SCIENCE sciencemag.org 18 JANUARY 2019 • VOL 363 ISSUE 6424 215
Iess and his team thought measuring those
shifts wouldn’t be feasible at Saturn be-
cause of the gravitational influence of its
rings. But that obstacle disappeared earlier
this decade, after the Cassini team decided
to end the mission by sending the craft
on a series of orbits, dubbed the Grand
Finale, that dipped below the rings and
eliminated their effects. As a result, Iess
and colleagues could use radio fluctuations
to map the shape of gravity fields at both
planets, allowing them to infer the density
and movements of material deep inside.
One goal was to probe the roots of the
powerful winds that whip clouds on the
gas giants into distinct horizontal bands.
Scientists assumed the winds would either
be shallow, like winds on Earth, or very
deep, penetrating tens of thousands of kilo-
meters into the planets, where extreme
pressure is expected to rip the electrons
from hydrogen, turning it into a metal-
like conductor. The results for Jupiter
were a puzzle: The 500-kilometer-per-hour
winds aren’t shallow, but they reach just
3000 kilometers into the planet, some 4%
of its radius. Saturn then delivered a differ-
ent mystery: Despite its smaller volume, its
surface winds, which top out at 1800 kilo-
meters per hour, go three times deeper, to
at least 9000 kilometers. “Everybody was
caught by surprise,” Iess says.
Scientists think the explanation for both
findings lies in the planets’ deep magnetic
fields. At pressures of about 100,000 times
that of Earth’s atmosphere—well short
of those that create metallic hydrogen—
hydrogen partially ionizes, turning it into
a semiconductor. That allows the mag-
netic field to control the movement of the
material, preventing it from crossing the
field lines. “The magnetic field freezes the
flow,” and the planet becomes rigid, says
Yohai Kaspi, a planetary scientist at the
Weizmann Institute of Science in Rehovot,
Israel, who worked with Iess. Jupiter has
three times Saturn’s mass, which causes
a far more rapid increase in atmospheric
pressure—about three times faster. “It’s ba-
sically the same result,” says Kaspi, but the
rigidity sets in at a shallower depth.
The Juno and Cassini data yield only
faint clues about greater depths. Scientists
once believed the gas giants formed much
like Earth, building up a rocky core before
vacuuming gas from the protoplanetary
disc. Such a stately process would have
likely led to distinct layers, including a
discrete core enriched in heavier elements.
But Juno’s measurements, interpreted
through models, suggested Jupiter’s core
has only a fuzzy boundary, its heavy ele-
ments tapering off for up to half its radius.
This suggests that rather than forming a
rocky core and then adding gas, Jupiter
might have taken shape from vaporized
rock and gas right from the start, says
Nadine Nettelmann, a planetary scientist
at the University of Rostock in Germany.
The picture is still murkier for Saturn.
Cassini data hint that its core could have a
mass of some 15 to 18 times that of Earth,
with a higher concentration of heavy ele-
ments than Jupiter’s, which could suggest
a clearer boundary. But that interpreta-
tion is tentative, says David Stevenson, a
planetary scientist at the California Insti-
tute of Technology in Pasadena and a co-
investigator on Juno. What’s more, Cas-
sini was tugged by something deep within
Saturn that could not be explained by the
winds, Iess says. “We call it the dark side of
Saturn’s gravity.” Whatever is causing this
tug, Stevenson adds, it’s not found on Ju-piter. “It is a major result. I don’t think we
understand it yet.”
Because Cassini’s mission ended with
the Grand Finale, which culminated with
the probe’s destruction in Saturn’s atmo-
sphere, “There’s not going to be a better
measurement anytime soon,” says Chris
Mankovich, a planetary scientist at the
University of California, Santa Cruz. But
although the rings complicated the gravity
measurements, they also offer an opportu-
nity. For some unknown reason—perhaps
its winds, perhaps the pull of its many
moons—Saturn vibrates. The gravitational
influence of those oscillations minutely
warps the shape of its rings into a pattern
like the spiraling arms of a galaxy. The re-
sult is a visible record of the vibrations,
like the trace on a seismograph, which sci-
entists can decipher to plumb the planet.
Mankovich says it’s clear that some of these
vibrations reach the deep interior, and he
has already used “ring seismology” to esti-
mate how fast Saturn’s interior rotates.
Cassini’s last gift may be to show how
fortunate scientists are to have the rings
as probes. Data from the spacecraft’s fi-
nal orbits enabled Iess’s team to show the
rings are low in mass, which means they
must be young, as little as 10 million years
old—otherwise, encroaching interplanetary
soot would have darkened them (Science,
22 December 2017, p. 1513). They continue
to rain material onto Saturn, the Cassini
team has found, which could one day lead
to their demise. But for now they stand
brilliant against the gas giant, with more
stories to tell. jMaterial thousands of kilometers below the clouds of
Jupiter and Saturn tugs subtly on orbiting spacecraft,
revealing hidden structure and motions.Published by AAASon January 21, 2019^http://science.sciencemag.org/Downloaded from