Astronomy Homework 10 Questions

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Michael Seeds Dana Backman

Chapter 7
The Outer Solar System

The worlds of the outer solar system can be studied from Earth.
However, much of what scientists know has been radioed back to Earth from robot spacecraft.

Voyager 2 flew past each of the outer planets in the 1970s and 1980s.
The Galileo spacecraft circled Jupiter dozens of times in the late 1990s.
The Cassini/Huygens orbiter and probe arrived at Saturn in 2004.
Throughout this discussion, you will find images and data returned by these robotic explorers.

You are about to visit five worlds that are truly unearthly.
This travel guide will warn you about what to expect.

A Travel Guide to the Outer Planets

The outermost planets in our solar system are Jupiter, Saturn, Uranus, and Neptune.
These are often called the “Jovian planets,” meaning that they are like Jupiter.
However, they have their own separate personalities.

The Outer Planets

The figure compares these four worlds.

The Outer Planets

Jupiter is the largest of the Jovian worlds.
It is over 11 times the diameter of Earth.

The Outer Planets

Saturn is a bit smaller than Jupiter.
Uranus and Neptune are quite a bit smaller than Jupiter.

The Outer Planets

Pluto, not included in the illustration, is smaller than Earth’s moon but was considered a planet from the time of its discovery in 1930 until a decision by the International Astronomical Union (IAU) in 2006 reclassified Pluto as a dwarf planet.

The Outer Planets

The other feature you will notice immediately is Saturn’s rings.
They are bright and beautiful and composed of billions of ice particles.

The Outer Planets

Jupiter, Uranus, and Neptune have rings too.
However, they are not easily detected from Earth and are not visible here.

The Outer Planets

Nevertheless, as you visit these worlds, you will be able to compare four different sets of planetary rings.

The Outer Planets

The four Jovian worlds have hydrogen-rich atmospheres filled with clouds.
On Jupiter and Saturn, you can see that the clouds form stripes that circle each planet.
You will find traces of these same types of features on Uranus and Neptune—but they are not very distinct.

Atmospheres and Interiors

Models based on observations indicate that, below their atmospheres, Jupiter and Saturn are mostly liquid.
So, the old fashioned term for these planets—the gas giants—should probably be changed to the liquid giants.

Atmospheres and Interiors

Uranus and Neptune are sometimes called the ice giants.
They are rich in water in both solid and liquid forms.

Atmospheres and Interiors

Only near their centers do the Jovian planets have cores of dense material with the composition of rock and metal.
None of the worlds has a definite solid surface on which you could walk.

Atmospheres and Interiors

You have learned that the Jovian planets have low density because they formed in the outer solar nebula where water vapor could freeze to form ice particles.
The ice accumulated into proto-planets with density lower than the rocky terrestrial planets and asteroids.
Once these planets grew massive enough, they could draw in even lower-density hydrogen and helium gas directly from the nebula by gravitational collapse.

Atmospheres and Interiors

You can’t really land your spaceship on the Jovian worlds.
You might, however, be able to land on one of their moons.
All the outer solar system planets have extensive moon systems.

Satellite Systems

In many cases, the moons interact gravitationally.
They mutually adjust their orbits.
They also affect the planetary ring systems.

Satellite Systems

Some of the moons are geologically active now.
Others show signs of past activity.
Of course, geological activity depends on heat flow from the interior.
So, you might ponder what could be heating the insides of these small objects.

Satellite Systems

Jupiter, named for the Roman king of the gods, can be very bright in the night sky.
Its cloud belts and four largest moons can be seen through even a small telescope.

Jupiter

Jupiter is the largest and most massive of the Jovian planets.
It contains 71 percent of all the planetary matter in the entire solar system.

Jupiter

You used Earth, the largest of the terrestrial planets, as the basis for comparison with the others.
Similarly, you can examine Jupiter in detail as a standard in your comparative study
of the other Jovian planets.

Jupiter

Jupiter is only 1.3 times denser than water.
For comparison, Earth is more than 5.5 times
denser than water.
This gives astronomers a clue about the average
composition of the planet’s interior.

The Interior

Jupiter’s shape also gives information about its interior.
Jupiter and the other Jovian planets are all slightly flattened.
A world with a large rocky core and mantle would not be flattened much by rotation.
An all-liquid planet, though, would flatten significantly.

The Interior

Thus, Jupiter’s oblateness—the fraction by which its equatorial diameter exceeds its polar diameter—combined with its average density helps astronomers model the interior.
Models indicate that the interior is mostly
liquid hydrogen.

