Why Auroras Are Red on Mars

NASA recently announced a surprising sight from its Martian observatory: An intense aurora had blanketed the planet’s northern hemisphere in a rippling veil of light. Such light shows aren’t uncommon on Earth—a solar storm last week made for spectacular views of the night sky over Alaska and elsewhere—but scientists have rarely seen what we’d call Northern Lights on Mars.

But when they do see them, the lights over Mars look different.

“If you look at the colors, there will be red and green, kind of like Christmas lights,” said Nicholas Schneider, the lead for the MAVEN spacecraft’s imaging ultraviolet spectrograph instrument. MAVEN scans Mars’s horizon for strange occurrences within its atmosphere.

Auroras appear when a wave of charged particles fired off from the sun hits a planet’s magnetic field. On Earth, the magnetic field funnels the plasma deluge into its poles. Mars’s atmosphere contains traces of oxygen and other particulates, which glow green and red when struck with stellar electrons, according to Schneider. Absent from Mars’s auroral palette is the color blue, a prominent hue in aurora borealis seen from Earth. That’s because blue comes from nitrogen, an element that is abundant in Earth’s atmosphere but scarce within Mars’s sky. Mars lost its complete magnetosphere billions of years ago, so when it gets blasted by the sun’s charged particles, its atmosphere paints a different picture than what we see in the Earth’s sky.

The other difference between our auroras and those that appear on the Red Planet is texture. Earth’s auroras are like a swirling river of light—an intricate pattern that results from the complexities within our planet’s magnetic fields. But Schneider says that on Mars the aurora appears as a singular, pulsating glow across the sky. That’s because it lacks a sophisticated magnetic sphere to alter incoming electrons. Auroras on Mars also stretch across the hemisphere because there is no magnetic field to direct it. Which means a Martian aurora could fill the sky, rather than collecting just around the planet’s poles like on Earth.

Although astronomers had observed auroras on the Red Planet before, they had never observed one so close to the surface of the planet, a clue that the solar forces that caused this one were particularly powerful. And when it was first detected, the aurora puzzled scientists back at mission control. MAVEN monitors the different light spectrums that the planet emits. During the daytime, the atmosphere releases a specific spectral fingerprint that differs from the spectrum it shines at night. So when MAVEN observed mysterious spectral readings that were as strong as what it sees during the daytime, scientists couldn’t make sense of the readings. The NASA astronomers who first found the odd spectrum were still in disbelief when they forwarded the data to Schneider.

“We were like, ‘Are you sure we’re on the night side? Because this can’t be happening,’” said Schneider. “It was as if we saw everything backwards. So we scratched our heads and checked our orbit again, and realized ‘Nope, we’re seeing this.’” Several weeks after receiving the strange spectral readings, the group deduced that what they had witnessed was a Martian aurora. “We felt like we were the luckiest scientists around,” Schneider said.