A (6)

18 AUSTRALIAN SKY & TELESCOPE JULY 2016

didn’t understand Io’s role until another spacecraft’s

brief visit in 1979.

That’s when Voyager 1 revealed Io’s prodigious

volcanic activity and found that more than 1 tonne of

sulfur dioxide (SO 2 ) escapes from this moon’s tenuous

atmosphere per second. These neutral molecules are

soon dissociated into sulfur and oxygen atoms, which

in turn become ionised and trapped by the magnetic

field. The resulting torus of plasma — a huge

doughnut of charged particles — roughly co-rotates

with Jupiter’s 10-hour spin period. The ions of sulfur

and oxygen become excited during their frequent

collisions with the magnetosphere’s trapped electrons,

and consequently they radiate about 1½ terawatts of

ultraviolet energy.

This plasma couples to Jupiter’s rotating

atmosphere (specifically, to its ionosphere) via electric

currents, and this coupling dominates the dynamics of

the magnetosphere. One consequence is that energetic

electrons bombard hydrogen molecules in the upper

atmosphere, triggering intense auroral emissions that

span the spectrum from X-ray to radio wavelengths.

Dramatic images from the Hubble Space Telescope’s

ultraviolet cameras show that Jupiter’s intense

aurorae are always active. Meanwhile, ground-based

observations of infrared emissions from the unusual

molecular ion H 3 +have revealed an auroral spot at the

foot of the magnetic field lines connected to Io. Aha!

This Jupiter-Io connection must be the source of the

radio bursts that astronomers have been monitoring

all these decades. Hubble images also show auroral

spots associated with Europa and Ganymede, indicating

further electrical current systems.

These various auroral emissions tell us that beams

of energetic electrons and ions are shooting into

Jupiter’s atmosphere. We suspect that these charged

SOPHISTICATED
SCIENCEJuno’s
seven scientific
instruments,
combined with
radio tracking
to map Jupiter’s
gravity field and its
ride-along imager,
should answer
many longstanding
questions about the
King of Planets.

spinaxisandwithapolarityoppositethatofEarth’s.

The magnetosphere’s most energetic electrons,

pumped to more than a million electron volts (1 MeV),

are trapped near the equator and close to the planet.

These very energetic particles pose a formidable hazard

for spacecraft exploring close to Jupiter.

Ground-based radio observations in 1964 revealed

thatIoplaysapeculiarrole,inthattheburstsofradio

emission were modulated by the position of this moon

along its 42-hour orbit around Jupiter. When Pioneers

10 and 11 flew past Jupiter in the mid-1970s, their

magnetometers and particle detectors revealed the

vastness of Jupiter’s magnetosphere and made direct

measurements of energetic ions and electrons. But we

Orbit 1

Orbit 16

Orbit 31

Perijove

(4,200 km

above clouds)

Trapped

Jupiter’s magnetosphere charged particles

LIGHT SHOWLeft:The complex aurorae at Jupiter’s north pole stand out clearly in this ultraviolet image obtained with the Hubble Space

Telescope’s Imaging Spectrograph (STIS) on November 26, 1998. Auroral ‘hot spots’ mark where electric currents magnetically tied to Io, Europa

and Ganymede intersect the planet’s upper atmosphere. Right: Juno’s orbit is the key to this mission’s success. By moving in a highly eccentric polar

orbit, the spacecraft minimises its exposure to high-energy charged particles yet gets near enough to study the planet at close range. But orbital

precession due to Jupiter’s gravity will eventually drag the orbit through those intense radiation zones, dooming the spacecraft.

Io footprint Polar aurora

Main

auroral oval

Ganymede

footprint

Europa

footprint

JunoCam

JIRAM

MWR

JEDI

Waves

UVS

JADE

JADE

Gravity Science

Magnetometer

S&T:

LEAH TISCIONE, SOURCE: NASA / JPL / SWRI

NASA / JOHN CLARKE

NASA / JPL /

SWRI (2)

Juno at Jupiter