Download Lecture 3

Space, time & Cosmos
Lecture 3:
Solar system - outer planets &
(a little bit) beyond
Prof. Ken Tsang
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The four gas giants against the Sun: Jupiter,
Saturn, Uranus, Neptune (Sizes to scale)
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Visible Planet Orbits
This diagram shows the relative size of the orbits of the
seven planets visible to the naked eye. All the orbits are nearly circular (but slightly
elliptical) and nearly in the same plane as Earth's orbit (called the ecliptic).
The diagram is from a view out of the ecliptic plane and away from the perpendicular axis that
goes through the Sun.
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Outer Planet Orbits
This shows the relative sizes and positions of the orbits
of the planets farther from the Sun than Earth. All the planets have orbits that are
ellipses with the Sun at one of the foci, and the ellipses are in different planes.
However, only Pluto has a noticeably noncircular orbit that lies in a different plane
from the other planets.
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Jupiter
based on a 1979
image from the
Voyager 1
spacecraft and
enhanced by the
U.S. Geological
Survey
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Jupiter
The Romans named the planet after the Roman god
Jupiter (the god of sky and thunder).
When viewed from Earth, Jupiter is the third brightest
object in the night sky after the Moon and Venus. (At certain
points in its orbit, Mars can briefly exceed Jupiter's brightness.)
The most massive planet in our solar system, with four
planet-sized moons and many smaller moons, Jupiter
forms a kind of miniature solar system. Jupiter resembles
a star in composition. In fact, if it had been about eighty
times more massive, it would have become a star rather
than a planet.
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"Jupiter et Thétis" by
Jean Ingres, 1811.
Jupiter or Jove was
the king of the gods,
and the god of sky and
thunder.
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Time-lapse sequence from the approach of Voyager I to Jupiter, showing the
motion of atmospheric bands, and circulation of the great red spot.
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Jupiter's upper atmosphere is composed of about 88-92% hydrogen
and 8-12% helium by percent volume or fraction of gas molecules, or
approximately 75% hydrogen and 24% helium by mass.
The interior contains denser materials such that the distribution is
roughly 71% hydrogen, 24% helium and five percent other elements
by mass. The atmosphere contains trace amounts of methane, water
vapor, ammonia, and silicon-based compounds.
Jupiter is 2.5 times more massive than all the other planets in our
Solar System combined. Jupiter's volume is equal to 1,317 Earths, yet
is only 318 times as massive.
Jupiter is thought to consist of a dense core with a mixture of
elements, a surrounding layer of liquid metallic hydrogen with some
helium, and an outer layer predominantly of molecular hydrogen.
Beyond this basic outline, there is still considerable uncertainty.
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This cut-away illustrates a model of Jupiter's interior.
In the upper layers
the atmosphere
transitions to a liquid
state above a thick
layer of metallic
hydrogen. In the
center there may be
a solid core of
heavier elements.
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Although Jupiter would need to be about 75 times as
massive to fuse hydrogen and become a star, the
smallest red dwarf is only about 30 percent larger in
radius than Jupiter. This has led some astronomers to
term it a "failed star“.
In the novel, 2010: Odyssey Two, written by Arthur C. Clarke in
1987, Jupiter's density was mysteriously increased until the planet
achieves nuclear fusion, becoming a mini-sun which Earth
eventually names "Lucifer“, and destroying the American space ship
Discovery entirely.
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This dramatic view of Jupiter's Great Red Spot and its surroundings was
obtained by Voyager 1 on February 25, 1979, when the spacecraft was
9.2 million km (5.7 million mi) from Jupiter. Cloud details as small as
160 km (100 mi) across can be seen here. The colorful, wavy cloud pattern
to the left of the Red Spot is a region of extraordinarily complex and
variable wave motion.
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Jupiter is perpetually covered with clouds composed of
ammonia crystals and possibly ammonium hydrosulfide.
The outer atmosphere is visibly segregated into several
bands at different latitudes, resulting in turbulence and
storms along their interacting boundaries.
The best known feature of Jupiter is the Great Red Spot, a
persistent anti-cyclonic storm located 22° south of the
equator that is larger than Earth. It is known to have been in
existence since at least 1831, and possibly since 1665.
Mathematical models suggest that the storm is stable and
may be a permanent feature of the planet. The storm is
large enough to be visible through Earth-based telescopes.
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Rings of Jupiter: third ring system discovered in the Solar System,
after those of Saturn and Uranus, first observed in 1979 by the Voyager 1 space
probe
Jupiter has a faint planetary ring system
composed of three main segments: an inner
torus of particles known as the halo, a relatively
bright main ring, and an outer "gossamer" ring.
These rings appear to be made of dust, rather
than ice as is the case for Saturn's rings.
