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

Heavy oxygen is so called because it has 10 neutrons., instead of the normal eight for the “main” oxygen, the way we breathe. Heavy oxygen is considered a sign of biological activity, common in the lower atmosphere. Both forms are by-products of photosynthesis, but the main oxygen is consumed more by breathing in living things 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 major to heavy oxygen in the mesosphere and lower thermosphere, providing the first spectroscopic detection of heavy oxygen outside 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 60 kilometers -so just the bottom of the mesosphere- and the question was, does it reach higher altitudes? And if that happens, 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, that 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 difficult-to-distinguish features.

This allowed the researchers to measure the ratio between primary and heavy oxygen up to 200 kilometers in 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 proportion on land.

“There are processes that change 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 may also help to better define a biologically relevant boundary of Earth’s atmosphere.

More ambitiously, future instruments sensitive to various oxygen signals could use similar techniques to measure oxygen ratios in exoplanets. A combination of high 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 lack.

“The idea is to first understand what’s happening on your doorstep before diving into deeper studies elsewhere,” said Wiesemeyer.

These observations are too low, even for low-orbiting satellites, but too sensitive to be made from the ground. Observations of stratospheric balloons may 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 completed its last scientific flight on September 29, 2022.