The Interior

However, if you jumped into Jupiter carrying a rubber raft expecting an ocean, you would be disappointed.
The base of the atmosphere is so hot and the pressure is so high that there is no sudden boundary between liquid and gas.
As you fell deeper through the atmosphere, you would find the gas density increased around you until you were sinking through a liquid.
You would, however, never splash into a distinct liquid surface.

The Interior

Under very high pressure, liquid hydrogen becomes liquid metallic hydrogen.
This is a very good conductor of electricity.
Most of Jupiter’s interior is composed of this material.

The Interior

That large mass of conducting liquid is stirred by convection currents and spun by the planet’s rapid rotation.
As a result, it drives the dynamo effect and generates a powerful magnetic field.
Jupiter’s field is over 10 times stronger than Earth’s.

The Interior

A planet’s magnetic field deflects the solar wind and dominates a volume of space around the planet called the magnetosphere.

The Interior

The strong magnetic field around Jupiter traps particles from the solar wind in radiation belts a billion times more intense than the Van Allen belts that surround Earth.

The Interior

The spacecraft that have flown through these regions received over 1000 times the radiation that would have been lethal for a human.

The Interior

At Jupiter’s center, a so-called rocky core contains heavier elements—such as iron, nickel, and silicon.
With a temperature four times hotter than the surface of the sun and a pressure of 50 million times Earth’s air pressure at sea level, this material is unlike any rock on Earth.
The term rocky core refers to the chemical composition, not to the properties of the material.

The Interior

Careful infrared measurements of the heat flowing out of Jupiter reveal that the planet emits about twice as much energy as it absorbs from the sun.
This energy appears to be heat left over from
the formation of the planet.

The Interior

There are three important ideas about Jupiter’s atmosphere.

Jupiter’s Complex Atmosphere

One, Jupiter’s extensive magnetosphere is responsible for auroras around the magnetic poles.
Jupiter's rings, discovered in 1979 by the Voyager 1 space probe, are close to the planet.

Jupiter’s Complex Atmosphere

Two, the pattern of colored cloud bands circling the planet like stripes on a child’s ball is called belt-zone circulation.
This pattern is related to the high- and low-pressure areas found in Earth’s atmosphere.

Jupiter’s Complex Atmosphere

Three, the positions of the cloud layers lie at certain temperatures within the atmosphere where ammonia (NH3), ammonium hydrosulfide (NH4SH), and water (H2O) can condense.

Jupiter’s Complex Atmosphere

Astronomers have known for centuries that Saturn has rings.
Jupiter’s ring, though, was not discovered until 1979—when the Voyager 1 spacecraft sent back photos.

Jupiter’s Ring

Less than 1 percent as bright as Saturn’s icy rings, Jupiter’s ring is very dark and reddish.
This indicates that it is rocky rather than icy.
Astronomers conclude that the ring particles are mostly microscopic.

Jupiter’s Ring

Photos show that it is very bright when illuminated from behind.
That is, it is scattering light forward.
Large particles do not scatter light forward.
So, a ring filled with basketball-size particles would look dark when illuminated from behind.

Jupiter’s Ring

Forward scattering of visible light shows you that the ring is mostly made of tiny grains, with diameters approximately equal to the wavelengths of visible light.
This would be about the size of particles in cigarette smoke.

Jupiter’s Ring

The rings orbit inside the Roche limit.
This is the distance from a planet within which a moon cannot hold itself together by its own gravity.
If a moon comes inside the Roche limit, the tidal forces overcome the moon’s gravity and pull the moon apart.
Also, raw material for a moon cannot coalesce inside the Roche limit.

Jupiter’s Ring

The Roche limit is about 2.4 times the planet’s radius—depending somewhat on the relative densities of the planet and the moon material.
Jupiter’s rings lie inside the limit for the planet.
Those of Saturn, Uranus, and Neptune too lie within the respective planetary limits.

Jupiter’s Ring

Now you can understand Jupiter’s dusty rings.
If a dust speck gets knocked loose from a larger rock inside the Roche limit, the rock’s gravity cannot hold the dust speck.
Also, the billions of dust specks in the ring can’t pull themselves together to make a new moon because of tidal forces inside the Roche limit.

