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ASTRONOMY
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MURCHISON
Chapter 7
The Jovian Planets
1) Jupiter, Saturn, and Uranus, and Neptune are giant planets, also called the Jovian planets
2) They are much bigger, more massive, and less dense that the inner planets
3) Their internal structure is entirely different from that of the four other planets
JUPITER
1) The largest and most massive planet
2) It alone contains two-thirds of the mass in the Solar System, outside the Sun, 318 times as much mass as
the Earth (but only 0.001 times the Sun’s mass)
3) Jupiter has at least 16 moons of its own and can be thought of as a miniature planetary system in itself
4) It is often seen as a bright object in our night sky
5) Jupiter is more than 11 times greater in diameter than the Earth
6) From its mass and volume, the density of Jupiter has been calculated to be 1.3 g/cm3, not much more
than the 1g/cm3 density of water
a) This low density indicates that any core of heavy elements (such as iron) makes up only a small
fraction of Jupiter’s mass
7) Jupiter is mainly composed of the elements hydrogen and helium
a) Jupiter’s chemical composition is closer to that of the Sun and stars than it is to that of the Earth,
so its origin can be traced directly back to the solar nebula with much less modification than the
terrestrial planets underwent
8) Jupiter has no crust.
9) At deeper and deeper levels, its gas just gets denser and denser, turning eventually mushy and eventually
liquefying about 20,000 km (15 per cent of the way) down
10) Jupiter’s surface rotates in about 10 hours, though different latitudes rotate at slightly different speeds.
a) Regions with different speeds corresponds to different bands
b) The clouds are in constant turmoil – the shapes and distribution of bands can change within days
11) the most prominent feature of Jupiter’s surface is a large reddish oval known as the Great Red Spot
a) It is two to three times larger than the Earth – other smaller spots are also present
12) Jupiter emits radio waves, which indicates that it has a strong magnetic field and strong “radiation belts”
– large scale versions of the Van Allen belts on Earth
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The Great Red Spot
1) The Great Red Spot is a gaseous island a few times larger across than the Earth
2) 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
3) From the sense of its rotation, measured from time-lapsed photographs, we can tell it is a pressure high
rather than a pressure low
a) It interacts with surrounding clouds and smaller spots
4) The Great Red Spot has been visible for at least 150 years and perhaps even 300 years
5) Sometimes it is relatively prominent and colorful, and at other times the color may even disappear for a
few years
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Spacecraft to Jupiter
1) In the 1970’s Pioneer 10 and Pioneer 11 and then Voyager 1 and Voyager 2 flew by
2) The Galileo spacecraft arrived in 1995 and dropped a probe through the atmosphere and went into orbit
in the Jupiter system
3) Cassini
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6) Why has the Great Red Spot lasted this long?
a) Heat, energy flows into the storm from below it, partly maintains its energy supply
b) The storm also contains more mass than hurricanes on Earth, which makes it more stable
c) Unlike Earth, Jupiter has no continents or other structure to break up the storm
Jupiter’s Atmosphere
1) Heat emanating from Jupiter’s interior churns the atmosphere – compared to Earth’s atmosphere where
most of the energy comes from the outside (the Sun)
2) The bright bands on Jupiter are rising currents of gas driven by this pressure of convection
a) The dark bands are falling gas
3) Winds velocities show that each hemisphere of Jupiter has a half-dozen currents blowing eastward or
westward
a) The Earth, in contrast, has only one westward current at low altitudes (the trade winds) and one
eastward current at middle latitudes (the jet stream)
4) On December 7, 1995, the Galileo probe transmitted data for 57 minutes as it fell through Jupiter’s
atmosphere
a) It went through about 600 km of Jupiter’s atmosphere, only about 1 per cent of Jupiter’s radius
b) The probe found that Jupiter’s winds were stronger than expected and increased with depth,
which shows that energy that drives them comes from below
5) Extensive lightning storms, including giant-sized lightning strikes called “super bolts” were discovered
from the Voyagers, as were giant aurorae
a) The Galileo spacecraft photographed giant thunderclouds on Jupiter, which indicate that some
regions are relatively wet and others relatively dry
Jupiter’s Interior
1) Most of Jupiter’s interior is in liquid form
2) Jupiter’s central temperature may be between 13,000 and 35,000 K
3) The central pressure is 100 million times the pressure of the Earth’ atmosphere measured at our sea
level because of Jupiter’s great mass pressing in
a) Because of this high pressure, Jupiter’s interior is probably composed of ultra compressed
hydrogen surrounding a rocky core consisting of perhaps 10 Earth masses of iron and silicates
4) Jupiter radiates 1.6 times as much heat as it receives from the Sun
a) It must have an internal energy source – perhaps the energy remaining from its collapse from a
primordial gas cloud 20 million km across
b) Jupiter is still contracting and this process also liberates energy
c) 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
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Jupiter’s Ring
1) Voyager 1 discovered a ring around Jupiter and Voyager 2 was programmed to take a series of
photographs of the ring.
