In the lower atmosphere of Earth, oxygen contains a higher fraction of the heavy isotope 18O than ocean water does. This enrichment is a signature of biological activity, set by the equilibrium between oxygenic photosynthesis and respiratory metabolisms in terrestrial and oceanic ecosystems. While the mixing between stratospheric and tropospheric oxygen leads to a slow isotopic homogenization, little is known about the isotopic oxygen enrichment in the mesosphere and thermosphere of Earth.
Heavy oxygen (18O) is so called because it has 10 neutrons, rather than the normal eight of main oxygen (16O), the form we breathe.
Heavy oxygen is seen as a signature of biological activity, common in the lower atmosphere.
Both forms are byproducts of photosynthesis, but main oxygen is consumed by the respiration of living things more than its heavy counterpart, leaving a larger concentration of heavy oxygen behind.
Little is known, however, about how this abundance of heavy oxygen permeates from the location of its creation near the ground into higher regions of the atmosphere.
With its high spectral resolution, the GREAT instrument onboard NASA’s Stratospheric Observatory for Infrared Astronomy (SOFIA) measured the ratio of main to heavy oxygen in the mesosphere and lower thermosphere, making the first spectroscopic detection of heavy oxygen outside a laboratory.
“It’s tracing biological activity — that’s well-proven,” said Dr. Helmut Wiesemeyer, a scientist at the Max Planck Institute for Radio Astronomy.
“So far, the altitude to which this signature extends was thought to be 60 km (37 miles) — so, barely the lower part of the mesosphere — and the question was, does it reach higher altitudes?”
“And if it does, because there are no living organisms up there, the only way to reach higher altitudes would be an efficient vertical mixing.”
In other words, the only explanation for large concentrations of heavy oxygen in these regions is the upward and…
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