Orbital and Physical Properties
Jupiter is named for the Roman king of the gods. It is the largest planet in the solar system.
The View from Earth
Jupiter is the 3rd brightest object in the night
sky after the Moon and Venus. Jupiter has
around 60+ moons and has no solid surface.
The 4 largest of these are called the Galilean Moons because
they were first seen by Galileo. When
you look at Jupiter you will see banding that goes across the planet. These are alternating cloud bands moving in
different directions and at different speeds.
Mass and Radius
After watching the Galilean Moons, we have been able to
determine the mass of Jupiter. It is
about 318 times more massive that the Earth.
In fact, Jupiter has more than twice the mass of all of the other
planets combined. Many scientists regard
the solar system as having 2 important objects: the Sun and Jupiter. These 2 objects exert a huge gravitational influence
on everything else in the solar system. The
radius o Jupiter is about 71,500 km or about 11.2 Earths in diameter. It would take 1400 Earths to fill the volume
of Jupiter. The density appears to be
about 1300 kg/m3. Our studies
of Jupiter show that it must be made up primarily of hydrogen and helium.
Rotation Rate
So far to determine the rotation rate all we really needed
to do was look at the planet and watch features. Jupiter is much different. The bands on Jupiter don’t all move at the same
rate. This is called differential rotation. This can’t happen in solid bodies, but since
Jupiter is a gas giant we get this effect.
To really calculate the rotation period we used the magnetosphere of
Jupiter. Jupiter has a strong magnetic
field so it was easy to track. Our
careful measurements show that Jupiter rotates in 9hours 55 minutes. This is the fastest rotation rate in the
solar system. Due to the fast rotation,
Jupiter bulges along the equator and is somewhat flattened in the polar
direction. The equatorial radius is
71,500 km while the polar radius is 66,900 km.
This also tells us something about the interior. If Jupiter were strictly hydrogen and helium
it would be flattened much more than it is.
This says that there must be an object about 5 – 10 times the mass of
the Earth at the center of Jupiter.
The Atmosphere of Jupiter
The thing that you will notice about Jupiter is that the
bands are ever-changing and that there is a disturbance called the Great Red Spot. In photographs you can see that the bands
show many colors to us. The Red Spot is
a hurricane that is about 2 times the size of the Earth. It has been visible since first being seen in
the 17th century.
Atmospheric Composition
Astronomers first studied the composition of the clouds by
looking at the spectra of reflected sunlight off the clouds. Later more advanced techniques gave us a
better understanding of Jupiter. The
clouds are made up of molecular hydrogen, 86.1%, and helium, 13.8%. That is over 99% of all material. Since Jupiter is so massive, it is thought
that very little of its original atmosphere has ever escaped out into
space.
Atmospheric Bands
The Jovian worlds are usually described as having a series
of bright zones and dark belts crossing the surface. The bright zones are thought to be the
upwelling of material from convection currents and the dark belts are the areas
that are falling back down into Jupiter.
This means that the zones are areas of high pressure and the belts are
areas of low pressure. The differences
in temperature and chemical composition are the reasons for the different
colors. There are underlying bands of
very stable air flow called zonal flow.
The equatorial region has material flowing faster due to the higher
rotation rate at the equator. It is
about 300 km/hr. This is very similar to
our jet streams on Earth. Jupiter has no
seasons (Why?) so the zones and belts remain pretty much intact even though
they can vary in their size.
Atmospheric Structure and Color
Scientists suspect that the variety of colors is due to
complex chemical reactions that are taking place in the atmosphere because
things like ammonia and water vapor and others wouldn’t produce the many colors
we see. Figure 11.6 is our best guess
due to available data and mathematical models.
The troposphere is considered to be at 0 km, which means we can have
negative altitudes on the drawing. The
clouds which are the result of convection currents are all at negative
altitudes. Just above the troposphere is
a thin layer of haze caused by photochemical
effects. These are reactions that
occur due to light. The temperature here
is about 110 K and it increases as you go up due to the absorption of UV.
There are 3 main layers in the atmosphere. At – 40 km you will find a layer of white
wispy clouds of ammonia ice. The
temperature here is about 125 – 150 K.
Below the clouds the temperature is around 200 K and the clouds are
probably made of drops of ammonium hydrosulfide. Below that you have clouds of water vapor or
water ice. The lowest cloud layer is
found at about -80 km. In the middle
layer you find the color is tawny. This
is where the atmospheric chemistry begins to occur. Many planetary scientists think this is where
sulfur plays an important part in color, especially the reds, browns, and
yellows.
