Europe's Mars Express spacecraft has made the first ever detection of an aurora on the Red Planet, Nature says.
The aurora was seen in a region called Terra Cimmeria
Aurorae are spectacular light shows often seen at high latitudes on Earth; they lie at the foot of planetary magnetic field lines near the poles.
The phenomena are produced by charged particles - electrons, protons or ions - cascading along these lines.
The Martian aurora is unique in the Solar System and is linked to anomalies in the planet's crustal magnetic field.
On Earth, as well as Jupiter, Saturn, Uranus and Neptune, aurorae are generated by particles accelerated by the magnetic field in the planet's core.
But Mars has no intrinsic magnetic field. Jean-Loup Bertaux and colleagues found that Martian aurorae are focused around areas of magnetised crustal rock. This makes them unique in the Solar System.
Aurorae have also been observed on the night side of Venus, a planet -like Mars - with no intrinsic (planetary) magnetic field.
Venusian aurorae are produced by electrons from the solar wind precipitating in the night-side atmosphere.
The glow from aurorae is caused by the charged particles hitting molecules of gas in the atmosphere.
The detection on Mars was made in observations by the Spicam (SPectroscopy for the Investigations and the Characteristics of the Atmosphere on Mars) instrument on Mars Express which were taken on 11 August 2004.