Juno at Jupiter: 7th December 2018

Dr Jonathan Nichols, Leicester University 

The Juno probe has been orbiting Jupiter since July 2016. All previous probes have stayed in the equatorial plane, but Juno is in a polar orbit passing inside the main radiation belts. The Ulysses probe is the only other spacecraft to have observed Jupiter’s poles, during its 1992 fly-by of Jupiter. In 1995 the Galileo mission sent a probe into the atmosphere to look for water, but found very little, probably because the entry point was a “downdraught hotspot”, equivalent to a high-pressure region on Earth. We know Jupiter formed somewhere outside the “ice line” and accreted both rocky and icy material, but determining Jupiter’s make-up will tell us more precisely where it formed. Juno’s mission goals are to investigate the origin of Jupiter, map the gravitational potential to study its internal structure, look below the cloud layers to measure the atmospheric composition and determine how much water is present, and the study the magnetosphere.

Juno was launched on August 5th 2011 and arrived at Jupiter on July 4th 2016. After orbit insertion, a further burn to reduce the orbital period from 53 to 14 days was abandoned due to a faulty fuel pump, but the scientific mission will still consist of 32 orbits as intended. Juno is the first outer-planet mission to be solar powered, and has three panels with a total of 18,000 solar cells providing 500 watts of electrical power. The polar orbit ensures Juno is in continuous sunlight, and allows close fly-bys, 5000 km above the cloud tops but below the most intense parts of the radiation belt. This orbit minimises exposure to damaging high-energy particles, but the most vulnerable electronics are mounted inside a 15mm thick titanium vault to give increased protection from radiation.

Juno’s instruments include: a gravity/radio science system; a multi-band microwave radiometer to measure emissions from Jupiter; detectors for plasma and charged particles; ultraviolet and infrared imaging spectrometers; a vector magnetometer and the JunoCam, a public-access imaging system. The magnetometer is mounted on the end of one solar panel, but the other instruments are inside the radiation vault. All the instruments can take data with the spacecraft oriented so the solar panels are in full sunlight. Juno also carries titanium Lego figures of Galileo, Juno and Jupiter.

The gravity instrument monitors the Doppler shift of radio signals, which allows mapping of the local gravitational field and hence mass variations. The results show that atmospheric structures extend at least 3000 km down from the cloud tops. Below that the planet appears to rotate at a constant speed similar to a solid body, which was not expected. The data also suggest that Jupiter has a small core of metallic hydrogen and helium, which is not fully separated from the mantle.

The radiometer can identify water by its absorption of the microwaves that Jupiter emits. The data show that visible cloud structures extend about 500 km down into the atmosphere. The instrument has also detected a huge equatorial plume of ammonia, seen a lot of structure in the auroras in the infrared and found that the auroral footprint of Io has a vortex structure. None of these features are understood at present. Infra-red images show that the polar vortex is surrounded by eight other storm systems, all rotating cyclonically. There are thus opposing flows where the systems meet, which is an unstable situation.

The polar orbit is optimum for measuring variations in the magnetic field, and also gives the best views of the auroras. Initial results show the field has twice the expected strength, and there seem to be two north magnetic poles, but only one south pole. This implies the field is generated close to the planet’s surface, and also that Jupiter has relatively high electrical conductivity.

JunoCam was a last-minute addition to the orbiter, and the camera is not protected inside the radiation vault. It has no science team: target selection and image processing are all done by public vote on the Juno website. Images show chaotic clouds around both poles, and no distinct belts at high latitudes. Storms are seen to have variable cloud colours: ammonia clouds are initially white, but become orange and yellow after UV light induces reactions with organic materials. Tall cumulus-type clouds have been observed to develop, casting long shadows, and these may be precursors of thunderstorms. Low pressure at the poles leads to convection and lightning: the separation of electrical charges implies that water is present there. The Great Red Spot is a huge high-pressure system that extends deep into the atmosphere. Its long duration has allowed plenty of time for photolysis reactions to generate the materials that give it its colour.

Juno has so far revealed many new and unexpected aspects of Jupiter, and is still only halfway through its mission. Eventually the radiation will cause the electronics to fail, after which, like Cassini at Saturn, the spacecraft will be de-orbited into Jupiter, to avoid contaminating any of the moons.

Notes and summary by Chris Hooker.