Studying this is difficult because the chemicals are
sensitive to changes in pressure and temperature. It is a place that is constantly churning
which constantly mixes different chemicals together. That means that the conditions change hour to
hour. The source of energy is from
internal heat, solar UV, and lightning in the clouds of Jupiter. IN 1995 we checked our models when Galileo
dropped a probe into the atmosphere. It
showed that most of our ideas about Jupiter were good. The probe was dropped into an equatorial band
which happened to be void of high clouds.
One of the things that were unexpected was the fact that the winds
continued down into Jupiter. This says
that heat within Jupiter not the Sun drives Jupiter’s weather patterns. Simple carbon-carbon molecules were detected,
but nothing more significant.
Weather on Jupiter
There are many small-scale weather patterns on Jupiter. The Great Red Spot is one example. Robert Hooke first saw this in the 17th
century and it has existed ever since.
When did it start? We have no
idea. It is very much like a whirlpool
or hurricane. The winds swirl
around. The size of the spot varies over
time, but at present it is about 25,000 km x 15,000 km. It lies at about 20°S latitude. It is unknown
why the spot is the color it is. It
rotates counterclockwise in about 6 days.
There are many storms smaller than the Great Red Spot that occur as
well. On the dark side of Jupiter we
have seen what appears to be lightning.
Also on Jupiter we have seen small white ovals that have appeared and
disappeared. There has also been a brown
oval that only appears at 20°N
latitude. We don’t know why. These spots can exist for years or decades.
Internal Structure
At its distance from the Sun astronomers thought that the
temperature at the cloud tops should be about 105K, but when we got there we
found the temperature to be about 125K.
This means that Jupiter releases about 2 times as much energy as it
receives. Where does the extra energy
come from? It is thought to be coming
from the escape of gravitational energy or in other words, the energy of
contraction. As the planet contracted,
the molecules bumped into each other causing friction and producing heat. It is that heat that is slowly leaking out
through the atmosphere. It has been
calculated that Jupiter loses a millionth of a K per year.
Jupiter’s clouds are probably only about 200 km thick. Below them the temperature and pressure
increases. We have very little evidence
about the interior of Jupiter. But since
it is made up of hydrogen and helium, 2 gases that we understand well, we
believe that our model of the interior is accurate.
As you deeper inside Jupiter, the gas becomes more
compressed. At a depth of a few thousand
km the gas slowly becomes a liquid.
Below about 20,000 km the pressure is thought to be about 3 million
times the Earths pressure. Here the
liquid hydrogen is compressed so much that it takes on the properties of being
a metal. This is called liquid metallic
hydrogen. This is why Jupiter has such a
large magnetic field.
Since the Galileo spacecraft went to Jupiter we now believe
that the core in Jupiter may be as small as 5 times the mass of the Earth. It is believed to be a rocky core, one that
is similar for all of the Jovian planets.
The pressure at the center of Jupiter is about 50 million times greater
than Earths. The core must be extremely
dense. It is thought to be 20,000 km in
diameter and have a temperature of about 40,000 K.
Jupiter’s Magnetosphere
Fro years we knew that Jupiter had a magnetic field, but it
wasn’t until Voyager and Galileo did we realize just how large it was. Jupiter is surrounded by a large number of
charged particles, mainly electrons and protons. The radiation that we could detect on the
Earth was due to the particles being accelerated to high speeds- near the speed
of light- by the magnetic field. The
radiation is several thousand times more intense than produced by the Earth’s
magnetic field. This means that
spacecraft we put into orbit around Jupiter needs to be protected from the
particles. The magnetosphere is about 30
million km across (remember the Earth’s magnetosphere is about 120,000 km
across). The size and shape of the
magnetosphere is determined by the solar wind.
Just like on the Earth, Jupiter’s magnetic field can cause aurora on
Jupiter. The axis of the magnetic field
is 10° off from the rotational
axis. Observations have determined that
it is probably 20,000 times stronger than the Earths magnetic field.
The Moons of Jupiter
The number of moons around Jupiter stands at 61 as of early
2004. Give it time. We are finding small moons still, but now
comes in the problem of how small you call it a moon. The Voyager spacecraft discovered 3 of the 4
small moons that lie closer than Io does.
The largest is only 300 km across.
Then you come to the Galilean moons and just beyond them you see 8 small
moons, 4 of which move in eccentric, inclined orbits and the other 4 have
orbits that are fairly eccentric, but orbit in a retrograde motion. These are all thought to be captured
bodies. They were probably from 2
objects that broke up due to Jupiter’s gravity into 4 pieces each.
The Galilean Moons as a Model of the Solar System
The Galilean Moons are very similar to the terrestrial
planets. The orbits are roughly circular
and lie close to the equatorial plane.
Their sizes range from just smaller than our Moon to larger than
Mercury. Like the inner solar system,
the densities decrease as you move away from Jupiter. Many scientists think that the Galilean moon
system may have mimicked the formation of the solar system. This could give us valuable insights to the
formation of our solar system.
