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Physics 217
An Introduction to Modern Astronomy
Part 6:
Planets
Figure 1: A schematic layout of solar system. Note sizes and distances are
not to scale.
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Introduction
The term solar system refers to the sun and all the objects orbiting around
it. The solar system can be divided into three main regions:
• The innermost region region containing the Sun and the small, terrestrial and rocky planets.
• The Gas giants. Large balls of gas orbited by their own systems of
moons
• The outer limits, inhabited by large empty wastelands with a scattering of icy worlds both large and small.
The Solar system is, as the name suggests, dominated by the Sun which
contains 99.86% of the mass of the Solar System, mostly in the form of
ionised Hydrogen and Helium. The gas giants account for about 90% of the
remaining mass with a tiny fraction left for the terrestrial planets.
To illustrate how small the planets are in comparison to the Sun imagine
if the Sun were modelled as a beach ball, Jupiter, the largest planet would
be a golf ball about 150 metres away and Earth would be smaller than a
pea and about 20m away.
A few things to note about the solar system:
• The orbits are roughly circular, apart from Mercury and Pluto.
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Figure 2: A schematic of the original solar nebula.
• The orbits are mostly coplanar, i.e. the planets all orbit in the same
plane.
• Most of the movements and spins are in the same direction.
• It obeys Keplers Laws
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The formation of the solar system
As mentioned above, the major planets follow almost circular orbits, all
going around in the same direction and in almost the same plane. This suggests a common origin from a flat disk-shaped gaseous cloud that encircled
the Sun. A collapsing protostar during its formation needs to shed angular
momentum, i.e. reduce its rotational inertia. One possible way is to develop a disk. Such disks appear to encircle a number of newly formed stars
observed today. It is therefore very likely that a disk formed around the sun.
This disk nebula, or solar nebula, was composed of the same material
as the Sun, i.e. 98% hydrogen and helium, 2% heavier elements (i.e. metals
in astronomy lingo). Isolated from the central heating within the protoSun, the disk began to cool. Elements with the highest temperatures of
vapourisation, particularly iron and silicates, condensed first, thereby forming swarms of tiny grains within the solar nebula
Like the dust in interstellar space, these grains, although accounting for
only a tiny fraction of the mass, would render the solar nebula opaque. Thus,
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shielded from the heat of the proto-Sun, the cooling in the outer part of the
disk accelerated. The condensation of ices may have made the grains sticky
since in time they adhered to one another forming tiny bodies or planetesimals. Those bodies that grew large enough used their mild gravitational
forces to attract their fellows. In time, each radial zone of the solar nebula
became dominated by a single proto planet that used its substantial gravity
to sweep up its competitors.
Yet, in competition to the force of gravity were thermal motions. In the
outer part of the nebula, the light hydrogen atoms had thermal velocities of
a few kilometres per second. Since this was soon less than the escape velocities of the outer proto-planets, the hydrogen was accumulated by those
proto-planets - likewise the helium. In the inner part of the nebula, the
thermal velocities of hydrogen and helium were around, ten kilometres per
second, greater or equal to the escape velocities of the proto-planets. Consequently the inner proto-planets were unable to draw in hydrogen or helium
(other than very small amounts) - even though this made up 98% of the
mass. The four giant outer planets are therefore similar in composition to
the Sun. The four inner planets, by comparison, are midgets - made almost
entirely of the 2% heavy elements. The gaseous remains of the inner solar
nebula have long since been blown away.
The above discussion is the generally accepted scenario for the distinction between the four small inner planets and the four giant liquid planets.
It has been somewhat thrown by the discovery of large Jupiter-like planets
very close to their suns in other solar systems.
Whilst each of the four inner terrestrial planets swept up all competitor
proto-planets (and a large collision may have knocked the material out of
the Earth to make the Moon), no fifth planet came together. Perhaps the
radial “zone” was too large for gravity to bring together a single body. Instead that zone is filled by minor planetary bodies - the asteroid belt - of
similar composition to the terrestrial planets.
In similar fashion, there is no fifth outer giant planet, but there appears
to be a Kuiper belt of smaller bodies - icy in composition. Pluto, its moon,
and other minor planets so far detected out there seem to be members of the
belt. Further out still, there may be a halo, the Oort Cloud, a reservoir of
small icy bodies that provide the seeds for comets.
The process of the formation of the solar system was repeated on reduced
scales as satellite systems formed around the giant outer planets. Obviously
such lesser bodies were either heavy elements or icy in composition. Each
outer planet today has a ring system - it is not clear whether this is material
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that never formed into satellites (due to the severe tidal forces) or whether
it originates from a satellite that broke up close to the planet.
An empirical relationship, the Bode-Titius law, gives the relative distances of the planets from the Sun. Take the series 0, 3, 6, 12, 24, 48,
96, 192, ... , add 4 and divide by 10. The latter results give very good
comparison to the mean distance (in A.U.) of the planets (including the
dwarf planet Ceres), except it does not work for Neptune and Pluto.
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A short tour of the solar system
Our solar system consists of the Sun, eight planets, moons, many dwarf
planets (or plutoids), an asteroid belt, comets, meteors, and others. The
sun is the centre of our solar system; the planets, their moons, a belt of
asteroids, comets, and other rocks and gas orbit the sun.
The eight planets that orbit the sun are (in order from the sun): Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune. Another large
body is Pluto, now classified as a dwarf planet or plutoid. A belt of asteroids (minor planets made of rock and metal) lies between Mars and Jupiter.