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Rings of Jupiter
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Triple Eclipse
On Earth, we witness a solar eclipse when our Moon's shadow sweeps across our planet's face as it
passes in front of our Sun. Jupiter, however, has four moons roughly the same size as Earth's Moon.
The shadows of three of them occasionally sweep simultaneously across Jupiter. The image was taken
March 28, 2004, with Hubble's Near Infrared Camera and Multi-Object Spectrometer.
Closer inspection by NASA's Hubble Space Telescope reveals that these spots are actually a rare
alignment of three of Jupiter's largest moons - Io, Ganymede, and Callisto - across the planet's face. In
this image, the telltale signatures of this alignment are the shadows [the three black circles] cast by the
moons. Io's shadow is located just above center and to the left; Ganymede's on the planet's left edge;
and Callisto's near the right edge. Only two of the moons, however, are visible in this image. Io is the
white circle in the center of the image, and Ganymede is the blue circle at upper right. Callisto is out of
the image and to the right.
Seeing three shadows on Jupiter happens only about once or twice a decade. Why is this triple eclipse
so unique? Io, Ganymede, and Callisto orbit Jupiter at different rates. Their shadows likewise cross
Jupiter's face at different rates. For example, the outermost moon Callisto orbits the slowest of the three
satellites. Callisto's shadow moves across the planet once for every 20 shadow crossings of Io. Add the
crossing rate of Ganymede's shadow and the possibility of a triple eclipse becomes even more rare.
Viewing the triple shadows in 2004 was even more special, because two of the moons were crossing
Jupiter's face at the same time as the three shadows.
Jupiter appears in pastel colors in this photo because the observation was taken in near-infrared light.
Astronomers combined images taken in three near-infrared wavelengths to make this color image. The
photo shows sunlight reflected from Jupiter's clouds. In the near infrared, methane gas in Jupiter's
atmosphere limits the penetration of sunlight, which causes clouds to appear in different colors
depending on their altitude.
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Jupiter has 63 named natural satellites. Of these, 47 are less than
10 kilometres in diameter and have only been discovered since 1975.
The four largest moons, known as the "Galilean moons", are Io, Europa,
Ganymede and Callisto.
Io
Callisto
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Jupiter's four largest moons, known
as the Galilean satellites. From top
to bottom, the moons shown are Io,
Europa, Ganymede and Callisto.
The Great Red Spot, a storm in
Jupiter's atmosphere, is at least 300
years old. Winds blow counterclockwise around the Great Red
Spot at about 400 kilometers per
hour (250 miles per hour). The
storm is larger than one Earth
diameter from north to south, and
more than two Earth diameters from
east to west.
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Facts about Jovian moons learned in
the past 30 years
Io is the most volcanically active body in our solar system.
Ganymede is the largest planetary moon and is the only
moon in the solar system known to have its own magnetic
field.
A liquid ocean may lie beneath the frozen crust of Europa.
Icy oceans may also lie deep beneath the crusts of Callisto
and Ganymede.
In 2003 alone, astronomers discovered 23 new moons
orbiting the giant planet, giving Jupiter a total moon count
of 49 officially named -- the most in the solar system. The
numerous small outer moons may be asteroids captured by
the giant planet's gravity.
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The Laplace resonance exhibited by three inner Galilean
moons. The ratios in the figure are of orbital periods.
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The Galilean moons seen with an amateur telescope
Photo : Bresson Thomas
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Jupiter with three of its Galilean satellites (from lower left to upper
right): Io, Europa, and Callisto. Sky & Telescope's editor in chief,
Rick Fienberg, recorded this scene on March 16, 2003, using a 12inch (30-centimeter) Meade Schmidt-Cassegrain telescope and a
Canon digital camera.
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Io and Ganymede
Date: 01.17.2007
The New Horizons Long Range Reconnaissance Imager
(LORRI) took this 4-millisecond exposure of Jupiter and two
of its moons at 01:41:04 UTC on January 17, 2007. The
spacecraft was 68.5 million kilometers (42.5 million miles)
from Jupiter, closing in on the giant planet at 41,500 miles
(66,790 kilometers) per hour. The volcanic moon Io is the
closest planet to the right of Jupiter; the icy moon Ganymede
is to Io's right. The shadows of each satellite are visible atop
Jupiter's clouds; Ganymede's shadow is draped over
Jupiter's northwestern limb.
Ganymede's average orbit distance from Jupiter is about
1.07 million kilometers (620,000 miles); Io's is 422,000
kilometers (262,000 miles). Both Io and Ganymede are larger
than Earth's moon; Ganymede is larger than the planet
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Mercury.
