A team of scientists using an advanced UCLA instrument reports the discovery of a planetary-scale "tug-of-war" of life, the deep Earth and the upper atmosphere that is expressed in atmospheric nitrogen.
Earth's atmosphere differs from the atmospheres of most other rocky planets and moons in our solar system in that it is rich in nitrogen gas or N2; Earth's atmosphere is 78 percent nitrogen gas. Titan, the largest of Saturn's more than 60 moons, is the other body of our solar system with a nitrogen-rich atmosphere that resembles ours.
Compared to other key elements of life, such as oxygen, hydrogen, and molecular carbon nitrogen, it is very stable. Two nitrogen atoms combine to form N2 molecules that remain in the atmosphere for millions of years.
Most nitrogen has an atomic mass of 14. Less than one percent of nitrogen has an additional neutron. While this heavy isotope, nitrogen-15, is rare, the N2 molecules that contain two nitrogen-15 – which the chemists call 15N15N – are the rarest N2 molecules.
The team of scientists measured the amount of 15N15N in the air and discovered that this rare form of nitrogen gas is much more abundant than scientists expected. The Earth's atmosphere contains approximately two percent more than 15N15N that can explain the geochemical processes that occur near the Earth's surface.
"This excess was not known before because nobody could measure it," said lead author Edward Young, a professor at UCLA's geochemistry and cosmochemistry. "Our unique Panorama mass spectrometer allows us to see this for the first time, and we carried out experiments that show that the only way this 15N15N excess occurs is through rare reactions in the upper atmosphere."
Young said that the enrichment of 15N15N in Earth's atmosphere is a signature that is unique in our planet. "But it also gives us a clue about how the signatures of other planets could be, especially if they are capable of sustaining life as we know it."
The research is published in the journal Science Advances .
"At first we did not believe in the measurements, and we spent about a year convincing them that they were accurate," said lead author Laurence Yeung, an assistant professor of Earth, Earth and Environmental Sciences at Rice University. 19659012] The study began four years ago when Yeung, a postdoctoral researcher at UCLA in Young's lab, heard about the first mass spectrometer that was being installed in Young's lab.
"At that time, no one had a reliable way to quantify 15N15N," said Yeung, who joined Rice's faculty in 2015. "He has an atomic mass of 30, just like nitric oxide. Nitric oxide signal usually overwhelm the 15N15N signal in mass spectrometers. "
The difference in ma ss between nitric oxide and 15 N 15 N is approximately two thousandths the mass of a neutron. When Yeung discovered that the new machine in Young's lab could discern this small difference, he requested funds from the National Science Foundation to know exactly how much 15N15N is in the Earth's atmosphere.
Coauthors Joshua Haslun and Nathaniel Ostrom at Michigan State University conducted experiments with N2 and N2 consuming bacteria that allowed the team to determine their 15N15N signatures.
These experiments suggested that one should see a little more 15N15N in the air than random matings of nitrogen-14 and nitrogen-15 would produce-an enrichment of about 1 part per 1,000, Yeung said.
"There was a bit of enrichment in the biological experiments, but not enough to account for what we had found in the atmosphere," Yeung said. "In fact, it meant that the process that causes 15N15N atmospheric enrichment has to fight against this biological signature, they are locked in a tug-of-war."
The team discovered that zapping air mixtures with electricity, which simulates the chemistry of the upper atmosphere, could produce enriched levels of 15N15N as measured in air samples.
Researchers tested air samples from ground level and from altitudes of approximately 20 miles, as well as dissolved air from shallow ocean water samples.
"We believe that 15N15N enrichment comes primarily from chemistry in the upper atmosphere, at altitudes close to the orbit of the International Space Station," Yeung said. "The tug of war comes from life pulling in the other direction, and we can see chemical evidence of that, we can see the tug of war everywhere."
The coauthors are Issaku Kohl and Edwin Schauble of UCLA; Huanting Hu of rice; Shuning Li, formerly of UCLA and Rice and now with Peking University in Beijing; and Tobias Fischer of the University of New Mexico.
Heavy nitrogen molecules reveal a tug of war on a planetary scale
Laurence Y. Yeung et al. Extreme enrichment in atmosphere 15 N 15 N, Science Advances (2017). DOI: 10.1126 / sciadv.aao6741