Researchers have observed a dramatic buildup of ozone inside Mars' north polar vortex, where extreme cold and darkness freeze out water vapor. This phenomenon, detected using data from ESA and NASA orbiters, offers insights into the planet's ancient atmospheric chemistry and potential habitability. The findings were presented at a joint meeting in Helsinki.
The north polar vortex on Mars creates harsh winter conditions, with temperatures plunging about 40 degrees Celsius colder than outside, from near the surface up to 30 kilometers high. This extreme cold, coupled with continuous darkness during the long Martian winter, causes the sparse water vapor in the atmosphere to freeze and settle onto the polar ice cap.
Normally, ultraviolet sunlight breaks apart water vapor molecules, which then react with and destroy ozone. However, when water vapor freezes out completely, these destructive reactions cease, allowing ozone levels to rise unchecked inside the vortex. "Ozone is a very important gas on Mars—it's a very reactive form of oxygen and tells us how fast chemistry is happening in the atmosphere," said Dr. Kevin Olsen of the University of Oxford, who led the study.
By measuring ozone variability, scientists can better understand how Mars' atmosphere has evolved over time, including whether it once had a protective ozone layer similar to Earth's. Such a layer could have shielded the surface from ultraviolet radiation, potentially making the planet more hospitable to life billions of years ago. The European Space Agency's ExoMars Rosalind Franklin rover, set to launch in 2028, aims to search for signs of ancient life.
The polar vortex forms as part of Mars' seasonal cycle, driven by its 25.2-degree axial tilt. Like Earth's, it can destabilize and drift, providing rare study opportunities. "Because winters at Mars' north pole experience total darkness, like on Earth, they are very hard to study," Olsen noted.
Observations came from ESA's ExoMars Trace Gas Orbiter, using its Atmospheric Chemistry Suite to analyze sunlight absorption through the atmosphere. To probe the dark vortex interior, researchers cross-referenced with NASA's Mars Reconnaissance Orbiter's Mars Climate Sounder, identifying sudden temperature drops as entry signs. "We looked for a sudden drop in temperature—a sure sign of being inside the vortex," Olsen explained. These results, presented at the EPSC-DPS2025 Joint Meeting in Helsinki, highlight differences in atmospheric chemistry inside versus outside the vortex.