Martian Meteorite Offers Scientists a Chemical Puzzle

NASA scientists studying the Martian meteorite discovered in Antarctica in 1984 are relying heavily on chemistry in their attempts to prove that primitive life once existed on Mars.

Unfortunately, each piece of evidence taken individually is ambiguous. Every chemical compound they have found in the meteorite could have been created by natural processes that don’t involve life.

It is only the fact that at least four different families of compounds were found together in one location that suggests life was once present, said planetary scientist David S. McKay of NASA’s Johnson Space Center.

But chemistry alone may never prove that life existed on Mars, he added, unless researchers find something more definitively linked to metabolic processes--such as a piece of DNA.

The NASA team found four distinct families of chemicals in the meteorite: carbonate globules, magnetite, iron sulfide and polycyclic aromatic hydrocarbons, commonly abbreviated as PAHs. Each has its own story to tell.

The carbonate globules are virtually identical to, albeit much smaller than, the antacid tablets in your bathroom cabinet--a mixture of calcium carbonate and magnesium carbonate. Calcium carbonate, more widely known as limestone, is commonly formed when the shells of dead crustaceans collect and are crushed together. When it is formed from microscopic sea-dwellers, it is called chalk.

But carbonates can also be formed when there is a lot of carbon dioxide dissolved in water, which causes carbonates to precipitate out.

Magnetite is a compound of iron and oxygen that is produced by bacteria, as well as many other organisms, such as whales, dolphins, bees and even humans. As the name suggests, the mineral is magnetic. Many researchers believe that bacteria, and even larger animals, use a small particle of magnetite in their bodies as a sense organ to help them navigate in the Earth’s magnetic field. When a cluster of bacteria dies, the magnetite particles can clump together, eventually forming large aggregations called lodestones.

Iron sulfide is similar to magnetite, but less magnetic, and is used by bacteria in the same way. Both can be formed without the presence of life, but require quite different conditions. Magnetite is formed when a lot of oxygen is present, while iron sulfide is formed when there is no oxygen. The two could not form in nature at the same time without having been produced by living organisms.

PAHs are found throughout the universe. They are produced whenever carbon-containing molecules are heated to high temperatures, such as when gasoline is burned in your car engine, or coal is burned in a power plant or a steak is cooked on the grill.

Finding all four classes of materials together is highly suggestive that life once existed on Mars, but is not conclusive, the team said. Chemical proof would require the detection of some compound that is unique to life, such as DNA or RNA, which store and transfer genetic information. Such materials have never been found where life did not exist.

Researchers are examining the meteorite to look for traces of DNA or RNA, McKay said. But he conceded that if they have not found them in the meteorite yet, they are not likely to do so. Their best hope, he said, would be to find them in the other 11 meteorites known to have come from Mars or in Martian meteorites as yet undiscovered.