Researchers have for the first time measured the oxygen isotope ratio in Earth’s mesosphere and thermosphere – the Earth’s outer gas envelope. Accordingly, the upper atmosphere contains more of the heavy oxygen isotope 18 O than previously suspected. This suggests that the upper, thin layers of Earth’s atmosphere are less affected by the solar wind than expected. Instead, the isotopic composition of the mesosphere and thermosphere more closely resembles that of the lower atmosphere.
Oxygen occurs on our planet in the two isotopes 16 O and 18 O. The relationship between the two is determined on the one hand by the oxygen ions that hit the earth with the solar wind: Solar oxygen has a 16 O/ 18 O ratio of about 529. In the lower earth’s atmosphere, on the other hand, the proportion of the heavier oxygen isotope is higher because Organisms selectively release more 18 O during respiration. As a result of this biosignature, the terrestrial ratio is only 498.7.
Riddles about the upper atmosphere
But what about the upper layers of the earth’s atmosphere? It was already known that the stratosphere is still clearly characterized by the oxygen biosignature of the lower air layers. The rising air carries lighter oxygen up into the ozone layer and beyond. Above that, in the mesosphere and thermosphere, only a limited amount of air from the lower layers is likely to arrive. For this reason, scientists previously assumed that these layers, beginning at about 50 kilometers, must be more strongly influenced by the solar wind and thus by lighter oxygen isotopes.
The problem, however, is that direct isotope measurements in the mesosphere and thermosphere are hardly possible because these layers are too high for measuring balloons but too deep for satellites. Helmut Wiesemeyer from the Max Planck Institute for Radio Astronomy in Bonn and his colleagues have now found a possible solution for this: They use the moon as a measuring aid.
Moonlight as a measuring aid
For their study, the researchers aimed at the moon from the stratosphere using a spectrometer from the airborne observatory SOFIA. As the light reflected from the bright lunar surface passes through Earth’s thermosphere and mesosphere, the different oxygen variants each leave distinct absorption lines in the lunar light spectrum. “We use a relativistic effect that splits the electronic ground state of atomic oxygen into three fine structure levels,” explains Wiesemeyer.
These fine structure lines reveal which oxygen isotopes caused them: “The spectral lines are further split if you add one or two neutrons to the nucleus: the center of gravity of the atom shifts, which leads to a slight change in the characteristic frequencies of the fine structure lines,” continues Wiesemeyer. “For the first time we were able to identify the spectroscopic signature of the isotope shift in spectral lines of atomic oxygen in nature.”
Significantly more 18 O than in the solar wind
The measurements showed that the uppermost layers of the earth’s atmosphere contain more heavy oxygen than previously thought. “Our measurement results for the isotope ratio of 16 O/ 18 O were 468 and 382 in February and November 2021. This is significantly lower than the value of almost 530 determined in the solar wind,” the researchers report. The upper atmosphere is therefore less strongly influenced by the solar wind than assumed. Previous assumptions about isotope fractionation in the mesosphere and thermosphere have to be corrected.
“From the measurements of the stratospheric observatory, we derive values that are typical for the lower atmosphere, but not for the solar wind, which dominates where the interplanetary magnetic field replaces that of the earth,” explains co-author Jürgen Stutzki from the University of Cologne. This could indicate that more air from the lower layers of the atmosphere is moving to high altitudes.
Explanation for contamination of the lunar regolith?
And the new measurements could also explain something else: During the isotope analysis of samples from the lunar surface, values slightly deviating from the solar wind were measured in some samples. But because the moon has no atmosphere or biosphere, its regolith should actually reflect the isotope ratio of the solar wind. But recently, scientists found that terrestrial hydrogen and oxygen atoms travel through Earth’s magnetotail to the moon each time the moon passes .
Because these atoms come from Earth’s mesosphere and thermosphere, they carry their isotope ratio all the way to the moon. The current measurements now show that these layers of the atmosphere also have the typical terrestrial excess of heavy oxygen. This could explain the local isotope contamination of the lunar regolith. (Physical Review Research, 2023; doi:10.1103/PhysRevResearch.5.013072 )
Source: Max Planck Institute for Radio Astronomy