Researchers in the US say they have found strong evidence to support the theory that life on Earth could have been seeded by meteorites from outer space. Sandra Pizzarello of Arizona State University and colleagues in California say that a primitive meteorite discovered in Antarctica is rich in nitrogen – a vital element for life.

Scientists are still unsure about how life emerged on Earth. Biologists agree, however, that for meteorites to have seeded life, they would need to contain nitrogen, a precursor to complex biological molecules such as amino acids and DNA. One possibility is that organic building blocks were brought to our planet by meteorites containing organic molecules, so-called carbonaceous chondrites, which have been bombarding our planet since it formed more than four billion years ago.

Many such meteorites have been studied over the last decade and several have been found to contain organic molecules that were almost certainly produced in a cosmochemical environment. However, these asteroidal bodies contain many different types of organic molecule and it is difficult to put forward a convincing theory that explains how any of these molecules could have eventually led to life.

Preserved in Antarctic ice

In recent years, scientists have begun analysing several primitive meteorites found in Antarctica that belong to a new category of carbonaceous chondrites, the "Renazzo-type" (or "CR)", and found that some do indeed contain nitrogen-based compounds. What is more, these compounds are water soluble, which is also essential because life as we know it always emerges in water.

Pizzarello's team studied the CR2 Grave Nunataks (GRA) 95229 meteorite, discovered in 1995, and found that it released abundant amounts of ammonia when treated chemically, accounting for 60% of the nitrogen contained within the carbonaceous material. They go on to speculate that the release of ammonia gas could have populated the Earth's emerging atmosphere with nitrogen, which could have led to the spark of life via a series of chemical processes.

"Any theory that tries to explain biogenesis has to account for an abundant supply of reduced (reactive) nitrogen", Pizzarello told Direct delivery of relatively large amounts of ammonia via a meteorite is prebiotically very attractive, she said.

Handled with care

As the ammonia was mixed with other materials and is volatile in some conditions the researchers had to careful when studying their 4 g sample not to allow the gas to escape. So they employed a number of techniques including gas chromatography, mass spectrometry and solid-state nuclear magnetic resonance to analyse meteorite powder samples that had been boiled in water and dissolved in acids.

"This result is excellent and could have profound implications in astrobiology," says Yi-Jehng Kuan of the National Taiwan Normal University. "However, large amounts of ammonia need to be found in other carbonaceous meteorites before we can back up the theory that ammonia in the atmosphere of the early Earth may have been supplied by these bodies and/or their parent asteroids." Pizzarello's team is now busy looking for ammonia in other such meteorites.

Antarctic meteorites were crucial for this study. "Carbonaceous meteorites are very similar to Earth rocks," explained Pizzarello, and this means that they can be hard to spot. In contrast, meteorites in the Antarctic are covered by snow as they fall and can be preserved for long periods. They don't usually surface until the ice, which constantly moves towards the sea, hits a mountain and churns over its content. Hundreds of these meteorites have been now been discovered and many new carbonaceous chondrite subgroups established.

The results were detailed in Proc. Natl. Acad. Sci.