Jupiter’s Ring

You can be sure that Jupiter’s ring particles are not old.
The pressure of sunlight and the planet’s powerful magnetic field alter the orbits of the particles.
Images show faint ring material extending down toward the cloud tops—evidently dust specks spiraling into the planet.
Dust is also destroyed by the intense radiation around Jupiter that grinds the dust specks down to nothing in a century or so.

Jupiter’s Ring

The rings you see today, therefore, can’t be material left over from the formation of Jupiter.
The rings of Jupiter must be continuously resupplied with new dust.
Observations made by the Galileo spacecraft provide evidence that the source of ring material is micrometeorites eroding small moons orbiting near, or within, the rings.

Jupiter’s Ring

The rings around Saturn, Uranus,
and Neptune are also known to be
short-lived.
They too must be resupplied by new material—probably eroded from nearby moons.

Jupiter’s Ring

In addition to supplying the rings with particles, moons:
Confine the rings
Keep them from spreading outward
Alter their shapes

Jupiter’s Ring

Jupiter has four large moons and at least 60 smaller moons.
Larger telescopes and modern techniques are rapidly finding more small moons orbiting the Jovian planets.

Jupiter’s Family of Moons

Most of the small moons are probably captured asteroids.
In contrast, the four largest moons are clearly related to each other and probably formed with Jupiter.
These moons are called Galilean moons—after their discoverer, Galileo.

Jupiter’s Family of Moons

The outermost Galilean moons, Ganymede and Callisto, are about the size of Mercury—one and a half times the size of Earth’s moon.
In fact, Ganymede is the largest moon in the solar system.

Jupiter’s Family of Moons

They have low densities—only 1.9 and 1.8 g/cm3 respectively.
This must mean that they consist roughly of half rock and half ice.

Jupiter’s Family of Moons

Observations of their gravitational fields by the Galileo spacecraft reveal that both have rocky or metallic cores and lower-density icy exteriors.
So, they have both differentiated.

Jupiter’s Family of Moons

Both moons interact with Jupiter’s magnetic field in a way that shows they probably have mineral-rich layers of liquid water 100 km or more below their icy crusts.

Jupiter’s Family of Moons

Callisto’s surface and most of Ganymede’s surface appear old.
This is because they are heavily cratered and very dark.

Jupiter’s Family of Moons

The continuous blast of micrometeorites evaporates surface ice, leaving behind embedded minerals to form a dark skin—like the grimy crust on an old snowbank.
So, surfaces get
darker with age.

Jupiter’s Family of Moons

More recent impacts dig up cleaner ice and leave bright craters.

Jupiter’s Family of Moons

Ganymede has some younger, brighter grooved terrain believed to be systems of faults in the brittle crust.
Some sets of grooves overlap other sets of grooves.
This suggests extended episodes of geological activity.

Jupiter’s Family of Moons

The density of the next moon inward, Europa, is 3 g/cm3.
This is high enough to mean that it is mostly rock with a thin icy crust.

Jupiter’s Family of Moons

Europa’s visible surface:
Is very clean ice
Contains very few craters
Has long cracks in the icy crust
Has complicated terrain that resembles blocks of ice in Earth’s Arctic Ocean

Jupiter’s Family of Moons

The pattern of mountainlike folds on the surface suggests that the icy crust breaks as the moon is flexed by tides.

Jupiter’s Family of Moons

Europa’s gravitational influence on the Galileo spacecraft reveals that a liquid-water ocean perhaps 200-km deep lies below the 10- to 100-km-thick crust.

Jupiter’s Family of Moons

The lack of craters shows you that it is an active world where craters are quickly erased.

Jupiter’s Family of Moons

Images from spacecraft reveal that Io, the innermost of the Galilean moons, has over 100 volcanic vents on its surface.

Jupiter’s Family of Moons

The active volcanoes throw sulfur-rich gas and ash high above the surface.
The ash falls back to bury the surface at a rate of a few millimeters a year.

Jupiter’s Family of Moons

That explains why you see no impact craters on Io.
They are covered up as fast
as they form.

Jupiter’s Family of Moons

Io’s density is 3.6 g/cm3.
Thus, it is not ice but rather rock and metal.
Its gravitational influence on the passing Galileo spacecraft revealed that it is differentiated into a large metallic core, a rocky mantle, and a low-density crust.

Jupiter’s Family of Moons

The activity seen in the Galilean moons must be driven by energy flowing outward.
Yet, these objects are too small to have remained hot from its formation.

