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Solar System:
1. Solar System Inventory
a. In Galileo Galilei's time (1574-1648), the terrestrial planets Mercury,
Venus, and Mars were known as well as the Jovian planets Jupiter and
Saturn. Galileo discovered the phases of Venus as well as the Moon's
craters and four moons orbiting Jupiter.
b. Subsequently Saturn's rings were discovered (1659), the planets Uranus
(1758) and Neptune (1846), many planetary moons, and the first asteroids,
which are minor "planets" orbiting the sun.
c. In the 20th century, the Kuiper belt objects were discovered in orbit
beyond Neptune, dozens of planetary moons and thousands of asteroids.
d. In addition planetary space explorations began in 1969 with our manned
landing on the moon.
e. Comparative planetology is the study of planetary properties.
2. Measuring the planets
a. Distance from the sun using Kepler's laws plus radar ranging from Venus.
b. Sidereal orbits.
c. Planets' radius by measuring its angular dimension, the solid angle it
subtends.
d. The masses of the planets can be determined by observing the orbits of
planetary moons except for Mercury and Venus as well as our own Moon
and the asteroid Ceres are determined by careful measurements of
gravitational effects on nearby bodies.
e. Rotational periods are measured by observing features or, when that is not
possible with the Jovian planets, by observing magnetic field effects,
which have an impact on radio radiation, and Doppler broadening of
various emission lines.
f. The density, which is a measure of the number of particles per unit volume,
is determined from the mass and radius: density = mass/volume.
3. Layout of Solar System
a. You can remember the names of the planets starting with Mercury from
the expression My Very Educated Mother Just Served Us Nine Pies,
which won't work now if Pluto is no longer considered a planet.
b. The planetary orbits about the Sun are approximately circular and
counterclockwise, as seen from about Earth's North Pole, and lie roughly
in a plane except for Mercury.
c. The inner terrestrial planets, Mercury, Venus, Earth and Mars, are similar
in density and composition to the Earth whereas the Jovian planets have
lower density and different compositions from the terrestrial planets.
There are additional differences in magnetic fields, rotations, moons and
rings.
d. Additionally there is interplanetary matter consisting of cosmic debris
ranging from relatively large asteroids and members of the Kuiper belt,
through smaller comets and even smaller meteoroids, down to the smallest
grains of interplanetary dust, which results from collisions.
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e. There's also the solar wind, which are charged particles emitted from the
sun, which interact with the planets and interplanetary materials.
4. Spacecraft Exploration
a. Beginning in the 1960s there have been numerous space missions that
have flown by and landed on planets and their moons, and in 2001 the
Near Earth Asteroid Rendezvous spacecraft landed on Eros, an asteroid
approximately 34 km long.
5. Solar System Formation
a. Nebular contraction: Early theory posited that either due to interstellar
collisions or the explosion of a star, a nebula started to contract under its
own gravitation, with the planets forming as the material aggregated at
certain points, which is to say that planets in this model are considered to
be the byproduct of star formation.
b. Planetary Condensation: In this model, which is an extension of nebular
contraction and addresses the aggregation dynamics, the planets condense
out as a function of the temperature of the stellar cloud. Closer to the
center of the solar nebula where the temperatures were hotter, rocky
byproducts were produced, which resulted in the terrestrial planets, but
farther out in the cloud where the temperatures were cooler, less dense
planets condensed.
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Comparison of the Terrestrial and Jovian Planets:
Terrestrial Planets
Jovian Planets
Close to Sun
Far from Sun
Closely spaced orbits
Widely spaced orbits
Small masses
Large masses
Small radii
Large radii
Predominantly rocky
Predominantly gaseous
Solid surface
No solid surface
High density
Low density
Slower rotation
Faster rotation
Weak magnetic fields
Strong magnetic fields
Few moons
Many moons
No rings
Many rings
Numerous missions to other planets and the Earth’s Moon began in the 1960s.
The Terrestrial Planets:
1. Earth:
a. The earth has multiple layers, including a molten core. The composition of
the earth is differentiated because as the earth was being formed, it was
bombarded with material, increasing its interior temperature. Radioactivity,
the decay of particles, increased the temperature of the core as well, as did
the gravitational energy as mass worked its way deeper into the earth. As a
result, the Earth’s central temperature is nearly equal to the surface
temperature of the Sun.
b. The Earth’s surface continues to be active, with plates shifting to produce
earthquakes.
c. In this molten core there are free electrons, so the rotation of the Earth
creates a current, which in turn creates a magnetic field.
d. The Earth has an atmosphere that decreases in density (mass/volume =
number of particles/volume) with increasing altitude. The composition of
the atmosphere, which is transparent to a narrow band of photon
frequencies, is responsible for maintaining the Earth’s livable temperature.
Prior to the existence of life some 3.5 billion years ago, which created
atmospheric oxygen, the atmosphere consisted primarily of nitrogen and
carbon dioxide.
e. The Earth is unique in that three-quarters of the surface is covered in water.
