One of our favorite topics here is the northern lights. Although auroras are uncommon in middle northern latitudes, you can always take a plane or train up to Churchill on Hudson Bay and see them nearly every night of the year. If you could get in a rocket ship and launch for Venus, Mars, Jupiter, Saturn, Uranus and Neptune, you’d see auroras in those places as well.
“See” might be too strong a term. Venusian auroras are very dim, while those on all the others, save Mars, are bright in ultraviolet light but would either not be visible or appear faint to the naked eye. Mars is the exception: during intense solar storms, an astronaut there would see deep blue aurora dabbed with hues of pink and green. Why blue? Excitation of carbon dioxide in Mars’ atmosphere by speeding electrons in the solar wind.
Auroras on Earth, Jupiter and Saturn have been well-studied but not so on the ice-giant planet Uranus. In 2011, the Hubble Space Telescope took the first-ever image of the auroras on Uranus. Then in 2012 and 2014 a team from the Paris Observatory took a second look at the auroras in ultraviolet light using the Space Telescope Imaging Spectrograph (STIS) on Hubble and stumbled upon the brightest auroras ever seen there.
The dot-like and patchy auroras look different from those we see on Earth. They also appear to be located at random spots in the planet’s upper atmosphere — but they’re not. The 2011 blobs were close to Uranus’s north magnetic pole; those in 2012 and 2014 near its south magnetic pole just as we see them on our home planet.
There are two reasons for the random look: Uranus is cloaked in nearly featureless, methane-tinted clouds, making it tricky to know at a glance where north or south is. The planet’s axis is also tilted 98°, so it rotates sideways not right side up as do all the other planets. Add the fact that Uranus’s magnetic field is inclined 59° to its spin axis, and you can see why it’s no simple matter to find the planet’s magnetic poles. Besides photographing the auroras, the team re-discovered the poles, which were lost shortly after their discovery by the Voyager 2 probe in 1986.
In 2012 and 2014, two powerful bursts of solar wind traveling from the sun to Uranus stoked those intense auroras. As on Earth, Uranian auroras occur when streams of charged particles like electrons carried by the solar wind gets caught in powerful magnetic fields surrounding a planet and channeled into the upper atmosphere. There, the particles interact with atmospheric gases and set the air aglow.
Here on Earth, they bash into oxygen and nitrogen atoms to create the familiar auroral pinks and greens. On Uranus, hydrogen, which comprises 82.5% of its atmosphere, sets the sky alight. Hydrogen auroras are faint or invisible to the naked eye but bright in ultraviolet light. The atmospheres of Jupiter and Saturn are also rich in hydrogen and display massive UV auroras.
An auroral display can last for hours here on the home planet, but in the case of the 2011 Uranian lights, they pulsed for just minutes before fading away.
Want to know more about the northern and southern lights on a planet nearly 2 billion miles away today? Read the team’s findings in detail here.