Atomic Oxygen Detected on Venus

For the past 50 years, many observations of Venus’s atmosphere have resulted in claims of an “indirect detection” of atomic oxygen (as well as a single direct detection that was never confirmed). Now, a group led by Heinz-Wilhelm Hübers (DLR, Germany) has identified atomic oxygen on both Venus’s dayside and nightside using observations from NASA’s and DLR’s  SOFIA airborne telescope. The team reports the results in Nature Communications, suggesting that the detection can help us to better understand Venus’s atmospheric circulation patterns, complementing future space missions to the planet.

For decades, instruments on satellites such as Venus Express, which orbited the planet from 2006 to 2014, have picked up the characteristic infrared airglow on Venus’s nightside. The airglow originates from molecular oxygen, and photochemical models show that the molecular oxygen in turn indicates a concentration of atomic oxygen.

So it has long been assumed that there was atomic oxygen hanging around on Venus, generated by sunlight’s destruction of carbon dioxide on the dayside. Winds then transport it towards the nightside. In fact, given how much carbon dioxide is in the atmosphere (96.5%), atomic oxygen pretty much has to be there.

But for the longest time, there was only one direct sighting, made more than 20 years ago on Venus’s nightside using the Keck Observatory on Mauna Kea, Hawai‘i. Far-infrared wavelengths are best for picking up the spectral lines of atomic oxygen, but Earth’s atmosphere absorbs most of them.

To pick up these wavelengths, astronomers designed the Stratospheric Observatory for Infrared Astronomy (SOFIA), a telescope that operated aboard an airplane between 2010 and 2022. With a perch higher than any mountain, it rose above Earth’s infrared-blocking atmosphere to see all kinds of celestial objects, such as star-forming regions, comets, nebulae, even peering into our galaxy’s center.

While the telescope has since been shut down, two important observations of Venus were made right near the end of SOFIA’s tenure. And on the same flight, in fact! The first was a nondetection of phosphine, a molecule that might have had biological importance. The second observation was that of atomic oxygen. These two studies on Venus will probably be some of the last research to come out of SOFIA.

Hübers group analyzed 17 different points on both Venus’s dayside and nightside, and they found atomic oxygen everywhere they looked. All the observations probed altitudes of around 100 km, a region sandwiched between two huge atmospheric circulation patterns.

Hübers explains that atomic oxygen is very reactive; that is, it disappears quickly into other chemical reactions. So its presence helps astronomers to understand the photochemistry that’s happening right now within the atmosphere.

The location of the oxygen they saw, between the two major wind fields, is significant as well. “It is like a probe, allowing us to better investigate the circulation patterns of the winds in these layers of the atmosphere,” he says.

The atomic oxygen on Venus shows up via a very narrow band of infrared light. The way this light distorts can be used as a probe to observe Doppler shifts, showing the speed at which winds transport oxygen across the planet. This in turn, will allow for a better characterization of atmospheric circulation cells.

Hüber’s results also match up well with previous indirect detections, providing good evidence that their general circulation models, and the models that predict how oxygen is generated on Venus, are trustworthy.

This study also bodes well for SOFIA’s nondetection of phosphine, which Martin Cordiner (Catholic University of America) and colleagues published last year. “For one-of-a-kind observations like these, there’s always doubt that creeps in, in the back of your mind,” Cordiner says. “Was the instrument working as well as it might have been? To see that they got these very definite, beautiful detections of oxygen from Venus is very encouraging.”