These objects all orbit the sun in roughly circular orbits that lie in the same
plane, the ecliptic (Pluto is an exception; it has an elliptical orbit tilted over
17 degrees from the ecliptic).
The largest planet is Jupiter. It is followed by Saturn, Uranus, Neptune,
Earth, Venus, Mars, Mercury, and finally, tiny Pluto (the largest of the
dwarf planets). Jupiter is so big that all the other planets could fit inside it.
A gas giant (sometimes also known as a Jovian planet after the planet
Jupiter, or giant planet) is a large planet that is not primarily composed of
rock or other solid matter. There are four gas giants in our Solar System:
Jupiter, Saturn, Uranus, and Neptune. Many extrasolar gas giants have
been identified orbiting other stars.
Planets above 10 Earth masses are termed giant planets. Below 10
Earth masses they are called super earths or, sometimes probably more
accurately for the higher mass examples, Gas Dwarfs. Objects large
enough to start deuterium fusion (above 13 Jupiter masses) are called brown
dwarfs and these occupy the mass range between that of large gas giant
planets and the lowest mass stars. The 13 Jupiter-mass cut-off is a rule of
thumb rather than something of precise physical significance.
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Figure 3: Visible image of the sun with sunspots.
3.1
The Sun
The Sun is the nearest star to Earth and the centre of our Solar system. It
is a large ball of gas composed of 74% Hydrogen, 24% Helium, 2% heavier
elements by mass. It has a diameter of about 1,392,000 km, about 109 times
that of Earth, and its mass is about 2 × 1030 kilograms, 330,000 times that
of Earth. The Sun is about 150,000,000 km from the earth. It takes sunlight
about 8 minutes to reach the earth.
The Sun, like most stars shines incredibly brightly, the energy it emits
is generated by nuclear fusion in the centre of the Suns core. This fusion
is combining Hydrogen nuclei (essentially protons) to form Helium nuclei.
The energy released from these reactions is in the form of photons, which
after moving slowly to the surface of the sun are eventually converted into
the photons we see on earth. The Sun’s surface temperature is about 6000 K.
The Sun possesses a generally weak magnetic field, but complications
arise because the Sun does not rotate as a solid body. The equatorial regions rotate once in about 25 days, whereas polar regions rotate in about
30 days. The gaseous material of the Sun, being ionised, drags the magnetic field with it and the uneven rotation tangles the field as suggested in
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Figure 4: Sunspots up close. See granular surface of sun.
figure ??. The “active bands” move slowly towards the Equator. Over an
11-year cycle, new bands start around latitudes of about 40 degrees north
and south, grow to a maximum, and eventually fade out at latitudes of about
5 degrees north and south, just as the next new pair of bands begin.
Much of the disturbance in the active bands consists of magnetic loops
breaking the surface of the Sun. If the magnetic field in these loops is strong
enough it will divert the heat flow from the interior resulting in cooler regions on the surface. These cooler regions appear darker; thus a pair of
sunspots, with opposite magnetic polarities, is created. The sense of polarity is reversed from northern to southern hemisphere, and one cycle to the
next. The last solar maximum - when the greatest number of sunspots are
visible on the photosphere - was be around 2013/14.
The magnetic disturbances extend well above the photospherc - great
prominences, some showing the loop structure, can occur. Energy is also
released, partly as short-duration flares of light - or by heating the outer
corona of the Sun to temperatures of millions of degrees. At such temperatures, the tenuous gas is no longer held by the Sun’s gravitational attraction,
but streams outward through the solar system. In general this solar wind
may “blow” at speeds of a few hundred kilometres a second, but can be enhanced in speed and strength during “storms”. Occasionally, flares may be
followed by mass coronal ejections, with enormous bubbles of material
erupting from the solar surface.
Very energetic charged particles originating from such ejections can disrupt satellite communications on Earth as well as power grids. As these
charged particles reach the earth, the earth’s magnetic field channels them
onto it’s magnetic poles. As the particles crash into the the atmosphere,
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Figure 5: Poster of the Aurorae Borealis (northern lights).
they ionize the gas atoms in the atmosphere. When these ions recombine
with electrons light is emitted. This hazy light called the aurora, can be
seen near from high latitude regions.
3.2
The inner planets versus the outer planets
The inner planets (those planets that orbit close to the sun) are quite different from the outer planets (those planets that orbit far from the sun).
• The inner planets are: Mercury, Venus, Earth, and Mars. They are
relatively small, composed mostly of rock, and have few or no moons.
• The outer planets include: Jupiter, Saturn, Uranus, Neptune. They
are mostly huge, mostly gaseous, ringed, and have many moons.
3.3
Mercury
Mercury is the smallest planet and also the closest to the Sun, it takes
about 88 Earth days to orbit the Sun. Mercury is quite bright when viewed
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Figure 6: Mercury.
from earth but it is not very easy to see as it is seldom very far from the Sun.
Compared to the other planets very little is known about Mercury. The
first of two spacecraft to visit the planet was Mariner 10, which mapped
about 45% of the planets surface from 1974 to 1975. The second is the Messenger spacecraft, which mapped a further 30% during its flyby of January
14, 2008. Messenger’s last flyby took place in September 2009 and it was
scheduled to attain orbit around Mercury in 2011, where it began mapping
the rest of the planet.
Mercury looks very similar to the Moon and is heavily cratered with
some regions of smooth plains. It also has no natural satellites and no
substantial atmosphere. However, unlike the Moon, it has a large iron core,
which generates a small magnetic field. Surface temperatures range from
about 90 to 700 K (-183 C to 427 C).
