MERCURY
Mercury, the
planet nearest the sun, is difficult to observe from the Earth because it rises
and sets within two hours of the sun. Consequently, little was known about
the planet until the Mariner 10 spacecraft made several flybys in 1974 and
1975.
Mercury's surface
has several different types of terrain. Planetary scientists can estimate the
age of a surface by the number of impact craters on it; in general, the older
the surface, the more craters it has. Some regions on Mercury are heavily
cratered, suggesting that they are very old surfaces that were probably formed
about 4 billion years ago. Between these regions are areas of gently rolling
plains that may have been smoothed by volcanic lava flows or by accumulated
deposits of fine material ejected from impacts. These plains are also old
enough to have accumulated a large number of impact craters. Elsewhere on the
planet are smooth, flat plains with few craters. These plains are probably
younger and volcanic in origin. Sometime between the formation of the
intercrater plains and the formation of the smooth plains, the whole planet may
have shrunk as it cooled, causing the crust to buckle and form the long, steep
cliffs called scarps.
The largest impact
basin on Mercury, Caloris, is
about 800 miles (1,300 Km) across and is surrounded by mountains that rise to
heights of about 1.2 miles (2 Km). It was probably created from the impact of a
large planetesimal when Mercury was forming. On the opposite side of the planet
from Caloris is an area of hilly, lineated terrain that probably resulted from
seismic waves caused by the same impact.
Like other airless, solid
bodies in the solar system, the entire surface of Mercury is covered with a
layer of rubble called regolith. Regolith is composed of material, ranging from
dust to boulders, that was scattered when impact craters were formed. This
debris was in turn broken up and redistributed by subsequent impacts.
Mercury is very
dense and has a magnetic field that is about 1 percent as strong as Earth's.
This suggests the existence of a core composed of iron and nickel and
constituting about 40 percent of the planet's volume. The surface gravity is
about one third as strong as Earth's. A thin atmosphere of hydrogen,
helium, potassium, and sulfur surrounds the planet. Radar images taken of
Mercury in 1991 show what are considered to be large ice patches at the
planet's north pole.
Mercury rotates
on its axis three times for every two revolutions around the sun and has a more
elliptical orbit than do most other planets. These two characteristics
combine to create effects unusual by Earth standards. At aphelion, the point
in Mercury's orbit when the planet is farthest from the sun, an observer on
Mercury would see a sun that appeared more than twice as large as it does from
Earth. At perihelion, when Mercury is nearest the sun, the sun would appear
almost four times as large as it does from Earth.
The sun would not
appear to move steadily across the sky instead, its apparent speed would
change, depending on the viewer's location on the planet and on the planet's
distance from the sun sometimes the sun would even appear to reverse its
course. Surface temperatures on Mercury range from about 675 K at "noon"
to 100 K just before "dawn"
It takes almost
59 Earth days for Mercury to complete one rotation about its axis, but the time
between one sunrise and the next is 176 Earth days. The reason for this is
that after one rotation, Mercury has completed two thirds of its orbit around
the sun, so that the sun is in a different place in Mercury's sky. It takes
three rotations, or two Mercurial years, for the sun to reappear in the same
place in the planet's sky.
VENUS
After the moon,
Venus is the most brilliant natural object in the nighttime sky. It is the
closest planet to the Earth and is also the most similar to Earth in size,
mass, and density. These similarities suggest that the two planets may have
had similar histories. Thus, scientists are intrigued by the question of why
Venus and Earth are now so different.
Venus
rotates once every 243 days in retrograde motion . The same side of Venus
is always facing Earth when the two planets pass in orbit. Although Venus is
close to Earth, the planet is difficult to observe because its surface is
completely obscured by thick layers of dense clouds. During the 1970s and
1980s NASA's Pioneer Venus orbiter and the Soviet Venera 15 and 16 orbiters
were able to obtain information about the Venusian clouds and surface
conditions.
Venus' atmosphere is
composed mainly of carbon dioxide, with droplets of sulfuric acid in the upper
clouds. The upper atmosphere moves rapidly, completely circling the planet
in four days, while the winds at the surface are gentle. The surface
temperature is approximately 750 K, even hotter than Mercury's "noon"
temperatures.
The large amount of
carbon dioxide in Venus' atmosphere accounts for the extremely high
temperatures near the planet's surface. Sunlight penetrates the atmosphere, is
absorbed by the planet's surface, and is reradiated in the form of heat. The
large amount of carbon dioxide in the atmosphere, however, absorbs and traps
this reradiated heat, preventing it from being released back into space. As a result
of this phenomenon, called the "greenhouse effect," the surface of
Venus is hot enough to melt lead, and the rocks may even glow faintly red from
their own heat.
