Image credit: David Crisp and the WFPC2 Science Team (Jet Propulsion Laboratory/ California Institute of Technology), Public domain, via Wikimedia Commons.

Planetary scientists believe they can now predict the green glow of an aurora in the night sky above Mars, and they have the images to prove it.

The first observations of a visible-light aurora from the surface of the Red Planet were made by NASA’s Perseverance Mars rover in 2024.

Now, presenting at the Europlanet Science Congress–Division of Planetary Science (EPSC–DPS) joint meeting in Helsinki this week, Dr Elise Wright Knutsen of the University of Oslo will reveal a second snapshot of the aurora by Perseverance and, more importantly, the tools to predict when an aurora will occur on Mars.

Knutsen, who was also the science lead for the first image of a Martian aurora seen from the ground, said: “The fact that we captured the aurora again demonstrates that our method for predicting aurorae on Mars and capturing them works.”

An artist’s impression of how the aurora might appear in the sky above the Perseverance rover. Credit: Alex McDougall-Page, University of Strathclyde/ AstrollCareers. Public Domain.

Charged particles

Aurorae are produced when a burst of energetic particles in the solar wind, belched out by a coronal mass ejection (CME) from the Sun, collide with molecules in the atmosphere, causing them to glow.

Mars’s aurorae glow green as a result of the charged particles colliding with oxygen atoms high above the Red Planet, and could be bright enough that astronauts on Mars would be able to see them with the naked eye.

Furthermore, because Mars does not have a magnetic field to direct the charged particles to the magnetic poles, which is where we generally see aurorae on Earth, the Martian aurorae are seen all across the night-side of the planet at the same as a glow in the sky.

This is called ‘diffuse’ aurora.

The same radiation that causes the aurora could also potentially be dangerous to astronauts without warning that they must take shelter, so having some idea of when a powerful solar storm will hit Mars is crucial if humans are going to one day survive on the surface.

Four images from Perseverance’s Mastcam-Z. The left hand-side images show both detections of the aurora, on 18 March and 18 May 2024. On the right are non-detections with comparable sky illumination (from Mars’s moons) to show the contrast in colours between a night with aurora and a night with no aurora. The coloured boxes show (from top to bottom): the theoretical aurora colour for these images, the average sky colour, and the bottom boxes show the sky colour with the aurora signal removed or added, for left and right column respectively. This is to show what the colour of the sky would have been, theoretically, with no aurora that night, or with aurora for the comparison images. If all conditions were identical, then the two bottom boxes should diagonally have the same colour, which worked close to perfectly for the May event. Below the images is the spectra from the rover’s SuperCam that identifies the green glow as the 557.7nm atomic oxygen auroral emission, indicated by the vertical green line. The solid lines are the real measurements for the two detections, while the dashed lines show our aurora model, demonstrating that the calculations estimating the aurora’s brightness from the surface with the measured dust amount corresponds very well with the observed aurora intensity. Credit: Elise Wright Knutsen et al. Public Domain.

Complex business

Nonetheless, predicting aurorae on Mars is a complex business. Observations have to be planned and uploaded to the rover three days ahead once a CME bursts out in the direction of Mars.

This means a lot of guesswork as to which solar storms will produce an aurora.

Knutsen’s team made eight attempts to view the aurora with Perseverance’s SuperCam and MastCam cameras between 2023 and 2024, and they found it to be a process of trial and error.

The first three attempts saw nothing, but by retrospectively analysing conditions as measured by NASA’s MAVEN and the ESA’s Mars Express orbiters, Knutsen and her colleagues realised that the velocities of those CMEs had likely not been fast enough to create a solar wind disturbance at Mars.

Knutsen said: “The faster the CME, the more likely it is to accelerate particles towards Mars that create aurorae, and the stronger the solar wind disturbance around Mars, the more likely it is that those particles make it into Mars’s nightside atmosphere.

“Later, we progressively targeted faster, more intense CMEs, and that’s when we found our first two detections.”

The final three CMEs also didn’t produce aurorae, even though they met the criteria that Knutsen was looking for. 

She said: “The last three non-detections are more curious.

“Statistically there is also a degree of randomness to these things, so sometimes we’re just unlucky.

“This perhaps isn’t that surprising, since predicting the aurora on Earth down to minute precision isn’t an exact science either.”

Growing dataset

Aurorae on Mars have previously been observed from orbit in ultraviolet light by ESA’s Mars Express and NASA’s MAVEN missions.

Now, with the addition of visible-light detections, there is a growing dataset of observations for improving the accuracy of the aurora predictions.

With further observations to come, they will hopefully help solve some ongoing mysteries about how the auroral lights are triggered on Mars. 

Knutsen said: “There is still much we don’t understand about how aurora occur on Mars as, unlike Earth, there is no global magnetic field to guide energetic solar particles onto the nightside where the aurora can be seen.

“Comparing the timing of solar wind disturbances, the arrival of solar energetic particles and the intensity and timing of aurora will advance our knowledge in this area.”