Earlier this month, NASA probes in orbit and on the Martian surface detected an incredibly bright global aurora as a powerful solar storm struck Mars. The aurora was accompanied by a powerful dose of radiation that would have posed a significant danger to humans exploring the Red Planet.
Between September 12 and 13, NASA's MAVEN spacecraft observed the Martian atmosphere as a magnificent ultraviolet aurora 25 times brighter than any seen by the probe since entering Mars' orbit in 2014 illuminated the planet.
The September 11 solar storm, known as a coronal mass ejection (CME), that prompted the aurora was so powerful that it was detected on Earth, despite the fact that our planet was on the opposite side of the Sun during the event.
A CME occurs when magnetic fields on the Sun explosively realign, throwing a colossal cloud of magnetized particles, also known as plasma, into space. This mass of plasma travels at millions of miles per hour and, upon colliding with a planet's magnetic field, can trigger a geomagnetic storm, during which particles trapped in a planet's atmosphere are released.
The release of the particles causes a reaction in the atmosphere, which triggers a release of photons that can create a breathtaking aurora. MAVEN observed the Martian aurora in ultraviolet light.
"When a solar storm hits the Martian atmosphere, it can trigger auroras that light up the whole planet in ultraviolet light," says Sonal Jain of the University of Colorado Boulder's Laboratory for Atmospheric and Space Physics, and member of MAVEN's Imaging Ultraviolet Spectrograph instrument team. "The recent one lit up Mars like a light bulb. An aurora on Mars can envelope the entire planet because Mars has no strong magnetic field like Earth's to concentrate the aurora near polar regions. The energetic particles from the Sun also can be absorbed by the upper atmosphere, increasing its temperature and causing it to swell up."
While MAVEN and a number of other spacecraft watched the fireworks from orbit, NASA's Curiosity rover recorded a more sinister aspect of the solar storm from the Red Planet's surface. As the solar storm struck Mars, the rover's Radiation Assessment Director (RAD) tracked the amount of radiation reaching its surface.
Solar storms are known to significantly heighten the amount of radiation penetrating the Martian atmosphere, and at the peak of the storm the RAD instrument detected surface radiation levels over double the intensity of any that it had ever detected since touching down on the Martian surface in 2012.
NASA is striving towards a long-term goal of putting humankind on Mars, and exposure to cosmic radiation is one of the key health issues to be faced by astronauts during any potential interplanetary mission. The agency and its partners will have to understand and cope with these radiation strikes if they hope to keep their astronauts healthy as they explore the surface of the Red Planet.
Had explorers been present on the planet at the time of such a storm, they would have been advised to seek shelter.
"If you were outdoors on a Mars walk and learned that an event like this was imminent, you would definitely want to take shelter, just as you would if you were on a space walk outside the International Space Station," says RAD Principal Investigator Don Hassler of the Southwest Research Institute's Boulder, Colorado, office. "To protect our astronauts on Mars in the future, we need to continue to provide this type of space weather monitoring there."
NASA believes that the observations made in the wake of the September 11 solar storm will lead to a greater understanding of how Mars' original atmosphere was lost to space, leaving it a dry, barren world. It will also inform scientists' understanding of the current Martian environment, and the threats posed to future explorers by powerful space weather events.