By Gareth Dorrian
Astronaut Alexander Gerst takes this image as the ISS flies through an aurora. Credit: ESA/NASA
Auroras paint Earth’s sky with streaks of greens, reds, and blues— but Earth isn’t the only planet to experience this surreal luminous spectacle. Scientists have observed auroras around other planets in our solar system. Although we do not have any extra-terrestrial based Aurorasaurus users to send us their photographs, we are still greatly interested in how and why this beautiful natural light phenomenon occurs on other worlds beyond ours.
Auroral emission results from high energy charged particles colliding with molecules of gas in the atmosphere of a planet. Certain regions—known as auroral ovals– have an increased likelihood of experiencing aurora. Earth’s auroral ovals are located close to the north and south poles where Earth’s magnetic field lines cluster tightly together and intersect the atmosphere vertically. (Hence why many aurora sightings, like those reported on Aurorasaurus, are in Alaska and Iceland!).
For space scientists, aurora provide a useful visible demonstration of the physical relationship a planet has with the environment of outer space. By comparing auroral behavior on other worlds with our own, we can place Earth in a solar system wide context. For example, variations in auroral emission on Earth compare well with variations in solar activity, suggesting the two are strongly linked. As we shall see, however, there is little correlation between the variation in auroral emission at Jupiter and variation in solar activity, suggesting that Jupiter’s aurora are dominated by physical processes which are non-solar in origin– an intriguing find!
For intense aurora to be observed in oval regions around a planet’s magnetic poles we need three principle ingredients: a planetary magnetic field, an atmosphere and a population of high-energy charged particles. Given that the Earth is not the only place in the solar system where these features are found, it stands to reason that auroras should be visible elsewhere, and indeed they are.
GIANT LIGHTS ON A GIANT PLANET
First we look at Jupiter. This massive planet is a great candidate for auroral emissions with its thick atmosphere and strong magnetic field. After the Sun, Jupiter is the single most massive object in the solar system, outweighing all of the other planets combined, and possesses a magnetic field about 20, 000 times stronger than Earth’s. Out of all the planets, Jupiter also rotates the fastest, completing one full day in just 10 hours and its magnetic field rotates with it. Because of the strength and rotation speed of Jupiter’s magnetic field, Jupiter creates a complex and highly energetic magnetic environment in which charged particles move.
Below in Figure 1, we can see bright auroral regions shining in ultra-violet light and encircling Jupiter’s magnetic poles. These images were captured using the Hubble Space Telescope.
As we mentioned, an atmosphere and magnetic field is not enough for auroral activity. A planet also needs high-energy particles. So where do these high-energy particles come from? The most significant single source of them in the solar system is the Sun which releases a constant stream of these particles out into the solar system, called the solar wind. At Jupiter however there is another significant source of particles — Jupiter’s moon Io.
Io is the innermost of the four Galilean Moons, orbiting Jupiter in just 42 hours, at a similar distance above Jupiter’s cloud tops as Earth’s Moon orbits above us. Further out, orbit the three other Galilean satellites, Europa, Ganymede, and Callisto. Io is repeatedly squeezed and stretched by the competing gravitational forces of huge Jupiter and the other large moons. These colossal forces release large amounts of heat in Io’s interior, enough even to melt it! In addition, the constant flexing of the outer crust of the moon weakens and fractures it, allowing the internal heat to escape in the form of active volcanoes which dot Io’s surface.
It is these volcanoes which serve as the primary source for energetic charged particles in the near-Jupiter space environment. Plumes of gas and dust erupted from vents rise under Io’s weak gravity to many hundreds of miles over its surface and are transported away from the moon and are swept up into Jupiter’s rapidly rotating magnetic field. In Figure 2 we can see an artistic impression of Io orbiting within Jupiter’s magnetic field. Material ejected from Io is represented by both the diffuse red clouds and a distinct polar current system (in yellow) which is called the Io plasma torus. This is an electric current which directly connects the moon with the poles of Jupiter and is responsible for the appearance of the bright auroral spot which can be seen to the North and South of the main auroral ovals in each hemisphere. When observed over time, these bright points can be seen to “orbit” the poles at the same rate as Io’s orbit around Jupiter. They are also observed to flare up at times of particularly intense eruptions on Io.
AURORA SHINES OVER SATURN
Scientists have also observed auroral activity on Saturn, as shown in Figure 1. Like Jupiter, Saturn is a massive gas giant planet with a thick atmosphere. It also has a substantial magnetic field of its own which, whilst still much stronger than Earth’s, is weaker than Jupiter’s magnetic field. Saturn’s magnetic field is perfectlyaxisymmetric – it is aligned with the planet’s rotation axis. As a result, the auroral oval regions on Saturn directly encircle the planet’s Northern geographic poles.
Saturn is also more affected by variations in the solar wind, resulting in a stronger correlation between auroral behaviour and solar activity. In short, Saturn represents a kind of middle case between Earth and Jupiter. However, like Jupiter, Saturn also has a family of moons, one of which, Enceladus, is also active with plasma filling Saturn’s magnetic field with charged particles.
Magnificent Lights Over Non-Magnetized Planets
Auroral ovals are a characteristic of all strongly magnetized planets with substantial atmospheres, such as Earth, Jupiter and Saturn, but what about aurora on non-magnetized planets? The planet Venus is an example of such; it has a thick atmosphere but no global magnetic field. Because Venus has no global magnetic field, there is no “funnelling” effect to concentrate energetic particles into auroral ovals over the poles. Instead, auroral emission on Venus is faint and more diffuse as the entire atmosphere is exposed to broadly the same intensity of particles from the solar wind—similar to if you evenly spread a coat of paint on a canvas.
Should you ever find yourself on a trip to one of these worlds don’t forget to take your camera as we’d love to see your extra-terrestrial auroral images!
Questions to Ponder:
- Besides Earth, what planet’s aurora would you like to see and why?
- Mars also lacks a magnetic field. What kind of aurora would you expect to see on Mars?
- How can studying a planet’s aurora help us learn about the planet?
Gareth Dorrian is a post-doctoral research associate at Lancaster University in the UK. He studies the relationship between short-term variations in the Earth’s magnetic field and space weather