Because the clouds
allow only about 15 percent of the sun's light to reach the Venus surface, the
days are dim and overcast. Because the dense atmosphere refracts, or bends,
light, some light may extend around to the night side of the planet, so the
nights may not be completely dark.
In many places the
surface has been severely fractured and folded by stresses caused by convection
of the Venusian mantle, the part of the planet's interior that lies just above
the core. Thus Venus may experience a form of plate tectonics, though the very
high surface temperature probably makes the tectonic style quite different from
that on Earth. The highest mountain on Venus, Maxwell Montes, is about 7.5
miles (12 Km) high. It may be a volcano. Radar images indicate that the
highlands on Venus have rougher surfaces than do the other terrains.
The Magellan spacecraft arrived at Venus in mid-1990, taking
up an orbit that carried it around the planet every three hours while it mapped
Venus' cloud-shrouded surface in the best detail ever achieved. The surface of
Venus is pocked with large meteor craters. Magellan also found evidence that
active volcanoes may exist on Venus and that the surface of the planet is
probably only 400 million years old. Further support for the possibility of
active volcanoes on Venus came from the Galileo spacecraft. Galileo flew by
Venus in early 1990 and found evidence of impulsive electromagnetic events
characteristic of lightning. Mission scientists suggested that these events
have a volcanic origin.
MARS
Since ancient times
Mars has been an object of great interest to astronomers. Unlike Venus, Mars
generally has no obscuring layer of clouds. In addition, it passes relatively
close to the Earth in its orbit. Thus it is a nearly ideal subject for
telescopic observation. Over the centuries observers have noted various unusual
phenomena on the planet's surface, including a seasonal growing and shrinking
of the polar caps and a wave of darkening that appears to sweep from pole to
equator during each hemisphere's spring. The explanation of most of these observations
had to await the exploratory space missions by the United States and Soviet
Union during the 1960s and 1970s.
Mars is about
half the size of Earth. Its atmosphere is composed mostly of carbon dioxide and
is very thin, exerting about 1/100 the surface pressure that the Earth's
atmosphere exerts. The temperature at the planet's surface varies widely
during the course of a Martian day, from about 190 K (-83o C)
just before dawn to about 240 K ( -33o C )in the afternoon. At
the center of the planet is probably a small iron or iron sulfide core. If Mars
has a magnetic field, it is so weak that no instrument has been able to detect
it.
Mars, like the
Earth, is tilted on its rotational axis. Consequently it is subject to seasonal
variations in climate as first one hemisphere and then the other receives more
sunlight during the planet's orbit around the sun.
Liquid water cannot
exist on Mars's surface because of the low temperature and pressure; water
exists only as ice deposited at the poles and perhaps trapped below the surface
and as vapor in the atmosphere. However, there is evidence that, in the past,
temperatures may have been warmer and the atmospheric pressure higher. Images
from the Viking orbiters show surface features that resemble dry riverbeds and
gullies. These might have been made by rainfall and runoff, but they might also
have been made by subsurface water that escaped to the surface.
Although it is
quiet now, Mars experienced a period of volcanic activity that peaked a few
billion years ago. The planet has the largest volcano in the solar system,
Olympus Mons. At a height of 17 miles (27 Km), the volcano is three times
higher than Earth's Mount Everest and covers an area the size of the state of
Arizona. The largest fracture system is Valles Marineris, a huge valley about
2,500 miles (4,000 Km) long and varying from 2 1/2 to 6 miles (4 to 10 Km) in
depth.
In general, once
each Martian year at the beginning of the Southern Hemisphere's spring season,
Mars is engulfed by global dust storms. Local temperature differences generate
strong winds that lift the dust from the surface to form thick clouds. The
clouds block the sunlight, gradually causing the surface temperatures to even
out and the winds to subside. Some of the atmospheric dust is deposited in a
snowfall of dust and ice in the winter hemisphere. The snow forms a winter cap
of carbon dioxide ice, water ice, and dust. During the spring most of the cap
evaporates, but some remains as a permanent deposit. As a result, a geologic
record of these storms and their variations over the planet's lifetime must be
preserved in the permanent layers of dust and ice at the Martian poles.
The phenomenon
known as the wave of darkening accompanies the seasonal waning of the polar
caps. Near the edge of either polar cap, a general darkening of the surface
markings appears in early spring as the cap begins to recede. The darkening
then moves away from the cap and sweeps across the equator, finally dissipating
in the opposite hemisphere. Although these waves have been well documented in
observations from Earth, attempts to study them from spacecraft have failed. No
surface changes have been detected that could be associated with this
phenomenon, and if it is not illusory, it seems most likely that it is some
kind of atmospheric effect.
