Download Explain why the jovian planets are so much different

Astronomy 201 Review 3 Answers
Explain why the jovian planets are so much different from the terrestrial planets and why Jupiter
has the most mass, followed by Saturn, and so forth.
The jovian planets formed in a much colder region of the solar system where ice could condense.
The terrestrial planets could only have cores of rocky material, whereas the jovian planets could
have cores of rock and ices making them much larger. These larger planetesimals accreted
surrounding hydrogen and helium, so much so that each jovian planetesimal was in effect a
miniature solar nebulae where the planet formed at the center and moons formed within the
surrounding swirling disk. This explains why the jovian planets have so many more moons than
the terrestrial planets. The jovian planets have about the same mass in their cores, but the
surrounding layers of hydrogen and helium decrease as you go outward from Jupiter because
accretion rates went much faster for Jupiter and decreased outward toward the more spread­out
regions. By the time the solar wind blew the gas away, Jupiter had had more time to accrete so
it grew the largest.
Compare the atmospheres of the jovian planets including defining characteristics, weather
patterns, and structure.
The atmospheres of the jovian planets are very dynamic and while the atmospheric structure is
very similar between each jovian planet, the primary difference among them is temperature, due
to the decreasing temperature with increasing distance from the Sun. Jupiter's atmosphere is
primarily hydrogen and helium, in addition to a small fraction of hydrogen compounds. The
hydrogen compounds, although accounting for a very small fraction of the atmospheric mass,
largely govern Jupiter's appearance. Jupiter has a thermosphere, stratosphere and troposphere
and three distinct cloud layers, each at an altitude where the temperature is just right for a
particular gas to condense (ie ammonia, ammonium hydrosulfide and water). Convection in the
troposphere is responsible for the different cloud layers. The regions of rising air are called
zones and the regions of falling air are called belts. The defining characteristic of Jupiter's
atmosphere is the Great Red Spot which is a mysterious high­pressure storm that has been
brewing for centuries. Saturn, Uranus and Neptune all have distinct cloud layers as well,
governed by the temperature at which different gases condense. Saturn has the same three cloud
layers as Jupiter, however, due to Saturn's atmosphere being colder, the clouds are deeper in the
atmosphere. Clouds on Uranus and Neptune are due to methane snowflakes. Saturn has stripes
similar to Jupiter, but they are much less bold. Neptune is also banded and had a high­pressure
storm similar to the Great Red Spot on Jupiter called the Great Dark Spot. Uranus also has
storms due probably to the extreme change in seasons due to being tipped on its side. Jupiter
has bold brown and red colors whereas Saturn is less distinctively tan and yellow. Uranus is
blue­green and Neptune is a deep blue color. Uranus and Neptune derive their colors from
methane, whereas the hydrogen compounds on Jupiter and Saturn are responsible for their
colors.
Compare the interiors of the jovian planets as well as their sizes.
Jupiter's composition is probably a nearly uniform combination of hydrogen and helium with
each layer defined by the phase of the material, namely, gaseous hydrogen, liquid hydrogen and
metallic hydrogen. The metallic hydrogen plays an important role in generating Jupiter's strong
magnetic field. Jupiter's core is made of rock, metal and hydrogen compounds and is very very
dense. The temperature, pressure and density are greatest at the core's center and decrease
outward. Jupiter has a tremendous amount of internal heat probably due to contraction
(gravitational potential energy converted to thermal energy) and it actually emits more heat than
it receives. Saturn is made also of hydrogen, helium and hydrogen compounds and is about the
same size as Jupiter, though one­third Jupiter's mass. Saturn also has metallic hydrogen, but
much deeper in comparison to Jupiter and therefore there is less overall metallic hydrogen on
Saturn. Saturn also has a significant source of internal heat, however, the mechanism driving
this heat is different from the contraction generating internal heat in Jupiter because Saturn is
too small to generate all of its heat by contraction. The helium rain supposition attempts to
explain Saturn's internal heat generation by contending that the relatively high density helium
rain droplets that condense at high levels in Saturn due to its pressure and low interior
temperature, slowly fall to the interior, thereby gradually providing a mechanism of
differentiation (denser things fall to the center and less dense things above that). Uranus and
Neptune have cores similar to Jupiter and Saturn, but the layers surrounding their cores are
different in that the pressures are too low to form liquid or metallic hydrogen around their
cores. That being said, their cores may be liquid making for oceans deep inside them. Uranus
emits practically no more heat than it receives which is expected, but Neptune unexpectedly
emits nearly twice the heat it receives. The source of Neptune's additional heat is a mystery and
it's unusual to think that Neptune would still be contracting to produce additional heat due to its
relatively small size.
