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PSCI 1414 GENERAL
ASTRONOMY
LECTURE 8: THE SOLAR SYSTEM
ALEXANDER C. SPAHN
THE SOLAR SYSTEM
Our ancestors long ago recognized the motions of the planets through the sky, but it has been
only a few hundred years since we learned that Earth is also a planet that orbits the Sun. Even
then, we knew little about the other planets until the advent of large telescopes. More
recently, the dawn of space exploration has brought us far greater understanding of other
worlds. We’ve lived in this solar system all along, but only now are we getting to know it.
In this chapter, we’ll explore our solar system like newcomers to the neighborhood. We’ll
begin by discussing what we hope to learn by studying the solar system, and in the process
take a brief tour of major features of the Sun and planets. We’ll also explore the major
patterns we observe in the solar system. Finally, we’ll discuss the use of spacecraft to explore
the solar system, examining how we are coming to learn so much more about our neighbors.
THE SOLAR SYSTEM
Sometimes we study the planets individually—for example, when we map the geography
of Mars or the atmospheric structure of Jupiter.
Other times we compare the worlds to one another, seeking to understand their
similarities and differences. This latter approach is called comparative planetology.
Note that astronomers use the term planetology broadly to include moons, asteroids, and
comets as well as planets.
THE SOLAR SYSTEM
A planet falls naturally into one of two
categories according to the size of its orbit.
The orbits of the four inner planets
(Mercury, Venus, Earth, and Mars) are
crowded in close to the Sun.
In contrast, the orbits of the four outer
planets (Jupiter, Saturn, Uranus, and
Neptune) are widely spaced at great
distances from the Sun.
THE SOLAR SYSTEM
All planetary orbits are nearly circular and lie nearly in the same plane.
All planets orbit the Sun in the same direction: counterclockwise as viewed from high
above Earth’s North Pole.
Most planets rotate in the same direction in which they orbit, with fairly small axis tilts.
The Sun also rotates in this direction.
Most of the solar system’s large moons exhibit similar properties in their orbits around
their planets, such as orbiting in their planet’s equatorial plane in the same direction as
the planet rotates.
THE SOLAR SYSTEM
THE SOLAR SYSTEM
CALCULATION CHECK 7-1
Which of the planets has an orbital path that is most nearly a perfect circle in shape?
The shape of a planet’s orbit is given by the value of its eccentricity. The closer this value
is to zero, the closer the orbit’s shape is to that of a perfect circle. According to the table,
the orbit of Venus has the eccentricity closest to zero (0.007), making it the most circlelike of all planetary orbits.
CONCEPT CHECK 7-3
A planet’s average density can be estimated by measuring its size and how much the
planet’s gravity deflects a nearby spacecraft’s path. If the density of rocks recovered
from a planet’s surface is lower than the planet’s average density, what can one infer
about the density of the planet’s core?
If the rocks on the surface have a density lower than the planet’s average density, then
the planet’s core has a density greater than the planet’s average. This is not surprising,
since gravity causes material of greater density to sink towards the center of a planet.
CALCULATION CHECK 7-2
If Earth’s diameter is 12,756 km and Saturn’s diameter is 120,536 km, how many Earths
could fit across the diameter of Saturn?
If we divide Saturn’s 120,536-km diameter by Earth’s 12,756-km diameter—we find that
120,536 km ÷ 12,756 km = 9.449, so about 9½ Earth’s would fit across Saturn’s diameter.
THE SOLAR SYSTEM
The terrestrial planets (terrestrial means “Earth-like”) are the four planets of the inner
solar system: Mercury, Venus, Earth, and Mars.
These planets are relatively small and dense, with rocky surfaces and an abundance of
metals in their cores. They have few moons, if any, and no rings.
THE SOLAR SYSTEM
The jovian planets (jovian means “Jupiter-like”) are
the four large planets of the outer solar system:
Jupiter, Saturn, Uranus, and Neptune.
The jovian planets are much larger in size and lower in
average density than the terrestrial planets, and they
have rings and many moons. They lack solid surfaces
and are made mostly of hydrogen, helium, and
hydrogen compounds.
