The Terrestrial Planets

Earth

A. The Earth formed in the inner solar nebula. It passed through the four stages or planetary development that other solid planets also experience to varying degrees. These four stages are:

1. Differentiation: The earth was originally molten, at which time matter separated according to density: the heavier iron sank to the core, while the lighter silicate minerals "floated" to the surface, thus forming the crust.
2. Cratering: The solid surface is bombarded with debris from the solar system.
3. Flooding: Decay of radioactive elements heats planet's interior, causing lava to well up through fissures in the crust and flood deeper basins. As the planet cools, water falls as rain and floods basins to form oceans.
4. Surface Evolution: Plate tectonics and erosion change surface features slowly.

B. The Earth's interior is differentiated. Because the density of the entire earth is 5.52 g/cm(3), and the crust is much less dense, the interior is made of very heavy elements. It is divided into four areas.

1. The crust is very thin relative to the radius of the Earth, only 35-60 Km deep. In fact, with respect to the size of the Earth, it is proportionally thinner than the skin on an apple.
2. The mantle is a layer of dense rock, which is very hot, and under a lot of pressure. The heat and pressure make the rock plastic, or malleable. The mantle is denser than the crust, which floats on it. As the mantle moves, the crust floating on it also moves, causing earthquakes.
3. The core has two regions, a liquid core and a solid core. The interior of the planet can be explored by monitoring shock waves from earthquakes. As the surface crust is dislocated, the shock of the motion spreads through the Earth. There are two types of waves that result. S-waves are shaking waves, like the shaking of jello. P-waves are pressure waves like sound waves. P-waves will travel through all materials, but S-waves only travel through solids. When an earthquake strikes one part of the Earth, S and P waves are felt nearby, but P-waves are also felt on the other side of the Earth. Therefore, the center of the Earth must be liquid. This liquid core is made of molten iron and nickel and has a density of about 14 g/cm(3), compared to 3.0 g/cm(3) for the crust and 4.4 g/cm(3) for the mantle. In the inner core, the pressure is so great that the iron and nickel become solid again.

C. Plate tectonics is the process by which the crust of the planet changes. The plastic mantle, which is always moving due to convection currents, causes the plates to move and rub against one another. A region where one plate is forcing another downward is called a subduction zone.

D. The Earth's atmosphere is called a secondary atmosphere. That is, the Earth did not have the atmosphere it has now when it was originally formed. The primeval atmosphere was rich in carbon dioxide, nitrogen and water vapor. During the flooding stage of planetary development, volcanoes bellowed up large amounts of gasses. The carbon dioxide was dissolved into the Earth's oceans and turned into carbonate rocks, thus removing it largely from the atmosphere. The atmosphere received its oxygen from green plants.

E. The Earth wobbles a tiny amount in its rotation about its axis. This wobbling is called precession, and takes about 26,000 years to complete one circle.

F. The Earth's moon is unique in the solar system in that it is so large relative to the Earth.

Earth’s Moon

There are larger moons, but they are orbiting proportionally larger planets. Therefore, the origin of our moon has been the focus of considerable debate. There are three modem theories of the origin of the moon. They are:

1. Capture Theory: The moon was captured by the gravitational field of the Earth, but this would not explain why the moon orbits the Earth in the same plane as the Earth orbits the sun. Many moons of other planets appear to be captured satellites, but they are very small and have very odd orbits.
2. Twin Theory: The moon formed alongside the Earth during its early development. However, one would then expect the Earth and moon to be made of the same material, which they are not.
3. Impact Theory: The early Earth was impacted by a large object, perhaps the size of Mercury. It then jettisoned a large amount of exterior material into space. This theory explains why the moon is made of mostly lighter material than the Earth, and why it is so large. The impact theory is now the most widely accepted.

The moon has several notable surface features. The first is its craters. These craters indicate that the moon does not have an atmosphere (otherwise the craters would have long ago eroded due to atmospheric conditions such as wind), and that the moon is not tectonically active. Each crater is named for a famous scientist, i.e., the Tycho Brahe crater and the Kepler crater. The dark regions on the moon are called Maria (Mare is Latin for sea), and each dark region is so named, i.e., the Sea of Tranquillity, the Sea of Storms. The Maria are made of newer, heavier material than the lighter, highland regions. They are therefore thought to be congealed lava pools created by large impacts. The highland regions are the light areas of the moon and are mostly made of anorthosite, which is a lighter but older type of rock.

G. Eclipses

1. A lunar eclipse is when the earth casts its shadow; on the moon.
2. A solar eclipse is when the moon casts its shadow on the earth.
3. These two types of eclipse often happen about two weeks apart, that being the time it takes the moon to travel from one side of the Earth to the other. Eclipses do not happen every month because the plane in which the moon orbits the Earth is inclined by 5 degrees' to the plane in which the Earth orbits the sun. Therefore, the shadow of the moon is usually above or below the Earth, and the shadow of the Earth is usually above or below the sun.
4. When one type of eclipse happens, the other will usually happen soon after. This is called an eclipse season, and happens somewhere on Earth at least once a year.

H. The moon orbits the Earth every 27.322 days. This is called the moon's sidereal (with reference to the stars) period. Because the Earth is revolving around the sun, however, it takes the moon 29.53 days to go through its phases as seen from Earth. This is called its synodic (with respect to the sun) period. 

I. There are eight names for the phases of the moon. 

• A new moon is completely dark. 
• Next comes waxing (growing brighter) crescent (1/4 lit), first quarter (1/2 lit), waxing gibbous (3/4 lit) and full. 
• A full moon is completely lit. 
• After the full moon the phases are waning (growing dimmer) gibbous (3/4 lit), third quarter (1/2 lit) and waning crescent (1/4 lit).