Eruption at Tvashtar Catena, Io, in color
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NASA's Galileo spacecraft caught this volcanic eruption in
action on Jupiter's moon Io on November 25, 1999. This
mosaic shows Tvashtar Catena, a chain of calderas, in
enhanced color. It combines low resolution (1.3 kilometers,
or .8 miles, per picture element) color images of Io taken
on July 3, 1999 with the much higher resolution (180
meters, or 197 yards, per picture element) black and white
images taken in November. The molten lava was hot
enough, and therefore bright enough, to saturate, or
overexpose, Galileo's camera (original image is inset in
lower right corner).
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Capturing Callisto
Date: 02.27.2007
The New Horizons Long Range Reconnaissance Imager (LORRI) captured
these two images of Jupiter's outermost large moon, Callisto, as the
spacecraft flew past Jupiter in late February. New Horizons' closest approach
distance to Jupiter was 2.3 million kilometers (1.4 million miles), not far
outside Callisto's orbit, which has a radius of 1.9 million kilometers (1.2
million miles). However, Callisto happened to be on the opposite side of
Jupiter during the spacecraft's pass through the Jupiter system, so these
images, taken from 4.7 million kilometers (3.0 million miles) and 4.2 million
kilometers (2.6 million miles) away, are the closest of Callisto that New
Horizons obtained.
Callisto's ancient, crater-scarred surface makes it very different from its three
more active sibling satellites, Io, Europa and Ganymede. Callisto, 4,800
kilometers (3000 miles) in diameter, displays no large-scale geological
features other than impact craters, and every bright spot in these images is a
crater. The largest impact feature on Callisto, the huge basin Valhalla, is
visible as a bright patch at the 10 o'clock position. The craters are bright
because they have excavated material relatively rich in water ice from
beneath the dark, dusty material that coats most of the surface.
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Magnetosphere of Jupiter
Jupiter's broad magnetic field is 14 times as strong as
the Earth's, ranging from 4.2 gauss (0.42 mT) at the
equator to 10–14 gauss (1.0–1.4 mT) at the poles,
making it the strongest among the Solar planets.
The field traps a sheet of ionized particles from the
solar wind, generating a highly-energetic magnetic field
outside the planet — the magnetosphere.
Electrons within the magnetosphere generate a strong
radio signature that produces bursts in the range of
0.6–30 MHz.
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Hubble Space Telescope image of Jupiter aurora in UV
Bright streaks and dots are caused by magnetic flux tubes connecting Jupiter
to its largest moons-- Io: bright streak on the far left; Ganymede: bright dot
below center; Europa: dot right of Ganymede dot
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Saturn
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A Hubble Space Telescope image of Saturn in
true color.
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Surely one of the most gorgeous sights the solar system has
to offer, Saturn sits enveloped by the full splendor of its rings.
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Saturn, named after the
Roman god of agriculture and
harvest, is the second largest
planet in the Solar System, the
most distant of the five planets
known to the ancients.
Saturn has a prominent
system of rings, consisting
mostly of ice particles with
a smaller amount of rocky
debris and dust.
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Saturn: Rings
While the other three gas planets in the solar system - Jupiter, Uranus
and Neptune - have rings orbiting around them, Saturn's are by far the
largest and most spectacular. With a thickness of about 1 kilometer
(3,200 feet) or less, they span up to 282,000 km.
Named alphabetically in the order they were discovered, the rings are
relatively close to each other, with the exception of the Cassini Division,
a gap measuring 4,700 kilometers (2,920 miles). The main rings are,
working outward from the planet, known as C, B, and A. The Cassini
Division is the largest gap in the rings and separates Rings B and
A. In addition a number of fainter rings have been discovered more
recently. The D Ring is exceedingly faint and closest to the planet. The F
Ring is a narrow feature just outside the A Ring. Beyond that are two far
fainter rings named G and E. The rings show a tremendous amount of
structure on all scales; some of this structure is related to gravitational
perturbations by Saturn's many moons, but much of it remains
unexplained.
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Feature
Radius (km)
Saturn equator
60 268
D inner edge
66 900
D outer edge
74 510
C inner edge
74 568
Titan ringlet
77 871
Maxwell gap
87 491
C outer edge
92 000
B inner edge
92 000
B outer edge
117 580
Cassini division
A inner edge
122 170
Encke gap
133 589
Keeler gap
136 530
A outer edge
136 775
F ring centre
140 180
G inner edge
170 000
G outer edge
175 000
E inner edge
181 000
E outer edge
483 000
Saturn's system of rings is
highly structured with many
interesting features. Alongside
the gaps between the rings
there are also moons that act
like shepherds keeping the
rings in shape and the gaps
clear of material.
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Encke gap
A ring
Cassini Division
B ring
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C ring
This amazing close-up of Saturn's rings reveals their incredible
variety. In some regions there are wavelike structures, while in other
places the rings' structure appears to be more chaotic.
This image shows (from top to bottom) the A ring with the Encke gap,
the Cassini Division, and the B and C rings.