Because these substances are gases under earthly
conditions, the jovian planets are sometimes called
“gas giants.”
THE SOLAR SYSTEM
All the planets except Mercury and Venus have moons (also called satellites).
At least 170 satellites are known: Earth has 1 (the Moon), Mars has 2, Jupiter has at least 66,
Saturn at least 62, Uranus at least 27, and Neptune at least 13.
Like the terrestrial planets, all of the satellites of the planets have solid surfaces.
THE SOLAR SYSTEM
Seven of the Jovian satellites are roughly as big as the planet Mercury!
Earth’s Moon and Jupiter’s satellites Io and Europa have relatively high average densities, indicating that
these moons are made primarily of rocky materials.
By contrast, the average densities of Ganymede, Callisto, Titan, and Triton are all relatively low. The
interiors of these four moons also contain substantial amounts of water ice, which is less dense than rock.
THE SOLAR SYSTEM
the average densities of the planets and satellites give us a crude measure of their chemical
compositions—that is, what substances they are made of.
The most accurate way to determine chemical composition is by directly analyzing samples
taken from a planet’s atmosphere and soil. Unfortunately, this isn’t possible for most worlds.
Astronomers instead analyze sunlight reflected from the distant planets and their satellites.
To do that, astronomers bring to bear one of their most powerful tools, spectroscopy, the
systematic study of spectra and spectral lines.
THE SOLAR SYSTEM
spectral line: In a light spectrum, an absorption or
emission feature that is at a particular wavelength.
dark spectral line arises when light at a specific
wavelength is at least partially absorbed so that
the spectrum appears darker; it is also called
an absorption line.
A bright spectral line arises because at least some
additional light is being emitted at a specific
wavelength; it is also called an emission line.
THE SOLAR SYSTEM
There is a direct connection between these
two types of spectra. These connections are
summarized in three statements about spectra
that are known as Kirchhoff’s laws.
A hot opaque body, such as a perfect
blackbody, or a hot, dense gas produces a
continuous spectrum—a complete rainbow of
colors without any spectral lines.
Note: a blackbody is an idealized type of dense
object that does not reflect any light at all;
instead, it absorbs all radiation falling on it.
THE SOLAR SYSTEM
A hot, transparent gas produces an emission line spectrum—a series of bright spectral lines against a
dark background.
THE SOLAR SYSTEM
A cool, transparent gas in front of a source of a continuous spectrum produces an absorption line
spectrum—a series of dark spectral lines among the colors of the continuous spectrum.
THE SOLAR SYSTEM
If a planet has an atmosphere, then sunlight reflected from that planet must have passed
through its atmosphere before coming back out.
During this passage, some of the wavelengths of sunlight will have been absorbed. Hence,
the spectrum of this reflected sunlight will have dark absorption lines.
Astronomers look at the particular wavelengths absorbed and the amount of light
absorbed at those wavelengths. Both of these depend on the kinds of chemicals present
in the planet’s atmosphere and the abundance of those chemicals.
THE SOLAR SYSTEM
For example, astronomers have
used spectroscopy to analyze the
atmosphere of Saturn’s largest
satellite, Titan.
THE SOLAR SYSTEM
Spectroscopy can also provide useful information about
the solid surfaces of planets and satellites without
atmospheres.
When light shines on a solid surface, some wavelengths
are absorbed while others are reflected.
By comparing such a spectrum with the spectra of
samples of different substances on Earth, astronomers
can infer the chemical composition of the surface of a
planet or satellite.
THE SOLAR SYSTEM
Spectroscopic observations from Earth and spacecraft show that
the outer layers of the Jovian planets are composed primarily of
the lightest gases, hydrogen and helium.
In contrast, chemical analysis of soil samples from Venus, Earth,
and Mars demonstrate that the terrestrial planets are made
mostly of heavier elements, such as iron, oxygen, silicon,
magnesium, nickel, and sulfur.
THE SOLAR SYSTEM
Temperature plays a major role in determining whether the materials of which planets are
made exist as solids, liquids, or gases.
