First detection of heavy oxygen in Earth’s upper atmosphere


He already retired Stratospheric Observatory for Infrared Astronomy (SOFIA) managed to make the first measurement of heavy atomic oxygen in the upper atmosphere of the Earth.

Heavy oxygen is so named because it has 10 neutrons., instead of the normal eight for “main” oxygen, the form we breathe. Heavy oxygen is considered a sign of biological activity, common in the lower atmosphere. Both forms are byproducts of photosynthesis, but the main oxygen is consumed more by living things’ respiration than its heavy counterpart, leaving a higher concentration of heavy oxygen.

However, little is known about how this abundance of heavy oxygen penetrates from the place of its creation, close to the ground, to the highest regions of the atmosphere. Thanks to its high spectral resolution, SOFIA’s GREAT instrument measured the ratio of main to heavy oxygen in the mesosphere and lower thermosphere, giving the first spectroscopic detection of heavy oxygen outside of a laboratory.

“The tracking of biological activity is proven,” he says it’s a statement Helmut Wiesemeyer, a scientist at the Max Planck Institute for Radio Astronomy. “Until now, the altitude at which this signature extended was thought to be 60 kilometers -so just the bottom of the mesosphere- and the question was, does it reach higher altitudes? And if it does, since there are no living organisms up there, the only way to reach higher altitudes would be efficient vertical mixing.”

In other words, the only explanation for the high concentrations of heavy oxygen in these regions is the upward and downward movement of air, which may have important implications for climate change.

Measuring heavy oxygen is complex because it is very similar to primary oxygen. From high in the stratosphere, SOFIA was able to separate the two elements against a lunar backdrop: the Moon’s brightness allowed maximum sensitivity to these hard-to-distinguish features.

This allowed the researchers to measure the ratio between primary and heavy oxygen up to 200 kilometers into the atmosphere. The results -published in Physical Review Research- ranged from a difference of 382 to 468 factors in the two types of oxygen, similar to the ratio on land.

“There are processes that alter these proportions. In the case of Earth, this process is oxygenic life,” says Wiesemeyer, although other possible chemical explanations must also be considered.

Wiesemeyer and his collaborators were very conservative in their uncertainty estimates, so they cannot completely attribute their heavy oxygen measurements to biology. The solar wind, for example, can also bring heavy oxygen to Earth, but its contribution is unlikely to be as important.

This pilot study measuring the balance between the two forms of oxygen tests a technique that atmospheric scientists could use to study vertical mixing. The results of the study they can also help to better define a biologically relevant boundary of the Earth’s atmosphere.

More ambitiously, future instruments sensitive to various oxygen signals could use similar techniques to measure oxygen ratios on exoplanets. A combination of elevated oxygen abundances with an understanding of vertical mixing on these exoplanets could indicate biological activity – although the group cautions that such a study would require enormous sensitivities that current technologies do not have.

“The idea is to first understand what’s going on at your own doorstep before delving into deeper studies elsewhere,” Wiesemeyer said.

These observations are too low even for low-orbit satellites, but too sensitive to be made from the ground. Observations from stratospheric balloons could offer possible follow-up studies in the future.

The SOFIA airborne observatory was a joint project of NASA and the German Space Agency (DLR). The plane was maintained and operated by NASA, in Palmdale, California. SOFIA reached its full operational capacity in 2014 and it concluded its last scientific flight on September 29, 2022.

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