Download File

Week #5
The Jovian Planets
Introduction



Jupiter, Saturn, Uranus, and Neptune
are giant planets; they are also
called the jovian planets.
They are much bigger, more massive,
and less dense than the inner,
terrestrial planets.
Their internal structure is entirely
different from that of the four inner
planets.
Jupiter



Jupiter, the largest and most
massive planet, dominates
the Sun’s planetary system.
It alone contains two-thirds
of the mass in the Solar
System outside of the Sun,
318 times as much mass as
the Earth (but only 0.001
times the Sun’s mass).
Jupiter has at least 52
moons of its own and so is a
miniature “planetary system”
in itself.
Jupiter


Jupiter is more than 11 times
greater in diameter than the
Earth.
From its mass and volume, we
calculate its density to be 1.3
g/cm3.


Jupiter, rather, is mainly
composed of the light elements
hydrogen and helium.
Jupiter’s chemical composition is
closer to that of the Sun and
stars than it is to that of the
Earth (see figure), so its origin
can be traced directly back to
the solar nebula with much less
modification than the terrestrial
planets underwent
Jupiter


Jupiter has no crust.
At deeper and deeper levels, its
gas just gets denser and denser,
turning mushy and eventually
liquefying about 20,000 km (15
per cent of the way) down.


Jupiter’s “surface” (actually, the
top of the clouds that we see)
rotates in about 10 hours,
though different latitudes rotate
at slightly different speeds.
Regions with different speeds
correspond to different bands;
Jupiter has a half-dozen jet
streams while Earth has only
one in each hemisphere.
Jupiter

Jupiter’s clouds are in constant
turmoil; the shapes and
distribution of bands can
change within days.


The most prominent feature of
Jupiter’s surface is a large
reddish oval known as the Great
Red Spot.


The bright bands are called
“zones” and the dark bands are
called “belts,” but the strongest
winds appear on the boundaries
between them.
It is two to three times larger in
diameter than the Earth.
Jupiter emits radio waves,
which indicates that it has a
strong magnetic field and strong
“radiation belts.
Spacecraft to Jupiter



Our understanding of Jupiter was revolutionized in the 1970s, when first
Pioneer 10 (1973) and Pioneer 11 (1974) and then Voyager 1 and
Voyager 2 (both in 1979) flew past it.
The Galileo spacecraft arrived at Jupiter in 1995, when it dropped a
probe into Jupiter’s atmosphere and went into orbit in the Jupiter system.
The Cassini spacecraft flew by Jupiter, en route to Saturn, in 2000 –2001
(see figure).
The Great Red Spot



The Great Red Spot is a gaseous “island” a few times larger across than
the Earth.
It is the vortex of a violent, long-lasting storm, similar to large storms on
Earth, and drifts about slowly with respect to the clouds as the planet
rotates.
We also see how it interacts with surrounding clouds and smaller spots.


The Great Red Spot has been visible for at least 150 years, and maybe even
300 years.
Sometimes it is relatively prominent and colorful, and at other times the
color may even disappear for a few years.
Jupiter’s Atmosphere

Heat emanating from Jupiter’s interior churns the atmosphere.



Pockets of gas rise and fall, through the process of convection.
The bright bands (“zones”) and dark bands (“belts”) on Jupiter represent
different cloud layers (see figure).
Wind velocities show that each hemisphere of Jupiter has a half-dozen
currents blowing eastward or westward.

The Earth, in contrast, has only one westward current at low latitudes (the
trade winds) and one eastward current at middle latitudes (the jet stream).
Jupiter’s Atmosphere



Jupiter’s winds were stronger
than expected and increased
with depth, which shows that
the energy that drives them
comes from below.
Extensive lightning storms,
including giant-sized lightning
strikes called “superbolts,” were
discovered from the Voyagers.
The Galileo spacecraft
photographed giant
thunderclouds on Jupiter, which
indicates that some regions are
relatively wet and others
relatively dry.
Jupiter’s Interior


Most of Jupiter’s interior is in liquid form. Jupiter’s central
temperature may be between 13,000 and 35,000 K.
The central pressure is 100 million times the pressure of the
Earth’s atmosphere measured at our sea level due to Jupiter’s
great mass pressing in. (The Earth’s central pressure is 4 million
times its atmosphere’s pressure, and Earth’s central
temperatures are several thousand degrees.)

