likely cause of thermal alteration would come from the heat of a meteorite impact.

On Mars, some light rings surrounding craters could be due to the play of light on uplifted crater rims. The raised rim edges can look bright to the Mars Orbital Camera, compared to lower lying areas. But these "bright" uplifted crater rims look quite different from the light rings being studied by Oehler and her colleagues.

"It is interesting to me that we don't see similar bright crater rings everywhere," says Oehler. "There are a lot of craters on Mars, but we've seen the bright crater rings mainly at Meridiani and some of the areas immediately adjacent to it. That suggests to me that Meridiani is somehow different from other regions on Mars."

Another cause of light crater rings on Mars could be an overlying crust of salt or other evaporite materials. Spectral analysis of these rings might be able to pinpoint exactly what the light rings are made of.

"If you ever could get samples of it, and found either carbonates, pyrite, or jarosite, I think that would be very interesting, because all of those things happen on Earth in zones of seeping hydrocarbons," says Oehler.

"If you found sulfate evaporate minerals like gypsum or anhydrite, or just salt, it would be less indicative of anything related to seeping methane."

If the light rings on Mars are the result of kerogen that was heated by impact, samples of that kerogen could provide information about past life on Mars. Paleontologists commonly analyze kerogen for detailed information about ancient life on Earth.

"You can look at kerogen and find microfossils in it," says Oehler. "When you look at it under a microscope you often can see preserved structures, such as bacteria and algae. You can look for remnants of cell walls and internal structures. You also can get some of the remnants of biochemicals."

The oldest structures date back 3.5 billion years, but because they are poorly preserved their origin is controversial. Some think the structures are the remains of ancient biology, while others think they are just carbon globs unrelated to life.

But Oehler says there is no debate about better preserved structures dating from slightly later periods in Earth's history - the remains of ancient life can clearly be seen, for instance, in the Gunflint chert, which dates back to 2 billion years ago.

Any kerogen on Mars probably would have been buried 2 or 3 billion years ago, when Mars may have still had bodies of liquid water on the surface.

Age is not the only factor affecting the quality of a kerogen sample. If kerogen has been subjected to high pressures and temperatures, such as kerogen in metamorphic rock, it will be poorly preserved.

So Oehler doesn't think the impact craters on Mars would yield kerogen that could reveal ancient structures, due to the high pressure and heat of impact.

"But," she notes, "if microorganisms were buried in the general area, all through the shallow Meridiani lake, you could look at the unaffected areas between the impact craters for samples of martian kerogen."

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