July 3, 2017

Juno unveils some of Jupiter's secrets

NASA’s Juno probe has been studying Jupiter’s structure and composition since July 2016. Its orbital trajectory brings it between the planet and its rings at an altitude of 5,000 km. During its mission, it will complete a total of 32 orbits before dropping into the planet’s atmosphere in 2021. Several papers have just been published about the first 2 flyovers. Tristan Guillot is an astrophysicist at the Observatoire de la Côte d’Azur in Nice, France; he co-authored 3 of these papers and helps us understand the probe’s first results.

Jupiter has already been observed by several probes; what’s new with Juno?

Tristan Guillot: Everything, actually. Juno is the first mission entirely focused on the planet itself instead of the entire Jupiter system with its moons and its environment. Most of the previously obtained data come from the Galileo, Pioneer, Voyager, and Cassini missions. These probes flew further from Jupiter and closer to its equatorial plane, whereas Juno flies over the poles. This trajectory is much more interesting when it comes to measuring the magnetic field and mapping the entire planet.

Some of the results from previous missions who measured the gravitational field contradicted each other. Juno’s observations are 10 times as accurate and can solve this mystery. As an example, the following figure compares Juno’s margin of error to that of some of the past missions.

Bolton et al., Science 356, 821–825 (2017) Jupiter’s interior and deep atmosphere: The initial pole-to-pole passes with the Juno spacecraft

These new observations are made using the Doppler Effect: as the probe gets closer to Earth, its radio signal shifts towards shorter wavelengths; and as it moves away, the wavelength increases. We can measure the probe’s exact speed and deduce the acceleration caused by Jupiter’s gravitational field.

Furthermore, Jupiter’s magnetic field turned out to be much more complex than what had been detected from further out. On Juno’s first flyover, it measured a magnetic field whose amplitude was 50% higher than previously estimated. This disparity stems from the fact that further out, instruments can only detect the primary dipole; as they get closer, they can detect smaller field loops. This helps better understand the dynamo effect that generates planets’ magnetic field. Since the field is generated by moving matter, we’re hoping to link this data with gravitational measurements and through them to the planet’s structure.

And finally, Juno is equipped with microwave antennae to sound Jupiter’s deep atmosphere for the first time ever.

In what way do these observations change our understanding of Jupiter?

TG: Previous models suggested that Jupiter contained a heavy core under a homogenous layer of hydrogen and helium. We know that these two elements represent close to 90% of the planet’s mass. Various estimations indicate the presence of heavy and solid elements up to 10 to 40 times the mass of the Earth. The latest gravitational data seems to indicate that these heavy elements aren’t in fact concentrated in a central core but rather distributed along outer layers. This could mean that they were captured over time during the formation of the planet or that they eroded slowly from the core. However, this theory has yet to be confirmed with more accurate analysis of the planet’s gravitational field and internal rotation, which can affect these observations. This is our primary objective for this year.

For now, in my opinion, Juno’s greatest achievement is the sounding of Jupiter’s deep atmosphere using microwave antennae. These observations track ammonia content, the molecule which composes Jupiter’s visible clouds. The planet seems to contain large amounts of ammonia near its poles, and less around the equator. This would indicate ascending movements near the equator and the opposite everywhere else, which is in complete opposition with what we previously imagined.

What about water on Jupiter?

TG: Jupiter’s water content has been one of the planet’s greatest secrets ever since Galileo measured only very small amounts during its fall through the atmosphere in 2003. Since then, we don’t know if Jupiter actually contains only small amounts of water vapour. It is possible that Galileo happened to fall through an abnormally dry area. But the theory is that water played a dominant role in the formation of planets in the Solar System. One of Juno’s primary objectives is to measure the presence of water on Jupiter. Unfortunately, for now, these measurements are impeded by the deep atmosphere’s unexpected complexity. But a lot of data has yet to be processed and we will do our best to determine the planet’s water content using data sent back by Juno.

The mission was originally scheduled to end in 2018. Why has it been extended to 2021?

At first, it was a malfunction. Instead of orbiting Jupiter every 14 days as planned, the probe had to stay on a 53-day orbit. When it was time to transfer to faster orbits, NASA detected a malfunction on the helium valves which support the primary engine: they didn’t open exactly when the command was given. NASA decided not to activate the engine for safety reasons, and Juno stayed on its 53-day orbit. Surprisingly, this orbit is proving to be more interesting from a scientific point of view. What started as a malfunction turned into an opportunity to sound the planet’s magnetosphere with more accuracy. The extended mission duration will allow Juno to measure variations over a longer timeframe. We will have extra time to detect magnetic field anomalies which could indicate convective motion inside the planet.

Apart from this incident, the mission is going smoothly. Damage suffered in the radiation belts proved less severe than anticipated so the spacecraft is in relatively good shape. The mission should operate without a hitch until 2021, after which it will dive into Jupiter’s atmosphere to avoid contaminating its moons, particularly Europa and its internal ocean where we hope to one day study the presence of life. Juno will use its descent to study Jupiter’s atmosphere closely one last time, just like the Cassini probe should be doing on Saturn next September.

Jupiter captured by Juno on 11th December 2016 ©NASA/JPL-Caltech/SwRI/MSSS/Gabriel Fiset


- Bolton et al., Science 356, 821–825 (2017) Jupiter’s interior and deep atmosphere: The initial pole-to-pole passes with the Juno spacecraft

- Wahl, S. M. et al. (2017), Comparing Jupiter interior structure models to Juno gravity measurements and the role of a dilute core, Geophys. Res. Lett., 44, doi:10.1002/2017GL073160. 

- Kaspi Y. et al., (2017), The effect of differential rotation on Jupiter's low-degree even gravity moments, Geophys. Res. Lett., 44, doi:10.1002/2017GL073629.


  • Tristan Guillot, CNRS research director at the Observatoire de la Côte d'Azur: tristan.guillot at oca.eu
  • Francis Rocard, CNES Solar System Programme Manager: francis.rocard at cnes.fr