For centuries
astronomers have considered the possibility that life might exist on Mars. As
telescopes became more powerful, the debate intensified. In 1877 the Italian
astronomer Giovanni Schiaparelli described a system of interconnecting channels
on the planet. The American astronomer Percival Lowell interpreted
Schiaparelli's word canali to mean canals and speculated that they were
structures that had been built by an advanced but dying Martian civilization.
Most astronomers could see no canals, however, and many doubted their reality.
The controversy was finally resolved only when pictures were returned from the
United States Mariner probes in 1969. The photographs showed many craters and
other features but nothing resembling a network of channels or canals.
Mars has two small
satellites, Phobos and Deimos,
that may be captured asteroids. Both are so small that they do not have enough
internal gravity to draw them into spherical shapes; instead, they are shaped
more or less like potatoes. Phobos is about 17 miles (27 Km) long; Deimos is
about 9 1/2 miles (15 Km) long. Both have rotational periods equal to their
orbital periods, so that they always point the same face toward Mars.
Phobos is very
close to Mars, and its orbit is gradually decaying, so that it is drawing
closer to the planet with each orbit. Astronomers estimate that Phobos may fall
to the Martian surface sometime in the next 100 million years. Deimos is in a
more distant orbit and is gradually moving away from the planet.
OBJECTIVE OF
CHAPTER SEVEN
1: How both chance and
calculation played major roles in the discovery of Uranus and Neptune.
2: The similarities and the
differences among the four jovian worlds.
3: Some of the processes
responsible for the properties of the jovian atmospheres.
4: How the internal
structure and composition of the jovian planets are inferred from external
measurements.
5: Why three of the four
jovian worlds radiate more energy into space than they receive from the Sun.
JUPITER
Jupiter is larger
than all the other planets combined. It gives off nearly twice as much energy
as it receives from the sun heat that was acquired during the planet's
accretion as well as heat that is generated as the planet gradually contracts. Jupiter
also has the strongest magnetic field of all the planets. The field extends
out to ten times the planet's radius and is the source of intense bursts of
radio noise.
Jupiter is
composed mostly of hydrogen and helium. It has no solid surface,
only layers of gaseous clouds. At the planet's center is probably a rocky core
with more than ten times the mass of the planet Earth. Temperatures in the core
may exceed 25,000 K. Surrounding the core is a liquid hydrogen-helium mixture
that has been squeezed into metallic form under the intense pressure of the
planet's upper layers. In October 1989 the Galileo spacecraft was sent into
orbit for a six-year journey to Jupiter.
When viewed through
a telescope, Jupiter's topmost clouds appear as dark belts and bright zones
that encircle the planet and range from tawny yellow to brown and gray. The
colors are most likely caused by ammonia-sulfur compounds. The most obvious
feature on the planet is Jupiter's famous Great Red Spot. It is a huge cyclonic
storm, as big as two Earth-sized planets placed side by side, and it has been
observed from the Earth for more than 300 years.
Jupiter spins
rapidly on its axis, completing one rotation in less than ten hours. Because of
the centrifugal force caused by this rapid rotation, Jupiter's diameter is
greater at the equator than it is from pole to pole, giving the planet the
shape of a slightly flattened sphere.
Jupiter and its 16
known satellites probably formed as a miniature solar system a large rotating
gaseous ball . Jupiter has a narrow system of rings, discovered by the Voyager
1 spacecraft in 1979, that are composed of tiny rocks and dust particles.
Jupiter's four brightest and largest moons: Io, Europa, Ganymede and Callisto; discovered independently by
Galileo and Marius the 17th century astronomer and called Galilean moons.
In 1992 the Ulysses
spacecraft flew by Jupiter to study its magnetic fields, plasma, dust, and X
rays. It found that the planet's magnetic field has a clocklike pulsing that
extends to the middle of the planet. It also observed that Io is less
volcanically active than was previously thought.
SATURN
Like Jupiter, Saturn
is a large, gaseous planet composed mostly of hydrogen and helium. It also
radiates more than twice as much heat as it receives from the sun. This excess
thermal energy is partly from primordial heat and partly from the friction
created by the heavier element, helium, gradually sinking through the hydrogen
toward the planet's center. Saturn has a magnetic field 1,000 times stronger
than Earth's but not as strong as Jupiter's. Saturn's density is so low
that it could float in an ocean of water. It probably has a core similar to
that of Jupiter. It is covered with cloud bands, some forming cyclonic patterns
like Jupiter's, but the colors appear more subdued than do Jupiter's because of
an atmospheric haze that covers the clouds. Saturn is surrounded by a
spectacular ring system. Galileo observed these rings in 1610, but he did not
identify them as rings he believed Saturn to be a triple planet.
