The planet Earth

The Earth is certainly the most familiar planet, though it has only been a few hundred years since we fully realized it was a planet. We begin our study of objects in the Solar System with the Earth because it is interesting in its own right, and it provides a test of many observing techniques that we wish to use for other objects in the Solar System.

The Earth is, at least by human standards, a beautiful planet, as the following images indicate.

Apollo 11 shot of Earth (Ref)	Galileo shot of the Earth and Moon	Galileo image of S. America (Ref)

Here is an interactive viewer that displays either a map of the Earth showing the day and night regions at this moment, or a view of the Earth from the Sun, the Moon, the night side of the Earth, above any location on the planet specified by latitude, longitude and altitude, from a satellite in Earth orbit, or above various cities around the globe (Credit: John Walker). Here is the NASA Earth from Space image gallery, where you can find many images of the Earth taken from space.

The Interior

The study of the Earth's surface and interior is the domain of geology. We know little directly about the interior of the Earth. Most of our information in that regard has come from seismic waves, which are vibrations in the body of the Earth.

Seismic Waves

There are two general categories of seismic waves.

1. P-waves, which are longitudinal pressure waves and can propagate in both solids and liquids.

2. S-waves, which are transverse waves that can propagate in solids but not in liquids

First, seismic waves have their direction of motion changed (refracted) by variations in the interior density. Thus, by studying the way such waves propagate in the Earth we can learn something about density variations. Second, the fact that P-waves propagate in liquids but S-waves do not allows us to determine if portions of the interior are liquid.

Structure of the Interior

Accumulated and detailed seismic studies, coupled with theoretical speculation, suggests the interior structure shown schematically on the left (the figure is not to scale). The Earth is believed to have a solid inner core, made mostly of iron and nickel. This is surrounded by a liquid outer core, also mostly iron and nickel. The diameter of the core is estimated to be 7000 km, compared with a 12,700 km diameter for the entire planet. The crust is only a few tens of kilometers thick. The region between the core and the crust is called the mantle. The upper part of the mantle and the crust together are called the lithosphere. Sitting just below the lithosphere is a region of plastic consistency called the aesthenosphere. We shall have more to say about the lithosphere and aesthenosphere shortly.

Geological Differentiation

The Earth did not have the interior structure described in the preceding section when it was formed. The geological process by which the Earth came to have its present interior structure is called differentiation, and is illustrated in the following figure.

The process of geological differentiation

Within about 1 billion years of its formation the Earth was melted by heat arising from a combination of sources:

1. Gravitational energy left from the formation of the planet,

2. Meteor bombardment

3. Decay of radioactive material trapped in the body of the Earth.

As we shall see, this has enormous implications for the subsequent geological history of the Earth because it produces conditions favorable for plate tectonics. One of the crucial questions that we will have of all solid bodies in the Solar System is whether they have ever been differentiated.

The Atmosphere

The present atmosphere of the Earth is probably not its original atmosphere. Our current atmosphere is what chemists would call an oxidizing atmosphere, while the original atmosphere was what chemists would call a reducing atmosphere. In particular, it probably did not contain oxygen.

Composition of the Atmosphere

The original atmosphere may have been similar to the composition of the solar nebula and close to the present composition of the Gas Giant planets, though this depends on the details of how the planets condensed from the solar nebula. That atmosphere was lost to space, and replaced by compounds outgassed from the crust or (in some more recent theories) much of the atmosphere may have come instead from the impacts of comets and other planetesimals rich in volatile materials.

The oxygen so characteristic of our atmosphere was almost all produced by plants (cyanobacteria or, more colloquially, blue-green algae). Thus, the present composition of the atmosphere is 79% nitrogen, 20% oxygen, and 1% other gases.

Layers of the Atmosphere

The atmosphere of the Earth may be divided into several distinct layers, as the following figure indicates.

Layers of the Earth's atmosphere

The Troposphere

The troposphere is where all weather takes place; it is the region of rising and falling packets of air. The air pressure at the top of the troposphere is only 10% of that at sea level (0.1 atmospheres). There is a thin buffer zone between the troposphere and the next layer called the tropopause.

The Stratosphere and Ozone Layer

Above the troposphere is the stratosphere, where air flow is mostly horizontal. The thin ozone layer in the upper stratosphere has a high concentration of ozone, a particularly reactive form of oxygen. This layer is primarily responsible for absorbing the ultraviolet radiation from the Sun. The formation of this layer is a delicate matter, since only when oxygen is produced in the atmosphere can an ozone layer form and prevent an intense flux of ultraviolet radiation from reaching the surface, where it is quite hazardous to the evolution of life. There is considerable recent concern that manmade flourocarbon compounds may be depleting the ozone layer, with dire future consequences for life on the Earth.

The Mesosphere and Ionosphere

Above the stratosphere is the mesosphere and above that is the ionosphere (or thermosphere), where many atoms are ionized (have gained or lost electrons so they have a net electrical charge). The ionosphere is very thin, but it is where aurora take place, and is also responsible for absorbing the most energetic photons from the Sun, and for reflecting radio waves, thereby making long-distance radio communication possible.

The structure of the ionosphere is strongly influenced by the charged particle wind from the Sun (solar wind), which is in turn governed by the level of Solar activity. One measure of the structure of the ionosphere is the free electron density, which is an indicator of the degree of ionization. Here are electron density contour maps of the ionosphere for months in 1957 to the present. Compare these simulations of the variation by month of the ionosphere for the year 1990 (a period of high solar activity with many sunspots) and 1996 (a period of low solar activity with few sunspots):

Weather and Climate

Not only does the Earth have a complex atmosphere, but that atmosphere has complicated motion and nontrivial behavior. We Earthlings call this weather in the short term and climate over the longer term. The following images illustrate some of the complex patterns that develop in Earth's atmosphere.