The image was taken in visible light with the Cassini spacecraft
narrow-angle camera on 26 April 2005 at a distance of approximately
2.3 million kilometres from Saturn. The image scale is 14 kilometres
per pixel.
There are thousands of rings made up of billions of particles of ice and
rock. The particles range in size from a grain of sugar to the size of a
house. The rings are believe to be pieces of comets, asteroids or shattered
moons that broke up before they reached the planet. Each ring orbits at a
different speed around the planet.
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This highly enhanced color view was assembled from clear, orange and
ultraviolet frames obtained August 17, 1981 from a distance of 8.9 million
kilometers (5.5 million miles).
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History of Saturn’s Rings
In 1610, Italian astronomer Galileo Galilei was the first to
gaze at Saturn through a telescope. To his surprise, he saw
a pair of objects on either side of the planet. He sketched
them as separate spheres and wrote that Saturn appeared
to be triple-bodied. Continuing his observations over the
next few years, Galileo drew the lateral bodies as arms or
handles attached to Saturn. In 1659, Dutch astronomer
Christiaan Huygens, using a more powerful telescope than
Galileo's, proposed that Saturn was surrounded by a thin,
flat ring. In 1675, Italian-born astronomer Jean-Dominique
Cassini discovered a 'division' between what are now called
the A and B rings. It is now known that the gravitational
influence of Saturn's moon Mimas is responsible for the
Cassini Division, which is 4,800 kilometers (3,000 miles)
wide.
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Like Jupiter, Saturn is made mostly of hydrogen and
helium. Its volume is 755 times greater than that of Earth.
Winds in the upper atmosphere reach 500 meters (1,600
feet) per second in the equatorial region. (In contrast, the
strongest hurricane-force winds on Earth top out at about
110 meters, or 360 feet, per second.) These super-fast
winds, combined with heat rising from within the planet's
interior, cause the yellow and gold bands visible in the
atmosphere.
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Though Saturn's magnetic field is not as huge as
Jupiter's, it is still 578 times as powerful as Earth's.
Saturn, the rings, and many of the satellites lie
totally within Saturn's enormous magnetosphere,
the region of space in which the behavior of
electrically charged particles is influenced more by
Saturn's magnetic field than by the solar wind.
Hubble Space Telescope images show that
Saturn's polar regions have aurorae similar to
Earth's. Aurorae occur when charged particles
spiral into a planet's atmosphere along magnetic
field lines.
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Saturn's Moons
Saturn currently has 60 known moons, five of which
have been discovered in data gathered by Cassini. Two
more possible discoveries by Cassini still await final
confirmation, as the objects might not be proper moons
but instead are transient in nature. When CassiniHuygens was launched in 1997 there were only 18
known saturnian moons. In addition to the Cassini
discoveries, reanalysis of data from the Voyager 2
spacecraft, as well as Hubble observations and groundbased observations of Saturn have led to the further
discoveries.
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Cassini–Huygens is a joint NASA/ESA robotic
spacecraft mission currently studying the planet Saturn
and its moons.
The spacecraft consists of two main elements: the NASA Cassini orbiter,
named after the Italian-French astronomer Giovanni Domenico Cassini,
and the ESA Huygens probe, named after the Dutch astronomer,
mathematician and physicist Christiaan Huygens. It was launched on
October 15, 1997 and entered into orbit around Saturn on July 1,
2004.
On December 25, 2004 the Huygens probe separated from the
orbiter at approximately 02:00 UTC; it reached Saturn's moon Titan
on January 14, 2005 where it made an atmospheric descent to the
surface and relayed scientific information. On April 18, 2008, NASA
announced a two year extension of the mission. Cassini is the first
spacecraft to orbit Saturn and the fourth to visit it.
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A Cassini spacecraft image of Titan's thick
atmosphere.
Saturn has 52 known natural
satellites (moons) and there are
probably many more waiting to be
discovered. Saturn's largest satellite,
Titan, is a bit bigger than the planet
Mercury. (Titan is the second-largest
moon in the solar system; only
Jupiter's moon Ganymede is bigger.)
Titan is shrouded in a thick, nitrogenrich atmosphere that might be similar
to what Earth's was like long ago.
Further study of this moon promises
to reveal much about planetary
formation and, perhaps, about the
early days of Earth. Saturn also has
many smaller 'icy' satellites.
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Titan's internal structure.
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Atmosphere of Titan
Visible light cannot escape from the veil of orange
smog that covers Titan's surface. The moon's dry cold
atmosphere causes a 300 km thick layer of smog to
build up. The smog, just like on Earth, forms when
sunlight interacts with hydrocarbon molecules.
Visit the NASA page for more detail on Titan:
http://saturn.jpl.nasa.gov/multimedia/flash/Titan/index.html
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Saturn and two of its moons, Tethys (above)
and Dione
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Cassini spacecraft captured this enduring portrait of a near-alignment of
four of Saturn's restless moons
Telesto
Prometheus
Dione
Titan
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Uranus
Discovered
March 13, 1781
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Uranus
is named after the ancient Greek deity of
the sky (Uranus, Οὐρανός).
