Mars has an aurora, and it's like none other in the solar system.

The European Space Agency's Mars Express orbiter detected the unique phenomenon with its ultraviolet instrument called SPICAM in August 2004. French, U.S. and Russian scientists are reporting the discovery today in Nature.

Mars Express was flying 270 kilometers (168 miles) above the planet when SPICAM's field of view was positioned just above the limb, or edge, of the planet during the Aug. 11, 2004 orbit. SPICAM, a spectrograph, detected a 30-kilometer wide (19-mile wide) auroral emission, which comes mainly from excited carbon monoxide molecules, 140 kilometers (87 miles) above the planet.

"Earth and all the giant planets have aurorae because they generate a global-scale magnetic field that extends great distances beyond the planet," said University of Arizona scientist Bill Sandel, a co-investigator on SPICAM. "Their planetary magnetic fields are so extensive that they accelerate and energize the charged particles that excite the auroras.

"That's not the case on Mars, and that's why this discovery is so interesting, " Sandel said. "Mars has no internally generated, planetary-scale magnetic field. It has what are called 'crustal magnetic anomalies' scattered around the martian surface, remnants of what presumably was Mars' planetary-scale magnetic field that was active when the planet was younger. These crustal pieces are the leftovers of that earlier field."

To visualize what's going on, think of magnetic lines as wires rising from patches of Mars' surface crust and reaching out beyond the planet. Electrons that have come from the solar wind flow down the "wires" toward Mars' surface, losing energy as they collide with molecules in Mars' thin atmosphere. The energy released on impact excites the auroral emission.

The aurora is brightest when the particles reach the densest part of the atmosphere, a narrow layer where the charged particles stop because they've lost all their energy in collisions with air molecules.

We on Earth, if at the right latitudes at the right time of year, see the aurora in spectacular nighttime Northern Light and Southern Light shows. If you had ultraviolet eyes and were standing at the right place on Mars, you'd see an aurora 100 times dimmer than Earth's, Sandel said.

"Now that we see the aurora, we'll get a better idea of what Mars' magnetic structure really is," Sandel said.

Sandel is a co-investigator on Voyager's ultraviolet spectrometer experiment that flew by the outer solar system planets in the 1970s and 1980s. That team was led by Lyle Broadfoot, also of UA's Lunar and Planetary Laboratory. "I'm personally excited about discovering an aurora on Mars because I'm part of the team that also discovered the auroras of Jupiter, Saturn, Uranus and Neptune," he said.

Mars Express's SPICAM saw the strongest aurora where NASA's Mars Global Surveyor orbiter previously detected the planet's strongest crustal anomalies, at about 180 degrees longitude and 50 degrees southern latitude.