NASA's Perseverance rover has observed an aurora on Mars in visible light for the first time, with the sky glowing softly in green in the first viewing of an aurora from any planetary surface other than Earth.
Scientists said the aurora occurred on March 18, 2024, when super-energetic particles from the sun encountered the Martian atmosphere, precipitating a reaction that created a faint glow across the entire night sky. Auroras have been observed previously on Mars by satellites from orbit in ultraviolet wavelengths, but not in visible light.
The sun three days earlier had unleashed a solar flare and an accompanying coronal mass ejection - a huge explosion of gas and magnetic energy that brings with it large amounts of solar energetic particles - that traveled outward through the solar system. Mars is the fourth planet from the sun, following Mercury, Venus and Earth.
Scientists had simulated the event in advance and prepared instruments on the rover to be ready to observe the expected aurora. Perseverance has two instruments that are sensitive to wavelengths in the visible range, meaning they detect colors human eyes can see. The researchers used the rover's SuperCam spectrometer instrument to identify exactly the wavelength of the green glow and then used its Mastcam-Z camera to take a snapshot of the softly glowing green sky.
An aurora forms on Mars the same way as on Earth, with energetic charged particles colliding with atoms and molecules in the atmosphere, exciting them, and causing subatomic particles called electrons to emit light particles called photons.
"But on Earth, the charged particles are channeled into the polar regions by our planet's global magnetic field," said Elise Wright Knutsen, a postdoctoral researcher at the University of Oslo's Center for Space Sensors and Systems and lead author of the study published this week in the journal Science Advances.
"Mars has no global magnetic field so the charged particles bombarded all of Mars at the same time, which leads to this planet-wide aurora," Reuters quoted Knutsen as saying.
The green color occurred because of the interaction between the charged particles from the sun and oxygen in the Martian atmosphere. While auroras can be brilliant, as often seen in Earth's northernmost and southernmost regions, the one observed on Mars was quite faint.
"This specific aurora we observed on March 18th of last year would have been too faint for humans to see directly. But if we get a more intense solar storm, it could become bright enough for future astronauts to see. And with a camera, such as an iPhone, you would clearly see it, rather like how an aurora on Earth is always brighter in images than with the naked eye," Knutsen said.
This particular event did not impact Earth.
All the planets with atmospheres in our solar system experience auroras.
"Various types and wavelengths of aurora have been observed previously from Mars-orbiting satellites. All previous observations have been in the UV, but they have had wildly different shapes. From the global, diffuse aurora we observed now, to discrete arcs and patches near the crustal fields (regional magnetic fields) in the south, and large-scale sinuous shapes," Knutsen said.
If astronauts from Earth visit Mars and perhaps establish a long-term presence on the planet's surface, they may be treated to a nighttime light show.
"During a more intense solar storm, producing a brighter aurora, I think a sky which glows green from horizon to horizon will be eerily beautiful," Knutsen said.
"The aurora will appear as a soft green glow covering more or less the whole sky," Knutsen added. "Dust in the lower part of the atmosphere would obscure some of the light towards the horizon, and if you looked straight up it would also be fainter simply because looking at a slant angle will allow you to see through a thicker section of the atmosphere that is emitting the aurora."
