Background Information 2 – The Earth

Tour of the Earth

480px-The_Earth_seen_from_Apollo_17Earth’s orbit is a nearly circular ellipse with an elongation of only 0.0167, but that means the nearest point to the sun (perihelion) is approximately 5 billion miles closer than the farthest point (aphelion). This results in an energy difference of 6.9%. The earth’s axis always points toward Polaris, the pole star, so that as it travels around the sun, sometimes the northern hemisphere points more toward the sun and sometimes the southern hemisphere points more toward the sun, creating our seasons. Southern hemisphere summers occur near perihelion (January 3) so they may be hotter. Similarly, winters in the southern hemisphere may be colder because they occur near aphelion (July 4). Greater ocean areas in the southern hemisphere help to mediate this effect.

High tides occur about every 12 hours on opposite sides of the earth because of the rotation of the earth under the gravitational influence of the sun and the moon which pull on the earth, the oceans and the atmosphere. During a month, the moon orbits the earth every 27 days so that when the moon and sun are on the same (new moon) or opposite (full moon) sides of the earth, tides are greater due to combined gravitational effects. Tides also occur in the earth and atmosphere, although less noticeably. Tides and wind circulate ocean waters and atmosphere to help cleanse and distribute waste, nutrients and useful gases like oxygen and carbon dioxide.

Earth’s Interior

The earth’s interior is separated into crust, mantle and core. The depth of the earth’s crust is, on average, 4 to 40 miles (6 to 64 km) thick in oceans and continents respectively. The crust is a thin layer on which lighter continental rock “floats” on top of heavier basaltic oceanic rock. In reality, the entire crust creeps across the surface of the earth from spreading volcanic cracks where new crust is forming, e.g. mid-ocean ridges, to subduction areas where crust is plunged back into the earth’s mantle to be recycled. The earth’s crust is divided into seven major tectonic plates and several minor ones. These plates can collide, causing new mountains to be pushed up, or be pulled apart forming rift valleys. The highest surface point is Mount Everest at 8848 meters (29,029 ft.), and the lowest point is the Challenger Deep in the Marianas Trench at 10,911 meters (35,797 ft.) below seal level. Even at that, compared to the bulk of the earth, the surface is extremely smooth, with maximum variance at only plus or minus 0.08%.

The crust makes up only 1.6% of the earth by volume. The mantle fills about 80% of the earth’s volume and consists of high temperature, high pressure molten or deformable rock. The mantle has convection currents from rising hotter and descending cooled material that drives the crustal movements and volcanism and can affect local gravity and magnetic fields. The core makes up about 16% of the earth by volume and consists of an outer liquid layer and an inner solid portion consisting mostly of iron with nickel. Note that the density rises from 2-3g/cm3 in the crust to 3.4-5.6g/cm3 in the mantle to 10-13g/cm3 in the outer and inner core, so that by weight, it would be more like an estimated 0.7% crust, 66% mantle and 33.3% core by weight.

The rotation of the earth creates a dynamo effect in the core that is responsible for earth’s magnetic field. The magnetic poles do not coincide with the axis poles around which the earth rotates. The magnetic poles wander so that the north magnetic pole presently resides about 5o south of the axis pole (true north) near northern Canada and is heading west-northwest at about 60 km (35 miles) per year. The north and south magnetic poles are not exactly opposite each other so that the south magnetic pole resides about 30o north of the axis pole (true south) off the coast of Antarctica and is heading northwest at about 15 km (10 miles) per year. The magnetic field lines between the poles on which our compasses depend also wander and have eddies and excursions caused by local factors or interior circulation, so that the US and international geologic services must redraw them every few years to keep marine navigation charts current. Without the proper correction factors, navigation by compass would be very inaccurate. Fortunately, we now have GPS satellites that can be used to augment navigation by triangulation.

The magnetic poles can flip on geologic (long) time scales so that north becomes south, and have done so several times in the past. The last confirmed reversal was about 780,000 years ago, as seen in bands of reversed magnetism in solidified lava at the mid-ocean ridges. Reversals are thought to take tens or hundreds of years to develop. During these periods, the magnetic field drops to a few percent of present levels. This can open the earth to deadly rays and particles from the solar wind and space that are normally deflected by the magnetosphere.