Cronus (Saturn) defeats his father Uranus
fresco by Giorgio Vasari and Cristofano
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Uranus
Though it is visible to the naked eye like the five
classical planets, it was never recognized as a
planet by ancient observers because of its dimness
and slow orbit. It was generally mistaken as a star.
Sir William Herschel announced its discovery on
March 13, 1781, expanding the known boundaries
of the solar system for the first time in modern
history.
This was the first discovery of a planet made using
a telescope.
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Once considered one of the blander-looking planets,
Uranus has been revealed as a dynamic world with some
of the brightest clouds in the outer solar system and 11
rings. The seventh planet from the Sun is so distant that it
takes 84 years to complete one orbit. Uranus, with no
solid surface, is one of the gas giant planets (the others
are Jupiter, Saturn, and Neptune).
The atmosphere of Uranus is composed primarily of hydrogen and helium,
with a small amount of methane and traces of water and ammonia. Uranus
gets its blue-green color from methane gas. Sunlight is reflected from
Uranus' cloud tops, which lie beneath a layer of methane gas. As the
reflected sunlight passes back through this layer, the methane gas absorbs
the red portion of the light, allowing the blue portion to pass through, resulting
in the blue-green color that we see. The planet's atmospheric details are very
difficult to see in visible light. The bulk (80 per-cent or more) of the mass of
Uranus is contained in an extended liquid core consisting primarily of 'icy'
materials (water, methane, and ammonia), with higher-density material at
depth.
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Uranus and Moons
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Taking its first peek at Uranus, NASA Hubble Space Telescope's Near Infrared
Camera and Multi-Object Spectrometer (NICMOS) has detected six distinct
clouds in images taken July 28,1997.
The image on the right, taken 90 minutes after the left-hand image, shows the
planet's rotation. Each image is a composite of three near-infrared images.
They are called false-color images because the human eye cannot detect
infrared light. Therefore, colors corresponding to visible light were assigned to
the images. (The wavelengths for the 'blue,' 'green,' and 'red' exposures are
1.1, 1.6, and 1.9 micrometers, respectively.)
At visible and near-infrared light, sunlight is reflected from hazes and clouds in
the atmosphere of Uranus. However, at near-infrared light, absorption by gases
in the Uranian atmosphere limits the view to different altitudes, causing intense
contrasts and colors.
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The power of the Keck telescope's adaptive optics system is clear in this image of
Uranus, its rings and the moon Miranda. Date: 07.09.2004
Upper: Uranus, its
rings and moon
Miranda at near
infrared
wavelengths of 2.2
microns.
Lower: Uranus and
its atmospheric
details as seen in
near infrared
wavelengths of 1.6
microns. The image
has been doubled in
size.
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This series of images from NASA's Hubble Space Telescope shows how the ring
system around the distant planet Uranus appears at ever more oblique
(shallower) tilts as viewed from Earth - culminating in the rings being seen edgeon in three observing opportunities in 2007. The best of these events appears in
the far right image taken with Hubble's Wide Field Planetary Camera 2 on August
14, 2007.
The edge-on rings appear as two spikes above and below the planet. The rings
cannot be seen running fully across the face of the planet because the bright
glare of the planet has been blocked out in the Hubble photo (a small amount of
residual glare appears as a fan- shaped image artifact). A much shorter color
exposure of the planet has been photo- composited to show its size and position
relative to the ring plane.
Earthbound astronomers only see the rings' edge every 42 years as the planet
follows a leisurely 84-year orbit about the Sun. However, the last time the rings
were tilted edge-on to Earth astronomers didn't even know they existed.
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The six best-known moons of Uranus compared at their proper
relative sizes.
From left to right:
Puck, Miranda, Ariel, Umbriel, Titania, and Oberon.
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Neptune
Discovered
Sep 23, 1846
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Neptune from Voyager 2
Neptune statue in Bologna
In Roman mythology, Neptune was the god
of the sea.
Neptune was the only planet found by
mathematical prediction rather than by
empirical observation. Unexpected changes in
the orbit of Uranus led astronomers to deduce
that its orbit was subject to gravitational
perturbation by an unknown planet.
Neptune is similar in composition to Uranus,
and both have compositions which differ from
those of the larger gas giants Jupiter and
Saturn. Neptune's atmosphere, while similar to
Jupiter's and Saturn's in that it is composed
primarily of hydrogen and helium, along with
traces of hydrocarbons and possibly nitrogen,
contains a higher proportion of "ices" such as
water, ammonia and methane.
