A meteorite found in Antarctica adds extra impact to the theory that the essential building blocks of life on Earth came from outer space, say US scientists.
The team from the University of Arizona say they have discovered a 'carbonaceous chondrite' meteorite - found in 1995 and called 'CR2 Grave Nunataks 59229' - contains relatively high amounts of ammonia and amino acids.
Carbonaceous chondrites meteorites contain abundant organic materials as they have not been melted, and much of their original chemical composition remains intact.
The research, led by Professor Sandra Pizzarello, is published today in Proceedings of the National Academy of Sciences.
Search for ammonia source
Researchers have been trying to explain the origin of the ammonia that triggered the formation of the first biomolecules on Earth.
It was initially thought that the early Earth's atmosphere would have freed up nitrogen to bond with other elements. But more recently, scientists have theorised that nitrogen atoms in the primordial atmosphere had a natural tendency to bond with each other, forming inert nitrogen gas N2. The bonds between the atoms in this gas are stable and strong, which makes it difficult to break the molecules down or to combine with other elements, such as hydrogen or carbon.
Under these conditions the nitrogen would not be available to bond with other elements in order to form the compounds and chains that form the building blocks of life.
"The current geochemical evidence of early Earth's atmosphere, combined with known photochemical destruction of ammonia, has left prebiotic scenarios struggling to account for a constant provision of ammonia," write Pizzarello and her co-authors.
That means scientists had to look for an alternative source.
Pizzarello and colleagues wanted to know if meteorites like 'CR2 Grave Nunataks 59229' could provide an answer.
They collected powder from the meterorite, treated it with water at high temperature and pressure, and analysed the resulting compounds.
They found the rock released compounds including hydrocarbon chains and a large amount of ammonia, which is rich in nitrogen.
This abundant release of ammonia from a carbonaceous chondrite meteorite is unprecedented, Pizzarello and colleagues write.
Chemical analysis of the nitrogen from the meteorite shows that the atomic isotope is not the same as those currently found on Earth. The researchers say that knocks out the possibility that the ammonia resulted from contamination during the experiment.
"The findings appear to trace CR2 meteorites' origin to to cosmochemical regimes where ammonia was pervasive," the authors write. That, they speculate, was the first step on the pathway to life on Earth.