3.4
Venus
Venus is the second planet from the Sun, its orbit takes about 225 Earth
days. After the Moon it is the brightest natural object in the night sky.
Venus reaches its maximum brightness shortly before sunrise or shortly after sunset, which is why it has been known as the Morning Star or Evening
Star.
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Figure 7: Venus. Top images show thick cloud layer. Bottom image shows
surface of the planet mapped using radar on the Magellan probe.
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Figure 8: Image of the earth from space, taken by Apollo 17 mission.
Venus is classified as a terrestrial planet and it is sometimes called
Earth’s “sister planet” due to the similar size, gravity, and composition.
Venus is covered with an thick layer of highly reflective clouds of sulfuric
acid, preventing its surface from being seen from space in visible light. Venus
has the densest atmosphere of all the terrestrial planets in our solar system,
consisting mostly of carbon dioxide. Venus’s surface is a dusty dry desert
scape with many slab-like rocks, periodically refreshed by volcanism. The
atmospheric pressure at the planet’s surface is 92 times that of the Earth.
The Venusian surface was a subject of speculation until some of its secrets were revealed by planetary science in the twentieth century. It was
mapped in detail by Project Magellan in 1990. The ground shows evidence
of extensive volcanism, and the sulphur in the atmosphere may indicate that
there have been some recent eruptions. The planet has few impact craters,
demonstrating that the surface is relatively young, approximately 300 to 600
million years old.
3.5
Earth
Earth is the third planet from the Sun and the densest of the eight planets in
the Solar System. It is also the largest of the Solar System’s four terrestrial
planets.
Home to millions of species including humans, Earth is currently the only
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Figure 9: The moon.
astronomical body where life is known to exist. The planet formed 4.54 billion years ago, and life appeared on its surface within a billion years. Earth’s
outer surface is divided into several rigid segments, or tectonic plates, that
migrate across the surface over periods of many millions of years. About
71% of the surface is covered with salt water oceans, the remainder consisting of continents and islands which together have many lakes and other
sources of water contributing to the hydrosphere.
Earth interacts with other objects in space, especially the Sun and the
Moon. At present, Earth orbits the Sun once for every roughly 366.26 times
it rotates about its axis, which is equal to 365.26 solar days, or one sidereal
year.
3.6
The Moon - Earth’s natural satellite.
The moon is Earth’s only natural satellite. It is a cold, dry orb with a surface that is riddled with craters and strewn with rocks and dust (this dust
is called regolith). The moon has no atmosphere. Recent lunar missions
indicate that there is some frozen ice at the poles.
As mention before the moon always show the same side towards the
Earth due to its synchronous rotation. The far side of the moon was first
observed by humans in 1959 when the unmanned Soviet Luna 3 mission
orbited the moon and photographed it. Neil Armstrong and Buzz Aldrin
(on NASA’s Apollo 11 mission, which also included Michael Collins) were
the first people to walk on the moon, on July 20, 1969.
If you were standing on the moon, the sky would always appear dark,
even during the daytime. This is because the moon has no atmosphere.
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Also, from any spot on the moon (except on the far side of the moon where
you cannot see the Earth), the Earth would always be in the same place in
the sky. The rotation of the earth will be apparent when viewed from the
moon, and it it will going through different the phases similar to the lunar
phases.
The moon is about 384,000 km from Earth on average. At its closest
approach (the lunar perigee) the moon is 356,410 km from the Earth. At
its farthest approach (its apogee) the moon is 406,700 km from the Earth.
The moon revolves around the Earth in about one month (27 days 8 hours).
It rotates around its own axis in the same amount of time. The same side
of the moon always faces the Earth; it is in a synchronous rotation with the
Earth.
The Moon’s orbit is expanding over time and its rotation rate is also
slowing down. As a fact of interest Earth’s rotation rate is also slowing
down. For example, a billion years ago, the Moon was much closer to the
Earth (roughly 200,000 kilometres) and took only 20 days to orbit the Earth.
Also, one Earth ‘day’ was about 18 hours long (instead of our 24 hour day).
The tides on Earth were also much stronger since the moon was closer to
the Earth.
3.7
Mars
Mars is the fourth planet from the Sun in the Solar System. It is often described as the “Red Planet”, as the iron oxide prevalent on its surface gives
it a reddish appearance. Mars is a terrestrial planet with a thin atmosphere,
having surface features reminiscent both of the impact craters of the Moon
and the volcanoes, valleys, deserts, and polar ice caps of Earth.
The rotational period and seasonal cycles of Mars are likewise similar to
those of Earth, as is the tilt that produces the seasons. Mars is the site of
Olympus Mons, the highest known mountain within the Solar System, and
of Valles Marineris, the largest canyon.
Until the first flyby of Mars occurred in 1965, by Mariner 4, many speculated about the presence of liquid water on the planet’s surface. Geological
evidence gathered by unmanned missions suggest that Mars once had largescale water coverage on its surface, while small geyser-like water flows may
have occurred during the past decade. In 2005, radar data revealed the
presence of large quantities of water ice at the poles, and at mid-latitudes.
The Phoenix lander directly sampled water ice in shallow Martian soil on
July 31, 2008. Mars has two moons, Phobos and Deimos, which are small
and irregularly shaped. These may be captured asteroids.
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Figure 10: Mars.
Mars is currently host to three functional orbiting spacecraft: the Mars
Reconnaissance Orbiter, the Mars Orbiter Mission (Mangalyaan) and MAVEN
(Mars Atmosphere and Volatile EvolutioN mission). On the surface are the
two Mars Exploration Rovers (Spirit and Opportunity) and several inert
landers and rovers, both successful and unsuccessful. The Phoenix lander
completed its mission on the surface in 2008. Mars can easily be seen from
Earth with the naked eye.