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Discovery of Neptune
In 1821, Alexis Bouvard published astronomical tables of the orbit of Neptune's
neighbor Uranus. Subsequent observations revealed substantial deviations from
the tables, leading Bouvard to hypothesize that an unknown body was perturbing
the orbit through gravitational interaction. In 1843, John Couch Adams calculated
the orbit of a hypothesized eighth planet that would account for Uranus's motion.
In 1845–46, Urbain Le Verrier, independently of Adams, rapidly developed his
own calculations but also experienced difficulties in stimulating any enthusiasm
in his compatriots.
Le Verrier urged Berlin Observatory astronomer Johann Gottfried Galle to
search with the observatory's refractor. Heinrich d'Arrest, a student at the
observatory, suggested to Galle that they could compare a recently drawn chart
of the sky in the region of Le Verrier's predicted location with the current sky to
seek the displacement characteristic of a planet, as opposed to a fixed star. The
very evening of the day of receipt of Le Verrier's letter on September 23, 1846,
Neptune was discovered within 1° of where Le Verrier had
predicted it to be.
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The outer planet takes more time to complete an orbit than the inner planet, so once
per orbit the inner planet overtakes the outer planet. When the planets are at a, the
outer planet exerts a gravitational perturbation that accelerates the inner planet,
advancing the body ahead of its normal path. When the planets reach b, the reverse
is true and the inner planet is decelarated. This perturbing influence is what led to
the discovery of the planet Neptune.
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The internal structure of Neptune:
1. Upper atmosphere, top clouds
2. Atmosphere consisting of hydrogen, helium and methane gas
3. Mantle consisting of water, ammonia and methane ices
4. Core consisting of rock and ice
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Neptune Hurricanes
Date: 08.24.1989
Voyager 2 sent back this stunning image of storms at work in Neptune's windy
atmosphere in August 1989.
This photograph of Neptune was reconstructed from two images taken by
Voyager 2's narrow-angle camera, through the green and clear filters. The
image shows three of the features that Voyager 2 photographed during its
Neptune flyby. At the north (top) is the Great Dark Spot, accompanied by bright,
white clouds that undergo rapid changes in appearance. To the south of the
Great Dark Spot is the bright feature that Voyager scientists nicknamed
"Scooter." Still farther south is the feature called "Dark Spot 2," which has a
bright core. Each feature moves eastward at a different velocity, so it is only
occasionally that they appear close to each other, such as at the time this
picture was taken.
Voyager 2 is the only spacecraft to visit Neptune.
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Moons of Neptune
Image of Triton from Voyager 2.
Neptune has thirteen known moons.
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The largest by far is Triton,
discovered by William Lassell just
seventeen days after the discovery
of Neptune itself.
About one hundred years later
American astronomer Gerard Kuiper
found Neptune's third-largest moon,
Nereid, in 1949. He missed Proteus,
the second-largest, because it's too
dark and too close to Neptune for
telescopes of that era.
No further moons were found until
Voyager 2 flew by Neptune in 1989
and discovered five new inner
moons, bringing the total of known
moons to eight.
From 2002 onwards, telescopic
surveys found the remaining five
outer moons.
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Pluto's Surface from 3 Billion Miles
Images of Pluto taken by the NASA Hubble Space Telescope with the
ESA Faint Object Camera. These images, taken in late June and early
July, 1994 are the first views which allow resolution of features on
Pluto's surface.
Pluto
Discovered February 18, 1930
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Discovery of Pluto
In the 1840s, using Newtonian mechanics, Urbain Le Verrier predicted the
position of the then-undiscovered planet Neptune after analyzing
perturbations in the orbit of Uranus.
Subsequent observations of Neptune in the
late 19th century caused astronomers to
speculate that Uranus' orbit was being
disturbed by another planet in addition to
Neptune. An extensive project in search of a
possible ninth planet, termed "Planet X“, was
started by some astronomers. Finally it was
discovered by a young American astronomer
Clyde Tombaugh on February 18, 1930, after
nearly a year of searching.
Pluto was the Roman god of the underworld.
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Moons of Pluto
Pluto has three known natural satellites: Charon, first
identified in 1978 by astronomer James Christy; and two
smaller moons, Nix and Hydra, both discovered in 2005.
The large size of Charon relative to Pluto has led some astronomers
to call it a dwarf double planet.
The system is also unusual among planetary systems in that each is
tidally locked to the other: Charon always presents the same face to
Pluto, and Pluto always presents the same face to Charon. If one
were standing on Pluto's near side, Charon would hover in the sky
without moving; if one were to travel to the far side, one would never
see Charon at all.
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Once known as the smallest, coldest, and most distant planet from the
Sun, Pluto has been enshrouded in controversy since its discovery in
1930.
On August 24, 2006, the International Astronomical
Union (IAU) formally downgraded Pluto from an official
planet to a dwarf planet.
