A clue to ancient life? What scientists found inside Mars’ frozen vortex

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

At these extreme lows, the small amount of water vapor in Mars' atmosphere freezes and settles onto the polar ice cap. This change has a striking effect on ozone levels. Normally, ozone is destroyed when it reacts with molecules formed as ultraviolet sunlight breaks apart water vapor. But when the vapor freezes out completely, those reactions stop. With nothing left to break it down, ozone begins to build 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," said Olsen. "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, scheduled to launch in 2028, will search for traces of ancient life on the planet. If Mars once had an ozone layer shielding its surface from ultraviolet radiation, that protective barrier could have made it far more hospitable to life billions of years ago.

How Mars' Polar Vortex Forms

Mars' polar vortex develops as part of its seasonal cycle, driven by the planet's 25.2-degree axial tilt. Like Earth, the Red Planet experiences seasonal shifts, and as northern summer ends, a swirling vortex forms above the pole and lingers until spring.

On Earth, the polar vortex can sometimes destabilize and drift south, sending cold air into lower latitudes. A similar process can occur on Mars, giving researchers valuable chances to study the vortex's interior.

"Because winters at Mars' north pole experience total darkness, like on Earth, they are very hard to study," says Olsen. "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."

Probing the Vortex

Olsen works with ESA's ExoMars Trace Gas Orbiter that is in orbit around Mars. In particular, the spacecraft's Atmospheric Chemistry Suite (ACS) studies Mars' atmosphere by gazing at the Red Planet's limb when the Sun is on the other side of the planet and is shining through the atmosphere. The wavelengths at which the sunlight is absorbed give away which molecules are present in the atmosphere and how high above the surface they are.

However, this technique doesn't work during the total darkness of martian winter when the Sun doesn't rise over the north pole. The only opportunities to glimpse inside the vortex are when it loses its circular shape but, to know exactly when and where this is happening, requires additional data.

For this, Olsen turned to the Mars Climate Sounder instrument on NASA's Mars Reconnaissance Orbiter to measure the extent of the vortex via temperature measurements.

"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."