Page 24 - Astronomy - October 2017 USA

Jupiter’s magnetic personality rings, but a mostly invisible region built from ionized gas and the
The largest structure in the jovian system isn’t the planet or its
gas giant’s strong magnetic field. The field sweeps charged gas
particles from Io’s atmosphere into a doughnut-shaped ring (red).
Other interactions power an electrical current (green) that arcs
from Jupiter to Io; similar interactions create weaker electrical
links with Europa and Ganymede. JOHN SPENCER

Magnetic field lines

Electrical current
Io torus Jupiter
Neutral
sodium cloud Europa
Io

Ganymede

Callisto

electrical circuit connecting Jupiter and its satellites do not disappoint. Discovered by explored a different solar system.”
moon Io, and it took Voyager 1 deep into Galileo Galilei in 1610, they are Io, Europa, “They’re quite distinct,” says Stone,
the most hazardous radiation belts in the Ganymede, and Callisto (in order of dis- “and I think the one thing we have learned
solar system. Based on measurements from tance from the planet). Ganymede, the is that nature is remarkably diverse, and
the Pioneers, the Voyager design included largest and most massive moon in the solar you don’t see replicas. Each body seems to
shielding that hardened sensitive electron- system, is slightly bigger than Mercury, have its own life history written on the sur-
ics to the bombardment of high-energy while Callisto is nearly the planet’s diame- face and in its interior.”
electrons, protons, and ions stored in ter. Both Io and Europa are roughly the There had long been hints that the big
Jupiter’s equivalent of Earth’s Van Allen size of Earth’s Moon. moons might be doing something interest-
The moons’ ing. Galileo’s 17th-century plots showed
AN EXTRAORDINARY PLANET DESERVES complexity and that Europa and Io always meet up on the
side of Jupiter exactly opposite from where
individuality, which
EXTRAORDINARY MOONS, AND JUPITER’S scientists hardly Europa and Ganymede do. In the 7.15 days
it takes Ganymede to go around Jupiter
suspected despite
FOUR BIG SATELLITES DO NOT DISAPPOINT. centuries of tele- once, Europa orbits twice and Io four
scopic observations, times. This 1:2:4 resonance forces the
proved a major sur- moons’ orbits to maintain a slight eccen-
Belts. But an unprotected human passenger prise of the Voyager missions. Reporting tricity, which in turn causes their bodies to
riding aboard Voyager 1 during closest their results in the journal Science three flex slightly due to tides raised by Jupiter’s
approach would have received a thousand months after the Voyager 1 encounter, the gravity. Just as repeatedly bending a paper
times the lethal radiation dose. imaging team noted that the large moons clip warms up the metal, this forced flexing
do not closely resemble either the planets warms the interiors of Jupiter’s big moons.
Four new worlds in the inner solar system or one another. Stanton Peale at the University of
An extraordinary planet deserves extraor- “The sense of novelty,” they wrote, “would California, Santa Barbara, and his col-
dinary moons, and Jupiter’s four big probably not have been greater had we leagues first described the mechanism in a

24 ASTRONOMY • OCTOBER 2017