Page 23 - Astronomy - October 2017 USA

UNDER THE VEIL

NASA’s Juno mission entered an elon- “On Jupiter, a gas planet, there is no
gated 53-day orbit around Jupiter on iron-bearing crust between us and the
July 4, 2016. The spacecraft is mapping dynamo, and the dynamo is much, much
the planet’s gravitational and magnetic closer to the surface of Jupiter than is the
fields in unprecedented detail, allowing case in the Earth,” says John Connerney
scientists to understand how material at NASA’s Goddard Space Flight Center,
moves deep within the planet. Juno also lead scientist for Juno’s magnetometer
is measuring atmospheric dynamics to instrument. Initially, scientists thought
pressures beyond 100 bars, far below Jupiter’s dynamo might be between
Jupiter’s visible clouds, and providing 15 and 25 percent of the way down, or
scientists with an improved upper limit 0.85 to 0.75 Jupiter radii from the plan-
on the mass of a possible solid core. et’s center. “More contemporary thinking
Meanwhile, Juno’s magnetometer is is that maybe the dynamo is actually
creating the first 3-D map of the planet’s closer, up to 0.93 jovian radii, in which
magnetic field, providing additional case it’s really just beneath the visible
insights into Jupiter’s internal structure. surface,” adds Connerney.
The movement of electrically charged To perform its mission, Juno flies over
material, called a dynamo, generates the Jupiter’s poles, passing about 2,600 miles
field somewhere within the planet. (4,100 kilometers) from its cloud tops.
Juno’s observations could provide sci- The elongated polar orbit helps limit the
entists with their most detailed look at a spacecraft’s exposure to Jupiter’s punish-
planetary dynamo. Earth’s dynamo lies ing radiation belts, but more protection
about halfway to its center, but the still was needed. So Juno’s critical elec- The Juno spacecraft’s camera preferentially targets
abundant iron-bearing materials in the tronics are placed in a radiation-shielded Jupiter’s poles. This view of the south pole reveals
crust makes mapping its location from “vault,” a titanium cube with walls a third several oval cyclones up to 600 miles (1,000km) in diameter.
orbit complicated. of an inch (1 centimeter) thick. — F. R. NASA/JPL-CALTECH/SWRI/MSSS/BETSY ASHER HALL/GERVASIO ROBLES

completely away, its high-density materials charged particles inside it, forming a vast, look up and identify Jupiter in the sky, its
dispersed throughout a larger portion of comet-shaped bubble — called a magneto- magnetosphere would be about the size of
the planet. One of the main goals of NASA’s sphere — that shields the planet from direct the Moon,” says Goddard’s John Connerney,
Juno mission, which has been orbiting the exposure to the solar wind. Pressure from deputy principal investigator for the Juno
planet since July 2016, is to answer the the solar wind pushes the Sun-facing side mission. Its bow shock typically lies about
many remaining questions about how the into a rounded bow shock that slows and 40 Jupiter diameters sunward of the planet
solar system’s largest world is put together. deflects most of the incoming charged par- and its magnetotail stretches some 3,500
(See “Under the veil,” above.) ticles in much the same way that water diameters behind it, nearly reaching Saturn’s
flows around the bow of a moving ship. orbit. We know this because as Voyager 2
Magnetic tango The opposite side tapers into an immense approached Saturn in 1981, two years and
Like Earth, Jupiter generates a magnetic magnetotail whose farthest portions wave 400 million miles (650 million km) after its
field, which at the cloud tops is about 15 and flap like the tattered end of a windsock. Jupiter encounter, the spacecraft slipped in
times stronger than our planet’s. The field Jupiter’s magnetosphere is the solar sys- and out of Jupiter’s frayed magnetotail
traps, stores, and controls the flow of tem’s largest planetary structure. “If you multiple times. (By comparison, Earth’s
bow shock lies three to five Earth diam-
eters toward the Sun, and its magnetotail
may extend hundreds of diameters.)
Between February 28 and March 2, 1979,
the magnetosphere seemed to be playing
hard to get with the approaching Voyager 1.
The solar wind was gusty, producing
unusually strong and variable pressures that
pushed the bow shock closer to the planet.
When the wind eased, the bow shock re-
expanded. Pioneers 10 and 11 made their
crossings 50 diameters from Jupiter, but
Voyager 1 crossed it five times, the last at
scarcely half that range (28 diameters).
On March 5, the spacecraft made its clos-
Scientists discovered the first volcanic eruptions Sinuous rivers of lava flow down the flanks est approach, passing within 128,400 miles
beyond Earth on this navigation image taken of Io’s volcano Ra Patera. The volcano’s vent (206,700km) of Jupiter’s cloud tops, barely
March 8, 1979. The photo shows a crescent Io measures about 19 miles (30km) across, while one-third the distance at which Voyager 2
with a volcanic plume rising more than 150 miles the lava flows extend up to 155 miles (250km)
(260km) above the limb. A second plume catches from there. The summit rises only about 0.6 mile would pass July 9. Scientists selected this
the rays of the rising Sun. NASA/JPL (1km) above the surroundings. NASA/JPL path so they could measure a hypothesized
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