Sky & Telescope - December 2019 - 18

Uranus and Neptune

t THE URANUS SYSTEM Uranus has 27 known moons and 13 rings
(blue). The inner 18 moons orbit the planet's equator; the others follow
highly inclined paths, suggesting they're captured objects.

Nor do we know whether the planets are layered. If they are,
each planet may have a rock-iron core, a water-methaneammonia mantle, and a hydrogen-helium envelope. But it's
also possible that the various substances mix together so that
no sharp boundaries exist.

CORDELIA
OPHELIA
BIANCA

MIRANDA

CRESSIDA
MAB

DESDEMONA

PUCK

JULIET
PORTIA

BELINDA
CUPID

Clues from Clouds

ROSALIND

UMBRIEL
ARIEL
MIRANDA

TITANIA

OBERON

MA R G A R

CA L I BA N

TRI NCUL

FERD

OBERON

STEPHA N

P ROSPE RO

D EC E M B E R 2 019 * SK Y & TELESCOPE

INAN

Despite their different distances from the Sun, Uranus and
Neptune have the same effective temperature of 59 Kelvin
(−353° Fahrenheit). This similarity actually betrays a key difference: Uranus radiates no more heat than it receives from
the Sun, whereas Neptune emits more than twice as much,
presumably leftover heat from its birth. Jupiter and Saturn
also give off more heat than they receive. No one knows why
Uranus is unique. Perhaps it lost its heat of formation soon
after birth, or, conversely, perhaps its interior is so stratiﬁed
that the heat can't escape.
The unexplained contrast in heat ﬂow does explain another
difference between the two ice giants: "Neptune is a lot more
dynamic than Uranus," de Pater says. Voyager showed Uranus
to be so bland that the planet made news when astronomers
later detected clouds and storms there. In contrast, Neptune has plenty of both, thanks to its vigorous internal heat.
Although the planet's best-known storm, the Great Dark Spot
that Voyager 2 saw, has vanished, similar storms have erupted
since. Fast winds race across both planets.
High white clouds often fringe the dark storms and probably consist of methane ice. These clouds resemble orographic
clouds on Earth, where air rises over mountains and cools,
causing water vapor to condense and form clouds. Likewise,
methane-laden air on Uranus and Neptune rises over the
storms and condenses, creating the high white clouds.
Below lies the main cloud deck. In 2018 Patrick Irwin
(University of Oxford, UK) and his colleagues reported the
infrared signature of hydrogen sulﬁde, a poisonous gas that
smells like rotten eggs. This work conﬁrmed earlier suggestions indicating that the main cloud deck consists of hydrogen
sulﬁde ice.
Why hydrogen sulﬁde? Beneath the hydrogen sulﬁde
clouds, ammonia (NH3) and hydrogen sulﬁde (H2S) join
to form yet another compound, ammonium hydrosulﬁde (NH4SH). Because each molecule has one atom
of nitrogen and one of sulfur, whichever element is
less abundant gets used up, leaving the other to form
a gas. An overabundance of sulfur explains the hydrogen sulﬁde in the air above.
The discovery means Uranus and Neptune have more
sulfur than nitrogen. That makes them unlike Jupiter and
Saturn and unlike the overall galaxy, which has nearly ﬁve
times as much nitrogen as sulfur. "This observation is quite
important," de Pater says. "It really does show that the forma-

G REGG DINDER M A N / S&T

PERDITA

Sky & Telescope - December 2019

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