When the Sun gets active the whole Solar System lit can light up the polar skies of Earth and other planets. On Mars, however, this has been found to manifest in a manner not seen on any other world, with exceptionally long and thin auroras that can stretch for thousands of kilometers, halfway across the planet. These add to the already diverse auroras found in the Red Planet's thin atmosphere.
On Earth, auroras occur when charged particles from solar outbursts are funneled by the magnetic field towards the poles, where they excite gasses in the atmosphere, which release light when they return to their ground state. Suspected records date back 3,000 years, although they wax and wane with the 11-year solar cycle.
Jupiter's auroras dwarf those on Earth, in keeping with everything about the gas giant being bigger. Mars, however, has no planetary magnetic field and almost no atmosphere, so even the discovery it has auroras was unexpected. The fact they come in such complex forms is stranger still, and now the Emirates Mars Mission (EMM) has revealed new types that larger nations' missions missed.
“When we first imaged Mars’ discrete aurora shortly after the Hope probe’s arrival at Mars in 2021, we knew we had unveiled new potential to make observations never before possible on this scale, and we took the decision to increase our focus on these auroras,” EMM's Dr Hessa Al Matroushi said in a statement.
Observations of Martian aurora began with widespread events across much of the planet seen only during the most powerful solar storms. Despite rare exceptions, these were usually faint. In contrast, highly localized auroras are thought to be the product of the solar wind interacting with fields produced by magnetized mineral deposits.
Four years ago NASA's MAVEN probe discovered Martian proton auroras. Analysis revealed some positively charged particles capture electrons allowing them to evade the bow shock that diverts charged particles around the planet, replenishing a little of the depleted Martian hydrogen.
However, the EMM's Hope probe has found something far more impressive; long curving streaks of energized electrons producing glowing light high in the Martian sky that stretch from the planet's dayside into the night. They're brighter than the diffuse auroras, and much more extensive than the previously seen discrete lights.
While their extreme length is impressive, it is the way these auroras bend that is most puzzling. The electrons responsible follow magnetic field lines, so these must also be curved. On Earth we see sometimes see auroras snake across the sky, but this is a function of the way multiple forces interact to influence the Earth's local magnetic field, and can't explain what Hope has recorded.
Yet these sinuous discrete auroras have been seen several times since Hope arrived at Mars last year. They were initially reported in July before the mission had even officially begun. The observing team couldn't immediately explain them, but it was expected understanding would come with time. Instead, the mystery has deepened. Dr Rob Lillis of the University of California, Berkeley, who is working Hope's Ultraviolet Spectrometer said the discovery “Has us scratching our heads and going back to the drawing board.”
Moreover, there's more than one version of the phenomenon to explain. Hope's operators describe what they call “closed” field lines that connect to the Martian crust at both ends and “open” counterparts that touch the crust at one end, while the other is connected to the solar wind, funneling electrons from outer space to the Martian atmosphere. There are also auroras that never reach the night side, making them unlikely to be seen without Hope's advanced instruments. The team refers to these as “draped” auroras.
The Hope probe completes an orbit of Mars every 55 hours and captures a full planetary data sample every nine days, releasing its data every three months so it's available to scientists and enthusiasts alike.