Left: GOES-7 image of weather patterns in the Americas and Eastern Pacific. Middle: Hurricane Fran from GOES-8 (Ref). Right: GOES-8 false-color image of water vapor in Earth's atmosphere (Ref).

For good measure, here are two galleries of hurricane images:

• Gallery of 1995 Atlantic Hurricane Images (DMSP)
• Gallery of 1995 Northwest Pacific Typhoon Images (DMSP)

• Current USIR (GOES 8)
• Current Western Hemisphere (GOES 8 visible-IR composite)
• Most Current GOES-9 IR MPEG of Eastern Pacific & Western U.S.

Consequences of Rotation for Weather

The Earth is a spinning globe where a point at the equator is travelling at around 1100 km/hour, but a point at the poles is not moved by the rotation. This fact means that projectiles moving across the Earth's surface are subject to Coriolis forces that cause apparent deflection of the motion.

Coriolis Forces

The following diagram illustrates the effect of Coriolis forces in the Northern and Southern hemispheres.

The Coriolis force deflects to the right in the Northern hemisphere and to the left in the Southern hemisphere when viewed along the line of motion.

Solar Heating and Coriolis Forces

Since winds are just molecules of air, they are also subject to Coriolis forces. Winds are basically driven by Solar heating. As the adjacent (highly idealized) image indicates, Solar heating on the Earth has the effect of producing three major convection zones in each hemisphere.

If solar heating were the only thing influencing the weather, we would then expect the prevailing winds along the Earth's surface to either be from the North or the South, depending on the latitude. However, the Coriolis force deflects these wind flows to the right in the Northern hemisphere and to the left in the Southern hemisphere. This produces the prevailing surface winds illustrated in the adjacent figure.

For example, between 30 degrees and 60 degrees North latitude the solar convection pattern would produce a prevailing surface wind from the South. However, the Coriolis force deflects this flow to the right and the prevailing winds at these latitudes are more from the West and Southwest. They are called the prevailing Westerlies.

Cyclones & Anticyclones

The swirling motions evident in the preceding animations are consequences of frontal systems anchored to high and low pressure systems, which are also called anticyclones and cyclones, respectively. The wind flow around high pressure (anticyclonic) systems is clockwise in the Northern hemisphere and counterclockwise in the Southern hemisphere. The corresponding flow around low pressure (cyclonic) systems is counterclockwise in the Northern hemisphere and clockwise in the Southern hemisphere. This is a consequence of the Coriolis force, as illustrated for the Northern hemisphere in the following figure.

Low pressure systems (left) and high pressure systems (right) in the Northern hemisphere

Here is a pronounced example of a cyclone: a movie of Hurricane Andrew (653 kB). This animation is a loop of infrared satellite images showing the path of Hurricane Andrew across Florida and into Louisiana from Sunday, August 23 through Thursday, August 27, 1992 (Credit: Nathan Gasser).

The Magnetic Field

The Earth has a substantial magnetic field, a fact of some historical importance because of the role of the magnetic compass in exploration of the planet.

Structure of the Field

The field lines defining the structure of the magnetic field are similar to those of a simple bar magnet, as illustrated in the following figure.

The Earth's magnetic field and Van Allen radiation belts

It is well known that the axis of the magnetic field is tipped with respect to the rotation axis of the Earth. Thus, true north (defined by the direction to the north rotational pole) does not coincide with magnetic north (defined by the direction to the north magnetic pole) and compass directions must be corrected by fixed amounts at given points on the surface of the Earth to yield true directions.

Van Allen Radiation Belts

A fundamental property of magnetic fields is that they exert forces on moving electrical charges. Thus, a magnetic field can trap charged particles such as electrons and protons as they are forced to execute a spiraling motion back and forth along the field lines.

As illustrated in the adjacent figure, the charged particles are reflected at "mirror points" where the field lines come close together and the spirals tighten. One of the first fruits of early space exploration was the discovery in the late 1950s that the Earth is surrounded by two regions of particularly high concentration of charged particles called the Van Allen radiation belts.

The inner and outer Van Allen belts are illustrated in the top figure. The primary source of these charged particles is the stream of particles emanating from the Sun that we call the solar wind. As we shall see in a subsequent section, the charged particles trapped in the Earth's magnetic field are responsible for the aurora (Northern and Southern Lights).

Origin of the Magnetic Field

Magnetic fields are produced by the motion of electrical charges. For example, the magnetic field of a bar magnet results from the motion of negatively charged electrons in the magnet. The origin of the Earth's magnetic field is not completely understood, but is thought to be associated with electrical currents produced by the coupling of convective effects and rotation in the spinning liquid metallic outer core of iron and nickel. This mechanism is termed the dynamo effect.

Rocks that are formed from the molten state contain indicators of the magnetic field at the time of their solidification. The study of such "magnetic fossils" indicates that the Earth's magnetic field reverses itself every million years or so (the north and south magnetic poles switch). This is but one detail of the magnetic field that is not well understood.

The Earth's Magnetosphere

The solar wind mentioned above is a stream of ionized gases that blows outward from the Sun at about 400 km/second and that varies in intensity with the amount of surface activity on the Sun. The Earth's magnetic field shields it from much of the solar wind. When the solar wind encounters Earth's magnetic field it is deflected like water around the bow of a ship, as illustrated in the adjacent image (Source).

The imaginary surface at which the solar wind is first deflected is called the bow shock. The corresponding region of space sitting behind the bow shock and surrounding the Earth is termed the magnetosphere; it represents a region of space dominated by the Earth's magnetic field in the sense that it largely prevents the solar wind from entering. However, some high energy charged particles from the solar wind leak into the magnetosphere and are the source of the charged particles trapped in the Van Allen belts.