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A rare glimpse into the wintry conditions of the Mars north polar vortex has shown that temperatures inside the vortex are far colder than outside, and that the permanent darkness that winter brings to the Martian north pole facilitates a surge in ozone in the atmosphere.

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

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 sunlight 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."

The European Space Agency's ExoMars 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.

How the Mars polar vortex forms

The polar vortex is a consequence of seasons on Mars, which occur because the red 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 the 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 the Mars polar vortex, and in doing so it provides an opportunity to probe its interior.

"Because winters at the 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 the Martian 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 atmospheric chemistry and how conditions change during the polar night to allow ozone to build up."