3.8
Asteroids and meteorites
Tens of thousand of minor planetary bodies, concentrated in the asteroid
belt, have been detected. Towards 20 000 asteroids with known orbits are
named and catalogued. They avoid orbits with periods 1/2, 1/3 etc. of
Jupiter’s period - the Kirkwood Gaps (likewise the gaps in rings of Saturn due to innermost moon). The exception are the Trojan asteroids
which have same period as Jupiter but keep 60 degrees ahead or 60 degrees
behind the planet.
A few Apollo asteroids have eccentric orbits that bring them within the
Earths orbit - it is not known what perturbed them into such orbits. In a
relatively short time ( million years) they will collide with one of the inner
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Figure 11: The asteroid Gaspra.
planets.
Smaller examples of Apollo asteroids are large meteors which survived
the fiery passage through the Earth’s atmosphere. Once landed they are
termed meteorites. They are divided into different types/classes depending on composition: the Chondrites (stoney, 86%), the Achrondrites (also
stoney, 7%), the Stoney-Irons (2%) and the Irons (5%). The latter number
statistics apply to those seen to fall. In comparison, most fallen meteorites
discovered are irons which is by far the easiest to recognise or detect.
The iron meteorites must be fragments of a larger body or bodies wish
was sufficiently big for differentiation to have occurred. The achrondrites
may be the mantle fragments Chrondrites are characterised by tiny round
bodies, chrondrules, packed together in a grey silicate matrix. Chrondrules
vary in size from that of a pinhead to a pea. Their presence implies no
differentiation and it is possible that they are original planetesimals.
3.9
Jupiter
Jupiter is the fifth planet from the Sun and the largest planet within the
Solar System. It is a gas giant with a mass slightly less than one-thousandth
of the Sun but is two and a half times the mass of all the other planets in
our Solar System combined. Jupiter is classified as a gas giant along with
Saturn, Uranus and Neptune. Together, these four planets are sometimes
referred to as the Jovian planets. The planet was known by astronomers of
ancient times and was associated with the mythology and religious beliefs of
many cultures. The Romans named the planet after the Roman god Jupiter.
When viewed from Earth, Jupiter is the third-brightest object in the night
sky after the Moon and Venus.
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Figure 12: Jupiter.
Jupiter is primarily composed of hydrogen with a quarter of its mass
being helium; it may also have a rocky core of heavier elements. The outer
atmosphere is divided into several bands at different latitudes, resulting in
turbulence and storms along their boundaries. A prominent result is the
Great Red Spot, a giant storm that is known to have existed since at least
the 17th century when it was first seen by telescope. Surrounding the planet
is a faint planetary ring system and a powerful magnetosphere. There are
also at least 63 moons, including the four large moons called the Galilean
moons that were first discovered by Galileo Galilei in 1610. Ganymede, the
largest of these moons, has a diameter greater than that of the planet Mercury.
Jupiter has been explored on several occasions by robotic spacecraft,
most notably during the early Pioneer and Voyager flyby missions and later
by the Galileo orbiter. The most recent probe to visit Jupiter was the Plutobound New Horizons spacecraft in late February 2007. The probe used the
gravity from Jupiter to increase its speed. Future targets for exploration in
the Jovian system include the possible ice-covered liquid ocean on the moon
Europa.
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Figure 13: Saturn.
3.10
Saturn
Saturn is the sixth planet from the Sun and the second largest planet in the
Solar System, after Jupiter. Saturn is named after the Roman god Saturn.
Saturn has an average radius about 9 times larger than the Earth’s.
While only 1/8 the average density of Earth, due to its larger volume, Saturn’s mass is just over 95 times greater than Earth’s. Because of Saturn’s
large mass and resulting gravitation, the conditions produced on Saturn are
extreme if compared to Earth. The interior of Saturn is probably composed
of a core of iron, nickel, silicon and oxygen compounds, surrounded by a
deep layer of metallic hydrogen, an intermediate layer of liquid hydrogen
and liquid helium and finally, an outer gaseous layer. Wind speeds on Saturn can reach 1,800 km/h, significantly faster than those on Jupiter.
Saturn has nine rings, consisting mostly of ice particles with a smaller
amount of rocky debris and dust. 62 known moons orbit the planet; fiftythree are officially named. This is not counting hundreds of “moonlets”
within the rings. Titan, Saturn’s largest and the Solar System’s second
largest moon (after Jupiter’s Ganymede), is larger than the planet Mercury
and is the only moon in the Solar System to possess a significant atmosphere.
3.11
Uranus
Uranus is the seventh planet from the Sun. It has the third-largest planetary radius and fourth-largest planetary mass in the Solar System. 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. Sir William Herschel announced its discovery on March 13, 1781,
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Figure 14: Uranus.
expanding the known boundaries of the Solar System for the first time in
modern history. Uranus was also the first planet discovered with a telescope.
Uranus is similar in composition to Neptune, and both are of different chemical composition than the larger gas giants Jupiter and Saturn.
Uranus’s atmosphere, while similar to Jupiter and Saturn’s in its primary
composition of hydrogen and helium, contains more “ices” such as water,
ammonia and methane, along with traces of hydrocarbons. It is the coldest
planetary atmosphere in the Solar System, with a minimum temperature
of 49 K (224 degrees Celsius). It has a complex, layered cloud structure,
with water thought to make up the lowest clouds, and methane thought to
make up the uppermost layer of clouds. In contrast the interior of Uranus
is mainly composed of ices and rock.
