Just because we can find ozone in the atmosphere of other planets doesn't mean there's life. Ozone is a sign of life on Earth, but its detection on Venus shows that it can also be produced abiotically. This indicates that there are different pathways for its creation, not only on Venus but also on other Venus-like exoplanets.
We know that ozone in Earth's atmosphere is a biosignature because we know Earth's history. Life may have started on our planet only four or five hundred million years after it formed. At first, life was restricted to the oceans. Dry land was off limits for Earth's simple life because the Sun's UV radiation blasted the surface unimpeded.
Then something changed around three billion years ago: photosynthetic organisms appeared.
These organisms produced excess oxygen as part of their energy-generating scheme, releasing it into the atmosphere. Oxygen is highly reactive, and at first, it combined with elements in the Earth's crust, forming minerals. Now, oxygen is the most abundant element in Earth's crust. When we mine things like iron and titanium, we mine their oxides.
Earth's crust absorbed the oxygen for a long time. Then, about 2.5 billion years ago, the Great Oxygenation Event began. The GOE saw the rise of free oxygen in Earth's shallow seas and atmosphere. The appearance of the ozone layer is a direct consequence of the GOE.
The red and green lines show the range of estimates for the GOE's progression. Time is shown in billions of years. Image Credit: By Oxygenation-atm.svg: Heinrich D. Hollandderivative work: Loudubewe (talk) - Oxygenation-atm.svg, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=12776502
As molecular oxygen (O2) appeared in the atmosphere, it was exposed to the Sun's powerful UV energy. Due to photodissociation, the molecules were split apart into separate atoms (O). Since oxygen atoms are reactive, they quickly combine with O2 molecules to form ozone (O3). The process is continuous, constantly creating and destroying ozone and lessening the amount of UV that can reach Earth's surface. With the ozone layer in place, life was able to colonize land.
With Earth's story as a backdrop, the detection of ozone in Venus' atmosphere generated significant interest. Ozone was considered an atmospheric biosignature, especially when found alongside water vapour and oxygen.
The ESA's Venus Express spacecraft orbited Venus for almost 8 years until its mission ended in 2014. It detected ozone in Venus' mesosphere, a transitional region between the planet's troposphere and thermosphere. The mesosphere is known to be dynamic, with photochemical processes, complex wind patterns, and temperature variations.
Ozone was first reported in a 2011 paper and was subjected to more analysis in a 2021 paper. Neither paper claimed that its presence indicated life on Venus. However, scientists have considered ozone a biosignature and a way to differentiate between different types of exoplanets. The issue is that Earth-like and Venus-like atmospheres can appear similar from a great distance, and both are in the Sun's habitable zone. In this respect, ozone was thought to be a good way to tell them apart.
This figure shows ozone detection in Venus' atmosphere between 2006 and 2014 by the ESA's Venus Express. Image Credit: Evdokimova et al. 2011. JGR Planets.
New research to be published in the Monthly Notices of the Royal Astronomical Society shows that Venus' ozone and ozone in the atmospheres of planets in the habitable zones of distant stars are not reliable indicators of life. The study is titled "Abiotic Ozone in the Observable Atmospheres of Venus and Venus-like Exoplanets," and the lead author is Robb Calder, a PhD student in the Institute of Astronomy at the University of Cambridge.
"Ozone is a potential biosignature and disambiguator between Earth-like and Venus-like exoplanets due to its association on Earth with photosynthetically produced oxygen (O)," the authors write. "However, the existence of ozone in Venus's observable atmosphere, a planet with no known life, raises the possibility of ozone biosignature false-positives on Venus-like exoplanets."
In their research, the authors used a photochemical model to investigate the ozone in Venus' mesosphere. The question at the heart of the study is: Are there other chemical pathways that create ozone, not only on Venus but also on exoplanets?
Venus' atmosphere contains only small traces of ozone. "O3 has been detected above Venus's cloud layer at concentrations ranging from 0.1-1 ppm," the authors write. The ozone was detected on Venus's nightside, though other researchers found it at even weaker concentrations on the dayside. Interestingly, photochemistry models predicted its presence before it was detected.
For ozone to appear, there has to be oxygen. Previous researchers suggested that oxygen atoms produced on the planet's dayside and then transported by winds to the nightside are behind the ozone detection.
However, Calder and his colleagues' model shows this is not the case. For that to happen, there would also have to be more O2 than has been observed. Even when they varied the chemistry in the lower atmosphere, the atmosphere's thermal structure, and the stellar flux, sufficient ozone wasn't produced to explain Venus' ozone.
"These results imply that a presently unknown chemical pathway is responsible for the ozone production in Venus's nightside mesosphere," the authors write. This is the part that really matters.
This figure summarizes the chemical cycles involved in O3 production. The observed ozone levels in Venus' mesosphere require another unknown pathway to ozone. Image Credit: Calder et al. 2025. MNRAS
If a chemical pathway that is unknown to us can produce ozone, its detection on an exoplanet isn't a reliable biosignature.
"Generalized to Venus-like exoplanets, known chemistry similarly fails to produce ozone in the abundance seen in the Venusian mesosphere," the authors write. "However, until the origin of Venus's ozone is understood, we cannot rule out that ozone production at concentrations observable with JWST will be common on abiotic Venus-like worlds, a possibility that limits the usefulness of ozone as a habsignature and as a biosignature."
Nature isn't making it easier for us to understand habitability on exoplanets. On Earth, ozone is a slam-dunk indicator of life. This research shows there's at least one other pathway to ozone that doesn't require life.
"Understanding the mechanism responsible for O3 production in Venus's atmosphere is required for accurately predicting O3 concentrations in the upper atmosphere of a Venus-like exoplanet, since our results show that none of the conventional chemical processes explored in this study can achieve observable O3 concentrations in the upper atmosphere on their own," the authors write in their conclusion.
All is not lost, however. Once this mechanism or pathway is identified on Venus, scientists will be better equipped to interpret its detection on other, Venus-like exoplanets, depending on how it's detected.
"This scenario makes an O3 false-positive less likely for direct imaging missions such as HWO and LIFE, but it remains a possibility for transmission spectroscopy with JWST," the authors conclude.