How Early Earth Got Its Nitrogen

An image of the Earth taken by the Russian weather satellite Elektro-L No.1.

Nitrogen may have arrived on Earth in ancient meteorites after the planet had already formed, according to a new study.

Reported in the journal Nature Geoscience, the research illustrates how meteorites -- often portrayed as portents of doom and destruction -- may actually have been key to the development of life on Earth.

Scientists have long puzzled over the origin of Earth's nitrogen because it has a different isotopic composition to nitrogen produced by the sun, and is also different to the nitrogen found in comets.

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"For a long time there had been a theory that comets brought water and nitrogen to Earth, but there is growing evidence that the isotopic ratios in comets are very different to that in Earth's atmosphere," said the study's lead author Dennis Harries of theFriedrich-Schiller-Universität in Jena, Germany.

The authors used an electron microscope to study crystals from two ancient carbonaceous chondrite meteorites named Yamato-791198 and Yamato-793321 which were recovered from Antarctica in 1979. They discovered an unusual mineralized form of nitrogen which matched Earth's nitrogen.

"We found nitrogen with a similar isotopic composition to nitrogen found in people and in Earth's atmosphere, in a very unusual mineral which was detected in two meteorites," said Harries. "This mineral shows us that there was another type of nitrogen in the early solar system billions of years ago, and this molecule was probably responsible for making the building blocks of life and bringing the nitrogen of our atmosphere to Earth."

"The discovery of this mineralized form of nitrogen is telling us something about how Earth got its nitrogen."

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Strange discovery

The meteorites examined by the authors contained a strange mineral called carlsbergite which is a compound of chromium and nitrogen.

"That's unusual because nitrogen is usually found as a gas in our atmosphere and is reluctant to enter mineral crystal structures," said Harries.

The authors discovered the carlsbergite by using an electron microscope to study crystals from the meteorites,

"The crystals we're looking at are only 100 nanometers in size, so they're not visible to the naked eye," said Harries.

They then used mass spectrometry to measure the isotopic composition of the nitrogen atoms in these very small mineral grains.

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How it formed

Harries believes there were high concentrations of ammonia in some parts of the solar nebula which formed the Sun and solar system 4.6 billion years ago. Under the right conditions, nitrogen can be produced out of molecules of ammonia.

"We think the ammonia was locked up with water in ices and was somehow evaporated by shock waves, or by collisions of small bodies near the inner solar system," said Harries.

This generated extremely high temperatures causing the ammonia to react with minerals, forming carlsbergite.

Harries speculates that the asteroids containing carlsbergite were flung into the inner solar system by gravitational perturbations caused by changes in the orbit of Jupiter early in the solar system's history. Eventually some of these asteroids impacted the Earth.

Most of the original bodies containing these ices would now be gone, but Harries thinks some ice could still be found when NASA's Dawn spacecraft enters orbit around the main asteroid belt dwarf planet Ceres on March 7, 2015

"This is the biggest body in the main asteroid belt, and may have retained some of its ices, a very interesting possibility which I'm looking forward to very much," said Harries.

"We've now found a tiny bit of this puzzle, allowing us to see what happened a long time ago just a little bit clearer," said Harries. "Our next step will be to try to recreate these conditions in our laboratory to see how these things happened, how fast they occurred, and what was produced, so we can refine the conditions responsible for the formation of these nitrogen minerals that we observed."