J. The tides are caused mostly by the moon, and moderately by the sun. The moon's gravity pulls on the Earth, causing a bulge of water. The Earth then spins beneath this bulge of water, causing the tides. Because there is a bulge at the side of Earth facing the moon, and the exact opposite side facing away from the moon, there are two high tides and two low tides every day. When the moon and the sun are working together to make very big tides, the effect is called Spring Tides. When they are working against one another (at right angles to one another) to produce very small tides, the effect is called Neap Tides.

Mercury

A. Mercury is named after the Roman messenger of the gods. Mercury is intermediate in size between the Earth and the moon. It orbits very close to the sun and so is hard to see. In photographs, Mercury looks like the moon, in that it is heavily cratered and has no atmosphere. Although it is closest to the sun, Mercury is not the hottest planet. That distinction belongs to Venus. However, Mercury does have the largest temperature differences of any planet, varying from -170 degrees C to 430 degrees C.

B. There is a large impact crater on Mercury called the Caloris Basin. On exactly the opposite side of the planet, there is an area, where the shock waves converge known as the weird terrain.

C. Mercury is made mostly of iron. The percentage of iron in the core of the planet indicates that it may have been hit in its early developmental stages by a very large object. This impact could have dislodged much of the lighter materials constituting the planet, and left it as the small iron sphere it is today. The planet also has long curved ridges called lobate scarps. These cliffs are wrinkles on the surface caused by the slow cooling of the iron core.

D. Mercury has an odd rotation about the sun. It rotates on its axis one and a half times for each orbit around the sun. That means there are three days in every two years on Mercury. Furthermore, the orbit of Mercury about the sun is notable in that it is very elliptical.

Venus

A. Venus is named after the Roman goddess of love. It is the planet closest in size to Earth. It is shrouded in a deep layer of clouds that reflect light very well. It is therefore a very bright object in our sky. Because they orbit very close to the sun, Mercury and Venus are called morning and evening "stars," as they can only be seen in the morning or evening. Aside from the sun and the moon, Venus is the brightest object in our sky.

B. The surface of Venus is often compared to the biblical notion of Hell. It is very hot (475 C), the air pressure is almost 100 times that of Earth, and it often rains sulfuric acid. The atmosphere is almost entirely carbon dioxide, which creates a greenhouse effect some 300,000 times that of Earth's. This is the cause of the intense heat. The atmosphere is also a secondary atmosphere. Its origin is in the plentiful volcanic eruptions on the planet. These eruptions can be seen indirectly from Earth since they spew large amounts of ions into the atmosphere, which tend to cause enormous lightning storms.

C. The surface of Venus is so hot that periodically the outer crust may melt. From radar images of the surface, it can be seen that there are many volcanoes. The surface is roughly divided up between 65 per cent low rolling plains, 25 per cent highlands, with the rest being volcanic areas. There are two major highland, or mountainous areas, Ishtar and Aphrodite (Ishtar is the Babylonian goddess of love, and Aphrodite, the Greek). Ishtar is about the size of Australia and Aphrodite is approximately the size of South America.

D. Although Venus has many volcanoes, there is no evidence of plate tectonic activity. The currents in the mantle are deforming the crust, and forming large surface bulges, called coronae, and mountains, such as the Maxwell Monte, which are almost twice as high as Mount Everest.

E. Venus is unique in the solar system because it spins, when viewed from a perspective looking down on Earth's North Pole, in a clockwise direction. All of the other planets, except Uranus, exhibit a counter-clockwise rotation with respect to our North Pole. This odd rotation makes Venus the slowest rotating planet, and contributes to its meteorological patterns. An impact with a large object was probably the cause for this aberrant behavior

Mars

A . Mars is named after the Roman god of war. It is about half the size of Earth, with about one-tenth the mass. In some ways, Mars is much like the Earth. It has a similar rotational period and its yearly orbit is only twice that of Earth’s. However, Mars is much colder than Earth, and its small size has affected its ability to retain an atmosphere.

B. Mars' atmosphere is very similar in composition to that of Venus, but much thinner. It is thinner because Mars has a very small gravitational field, and hence cannot hold onto light gasses. The thin CO2 atmosphere therefore does not contribute greatly to any greenhouse effect. Mars does have polar ice caps which are composed of a combination of water ice and carbon dioxide ice (dry ice).

C. The dried-out river channels on Mars give evidence that Mars did in fact once have water on its surface. When Mars was cooling after its initial formation, water vapor was probably outgassed. The water then condensed into clouds and rained down to the surface to form rivers and lakes. In addition, large quantities of water could have been released in the form of mud slides and the like. The lack of an ozone layer, however, means that the water molecules could be easily broken up into their constitutive elements, and these gasses could escape Mars' small gravitational field. Therefore, Mars now has no water existing in the liquid state. However frozen water exists in the polar ice caps and as permafrost beneath the surface.

D. Mars has a very thick outer crust. This determines many of its geological features. The largest volcano in the solar system is on Mars, the Olympus Mons. Its base is approximately the size of the state of Missouri, and it is more than twice as tall as the largest volcano on Earth. The largest valley in the solar system is also on Mars. In a region called Tharsis, a massive bulge about 10 Km above the surface has formed. Near this bulge is the Valles Marineris (named after the Mariner Space Probe which discovered it). The Valles Marineris is long enough to reach from New York to Los Angeles, and is at some spots, over 4 miles deep. The Tharsis bulge and the Valles Marineris are thought to be causally related to one another.

E. Mars is red because it has a great deal of oxidized iron on the surface; i.e. because it is rusty.

E Mars has two small moons, Phobos (fear) and Deimos (panic). Diemos is the smallest cataloged satellite in the solar system.

Comparative Data on the Terrestrial Planets

Satellites of Planets in the Solar System