Like the other giant planets, Uranus has a ring system, a magnetosphere,
and numerous moons. The Uranian system has a unique configuration
among the planets because its axis of rotation is tilted sideways, nearly
into the plane of its revolution about the Sun. As such, its north and south
poles lie where most other planets have their equators. Seen from Earth,
Uranus’s rings can sometimes appear to circle the planet like an archery
target and its moons revolve around it like the hands of a clock. The wind
speeds on Uranus can reach 900 km/h.
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Figure 15: Neptune.
3.12
Neptune
Neptune is the eighth and farthest planet from the Sun in our Solar System. Named for the Roman god of the sea, it is the fourth-largest planet
by diameter and the third-largest by mass. Neptune is 17 times the mass
of Earth and is slightly more massive than Uranus. On average, Neptune
orbits the Sun at a distance of 30.1 AU, approximately 30 times the EarthSun distance.
Discovered on September 23, 1846, Neptune was the first planet found by
mathematical prediction rather than by empirical observation. Unexpected
changes in the orbit of Uranus led Alexis Bouvard to deduce that its orbit
was subject to gravitational perturbation by an unknown planet. Neptune
was subsequently observed by Johann Calle within a degree of the position
predicted by Urbain Le Verrier, and its largest moon, Triton, was discovered shortly thereafter, though none of the planet’s remaining 12 moons were
located telescopically until the 20th century. Neptune has been visited by
only one spacecraft, Voyager 2, which flew by the planet on August 25, 1989.
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
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Figure 16: The comet Hale-Bopp.
water, ammonia and methane. The interior of Neptune, like that of Uranus,
is primarily composed of ices and rock. Traces of methane in the outermost
regions in part account for the planet’s blue appearance.
In contrast to the relatively featureless atmosphere of Uranus, Neptune’s
atmosphere is notable for its active and visible weather patterns. At the time
of the 1989 Voyager 2 flyby, for example, the planet’s southern hemisphere
possessed a Great Dark Spot comparable to the Great Red Spot on Jupiter.
These weather patterns are driven by the strongest sustained winds of any
planet in the Solar System, with recorded wind speeds as high as 2,100 km/h.
Because of its great distance from the Sun, Neptune’s outer atmosphere is
one of the coldest places in the Solar System, with temperatures at its cloud
tops approaching 218 degrees Celsius (55 K). Temperatures at the planet’s
centre, however, are approximately 5,400 K (5,000 C). Neptune has a faint
and fragmented ring system, which may have been detected during the 1960s
but was only indisputably confirmed in 1989 by Voyager 2.
3.13
Icy bodies and comets
There is apparently a vast reservoir of small icy bodies in the Kuiper Belt
and Oort Cloud. On occasions, these are somehow perturbed and fall in
towards the Sun. They follow a virtual parabolic orbit around the Sun and
return to the outer regions. In certain cases, an encounter with a planet
may slow down the body such that it ends up in a short period eccentric
orbit - such as the nucleus of Comet Halley with a period of only 75 years.
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These object are low density, partly uffy in composition. Once described
as “dirty snowballs”. The 2005 impact of a spacecraft with Comet Tempel
2 indicates that “snowy dirtball” might be more appropriate. During the
time when the body is close to the Sun, the heat of the sun sublimates the
ice (turns directly into water vapour). Hampered by the build up of rocky
debris on the surface, the water vapour so formed may blast out through
crevices in the crust carrying ner particles with it. Halley has now shrunk
to about 11 × 7 km in size and probably loses about 6m of surface ice each
perihelion passage. Other cometary nuclei that pass closer to the Sun may
not even survive the passage intact.
The water vapour and dust also form a spherical tenuous coma, sometimes millions of kilometres across during this phase in the comets orbit.
The coma is the most obvious feature of a visible comet. Dissociation of
water molecules releases hydrogen that forms an even larger cloud. From
the fragile coma, two forms of tails develop. Solar radiation pushes the dust
to form an amorphous yellow tail. The solar wind drives ionised gas to produce a structured blue tail. Obviously, both tails point away from the Sun.
Over time (maybe several orbits) after which most of the water/ice has
been removed, eventually what will remain is a swarm of particles. At times
the Earth has encountered such swarms (they can even be predicted) and a
meteor shower is observed. Since they all come from a common direction,
they will appear to come from a common radiant in the sky. None of these
particles is large enough to reach the Earth’s surface.
Comet-planet collisions can and doe occur. At least one comet (Shoemaker Levy 9) was captured in orbit around Jupiter and eventually collided
with that planet. It is not known if the explosion in the atmosphere over
Siberia in l908 was caused by the impact of a small comet or asteroid. Many
comets fall straight into the Sun.
Comets have a wide range of orbital periods, ranging from a few years to
hundreds of thousands of years. Short-period comets originate in the Kuiper
belt, or its associated scattered disc, which lie beyond the orbit of Neptune.
Longer-period comets are thought to originate in the Oort Cloud. The
long-period comets plunge towards the Sun from the Oort Cloud because
of gravitational perturbations caused by either the massive outer planets of
the Solar System (Jupiter, Saturn, Uranus, and Neptune), or passing stars.
Rare hyperbolic comets pass once through the inner Solar System before
being thrown out into interstellar space along hyperbolic trajectories.
Currently there are more than 5000 known comets. This number is
steadily increasing. However, this represents only a tiny fraction of the
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Figure 17: A schematic of the interior of terestrial planet.
total potential comet population: the reservoir of comet-like bodies in the
outer solar system may number one trillion. The number visible to the
naked eye averages roughly one per year, though many of these are faint
and unspectacular.
