Take a deep breath. Can you taste the flavour of ancient space? Nitrogen in Earth's atmosphere has been traced back to the spinning disc of dust and gas that formed our solar system, and may even have yielded ammonia to fuel organic reactions. This all comes courtesy of a meteorite found in Antarctica named after a popular brand of beer.
"Our [meteorite] samples were collected in Antarctica in the late 1970s," says Dennis Harries of The Friedrich-Schiller University in Jena, Germany. "They fell there hundreds or thousands of years ago." Known as chondritic meteorites, their history goes back some 4.6 billion years. At that time, our solar system was a vast disc of dust and gas, called the protoplanetary disc, spinning around the sun.
Harries and his colleagues were studying the make-up of the meteorites when they found a mineral called carlsbergite, named after the Carlsberg Foundation, an offshoot of the Danish brewery, which funded previous work on it.
Carlsbergite is a rare composite of chromium and nitrogen. Because of the meteorite's age, it acts like a time capsule, telling us about the form these elements were in while our planet was forming. Looking at the ratio of stable and unstable isotopes in the nitrogen, Harries found that it was very close to the ratio in the nitrogen that makes up two-thirds of Earth's atmosphere today. That suggests they have a common origin, and the nitrogen in our atmosphere came from the protoplanetary disc.
From a cold start
As for the formation of the carlsbergite itself, Harries imagines "a dusty volume of space in which dust grains were freely floating in a very thin gas – almost a vacuum. These grains may have been covered by thin shells of ice containing ammonia and other compounds."
He says a large body moving through this thin, icy gas could have created a shock wave – similar to a massive sonic boom – which heated the gas, evaporated the ice and kick-started chemical reactions that eventually led to the formation of the strange carlsbergite mineral.
Louis Le Sergeant d'Hendecourt of the University of Paris-South, France, is cautious – he says the scenario is not impossible, but difficult to test.
Makes all the difference
Harries also looked at the shape of the carlsbergite crystals. He found that they must have been formed under very high temperatures, in the presence of ammonia gas – which is in keeping with the evaporating ice scenario (Nature Geoscience, DOI: 10.1038/NGEO2339).
Knowing the molecular composition of a given element at the time Earth was formed matters, says d'Hendecourt. Nitrogen, for instance, can come as pure nitrogen gas or as ammonia, which also contains hydrogen. "This makes a hell of a difference, particularly if you are interested in prebiotic molecules like amino acids," he says.
Pure nitrogen is stable and unreactive, so an unlikely source for organic molecules, but ammonia can easily react to help form the organic chemistry that underpins life. Discovering that it was present even before our planet was formed may tell us something about the origins of life – although whether it helped trigger the formation of the precursors to life is still complete speculation.
"The presence of ammonia could have acted as an active ingredient in some of the chemistry needed to eventually get to life," says Hope Ishii of the University of Hawaii. "But it's still a long way between having ammonia and having life."
What about nitrogen compounds elsewhere in our solar system? "It seems possible that ammonia and organic molecules were brought to other bodies like Mars and Europa," Harries says.
Journal reference: Nature Geoscience, DOI: 10.1038/NGEO2339
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