British astronomers from the University of Cambridge have discovered that ozone in Venus’ atmosphere can form without biological processes, casting doubt on its reliability as a biosignature of life. The study, published in the Monthly Notices of the Royal Astronomical Society (MNRAS), complicates the search for extraterrestrial life and calls for a reassessment of approaches in astrobiology.

Traces of Ozone on Venus

The European spacecraft Venus Express detected traces of ozone in Venus’ mesosphere — about 100 km above the surface — as early as 2008. The concentration was extremely low (0.1–1 part per million), but its very presence was unexpected, given the lack of any signs of life on the planet. Venus, with its extreme surface temperatures (up to 460°C), atmospheric pressure 92 times higher than Earth’s, and a carbon dioxide–rich atmosphere (96.5%), is considered uninhabitable.

"On Earth, ozone forms thanks to photosynthesis, which produces oxygen that is then converted to O₃ under ultraviolet radiation. Venus shows that there are alternative pathways," said Rob Calder, lead author of the study.

The Photochemistry Problem

Scientists analyzed the photochemical processes that could explain the formation of ozone on Venus but faced several challenges:

  • Lack of oxygen: Venus’ atmosphere contains only trace amounts of O₂ (less than 0.001%), insufficient for traditional ozone formation models.
  • Atom transport: The mechanism for transporting atomic oxygen from the planet’s dayside (where UV radiation breaks down CO₂) to the nightside proved inefficient. Ozone was primarily detected in the nighttime mesosphere, contradicting existing models.
  • Alternative sources: Volcanic activity and lightning were considered but do not explain the observed concentrations.

The team proposed that ozone may form through rare photochemical reactions involving excited CO₂ molecules or trace gases such as SO₂. However, the exact mechanism remains unknown and requires further investigation.

Implications for the Search for Life

This discovery has far-reaching consequences for astrobiology:

  • Ozone as a biosignature: On Earth, ozone (O₃) is considered a marker of life because its presence is linked to biogenic oxygen. Venus demonstrates that ozone can be abiogenic, reducing its reliability as a sign of life on exoplanets.
  • Rethinking data: Observations by the James Webb Space Telescope (JWST), which looks for biosignatures in exoplanet atmospheres, now need more cautious interpretation. For instance, detecting ozone on a planet with Venus-like conditions could produce a false positive.
  • New missions: Future projects such as the Habitable Worlds Observatory (HWO) and Large Interferometer For Exoplanets (LIFE) remain more promising for life detection, as they rely on direct imaging and detailed spectral analysis.

Venus as a Key to Astrobiology

The study highlights Venus’ value as a "laboratory" for studying atmospheric processes. Despite being lifeless, the planet helps scientists understand how chemical reactions in extreme conditions can mimic biosignatures. This is especially important for interpreting data from exoplanets, where conditions may differ drastically from Earth’s.

In 2021, the detection of phosphine in Venus’ atmosphere also sparked debate about possible life but was later attributed to volcanic processes. Ozone adds another layer of complexity, showing that even "classic" biosignatures require contextual analysis.

What’s Next?

Scientists plan to:

  • Conduct further observations of Venus using JWST and ground-based telescopes like ALMA to refine the chemical profile of the mesosphere.
  • Model photochemical reactions based on new data involving SO₂ and CO₂.
  • Develop more sophisticated criteria for biosignatures, combining ozone with other gases like methane or oxygen to rule out false positives.

Upcoming missions, such as DAVINCI (NASA, launch in 2029) and EnVision (ESA, 2031), will provide new insights into Venus' atmospheric chemistry and help solve the ozone mystery.