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4.1
Planetary structures
Internal composition and structure
The scenario presented above indicates that planets can be classied as either
“giant” or “terrestrial” - the latter class can include the large numbers of
Moons in the solar system. Planets differ greatly in composition - an indication is provided by their average density. It is also possible to gauge the
internal mass distribution from their period of rotation and the resulting
degree of attening.
Terrestrial Planets
The inner planets of the solar system are composed almost entirely of heavy
elements - including some radioactive isotopes, with very long half lives.
The decay of these isotopes has released a great deal of heat - for example,
the centre of the Earth is believed to have a temperature of 6600 K. Under
such conditions the interiors are molten and differentiation can occur whereby elements with higher density (chiefly iron and nickel) sink to the
centre to form a core (see figure 17).
Slow convective currents in the core then generate weak magnetic elds
(much weaker than those of the giant planets). We would expect Venus to
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Figure 18: A schematic of the interior of an icy moon.
have a core similar to the Earth, but it lacks a magnetic field and its very
slow rotation rate does not allow us to gauge its internal mass distribution.
Mercury has a large core, almost as if part of its outer mantle had been
lost. Mars has a weak cores and the Moon no core. That the density and
composition of the Moon seems to match that of the Earth’s outer mantle
supports the theory that the material used to make the Moon broke from
the Earth’s mantle, possible during a catastrophic collision.
Europa, Ganymede and Callisto (the three biggest moons of Jupiter)
lie in a much cooler part of the solar system. They appear to have silicate
cores, but surrounded by substantial mantles of water ice (or perhaps water,
in the case of Europa and Ganymede). Ganymede is the largest moon in
the Solar System, larger even than planet Mercury. Lying closer to Jupiter,
tidal stresses have heated the interior of Io - so much so that it has lost its
water mantle and is subject to severe volcanism that is constantly “turning
the moon inside out”.
The density of most of the moons of Saturn (Titan excluded) Uranus
and Neptune suggests that they are almost completely water ice - or rather
“dirty ice”. So peculiar is the appearance of Miranda (a moon of Uranus),
that some researchers have even suggested that this moon broke apart into
a number of pieces and then re-assembled.
Gas planets
The two largest planets, Jupiter and Saturn, are mainly hydrogen and helium in liquid form (and the description “gas giant”, often applied, is misleading) - what heavy elements they have are congregated in the centres;
Under the extreme pressures towards the centre, the liquid hydrogen behaves as a metal (i.e. electrical currents can ow in their cores). Jupiter
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Figure 19: A schematic of the interior of gas giant planet.
and Saturn are “would be stars”; their interiors are hot and they radiate
out to space about twice the heat they receive from the Sun - although it
is obviously in the far infrared. The flow of heat outward sets up convective motions within the planets’ interiors. The details are not understood,
but the motions within the metallic region are thought to generate the very
strong magnetic fields possessed by these planets.
Uranus and Neptune are much smaller and their interiors much cooler.
Some models suggest that their cores could be surrounded by ices.
4.2
Atmospheres
Terrestrial planets
The primary atmosphere formed with the planet - hydrogen and hydrogen compounds (methane, ammonia etc.) - but was gradually lost to space
because the thermal molecular motions approach escape velocity - especially
molecular hydrogen and helium. The Moon’s gravity was too low to retain
any atmosphere, virtually the same for Mercury.
The secondary atmosphere, formed from outgassing from the crust
(volcanism), is almost entirely carbon dioxide - as in atmospheres of Venus
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and Mars - with a small amount of nitrogen. The Earth has lost 99% of its
atmosphere - almost all carbon dioxide washed out by rain to form carbonate rocks. Only nitrogen remains, plus oxygen from plant photosynthesis.
Weather is common phenomenon in terestrial planetary asmospheres.
Hot air rises where the solar heating is greatest (near equators) and descends
at higher latitudes. This circulation is forms Hadley cells (seen on Earth
and Venus). On Earth, variations in weather are largely driven by the heat
stored in the oceans. Both Earth and Venus show weather patterns rotating
faster than the planet.
Violent atmospheric motions can occur if substantial energy can be directly inserted into atmosphere. This happens when condensing water releases large amount of latent heat which drives thunderstorms, hurricanes
etc. Also dust particles suspended in atmosphere absorb solar heat directly
which leads to sandstorms. In 1971 and 2002, the whole of Mars was covered
by a single sandstorm.
Gas Planets
The gaseous outer regions of the giant planets are, of course, mainly hydrogen and helium. However, a variety of other components create cloud layers
and colours. The deep blue colour of Neptune is from methane absorption,
but the reddish colours in the clouds of Jupiter and Saturn are not yet explained.
Rising convective motions push clouds higher (lighter bands on Jupiter
and Saturn), sinking motions make clouds lower (darker bands on Jupiter
and Saturn). Why these should form longitudinal bands, each rotating at
a speed different to each other, is not properly understood either. The
driving forces may be cyclonic storms, the most conspicuous being the great
red spot of Jupiter (which has been there since telescopes were first used)
the great dark spots of Uranus and Neptune and the white spots on Jupiter
and Saturn (which come and go). How these storms are powered is not
known.
4.3
Magnetospheres
Planets with magnetic fields (Mercury, Earth, Jupiter, Saturn, Uranus and
Neptune) are surrounded by magnetospheres. The stronger the eld, the more
extensive the magnetosphere. Magnetospheres are bounded and deformed
by the solar wind (charged particles) blowing past. Surges in the solar wind
may compress the magnetosphere and set up massive electrical currents and
causing aurora displays.