Because of this high pressure, Jupiter’s
interior is probably composed of ultracompressed hydrogen surrounding a
rocky core consisting of perhaps 10
Earth masses of iron and silicates (see
figure).
Jupiter’s Interior



Jupiter radiates 1.6 times as
much heat as it receives
from the Sun.
It must have an internal
energy source
It lacks the mass necessary
by a factor of about 75,
however, to have heated up
enough to become a star,
generating energy by nuclear
processes (see Chapter 12).

It is therefore not “almost a
star,” contrary to some
popular accounts.
upiter’s Magnetic Field


The space missions showed that Jupiter’s tremendous
magnetic field is even more intense than many scientists
had expected (see figure).
At the height of Jupiter’s clouds, the magnetic field
strength is 10 times that of the Earth, which itself has a
rather strong field.
Jupiter’s Ring



Though Jupiter wasn’t expected
to have a ring, Voyager 1 was
programmed to look for one just
in case; Saturn’s rings, of
course, were well known, and
Uranus’s rings had been
discovered only a few years
earlier during ground-based
observations.
The Voyager 1 photograph
indeed showed a wispy ring of
material around Jupiter at about
1.8 times Jupiter’s radius, inside
the orbit of its innermost moon.
From the far side looking back,
the ring appeared unexpectedly
bright, probably because small
particles in the ring scattered
the light toward the spacecraft.
Jupiter’s Amazing Satellites


Four of the innermost satellites were discovered by Galileo
in 1610 when he first looked at Jupiter through his small
telescope.
These four moons (Io, Europa, Ganymede, and Callisto)
are called the Galilean satellites (see figure).

One of these moons, Ganymede, 5276 km in diameter, is the
largest satellite in the Solar System and is larger than the
planet Mercury.
Jupiter’s Amazing Satellites



Through first Voyager-spacecraft and
then Galileo-spacecraft close-ups
(see figure)
The four Galilean satellites, in
particular, were formerly known only
as dots of light.
The Galilean satellites, range
between 0.9 and 1.5 times the size
of our own Moon
Pizza-like Io



Io, the innermost Galilean
satellite, provided the biggest
surprises.
Scientists knew that Io gave off
particles as it went around
Jupiter, and other scientists had
predicted that Io’s interior
would be heated by its flexing.

Voyager 1 discovered that
these particles resulted from
active volcanoes on the
satellite, a nice confirmation
of the earlier ideas.
Eight volcanoes were seen
actually erupting, many more
than erupt on the Earth at any
one time.
Pizza-like Io


Though the Galileo spacecraft could not go close to Io for most of
its mission, for fear of getting ruined because of Jupiter’s strong
radiation field in that region,
Io’s surface has been transformed by the volcanoes, and is by far
the youngest surface we have observed in the Solar System.
Pizza-like Io


Why does Io have so many
active volcanoes?
Gravitational forces from
Ganymede and Europa
distort Io’s orbit slightly,
which changes the tidal force
on it from Jupiter in a
varying fashion.

This changing tidal force
flexes Io, creating heat from
friction that heats the
interior and leads to the
volcanism.
Europa,
a Possible Abode for Life


Europa, Jupiter’s Galilean satellite with the highest albedo
(reflectivity), has a very smooth surface and is covered with narrow,
dark stripes.
The lack of surface relief, mapped by the Galileo spacecraft to be no
more than a couple of hundred meters high, suggests that the surface
we see is ice.
Europa,
a Possible Abode for Life

Few craters are visible,
suggesting that the ice was soft
enough below the crust to close
in the craters.


Gravitational tidal heating like
that inside Io provides the heat
to soften the ice.
Because Europa possibly has a
liquid-water ocean and extra
heating, many scientists
consider it a worthy location to
check for signs of life.

We can only hope that the ice
crust, which may be about 10–
50 km thick, is thin enough in
some locations for us to be able
to penetrate it to reach the
ocean that may lie below.
Giant Ganymede


The largest satellite in the Solar
System, Ganymede, shows many
craters alongside weird, grooved
terrain.
Ganymede is bigger than Mercury
but less dense; it contains large
amounts of water ice surrounding a
rocky core.


