Page 25 - Astronomy - October 2017 USA

The Galilean moons

Io Europa Ganymede Callisto
The solar system’s most Likely supports an The solar system’s largest The solar system’s third-
volcanic body; 5 percent ocean beneath an icy satellite, 8 percent larger largest moon, 1 percent
larger than Earth’s Moon. crust; 90 percent the than the planet Mercury; smaller than Mercury; it
size of Earth’s Moon. it is the only one known to has the oldest surface and
0 1,000 2,000 miles generate a magnetic field. is nearly saturated with
impact craters.
3,000km
2,160 miles
Diameter of Earth’s Moon The Voyager flybys brought Jupiter’s diverse family of moons into sharp
focus. Observations of the four Galilean satellites proved to be as much
3,476km 3,270 miles
Diameter of Ganymede of a revelation as those of Jupiter itself. NASA/JPL
0 5,562km

paper submitted to Science in January 1979.
They presciently concluded: “Voyager
images of Io may reveal evidence for a plan-
etary structure and history dramatically
different from any previously observed.”
The paper was published March 2, three
days before Voyager 1’s closest approach.
By then, images of the weirdly colored
moon, reminiscent of a cheese pizza,
already were making headlines. Scientists
had expected Io to resemble our Moon,
but it lacked anything that looked like an
impact crater and was coated with richly
colored deposits that wouldn’t look out of
place around active fumaroles on Earth.
On March 8, as Voyager 1 raced out of Giant Ganymede — larger than Mercury —
the Jupiter system, its camera captured a sports two different types of terrain. The dark
material is oldest, while the lighter bands cut
routine navigation image of a crescent Io as by grooves and ridges are significantly younger.
part of a program to refine knowledge of A relatively recent impact threw out the bright The Voyagers revealed dark streaks crisscrossing
the spacecraft’s trajectory. The next day, material at bottom. NASA/JPL the surface of Europa, the smallest of Jupiter’s
JPL engineer Linda Morabito enhanced Galilean moons. In some cases, the streaks
this image to locate background stars and stretch more than 1,000 miles (1,600km).
Scientists later deduced that the streaks are
uncovered another crescent shape on Io’s ejecta associated with a heart-shaped vol- cracks in the moon’s icy crust filled with material
sunlit limb. It looked like the edge of cano now known as Pele. Remarkably, the from its interior. NASA/JPL/USGS
another satellite peeking out from behind same picture also held a second plume, one
Io, but scientists determined that an erupting from a feature now called Loki
unknown satellite so large would have been and catching sunlight as it rose high over paint Io’s terrain in garish hues, and the
detectable from Earth. “No one understood Io’s night side. rest forms a thin, distended atmosphere
what they were seeing, reinforcing the Scientists now know of at least 150 around the moon. Particle interactions ion-
degree of difficulty associated with inter- active volcanoes on Io. Some of them blast ize some of these atoms, and they then
preting this image,” she later wrote. umbrella-shaped plumes containing become swept up in Jupiter’s fast-moving
By March 12, the imaging team had sodium, potassium, sulfur, sulfur dioxide, magnetic field, which rotates with the
confirmed the presence of active volcanoes. and more to altitudes as high as 310 miles planet’s 10-hour rotation. “About a ton per
What Morabito had noticed was a plume of (500km). Some of the ejecta falls back to second of that material is picked up by the

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