Download Comparing Earth, Sun and Jupiter

The solar system
 Mass dominated by the Sun: an average star
 Planets are solid, almost spherical bodies
orbiting the Sun:
 Mercury
 Venus
 Earth
 Mars
 Jupiter
 Saturn
 Neptune
 Uranus
 Pluto
 Smaller bodies found in the asteroid belt
between Mars and Jupiter.
 Often nonspherical
 Most of the planets rotate in the same
direction as they orbit the Sun, and in the
same plane
 Exceptions are Venus, Uranus, Pluto
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The Sun
Mostly Hydrogen and Helium
High luminosity due to central fusion reactions
Sunspots: cooler regions where magnetic field
is strongest
More features visible at other wavelengths
Types of Stars
Characterised mainly by their luminosity and
colour
Most stars lie on a tight correlation between L
and colour: this is the main sequence
Main sequence stars range from very bright,
blue stars, to very faint, red ones
Sun is a greenish star on the main sequence
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Star systems
Most stars are part of a binary system. By carefully
measuring their orbits around one another we can
measure their masses
Star clusters are gravitationally bound groups of stars.
 Open clusters are young clusters of a few 100 stars
 Globular clusters are much older systems and
contain tens of thousands of stars
A galaxy is a system of several billion stars and star
clusters, gravitationally bound and orbiting a common
centre of mass
Galaxies themselves form groups and clusters
The Milky Way
 Our galaxy is characterized by a relatively thin disk of
stars and dust. This appears as a roughly linear
feature on the sky
 The dust obscures most of the light from these
stars. If there were no dust, the centre of the galaxy
would be as bright as the full moon
 Looking in infrared light, which is less affected by
dust, we can see the full structure of our Galaxy.
 Young stars and open clusters, in roughly circular
orbits about the centre, lie in the disk.
 Globular clusters and older stars form the halo, a
roughly spherical collection of stars, much larger than
the disk, on high velocity, radial orbits.
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Mercury
Small planet, very close to the Sun
Hard to see because always a small angular
distance from the Sun.
Visited by Mariner 10 in 1974
Heavily cratered, moon-like surface
No detectable atmosphere
Venus
Most Earthlike planet in terms of size,
composition
Retrograde rotation
Only direct views of surface from the Soviet
Venera missions, in 1970s
However it is covered in very thick clouds
which produce an intense Greenhouse effect
Surface heated to 700K
Most information about surface comes from
radar
Low-relief surface, with some mountains likely
volcanic in origin
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Earth-Moon
Only planet with liquid surface water
 2/3 of surface covered in water
bluish colour due to Rayleigh scattering
Anomalously large moon shows same face
 Near-side dominated by large maria
 Far side more heavily cratered
Covered with fine dust known as regolith
Lunar cycle every 28 days due to relative
position of Sun and Earth
Slight wobble due to eccentricity of orbit
Rilles found all over moon’s surface: may be
evidence of liquid lava flow at earlier time
Mars
Red surface, due to high iron content
Polar caps clearly visible: indicate water
content
 Change with seasons
Atmosphere very thin and dry; planet is too
small to hold on to an atmosphere
Surface now well explored by robotic craft
Desert-like terrain covered with lavalike rocks
Two small moons, Phobos and Deimos, probably
captured asteroids.
The Asteroid Belt
 Thousands of many small (<1000 km diameter),
rocky bodies
 In contrast to planets, most are non-spherical
 Orbits lie more or less in the ecliptic plane;
a little more inclined and eccentric than
most planets
 Almost certainly pieces left over from the era
of planet-formation.
 Source of most meteors
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Jupiter
The nearest of the gas giants
Huge atmosphere of mostly hydrogen and helium.
Has more mass than all the other planets put
together. Also has most of the angular momentum
May have a small, solid core
Many (>60) small moons, and also a small ring system
 Callisto: similar to a terrestrial planet
 Dark surface, composed of carbonaceous
soil and water ice
 Interior probably contains water ice
 Ganymede: largest moon of Jupiter
 Bright surface, containing ~90% water ice
 Europa: 12% smaller than our Moon
 Bright, icy surface with many “scratches”
and no impact craters
 Young surface of pack ice floating on
underlying ocean
 Some unknown internal source of heat
 Io: slightly larger than the moon
 Volcanoes were predicted before the
Voyager mission found them
 Most volcanically active body in the SS
 Volcanoes have driven off water and other
volatiles, leaving rocky surface
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Saturn
Similar composition to Jupiter, dominated by a
hydrogen/helium atmosphere
Has the lowest mean density of all planets
Rings are made up of rocky bodies orbiting in a thin
plane
 Structure in the rings due to orbital resonances
with the planet and moons
Like Jupiter, Saturn has many moons
 Phoebe is outermost moon and probably a
captured, interplanetary body
 Iapetus is strange because one hemisphere is
covered with very dark soil, while the other is
covered with bright frost.
 Tethys, Dione, Rhea have bright, icy surfaces,
probably mixed with rocky material. May have
been some resurfacing, indicating a source of
internal heat
 Enceladus is mostly icy, with some fractured,
cratered regions. Has unknown heat source.
 Titan is the second-largest moon in the SS, and
the only one with a thick atmosphere
 Atmosphere is opaque, containing methane
and photochemical smog
 Explored by Huygens probe last year
 Mimas is a heavily cratered, icy planet
 One very large crater suggests a collision
which nearly destroyed the moon.
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Uranus
Also has a dense atmosphere of H and He
Has a thin ring system and many satellites
Voyager 2 is the only spacecraft to visit
Uranus
Bluish colour due to Rayleigh scattering of
blue light and strong absorption of red light by
methane
Axis of rotation lies nearly in plane of the SS
Neptune
Only discovered in 1846
Also visited by Voyager 2
Similar colour to Neptune, for the same reason.
But more active atmosphere, including a large,
oval storm.
Also has a ring system and many satellites
 Triton is a large moon with a retrograde,
inclined orbit and a thin atmosphere of N2
and methane.
 Voyager 2 observed geyserlike plumes
of dark smoke, indicating internal
heating.
 Cold surface of nitrogen and methane
ice
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Pluto
Pluto and Charon a twin system; Pluto is about
7 times more massive
Only unvisited planet in the SS
Similar to Triton in mass and density
Pluto’s period is 3/2 that of Neptune.
 Probably resonantly captured Kuiper belt
objects
Infrared spectra show Pluto has a surface of
nitrogen and methane ice.
 Charon has a more neutral gray colour and
is rich in water ice. Why so different?
Comets
 Similar to asteroids, but originate from much
farther out, beyond the orbit of Neptune
 Highly eccentric orbits bring them in to the
inner solar system
 Have icy components in addition to rock
 This ice sublimes when comet approaches
the Sun