'Stolen' electrons spark unusual auroras on Mars

A new type of aurora has been spotted on Mars, and thieving streams of solar wind may be to blame, a new study shows.

Unlike auroras on Earth, which are caused by electrons and are generally seen near the poles, the auroras on Mars are triggered by an influx of protons and occur over much of the dayside of the Red Planet, where auroras are very hard to see.

Using the Imaging Ultraviolet Spectrograph on NASA's Mars Atmosphere and Volatile Evolution (MAVEN) orbiter, astronomers observed that, on occasion, the ultraviolet light coming from hydrogen gas in Mars' upper atmosphere would mysteriously brighten for a few hours. These brightening events happened at the same time that the orbiter's Solar Wind Ion Analyzer (SWIA) instrument measured an increase in solar-wind protons (charged particles released from the sun), according to the study.

This image shows the formation of a proton aurora at Mars. A solar-wind proton races toward Mars, hitting a cloud of hydrogen surrounding the planet and stealing an electron from one of those hydrogen atoms to become neutral. The atom can then pass through the magnetic bow shock surrounding Mars because it no longer carries a charge, and it collides with gas molecules in Mars' atmosphere to produce ultraviolet light.Credit: Dan Gallagher/NASA/MAVEN/Goddard Space Flight Center

Generally, Mars' "bow shock" — a magnetic obstacle surrounding the planet — would divert charged particles from the solar wind. However, the researchers found that, in this case, the charged particles "steal" electrons in order to sneak into the planet's upper atmosphere.

"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," Justin Deighan, a researcher at the Laboratory for Atmospheric and Space Physics at the University of Colorado Boulder and lead author of the new study, said in a statementfrom NASA Monday (July 23). "The bow shock can only divert charged particles, so these neutral atoms continue right on through."