Mars polar vortex traps cold and builds seasonal ozone layer

Mars polar vortex traps cold and builds seasonal ozone layer
by Robert Schreiber
Berlin, Germany (SPX) Sep 22, 2025

A rare look inside the wintry north polar vortex of Mars has revealed that conditions are far colder than outside its boundary, and that the months of complete darkness allow ozone to accumulate in the martian atmosphere.

"The atmosphere inside the polar vortex, from near the surface to about 30 kilometres high, is characterised by extreme cold temperatures, about 40 degrees Celsius colder than outside the vortex," explained Dr Kevin Olsen of the University of Oxford, who presented the findings at the EPSC-DPS2025 Joint Meeting in Helsinki.

At these low temperatures, the scant water vapour in Mars's atmosphere condenses and falls onto the ice cap. This halts the usual process where ozone is destroyed by reactions with molecules formed when sunlight splits water vapour. With the water gone, ozone instead builds up 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," Olsen noted. "By understanding how much ozone there is and how variable it is, we know more about how the atmosphere changed over time, and even whether Mars once had a protective ozone layer like on Earth."

The European Space Agency's ExoMars Rosalind Franklin rover, planned for launch in 2028, will search for evidence of past life. If Mars once possessed an ozone shield protecting its surface from harmful ultraviolet radiation, the chances of ancient life surviving would have been significantly higher.

Mars's polar vortex forms as a result of its 25.2 degree axial tilt, which drives the planet's seasons. At the close of northern summer, a vortex develops over the north pole and endures until spring. Like Earth's polar vortex, it can become unstable and drift south, exposing its interior to spacecraft instruments.

"Because winters at Mars's north pole experience total darkness, like on Earth, they are very hard to study," Olsen said. "By being able to measure the vortex and determine whether our observations are inside or outside of the dark vortex, we can really tell what is going on."

Olsen uses ESA's ExoMars Trace Gas Orbiter and its Atmospheric Chemistry Suite (ACS) to probe Mars's atmosphere, identifying gases by their absorption of sunlight. But during polar night, when the Sun never rises, the technique fails. Only when the vortex distorts from its circular shape do opportunities to peer inside arise.

To pinpoint these occasions, Olsen relied on temperature data from the Mars Climate Sounder aboard NASA's Mars Reconnaissance Orbiter.

"We looked for a sudden drop in temperature - a sure sign of being inside the vortex," said Olsen. "Comparing the ACS observations with the results from the Mars Climate Sounder shows clear differences in the atmosphere inside the vortex compared to outside. This is a fascinating opportunity to learn more about martian atmosphere chemistry and how conditions change during the polar night to allow ozone to build up."