According to the new rules a planet meets three criteria: it must orbit the
Sun, it must be big enough for gravity to squash it into a round ball, and
it must have cleared other things out of the way in its orbital
neighborhood. The latter measure knocks out Pluto and 2003UB313
(Eris), which orbit among the icy wrecks of the Kuiper Belt, and Ceres,
which is in the asteroid belt.
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Planet or Dwarf Planet ?
(1) A "planet" is a celestial body that (a) is in orbit around the Sun, (b)
has sufficient mass for its self-gravity to overcome rigid body forces so
that it assumes a hydrostatic equilibrium (nearly round) shape, and (c)
has cleared the neighborhood around its orbit.
(2) A "dwarf planet" is a celestial body that (a) is in orbit around the
Sun, (b) has sufficient mass for its self-gravity to overcome rigid body
forces so that it assumes a hydrostatic equilibrium (nearly round)
shape, (c) has not cleared the neighborhood around its orbit, and (d)
is not a satellite.
(3) All other objects except satellites orbiting the Sun shall be referred
to collectively as "Small Solar-System Bodies".
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Kuiper belt and Kuiper Belt objects
Pluto, is the second-largest known dwarf planet in the Solar
System (after Eris) and the tenth-largest body observed
directly orbiting the Sun.
Originally classified as a planet, Pluto is now considered the
largest member of a distinct population called the Kuiper
belt.
Like other members of the Kuiper belt, Pluto is composed
primarily of rock and ice and is relatively small:
approximately a fifth the mass of the Earth's Moon and a
third its volume. It has a highly eccentric and highly inclined
orbit.
Some astronomers believe that Triton, Charon are merely the largest examples
of Kuiper Belt objects.
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The Kuiper belt, is a disk-shaped region of the Solar
System beyond the planets extending from the orbit of
Neptune (at 30 AU) to approximately 55 AU from the Sun. It
is similar to the asteroid belt, although it is far larger -- 20
times as wide and 20–200 times as massive.
It is home to at least three dwarf planets – Pluto, Haumea
and Makemake. But while the asteroid belt is composed
primarily of rock and metal, the Kuiper belt objects are
composed largely of frozen volatiles (dubbed "ices"), such as
methane, ammonia and water. It is now considered to be the
source of the short-period comets.
astronomical unit (AU) = 149,597,870.691 km; the average distance from the Earth to the
Sun. 1 AU is a long way -- at 100 miles per hour (160 kmph) it would take over 100 years
to go 1 AU.
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Positions are accurate for January 1st, 2000
Green:
Kuiper belt object
distances in AU
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Plutoids
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Plutoids
Pluto and Eris are the first objects to be classified as Plutoids, a
special class of dwarf planets that orbit our Sun in the icy region beyond the orbit
of Neptune.
The name plutoid was proposed by the members of the IAU Committee on Small
Body Nomenclature (CSBN), and approved by the IAU Executive Committee at a
June 2008 meeting in Oslo, Norway.
Plutoids are celestial bodies in orbit around the Sun at a semi-major axis greater
than that of Neptune that have sufficient mass for their self-gravity to overcome
rigid body forces so that they assume a hydrostatic equilibrium (near-spherical)
shape, and that have not cleared the neighbourhood around their orbit. Satellites
of plutoids are not plutoids themselves, even if they are massive enough that their
shape is dictated by self-gravity.
The dwarf planet Ceres is not a plutoid as it is located in the asteroid belt
between Mars and Jupiter. Current scientific knowledge lends credence to the
belief that Ceres is the only object of its kind.
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Eris
is the largest known dwarf planet in the Solar System and the
ninth-largest body known to orbit the Sun directly, roughly
2 500 km in diameter and 27% more massive than Pluto.
Eris (centre) and Dysnomia (left of centre, the only
known moon of Eris). --Hubble Space Telescope
Because Eris is larger than Pluto, its
discoverers and NASA called it the Solar
system’s tenth planet. This, along with
the prospect of other similarly sized
objects being discovered in the future,
motivated the International Astronomical
Union (IAU) to define the term planet for
the first time.
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The IAU's formal definition of 'plutoid,' announced 11 June 2008, is:
Plutoids are celestial bodies in orbit around the Sun at a semi-major axis
greater than that of Neptune that have sufficient mass for their self-gravity to
overcome rigid body forces so that they assume a hydrostatic equilibrium
(near-spherical) shape, and that have not cleared the neighborhood around
their orbit. Satellites of plutoids are not plutoids themselves.
Accordingly, plutoids can be thought of as the intersection of the set of dwarf
planets and the set of trans-Neptunian objects. As of 2008[update], Pluto,
Eris, Haumea, and Makemake are the only objects classified as plutoids.
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The inner Solar System, from the Sun to Jupiter.