But an icy surface is as hard as steel
in the cold conditions that far from
the Sun, so it retains the craters
from perhaps 4 billion years ago.
The grooved terrain is younger.
Giant Ganymede

Ganymede shows many
lateral displacements, where
grooves have slid sideways,
like those that occur in some
places on Earth (for
example, the San Andreas
fault in California).

It is the only place besides
the Earth where such faults
have been found.
Pockmarked Callisto

Callisto, the outermost of Jupiter’s Galilean
satellites, has so many craters (see figure) that
its surface must also be the oldest.


Callisto, like Europa and Ganymede, is covered
with ice.
A huge bull’s-eye formation, Valhalla, contains
about 10 concentric rings, no doubt resulting
from an enormous impact.

Perhaps ripples spreading from the impact froze
into the ice to make Valhalla.
Saturn


Saturn, like Jupiter, Uranus, and Neptune, is a giant planet.
Its diameter, without its rings, is 9 times that of Earth; its mass
is 95 Earth masses.


It is a truly beautiful object in a telescope of any size.
The view from the Cassini spacecraft, now in orbit around it, is
breathtaking.
Saturn
The giant planets have low
densities.
 Saturn’s is only 0.7
g/cm3, 70 per cent the
density of water (see
figure).

The bulk of Saturn is
hydrogen molecules and
helium, reflecting Saturn’s
formation directly from the
solar nebula.
Saturn is thought to have a core of heavy elements, including rocky
material, making up about the inner 20 per cent of its diameter.


Saturn’s Rings


The rings extend far out in Saturn’s equatorial plane, and are
inclined to the planet’s orbit.
Over a 30-year period, we sometimes see them from above
their northern side, sometimes from below their southern side,
and at intermediate angles in between.

When seen edge-on, they are almost invisible.
Saturn’s Rings


The rings of Saturn consist
of material that was torn
apart by Saturn’s gravity or
material that failed to collect
into a moon at the time
when the planet and its
moons were forming.
Every massive object has a
sphere, called its Roche
limit, inside of which blobs
of matter do not hold
together by their mutual
gravity.

The forces that tend to tear
the blobs apart from each
other are tidal forces.
Saturn’s Rings



The radius of the Roche limit is
usually 2½ to 3 times the radius
of the larger body, closer to the
latter for the relative densities
of Saturn and its moons.

The Sun also has a Roche
limit, but all the planets lie
outside it.
The natural moons of the
various planets lie outside their
respective Roche limits.
Saturn’s rings lie inside Saturn’s
Roche limit, so it is not
surprising that the material in
the rings is spread out rather
than collected into a single
orbiting satellite.
Saturn’s Rings




Saturn has several concentric
major rings visible from Earth.
The brightest ring is separated
from a fainter broad outer ring
by an apparent gap called
Cassini’s division.
We know that the rings are not
solid objects, because the
rotation speed of the outer rings
is slower than that of rings
closer to Saturn.
Radar waves bounced off the
rings show that the particles in
the rings are at least a few
centimeters, and possibly a
meter, across.
Saturn’s Rings

Each of the known rings was
actually divided into many
thinner rings.


The number of these rings
(sometimes called “ringlets”)
is in the hundreds of
thousands.
The images from the Cassini
spacecraft (see figure)
surpassed even Voyager’s
views of ringlets.
Saturn’s Rings



The outer major ring turns out
to be kept in place by a tiny
satellite orbiting just outside it.
At least some of the rings are
kept narrow by “shepherding”
satellites that gravitationally
affect the ring material, a
concept that we can apply to
rings of other planets.
Density waves (see figure) were
seen in the ring.
Saturn’s Atmosphere


Like Jupiter, Saturn rotates
quickly on its axis; a
complete period is only 10
hours, in spite of Saturn’s
diameter being over 9 times
greater than Earth’s.
The rapid rotation causes
Saturn to be larger across
the equator than from pole
to pole.


