Orbiting spacecraft have peered inside the swirling vortex which encircles Mars' north pole during winter and found an unexpected surge in ozone, raising questions as to whether the Red Planet once had a protective layer like Earth.
"The atmosphere inside the polar vortex, from near the surface to about 30km high, is characterised by extreme cold temperatures, about 40°C colder than outside the vortex," says Dr Kevin Olsen of the University of Oxford, UK.
Olsen presented the analysis last week at the Joint Meeting of the Europlanet Science Congress and the Division of Planetary Sciences (EPSC-DPS) in Helsinki, Finland.
Mars' axis is tilted at an angle of 25.2° which is similar to Earth's (23.4°). This means the planet experiences similar seasons and its polar regions are also plunged into total darkness during autumn and winter.
These conditions gives rise to a vortex of extremely cold and dry air that encircles the north.
The frigid temperatures, which reach as cold as -148°C (125K), mean that the little water vapour present in the Martian atmosphere freezes and snows down onto the ice cap.
Normally, ultraviolet sunlight breaks down water vapour in the atmosphere to produce reactive hydrogen oxide molecules which destroy ozone. Without these conditions, ozone is left 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," says 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 possibility that Mars' surface was once shielded from the deadly influx of ultraviolet radiation from space would increase the chances that life could have once survived there, billions of years ago.
In 2028, the European Space Agency's ExoMars Rosalind Franklin rover will launch to search for evidence of this past life.