Astronomers Mystified by Eerie Phenomenon on Mars: Ultraviolet “Nightglow” Spreads Across the Planet’s Sky Every Night

This is an image of the ultraviolet “nightglow” in the Martian atmosphere over the south pole. Green and white false colors represent the intensity of ultraviolet light, with white being the brightest. The nightglow was measured at about 70 kilometers (approximately 40 miles) altitude by the Imaging UltraViolet Spectrograph instrument on NASA’s MAVEN spacecraft. A simulated view of the Mars globe is added digitally for context, and the faint white area in the center of the image is the polar ice cap. The image shows an unexpectedly bright glowing spiral in Mars’ nightside atmosphere. The cause of the spiral pattern is unknown. Credit: NASA/MAVEN/Goddard Space Flight Center/CU/LASP

Every night on Mars, when the sun sets and temperatures fall to minus 80 degrees Fahrenheit and below, an eerie phenomenon spreads across much of the planet’s sky: a soft glow created by chemical reactions occurring tens of miles above the surface.

An astronaut standing on Mars couldn’t see this “nightglow”—it shows up only as ultraviolet light. But it may one day help scientists to better predict the churn of Mars’ surprisingly complex atmosphere.

Mars’ nightside atmosphere glows and pulsates in this data animation from MAVEN spacecraft observations. Green-to-white false color shows the enhanced brightenings on Mars’ ultraviolet “nightglow” measured by MAVEN’s Imaging UltraViolet Spectrograph at about 70 kilometers (approximately 40 miles) altitude. A simulated view of the Mars globe is added digitally for context, with ice caps visible at the poles. Three nightglow brightenings occur over one Mars rotation, the first much brighter than the other two. All three brightenings occur shortly after sunset, appearing on the left of this view of the night side of the planet. The pulsations are caused by downwards winds which enhance the chemical reaction creating nitric oxide which causes the glow. Months of data were averaged to identify these patterns, indicating they repeat nightly. Credit: NASA/MAVEN/Goddard Space Flight Center/CU/LASP

“If we’re going to send people to Mars, we better understand what’s going on in the atmosphere,” said Zachariah Milby, a professional research assistant at the Laboratory for Atmospheric and Space Physics (LASP) at CU Boulder.

In a study published earlier this month in the Journal of Geophysical Research: Space Physics, Milby and his colleagues set their sights on understanding the phenomenon. They drew on data from NASA’s Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft to map the planet’s nightglow in greater detail than ever before. 

The team’s findings show how this light display ebbs and flows over Mars’ seasons. The group also discovered something unusual: an unexpectedly bright spot that appears in the planet’s atmosphere just above its equator. 

Mars, in other words, still has a few surprises in store for scientists, said LASP’s Nick Schneider, lead author of the new study. 

“The behavior of the Martian atmosphere is every bit as complicated and insightful as that of Earth’s atmosphere,” said Schneider, also a professor in the Department of Astrophysical and Planetary Sciences.

Full picture

MAVEN wasn’t the first spacecraft to spot the nightglow on Mars, a phenomenon that resembles similar glows seen on Earth and Venus. That honor belongs to the European Space Agency’s Mars Express Mission, which entered orbit around Mars in 2003.

But the mission was the first to capture the nightglow for what it is—a dynamic and constantly evolving phenomenon.

“It wasn’t until MAVEN came along in 2014 that we could actually snap this full picture five times a day as the planet rotates,” Schneider said.

The diagram explains the cause of Mars’ glowing nightside atmosphere. On Mars’ dayside, molecules are torn apart by energetic solar photons. Global circulation patterns carry the atomic fragments to the nightside, where downward winds increase the reaction rate for the atoms to reform molecules. The downwards winds occur near the poles at some seasons and in the equatorial regions at others. The new molecules hold extra energy which they emit as ultraviolet light. Credit: NASA/MAVEN/Goddard Space Flight Center/CU/LASP

In the new study, researchers used MAVEN’s Imaging Ultraviolet Spectrograph (IUVS)—an instrument designed and built at LASP—to snap images of Mars from a distance of 3,700 miles. Those far-flung recordings allowed the team to trace the path of nightglow as it moved across the entire planet. 

Milby led the data analysis for the research while he was still an undergraduate student at CU Boulder.

He explained that the eerie aura appears when air currents high in Mars’ atmosphere plunge to about 40 miles above the planet’s surface. When that happens, lone nitrogen and oxygen atoms in the atmosphere combine to form molecules of nitric oxide, giving off small bursts of ultraviolet light in the process.  

Put differently, when its atmosphere drops, Mars shines.

“It’s a great tracer for dynamics between the layers of the atmosphere,” Milby said.

Bright spots

Milby added that, like on Earth, those dynamics can shift with the seasons. The MAVEN team found, for example, that Mars’ nightglow seems to be brightest at the height of the planet’s northern and southern winters when hotter currents rush away from the equator and toward Mars’ poles.

Milby also found something he wasn’t expecting in the data: an extra-bright blob of nightglow that appeared and disappeared from almost exactly above 0 degrees longitude and 0 degrees latitude on Mars.

“We spent weeks thinking there was a bug in our code somewhere,” Milby said. 

This is an image of the ultraviolet “nightglow” in the Martian atmosphere. Green and white false colors represent the intensity of ultraviolet light, with white being the brightest. The nightglow was measured at about 70 kilometers (approximately 40 miles) altitude by the Imaging UltraViolet Spectrograph instrument on NASA’s MAVEN spacecraft. A simulated view of the Mars globe is added digitally for context. The image shows an intense brightening in Mars’ nightside atmosphere. The brightenings occur regularly after sunset on Martian evenings during fall and winter seasons, and fade by midnight. The brightening is caused by increased downwards winds which enhance the chemical reaction creating nitric oxide which causes the glow. Credit: NASA/MAVEN/Goddard Space Flight Center/CU/LASP

There wasn’t a bug. The researchers still aren’t sure why Mars is glowing so much at that unusual spot—it may have something to do with the shape of the terrain underneath. But Schneider said that observations like this can help scientists improve their computer models of how the planet’s atmosphere works.

And that could lead to something that every astronaut might use: more accurate weather reports on Mars. 

“We use supercomputers to predict weather on Earth so that you can plan for your vacation or growing crops,” Schneider said. “The same computer models can be spun up for Mars and all the other planets.”

For more on this study, read NASA’s MAVEN Spacecraft Observes Weird Glowing and Pulsing in Mars’ Night Sky.