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Jupiter’s Magnetic Field
1) At the height of Jupiter’s clouds the magnetic field strength is 10 times that of the Earth
2) The inner field is shaped like a doughnut, containing several shells of charged particles, like giant versions
of the Earth’s Van Allen belts.
3) The outer region of Jupiter’s magnetic field interacts with the particles flowing outward from the Sun
a) When this solar wind is strong, Jupiter’s outer magnetic field (shaped like a flattened pancake) is
pushed in
4) When the high-energy particles interact with Jupiter’s magnetic field, radio emission results
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Jupiter’s Galilean Satellites
1) Four of the innermost satellites were discovered by Galileo in 1610
2) These four main moons (Io, Europa, Ganymede, and Callisto) are called the Galilean satellites
3) The Galilean satellites played an important role in supporting Copernicus’s Sun-centered model of the
Solar System
4) Jupiter also has another dozen satellites, but none of these other satellites is even 10 per cent of the
diameter of the smallest Galilean satellite
5) The satellites of Jupiter have become known to us as worlds with personalities of their own
6) Io is the innermost Galilean satellite
a) Io gives off particles as it goes round Jupiter and Voyager 1 discovered that these particles
resulted from active volcanoes on the satellite
b) The Galileo satellite went within a few hundred kilometers of Io’s surface and detected 100
volcanoes erupting simultaneously
c) Io’s surface has been transformed by the volcanoes and is by far the youngest surface we have
observed in our Solar System
d) Gravitational forces from the other Galilean satellites distort Io’s orbit slightly, which changes the
tidal force on it from Jupiter in a varying fashion
i) This changing tidal force flexes Io, creating heat from friction that heats the interior and
leads to the volcanism
e) The surface of Io is covered with sulfur and sulfur compounds, including frozen sulfur dioxide and
the thin atmosphere is full of sulfur dioxide
7) Europa has the highest albedo, has a very smooth surface and is covered with narrow dark stripes
a) The lack of surface relied, mapped by the Galileo spacecraft to be no more than a couple of
hundred meters high, suggests that the surface we see is ice
b) The markings may be fracture systems in the ice, like fractures of sea ice near the Earth’s north
pole, as verified in the Galileo close-ups
c) Few craters are visible, suggesting that the ice was soft enough below the crust to close in the
craters
i) Either internal radioactivity or gravitational tidal heating provides the heat to soften the
ice
d) Because Europa possibly has a liquid-water ocean and extra heating, many scientists consider it a
worthy place to check for life
8) Ganymede – the largest satellite in the Solar System – shows many craters alongside weird grooved
terrain
a) Ganymede is larger than Mercury but less dense – it contains large amounts of water-ice
surrounding a rocky core
i) In the cold conditions so far from the Sun, an icy surface is as hard as steel
ii) Therefore Ganymede retains craters from perhaps 4 billion years ago
iii) The grooved terrain is younger
b) Ganymede shows many lateral displacements similar to features on Earth like the San Andreas
fault in California
i) It is the only other place in the Solar System where such faults have been found
ii) Further studies of Ganymede may help our understanding of terrestrial earthquakes
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a) From the far side looking back, the ring appeared unexpectedly bright
b) Within the main ring, fainter material appears to extend down to Jupiter’s cloud tops
2) The ring particles may come from Jupiter’s moon Io, or they may come from comet and meteor debris or
from material knocked off the innermost moons by meteorites
3) Whatever their origin, the individual particles probably remain in the ring only temporarily
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9) Callisto – the outermost of Jupiter’s Galilean satellites, has so many craters that its surface must also be
the oldest
a) Callisto (like Europa and Ganymede) is covered with ice
b) A huge bull’s-eye formation - Valhalla – contains about 10 concentric rings, probably resulting
from a large impact
c) Galileo’s measurements of Callisto’s gravity from place to place show that its mass is
concentrated more toward its center that had been thought
i) This concentration indicates that heavier materials inside have sunk, and perhaps even
indicates that there is an ocean below Callisto’s surface
iii) Callisto’s interactions with Jupiter’s magnetic field have been interpreted to back up the