This equatorial bulging
makes Saturn look slightly
“flattened.”
Jupiter also looks flattened,
or oblate, for this reason.
Saturn’s Atmosphere

Saturn has extremely high winds,
up to 1800 km /hr, 4 times faster
than the winds on Jupiter.


Cassini is tracking the winds with
higher precision than was
previously possible.
On Saturn, the variations in wind
speed do not seem to correlate
with the positions of bright and
dark bands, unlike the case with
Jupiter (see figure).
Saturn’s
Interior and Magnetic Field


Saturn radiates about twice as much energy as it absorbs
from the Sun, a greater factor than for Jupiter.
One interpretation is that only ⅔ of Saturn’s internal
energy remains from its formation and from its continuing
contraction under gravity.


The rest would be generated by the gravitational energy
released by helium sinking through the liquid hydrogen in
Saturn’s interior.
The helium that sinks has condensed
because Saturn, unlike Jupiter,
is cold enough.
Saturn’s
Interior and Magnetic Field


Saturn gives off radio signals, as does Jupiter, a pre-Voyager
indication to earthbound astronomers that Saturn also has a magnetic
field.
The Voyagers found that the magnetic field at Saturn’s equator is only
⅔ of the field present at the Earth’s equator.

Saturn’s magnetic field contains belts of charged
particles (analogous to Van Allen belts), which
are larger than Earth’s but smaller than Jupiter’s.
(Saturn’s surface magnetic field is 20 times
weaker than Jupiter’s.)

These particles interact with the atmosphere near
the poles and produce auroras (see figure).
Saturn’s Moon Titan



At 40 per cent the diameter of
the Earth, Titan is an intriguing
body for a number of reasons.
Titan has an atmosphere that
was detected from Earth preVoyager.
Studies of how the radio signals
faded when Voyager 1 went
behind Titan showed that
Titan’s atmosphere is denser
than Earth’s.

The surface pressure on
Titan is 1½ times that on
Earth.
Saturn’s Moon Titan


Titan’s atmosphere is opaque, apparently because of the action of
sunlight on chemicals in it, forming a sort of “smog” and giving it its
reddish tint.
The Voyagers showed (see figure) several layers of haze.



They detected nitrogen, which makes up the bulk of Titan’s atmosphere, as it
does Earth’s.
Methane is a minor constituent, perhaps 1 per cent.
A greenhouse effect is present, making some scientists wonder whether
Titan’s surface may have been warmed enough for life to have evolved
there.
Saturn’s Moon Titan



The temperature near the
surface is only about
180°C (93 K), somewhat
warmed (12°C) by a
combination of the
greenhouse effect and the
anti-greenhouse effect.
This temperature is near that
of methane’s “triple point,” at
which it can be in any of the
three physical states—solid,
liquid, or gas.
So methane may play the
role on Titan that water does
on Earth.
Saturn’s Moon Titan


Scientists wanted to know whether Titan is covered with lakes
or oceans of methane mixed with ethane, and whether other
parts are covered with methane ice or snow.
Also, using filters in the near-infrared, first the Hubble Space
Telescope and then ground-based telescopes have been able to
penetrate Titan’s haze to reveal some structure on its surface
(see figure), though no unambiguous lakes were found.
Saturn’s Moon Titan


Titan is so intriguing, and
potentially so important, that
the lander of the Cassini mission
was sent to plunge through its
atmosphere.
So while the lander of Jupiter’s
Galileo mission went into the
planet itself, this lander, known
as Huygens, penetrated the
clouds around Saturn’s largest
moon when it arrived on
January 14, 2005.

At higher levels, the
Huygens probe was
buffeted by strong winds
that ranged up to 400 km
/h, though the winds
calmed near the surface.
Saturn’s Moon Titan

As the Huygens lander drifted downward on its
parachute for 2½ hours, it measured the
chemical composition of Titan’s atmosphere.




It then imaged the surface below it when it got
sufficiently far below the haze.
The shape of a shoreline is visible (see figure).
No liquid substances were seen (there was
never a glint of reflection of the type that
occurs off a shiny body, for example), even
though methane or ethane had been expected.
The dark material is thought to be tar-like and
has probably settled out of the atmosphere.
Saturn’s Moon Titan

A final triumph came when the probe
survived for 90 minutes on the surface,
sending back pictures of ice blocks.




