Auroras on Earth and four other planets (Jupiter, Saturn, Uranus and Neptune) result from the interaction between charged particles from the solar wind with the planets' magnetic fields, which subsequently excites atmospheric molecules. On Venus, which lacks a magnetic field, auroras arise from atmospheric molecules energized directly by solar wind particles. Using the ultraviolet spectrometer (SPICAM) on board the Mars Express satellite, Jean-Loup Bertaux of the Service d'Aeronomie du CNRS in France and his colleagues observed a Martian aurora. Unlike the other large planets' celestial shows, which occur near the poles, the light show above the Red Planet manifests around areas of magnetized rock in the planet's crust. "Mars has no internally generated, planetary-scale magnetic field," explains study co-author Bill Sandel of the University of Arizona. "It has what are called 'crustal magnetic anomalies' scattered around the Martian surface, remnants of what presumably was Mars's planetary-scale magnetic field that was active when the planet was younger."
The aurora measured some 30 kilometers across and appeared about 130 kilometers above the surface, according to a report published today in the journal Nature. Note the authors: "It corresponds to a distinct type of aurora not seen before in the solar system."