Meteorites or Comets are the source of Life on Earth?

A chemical vital for life on Earth may have arrived ready-made from space. Unexpectedly, a chondritic meteorite has been found to contain large amounts of ammonia, a nitrogen-rich chemical needed to form the basic building blocks of life, including proteins, DNA and RNA.

Explaining how the early Earth gained enough ammonia for life to begin has been a puzzle, because sunlight rapidly destroys ammonia gas in the atmosphere. Furthermore, geological samples suggest that the environment was chemically neutral, rather than the reducing conditions that would have favoured ammonia formation.

Now, Sandra Pizzarello of Arizona State University in Tempe and her colleagues think they have solved that puzzle.

The team took dust from a chondritic meteorite and exposed it to water heated to 300 °C, before compressing it beneath 100 megapascals of pressure. The process "was designed to mimic early Earth conditions, which led to the formation of clays", she says.

To the researchers' surprise, the treatment liberated huge amounts of ammonia bound up in the debris; so much so that it accounted for 60 per cent of the nitrogen in the powder.

Pre-life ammonia

By analysing the nitrogen isotopes within the ammonia, they confirmed that it came from outer space and not from terrestrial contamination.

"Any theory that tries to explain biogenesis has to account for a supply of reduced nitrogen as ammonia," says Pizzarello. "The direct delivery of relatively large amounts of ammonia prebiotically is attractive" as an explanation, she adds.

"To see such available ammonia at twice the concentration levels as amino acids makes such primitive meteorites potentially significant exogenous sources of reactive nitrogen," says Terence Kee, who studies prebiotic chemistry at the University of Leeds in the UK.

Kee says that buried within the meteorite dust, the ammonia would potentially be shielded from destruction by sunlight, and therefore able to participate in reactions to create the building blocks of life.

Pizzarello speculates that the meteorite must have originated from asteroids or other environments where ammonia was a dominant chemical.

Journal reference: Proceedings of the National Academy of Sciences, DOI: 10.1073/pnas.1014961108