Sky and Telescope - July 2016 - 22

Juno at Jupiter

Waves

Magnetometer
JIRAM

NASA / JPL / SWRI (2)

Gravity Science

JADE

JunoCam

UVS

SOPHISTICATED SCIENCE Juno'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.

Astronomers ﬁrst detected the magnetosphere of
Jupiter in 1954, four years before Explorer 1's discovery of
the Van Allen radiation belts around Earth, via bursts of
radio emission. These early radio measurements showed
that Jupiter has a strong magnetic ﬁeld with an axis
tilted about 10° from the spin axis and with a polarity
opposite that of Earth'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.
Io footprint

Polar aurora

Ground-based radio observations in 1964 revealed
that Io plays a peculiar role, in that the bursts of radio
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 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 ton of sulfur
dioxide (SO2) escapes from this moon's tenuous atmosphere per second. These neutral molecules are soon
dissociated into sulfur and oxygen atoms, which in turn
become ionized 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 auroras are
constantly active. Meanwhile, ground-based observations
of infrared emissions from the unusual molecular ion H3+
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 Gany-

Orbit 1

Perijove
(4,200 km
above clouds)

Orbit 16

NASA / JOHN CLARKE

Ganymede
footprint

Main
auroral oval

Orbit 31

Europa
footprint
Jupiter's magnetosphere

Trapped
charged particles

LIGHT SHOW Left: The complex auroras 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 minimizes 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.

July 2016 sky & telescope

S&T: LEAH TISCIONE, SOURCE: NASA / JPL / SWRI

MWR
JEDI

Sky and Telescope - July 2016

Table of Contents for the Digital Edition of Sky and Telescope - July 2016