Jupiter’s Family of Moons

Io’s volcanism seems to be driven
by tidal heating.
Io follows a slightly elliptical orbit—caused by its interactions with the other moons.
Jupiter’s gravitational field flexes Io with tides.
The resulting friction heats its interior.
That heat flowing outward causes
the volcanism.

Jupiter’s Family of Moons

Europa is not as active as Io.
However, it too must have a heat source—presumably tidal heating.

Jupiter’s Family of Moons

Ganymede is no longer active.
When it was younger, though,
it must have had internal heat
to break the crust and produce
the grooved terrain.

Jupiter’s Family of Moons

Can you put all the evidence together and tell Jupiter’s story?
Creating such a logical argument of evidence and hypotheses is the ultimate goal of planetary astronomy.

The History of Jupiter

Jupiter formed far enough from the sun to incorporate large numbers of icy planetesimals.
It must have grown rapidly.
Once it was about 10 to 15 times more massive than Earth, it could grow by gravitational collapse—capturing gas directly from the solar nebula.
Thus, it grew rich in hydrogen and helium from the solar nebula.

The History of Jupiter

Its present composition resembles the composition of the solar nebula and is also quite sunlike.
Jupiter’s gravity is strong enough to hold onto all its gases—even hydrogen.

The History of Jupiter

The large family of moons may be mostly captured asteroids.
Jupiter may still encounter a wandering asteroid or comet now and then.

The History of Jupiter

Some asteroids and comets are deflected.
Some are captured into orbit.
Some actually fall into the planet.
An example is the comet that struck Jupiter in 1994 and an unidentified object in 2009.

The History of Jupiter

Dust blasted off of the inner
moons by micrometeorites settles into the equatorial plane to form Jupiter’s rings.

The History of Jupiter

The four Galilean moons seem to have formed like a mini-solar system in a disk of gas and dust around the forming planet.
The innermost, Io, is densest.
The densities of the others decrease as you move away from Jupiter—similar to the way the densities of the planets decrease with distance from the sun.

The History of Jupiter

Perhaps the inner moons incorporated less ice because they formed closer to the heat of the growing planet.
You can recognize that tidal heating also has been important—and the intense warming of the inner moons could have driven off much of their ices.
Thus, two processes together may be responsible for the differences in compositions of the Galilean moons.

The History of Jupiter

The Roman god Saturn, protector of the sowing of seed, was celebrated in a weeklong Saturnalia at the time of the winter solstice in late December.
Early Christians took over the holiday to celebrate Christmas.

Saturn

Saturn is most famous for its beautiful rings.
These are easily visible through the telescopes of modern amateur astronomers.

Saturn

Large Earth-based telescopes have explored the planet’s atmosphere, rings, and moons.
The two Voyager spacecraft flew past Saturn in 1979.

Saturn

The Cassini spacecraft went into orbit around Saturn in 2004 on an extended exploration of the planet, its rings, and its moons.

Saturn

Saturn shows only faint belt-zone circulation.
However, Voyager, Hubble Space Telescope, and Cassini images show that belts and zones are present and that the associated winds blow up to three times faster than on Jupiter.

Saturn the Planet

Belts and zones on Saturn are less visible because they occur deeper in the cold atmosphere—below a layer of methane haze.

Saturn the Planet

Saturn is less dense than water—it would float.
This suggests that it is, like Jupiter, rich in hydrogen and helium.
Photos show that Saturn is the most oblate of the planets.
That evidence shows that its interior is mostly liquid with a small core of heavy elements.

Saturn the Planet

As its internal pressure is lower, Saturn has less liquid metallic hydrogen than Jupiter.
Perhaps this is why its magnetic field is 20 times weaker than Jupiter’s.
Like Jupiter, it radiates more energy than it receives from the sun.
Models predict that it has a very hot interior.

Saturn the Planet

There are three important points to note about the icy rings of Saturn.

Saturn’s Rings

One, the rings are made up of billions of ice particles—each in its own orbit around the planet.
However, the ring particles you observe now can’t be as old as Saturn.
They must be replenished now and then by impacts on Saturn’s moons or other processes.
The same is true of the rings around the other Jovian planets.

Saturn’s Rings

Two, the gravitational effects of small moons can confine some rings in narrow strands or keep the edges of rings sharp.
Moons can also produce waves in the rings that are visible as tightly wound ringlets.

Saturn’s Rings

Three, the ring particles are confined in a thin plane spread among small moons and confined by gravitational interactions with larger moons.
The rings of Saturn, and the rings of the other Jovian worlds, are created by and controlled by the planet’s moons.
Without the moons, there would be no rings.

