If we ever manage to make the giant leap across the solar system, humans standing on the Red Planet soil may one day look up at the dark night sky and be in for a real visual treat: a Martian aurora. We already know that, unlike our own, sunsets on Mars are blue, so what color would a Martian aurora be? An international team of scientists think they might have the answer.
Using the available data from Mars, as well as experiments in Earth laboratories, we know that, like their beautiful sunsets, Martian aurorae are probably blue. That's because the atmosphere of Mars is composed of mostly carbon dioxide; when these molecules are stimulated with electrons, like during an aurora, they emit blue light.
Of course, the Martian atmosphere isn't only composed of CO2, there are also hints of oxygen just like on Earth. So there may also be familiar hues of green and even red in a Martian aurora during an especially powerful solar event.
For an aurora to light up the sky, the planet needs to have a magnetic field. The sun occasionally spits out a flurry of charged particles, for example electrons, and when these whizz past a planet, its magnetic field directs them away from the surface. The particles then travel along the magnetic field lines, eventually reaching the planet's atmosphere. Here, they interact with the gas particles that are floating around, such as carbon dioxide, nitrogen or even oxygen in Mars' case. These interactions give the gas particles energy that are then converted to light, resulting in the dazzling display that is an aurora.
But some of you may have already noticed a conundrum: Mars does not have a magnetic field. So how can this phenomenon ever be seen? When Mars was 'born' 3.5 billion years ago, the Red Planet did have a brimming magnetic field. It has faded now, but the remnants of this field are still strong enough to cause an aurora.
There are several space missions orbiting Mars that have contributed to the discovery: The European Space Agency's (ESA's) Mars Express and NASA's MAVEN. The Mars Express satellite first spotted aurorae on Mars from space. MAVEN then confirmed these observations during its 1,000 orbits around the planet.
However, both of these crafts have a fatal flaw when it comes to replicating human vision: They can't observe visible light. You might think it's bizarre that these space missions don't take photos in the visible spectrum, but the truth is that there's lots more interesting information in spectrums such as ultraviolet and infrared. Fortunately, scientists have other ways to confirm the actual color we would see in the aurorae.
To simulate the phenomenon back on Earth, scientists used a remarkable little device known as the 'Planeterella.' This consists of a large sphere representing the sun and a small sphere representing a planet. The experiment is set up in a vacuum to mimic space conditions. The team started off by adding little CO2 ice crystals onto the planet-sphere to simulate the Martian atmosphere. The 'sun' then shoots a stream of electrons at the 'planet' and simulates an aurora. The main photograph is from one of these experiments, showcasing a deep blue hue.
Video demonstrating how the Planeterella works via University of Leicester