According to the new mission results, the Martian aurora, first identified by a NASA spacecraft in 2016, is actually the most common form of aurora found on the Red Planet.
On Earth, auroras are usually seen as bright manifestations of light in the night sky near the polar regions, where they are called the aurora borealis and southern lights. Proton aurora on Mars occurs during the day and emits ultraviolet radiation, so it is invisible to the human eye, but can be detected using the Imaging Ultraviolet Spectrograph (IUVS) instrument on the MAVEN spacecraft.
MAVEN's mission is to explore how the Red Planet lost most of its atmosphere and water, changing its climate from a climate that could support life to a climate that is cold, dry and inhospitable. Because the proton aurora is indirectly generated by hydrogen sourced from Martian water that is in the process of leaking out into space, the aurora can be used to track the ongoing loss of water in Mars.
"In a new study using MAVEN / IUVS data from several years on Mars, the team found that periods of increased atmospheric emission correspond to increased proton aurora intensity," said lead author Andrea Hughes of Embry-Riddle Aviation University, Florida. “Perhaps one day, when interplanetary travel becomes commonplace, travelers arriving on Mars during the southern summer will be given front row seats to see the Martian proton light dancing majestically across the daytime planet (using UV-sensitive goggles, of course) … Travelers will see with their own eyes the last stages of Mars, which is losing the rest of the water.
Different phenomena give rise to different types of auroras. All auroras on Earth and Mars are powered by solar activity, be it explosions of high-speed particles known as solar storms, eruptions of gas and magnetic fields known as coronal mass ejections, or gusts of solar wind that continuously blows through space at about a million miles. in hour. For example, the northern and southern lights on Earth occur when intense solar activity disrupts the Earth's magnetosphere, forcing high-speed electrons to slam into gas particles in Earth's nighttime upper atmosphere and cause them to glow. Similar processes give rise to discrete and diffuse auroras on Mars, two types of auroras previously observed on the Martian night side.
Proton auroras are formed when solar wind protons (which are hydrogen atoms stripped of electrons by intense heating) interact with the upper atmosphere on the daytime side of Mars. As they approach Mars, protons from the solar wind turn into neutral atoms, stealing electrons from hydrogen atoms on the outer edge of the Martian hydrogen corona, the huge cloud of hydrogen that surrounds the planet. When high-speed incoming atoms enter the atmosphere, some of their energy is emitted as ultraviolet light.
When the MAVEN team first saw the proton radiance, they thought it was an unusual phenomenon. “At first, we thought these events were quite rare because we weren't looking at the right time and place,” said Mike Chaffin, a researcher at the University of Colorado's Laboratory of Atmospheric and Space Physics (LASP) and the study's second author. "But upon closer inspection, we found that proton auroras are much more common in daytime southern observations during the summer than we originally expected."
The team found proton aurora in about 14% of daytime observations, increasing in over 80% of cases when only daytime southern summer observations are considered. “By comparison, the IUVS has detected diffuse auroras on Mars in a few percent of favorable geometries, and discrete auroras are even rarer,” said Nick Schneider, co-author and team leader of the IUVS at LASP.
Images of the proton radiance of Mars. The MAVEN ultraviolet spectrograph observes the atmosphere of Mars, capturing images of neutral hydrogen and proton aurora simultaneously (left). Observations under normal conditions show the presence of hydrogen on the disk and in the expanded atmosphere of the planet from a vantage point on the night side (middle). Proton radiance is seen as significant lightening on the disk (right); By subtracting the contribution of neutral hydrogen, the proton aurora distribution is found when it peaks in brightness in the immediate vicinity of the Martian disk.
Correlation with southern summers has made it clear why proton auroras are so common and how they can be used to track water loss. During the southern summer on Mars, the planet is also close to the Sun in its orbit, and huge dust storms can occur. Summer warming and dusty activity appear to trigger proton auroras, causing water vapor to rise high into the atmosphere. The sun's ultraviolet light breaks down water into its constituents, hydrogen and oxygen. Light hydrogen is loosely bound by the gravity of Mars and strengthens the hydrogen corona surrounding Mars, increasing the loss of hydrogen into space. The more hydrogen in the corona makes interactions with the protons in the solar wind more common, creating a more frequent and brighter proton glow.
"All the conditions needed to create a Martian proton aurora (such as solar wind protons, an expanded hydrogen atmosphere, and the absence of a global dipole magnetic field) are more available on Mars than those needed to create other types of aurora," Hughes said. "In addition, the link between the MAVEN observations means that proton aurora can actually be used for what happens in the hydrogen corona surrounding Mars, and thus temporarily increase atmospheric emissions and water loss."