Saturn’s Rings

Saturn has more than 60 known moons.
They contain mixtures of ice and rock.
Many are small.
Many are probably captured objects.

Saturn’s Family of Moons

The largest of Saturn’s moons is Titan.
It is a bit larger than Mercury.

Saturn’s Family of Moons

Its density suggests that it must contain a rocky core under a thick mantle of ices.

Saturn’s Family of Moons

Titan is so cold that its gas molecules do not travel fast enough to escape.
So, it has an atmosphere composed mostly of nitrogen, with traces of argon and methane.

Saturn’s Family of Moons

Sunlight converts some of the methane into complex carbon-rich molecules.
These collect into small particles—filling the atmosphere with orange smog.

Saturn’s Family of Moons

These particles settle slowly downward to coat the surface with what has been described as dark organic goo.
That is, it is composed of carbon-rich molecules.

Saturn’s Family of Moons

Titan’s surface is mainly composed of ices of water and methane at –180°C (–290°F).

Saturn’s Family of Moons

The Cassini spacecraft dropped the Huygens probe into Titan’s atmosphere.
It photographed dark drainage channels.

Saturn’s Family of Moons

This suggests that liquid methane falls as rain, washes the dark goo off of the higher terrain, and drains into the lowlands.

Saturn’s Family of Moons

Such methane downpours may be rare, though.
No direct evidence of liquid methane was detected as the probe descended.

Saturn’s Family of Moons

However, later radar images made by the Cassini orbiter have detected what appear to be lakes presumably containing liquid methane.
Infrared images suggest the presence of methane volcanoes that replenish the methane in the atmosphere.
So, Titan must have some internal heat source to power the activity.

Saturn’s Family of Moons

Most of the remaining moons of Saturn:
Are small and icy
Have no atmospheres
Are heavily cratered
Have dark, ancient surfaces

Saturn’s Family of Moons

The moon Enceladus, however, shows signs of recent geological activity.

Saturn’s Family of Moons

Some parts of its surface contain 1,000 times fewer craters than other regions.

Saturn’s Family of Moons

Infrared observations show that its south polar region is unusually warm and venting water
and ice geysers.

Saturn’s Family of Moons

Evidently, a reservoir of liquid waters lies only tens of meters below the surface.
At some point in its
history, the moon
must have been
caught in a resonance
with another moon
and was warmed by
tidal heating.

Saturn’s Family of Moons

Enceladus appears to maintain the faint E ring that extends far beyond the visible rings.
In 2009 astronomers detected infrared radiation from a dark ring 13 million km (8 million mi) in radius.
This is beyond the orbits of most of Saturn’s moons.

Saturn’s Family of Moons

Now that you are familiar with the gas giants in our solar system, you will be able to appreciate how weird the ice giants—Uranus and Neptune—are.

Uranus

Uranus, especially, seems to have forgotten how to behave like a planet.

Uranus

Uranus was discovered in 1781 by the scientist William Herschel, a German expatriate living in England.
He named it Georgium Sidus (George’s Star)—
after the English King George III.

Uranus

European astronomers—especially the French—refused to accept a planet named after an English king.
They called it Herschel.

Uranus

Years later, German astronomer J. E. Bode suggested Uranus—the oldest of the Greek gods.

Uranus

Uranus is only a third the diameter of Jupiter and only a twentieth as massive.
Being four times farther from the sun, its atmosphere is over 100° C colder than Jupiter’s.

Planet Uranus

Uranus never grew massive enough to capture large amounts of gas from the nebula as Jupiter and Saturn did.
So, it has much less hydrogen and helium.
Its internal pressure is enough lower than Jupiter’s that it should not contain any liquid metallic hydrogen.

Planet Uranus

Models of Uranus, based in part on its density and oblateness, suggest that it has a small core of heavy elements and a deep mantle of partly solid water.

Planet Uranus

Although referred to as ice, this material would not be anything like ice on Earth—given the temperatures and pressures inside Uranus.

Planet Uranus

The mantle also contains rocky material and dissolved ammonia and methane.
Circulation in this electrically conducting mantle may generate the planet’s peculiar magnetic field—which is highly inclined to its axis of rotation.

Planet Uranus

Above the mantle lies the
deep hydrogen and helium atmosphere.