idea of an ocean
SATURN
Saturn’s diameter, without its rings, is 9 times greater than Earth’s; its mass is 95 times greater
Saturn’s is only 0.7 g/cm3, 70 per cent the density of water (Saturn would float in water)
The bulk of Saturn is hydrogen molecules and helium, reflects Saturn’s formation from the solar nebula
Saturn is thought to have a core of heavy elements including rocky material, making up the inner 20 per
cent of its diameter
5) Voyagers 1 and 2 flew by Saturn in 1980 and 1981 and Cassini arrived in 2004
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Saturn’s Rings
1) The rings extend far out in Saturn’s equatorial plane, and are inclined to the planet’s orbit
2) 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
a) When seen edge-on, they are almost invisible
3) The rings of Saturn were originally 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; the actually particles in
the ring now are more newly arrived
a) Every massive object has a sphere – called a Roche limit – inside of which blobs of matter do not
hold together by their mutual gravity
b) The forces that tend to tear the blobs apart from each other are tidal forces – they arise because
some parts of an object are closer to the planet that others and are subject to higher gravity
i) The difference between the gravity force farther in and the gravity force farther out is
the tidal force
c) The radius of the Roche limit is usually 2 ½ to 3 times the radius of the larger body, closer to the
latter for the relative densities of Saturn and its moons.
i) The Sun also has a Roche limit, but all the planets lie outside it
ii) The natural moons of the various planets lie outside the respective Roche limits
d) Saturn’s rings lie inside the Roche limit, and therefore the material in the rings is spread out
rather than collected into a single orbiting satellite
i) Artificial satellites sent up into orbit are constructed out of sufficiently rigid material that
they do not break up even though they are within the Earth’s Roche limit – they are held
together by forces stronger that gravity
4) Saturn has several concentric major rings visible from Earth
a) The brightest ring is separated from a fainter broad outer ring by an apparent gap called Cassini’s
division
b) Another ring is inside the brightest ring
c) The rings are not solid objects, because their rotation speed decreases as their distances from
the planet increase
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d) Radar waves bounced off the rings show that the particles in the rings are at least a few
centimeters and possibly a meter across. Infrared studies show that at least their outsides are ice
e) The rings are thin, for when they pass in front of stars, the starlight easily shines through
i) Studies of the changes in the radio signals from the Voyagers when they went behind the
rings showed that the rings are only 20 km thick – relative to the diameter of the rings,
this is equivalent to a CD that is 30 km across though still its normal thickness
iii) Each of the major rings are actually divided into many thinner rings
iv) The number of these “ringlets” is in the hundreds of thousands
f) Expectations were that the collisions between particles in Saturn’s rings would make each ring
perfectly uniform
i) Voyager 1 discovered radial spokes – the spokes look dark from the side illuminated by
the Sun but look bright from behind
ii) This showed that the spokes were very small – about 1 micron in diameter – since only
small particles, such as terrestrial dust in a sunbeam, reflects in this way
g) The outer major ring turns out to be kept in place by a tiny satellite orbiting just outside it
i) At least some of the rings are kept narrow by “shepherding” satellites that gravitationally
affect the ring material, a concept that can be applied to the rings of other planets
iii) A post-Voyager theory stated that many of the narrowest gaps may be swept clean by a
variety of small moons
iv) These objects would be embedded in the rings in addition to the icy snowballs that make
up most of the ring material
v) The tiny moon Pan has been observed cleaning out the Encke Gap in this way
vi) These bodies have not been discovered by the Voyagers
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Saturn’s Interior and Magnetic Field
1) Saturn radiates about twice as much energy as it absorbs from the sun, a greater factor than for Jupiter
a) One interpretation is that only 1/3 of Saturn’s internal energy remains from its formation and
from its continuing contraction under gravity
b) The rest would be generated by the gravitational energy released by helium sinking through the
liquid hydrogen in Saturn’s interior.