These pieces of ice are rounded,
apparently also revealing the past
presence of flowing liquid.
Later analysis of infrared images seems
to show a 30-km-wide structure that
may be a volcanic dome.
Such an ice volcano could be caused by the energy generated by
tidal stresses within Titan caused by its elliptical orbit.
It would release methane to the atmosphere.

So the methane long measured in Titan’s atmosphere might not be
from a methane-rich hydrocarbon ocean after all.
Saturn’s Other Satellites


So many of Saturn’s other
moons proved to have
interesting surfaces when seen
close up from Cassini that we
show a variety of images (see
figures).
Also, as was the case for
Jupiter, dozens of small,
irregular moons continue to be
found from space images and
from Earth-based telescopes.
Uranus

The two other giant planets beyond Saturn—Uranus
(pronounced “U´ran-us”) and Neptune—are each about 4 times
the diameter of (and about 15 times more massive than) the
Earth.



Like Jupiter and Saturn, Uranus and Neptune don’t have solid
surfaces.
Their atmospheres are also mostly hydrogen and helium, but
they have a higher proportion of heavier elements.


They reflect most of the sunlight that hits them, which indicates
that they are covered with clouds.
Some of the hydrogen may be in a liquid mantle of water,
methane, and ammonia.
At the planets’ centers, a rocky core contains mostly silicon and
iron, probably surrounded by ices.
Uranus


Uranus was the first planet to be discovered that had
not been known to the ancients.
The English astronomer and musician William Herschel
reported the discovery in 1781.



Actually, Uranus had been plotted as a star on several sky
maps during the hundred years prior to Herschel’s
discovery, but had not been singled out as anything other
than an ordinary star.
Uranus revolves around the Sun in 84 years at an
average distance of more than 19 A.U.
Uranus is apparently surrounded by thick clouds of
methane ice crystals (see figure), with a clear
atmosphere of molecular hydrogen above them.

The trace of methane gas mixed in with the hydrogen
makes Uranus look greenish.
Uranus

Uranus is so far from the Sun that its outer layers are very cold.



Studies of its infrared radiation give a temperature of -215°C (58 K).
There is no evidence for an internal heat source, unlike the case for
Jupiter, Saturn, and Neptune.
Uranus Rotation - for its axis of rotation is roughly perpendicular to the
other planetary axes, lying only 8° from the plane of its orbit (see figure).
Uranus

Sometimes one of Uranus’s
poles faces the Earth, 21
years later its equator
crosses our field of view, and
then another 21 years later
the other pole faces the
Earth.


Polar regions remain
alternately in sunlight and in
darkness for decades.
Spring comes once in every
84 years
Uranus



Voyager 2 reached Uranus in 1986.
It revealed most of our current understanding of Uranus, its
rings, and its moons. (the figure (below) shows a more
recent view.)
Its moon Miranda, for example, though
relatively small, has a surface that is
extremely varied and interesting (see
figure, right).
Uranus’s Atmosphere

Even though Voyager 2 came very close to Uranus’s surface, as
close as 107,000 km (a quarter of the distance from the Earth to
the Moon), it saw very little detail on it.




Thus Uranus’s surface is very bland.
Uranus’s clouds form relatively deep in the atmosphere.
A dark polar cap was seen on Uranus
At lower levels, the abundance of methane gas (CH4) increases.

Thus most of the light that is reflected back at us is blue-green.
Uranus’s Rings


In 1977, astronomers on Earth watched as Uranus occulted
(passed in front of ) a faint star.
Predictions showed that the occultation would be visible only
from the Indian Ocean southwest of Australia.


The scientists who went to study the
occultation from an instrumented airplane
turned on their equipment early, to be sure
they caught the event.
Surprisingly, about half an hour before the
predicted time of occultation, they detected a
few slight dips in the star’s brightness (see
figure).

They recorded similar dips, in the reverse
order, about half an hour after the occultation.
Uranus’s Rings

The dips indicated that Uranus is surrounded by several rings,
some of which have since been photographed by Voyager 2 and
with the Hubble Space Telescope.