The gravitational force of the moon and sun are responsible for the tides,
which are high and low twice in each 24-hour day, varying in height
because of the sun-moon interaction with the earth.
f. The tidal forces are also responsible for the slowing of the earth’s rotation.
Some half a billion years ago a day lasted 22 hours and the rotation around
the sun took some 397 days. This process, caused by the tidal drag of the
Moon on the Earth, will continue until the moon orbits always about the
same place on earth. When this happens in billions of years from now, the
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Earth’s day will be 47 times longer and the Moon will be 43 percent
farther away than it currently is.
2. Mercury and the Moon:
a. Mercury, .38 Earth radii, and the Moon, .27 Earth radii, are comparable in
size.
b. Because Mercury is an inferior planet, i.e., its orbit is nearer the Sun, the
planet’s orbit keeps it quite close to the Sun.
c. Mercury has roughly the same density as the earth while the Moon has a
lower density since it contains fewer heavy elements such as iron.
d. Mercury and the Moon have surfaces that are strikingly similar, both
resulting from frequent meteor impacts.
e. The Moon’s orbital rate, 27.3 days, exactly matches its rotation about the
Earth because the Earth-Moon gravitational force has synchronized the
two. It was initially believed that Mercury was similarly synchronized
with the Sun, but later, through radar Doppler measurements, it was
determined that Mercury’s rotation period is 59 days while its orbital
period about the Sun is 88 days. This results because Mercury has a highly
eccentric orbit, i.e., it is not circular like the Moon’s about Earth.
f. Even though the Moon may have a molten core, its slow rotation results in
a very small magnetic field. Mercury’s magnetic field is small, about a
hundredth that of Earth’s, but it was surprising it had one at all and
suggests that Mercury contains a rapidly rotating molten core.
g. It’s believed that the Moon was formed when a large, Mars-sized object
collided, with more a glancing blow than a direct hit, with a youthful Earth.
3. Venus:
a. Venus is the third brightest object in the sky after the Sun and Moon
because its surface is highly reflective.
b. Like Mercury, Venus has no moon, and Venus’s radius is 0.95 the Earth’s
radius. Since its density is similar to the Earth’s as well, Venus is called
Earth’s sister planet.
c. Venus’s rotation, unlike Earth’s, is retrograde, moving the opposite
direction, and very slowly, taking 243 days for a single rotation. It is
believed that this slow retrograde rotation is the result of a collision with a
large body.
d. While Venus’s atmosphere, which at high altitudes rotates about the planet
every four days, is opaque to visible radiation, it’s relatively transparent to
longer radar wavelengths. This allows the Arecebo observatory to map the
surface.
e. While it was thought initially that the temperature on the surface of Venus,
measured to be roughly 240 K, might be similar to Earth’s, the blackbody
temperature was later measured to be 730K, more than twice Earth’s
temperature.
f. Venus’s carbon dioxide atmosphere, with just a hint of nitrogren (3.5
percent) is 90 times that of Earth’s. While it might have had a similar
origin, the absence of life might account for the absence of oxygen. And
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g. As we might expect, Venus has no magnetic field because of its extremely
slow rotation.
4. Mars:
a. Mars has an orbit with greater eccentricity than most other planets’ except
for Mercury.
b. Mars has two small moons, Phobos (Fear) and Deimos (Panic), which are
visible from Earth and are little more than large rocks (some tens of
kilometers in size), are asteroids trapped in Mars’s gravitational field.
c. Mars rotates on its axis once every 24.6 hours, about the same as the
Earth’s rotation, and its axis is inclined by 24 degrees, which is quite
similar to the Earth’s 23.5 degree tilt.
d. Mars’s northern hemisphere is composed of rolling volcanic planes,
similar to the Moon’s planes, which are much less cratered than the
southern hemisphere.
e. While Mars today is dry and desolate, there are hints in features such a
runoff channels, that there was once running water.
f. Mars does have polar caps which are dry ice, frozen carbon dioxide.
g. The average surface temperature on Mars is about 50 K cooler than on
Earth though during the summer, surface temperatures may reach 300 K.
h. Mars’s atmosphere is over 10,000 times thinner than Venus’s. Sometime
during. Some of it may have leaked away because of impacts and also
because Mars’s gravity is significantly less than Earth’s.
i. Mars does not have a magnetic field probably because it does not have a
molten core. And this probably resulted because of its smaller size.