In 1655, using a
more powerful telescope, the Dutch astronomer Christiaan Huygens was able to
see a flat, apparently solid ring around Saturn. Later astronomers were able to
identify separate rings.
The cameras of
Voyagers 1 and 2 revealed that there are really tens of thousands of rings
extending from about 4,300 miles (7,000 kilometers) to 46,000 miles (74,000
kilometers) beyond Saturn's atmosphere. They are made of ice and ice-covered
particles that range from the size of a speck of dust to the size of a house.
The rings occur in groups, which are referred to as the A ring, the B ring, and
so on inward. The gap between the A and B rings is called the Cassini Division.
The Voyager cameras observed occasional dark radial spokes in the B ring. These
began as thin lines and were then stretched into wedge shapes as the inner
rings, which move faster, passed the outer rings. The spokes would disappear
after a few hours. Astronomers believe the spokes are probably made of fine
particles that have been raised slightly above the rings by electrostatic
forces.
At least twenty satellites orbit Saturn. The largest of these is Titan,
intermediate in size between the planets Mercury and Mars. Titan is half rock
and half ice, with an atmosphere of nitrogen and methane that exerts about 1
1/2 times the surface pressure of the Earth's atmosphere. Its surface is very
cold and is obscured by haze. It may be covered by oceans of liquid methane.
The six other major satellites are Mimas, Enceladus, Tethys, Dione, Rhea, and
Iapetus. Most of these have icy, cratered surfaces. Enceladus has a smooth,
bright surface of apparently pure ice. Iapetus has a large patch of material as
dark as asphalt that nearly covers the leading hemisphere (the side of the
satellite facing in the direction of orbital motion).
The remaining
satellites of Saturn are all small, icy, and irregular in shape. Some are
called shepherd satellites because their orbits are located at the edges of
rings, apparently helping to keep the ring material in place. The F ring of
Saturn has two shepherd satellites whose gravitational forces may be
responsible for the ring's braided, or twisted, appearance.
URANUS
Uranus is another
large gaseous planet. It is denser than Jupiter and Saturn and is composed of
hydrogen, helium, substantial amounts of water, and probably some methane,
ammonia, rock, and metal. Trace amounts of methane in its upper atmosphere give
it a blue-green color. The temperature in the upper atmosphere is only about 60
K, but the temperature increases with atmospheric depth. Underneath the thick
clouds there may be an immense ocean of water that, though it is heated to
several thousand degrees Kelvin, does not boil away because of the intense
pressure from the atmosphere above it. The core of the planet is most likely
rock and metal.
Uranus' rotational
axis is tilted an unusually great 98 degrees from a hypothetical line perpendicular
to the ecliptic plane.. Uranus rotates in retrograde, or clockwise, motion
about once every 17 hours.
Uranus has a strong
magnetic field in which the magnetic north pole is tilted an exceptionally
great 60 degrees from the rotational north pole.
Uranus has 15
known satellites, which are composed mostly of ice and are heavily cratered.
The five major satellites are Miranda, Ariel, Umbriel, Titania, and Oberon.
Oberon's surface is very old and heavily cratered, indicating that the body has
been geologically inactive during most of its existence.
Uranus has a system of
narrow, sharp-edged rings made of some unusually dark material very unlike
Saturn's bright, icy rings. They are not uniformly thick, and in some places
certain rings are so thin that they disappear. It is possible that Uranus'
rings are relatively young compared to Saturn's and are still being formed.
Voyager 2 recorded two small shepherd satellites in orbit close to the rings.
There are probably more such satellites, but they are too small and dark to be
seen. Some astronomers have suggested that the dark material of the rings and
the small satellites, and possibly the dark coating on Umbriel, may be
carbonaceous chondritic material.
NEPTUNE
Little was known
about the planet Neptune, which was discovered in 1846, until the Voyager 2
encounter in 1989. Its mass is comparable to that of Uranus, and it has a
similar composition. Its thick atmosphere of hydrogen, helium, and some methane
and ammonia gives it a bluish color.
Like the other gaseous planets, Neptune rotates rapidly, once
every 16.2 hours, and has a slightly larger diameter at the equator than at the
poles. The atmospheric temperature has been found to be at about 60 K, higher
than expected for a body so far from the sun. Its high temperature suggests
that Neptune has another, possibly internal, heat source. The planet probably
has a rocky core surrounded by water ice and liquid methane, which in turn are
surrounded by hydrogen and helium gases.