Also includes the Main
Asteroid Belt (the white
donut-shaped cloud), the
Hildas (the orange
"triangle" just inside the
orbit of Jupiter) and the
Jovian Trojans (green). The
group that leads Jupiter are
called the "Greeks" and the
trailing group are called the
"Trojans"
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The asteroid belt is the region of the Solar System located roughly
between the orbits of the planets Mars and Jupiter. It is occupied by
numerous irregularly shaped bodies called asteroids or minor planets. The
asteroid belt region is also termed the main belt to distinguish it from
other concentrations of minor planets within the Solar System, such as the
Kuiper belt and scattered disc.
More than half the mass of the main belt is contained in the four largest
objects: Ceres, 4 Vesta, 2 Pallas, and 10 Hygiea. All of these have mean
Ceres, the main belt's only
dwarf planet, is about 950 km in diameter.
diameters of more than 400 km, while
The asteroid belt formed from the primordial solar nebula as a group of
planetesimals, the smaller precursors of the planets. Between Mars and
Jupiter, however, gravitational perturbations from the giant planet imbued the
planetesimals with too much orbital energy for them to accrete into a planet.
Collisions became too violent, and instead of sticking together, the
planetesimals shattered.
Some fragments can eventually find their way into the inner Solar System,
leading to meteorite impacts with the inner planets.
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The asteroid 951 Gaspra, the first ever imaged by a
spacecraft, taken by Galileo as it passed by it in 1991
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The Hilda family of asteroids consists of asteroids with a semimajor axis between 3.7 AU and 4.2 AU, an eccentricity greater than 0.07, and an
inclination less than 20°.
Hildas move in their elliptical orbits so that their aphelia (point on the orbit of a
celestial body that is farthest from the sun) put them opposite Jupiter, or 60
degrees ahead of or behind Jupiter.
The Jupiter Trojans, commonly called simply Trojans or Trojan
asteroids, are a large group of objects that share the orbit of the planet Jupiter
around the Sun.
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The Hildas (black) and the Trojans viewed
from ecliptic plane.
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Positions are accurate for January 1st, 2000
Green:
Kuiper belt object
Scattered disc object
or Centaur in orange
distances in AU
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Centaur (minor planet)
The common definition of a Centaur is an object that orbits
the Sun between Jupiter and Neptune. They generally have
a semi-major axis between Jupiter and Neptune, crossing
the orbits of one or more the giant planets. and have
dynamical lifetimes of a few million years. The centaurs are an
unstable orbital class of minor planets. The name was chosen because they
behave as half asteroid and half comet.
No centaur has been photographed up close, although
there is evidence that Saturn's moon Phoebe, imaged by
the Cassini probe in 2004, may be a captured centaur.
Any centaur that is perturbed close enough to the Sun is
expected to become a comet.
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90377 Sedna is a trans-Neptunian object and a likely dwarf planet, discovered
by Michael Brown (Caltech), Chad Trujillo (Gemini Observatory) and David Rabinowitz
(Yale University) on November 14, 2003. It is currently 88 AU from the Sun, about three
times farther than Neptune. For most of its orbit Sedna is farther from the Sun than any
other known dwarf planet candidate.
Sedna, highlighted
by the green circle.
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Oort Cloud
In 1950 Jan Oort noticed that
--no comet has been observed with an orbit that indicates that it
came from interstellar space,
--there is a strong tendency for aphelia of long period comet
orbits to lie at a distance of about 50,000 AU, and
--there is no preferential direction from which comets come.
From this he proposed that comets reside in a vast cloud at the outer
reaches of the solar system. This has come to be known as the Oort
Cloud. The statistics imply that it may contain as many as a trillion
(1e12) comets. Unfortunately, since the individual comets are so
small and at such large distances, we have no direct evidence
about the Oort Cloud yet.
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In 2004, the discovery of an object known as 2003 VB12
"Sedna" was announced. Its orbit is intermediate between
the Kuiper Belt and what was previously thought to be the
inner part of the Oort Cloud. Perhaps this object is the first
of a new class of "inner Oort Cloud" objects.
Some astronomers believe the Oort Cloud may account for
a significant fraction of the mass of the solar system,
perhaps as much or even more than Jupiter.
But Kuiper Belt and the Oort Cloud are more than distant curiosities. They are
relatively pristine remnants of the nebula from which the entire solar system
was formed. Their composition and distribution places important constraints on
models of the early evolution of the solar system.
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Artists rendering of the Kuiper
Belt and Oort Cloud.
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The presumed
distance of the Oort
cloud compared to
the rest of the Solar
System
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Home work for Lecture 3:
Go to the following web-site and follow the instruction there to
download the needed software so that you can start the Cassini
Virtual Tour.
http://saturn.jpl.nasa.gov/video/cassinivirtualtour/
Enjoy flying around Saturn and investigate the moons of Saturn in
your Virtual Tour. Write a report on the Saturn moons.
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