Planet Uranus

Uranus rotates on its side—with its equator inclined 98° to its orbit.
With an orbital period of 84 years, each of its four seasons lasts 21 years.
The winter–summer contrast is extreme.
During a season when one of its poles is pointed nearly at the sun (a solstice), a citizen of Uranus would never see the sun rise or set.

Planet Uranus

Uranus’s odd rotation may have been produced when it was struck by a very
large planetesimal late in its formation.
Alternatively, it could due to tidal
interactions with the other giant planets,
as it migrated outward early in the history
of the solar system.

Planet Uranus

Voyager 2 photos show a nearly featureless ball.
The atmosphere is mostly
hydrogen and helium.
However, traces of methane
absorb red light—making
the atmosphere look
green-blue.

Planet Uranus

There is no belt-zone circulation visible in the Voyager photos.

Planet Uranus

However, extreme computer enhancement revealed
a few clouds and
bands around
the south pole.

Planet Uranus

In the decades since Voyager 2 flew past Uranus, spring has come to the northern hemisphere of Uranus and autumn to the southern hemisphere.

Planet Uranus

Images made by the Hubble Space Telescope and modern Earth-based telescopes reveal
changing clouds and
cloud bands in
both hemispheres.

Planet Uranus

Infrared measurements show that Uranus is radiating about the same amount of energy that it receives from the sun.
Thus, it has much less heat flowing out of its interior than Jupiter or Saturn (or Neptune).

Planet Uranus

This may account for its limited atmospheric activity.
Astronomers are not sure why it differs in this respect from the other Jovian worlds.

Planet Uranus

Until recently, astronomers could see only five moons orbiting Uranus.
However, Voyager 2 discovered 10 small moons in 1986.
More have been found in images recorded by new, giant telescopes on Earth.

The Uranian Moons

The five major moons of Uranus are smaller than Earth’s moon and have old, dark, cratered surfaces.
A few have deep cracks—produced, perhaps, when the interior froze and expanded.
In some cases, liquid water “lava” appears to have erupted and smoothed over some regions.

The Uranian Moons

Miranda, the innermost moon, is only 14 percent the diameter of Earth’s moon.
Its surface is marked by grooves called ovoids.

The Uranian Moons

The ovoids may have been caused by internal heat driving convection in the icy mantle.
By counting craters on
the ovoids, astronomers
conclude that the entire
surface is old, and the
moon is no longer active.

The Uranian Moons

The rings of Uranus:
Are dark and faint
Contain little dust
Are confined by shepherd
satellites
Must be continuously
resupplied with material
from the moons

The Uranian Rings

The rings are not easily visible from Earth.
The first hint that Uranus had rings came from occultations.
This is the passage of the planet in front of a star.

The Uranian Rings

Most of what astronomers know about the rings comes from the observations of the Voyager 2 spacecraft.
Their composition appears to be water ice mixed with methane that has been darkened by exposure to radiation.

The Uranian Rings

In 2006, astronomers found two new, very faint rings orbiting far outside the previously known rings.

The Uranian Rings

The newly discovered satellite Mab appears to be the source of particles for the larger ring.
The smaller of the new rings is confined between the orbits of the moons Portia and Rosalind.
Note that the International Astronomical Union (IAU) has declared that the moons of Uranus are to be named after characters in Shakespeare’s plays.

The Uranian Rings

A British and a French astronomer independently calculated the existence and location of Neptune from irregularities in the motion of Uranus.
British astronomers were too slow to respond.
Neptune was discovered in 1846.
The French astronomer got the credit.

Neptune

Neptune looks like a tiny blue dot with no visible cloud features.
Thus, astronomers named it
after the god of the sea.

Neptune

In 1989, Voyager 2 flew past and revealed some of Neptune’s secrets.

Neptune

Neptune is almost exactly the same size as Uranus.
It has a similar interior too.

Planet Neptune

A small core of heavy elements lies within a slushy mantle of water, ices, and minerals (rock) below a hydrogen-rich atmosphere.

Planet Neptune

However, Neptune looks quite different.
It is dramatically blue.
It has active cloud formations.

Planet Neptune

The dark-blue tint to the atmosphere is understandable.
Its atmosphere contains
one and a half times more
methane than Uranus.

Planet Neptune

Methane absorbs red photons better than blue and scatters blue photons better than red.
This gives Neptune
a blue color and Uranus
a green-blue color.

Planet Neptune

Atmospheric circulation on Neptune is much more dramatic than on Uranus.