Neptune has eight
known satellites. The two largest are Triton and Nereid. The largest satellite,
Triton, revolves around Neptune in a direction opposite to that of most other
satellites in the solar system. Nereid revolves in direct motion in a very
eccentric orbit.
OBJECTIVE OF
CHAPTER NINE
1: The overall properties of
the Sun.
2: How energy travels from
the solar core, through the interior, and out into space.
3: The Sun's outer layers
and what those layers tell us about the Sun's surface composition and temperature.
4: The nature of the Sun's
magnetic field and its relationship to the various types of solar activity.
5: Outline the process by
which energy is produced in the Sun's interior.
6: How observations of the
Sun's core challenge our present understanding of solar physics.
STAR: A glowing ball of gas held together by its own gravity and powered by
nuclear fusion in its core.
SUN: The Sun is the most prominent feature in our solar system. It is the
largest object and contains approximately 98% of the total solar system mass.
One hundred and nine Earth’s would be required to fit across the Sun's disk,
and its interior could hold over 1.3 million Earth’s. Table 9.1
Table 6.1 Some
Solar Properties.
Radius 6.96 x 108 m
Mass 1.99 x 1030 Kg
Average Density 1410
Kg/m3
Rotational Period 24.9
( equator)
Surface Tempreture 5780
K
Luminosity 3.86
x 1026 W
SIX MAIN REGION OF SUN:
1: CORE: Solar energy is created deep within the core of the Sun. It is here
that the Sun generates energy by process of nuclear fusion.
2: INTERIOR
3: CONVECTION ZONE
4: PHOTOSPHERE Sun's outer visible layer is called the photosphere and has a
temperature of 6,000°C. This layer has a mottled appearance due to the
turbulent eruptions of energy at the surface.
5: CHROMOSPHERE: The chromosphere is above the photosphere. Solar energy passes
through this region on its way out from the center of the Sun.
6: CORONA: The corona is
the outer part of the Sun's atmosphere. It is in this region that prominence
appears.
(Fig. 9.2 ) Table 9.2
PROMINENCES: Prominences are immense clouds of glowing gas that erupt from the
upper chromosphere
LUMINOSITY: The total amount of energy radiated from the star surface each second
is called Luminosity.
SOLAR CONSTANT: The amount of solar energy reaching the Earth per unit area per unit
time is called Solar constant, approximately 1400 w/m2
HELIOSEISMOLOGY: The study of conditions far below the Sun’s surface through the analysis
of internal “ sound “ waves that repeatedly cross the solar interior.
GRANULATION: A pattern of small cells seen on the surface of the Sun caused by the
convective motions of the hot solar gas.
SUPERGRANULATION: Large-scale flow pattern on the surface of the Sun, consisting of cells
measuring up to 30.000 Km acros, belived to be the imprint of large convective
cells deep in the solar interior.
SUN SPOTS: sunspot An Earth-sized dark blemish found on the surface of the Sun.
The dark color of the sunspot indicates that it is a region of lower
temperature than its surroundings. (Fig. 9.16, Fig. 9.17 Fig. 9.8 )
1: NUCLEAR FISSION
NUCLEAR REACTION
2: NUCLEAR FUSION
1: NUCLEAR
FISSION: Mechanism of energy
generation, in which heavier nucles are break down, into lighter nuclie,
releasing energy in the process.
nucles A ̃ nucles 1 + nucles 2 nucles
3 + ooo + energy
n + 23592
U ̃ 23692 U* ̃ 14054 Xe + 9438 Sr + 2 n +
energy
14054 Xe ̃ 14055 Cs ̃ 14056 Ba ̃ 14057
La ̃ 14055 Cs (stable)
NUCLEAR FUSION: Mechanism of energy generation in the core of sun, in which light
nuclie are combined, or fused, into heavier ones, releasing energy in the
process. (Fig. 9.23)
nucles 1 + nucles
2 ̃ nucles 3 + energy
H + H ̃ He + energy
OBJECTIVE OF
CHAPTER TEN
Venus 4.9E24 6.82 6100 0.95 5300 0.90 10.4 -243f 730 90
Earth 6.0E24 1.00 6400 1.00 5500 1.00 11.2 1.00 290
1.00
Mars 6.4E23 0.11 3400 0.53 3900 0.38 5.0 1.03 180-270 0.007
Moon 7.3E22 0.012 1700 0.27 3300 o.17 2.4 27.3 100-400 ---
