NASA’s MAVEN (Mars Atmosphere and Volatile Evolution) spacecraft has found a new type of Martian aurora that occurs over much of the day side of the planet. A paper reporting this discovery is published in the journal Nature Astronomy.
On Earth, the Northern and Southern lights occur when the solar wind (electrically charged particles from the Sun) follow our planet’s geomagnetic field lines to the poles and collide with the upper atmosphere. Mars lacks a global magnetic field, so instead the solar wind piles up in front of Mars in a bow shock, which blocks charged particles from reaching the bulk of the atmosphere. However, in a process first observed by NASA’s MAVEN spacecraft, some solar wind protons can slip past the bow shock by first bonding with electrons from the Martian upper atmosphere to form hydrogen atoms. Because these hydrogen atoms are electrically neutral, they can pass through the bow shock and go on to create an ultraviolet proton aurora on the dayside of Mars. Image credit: NASA’s Goddard Space Flight Center.
MAVEN project scientists were studying Mars’ atmosphere with the Imaging UltraViolet Spectrograph (IUVS), and observed that on occasion, the ultraviolet light coming from hydrogen gas in the planet’s upper atmosphere would mysteriously brighten for a few hours.
They then noticed that the brightening events occurred when another MAVEN instrument, the Solar Wind Ion Analyzer (SWIA), measured enhanced solar wind protons.
But two puzzles make this type of aurora seem impossible at first glance: how did these protons get past the planet’s ‘bow shock,’ a magnetic obstacle which normally diverts the solar wind’s charged particles around the planet? And how could the protons give off light, since atoms need electrons to do so?
“The answer was thievery. As they approach Mars, the protons coming in with the solar wind transform into neutral atoms by stealing electrons from the outer edge of the huge cloud of hydrogen surrounding the planet,” said Dr. Justin Deighan, from the University of Colorado, Boulder.
“The bow shock can only divert charged particles, so these neutral atoms continue right on through.”
When those high-speed incoming atoms hit the Martian atmosphere, some of their energy was emitted as ultraviolet light, which is invisible to the human eye but detectable to instruments like IUVS.
In fact, one incoming atom can collide with molecules in the atmosphere hundreds of times before it slows down, giving off a slew of ultraviolet photons.
“The Martian proton auroras are more than a light show,” said Dr. Jasper Halekas, from the University of Iowa.
“They reveal that the solar wind is not completely diverted around Mars, by showing how solar wind protons can sneak past the bow shock and impact the atmosphere, depositing energy and even enhancing the hydrogen content there.”
Proton auroras do occur at Earth, but not as often as at Mars.
One key difference is Earth’s strong magnetic field, which diverts the solar wind away from Earth to a much greater degree than at Mars.
On Earth, proton auroras only occur in very small regions near the poles, whereas at Mars they can happen everywhere.
However, proton auroras could be common on Venus and on Saturn’s moon Titan.
Like Mars, these two worlds lack their own magnetic fields, and have lots of hydrogen in their upper atmospheres — with plenty of electrons to share.
Looking further, it’s likely that many planets orbiting other stars have the same favorable conditions, and would be likely to have proton auroras too.
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J. Deighan et al. Discovery of a proton aurora at Mars. Nature Astronomy, published online July 23, 2018; doi: 10.1038/s41550-018-0538-5
