High up above the surface, the atmosphere begins to glow with an ultraviolet light, sometimes pulsing, as nitrogen and oxygen combine into nitric oxide.

This invisible glow, first revealed by the Mars Orbiter mission in 2005, has now been characterised in detail - and its surprising behaviour is revealing how the Martian atmosphere circulates and changes over the course of the year.

"If we're going to send people to Mars," said atmospheric scientist Zachariah Milby of the University of Colorado Boulder's Laboratory for Atmospheric and Space Physics, "we better understand what's going on in the atmosphere.".

Five times a day, MAVEN took detailed images of Mars using its Imaging Ultraviolet Spectrograph; with these observations, scientists could track the movements of the ultraviolet glow for the first time.

"The ultraviolet glow comes mostly from an altitude of about 70 kilometres (44 miles), with the brightest spot about 1,000 kilometres (621 miles) across, and is as bright in the ultraviolet as Earth's northern lights," Milby said.

There are still a number of questions to be answered, but the research is a step in the right direction in understanding the complex behaviour of the atmosphere of Mars

"The behaviour of the Martian atmosphere is every bit as complicated and insightful as that of Earth's atmosphere," said lead author and planetary scientist Nick Schneider of CU Boulder