c) the helium that sinks has condensed because Saturn, unlike Jupiter, is cold enough
2) Saturn gives off radio signals, an indication that Saturn also has a magnetic field
a) The Voyagers found that the magnetic field at Saturn’s equator is only 2/3 present of the field
present at the Earth’s equator
b) Saturn is much larger than the Earth and so its equator is much farther from its center
i) Saturn’s magnetic field contains Van Allen belts which are larger than Earth’s but smaller
than Jupiter’s
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Saturn’s Atmosphere
1) Saturn rotates quickly on its axis
2) A complete period is only 10 hours in spite of Saturn’s diameter being over 9 times greater than Earth’s
a) This makes Saturn look slightly flattened – it bulges out more at the equator that at its poles
3) The structure in Saturn’s clouds is of much lower contrast than that in Jupiter’s clouds – the chemical
reactions are different because Saturn is colder
4) Saturn has extremely high winds, up to 1800 km/hr, 4 time faster than the winds on Jupiter
5) The variations in wind speed do not seem to correlate with the positions of bright and dark bands, unlike
the Jupiter
a) As on Jupiter, the winds seem to be driven by rotating eddies which in turn get most of their
energy form the planet’s interior.
i) Such differences provide a better understanding of storm systems in Earth’s atmosphere
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ii) These particles interact with the atmosphere near the poles and produce aurorae
Saturn’s Moon Titan
1) Titan is Saturn’s largest moon and is larger than the planet Mercury, and is 40 percent the diameter of
Earth and is an intriguing body
2) Titan has an atmosphere
a) Titan’s atmosphere is denser than Earth’s
b) The surface pressure on Titan is 1 ½ times that on Earth
c) Titan’s atmosphere is opaque, because of the action of sunlight on chemicals in it, forming a sort
of “smog” and giving it its reddish tint – smog on Earth forms in a similar way
d) The Voyagers showed several layers of haze
e) They detected nitrogen which makes up the bulk of Titan’s atmosphere; methane is about 1
percent of the atmosphere
i) A greenhouse effect is present, leading scientists to believe that Titan’s surface may have
been warmed enough for life to have evolved there
ii) The temperature near the surface is only about -180oC (93 K), warmed by the
greenhouse effect, but still very cold
iii) This temperature is near that of methane’s “triple point” at which methane can exist in
any of the three physical states – solid, liquid, or gas
iv) Parts of Titan may be covered with lakes or oceans of methane mixed with ethane, and
other parts may be covered with methane ice or snow
f) Some of the organic molecules formed in Titan’s atmosphere from lightening storms and other
processes may rain down on its surface
i) Therefore the surface (largely hidden from our view) may be covered with an organic
crust about a kilometer thick, perhaps partly dissolved in liquid methane
ii) These chemicals are similar to those from which we think life evolved on the primitive
Earth – but it is probably too cold on Titan for life to begin
g) Titan is so potentially important the Huygens Lander of the Cassini mission penetrated the
clouds around Saturn’s largest moon
URANUS
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Uranus was the first plant to be discovered that had not been known to the ancients.
Uranus revolves around the Sun in 84 Earth years at an average distance of more that 19A.U.
Uranus appears so tiny that it is not much bigger that the resolution we are allowed by our atmosphere
Uranus is apparently surrounded by thick clouds of methane ice crystals with a clear atmosphere of
molecular hydrogen above them
a) The trace of methane gas mixed in with the hydrogen makes Uranus look greenish
5) Uranus is so far from the Sun that its outer layers are very cold with temperatures of -215oC (58 K)
a) There is no evidence for an internal heat source (unlike the case for Jupiter, Saturn, and Neptune)
6) The other planets rotate so that their axes of rotation are very roughly parallel to their axes of revolution
around the Sun
a) Uranus is different for its axis of rotation is roughly perpendicular to the other planetary axes,
lying only 8o from the plane of its orbit
b) 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 Earth
c) Polar regions remain alternately in sunlight and in darkness for decades
7) When we understand how the seasonal changes in heating affect the clouds, we will better understand
the Earth’s climate
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Uranus Atmosphere
1) Voyager 2 come very close to Uranus’s surface (a quarter of the distance from the Earth to the Moon) it
saw very little detail on it
