Sky & Telescope - December 2019 - 17

NEPT UNE CLOUDS: N ASA / JPL; STOR MS ON UR A NUS:
L AWRENCE SRO M OVSK Y / UNIV ERSIT Y OF WISCONSIN,
M A DISON / W. W. K ECK OBSERVATORY

helium envelope surrounds these centers. Yet this similarity points to a great
difference. For Uranus and Neptune, the
hydrogen-helium envelope accounts for
just a small fraction of each planet's mass.
Jupiter and Saturn, on the other hand, are
mostly hydrogen and helium.
When Jupiter and Saturn were forming,
they must have grabbed lots of hydrogen
and helium from the protoplanetary disk
around the newborn Sun, whereas Uranus
and Neptune took just a little. "You might
think of Uranus and Neptune as being
naked Jupiter and Saturn," Stevenson
says. McKinnon adds, "They're baby giant
planets that didn't get to grow up." Indeed,
each of these "baby giants" is only 5%
as massive as Jupiter and, at four times
Earth's diameter, about a third as wide.
Because hydrogen and helium are gases
on Earth, planetary scientists call Jupiter and Saturn gas
giants, but their great gravity actually squeezes most of these
gases into a fluid. In contrast, Uranus and Neptune are ice
giants, so named because they have large quantities of three
compounds that were frozen solid in the cold outer solar
nebula: water (H2O), methane (CH4), and ammonia (NH3),
in unknown proportions. Because oxygen is more common in
the cosmos than carbon and nitrogen, the main component

t Top: Voyager 2 caught these linear clouds on
Neptune, where they stretched approximately
along lines of constant latitude. Bottom: Several
storm systems appear in these infrared composites of Uranus taken by the Keck II telescope.

in both planets may be water. And despite
the term "ice giant," this water is mostly
liquid, because the interiors are so hot.
At the greatest depths, however,
intense pressure may squeeze the water
into superionic ice, which Burkhard Militzer (University of California, Berkeley)
compares to a soft solid, like chalk. "It's
neither completely solid nor completely
liquid," he says. In superionic water, the
oxygen atoms are locked into place but the
smaller hydrogen nuclei move like people
walking through a parking lot full of stationary cars, he explains.
Hydrogen nuclei are protons, and a
current of protons through each planet's mantle may explain
one of Voyager's strangest findings: Both planets have tilted
magnetic fields that arise far from the planets' centers. In
contrast, Earth's magnetic field comes from the flow of electrons through molten iron in our planet's outer core.
Uranus and Neptune probably have more "ice" - that is,
water, methane, and ammonia - than rock and metal combined, but Stevenson says this is not known with certainty.

CREDIT.RIG H T X X X X X X X X X X X X X X X

cted
Planets
sk yandtele scope.com * D EC E M B E R 2 019

17



http://www.skyandtelescope.com

Sky & Telescope - December 2019

Table of Contents for the Digital Edition of Sky & Telescope - December 2019

Contents
Sky & Telescope - December 2019 - Cover1
Sky & Telescope - December 2019 - Cover2
Sky & Telescope - December 2019 - 1
Sky & Telescope - December 2019 - Contents
Sky & Telescope - December 2019 - 3
Sky & Telescope - December 2019 - 4
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