Page 26 - Astronomy - October 2017 USA

A giant’s ghostly rings Jupiter’s ghostly rings lie well inside the orbits of the Galilean moons, though four much
smaller moons orbit among the tiny dust particles. The Voyager spacecraft discovered
these rings along with the moons Metis, Adrastea, and Thebe. ASTRONOMY: ROEN KELLY; IMAGES: NASA/JPL

Main ring

NASA’S New Horizons probe found
Halo a gap in the main ring during its
2007 flyby. This may be caused by
Gap an unknown moon. NASA/JHUAPL/SWRI

Main ring Thebe ring Thebe extension
Halo ring
Amalthea ring

Adrastea Amalthea Thebe

Metis

0 5,000 10,000 miles
0 300 miles
0 10,000 20,000km
Thebe
Adrastea
0 500km
Rings are labeled in yellow.
Moons are labeled in white. Amalthea Sulfur debris from Io’s eruptions
Metis
may cause the moons’ red color.

jovian magnetic field, and that mass of stuff a spot of infrared emission in Jupiter’s and its images revealed frozen plains criss-
inflates the Jupiter magnetosphere to about polar atmosphere. The glow tracked with crossed by dark streaks, giving it the look
twice the size it should be,” Stone says. Io in its orbit and arose from energy cours- of a cracked egg. Europa’s surface is the
The ionized gas spreads along Io’s orbit ing down the flux tube. But Io isn’t alone in smoothest in the solar system. Features
to form a doughnut-shaped cloud called this regard. In 2002, Hubble imaged Io’s display so little topographical relief that
the Io plasma torus. Some of the heavy ions spot in the UV and found two more glows imaging team member Larry Soderblom
in the torus migrate outward, and their from Europa and Ganymede, showing they compared the moon to a billiard ball. Later
pressure supersizes the magnetosphere. As generate their own flux tubes. “The system that year, the scientists who explained the
Io moves through the torus, it continuously is highly coupled and connected, where the heating of Io suggested that tidal flexing of
magnetic fields and Europa could provide enough heat to sus-
THE JUPITER FLYBYS MARK THE FIRST the particles are all tain an ocean beneath its icy shell, which is
interacting with the
widely thought to be 10 to 15 miles (16 to
CHAPTER IN THE VOYAGER’S EXPLORATION moons,” Stone says. 24km) thick. The ocean itself may be at
And the phenom-
least 30 miles (48km) deep, or more than
OF THE OUTER SOLAR SYSTEM. enon isn’t unique to 10 times the average depth of Earth’s seas.
Jupiter. In 2011, sci- In fact, several lines of evidence now
entists identified a support the presence of briny global oceans
generates an electrical current that flows UV spot associated with the active moon within Europa, Ganymede, and Callisto,
along a conduit, called the Io flux tube, Enceladus in images from the Cassini mis- each containing more water than Earth’s
linked to Jupiter’s upper atmosphere. Two sion orbiting Saturn. seas. NASA’s Galileo spacecraft, which in
billion kilowatts flow through the flux tube, 1995 became the first to orbit Jupiter, flew
comparable to the average global power The outer trio close to these moons and found that
consumption on Earth. The Voyager 1 Europa, the next moon out from Jupiter, Jupiter’s rotating magnetic field induces
team deliberately tried to pass through the couldn’t be more different from its siblings. currents in electrically conducting layers
tube, but the material around Io shifted its Low-resolution images from Voyager 1 within them. These currents, in turn, gen-
position from what was expected, and the showed a bright surface of frozen water erate secondary magnetic fields Galileo
spacecraft instead flew alongside it. with no discernible craters, along with could detect. Europa’s induced response
While imaging from Earth in 1993, hints of dark linear features. Voyager 2 matches what researchers would expect for
Connerney and his colleagues discovered passed much closer to Europa on July 9, a salty subsurface ocean many miles thick.

26 ASTRONOMY • OCTOBER 2017