Planet Neptune

When Voyager 2 flew by Neptune in 1989, the largest feature was the Great Dark Spot.
Roughly the size of Earth,
the spot seemed to be an
atmospheric circulation—
much like Jupiter’s
Great Red Spot.

Planet Neptune

Smaller spots were visible in Neptune’s atmosphere.
Photos showed they were
circulating like hurricanes.

Planet Neptune

Recently, the Hubble Space Telescope photographed Neptune and found that the Great Dark Spot is gone and new cloud formations have appeared.
Evidently, the weather on Neptune is changeable.

Planet Neptune

The atmospheric activity on Neptune is apparently driven by heat flowing from the interior plus some contribution from dim light from the sun 30 AU away.

Planet Neptune

Neptune may have more atmospheric activity than Uranus because it has more heat flowing out of its interior.
The reasons for this, though, are unclear.

Planet Neptune

Like Uranus, Neptune has a highly inclined magnetic field that must be linked to circulation in the interior.
In both cases, astronomers suspect that ammonia dissolved in the liquid water mantle makes the mantle a good electrical conductor.
That convection in the water, coupled with the rotation of the planet, drives the dynamo effect and generates the magnetic field.

Planet Neptune

Neptune has two moons that were discovered from Earth before
Voyager 2 flew past in 1989.
The passing spacecraft discovered six more very
small moons.
Since then a few more small moons have been found by astronomers using large Earth-based telescopes.

The Neptunian Moons

The two largest moons have peculiar orbits.
Nereid, about a tenth the size of Earth’s moon, follows a large, elliptical orbit—taking nearly an Earth year to circle Neptune once.
Triton, nearly 80 percent the size of Earth’s moon, orbits Neptune backward—clockwise as seen from the north.

The Neptunian Moons

These odd orbits suggest that the system was disturbed long ago in an interaction with some other body—such as a massive planetesimal.

The Neptunian Moons

With a temperature of 37 K (–393°F), Triton has an atmosphere of nitrogen and methane about 105 times less dense than Earth’s.

The Neptunian Moons

A significant part of Triton
is ice.
Deposits of nitrogen frost
are visible at the southern
pole.

The Neptunian Moons

Many features on Triton suggest it has had an active past.
It has few craters, but it does have long faults that appear to have formed when the icy crust broke.
Also, there are large basins that seem to have been flooded repeatedly by liquids from the interior.

The Neptunian Moons

Even more interesting are the dark smudges visible in the southern polar cap.
These are interpreted as
sunlight-darkened
deposits of methane
erupted out of liquid
nitrogen geysers.

The Neptunian Moons

Neptune’s rings are faint and very hard to detect from Earth.
However, they
illustrate some
interesting
processes of
comparative
planetology.

The Neptunian Rings

Neptune’s rings, named after the astronomers involved in the discovery of the planet, are similar to those of Uranus—but contain more dust.
One of Neptune’s moons is producing short arcs in the outermost ring.

The Neptunian Rings

Neptune’s ring system, like the others, is apparently resupplied by impacts on moons scattering debris that fall into the most stable places among the orbits of the moons.

The Neptunian Rings

Out on the edge of the solar system orbits a family of small, icy worlds.
Pluto was the first to be discovered—in 1930.
However, modern telescopes have found more.

Pluto: Planet No More

You may have learned in school that there are nine planets in our solar system.
However, in 2006, the International Astronomical Union voted to remove Pluto from the list of planets.

Pluto: Planet No More

Pluto is a small, icy world.
It isn’t Jovian.
It isn’t terrestrial.

Pluto: Planet No More

Its orbit is highly inclined and so elliptical that it actually comes closer to the sun than Neptune at times.

Pluto: Planet No More

To understand Pluto’s status, you must use comparative planetology to analyze Pluto and then compare it with its neighbors.

Pluto: Planet No More

Pluto is very difficult to observe from Earth.
It has only 65 percent the diameter of Earth’s moon.
In Earth-based telescopes, it never looks like more than a faint point of light.
Even in space telescope images, it shows little detail.

Pluto: Planet No More

Orbiting so far from the sun, it is cold enough to freeze most compounds you think of as gases.
Spectroscopic observations have found evidence of nitrogen ice.
It has a thin atmosphere of nitrogen and carbon monoxide with small amounts of methane.

Pluto: Planet No More

Pluto has three moons.
Two—Nix and Hydra—are quite small.
Charon, though, is relatively large—half
of Pluto’s diameter.