Describe the magnetospheres of the jovian planets.
A magnetosphere traps charged particles causing them to spiral along magnetic field lines and
in doing so, the more energetic particles collide with atoms and molecules in the upper
atmosphere causing them to radiate and produce an aurora. Jupiter has the brightest auroras
due to the large number of trapped charged particles. Each jovian planet is surrounded by a
magnetosphere. The size of a magnetosphere depends on both the magnetic field strength and
the pressure of the solar wind against it. Just as the further away from the sun a planet is the
lower the temperature, larger distances correspond to weaker solar wind pressure on a distant
planet, so Uranus and Neptune, despite intrinsically weak magnetic fields, have moderately sized
magnetospheres. Jupiter's magnetic field is about 20,000 times stronger than Earth's due to the
thick layer of metallic hydrogen. Jupiter also has two sources of charged particles interacting
with its magnetosphere: the solar wind and gas coming from Io. Io loses atmospheric gas very
fast due to the constant bombardment of charged particles within Jupiter's magnetosphere. The
magnetic axis of Jupiter is roughly aligned with its rotation axis as would be expected. Saturn
has a weaker magnetic field than Jupiter because it has less metallic hydrogen. Saturn's
magnetic axis is also aligned with its axis of rotation. Neptune and Uranus have no metallic
hydrogen and therefore have decidedly weak magnetic fields probably caused by liquid
materials in their cores. Unexpectedly, the magnetic axes of Neptune and Uranus are not
aligned with their respective rotation axes.
Where did the jovian moons come from?
The larger moons probably formed around the swirling disk of material surrounding each jovian
planetesimal during formation and this is supported by the fact that the large majority of the
moons orbit the same direction that their planet rotates and lie close to the equatorial plane
orbiting in nearly circular orbits. Most of the moons also rotate synchronously due to the strong
tidal forces exerted by the jovian planets. Most of the small moons are probably captured
asteroids with eccentric orbits and often large tilts relative to the equatorial plane.
Why are jovian moons generally more geologically active than terrestrial planets?
In some cases (Io and Europa), the moons have a continual source of internal heat due to tidal
heating (from Jupiter) whereby any deviation from a circular (synchronous) orbit means a
change in the strength and direction of the tidal force, altering the sizes and orientations of tidal
bulges on the moons and generating internal friction. Additionally, the composition of the
jovian moons includes a higher proportion of ice (directly related to where they formed in the
solar system), relative to the terrestrial planets, which can undergo geological change at lower
temperatures than the rocky materials that make up the terrestrial planets. Accordingly, the
moons of the more distant planets are composed of a higher fraction of ice.
What are the Galilean moons? Briefly describe them.
The Galilean moons include Io, Europa, Ganymede, and Callisto. Io is the most volcanically
active celestial body in the solar system and therefore has an extremely young surface. Io has a
thin atmosphere of sulfur dioxide produced through outgassing, which is also accountable for
the additional ionized gas particles within Jupiter's magnetosphere. Sulfur is also the reason for
the red and orange colors of Io. Io is also thought to have tectonic processes. Europa's surface
and crust are made primarily of water ice and its surface is devoid of impacts. Europa might
have an ocean of liquid water deep under its surface due to tidal heating which would be nice
and would explain the lack of impacts. Europa has a magnetic field that is probably induced by
Jupiter's strong magnetic field sweeping past and it changes as Jupiter rotates. A salty liquid
ocean could drive such a magnetic field. Ganymede has a surface of water ice and regions of
very young surface, as well as regions of very old surface. The regions of young surface are
thought to be replenished by upwellings of liquid water that fill the craters prior to freezing.
Ganymede may also have an ocean of liquid water supported by magnetic field measurements.
Callisto is also covered with ice, but is heavily cratered. Callisto lacks volcanic and tectonic
activity suggesting very weak internal heating. Callisto is not in an orbital resonance and thus
has no tidal heating. Callisto's interior never underwent differentiation due to its extremely cool
interior. Callisto has a magnetic field suggestive of a submerged salty liquid ocean. Ganymede,
Europa, and Io are in a 1:2:4 orbital resonance and all are subject to tidal heating due to their
slightly elliptical orbits, though the degree of tidal heating depends on the moon's distance from
Jupiter.
Describe Saturn's largest moon Titan.
