According to the latest online publication in the journal Science Advances, a study led by Rice University scientists has identified one of Earth's atmosphere signature – the enrichment of rare heavy nitrogen-15 (N-15) gas molecules.
Simulated high-energy chemistry in Earth’s upper atmosphere to reproduce enriched levels of N-15 nitrogen molecule. Credit: Laurence Yeung/Rice University
There are two stable nitrogen isotopes, nitrogen-14 (99.6% of naturally occurring nitrogen), and nitrogen-15 (0.4% of naturally occurring nitrogen). Nitrogen-15 is a rare stable isotope of nitrogen. Due to its unique chemical nature (it has one more neutron but chemically behave like Nitrogen-14), the tracking of Nitrogen-15 is a popular technique used to study the nitrogen cycle.
The nitrogen cycle is the biogeochemical cycle through which nitrogen is converted into multiple chemical forms as it circulates among the atmosphere, terrestrial, and marine ecosystems. The chemical reactions that nitrogen go through can be carried out through by biological beings like plants and bacteria, and physical processes like the high energy particle bombardment in Earth’s atmosphere.
The new study shows that the concentration of N-15 nitrogen molecule in Earth’s atmosphere is 20 times higher than anywhere else on earth.
“We think the N-15 nitrogen molecule enrichment fundamentally comes from chemistry in the upper atmosphere, at altitudes close to the orbit of the International Space Station,” said Lawrence Yeung, a chemist from Rice University and the supervisory author of the paper. “The tug-of-war comes from life pulling in the other direction, and we can see chemical evidence of that.”
Because biological processes are much more efficient at cycling nitrogen through the atmosphere than are geologic processes, one would expect that the atmosphere would reflect these biological cycles. But what the researchers found surprised them.
“There was a bit of enrichment in the biological experiments, but not nearly enough to account for what we’d found in the atmosphere,” Yeung said. “In fact, it meant that the process causing the atmospheric N-15 nitrogen molecule enrichment has to fight against this biological signature. They are locked in a tug-of-war.”
To simulate the chemical/physical processes happening in Earth’s upper atmosphere, scientists chose to electrify different mixtures of gas. The normal air mixture produced slightly enriched levels of N-15 nitrogen molecule. Mixtures of pure nitrogen gas, on the hand, produced very little enrichment. The mixtures resembling gases in Earth’s upper atmosphere could produce a signal that is even higher than the one observed in air.
“So far, we’ve tested natural air samples from ground level and altitudes of 32 kilometers, as well as dissolved air from shallow ocean water samples,” Yeung said. “We’ve found the same enrichment in all of them. We can see the tug-of-war everywhere.”
Commenting on the significance of the study on planetary science and the search of exoplanet, Edward Young, a professor of Earth, planetary and space sciences at UCLA and a co-author said, “The enrichment of N-15 nitrogen molecule in Earth’s atmosphere reflects a balance between the nitrogen chemistry that occurs in the atmosphere, at the surface due to life and within the planet itself. It’s a signature unique to Earth, but it also gives us a clue about what signatures of other planets might look like, especially if they are capable of supporting life as we know it.”
Heavy-nitrogen molecules reveal planetary-scale tug-of-war. Credit: Rice University
Source: Rice University