The polar vortex is a consequence of Martian seasons, which occur because the planet’s axis is tilted at an angle of 25.2 degrees, according to University of Oxford’s Dr. Kevin Olsen and his colleagues from LATMOS, CNRS, Space Research Institute, The Open University and NASA’s Jet Propulsion Laboratory.
Perspective view of Mars’ north polar ice cap and its distinctive dark troughs forming a spiral-like pattern. The view is based on images taken by ESA’s Mars Express and generated using elevation data from the Mars Orbiter Laser Altimeter on board NASA’s Mars Global Surveyor. Image credit: ESA / DLR / FU Berlin / NASA / MGS / MOLA Science Team.
“The atmosphere inside the polar vortex, from near the surface to about 30 km high, is characterized by extreme cold temperatures, about 40 degrees Celsius colder than outside the vortex,” Dr. Olsen said.
“At such frigid temperatures, what little water vapor there is in the atmosphere freezes out and is deposited onto the ice cap, but this leads to consequences for ozone in the vortex.”
Ordinarily ozone is destroyed by reacting with molecules produced when ultraviolet light breaks down water vapor.
However, with all the water vapor gone, there’s nothing for the ozone to react with. Instead, ozone is able to accumulate within 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.”
ESA’s Rosalind Franklin rover, which is currently scheduled to launch in 2028, will search for evidence of past life on Mars.
The possibility that Mars once had an ozone layer protecting the planet’s surface from the deadly influx of ultraviolet radiation from space would boost the chances that life could have survived on Mars billions of years ago substantially.
The polar vortex is a consequence of Mars’ seasons, which occur because the planet’s axis is tilted at an angle of 25.2 degrees.
Just like on Earth, the end of northern summer sees an atmospheric vortex develop over Mars’ north pole and last through to the spring.
On Earth the polar vortex can sometimes become unstable, lose its shape and descend southwards, bringing colder weather to the mid-latitudes.
The same can happen to Mars’ polar vortex, and in doing so it provides an opportunity to probe its interior.
“Because winters at Mars’ north pole experience total darkness, like on Earth, they are very hard to study,” Dr. 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.”
The Atmospheric Chemistry Suite onboard ESA’s Trace Gas Orbiter studies the Martian atmosphere by gazing at the 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, the researchers 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,” Dr. Olsen said.
“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.”
The scientists presented their results this month at the EPSC-DPS2025 Joint Meeting in Helsinki, Finland.
_____
K. Olsen et al. 2025. What goes on inside the Mars north polar vortex? EPSC Abstracts 18: EPSC-DPS2025-1438; doi: 10.5194/epsc-dps2025-1438