2) Uranus’s surface is very bland and apparently chemical reactions are more limited than on Jupiter and
Saturn perhaps because Uranus is colder
3) Uranus’s clouds from relatively deep in the atmosphere
4) A dark polar cap was seen on Uranus, perhaps are a result of a high-level photochemical haze added to
the affect of sunlight scattered by hydrogen molecules and helium atoms
5) At lower levels, the abundance of methane gas (CH4) increases
a) It is this gas that absorbs the orange and red wavelengths from the sunlight that hits Uranus and
therefore most of the light that is reflected back at us is blue-green
6) It was a surprise that both of Uranus’s poles, even the one out of sunlight are about the same
temperature and the equator is nearly as warm
7) Comparing such a strange atmosphere with our own will help us understand Earth’s weather and climate
and better
Uranus’s Rings
1) Uranus is surrounded by several rings some of which have since been photographed by Voyager 2 and
with the Hubble Space Telescope
2) Nine rings are now known – they are quite dark, reflecting only about 2 per cent of the sunlight that hits
them
3) The rings have radii 1.7 to 2.1 times the radius of the planet
4) They are very narrow from side to side, only a few km wide
5) The discovery of Uranus’s narrow rings led to the suggestion that a small satellite (a shepherd moon) in
each ring keeps the particles together – this model turned out to be applicable to at least some of the
narrowest ringlets of Saturn later discovered by the Voyagers
6) Interpreting the small color differences is important for understanding the composition of the ring
material
7) 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
8) The rings of Uranus are apparently younger than 100 million years of age, since the satellites that hold
them in place are too small to hold them longer
9) That source may have been a small moon destroyed by a meteoroid or comet
10) The larger particles seen only when the rings were back lighted may have come from a different source
a) Perhaps they came from the surface of Uranus’s current moons
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Uranus’s Interior and Magnetic Field
1) Voyager 2 discovered Uranus’s magnetic field and it is intrinsically 50 times stronger than Earth’s
2) It is tipped 60o with respect to Uranus’s axis of rotation
3) it is centered on a point offset from Uranus’s center by 1/3 its radius
4) Since Uranus’s field is so tilted, it winds up like a corkscrew as Uranus rotates
5) Uranus’s magnetosphere contains belts of protons and electrons, similar the Van Allen belts
6) Voyager also detected radio bursts from Uranus every 17.24 hours.
a) These bursts apparently come from locations carried by Uranus’s interior by the magnetic field as
the planet rotates.
b) This provides evidence that Uranus’s interior rotates slightly more slowly than its atmosphere
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NEPTUNE
1) Neptune is farther from the Sun than Uranus, 30 A.U. compared to Uranus which is 19 A.U.
2) Neptune takes 164 years to orbit the Sun
3) The discovery of Neptune was a result of modern Newtonian astronomy
a) Mathematicians analyzed the amount that Uranus (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
b) The small deviations could have been caused by gravitational interaction with another planet
4) Neptune has not yet made a full orbit since it was located in 1846
a) But, it seems that Galileo inadvertently observed Neptune in 1613 and recorded its position,
which more than doubles the period time over which it has been studied
b) We have used the positions he measured to improve our knowledge of Neptune’s orbit
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Neptune’s Interior and Magnetic Field
1) Voyager 2 measured Neptune’s average temperature: 59 K – that is 59oC above absolute zero
2) This temperature is higher than would be expected on the basis of solar radiation alone
a) Neptune gives off about 2.7 times as much energy as it absorbs from the Sun and therefore there
is an internal source of heating
3) The average density of Uranus and Neptune is higher than that of Jupiter and Saturn.
a) This may reflect slight differences in the origins of those planets
b) Their densities show that Uranus and Neptune have a higher percentage of heavy elements than
Jupiter and Saturn
c) One possible explanation is that the rocky cores they built up from the solar nebula were smaller,
giving them less gravity and therefore attracting less hydrogen and helium or perhaps the
positions of Neptune and Uranus farther out in the solar nebula was less dense or their slower
orbital motion moved them through less gas
4) Voyager detected radio bursts from Neptune every 16.11 hours and thus Neptune's interior must rotate
with this rate.
