Page 27 - Astronomy - October 2017 USA

The multiple-ring impact basin Valhalla dominates this Voyager 1 view Voyager 1 discovered Jupiter’s faint, dusty rings during its flyby, and they
of Jupiter’s outermost major moon, Callisto. The lighter, central area posed for this Voyager 2 portrait once the probe passed into the planet’s
spans about 375 miles (600km) while the outer rings extend about shadow. The curved rings appear orange-red; Jupiter’s multicolored limb
1,100 miles (1,800km) from the center. NASA/JPL comes from the long exposure through two filters. NASA/JPL

And different teams using Hubble in 2012 suggested that tidal heating had powered a frozen remnant of a giant impact. Galileo
and 2016 discovered tantalizing evidence brief period of intense tectonic activity in spacecraft observations indicate the pres-
that Europa occasionally erupts plumes of Ganymede’s icy crust early in its history. ence of a salty, subsurface global ocean
water vapor reaching heights of 125 miles The Galileo mission revealed in 1996 despite little tidal heating at Callisto now.
(200km), suggesting the icy shell may be that Ganymede generates its own perma- Perhaps ammonia and other contaminants
quite thin in some locations. nent magnetic field, the only moon in the lower the freezing point enough for a liquid
The first low-resolution glimpses of solar system known to do so, and therefore layer to survive.
Ganymede revealed two starkly different makes its own miniature magnetosphere.
terrain types. A dark material covers about This complicates the interpretation of its Opening act
35 percent of the surface. Peppered with induced field, but recent models, as well The Jupiter flybys mark the first chapter
impact craters and their bright halos of icy as Hubble observations of Ganymede’s in the Voyagers’ exploration of the outer
ejecta, this is the moon’s geologically oldest aurorae, suggest the interior contains solar system. They provided new views
surface. The rest of the moon features light shells of different phases of water ice sepa- of an enormous, complex, and dynamic
materials crosscut by an intricate patchwork rated by salty seas. atmosphere that is still far from under-
of grooves and ridges. Voyager 1 images Callisto, the farthest of Jupiter’s big stood. They explored a vast magnetosphere
showed that fault lines cut some of the light moons, hosts the solar system’s most heav- loaded with particles from its moons,
bands, which were then offset by surface ily cratered and geologically ancient sur- especially Io, and intimately connected
movement. “There is transverse motion face. Its terrain is nearly saturated with to them. Close-ups of unique new worlds
along these faults,” Soderblom said at a bright impact craters. The largest visible uncovered incredible properties, including
March 6 briefing. “Things get offset, appar- feature, named Valhalla, resembles a bull’s- the first example of active extraterrestrial
ently, for hundreds of kilometers.” This eye about 2,200 miles (3,600km) across, the volcanism and the first clues that frozen
moons could sport internal seas. Further
SPACECRAFT VISITS TO JUPITER discoveries included a faint ring of dust
extending 80,000 miles (129,000km) from
the planet’s center, and two new moons,
Probe Status Agency
Metis and Adrastea, orbiting just beyond
Pioneer 10 Flyby and gravity assist, 1973 NASA
it. The probes also found a third satellite,
Pioneer 11 Flyby and gravity assist, 1974 NASA
Thebe, in a more distant orbit, though still
Voyager 1 Flyby and gravity assist, 1979 NASA well inside Io’s.
Voyager 2 Flyby and gravity assist, 1979 NASA With Jupiter now in the rearview mir-
Ulysses Flyby for gravity assist, 1992 and 2004 NASA/ESA ror, Voyager scientists could begin digging
Galileo Orbiter, 1995 to 2003 NASA deeper into the data — and wondering
what awaited them at their next destina-
Cassini-Huygens Flyby for gravity assist, 2000 NASA/ESA
tion, Saturn.
New Horizons Flyby for gravity assist, 2007 NASA
Juno Orbiter, reached Jupiter in 2016 NASA
Francis Reddy is the senior science writer for
Jupiter Icy Moons Orbiter Orbiter, in development for 2022 launch ESA the Astrophysics Science Division at NASA’s
Europa Clipper Orbiter, in development for 2020s launch NASA Goddard Space Flight Center in Greenbelt,
Maryland.

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