The aurora borealis is one of the most dazzling light shows on Earth, but such events are by no means unique to our world. You might witness a similar phenomena on the surface of Mars—except instead of green and red streamers, you'd see a tapestry of electric blue.
"An astronaut looking up while walking on the red Martian soil would be able, after intense solar eruptions, to see [an aurora] with the naked eye," said planetary scientist Cyril Simon Wedlund in a statement. Wedlund is a co-author on a new study which, for the first time, shows that an aurora could be visible on a planet other than Earth. The finding is not only intrinsically fascinating, it could help scientists understand how the Martian atmosphere—and perhaps, an ancient biosphere—fizzled away over time.
Few space weather events fascinate us like the northern lights. But while countless generations of humans have gazed in wonder at the aurora's brilliant displays of green, red and purple light, the underlying forces at work were a mystery until the 1960s, when scientists began to observe streams of electrons raining down on Earth's atmosphere from above. We've since learned that aurorae are the result of solar winds—rivers of charged particles emanating from our Sun—interacting with Earth's magnetic field and entering our planet's upper atmosphere.
Once in our atmosphere, solar wind particles collide with nitrogen and oxygen atoms, imparting energy that's released as visible light. Because different molecules in the atmosphere absorb and emit different amounts of energy, this electric space rain produces a vibrant, color-changing display.
The ingredients needed to produce aurorae—an atmosphere, magnetic activity, and solar wind—are found throughout our solar system, so it's no great surprise that the Hubble Space Telescope has, in recent years, detected similar light shows on Jupiter and Saturn.
But if you're not a planetary scientist, you probably won't find any of these extrasolar auroras as compelling as our own—because we can't see them. Until now, we hadn't found a single aurora beyond Earth that emits visible light. Mostly, we've witnessed their distant glows in the ultraviolet.
In 2005, the ESA Mars Express discovered the first hints of an ultraviolet aurora in Mars's upper atmosphere. While Mars lacks a global magnetic field, patches of magnetism originating in the planet's crust can apparently catalyze auroras, as well.
To investigate the possibility of a visible aurora on Mars, Wedlund and his colleagues used a Planeterrella, a type of laboratory experiment designed to simulate atmospheres and study auroras, in conjunction with numerical models.
"We replicated the gas of the atmosphere with the most common component on Mars, which is carbon dioxide, after which an electrical discharge was created in a vacuum reminiscent of the Martian upper atmosphere," Wedlund explained. This, it turns out, results in a brilliant blue aurora. The researchers also observed glimmers of green and red, due to traces of oxygen in the simulated Martian atmosphere.
Cooking up an aurora in the lab seems like a generally awesome thing to do, but there's a deeper scientific purpose here. In some ways, aurorae are a signpost for an planetary evolution, because some of the energy that produces them also helps molecules in the atmosphere escape into space.
"This is a tool that we can use to observe the escape of the [Martian] atmosphere," study co-author Guillaume Gronoff of NASA's Langley Research Center told me over the phone. "And when you understand how the atmosphere is escaping today, you can go back in time and see what it was like before."
Indeed, Mars' present atmosphere—really just a thin wisp of CO2—is thought to be a shadow of its former self. Billions of years ago, the air was probably much thicker, perhaps rich in water vapor and oxygen.
"We know that Mars used to have liquid water, which means the atmosphere must have been evaporating," Gronoff told me. "The strength of the aurora can tell us about how charged particles from the sun helped it escape."
Based on Mars's magnetic activity, the new study predicts that the best place to see the aurora today is in the planet's southern hemisphere, shortly after sunset or before sunrise. In the grand scheme of factors to consider when choosing a place to settle on Mars, a prime view of the aurora may seem trivial. And yet, on cold, barren planet millions of miles away, one beautiful reminder of home could make a world's difference.