Eleven rings are now known.
They are quite dark, reflecting only about 2 percent of the
sunlight that hits them.
They are very narrow from side to side; some are only a few km
wide.
Uranus’s Rings




Voyager provided detailed ring images
(see figure).
Interpreting the small color differences is
important for understanding the
composition of the ring material.
Quite significant was the single longexposure, backlighted view taken by
Voyager.
Study of these data has shown that less
of the dust in Uranus’s rings is very small
particles compared with the dust in the
rings of Saturn and Jupiter.
Uranus’s Interior
and Magnetic Field





Voyager 2 detected Uranus’s
magnetic field.

It is intrinsically about 50
times stronger than Earth’s.
Since Uranus’s field is so tilted,
it winds up like a corkscrew as
Uranus rotates.
Uranus’s magnetosphere
contains belts of protons and
electrons, similar to Earth’s Van
Allen belts.
Voyager also detected radio
bursts from Uranus every 17.24
hours.
Thus Uranus’s interior rotates
slightly more slowly than its
atmosphere.
Neptune

Neptune is even farther from the Sun than Uranus, 30
A.U. compared to about 19 A.U.



Neptune takes 164 years to orbit the Sun.
Its discovery was a triumph of the modern era of
Newtonian astronomy.
Mathematicians analyzed the amount that Uranus (then
the outermost known planet) deviated from the orbit it
would follow if gravity from only the Sun and the other
known planets were acting on it.

The small deviations could have been caused by
gravitational interaction with another, as yet unknown,
planet.
Neptune

A year later, the French astronomer Urbain Leverrier
independently worked out the position of the
undetected planet.



The French astronomers didn’t test his prediction right
away either.
Leverrier sent his predictions to an acquaintance at
the observatory in Berlin, where a star atlas had
recently been completed.
The Berlin observer, Johann Galle, discovered
Neptune within hours by comparing the sky against
the new atlas.
Neptune



Neptune has not yet made a full orbit since it was
located in 1846.
But it now seems that Galileo inadvertently observed
Neptune in 1613 and recorded its position, which more
than doubles the period of time over which it has been
studied.
We have used the positions he measured to improve our
knowledge of Neptune’s orbit!
Neptune’s Atmosphere



Neptune, like Uranus, appears
greenish in a telescope because of its
atmospheric methane (see figure,
top).
Some faint markings could be
detected on Neptune even before
adaptive optics systems became
available on Earth (see figure,
bottom).
It was thus known before Voyager
that Neptune’s surface was more
interesting than Uranus’s.

Still, given its position in the cold
outer Solar System, nobody was
prepared for the amount of activity
that Voyager discovered.
Neptune’s Atmosphere



As Voyager approached Neptune, active weather systems became
apparent (see figure, left).
An Earth-sized region that was soon called the Great Dark Spot (see
figure, center) became apparent.
Though colorless, the Great Dark Spot seemed analogous to Jupiter’s
Great Red Spot in several ways.


For example, it was about the same size relative to its planet, and it was in
the same general position in its planet’s southern hemisphere (see figure,
right).
Putting together a series of observations into a movie, scientists
discovered that it rotated counterclockwise, as does the Great Red Spot.

Thus it was
anti-cyclonic,
which made
it a highpressure
region.
Neptune’s Atmosphere

Clouds of ice crystals, similar to Earth’s cirrus but made of
methane, form at the edge of the Great Dark Spot as the
high pressure forces methane-rich gas upward.

But this Great Dark Spot had disappeared when the Hubble Space
Telescope photographed Neptune a few years later, so it was much
less long lived than Jupiter’s Great Red Spot.
Neptune’s
Interior and Magnetic Field

Voyager 2 measured
Neptune’s average
temperature: 59 K, that is,
59°C above absolute zero (214°C).


This temperature, though
low, is higher than would be
expected on the basis of
solar radiation alone.
Neptune gives off about 2.7
times as much energy as it
absorbs from the Sun.