Deeper is hotter in earth’s crust due to heat from the mantle, radioactive decay and pressure from gravity. At a few tens of meters, caves often are near a constant 50oF (10oC) due to air and water circulation and absence of direct solar heating, but in deep mines and natural caves the temperature rises to the point that life is unbearable without external cooling devices. On average the temperature increases at 25oC/km (124oF/mile) depth. At the Kola Peninsula experimental borehole[1], at a depth of 12,262 meters (40,230 ft. or 7.6 miles) the temperature rose to a whopping 180oC (356oF) although it was expected to only reach 100oC (212oF) in that location.   Work was stopped because the drilling equipment could not stand any higher heat.


In the oceans, the pressure steadily increases with depth. Pressure increases by one atmosphere for each 10 meters (33 ft.) depth, so that at 200 meters, the pressure is 20 atmospheres. At the deepest part of the ocean[2] the pressure is about 1100 atmospheres. Light is reduced at greater depths so that below 200 meters, light is reduced to the point where photosynthesis is not possible. The oceans cover over 70% of the earth, including 60% covered by waters deeper than 200 meters depth. The average depth is over 3650 meters (11975 ft.).

Temperature also declines with depth so that by about 1000 meters (3300 ft.) the average temperature is below 12oC (54oF). Average temperature at the bottom of the deep ocean is around 2oC (36oF), and at the deepest point in the oceans the temperature is about 1oC (34oF). The exceptions to this are the few volcanic regions such as hydrothermal vents in which the temperature of liquid[3] water may be as high as 450oC (842oF), supporting a rich community of life forms in the surrounding waters.   Living creatures, from microbes[4] to crustaceans and fish, are present at all depths in the oceans.


The atmosphere is divided into zones called the troposphere, stratosphere, mesosphere, thermosphere and exosphere, but the temperature profile is a little more complicated than that of the interior of the earth or the oceans. Although, as a rule, higher is colder due to heating at the earth’s surface, it is not a steady gradient. At altitudes of commercial airlines, typically 10 to 15km (6 to 9 miles), the temperatures are in the range of -50oC to -60oC (-58oF to -76oF). However, temperatures rise from there to a maximum of around 0oC (32oF) in the stratosphere (50 km or 30 miles) due to heating by absorption of UV radiation in the ozone layer. Temperatures again dip down in the ionosphere (80 to 90 km or 50 to 56 miles), (which is in the mesosphere), to -90oC (-130oF). At the altitude of the thermosphere and upward, the air molecules are so thinned out that gas properties and temperatures as we know them are meaningless. While molecules, practically unimpeded by collisions with other molecules, may be moving so fast that their individual “temperatures” may reach several thousand degrees, their overall effect still results in extremely cold measured temperatures near -90oC (-130oF).

Average atmospheric pressure at sea level is 14.7 Psi (1.03 kg/cm)[5], but rapidly declines with altitude due to reduced density of air molecules and reduced depth of the air column. On the highest mountains the pressure, density and therefore the oxygen content reduction require supplemental oxygen. Airliners must be pressurized at most altitudes. Just as many deep ocean creatures require the crushing pressure of the depths to survive, complex life on the surface of the earth requires the pressure of the atmosphere to survive. We live at the bottom of a sea of atmosphere. Some birds migrate at altitudes that would suffocate us, and some bacteria are found as high as the Stratosphere, but as a general rule, we depend on the surface pressure, temperature and mixture of gases to keep us alive.

Habitability Zone

Only in this thin band of atmosphere and crust are conditions truly habitable for us. Although life exists in all climates and conditions, most complex life, from quadrupeds to humans requires a narrow range of temperatures, and plant life (crops) also require sufficient water and nutrients in the soil. If we assume that 70% of the earth is covered by water, another 12% is too cold, hot or dry and another 12% has soil that is too poor. That leaves around 3% capable of growing crops to feed 7.2 billion people who live on only 3% of the land. The oceans add about 5% to the protein consumed by humans. The rest of the earth, the solar system and indeed the universe as a whole are completely hostile to us.


[1] Soviet Union Kola super deep borehole project 1970 to 1989.

[2] Mariana Trench, Challenger Deep is 10.9km (6.8 mi.)

[3] Water can remain liquid at higher temperatures due to high pressure at these depths.

[4] Since classification has divided “bacteria” into two types, bacteria and archaea, for simplicity I will use the word microbe to mean either or both.

[5] PSI means pounds per square inch.


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