Researchers detect heavy oxygen isotope in Earth’s stratosphere

The study of the composition of the upper atmosphere ¹⁸O is a heavier oxygen isotope with 10 neutrons instead of 8. Helmut Wiesemeyer (MPIfR Bonn) and his colleagues ¹⁸Using the GREAT instrument on board SOFIA, we investigated the O-part of the upper mesosphere/lower thermosphere for the first time and ¹⁸Fraction of O close to the lower atmosphere.

how much understanding biological effect By penetrating Earth’s atmosphere, researchers may one day be able to improve their search for potential signs of life on other planets.

Where is the boundary between Earth’s atmosphere and space? A seemingly simple question with no easy answer. Aerospace applications refer to the so-called Karman line, defined at an altitude of 100 km above sea level. This is the altitude at which hydrodynamic lift reliably ceases, or the altitude at which the satellite cannot make a complete orbit around the Earth due to friction with the air around the Earth. upper atmosphere.

Magnetospheric winds, on the other hand, were only recently discovered, traveling all the way from the Earth’s ionosphere to the Moon, where they pollute the isotopic composition of soils and are exposed to the sun. Solar wind.

This terrestrial fingerprint can be considered unique in the solar system, as it may carry traces of respiratory metabolism.The Dole effect explains the ratio inequality of heavy isotopes ¹⁸oh to the writer ¹⁶O, measured in air and seawater. Here’s why. Oxygen produced as a waste product of photosynthesis predominantly inherits the isotopic composition of the water supply, whereas respiration preferentially destroys lighter versions of oxygen.

Efficient vertical mixing carries this well-studied biosignature into the stratosphere. Further mixing of air into the higher atmospheric layers (mesosphere and thermosphere) has been documented for decades. The thermosphere is the origin of the oxygen ion wind emerging in the Earth’s plasma sheet, but its isotopic oxygen composition is not yet known.

“In our attempt to remotely measure isotopic composition Studies of oxygen in the Earth’s mesosphere and lower thermosphere take advantage of relativistic effects in which the electronic ground state of atomic oxygen splits into three fine structure levels,” said MPIfR, Max Planck Institute for Radio Astronomy. Helmut Wiesemeyer said. First author of the publication.

“radiative transition from 1 quantum state generate another infrared spectral linesAdding one or two neutrons to the nucleus splits them further. The atomic centroids move, slightly altering the line frequencies of the characteristic fine structures. ”

These spectral lines, originating from the Earth’s mesosphere and lower thermosphere, exhibit strong absorption against bright background sources of infrared light, thus providing valuable signatures of the chemistry of this region.

“For the first time, we have been able to identify the spectroscopic signature of this isotope shift in the spectral lines of atomic oxygen in nature. It’s hard to access research in. It’s too high for balloons. Too low for Earth-orbiting satellites.” MPIfR’s Rolf Güsten explains.

“Our observations allow us to identify the spectral lines of ¹⁸O. in the terahertz region of absorption for the moon.”

“Here we come full circle: intensity of spectral lines from heavy ¹⁸O, with respect to equivalents from the main isotopes ¹⁶Thanks to this, we are now able to measure the relative abundance of both species remotely,” adds Jürgen Stutzki from the University of Cologne, who took over PI of the GREAT instrument in October 2018.

“Based on measurements from the Stratospheric Observatory, we infer values ​​that are representative of the lower atmosphere, not the solar wind, which governs where the interplanetary magnetic field takes over from the Earth’s magnetic field.”

It remains to be determined whether the sensitivities achieved so far track the biogenic isotope ratios that characterize the oxygen molecules that dominate the lower atmosphere or the bioisotopic ratios of stratospheric ozone. Achieving higher sensitivity requires more measurements. It is also a challenging endeavor because the origin of the ozone isotopic record is not yet fully understood. It is believed to result from a series of rapid chemical reactions that exchange isotopes between partners.

“In the mesosphere and lower thermosphere, we show that these reactions compete with inelastic collisions that excite quantum states without altering charge or chemical bonding. with a non-equilibrium population of ¹⁸O, not considered in previous studies, contrasts with the thermodynamic equilibrium found in ¹⁶O,” said Heinz-Wilhelm Hübers of the DLR Light Sensor Systems Institute in Berlin.

“The relative strength of the measured spectral lines is important for proving different population distributions. empirical data Atomic and molecular concentrations in the upper atmosphere air, which is sufficient to correct the isotope fractionation determination. Our observations with the balloon experiment OSAS-B are heading in that direction. ”

At first glance, the need for such corrections seems to unnecessarily complicate the analysis. At first glance, it provides a tool for studying the effects of isotope exchange reactions between atoms. molecular oxygen It occurs before ozone is formed by well-known chemistry. This requires a third celestial body to act as a catalyst. It is abundant in the stratosphere but becomes increasingly scarce at higher altitudes of interest in both the mesosphere and thermosphere.

Last but not least, the selection rules imposed by quantum theory imply that the rate of collisional excitation is strongly dependent on temperature, in competition with isotope exchange. This effect may ultimately be used as a complement to corroborate empirical models of the upper atmosphere.

“At the time of writing these lines, we are not ready yet. Future experiments will be needed to continue our successful airborne observation program to monitor the infrared sky for definitive results. ” concludes Helmut Wiesemeier.

The research is published in the journal Physical review study.