Atoms: Molecules: Molecular Bonding

Newton's Cradle Potential Energy Pendulum

Gyroscopic Motion


What are the gases that make up the earth's atmosphere?
Two gases make up the bulk of the earth's atmosphere: nitrogen ( ), which comprises 78% of the atmosphere, and oxygen ( ), which accounts for 21%. Various trace gases make up the remainder. Based on temperature, the atmosphere is divided into four layers: the troposphere, stratosphere, mesosphere, and thermosphere.
What is the earth's atmosphere?
The Earth's atmosphere is a thin layer of gases that surrounds theEarth. It composed of 78% nitrogen, 21% oxygen, 0.9% argon, 0.03% carbon dioxide, and trace amounts of other gases.
How thick is the Earth's atmosphere?
Earth's atmosphere is about 300 miles (480 kilometers) thick, but most of it is within 10 miles (16 km) the surface. Air pressure decreases with altitude. At sea level, air pressure is about 14.7 pounds per square inch (1 kilogram per square centimeter).
How much of the earth's atmosphere is made up of nitrogen and oxygen?
Earth's atmosphere is 78% nitrogen, 21% oxygen, 0.9% argon, and 0.03% carbon dioxide with very small percentages of other elements. Our atmosphere also contains water vapor.

he 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):

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.

Realistic Weather Patterns

The adjacent animation shows GOES-8 weather satellite images over a 72-hour period from Dec. 29, 1996, through Jan. 1, 1997. This is a geosynchrous satellite, which means that it orbits the Earth with the same period as the Earth's rotation and therefore appears to be essentially motionless over a fixed position on the Earth's surface. For GOES-8 this fixed position looks down on North and South America.

In these composite images red indicates visible light (reflected sunlight), green indicates the 11 micron IR channel (thermal emission), and blue indicates the 3.9 micron channel (thermal + sunlight). At night the images are blue and green. The three periods of daylight in this 72 hour sequence are clearly visible as red-orange regions moving from East to West (right to left). In the IR channels, the natural intensity pattern has been inverted: warmer is darker, so that cool cloudtops stand out brightly.

One can see clearly the pronounced cloud flows associated with the strong westerlies at mid-latitudes in each hemisphere. (This is taken in Northern hemisphere Winter, so the heavier cloud cover in that hemisphere is not surprising.) Less obvious are the easterly trade winds and the polar easterlies, though one can see vestiges of each if one looks carefully. Also apparent are the swirling motions associated with frontal systems. These are particularly pronounced at the boundaries between the mid-latitude westerly and polar wind flows in each hemisphere.

Here is a similar weather animation (1.49 MB animated GIF) using GOES-8/9 IR images for North America over a 2 day period from December 31, 1996 through January 1, 1997. The large weather systems that move ashore from the Pacific in this animation produced catastrophic flooding in California, Oregon, and Washington in early January, 1997.

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

Auroras: the Northern and Southern Lights 


The aurora, or northern and southern lights, are often visible from the surface of the Earth at high northern or southern latitudes. Auroras typically appear as luminous bands or streamers that can extend to altitudes of 200 miles (well into the ionosphere).

Northern and Southern Lights

The following figures show three examples of the often spectacular visible light display associated with auroras.


Southern aurora from the Space Shuttle Endeavor (Ref) Northern aurora over Lake Superior (Ref) Northern aurora over Circle, Alaska (Ref)


Here is another image of the southern aurora taken from the Space Shuttle. The aurora changes with time, often looking like moving curtains of light. Here are some MPEG and QuickTime film clips that illustrate the time dependence of the auroral display.

The Cause of Auroras

Auroras are caused by high energy particles from the solar wind that are trapped in the Earth's magnetic field. As these particles spiral back and forth along the magnetic field lines, they come down into the atmosphere near the north and south magnetic poles where the magnetic field lines disappear into the body of the Earth.

The delicate colors are caused by energetic electrons colliding with oxygen and nitrogen molecules in the atmosphere. This excites the molecules, and when they decay from the excited states they emit the light that we see in the aurora.


The collisions of trapped charged particles with atmospheric molecules causes spectacular effects in the visible spectrum, but these excited molecules can also emit radiation in other wavelength bands. The following figures show aurora imaged in the ultraviolet (UV) and X-ray regions of the spectrum.


Imaging the Earth 


The Earth in Visible Light

We have seen a number of images of Earth in visible light, but mostly at large scale from a great distance. Here are two images from space of smaller objects on the Earth that we will be interested in looking for on other planets and moons: a canyon system, and an active volcano.


The Grand Canyon from space (Source) Mt. Etna from space (Source)


We shall find canyon systems and active volcanoes on other objects in the Solar System to are not too different from these images. Here is a set of links to satellite imaging of the Earth.

The Earth at Night

There is one aspect of the Earth's appearance that we do not expect to be repeated in the near future for other objects in the Solar System: at night the artificial light associated with human civilization is very visible from space. The following image shows the appearance of the United States at night as observed from a composite of many satellite passes.


The USA at night


The major source of light is from cities, but by looking carefully you can even see things like lights scattered in the Gulf of Mexico south of Louisiana that are from oil platforms (Ref). The photograph is from Defense Meteorological Satellite Program (DMSP) images.

Imaging in Ways other than Visible Light

Because our eyes are sensitive to visible light, our prejudice is to view things at those wavelengths. However, we now have instruments at our disposal that permit observations in many wasy other than the visible light region of the electromagnetic spectrum. These often offer considerable advantage; for example radar cuts through the ever-present thick cloud cover to give us images the surface of Venus that we could not obtain at visible wavelengths.


