Strong aurorae on Jupiter: a heater for the gas planet

For around half a century, planetary researchers have been wondering why the upper layers of the atmosphere of Jupiter are so hot.

The gas planet is around five times as far from the sun as the earth and therefore receives only a fraction of the radiation energy from our central star.

Therefore, its upper atmosphere should actually have a temperature below minus 70 degrees Celsius.

Instead, it is around 420 degrees on average.

There are now several theories about what makes the upper gas layers on the giant planet so hot. A popular hypothesis consists of gravity waves, i.e. wave-like fluctuations between the various layers of air. In the turbulent atmosphere of a gas giant, such waves could transport large amounts of energy and greatly heat the uppermost layers of the atmosphere. It has also been speculated that cyclones like the Great Red Spot could shovel energy into the upper atmosphere.

Another hypothesis indicated the influence of the strong polar lights (aurorae) on the polar ice caps.

These are - due to the enormous magnetic fields of Jupiter - the strongest in our solar system.

However, until now it was unclear whether the energy that is released in this way at the poles can also spread through the rest of the high atmosphere.

Finally, various atmospheric models predict that strong winds along the latitudes should prevent the hot gas masses from being distributed over most of the planet.

High resolution temperature maps

An international group of researchers has now found strong evidence that the northern lights are indeed responsible for the high temperatures. The researchers report this in the journal Nature. To do this, they had to create the most detailed temperature maps of Jupiter to date, for which they combined data from the space probes Juno and Hisaki as well as two multi-hour observations with the ten-meter “Keck” telescope in Hawaii. The last such temperature maps came from the 1990s and were only a few pixels in size, which is why no clear information on the temperature distribution on Jupiter, especially between its poles and the equator, could be obtained from them.Thanks to the technological leaps that have been achieved in the meantime, the new maps have a good ten times higher resolution of around two degrees of latitude and longitude on Jupiter, and the temperature data are also much more accurate.

These maps not only clearly show that the aurorae at the poles have very high temperatures of over 700 degrees Celsius. There was also - as is sometimes necessary in science - a bit of luck: On one of the two high-precision images with the Keck telescope it became apparent that hot gas masses were flowing from the pole towards the equator. This contradicts some atmospheric models, according to which strong winds along the latitudes should suppress such a spread. If the recording had taken place on another day when the flow conditions would have been weaker, this would have been barely visible, according to the researchers.

"Most global circulation models of Jupiter's upper atmosphere cannot explain the distribution of heat from the aurorae to the rest of the planet," says James O'Donoghue of the Japanese space agency JAXA and first author of the publication.

Because the gas masses on Jupiter rotate extremely quickly - a day there has less than ten hours despite the planet's size.

The footprint of Jupiter's moons

The new results indicate that the circulation models need to be supplemented, as they do not adequately take into account the gas transport away from the poles.

Because the northern lights convert huge amounts of energy.

The origin of the bright luminous phenomena is not only - as on earth - in the solar wind, which is channeled to the poles by the magnetic field lines.

Jupiter's moons also play an important role. Above all, the volcanically active Io feeds many particles into the orbit of the gas giant, especially sulfur and oxygen ions. Since Jupiter's magnetic field is around twenty times stronger than that of Earth, this creates spectacular auroras when these particles hit the upper layers of the atmosphere. But other moons such as Europe, Ganymede or Callisto also leave their “footprints” in Jupiter's northern lights, which therefore shine more constantly than those on Earth.

In order to be able to better understand the flow conditions on Jupiter, longer-term, high-resolution measurement campaigns will be necessary in the future. "The whole planet shows a fairly high variability, so that in addition to a constant heating process, individual phases with strong currents could also be responsible for the transport of energy to the equator," says O'Donoghue. This can only be captured with measurements over many days.