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On Mars, Auroras Glow All Summer Long in UV Light

SAN FRANCISCO — Auroras unfurling in Martian skies are stunning displays, and they also offer important clues about how the Red Planet's water escapes into its atmosphere, scientists reported on Dec. 12 here at the annual meeting of the American Geophysical Union (AGU). 

A type of aurora called a proton aurora was first identified on Mars in 2016, using data from the Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft. This aurora, which occurs during daytime and produces ultraviolet light, is invisible to the naked eye, but was spotted by MAVEN's Imaging UltraViolet Spectrograph (IUVS) instrument. 

Recently, researchers took a closer look at Martian proton auroras, analyzing data accumulated over years of observations and describing their findings in a new study. They found that these auroras aren't as rare as originally thought. In fact, they are Mars' most common aurora and they are astonishingly frequent, "with almost 100% occurrence rate on the day side of the planet in southern summer," said lead study author Andréa Hughes, a doctoral candidate in engineering physics at Embry-Riddle Aeronautical University in Daytona Beach, Florida.

Related: Aurora Photos: Northern Lights Dazzle in Night-Sky Images (opens in new tab)

"To think that the probability of seeing proton aurora during southern summer on the day side approaches 100%!" Hughes told Live Science. "That, I think, was most surprising to me and, I think, to most members of the team as well." 

MAVEN's Imaging Ultraviolet Spectrograph observes the atmosphere of Mars, making images of neutral hydrogen and proton aurora simultaneously (left). Observations under normal conditions show hydrogen on the disk and in the extended atmosphere of the planet from a vantage point on the nightside (middle). Proton aurora is visible as a significant brightening on the limb and disk (right).  (Image credit: Embry-Riddle Aeronautical University/LASP, CU Boulder)
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Auroras on Earth typically appear when streams of charged particles from the sun — also known as solar winds, traveling at approximately 1 million mph (1.6 million km/h) — slam into our planet's magnetic field. The high-energy collisions between solar particles and atmospheric gas particles create sky glows such as the northern and southern lights.

Mars' proton auroras also start with solar winds. But in this case, charged protons collide with a cloud of hydrogen surrounding Mars. There, they slurp electrons away from hydrogen atoms, which neutralizes the protons. When those energetic neutral atoms enter the lower atmosphere of Mars, their collisions with molecules produce ultraviolet glows — proton auroras, Hughes explained at AGU.

Why are these auroras so common during the Martian southern summer? "We have known for several years now about seasonal variation in the hydrogen corona" — the cloud of hydrogen surrounding Mars — which is at its highest altitude around the summer solstice, Hughes said. In other words, summer months are when Mars' hydrogen cloud is perfectly positioned to interact frequently with solar winds and produce near-constant proton auroras.

A solar wind proton approaches Mars at high speed; it steals an electron from an escaped Martian hydrogen atom and passes through a magnetic field surrounding Mars. The now-neutral atom then enters the planet's atmosphere and collides with gas molecules, which causes the atom to emit ultraviolet light. (Image credit: NASA/MAVEN/Goddard Space Flight Center/Dan Gallagher)

And that's not all the researchers discovered. As temperatures climb during summer months, rising dust clouds carry water vapor away from the Martian surface. "That causes hydrogen to break apart into hydrogen and oxygen, and that causes it to escape," Hughes said. "Because of that — and because of the connection between solar wind protons interacting with the hydrogen in Mars' atmosphere — we know that when we're seeing proton aurora, the source of that is not only the solar wind but also this water that's breaking apart and being lost to space."

At this point, you might be wondering if you'd be able to witness a proton aurora if you were standing on Mars — but you wouldn't, unfortunately.

"It's not something that could be observed from the surface, because we're looking at this in ultraviolet light, and ultraviolet is absorbed in the atmosphere. So by the time it gets to the surface, you wouldn't see it," Hughes said. 

However, let's say you were a space traveler arriving at Mars on the daytime side during summer in the southern hemisphere. If you were to don ultraviolet goggles during your spacecraft's approach, "then yes, you would absolutely be able to see this beautiful emission enhancement — and perhaps proton aurora dancing around in the atmosphere," Hughes said. 

But as human spaceflight to Mars is still a distant dream, it could be a while before people experience that awe-inspiring sight firsthand.

The findings were published online Dec. 12 in the journal JGR Space Physics.

Originally published on Live Science.

Originally published on Live Science.

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Mindy Weisberger is a senior writer for Live Science covering general science topics, especially those relating to brains, bodies, and behaviors in humans and other animals — living and extinct. Mindy studied filmmaking at Columbia University; her videos about dinosaurs, biodiversity, human origins, evolution, and astrophysics appear in the American Museum of Natural History, on YouTube, and in museums and science centers worldwide. Follow Mindy on Twitter.