Radar Imaging of the Earth's Surface

The adjacent images show a comparison of the Mt. Everest region (border of Nepal and Tibet). The top image was taken through thick cloud cover with synthetic aperture radar on the space shuttle Endeavor; The bottom figure is an optical image of the same region taken from Endeavor (Ref). One can see many of the same features in the two photographs (the photographs were taken at different times of the year, so they have different snow covers).

The curving and branching features are glaciers. The radar technique used is sensitive to characteristics of the glacier surfaces such as the ice roughness and water content. Thus the glaciers show a variety of colors in the radar image but are a rather featureless gray or white in the optical photograph.

Radar (upper) and visual (lower) images of Mount Everest


Infrared and more Exotic Imaging

We have seen in the preceding sections examples of imaging the Earth in the infrared, ultraviolet, and X-ray regions of the spectrum. Here we show additional examples of IR images, and a more exotic technique combining magnetic and gravitational data that can even locate objects beneath the surface of the planet.


San Francisco Bay imaged in IR from space Fossil crater imaged with representation of gravity and magnetic field data GOES-8 IR satellite image of water vapor in Earth's atmosphere


The left figure shows the San Francisco Bay area imaged from space in the infrared (IR). Click on the image for a larger version revealing quite fine details such as bridges and highways (Source).

The middle figure shows a composite of local gravity and magnetic field variation data to image a 112-mile wide relic meteor crater in Yucatan that presently lies below several hundred meters of sedimentary rock. This crater, called Chicxulub, is famous because it is the leading candidate for the site of the asteroid impact that is thought to have killed the dinosaurs 65 million years ago in the K-T extinction (Ref).

The right image shows a GOES-8 weather satellite image in the 6.7 micron IR channel that is sensitive to the distribution of water vapor in Earth's atmosphere (Ref). The imager on this satellite records radiation emitted by water vapor in the upper troposphere. Regions with high concentrations of water vapor are bright, while dark spots signal lower water vapor concentrations.

Surface Temperature Maps

Infrared radiation is basically radiant heat. Therefore, IR detected from satellites can be used to determine the temperature of objects. The following image shows a color-coded map constructed from a composite of satellite data and surface observations giving surface temperatures on the Earth (Ref).


Surface temperatures on Earth for January 30, 1997


Here is a link to the current temperature map (updated on a 6 hour cycle), and here is a movie (2MB MPEG---slow download) of the temperature varation over the past week.



Sea Surface Temperature Maps

Similar methods as described above may be used to construct color coded maps of surface seawater temperatures. Here is an example (Ref):


Sea surface temperatures on Earth for January 29, 1997



  • This reciprocal induction of tides in the body of the Earth and the Moon leads to a complicated coupling of the rotational and orbital motions of the two objects. These tidal forces and associated couplings have the following general effects:


    • The interior of the Earth and Moon are heated by the tides in their bodies, just as a paper clip is heated by constant bending. This effect is very small for the Earth and Moon, but we shall see that it can be dramatic for other objects that experience much larger differential gravitational forces and therefore much larger tidal forces. For example, we shall see that the tidal forces exerted by Jupiter on its moon Io are so large that the solid surface of Io is raised and lowered by hundreds of meters twice in each rotational period. This motion so heats the interior of Io that it is probably mostly molten; as a consequence, Io is covered with active volcanos and is the geologically most active object in the Solar System.
    • The tidal coupling of the orbital and rotational motion tends to synchronize them. In the simplest instance, the period of rotation for the two bodies and the orbital period eventually become exactly equal because of this tidal coupling (and as a result, the size of the orbit is changed in such a way as to conserve angular momentum for the entire system). This is calledgravitational (or tidal) locking, because as the two objects revolve around their common center of mass each keeps the same side turned toward the other

Viking: the
Search for Life

In 1976 the Viking 1 and 2 landers undertook searches on the Martian surface for the chemical evidence of present or past life on Mars. The images shown below give a picture of one of the backup landers, and two different views of the Martian surface as photographed from Viking 1.


Viking lander (Ref) and two views of the Martian surface from Viking 1(Ref)


In addition to photgraphing the surface, the Viking landers undertook a series of experiments at two points on the surface to find evidence for life.

The Experiments

The 4 basic experiments that the Vikings carried out to search for evidence of life were:
  1. Gas Metabolism: look for changes in the atmosphere induced by metabolism in the Martian soil.
  2. Labeled Release: Look for release of radioactive carbon dioxide by metabolism from organic material labeled by radioactive carbon.
  3. Pyrolytic Release: Search for radioactive compounds in soil by heating soil exposed to radioactive carbon dioxide.
  4. Mass Spectrometer: Search directly in Martian soil for organic compounds known to be essential to Earth life.
These experiments were built around the hypothesis that if there were life on Mars it would have a similar metabolism to life on Earth, and that it would have a similar biochemistry based on the same organic compounds important to life on Earth.

The Results

The results of these experiments were complex. The first three gave positive results, but the complete absence of any organic compounds in the Martian soil according to the mass spectrometer experiment suggests that the positive results for the first three were not evidence for life, but rather evidence for a complex inorganic chemistry in the Martian soil. Thus, the Viking verdict was that there was no evidence for present or past life on Mars.


Renewed Interest in Martian Life

This issue has been given renewed impetus by the recent claim (see also this and this) that a meteorite found on the Earth was once part of Mars (because of detailed chemical composition), and that there may be evidence in this rock for past organic activity. However, this is a very open topic at the moment, since there potentially are other explanations of the meteorite's content. We will have to wait on further evidence to clarify this issue.


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