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Figure 20: A schematic of the magnetosphere of a planet.
Magnetospheres serve as a shield to cosmic rays - but such charges particles tend to accumulate in particle belts (van Allen belts). Such regions are dangerous to humans and (in the case of Jupiter and Saturn) even
spacecraft. In the case of Io (innermost of Galilean moons) moves through
magnetosphere and stirs up particle belts causing radio radiation.
4.4
Surface gravities
The force of gravity experienced on the surface of a planet varies according
to the mass of the planet and the inverse square of its radius. Consequently
the surface gravities of the Moon, Mercury and Mars are much lower than
the Earth’s, while Jupiter’s is much higher.
5
Planetary surface features
The four terrestrial planets and all the moons and minor planets of the Solar
System have hard surfaces exhibiting many common features. Mercury is
very cratered, Venus has elevated “continents”, volcanoes and some impact
craters. The earth have moving continents, mountain ranges, and surface
largely sculpted by water, deserts and ice sheets. Mars has an elevated
southern hemisphere heavily cratered, has desert and ice sheets. The moon,
like Mercury, is heavily cratered and show volcanic maria.
The moons of Jupiter shows a wide variety of surface features. lo has
volcanoes, while Europa has an ice covered ocean. Ganymede has a thick
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Figure 21: A schematic of the crater formation process.
ice cover and Callisto is heavily cratered. Titan shows dry rivers and lakes
while Triton have icy geysers.
5.1
Impact cratering
Impact craters are the most ubiquitous of surface features on planets, Some
planets, like the Moon, have so many impact craters that there is no place
on their surface that is not an impact crater, and newer craters obliterate
older craters. As such they represent the “sweeping up” of minor bodies in
the solar system. Their ages conrm that most of the cratering (including
some major impacts) took place during the first billion years of the solar
system.
The cratering rate has declined dramatically as the population of minor
bodies was decimated. However, even today, cratering continues - though
the last major impact on Earth was about 50 000 years ago. The source for
the impacting material is apparently the asteroid belt. Figure 21 illustrates
what happens during an impact.
All planets is believed to have received similar numbers of impacts - yet
on some planets, erosion has removed the craters. The Earth, with only one
intact impact crater (in Arizona), is an example of severe erosion, though
Io has none. Many planets, like Venus or Mars, show limited numbers of
impact craters. The southern hemisphere of Mars has much less cratering
than the Moon, while Venus only shows cratering from the last half billion
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Figure 22: A schematic showing difference in structure between craters and
volcanoes.
years. Thus the degree of cratering on a planetary surface can he used as
an indication of the age of the surface (the heavier the cratering, the older
surface).
Stratigraphy concerns the relative chronological sequence of surface
features and is particularly applicable to the Moon and Mars. A newer feature will cover or partially obliterate an older feature. Also older craters are
more subdued and dark. Fresh craters show sharp proles, rugged appearance, light materials or ray systems.
5.2
Volcanism
Volcanic vents emit lava and gas. Thick lava builds elevated shield volcanoes, especially on Venus, Earth and Mars. The largest is Olympus Mons
on Mars - almost 25 km high. Thinner lava seems to have gradually ooded
large impact basins, forming maria basins, the most conspicuous features
on the Moon’s near side also found on Mercury and Mars. The flows are
complex, building up over millions of years. The thin lava comes to the
surface either through fractures or from craters that mimic impact craters,
but are different in their cross sections (see figure 22).
High speed ejection ( 1 km/s) is constantly occurring from several volcanoes on Io. Much sulphur is emitted and rains down on the surface almost
completely covering the planet. Lakes of hot liquid sulphur and lava ows
are apparent; it seems that the underlying mechanism must be different to
terrestrial volcanoes.
5.3
Erosion
Planets with no atmosphere suffer bombardment from tiny meteors, which
tends to pulverise surface layers - hence making a regolith resembling dry
Portland cement. It also contains stones and microscopic glass beads (solidified from drops of molten material flung skyward during impacts). The
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beads are an excellent light backscatterer (esp. rays from craters on Moon
best visible near Full Moon).
Planets with some atmosphere - wind erosion results from abrasive action of suspended particles. Sand shifts with wind patterns, but collects to
form dune fields in low lying basins.
Water is by far the most powerful erosive agent. The surfaces of the
continents of the Earth are subject to severe water erosion. Smaller features
(e.g. impact craters) are rapidly erased, even major mountain ranges only
survive for perhaps only a tenth of the Earth’s lifetime. Sedimentary basins;
later elevated, are signicant features (e.g. Karoo).
5.4
Valleys and Rilles
Rivers rapidly carve valleys or, in dryer climates, canyons with characteristic dendritic pattern - esp. Earth, Mars and Titan. The climate of Mars
is currently so cold that virtually no water exists in liquid form; apparently
it was warmer in the past. The surface of Titan is so cold that water only
exists as hard “rocks”, but liquid methane was probably responsible for the
valleys and lakes (now dry).
The Moon, Venus and Mars possess many valleys clearly not made by
running liquid, at least not on the surface. Rilles - some straight and some
sinuous - are somehow related to volcanism, partly by collapse. Africa has
a giant rift valley where the continent is currently splitting into two pieces.
Mars has gigantic rift valley (Valles Marinus), thousands of kilometres long
and up to 6 km deep, a great split in its surface. In places it is modified by
water erosion, but it is mainly growing by the surrounding elevated plateau
collapsing.