Hydrogen (H2) and helium (He) are gaseous except at extremely low temperatures and
extraordinarily high pressures. By contrast, rock-forming substances such as iron and silicon
are solids except at temperatures well above 1000 K.
As you might expect, a planet’s surface temperature is related to its distance from the Sun.
The four inner planets are quite warm. For example, midday temperatures on Mercury may
climb to 700 K (801°F), and during midsummer on Mars, it is sometimes as warm as 290 K
(63°F). The outer planets, which receive much less solar radiation, are cooler. Typical
temperatures range from about 125 K (−234°F) in Jupiter’s upper atmosphere to about 55 K
(−360°F) at the tops of Neptune’s clouds.
THE SOLAR SYSTEM
In addition to the eight planets, many smaller objects orbit the Sun.
These objects fall into three broad categories: asteroids, transNeptunian objects, and comets.
Asteroids (minor planets) are rocky bodies that orbit the Sun much
like planets, but they are much smaller.
Most known asteroids are found within the asteroid belt between
the orbits of Mars and Jupiter (2.0 – 3.5 AU).
The asteroid shown in this image, 433
Eros, is only 33 km (21 mi) long and 13
km (8 mi) wide—about the same size as
the island of Manhattan. Because Eros
is so small, its gravity is too weak to
have pulled it into a spherical shape
THE SOLAR SYSTEM
The outer solar system hosts what are
known as trans-Neptunian objects. As
the name suggests, these are small
bodies whose orbits lie beyond the orbit
of Neptune.
Since 1992 astronomers have discovered
more than 900 other trans-Neptunian
objects, including Pluto and Eris.
Just as most asteroids lie in the asteroid
belt, most trans-Neptunian objects orbit
within a band called the Kuiper belt that
extends from 30 AU to 50 AU.
THE SOLAR SYSTEM
Objects in the Kuiper belt can collide if their orbits cross each other. When
this happens, a fragment a few kilometers across can be knocked off one of
the colliding objects and be diverted into an elongated orbit that brings it
close to the Sun. Such small objects, each a combination of rock and ice,
are called comets.
When a comet comes close enough to the Sun, the Sun’s radiation vaporizes
some of the comet’s ices, producing long tails of gas and dust particles.
Some comets appear to originate from locations far beyond the Kuiper belt.
The source of these is thought to be a swarm of comets that forms a
spherical “halo” around the solar system called the Oort cloud. This
hypothesized “halo” extends to 50,000 AU from the Sun.
THE SOLAR SYSTEM
We can gather important clues about the interiors of terrestrial planets and satellites by
studying the extent to which their surfaces are covered with craters.
The planets orbit the Sun in roughly circular orbits. But many asteroids and comets are in more
elongated orbits. Such an elongated orbit can put these small objects on a collision course with
a planet or satellite.
If the object collides with the solid surface of a terrestrial planet or a satellite, the result is an
impact crater – a circular depression on a planet or satellite caused by the impact of a
meteoroid.
THE SOLAR SYSTEM
The Moon’s surface has craters of
all sizes. The large crater near the
middle of this image is about 80
km (50 mi) in diameter, equal to
the length of San Francisco Bay.
This Reservoir in Quebec is the
relic of a crater formed by an
impact more than 200 million
years ago. The crater was eroded
over the ages, leaving a ring lake
100 km (60 mi) across.
Lowell Crater of Mars is 201 km
(125 mi) across. There are craters
on top of craters. Note the lightcolored frost formed by
condensation of carbon dioxide
from the Martian atmosphere.
THE SOLAR SYSTEM
These smaller craters are thought to have been caused by impacts of relatively small objects
called meteoroids, which range in size from a few hundred meters across to the size of a
pebble or smaller.
Meteoroids are the result of collisions between asteroids, whose orbits sometimes cross.
The chunks of rock that result from these collisions go into independent orbits around the Sun,
which can lead them to collide with the Moon or another world.
THE SOLAR SYSTEM
Why are craters so much rarer on Earth than on the Moon if both have been subjected
to the same amount of bombardment?
The answer is that Earth is a geologically active planet.