Titan's atmosphere is primarily nitrogen with the rest being argon, methane, and ethane. The
latter two are greenhouse gases that significantly warm Titan. Titan is composed mostly of ices
including methane and ammonia ice that, when sublimation occurs, adds to the atmosphere.
Ultraviolet light may have substantially reduced the amount of methane by converting it to
ethane, which may in turn form clouds and rain liquid ethane. The surface temperature and
pressure on Titan are just about right for ethane to exist at its triple­point, where it can exist as
a gas, liquid and solid simultaneously. This, in addition to surface brightness variations, lend
evidence to support oceans of liquid ethane.
What is interesting about the moons of Uranus?
Uranus has five medium­sized moons. Specifically, there are two pairs of similar­sized (“twin”)
moons that vary significantly in the context of their geological activity and this continues to be a
mystery.
What are the two major moons of Neptune and why is Triton unique?
Nereid is a medium­sized moon orbiting Neptune and Triton is the larger and much colder moon
in retrograde rotation. Triton also orbits highly inclined relative to the equatorial plane of
Neptune which suggests it was captured. Triton is icy, spherical, and large and it is thought that
Triton orbited the Sun at one point instead of Neptune. Triton has a large number of impacts,
but also shows evidence of past icy volcanism. Triton also shows evidence for past geologic
activity in the “cantaloupe terrain” which comprises a region of wrinkly ridges. The source of
internal heat powering that geological activity may have involved tidal heating, but it is not
known for sure. Triton also undergoes extreme seasonal changes. Since Triton orbits in
retrograde motion, it is being slowed down by tidal bulges and is slowly spiraling in.
Eventually, it will reach the Roche zone and an extraordinary broad ring system will surround
Neptune.
Which of the jovian planets have rings? What accounts for gaps in ring systems? What is the
origin of rings?
All four jovian planets have rings. Saturn's rings are the most well known and they are made of
individual particles consisting primarily of reflective water ice. Each particle orbits Saturn
independently according to Kepler's laws. Gaps within rings are primarily due to orbital
resonances which means that every time a given particle enters a specific point in its orbit, it
feels a consistent gravitational push that clears it out and creates a gap. The Mimas 2:1
resonance is responsible for the Cassini division (which we know is not completely devoid of
particles). Jupiter, Uranus, and Neptune also have rings, but they are much fainter and smaller
than those surrounding Saturn. The rings around Uranus were actually discovered during
observations of stellar occultation. The rings around the jovian planets lie close to their planets
where tidal forces are strong. When the tidal forces exerted on an object become comparable to
the gravitational forces holding it together, the object has reached the Roche zone and will be
torn apart. Another explanation is that the constant bombardment of micrometeorites on small
moons within rings provides a continual source of ring particles.
What are the three categories of “small bodies” in the solar system? Describe asteroids and
comets. Describe the origin of the asteroid belt.
Asteroids, Kuiper belt comets, and Oort cloud comets make up the smaller bodies of the solar
system. Asteroids are rocky chunks of material thought to be leftover from the formation of the
solar system. A comet is an icy leftover orbiting the Sun usually with a very long period
compared to asteroids. Meteors are pieces of material that streak across the sky and a meteorite
is a piece of material that actually reaches the surface of Earth. The majority of asteroids are
located in the asteroid belt between Mars and Jupiter and this is probably due to orbital
resonances between the asteroids and Jupiter. Orbital resonances with Jupiter also probably
prevented a planet from forming where the asteroid belt is because the planetesimals that would
have accreted into a planet there were not allowed to accrete due to the orbital resonances
throwing material out of the region. Gaps in the asteroid belt are known as Kirkwood gaps.
Comets consist of a nucleus, a coma, and a tail which consists of gas and dust; actually there
are two tails, the plasma tail is made of ionized gas and the dust tail is made of small particles.
Will Pluto and Neptune ever collide? Why?
Neptune and Pluto are in a 3:2 orbit resonance which translates to Neptune always being a safe
distance from Pluto. Also, Pluto is inclined roughly seventeen degrees relative to the equatorial
plane of Neptune. What is the explanation for Pluto's higher­than­expected density? What evidence suggests Pluto
is in fact a Kuiper belt object?
It is thought that a giant impact with Pluto may have blasted away the low­density outer regions
of Pluto which ultimately led to the formation of its moon Charon. Pluto is about the size of a
large Kuiper belt object and its inclined orbit is different from the relatively planar orbits of the
planets.