Juno mission will study hostile Jupiter

Thursday, 23 June 2016

Next month, NASA's Juno spacecraft will arrive at Jupiter, the biggest planet in the Solar System, where it will move into an orbit that passes over both poles. Using an array of instruments, the goal of the mission is to study the planet's magnetic field and internal structure.

Jupiter is a monster - 2.5 times the mass of all the other seven planets combined and with a diameter nearly 11 times that of the Earth.

A non-stop flight in a jet aircraft from Auckland to London - that is, diagonally across the face of the Earth - would take 21 hours; a similar flight across the face of Jupiter would take 9.5 days. It is a gas giant, which means it is composed largely of low boiling-point materials - about 75 per cent of Jupiter is hydrogen and 24 per cent helium with traces of other gases.

Its gravitational field is powerful enough to have captured 67 moons of which the biggest, the Galilean moons, were discovered by Galileo in 1610. Only one day later, astronomer Simon Marius independently saw these four moons and named them Io, Europa, Ganymede and Callisto.

Jupiter has a dense core about 30 times the mass of the Earth. The composition of the core is not known for certain but it is surrounded by a layer of metallic hydrogen at a temperature of 37,000 degrees Celsius.

The pressure at the core is enormous - 40 million times that of the Earth's atmospheric pressure. Most of the remainder of the planet is molecular hydrogen with the outer 50km comprising cloud layers containing ammonia gas, ammonia crystals and some water vapour.

Jupiter is the fastest spinning planet in the Solar System. It is 318 times more massive than the Earth but it spins its huge bulk on its axis once every 10 hours – that means the equatorial rotation velocity is 45,300km/h, compared to the Earth's 1674km/h. It is spinning fast enough to cause its equator to bulge – the diameter measured at the equator is 9250km larger than that measured pole to pole.

Jupiter is not a solid sphere, which means the rotation of the polar atmosphere is about 5 minutes longer than the equatorial atmosphere. This differential rotation coupled with the high rotation speed generates enormous winds and vortices. Winds of 600km/h are common and there are numerous vortices, the most obvious being the Great Red Spot.

This is an anticyclonic storm, persistent and possibly permanent, and located just south of Jupiter's equator, circulating anticlockwise. The Great Red Spot is about three Earth diameters across and one rotation takes about six days.

Humans might be tempted in the future to approach Jupiter and witness the mesmerising dynamics of its upper atmosphere and Great Red Spot. That would be a mistake because Jupiter has a powerful magnetic field. The source of this is thought to be swirling currents within its metallic hydrogen interior. At the poles, interaction of the magnetic field with charged particles from space can produce very high levels of radiation, enough to destroy the instruments on visiting spacecraft.

The numerous volcanoes on the moon Io and Jupiter's magnetic field are responsible for an intriguing phenomenon. Io's volcanoes emit sulphur dioxide gas which escapes into space and interacts with Jupiter's strong magnetic field to produce charged particles. These charged particles and hydrogen from Jupiter create a hot plasma which forms a torus (doughnut shape) around Jupiter's equator. This plasma torus intermittently emits very strong and damaging bursts of electromagnetic energy.

The Juno spacecraft will take 14 days to complete each orbit. Thirty-seven orbits are planned but, despite the presence of a thick titanium shield to protect the instruments, it is expected some of the equipment will only last about eight orbits before failing due to radiation damage.

The spacecraft has an unusual payload, in addition to a plaque with an illustration of Galileo, Juno also has three specially made Lego pieces depicting Galileo, the Roman god Jupiter and Jupiter's wife Juno. Because of the extreme conditions the spacecraft will encounter the Lego figurines are not made of plastic but aluminium.

All going well, Juno will arrive at Jupiter on July 4.

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Dr Roger Hanson is a New Plymouth-based chemical engineer with a PhD from the University of Cambridge.