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The Jovian Planets:
5. Jupiter:
a. Jupiter’s rotation period is .41 days, and its density is 1300/5500 that of
earth’s, or about ¼ that of earth’s.
b. Like a star, Jupiter’s atmosphere is predominantly hydrogen and helium -H2 makes up 86.1% and He comprises 13.8%. Because of its large mass,
318 times Earth’s, Jupiter still retains the hydrogen it had 4.6 billion years
ago when the planet formed.
c. The bands of Jupiter, which include the Great Red Spot, appear to result
from thermal gradients in the gaseous atmostphere. The colors appear to
result from sulfur-containing compounds.
d. The magnetic field of Jupiter is a million times more voluminous and
20,000 times greater than Earth’s.
e. While Jupiter has 63 moons, the four Galilean satellites, Io, Europa,
Ganymede and Callisto, are similar in size to Earth’s moon. It is thought
that the formation of Jupiter evolved in a way similar to the formation of
the Sun and its planets.
f. Both Europa and Ganymede contain water.
g. The 1979 Voyager mission discovered a thin ring that lies roughly 50,000
km above the top cloud layer and inside the innermost moon.
6. Saturn:
a. Saturn’s rotation period is .44 Earth days, and its density is 700/5500 that
of earth’s or about 1/8 that of Earth.
b. Saturn is much less colorful than Jupiter and consists almost entirely of
hydrogen, about 92.4%, with 7.4% helium.
c. While Galileo viewed Saturn in 1610, the resolution of his small telescope
didn’t enable him to resolve the rings, which looked to him like bumps or
a triple planet system. The rings are only 10 to 15 meters thick and are
composed primarily of water ice. It is believed that the rings were formed
when a moon was ripped apart by tidal forces.
d. The moons of Saturn, of which there are 56, gravitationally interact with
the rings to create various patterns, like kinks and waves, in the ring
structure.
e. Titan, with almost twice the mass of Earth’s Moon, is the most interesting
of Saturn’s satellites because it has a thick atmosphere and a surface with
liquid methane lakes. It is also believed that the interior of this rocky
planet contains water.
f. Cassini’s multiple flybys of the moons and rings of Saturn that will end in
June of next year will provide more information.
7. Uranus & Neptune:
a. These planets have atmospheres that are quite similar to Jupiter’s and
Saturn’s.
b. While Uranus rotates counterclockwise like the all the other planets save
Venus, Uranus’s rotation axis lies slightly below the plane of the ecliptic,
the path of the planet’s motion about the sun. It is thought that this
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extreme tilt is the result of a catastrophic glancing collision with another
object.
c. Both Uranus and Neptune have fairly strong magnetic fields as we would
expect given their rotation rates and size much larger than Earth.
d. Both Uranus and Neptune have rings.
8. Solar System Debris:
a. Asteroids:
i. Asteroids, which means star-like bodies, are predominantly rocky
objects that revolve around the Sun.
ii. Astronomers have cataloged over 75,000 asteroids with welldetermined orbits and another 125,000 for which the orbits have
not been determined. The vast majority are located in the asteroid
belt lying between 2.1 and 3.3 AU from the Sun, roughly midway
between the orbits of Mars (1.5 AU) and Jupiter (5.2 AU).
iii. The largest asteroids are Ceres, Pallas, and Vesta with diameters of
940 km, 580 km, and 540 km.
iv. As of early 2007, more than 4400 Earth-crossing asteroids are
known, with more than 800 designated as potentially hazardous,
meaning that they are more than 150 m in diameter and move in
orbits that bring them within 7.5 km of Earth. In 2002 a 100 m
asteroid came within 1/3 the distance of the Moon and was
detected three days after it had passed the Earth.
v. While none are expected to collide in the next century, calculations
indicate that most Earth-crossing asteroids will eventually collide
with Earth at the rate of about three each million years.
b. Comets:
i. Comets are small objects, usually no more than a few km in
diameter, which consist primarily of dust particles trapped within a
mixture of methane, ammonia, carbon dioxide, and ordinary water
ice, which is similar to the composition of the small moons in the
outer solar system.
ii. As they approach within 2 or 3 AU of the Sun, the comet surface
becomes gaseous and creates a coma, the tail that you see.
iii. There have been several comet flybys, the closest being within 600
km of the nucleus. And in 2004 the Stardust spacecraft captured
comet coma particles in aerogel that it brought back to earth in
2006.
c. Trans-Neptunian objects:
i. Any body orbiting beyond Neptune is called a Trans-Neptunian
object, which are collectively referred to as Kuiper belt objects, the
largest of which is Pluto. Currently there are 1000 objects.
ii. Originally it was claimed that Pluto was necessary to explain
irregularities in the motions of Uranus and Neptune; however,
these irregularities were later shown not to exist, and the mass of
Pluto was discovered to be too small to have been responsible for
such effects on these planets.
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d. Meteroids:
i. Smaller meteoroids are mainly the rocky remains of broken-up
comets that have passed near the Sun.
ii. If Earth’s orbit happens to intersect a cluster of meteoroids, which
is the distribution of comet debris along its orbit, a meteor shower
results.
iii. The world’s largest meteorite weighs 34 tons and is on display at
the American Museum of Natural History in New York.
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