5.5
Water and Polar Caps
These are abundant stores of water ice. Mars had a wet episodes in its past,
the low lying northern hemisphere (devoid of ancient craters) may have at
times been a shallow ocean. Currently Martian water is frozen in the soil,
and thick layers of ice appear to have accumulated at the poles. These are
covered by sand. The conspicuous white polar caps on Mars that grow and
shrink with the seasons are formed from thin coatings of carbon dioxide
(when about a quarter of the Martian atmosphere condenses). Earth has
ice caps kilometres thick on Antarctica and Greenland.
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Figure 23: A schematic of a continent.
5.6
Crustal Materials
The crusts of terrestrial planets are predominantly silicates. Silicon atoms
are usually linked by oxygen atoms (silica - acidic), otherwise metal ions
intervene (olivines - basic). There are intermediate cases, such as feldspars
and pyroxenes.
There is a general tendency for original crustal materials to be acidic
or intermediate. Materials deeper down, brought up in lava, tend to be
basic. The marias of the Moon are basalts (basic) rich in iron and titanium.
Materials elsewhere on the Moon are complex - usually feldspathic breccias
(partial melting and recrystallization) as expected from impact ejecta. Mars
also has basalts. The high iron content in Mars’ crustal materials gives its
rocks and sand a reddish colour.
5.7
Continents
Both Earth and Venus have continents - bodies of material elevated above
the surrounding crust - apparently masses of lighter material being in isostatic equilibrium and oating higher. On Earth, it is of course the continents
that generally project above the surface of the oceans (see figure 23). Most
continents have a hard core termed a craton (heavy stippling in figure 23)
surrounded by softer orogenic zones (light stippling).
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Figure 24: A schematic of Earth’s tectonic plate movements.
The tectonic plates of the Earth
The thin ocean beds are not permanent but are constantly replenished and
replaced by new material from the Earth’s interior. The new ocean bed
pushes and spreads sideways from mid-oceanic ridges. Old ocean beds are
subducted back into the Earth’s interior in deep trenches close to the continents. The surface of the Earth is fragmented into segments, the tectonic
plates (see figure 24).
The continents ride with the plates sometimes separating and sometimes
colliding with one another - the soft orogenic zones getting crumpled and
the continents getting sutured together (c,g. the collision between India and
Asia causing the Himalayas - a collision still in progress). The Urals were
formed when Europe and Asia were sutured, the Appalachians when Africa
and North America collided.
At intervals of a few hundred million years, the continents tend to gather
into a single supercontinent. Some 300 million years ago, most of the continental masses came together to form Pangea, Pangea became unstable,
possibly because too much heat built up beneath it, and split into Laurasia and Gondwanaland. Laurasia split into North America and Eurasia,
Gondwanaland into South America, Africa, Antartica, India and Australia.
Africa is soon to split again (along the rift valley). The Atlantic Ocean
(only 150 million years old) is rapidly opening up - the Pacific is being
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closed on all sides - its plate is slowly being subducted. In less than another
200 million years, this is where all the continents will regroup to form a new
supercontinent. And so the cycle will repeat, as it must have already done
so many times in the Earth’s history.
Note that oceans are relatively shallow over the mid-oceanic ridges where
new material comes up. The mid-Atlantic ridge actually rises above sea level
at one point - Iceland. One other place of interest is California where the
infamous San Andreas fault marks the boundary between the Pacific plate
(moving laterally to the north) and the N. American plate (moving to the
south).
The Earth is, of course, the only planet known to have this plate motion.
6
Extrasolar planets
An extrasolar planet, or exoplanet, is a planet outside the solar system. It
is now known that a substantial fraction of stars have planetary systems,
including at least around 10% of sun-like stars. The true proportion may
be much higher. It follows that billions of exoplanets must exist in our own
galaxy alone. Even more exciting are that current estimates suggest that
about 10% of all sun-like stars probably harbour earth-size planets in the
habital zone. Broadly speaking, the habital zone is defined as the region
around a star where the conditions (e.g. planetary surface temperature)
would be suitable for liquid water (and thus making life as we know it possible) to exist.
Extrasolar planets became an object of scientific investigation in the
nineteenth century. Many astronomers supposed that they existed, but there
was no way of knowing how common they were or how similar they might be
to the planets of our solar system. The first confirmed detection was made in
1992, with the discovery of several terrestrial-mass planets orbiting the pulsar PSR B1257+12. The first confirmed detection of an exoplanet orbiting
a main sequence star was made in 1995, when a giant planet, 51 Pegasi b,
was found in a four-day orbit around the nearby G-type star 51 Pegasi. The
frequency of detections has tended to increase on an annual basis since then.
The number of exoplanets detected and confirmed is now well over the
1000 mark. There are at least another several thousand more planet candidates awaiting confirmation by more detailed investigations. Exoplanets
have been detected through various techniques. The most successful of these
detects a planet indirectly, due to its (very small) gravitational influence on
its host star. The planet causes a slight wobble in the motion of the host
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star, which is detected as a very small line-of-sight velocity change. The
technique obviously favours massive Jupiter-like planets close to their parent stars, and these are the majority found. For example our Sun moves in
a circle at 16 metres a second as Jupiter (by far the most massive planet in
our Solar system) orbits around it.
Some extra-solar planets are found because their orbits are seen edgeon and they “eclipse” their parent star, causing a small drop in the light
we receive. The Kepler mission has found thousands of such planets
using this technique, many of which are are of super-earth size and smaller.
For more on this exciting field in modern astronomy explore the website
exoplanets.org.
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