What causes auroras?
Auroras are a phenomenon caused when highly energetic particles are released by the Sun in what is called the solar wind. When these particles approach Earth they are attracted towards the Earth’s magnetic poles and will become concentrated there. They interact with the atmosphere and excite the gas molecules, causing them to emit light in a variety of colours. But how does this phenomenon affect the other planets in our solar system?
Mercury, being so small and so close to the Sun, does not have an atmosphere. Even though it is constantly blasted with solar radiation, with no atmosphere to interact with the particles cannot cause any auroras.
Venus does not have its own magnetic field, however that does not mean it cannot have auroras. There is a layer of Venus’ atmosphere called the ionosphere which is a layer that is composed of mostly charged particles. When solar wind reaches this layer, it is thought that it can induce a magnetic field. This was not thought possible until 2012, when ESA’s Venus Express spacecraft measured plasma in the planet’s magnetotail (see https://en.wikipedia.org/wiki/Magnetosphere) in a process called magnetic reconnection. This helped to explain dim lights seen on the planet and supports the theory.
Like Venus, Mars does not have its own magnetic field; yet it still forms auroras. Instead of the energetic particles being attracted towards the poles, on Mars there are no poles so the particles will hit wherever. It is thought that due to Mars’ carbon dioxide-heavy atmosphere, the aurorae would appear green to the observer.
Below is an image from NASA’s MAVEN spacecraft, showing the distribution of aurorae in the northern hemisphere of Mars in 2014. This is a much more widespread distribution of events than Earth.
As the largest planet, it is no surprise that Jupiter has the strongest magnetic fields and in turn some of the most impressive aurorae in the solar system. However, unlike the other planets, it is believed that they are not only governed by the solar wind. Using Japan’s Hisaki satellite, the Hubble Space Telescope and the Juno spacecraft, scientists have found that the moon Io may have an unexpected effect on the formation of aurorae. Io is the most active volcanic object in the solar system and the sulphur gas emitted is driven towards Jupiter’s polar region, amplifying the auroras.
Saturn experiences strong aurorae, however these can only be seen in ultraviolet light. The picture below was taken by the Hubble Space Telescope in 2004.
The picture below was taken by Cassini.
Spotting an aurora on Uranus is a rare event. The Hubble Space Telescope has only caught the event twice, both in 2011. Scientists tracked two large solar wind bursts to monitor their effect on the planet. It is so difficult to catch the auroras as the magnetic field of the planet is angled at 59 degrees from the axis of rotation and therefore the auroras form far away from the north and south poles.
The auroras on Neptune are estimated to only be half the strength of those on Earth. Like Uranus, the rotation is not inline with the magnetic field making detection difficult.