New research using a decade of data from ESA’s Mars Express has found clear signs of the complex martian atmosphere acting as a single, interconnected system, with processes occurring at low and mid levels significantly affecting those seen higher up.

A new study based on 10 years of data from the radar instrument on Mars Express now offers clear evidence of a sought-after link between the upper and lower atmospheres of the planet.

“The lower and middle levels of Mars’ atmosphere appear to be coupled to the upper levels: there’s a clear link between them throughout the martian year,” says lead author Beatriz Sánchez-Cano of the University of Leicester, UK. 

“We found this link by tracking the amount of electrons in the upper atmosphere — a property that has been measured by the MARSIS radar for over a decade across different seasons, areas of Mars, times of day, and more — and correlating it with the atmospheric parameters measured by other instruments on Mars Express.”

The amount of charged particles in Mars’ upper atmosphere – at altitudes of between 100 and 200km – is known to change with season and local time, driven by changes in solar illumination and activity, and, crucially for this study, the varying composition and density of the atmosphere itself. But the scientists found more changes than they were expecting.

“We discovered a surprising and significant increase in the amount of charged particles in the upper atmosphere during springtime in the Northern hemisphere, which is when the mass in the lower atmosphere is growing as ice sublimates from the northern polar cap,” adds Beatriz.

Mars’ polar caps are made up of a mix of water ice and frozen carbon dioxide. Each winter, up to a third of the mass in Mars’ atmosphere condenses to form an icy layer at each of the planet’s poles. Every spring, some of the mass within these caps sublimates to rejoin the atmosphere, and the caps visibly shrink as a result. 

“This sublimation process was thought to mostly only affect the lower atmosphere – we didn’t expect to see its effects clearly propagating upwards to higher levels,” says co-author Olivier Witasse of the European Space Agency, and former ESA Project Scientist for Mars Express. 

The finding suggests that the atmosphere of Mars behaves as a single system. 

This could potentially help scientists to understand how Mars’ atmosphere evolves over time — not only with respect to external disturbances such as space weather and the activity of the Sun, but also with respect to Mars’ own strong internal variability and surface processes.