Extra-terrestrial aurora

The Earth isn’t the only planet in our solar system to possess a magnetic shield or play host to auroral lights. UK scientists are at the forefront of efforts to understand the similarities between aurorae at Earth and some of our planetary neighbours.

Most major solar system bodies generate a substantial magnetic field (Mercury, Earth, Jupiter, Saturn, Uranus, and Neptune), while those that definitely don’t are Venus, the Earth’s moon, comets, and asteroids. Mars seems to be somewhere in the middle, producing a weak field that partly deflects the solar wind in some regions, while Pluto remains a mystery.

Jupiter and Saturn, both gas giants with no solid surface, generate such strong magnetic fields in their interiors that they are surrounded by giant magnetospheres. Magnetic fields are invisible, but if you could see Jupiter’s magnetosphere from Earth it would look the same size in the sky as the Sun, even though it is five times further away!

Jupiter and Saturn's magnetospheres are so large that they contain many of the moons that orbit those planets. One of Jupiter’s moons (Ganymede) is the only planetary moon that is known to possess its own magnetic field and to produce a ‘mini-magnetosphere’ within Jupiter’s own (much larger) margnetosphere. Some moons, such as the volcanic Jovian moon Io, and the Icy Saturnian moon Enceladus, produce huge amounts of gas. In turn this creates dense doughnut-shaped belts of neutral and ionised (electrically charged) material around their host planets. Eventually, this material dissipates, but exactly how this happens is uncertain.

The giant planets rotate very rapidly about their axes. Jupiter revolves approximately once every 10 hours while Saturn is only slightly slower, taking just 11 hours to rotate once. Coupling between the rapidly spinning planet’s ionosphere and magnetic field causes a rotational motion that dominates the magnetosphere. Plasma created in the inner magnetosphere is transported outward by centrifugal force, but the conservation of angular momentum causes its rotational speed to fall below that of the planet. (Think of a spinning ice skater who stretches out her arms to slow down her spin.)

When the ‘coupling’ with the planet’s magnetic field tries to speed the plasma back up, the magnetic field lines become bent, resulting in electrical currents of millions of Amps flowing between the magnetosphere and ionosphere. At Jupiter, these currents are powerful enough to generate an auroral oval around each pole, bright enough to be observed by the Hubble Space Telescope. In Saturn’s case, the currents are weaker and the effect less marked.

Data about plasma processes at Jupiter and Saturn come not only from HST images, but also from fly-by and orbiting spacecraft. Fly-by missions to Jupiter and Saturn have included Pioneer, Voyager, and Ulysses, while most in situ data have been acquired by the Galileo orbiter at Jupiter (1995-2003), and the Cassini orbiter at Saturn (2004-present). UK teams currently involved in Cassini studies of Saturn’s plasma environment include those at Imperial College London, University College London, and the University of Leicester.