Thus there is an internal
source of heating, unlike the
case of Uranus, which
otherwise seems like a
similar planet.
Neptune’s
Interior and Magnetic Field



Why is the average density of Uranus and Neptune higher
than that of Jupiter and Saturn?
Their densities show that Uranus and Neptune have a
higher percentage of heavy elements than Jupiter and
Saturn.
Voyager detected radio bursts from Neptune every 16.11
hours.


Thus Neptune’s interior must rotate with this rate.
On Neptune and Uranus, as on the Earth, equatorial winds
blow more slowly than the interior rotates.

By contrast, equatorial winds on Venus, Jupiter, Saturn, and
the Sun blow more rapidly than the interior rotates.
Neptune’s
Interior and Magnetic Field



The field, as for Uranus, turned out to be both greatly tipped and offset
from Neptune’s center (see figure).
Astronomers favor the explanation that the magnetic field is formed in an
electrically conducting shell outside the planets’ cores.
The fields of Earth and
Jupiter, in contrast, are
thought to be formed deep
within the core.
Neptune’s Rings




Before Voyager’s arrival, astronomers wanted to know whether
Neptune has rings, like the other giant planets.
 There was no obvious reason why it shouldn’t.
Some observations from Earth detected dips during occultations
of stars, similar to those produced by Uranus’s rings, while others
didn’t.
 It was thought that perhaps Neptune had incomplete rings,
“ring arcs.”
As Voyager came close to Neptune, it radioed back images that
showed conclusively that Neptune has narrow rings.
 Further, it showed the rings going all the way around
Neptune.
The material in the densest of Neptune’s rings is very clumpy.
Neptune’s Rings


The clumps had led to the incorrect idea of “ring arcs.”
The clumpy parts of the ring had blocked starlight, while
the other parts and the other rings were too thin to do so
(see figures).

The rings can now be studied from the Hubble Space
Telescope and with ground-based telescopes having
adaptive optics (see discussion in Chapter 3).
Neptune’s Rings

The fact that Neptune’s rings
are so much brighter when seen
backlighted tells us about the
sizes of particles in them.


The most detectable parts of
the rings have at least a
hundred times more dust-sized
grains than most of the rings of
Uranus and Saturn.
Since dust particles settle out of
the rings, new sources must
continually be active.

Probably moonlets collide and
are destroyed.
Neptune’s Moon Triton

Neptune’s largest moon, Triton, is a little larger than our Moon
and has a retrograde (backward) orbit.



It is massive enough to have a melted interior.
Its density is 2.07 grams /cm3, so it is probably about 70 per
cent rock and 30 per cent water ice.



It is named after a sea god who was a son of Poseidon.
It is denser than any jovian-planet satellite except Io and Europa.
Even before Voyager 2 visited, it was known that Triton has an
atmosphere.
The scientists waited eagerly to learn if Triton’s atmosphere
would be transparent enough to see the surface. It was.

The atmosphere is mostly nitrogen gas, like Earth’s.
Neptune’s Moon Triton

Triton’s surface is incredibly varied. Much of the region Voyager
2 imaged was near Triton’s south polar cap (see figure).

The ice appeared slightly reddish.

The color probably shows
the presence of organic
material formed by the
action of solar ultraviolet
light and particles from
Neptune’s magnetosphere
hitting methane in Triton’s
atmosphere and surface.

Nearer to Triton’s equator,
nitrogen frost was seen.
Neptune’s Moon Triton

Many craters and cliffs were seen.



They could not survive if they were made of only methane ice, so water ice
(which is stronger) must be the major component.
Since Neptune’s gravity captures many comets in that part of the Solar
System, most of the craters are thought to result from collisions with
comets.
Triton’s surface showed about 50 dark streaks parallel to each other as is
readily visible near the bottom of the figure.


They are apparently dark material vented from below.
The material is spread out by winds.
Neptune’s Moon Triton

A leading model is that the Sun heats darkened methane ice on
Triton’s surface.


A couple of these “ice volcanoes” were
erupting when Voyager 2 went by.


This heating vaporizes the underlying nitrogen ice, which escapes
through vents in the surface.
Since the streaks are on top of
seasonal ice, they are all probably less
than 100 years old.
Much of Triton is so puckered that it is
called the “cantaloupe terrain.”

It contains depressions 30 km in
diameter, crisscrossed by ridges (see
figure).