Sky & Telescope - December 2019 - 19

mers saw hints of one in the 1980s
(S&T: June 1989, p. 606), but Voyager
definitively discovered five. A sixth
ring, which may not fully wrap around
the planet, also appears among the
AVERAGE
MEAN DENSITY
DISTANCE FROM
(WATER = 1.00)
others in the Voyager data. In addition,
THE SUN
1.27
the spacecraft saw enhancements of
2.87 billion km
KNOWN MOONS
material in four sections of the outer(19.2 a.u.)
27
most ring. How these ring arcs arose
YEAR
RINGS
is unknown, but they may owe their
84 Earth years
13
existence to gravitational resonances
ESTIMATED DAY
DISCOVERED
with various satellites or to collisions
17 hours 14 minutes
March 13, 1781
in the ring. Since Voyager's visit, two
AXIAL TILT
of the four ring arcs have disappeared;
97.8°
perhaps new ring arcs will form in
MASS
the future.
14.54 Earths
Whereas Saturn's stunning
Rings and Things
RADIUS
rings glisten with water ice,
Both Uranus and Neptune spin fast
4.0 Earth radii
those of Uranus and Neptune
but have very different axial tilts. Voyare dark, probably due to carbon
ager found that Uranus spins every 17
compounds. Small moons tug on
hours or so and Neptune about every
the rings and actually spawn others.
16 hours - faster than Earth but more slowly than Jupiter
For example, the moons Cordelia and Ophelia orbit Uranus
and Saturn. These figures constrain models of planetary inteon either side of its brightest ring, named Epsilon, their gravriors; however, with only one spacecraft measurement, some
ity keeping that ring narrow, while Neptune's moon Galatea
scientists have questioned the numbers.
probably sprinkles material along its orbit and creates the
Although Uranus and Neptune have similar spin peridiffuse ring it inhabits.
ods, their rotation axes are another story. Early in Uranus's
life, an object roughly twice as massive as Earth might have
slammed into the planet, knocking it over. Or that object
It's a Mab Mab World
may have whizzed by the planet, twirling it around via gravIndeed, both planets have lots of moons. When Voyager flew
ity. Either way, Uranus now lies on its side as it spins, with
past, it tripled the number known at Uranus from 5 to 15 and
an axial tilt of 98°. In contrast, Neptune's axis tilts only 28°, quadrupled the number known at Neptune from 2 to 8. This
similar to Earth's.
created the pleasing coincidence that the eighth planet from
Both Uranus and Neptune have rings, though they're
the Sun had eight known moons.
much darker than those around Saturn. Astronomers first
But Hubble and ground-based telescopes have nearly
detected Uranus's rings in 1977, when the planet passed in
doubled the numbers again. Today, Uranus has 27 known
front of a star and 5 narrow rings blocked the star's light
moons and Neptune 14.
before and after the planet did. Subsequent discoveries have
On its visit to Uranus, Voyager passed closest to Miranda,
boosted the total number of known rings to 13. Because of
the smallest and innermost of the five classical Uranian
its drastic axial tilt, Uranus points one pole almost directly
moons, and astonished scientists with pictures of radically
sunward during summer and winter solstices, giving observdifferent terrain types, some ancient, others young. "It's a
ers on Earth a face-on view of the rings, first from the top,
kind of schizophrenic world," McKinnon says.
then decades later from the bottom.
Thirteen additional moons, most found by Voyager, lie
Neptune has six rings.
inside Miranda's orbit. All are smaller than Miranda. One
Ground-based astronoof the most intriguing is Mab, which Mark Showalter (SETI
Institute) and Jack Lissauer (NASA Ames) found in 2003.
t MIRANDA This 1986 image
Mab lies in the outermost ring, named Mu. In like fashion,
from Voyager 2 shows the
the moon Enceladus, which orbits Saturn and is about the
varied terrain on Uranus's icy
size of Miranda, lies in that planet's E ring. The E ring comes
moon Miranda. Features that
from geysers on Enceladus that spew water along its orbit.
look like compressional folded
ridges mix with faults, and
Both rings are blue, so Mab might be doing something similar
some of the scarps are up to
around Uranus.
5 km high - higher than the
But there's a problem. "Mab is tiny," Showalter says.
walls of the Grand Canyon.
It's much smaller than Enceladus. How can a moon that's
The moon itself is less than
roughly a dozen miles across be geologically active? No one
500 km wide.

UR A NUS PL AYING CA RD: TERRI DUBÉ / S&T, BACKG ROUND
IM AG E: A LLIES IN TER ACTIV E / SHU T TERSTOCK .CO M, UR A NUS
IM AG E: N ASA / JPL- CA LTECH; MIR A NDA: N ASA / JPL

tion of the planets isn't as simple as
people initially had thought."
If instead nitrogen were more common in the atmosphere, ammonia
clouds would form, as they do on Jupiter and Saturn. Perhaps, at the greater
distance and colder temperatures of
Uranus and Neptune, water ice in the
planetesimals that built the two worlds
trapped more sulfur-bearing gases
than nitrogen-bearing ones.
Far below all these clouds, temperatures become warmer, and scientists
expect good old-fashioned water clouds
to exist. But no one has yet seen them.

URANUS STATS

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

19


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Sky & Telescope - December 2019

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

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