5) On Neptune and Uranus, as on the Earth, equatorial wind blow more slowly than the interior rotates
a) By contrast, equatorial winds on Venus, Jupiter, Saturn, and the Sun blow more rapidly than the
interior rotates
6) This gives us a variety of planetary atmospheres to help understand the basic causes of circulation
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Neptune’s Atmosphere
1) Neptune (like Uranus) appears greenish in a telescope because of its atmospheric methane
2) Some faint markings can be detected on Neptune even from Earth
3) As Voyager approached Neptune, active weather systems became apparent
a) An Earth sized region called the Great Dark Spot, analogous to Jupiter’s Great Red Spot in several
ways
b) 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
i) Scientists discovered that it rotated counterclockwise, as does the Great Red Spot
ii) Therefore it is an anticyclonic storm which makes it a high-pressure region
iii) Clouds of ice crystals made of methane, form at the edge of the Great Dark Spot as the
high pressure forces methane-rich gas upward
iv) The Great Dark Spot had disappeared when the Hubble Space Telescope photographed
Neptune a few years later, and therefore much less long lived that Jupiter’s Great Red
Spot
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7) Voyage discovered and measured Neptune's magnetic field and it turned out to be greatly tipped and
offset from Neptune's center
a) The tentative explanation for Uranus that its field was tilted by a collision is not plausible since
such a rare event would not be expected to happen twice
b) Astronomers favor the explanation that the magnetic field is formed in an electrically conducting
shell outside the planets' core
Neptune's Rings
1) As Voyager came close to Neptune it radioed back images that showed conclusively that Neptune had
narrow rings
2) It showed the rings going all the way around Neptune
3) The material in the densest of Neptune's rings is very clumpy
1) the fact that Neptune's rings are so much brighter when seen back lighted tells us more about the sizes
of particles in them
4) The most detectable parts of the rings have at least a hundred times more dust-size grains than most of
the rings of Uranus and Saturn
5) Since dust particles settle out of the rings, new source must be continually active
6) Probably moonlets collide and are destroyed
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Neptune's Moon Triton
1) Neptune's largest moon Triton is a little larger than our Moon and has a retrograde orbit
2) It is named after a sea god who was the son of Poseidon
3) It is massive enough to have a melted interior
4) Its density is 2.07 grams/cm3, so it is probably about 70 per cent rock and 30 per cent water-ice.
5) It is denser than any Jovian-planet satellite except Io and Europa
6) Triton has an atmosphere which is mostly nitrogen gas like Earths
7) Triton surface is incredibly varied. Much of the region Voyage imaged was near Triton's south polar cap
a) The ice appears reddish – this color probably shows the presence of organic material formed by
the action of solar ultraviolet light particles from Neptune's magnetosphere hitting methane in
Triton's atmosphere and surface
b) Near Triton's equator, nitrogen frost was seen
c) Many craters and cliffs were seen
8) Neptune's gravity captures many comets in that part of the Solar System; most the craters are thought to
result from collisions with comets
9) Triton's surface showed about 50 dark streaks parallel to each other
a) They are apparently dark material vented from below. The material is spread out by winds
b) Much of Triton is puckered giving it a so called cantaloupe terrain
10) Triton has been geologically active
11) Since it is in a retrograde (backward) orbit around Neptune, it was probably born elsewhere in the Solar
System and later captured by Neptune
12) Tidal forces from Neptune would have kept Triton molten until its orbit became circular
13) While it was molten, the heavier rocky material would have settled to form a 2000-km-diameter core
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Extra Solar Planets
1) Planets around other starts would be so faint that they would be extremely difficult to see in the glare of
the parent star with current technology
2) The search for planets has not concentrated on the visible sighting of these planets but instead
depending on watching for motions in the star that would have to be caused by something orbiting
3) The first extra-solar planets were discovered around a pulsar (a type of collapsed star) that gives off
extremely regular pulses of radio waves with a period that is a fraction of a second
4) The pulses came more frequently for a time and then less frequently in a regular pattern
a) So the planet around this pulsar must be first pulling the pulsar in our direction, making the
pulses come more rapidly, and then pulling it in the opposite direction, making the pulses come
more seldom
5) In the 1990, techniques were developed using the Doppler Effect – the Doppler Effect is a change in the
wavelengths of light that has been emitted, cause by motion of the source or the receiver along the line
of sight
6) The discovery, in 1999, of a system of three planets around the star Upsilon Andromedae clinched the
case for planets
a) It seems most unlikely that a system would have formed with four stars (or brown dwarfs) in it,
while a system with one star and three planets is reasonable
b) One of the planets even has an orbit that would correspond to Venus's in our solar system, not as
elliptical as the orbits of other planets around other stars
c) This planet is even in a zone that may not be too hot for life nor too cold for it
d) Though such a massive planet would be gaseous, and so not have a surface, and so not have a
surface for life to live on, it could have a moon with a solid surface, just as Jupiter and the other
giant planets in our Solar System have moons
7) No telescope in space currently has enough resolution to detect a planet orbiting another star
8) Though we now only detect only giant planets, we presume that small planets exist in those systems as
well.
9) It will take even longer to detect them – NASA's planned Terrestrial Planet Finder, with optical telescopes
flying in space kilometers apart but used together, may be able to image small planets in the second
decade of the century
10) The discovery of several planetary systems instead of just our own will obviously change the models for
how they are formed