In order to better understand the planets within our solar system we must first study the objects we know best. In this case we mean Earth and her Moon.

How is the interior of the Earth structured? The Earth is comprised of three layers. The outer most layer is referred to as the crust. The crust is a thin layer less than 30 miles thick. Direct sampling of the crust tells us that its material has an average density of about three grams per cubic centimeter (cc). The density is a measure of a material's mass per unit volume. For example, a cup of water has less mass (or in our experience, less weight) then a cup of iron. Water has a density of one gram per cc, iron has a density of almost eight grams per cc, most woods have a density of less than one gram per cubic centimeter. As you know, most wood floats in water, whereas iron will sink in water. For this reason we believe the crust has the least density of the material comprising the Earth. Indeed, measurements of the Earth's mass and volume indicate an average density of roughly 5.5 grams per cc. This is in line with the idea that the crust has floated to the top of the Earth.

The crust of the Earth is comprised of separate plates. The motion of these plates is referred to as plate tectonics. These plates are known to move, as is evidenced by earthquakes along the borders of these plates. These borders may also give rise to mountain ranges, rift zones (such as the mid-Atlantic ridge), and volcanoes. Study of plate tectonics on Earth will indicate whether similar activity occurs on other planets. Similarly, study of plate tectonics on other planets may give us some insight into tectonic activity on Earth.

The second layer of the Earth is referred to as the mantle. The thickness of the mantle is roughly 1/2 the entire radius of the Earth. It's density is higher than the density of the crust, and therefore the crust is able to float upon the mantle.

The final layer of the Earth is referred to as the core. And it seems to be divided into an inner and an outer core. The outer core is apparently liquid while the inner core is more solid. The density of the core may exceed 10 grams per cc. These heaviest elements, probably iron and nickel, sank to the center of the Earth. This differentiation (separation) of the Earth's layers would have occurred early in the Earth's history, while the Earth was still molten.

The liquid outer core of the Earth is apparently critical in the creation of the Earth's magnetic field. The magnetic field is probably created by currents of electricity circulating within the core. This magnetic field allows the Earth to act like a giant bar magnet. The north and south pole of this bar magnet however, are not exactly aligned with the rotational north and south pole of the Earth. In fact, it is known that the magnetic field has altered its orientation over the centuries. We will find most (but not all) of the other planets also have magnetic fields.

One spectacular side effect of this magnetic field is the creation of the Aurora Borealis (northern hemisphere) and the Aurora Australis (southern hemisphere). The Aurora is a brilliant light show (see the image below) created when charged particles, usually from the sun, move along the Earth's magnetic field and interact with the atoms in the atmosphere. The atoms become excited by the interaction and give off light when the electrons go back to lower energy states. (See the discussion of atomic radiation.) This interaction is generally seen near the magnetic poles although on occasion it will expand to lower latitudes.

The atmosphere of the Earth is also layered. The characteristics of these layers, such as temperature and chemical composition, vary from one to the next. The chief components of the Earth's atmosphere are nitrogen and oxygen. Once again, study of our atmosphere and its layers will give rise to understanding the atmosphere and the layering of other planets, and vice versa.

The as we turn our attention to the Moon we shall concentrate mostly on the exterior surface of the Moon. The interior of the Moon is very similar to the interior of the Earth. There is an iron rich core, a plastic asthenosphere, followed by a thick mantle, and finally a thin crust.

As we view the surface of the Moon from the Earth we notice much of the surface is white, with large gray regions. When viewed by Galileo, the gray regions were thought to resemble seas. For this reason those regions are referred to as mare, which means sea. The white regions, known as highlands, are known to be pock marked by thousands of craters.

Where did the mare and the craters come from? The origins of these two features are interconnected. The craters come from meteorite activity bombarding the Moon. The craters form when the rocky debris impacts the surface of the Moon. Early in the Moon's history when the crust was thin, a large meteorite impact could crack the surface of the Moon allowing lava to seep out from underneath and flood the Moon's surface. This flooding would erase all previous cratering, like an eraser eliminating the writing on a chalk board. As time went on the bombardment rate decreased. This is why only a few craters exist within the mare. The white regions are highlands. These regions were too high in altitude to be flooded by the lava. Therefore, craters created early in the Moon's history are still there unless replaced by more recent craters.

As we view the surface of the Moon from the Earth we note that we see the same features all the time, as the phases allow. This is because the same side of the Moon always faces the Earth. This characteristic is referred to as synchronous rotation. It is a common misconception to believe that the Moon does not rotate. In fact it does rotate, but the period of rotation is identical to the period of orbit. Therefore, the far side of the Moon was unknown to humans until satellites photographed the far side. The far side of the Moon is almost completely covered by craters with virtually no mare. This is likely due to the crust being thicker on the far side of the moon than on the near side. The thicker crust would be more difficult to crack.

Where did the Moon come from? As with the planets it was thought possible that the Moon was captured by the gravitational pull of the Earth. However, given its composition and orbital path, this would be unlikely. Most likely, the Moon was formed during the formation of Earth. Current theory believes that an asteroid or some other large piece of space debris impacted the Earth and caused the piece of the Earth to break away and eventually form the Moon.

This page was last updated on 08/25/04.