Through plate tectonics—the motion of rocky plates over Earth’s surface—the
continents slowly change their positions over eons, new material flows onto the surface
from the interior, and old surface material is pushed back into the interior.
These processes, coupled with erosion from wind and water, cause craters on Earth to
be erased over time.
THE SOLAR SYSTEM
The Moon, by contrast, is geologically inactive.
There are no volcanoes and no motion of continents (and, indeed, no continents).
Furthermore, the Moon has neither oceans nor an atmosphere, so there is no erosion as
we know it on Earth.
With none of the processes that tend to erase craters on Earth, the Moon’s surface
remains pockmarked with the scars of billions of years of impacts.
THE SOLAR SYSTEM
In order for a planet to be geologically active, its interior must be at least partially molten.
Even if the surface does not undergo plate tectonics—as in the case of Jupiter’s moon Io—
a semi-molten interior is required to generate volcanoes with molten lava.
Therefore, we would expect geologically inactive (and hence heavily cratered) worlds like
the Moon to have less molten material in their interiors than does Earth.
THE SOLAR SYSTEM
The smaller the terrestrial world, the less internal
heat it is likely to have retained, and, thus, the less
geologic activity it will display on its surface. The less
geologically active the world, the older and hence
more heavily cratered its surface.
This rule means that we can use the amount of
cratering visible on a planet or satellite to estimate
the age of its surface and how geologically active it is.
THE SOLAR SYSTEM
Another, more direct tool for probing the interior of any planet or satellite is an ordinary
compass, which senses the magnetic field outside the planet or satellite.
The needle of a compass on Earth points north because it aligns with Earth’s magnetic field.
Magnetic fields arise whenever electrically charged particles are in motion.
The consensus among geologists is that the Earth’s magnetic field is caused by the motion
of the liquid portions of its interior.
THE SOLAR SYSTEM
THE SOLAR SYSTEM
A planet or satellite with a global magnetic field has
liquid material in its interior that conducts electricity
and is in motion, generating the magnetic field.
This process for producing a magnetic field is called a
dynamo.
A planet’s rotation helps to sustain the magnetic field.
Thus, by studying the magnetic field of a planet or
satellite, we can learn about that world’s interior.
Many spacecraft carry devices called magnetometers
to measure magnetic fields.
THE SOLAR SYSTEM
Mars has a very interesting magnetic field that
sheds light on its early history.
Mars has no planet-wide magnetic field like Earth,
but portions of its rocky surface are magnetized.
This information actually tells us a lot about the Martian past. During the planet’s first billion years or so,
the interior was still hot and molten. Electric currents in the flowing molten material could then produce a
planet-wide magnetic field.
As surface material cooled and solidified, the crust became magnetized by the planet-wide field, and
remained magnetized even after the internally generated planet-wide field shut down.
THE SOLAR SYSTEM
The Jovian planets are composed primarily of hydrogen and helium, not substances like
iron that conduct electricity. How, then, can they have a dynamo that generates strong
magnetic fields?
Deep inside Jupiter and Saturn, the pressure is so great and hydrogen atoms are squeezed
so close together that electrons can hop from one atom to another.
This hopping motion creates an electric current, just as the ordered movement of
electrons in the copper wires of a flashlight constitutes an electric current.
In other words, the highly compressed hydrogen deep inside Jupiter behaves like an
electrically conducting metal; thus, it is called liquid metallic hydrogen.
THE SOLAR SYSTEM
Uranus and Neptune also have magnetic fields, but they cannot be produced in the same
way: Because these planets are relatively small, the internal pressure is not great enough to
turn liquid hydrogen into a metal.
Instead, it is thought that both Uranus and Neptune have large amounts of liquid water in
their interiors and that this water has molecules of ammonia and other substances dissolved
in it.
Under the pressures found in this interior water, the dissolved molecules lose one or more
electrons and become electrically charged.
FOR NEXT TIME…
• Reading: Finish chapter 7 (7.6 – 7.8) and read the first few sections of chapter 8
• Homework: Homework 6 (Chapter 7 questions 4, 7, 8, 10, 11, 33. Due Wednesday)