Interesting... Our Nitrogen from Jupiter, huh? Read on to find out more...
Take a deep breath. About 78 percent of the air you inhaled is the most abundant pure element found on Earth. Besides its role in the atmosphere, it’s used in all sorts of products: fertilizers, propellants, you name it. It's also an essential component of DNA and proteins. It’s called nitrogen.
But it's something of a mystery. The nitrogen found on Earth doesn’t match the nitrogen found in the Sun or in the tails of comets. Those sources have nitrogen isotope fractions that differ from those on Earth. So how did nitrogen get to Earth in the first place, and where did it come from? One clue is that some very ancient meteorites do match the Earth’s isotopic abundances very closely, implying that the nitrogen may have come from an ancient source that wasn't so much interplanetary, but existed before the planets formed.
In a new study, researchers examined an ancient meteorite using techniques called transmission electron microscopy and secondary ion mass spectrometry. These provide a glimpse of the material it contains and revealed that the meteorite contains a mineral called carlsbergite.
The carlsbergite’s characteristics suggest that it was likely formed out of a chromium-bearing metal in the presence of reactive ammonia. The ammonia necessary to form the carlsbergite likely came from ices in the protoplanetary disk, which orbited the early Sun. But, for that reaction to take place, the ammonia would have needed to be melted out of the ice.
The most likely way for the ices to melt would be in the wake of shock waves through the disk. This would have subjected the ices to relatively gentle, yet rapid, heating—allowing them to melt in just the right way to produce this mineral. These shock waves would have been produced by activity in the early Sun and are thought to contribute to the formation of proto-planetary rings, which ultimately condensed into the planets.
The researchers propose in their paper that the ices might have been delivered to the inner part of the Solar System by Jupiter, during its “Grand Tack.” The Grand Tack is a model which proposes that Jupiter went through a period of migration early in its history, toward and then away from the inner Solar System. If so, it could have dragged the nitrogen-bearing ices with it, depositing them where they could later accrete onto the forming Earth.
Beyond that, there are a few possibilities as to where the nitrogen in the ices came from. They could be the result of the Sun’s nitrogen mixing with the nitrogen found in comets. As mentioned before, both sources are different from Earth in terms of the relative abundances of nitrogen isotopes. They’re also different from each other, as the Sun has more light isotopes and the comets have heavier isotopes. If these sources were somehow mixed together, they might have created the nitrogen seen on Earth today.
It’s also possible that ammonia from outside the Solar System, which started out with very little nitrogen, absorbed some through its exposure to the nitrogen here—only it absorbed a moderate amount, not enough for it to match the nitrogen in comets or in the Sun.
Whatever happened, it also played an important role in the arrival of organic molecules on Earth, which makes it an important piece of the puzzle that is the origin of life. Simple amino acids have been found in a number of settings outside of the Earth. These compounds, arising before the formation of the planet, could have contributed to the origin of life on Earth, making the understanding of the origins of Earth’s nitrogen important to the understanding of the history of life itself.