Pluto: Planet No More

Charon orbits Pluto with a period of 6.4 days in an orbit highly inclined to the ecliptic.

Pluto: Planet No More

Pluto and Charon are tidally locked to face each other.
So, Pluto’s rotation is also highly inclined.

Pluto: Planet No More

Charon’s orbit size and period plus Kepler’s third law reveal that the mass of the system is only about 0.002 Earth mass.
Most of the mass is Pluto—about 12 times the mass of Charon.

Pluto: Planet No More

Knowing the diameters and masses of Pluto and Charon allows astronomers to calculate that their densities are both about 2 g/cm3.
Thus, Pluto and Charon must contain about 35 percent ice and 65 percent rock.

Pluto: Planet No More

The best photos by the Hubble Space Telescope reveal almost no surface detail.
However, you know enough about icy moons to guess that Pluto has craters and probably shows signs of tidal heating caused by interaction with its large moon Charon.

Pluto: Planet No More

The New Horizons spacecraft will fly past Pluto in July 2015.
The images radioed back to Earth will certainly show that Pluto is an interesting world.

Pluto: Planet No More

To understand why Pluto is no longer considered a planet, you should recall the Kuiper belt.
Since 1992, new, large telescopes have discovered roughly a thousand icy bodies orbiting beyond Neptune.
There may be as many as 100 million objects in the Kuiper belt larger than 1 km in diameter.
They are understood to be icy bodies left over from the outer solar system.

What Defines a Planet?

Some of the Kuiper-belt objects are quite large.
One, named Eris, is 5 percent larger in diameter than Pluto.
Three other Kuiper-belt objects found so far—Sedna, Orcus, and Quaoar—are half the size of Pluto or larger.

What Defines a Planet?

Some of these objects have moons of their own.
In that way, they resemble Pluto and its three moons.

What Defines a Planet?

A bit of comparative planetology shows that Pluto is not related to the Jovian or terrestrial planets.
It is obviously a member of a newfound family of icy worlds that orbit beyond Neptune.

What Defines a Planet?

These bodies must have formed at about the same time as the eight classical planets of the solar system.
However, they did not grow massive enough to clear their orbital zones of remnant planetesimals and remain embedded among a swarm of other objects in the Kuiper belt.

What Defines a Planet?

One of the IAU’s criteria for planet status is:
An object must be large enough to dominate and gravitationally clear its orbital region of most or all other objects.

What Defines a Planet?

Xena and Pluto—the largest objects found so far in the Kuiper belt—do not meet the standard.
Nor does Ceres—the largest object in the asteroid belt.

What Defines a Planet?

However, all three are large enough for their gravities to have pulled them into spherical shapes.
Hence, they are the prototypes of a new class of objects defined by the IAU as dwarf planets.

What Defines a Planet?

No, this section is not about a 1950s rock and roll band.
It is about the history of the dwarf planets.
It will take you back 4.6 billion years to watch the outer planets form.

Pluto and the Plutinos

Over a dozen Kuiper-belt objects are known that are caught with Pluto in a 3:2 resonance with Neptune.
That is, they orbit the sun twice—whereas Neptune orbits three times.
These Kuiper-belt objects have been named plutinos.

Pluto and the Plutinos

The plutinos formed in the outer solar nebula.
So, how did they get caught in resonances with Neptune?

Pluto and the Plutinos

Models of the formation of the planets suggest that Uranus and Neptune may have formed closer to the sun.
Sometime later, gravitational interactions among the Jovian planets could have gradually shifted Uranus and Neptune outward.
As Neptune migrated outward, its orbital resonances could have swept up small objects like a strange kind of snowplow.

Pluto and the Plutinos

The plutinos are caught in the 3:2 resonance.
Other Kuiper-belt objects are caught in other resonances.
This appears to support the models that predict that Uranus and Neptune migrated outward.

Pluto and the Plutinos

The migration of the outer planets would have dramatically upset the motion of some of these Kuiper-belt objects.

Pluto and the Plutinos

Some could have been thrown inward—where they could interact with the Jovian planets.
Some may have been captured as moons.
Astronomers wonder if moons such as Neptune’s Triton could have started life as Kuiper-belt objects.

Pluto and the Plutinos

Other objects may have impacted bodies in the inner solar system and caused the late heavy bombardment episode especially evident on the surface of Earth’s moon.

Pluto and the Plutinos

The small frozen worlds on the fringes of the solar system may hold clues to the formation of the planets 4.6 billion years ago.

Pluto and the Plutinos