Heavy oxygen is so called because it has 10 neutrons, rather than the normal eight of "main" oxygen, 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, SOFIA's GREAT instrument 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 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 kilometers [around 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 downward motion of air, which can have important implications for climate change.
Measuring heavy oxygen is complex because it looks so similar to main oxygen. From up in the stratosphere, SOFIA could separate the two against a lunar backdrop: the Moon's brightness enabled the highest sensitivity to these hard-to-distinguish features.
This allowed the researchers to measure the main-to-heavy oxygen ratios up to 200 kilometers in the atmosphere. The results - published in Physical Review Research - ranged from a 382 to 468 factor difference in the two types of oxygen, similar to the ratio on the ground.
"There are processes that are altering these ratios. For Earth, this process is oxygenic life," Wiesemeyer said - though there are other potential chemical explanations to be considered as well.
Wiesemeyer and his collaborators were very conservative in their uncertainty estimates, so they cannot completely attribute their heavy oxygen measurements to biology. Solar wind, for example, can also deliver heavy oxygen to Earth, but it is unlikely to make such a large contribution.
This pilot study measuring the balance between the two forms of oxygen proves a technique that atmospheric scientists could use to study vertical mixing. The study's findings can also help better define a biologically relevant boundary of Earth's atmosphere.
More ambitiously, future instruments that may be sensitive to various oxygen signatures can potentially use similar techniques to measure oxygen ratios in exoplanets. A combination of high oxygen abundances with an understanding of the vertical mixing on these exoplanets could indicate biological activity - though the group warns such a study would require huge sensitivities that current technologies do not have.
"The idea is to first understand what happens in front of your own door before you go into deeper studies elsewhere," Wiesemeyer said.
These observations are too low for even low-orbit satellites, but too sensitive to be done from the ground. Stratospheric balloon-based observations may be able to offer potential follow-up studies in the future.
SOFIA was a joint project of NASA and the German Space Agency at DLR. DLR provided the telescope, scheduled aircraft maintenance, and other support for the mission. NASA's Ames Research Center in California's Silicon Valley managed the SOFIA program, science, and mission operations in cooperation with the Universities Space Research Association, headquartered in Columbia, Maryland, and the German SOFIA Institute at the University of Stuttgart. The aircraft was maintained and operated by NASA's Armstrong Flight Research Center Building 703, in Palmdale, California. SOFIA achieved full operational capability in 2014 and concluded its final science flight on Sept. 29, 2022.
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