Io: the Most Active Moon
Io is the most active moon in the solar system. It is similar in size and mass as our Moon,
but that’s where it ends. Io has active
volcanoes. There have been over 80
active volcanoes identified on Io. The
largest is Loki which is bigger than
and emits more energy than all of the volcanoes on Earth. The lava at some volcanoes have temperatures
of 2000 K which makes it hotter than any volcano on Earth. The orange color you see on Io is due to the
sulfur compounds in the ejecta. Except
for the volcanoes, there really isn’t any other features because they have been
filled in by the lava.
The volcanoes on Io affect the magnetosphere of
Jupiter. Io is releasing many charged
particles and they are being trapped by the magnetosphere. The particles are swept up and accelerated. This gives rise to the Io plasma torus. Studies of
the torus show that sulfur is a major part of the torus. This torus would be lethal to humans. Why is a world as small as Io so active? There are tremendous tidal forces tugging at
Io. Jupiter pulls on Io while the other
3 Galilean Moons tug on Io in the opposite direction, The inside of Io is being pulled and tugged
by gravity which makes the inside of Io extremely hot. Because of this the inside of Io remains hot
constantly so that you have volcanoes erupting constantly. This means that the surface of Io is the
youngest surface in the solar system.
Europa: Liquid Water Locked in Ice
Europa is very different from Io. Io is a volcanic world while Europa is
covered in ice. There are very few
craters on Europa which suggests that some sort of geologic activity has taken
place. This means that the surface is
probably only a few million years old.
There are lines that run across the surface of Europa that resemble
pressure ridges that develop in the ice floes at the Earth’s poles. It is thought that Europa has an ocean that
is covered by a thin layer of ice. It is
due to the tidal forces of Jupiter and the other Galilean moons that these
cracks have formed. The ice layer is
thought to be several kilometers thick and the ocean may be 100 km thick. There are many areas that show what looks to
be localized flooding of the region. The
moon has been found to have a small magnetic field. It apparently has developed because the
magnetic field produced by Jupiter is acting on an electrically conducting
fluid about 100 km under the ice- specifically the salty layer of liquid
water. This causes the field to change
strength and direction. The possibly of
liquid water brings up the possibility of life on Europa. The Earth is the only world that we know of
that has liquid water. If Europa has
liquid water it may have more water than the Earth! But remember that it is still a very hostile
environment compared to the Earth.
Ganymeade and Callisto: Fraternal Twins
The last 2 Galilean moons are Ganymeade and Callisto. Their density is only about 2000 kg/m3. This says that they must contain a
significant amount of ice. Ganymeade is
the largest moon in the solar system. It
is even bigger than Mercury and Pluto.
It has many craters and areas of light and dark just like on the
Moon. The dark areas are the oldest
parts of Ganymeade. It is covered by
micrometeorite dust which darkens the surface.
The light areas are much younger.
These areas may have formed when intense meteoric activity caused liquid
water to well up from below and flood the area.
There are grooves and cracks that appear to be some type of tectonic
activity, much as Earth’s surface undergoes mountain building and faulting at
plate boundaries. Ganymeade appears to
have had plate tectonics early in its history, ending abut 3 billion years
ago. The Galileo spacecraft in 1996 detected
a weak magnetic field around Ganymeade. This
implies that Ganymeade has a modest iron rich core. The magnetic field is about 1% of the
Earth’s. In 2000 the field was seen to
show fluctuations just like Europa, so maybe Ganymeade has a layer of slushy
water under its surface.
Callisto is similar to Ganymeade in appearance. It has 2 large basins, the largest of which
is 3000 km across. Callisto apparently
froze before any plate tectonics occurred.
Callisto’s surface is probably about 4 billion years old, making it the
oldest of the 4 Galilean moons. There
are hints from Galileo’s magnetometers that there may be a thin layer of water
or slush deep below the surface. Unlike
Ganymeade that was differentiated in the past, Callisto has not
differentiated. Scientists are uncertain
why 2 such similar bodies have evolved so differently. It is thought the heating and melting on
Ganymeade occurred in the last billion years since it may still be slightly
warm inside. Ganymeade may have been
much closer to Jupiter and the tidal forces heated the insides. What happened? One idea is that Ganymeade’s orbit has
changed due to interactions with the other moons about a billion years
ago. This would explain why it is still
warm.
Jupiter’s Rings
In 1979 Voyager discovered that Jupiter has a series of
faint rings around it. They lie about
50,000 km above the planet. The rings
are perpendicular to the equatorial plane of Jupiter. The rings are dark and may be due to pieces
chipped of 2 small moons that are nearby.