Giving thanks for dirt and for air

Giving thanks for dirt and for air November 23, 2023


NASA photo of terrestrial atmosphere
The thin line of Earth’s atmosphere and the setting sun are featured in this NASA public domain photograph taken on 25 November 2009 by the crew of the International Space Station while space shuttle Atlantis on the STS-129 mission was docked with the station.


Happy Thanksgiving!

Both of these columns appeared in the Deseret News in connection with Thanksgiving 2019.  Although I have a few more of these, they are (mercifully) the last of the Thanksgiving-themed columns that I intend to resurrect here this year:


NASA ISS photo of Earth's atmosphere
A setting last quarter crescent moon and the thin line of Earth’s atmosphere, photographed on 4 September 2010 by a member of the Expedition 24 crew as the International Space Station passed over central Asia.

(NASA public domain photograph)


As we in the United States approach the national Thanksgiving holiday for 2019, it’s appropriate to consider things for which we should express our gratitude.  Obviously, of course, there’s the good food that many of us will be eating.  There are the family members with whom many of us will be gathering to share it.  However, there is much, much more. Indeed, our reasons for gratitude are virtually infinite.  Here, let me suggest one vital factor in our lives that we almost always take for granted:

The phrase “thin blue line” is sometimes used to refer to the role of the police in society, who hold chaos at bay and thus permit order and civilization to flourish.  The term could perhaps be used even more appropriately to describe the function of our terrestrial atmosphere, which allows not only civilization and order but sheer physical survival.

Our atmosphere as it exists today probably derives (as our oceans also do) from the “degassing” of the primitive semi-molten earth, supplemented by later additions belched up from volcanoes and emitted by hot springs.  The atmosphere of early geologic times was composed of such gases as hydrogen, nitrogen, carbon monoxide, carbon dioxide, water vapor, and various forms of hydrogen chloride.  We couldn’t have survived those conditions.  However, the lighter gases (e.g., hydrogen and helium) presumably escaped toward space.  Five hundred miles above the earth, our “atmosphere,” if it can still be called that, is composed of 50% helium and 50% hydrogen.

Somewhat later in our planet’s history, living organisms developed that were capable of photosynthesis.  They provided the oxygen that then permitted animal respiration and eventually the colonization of land, as well as providing the famous ozone layer that shields Earth (and us) from the sun’s ultraviolet radiation.

Evidence for this sequence of atmospheric development can be found, to some degree at least, in Precambrian rocks and a few fossils, which seem to suggest a transition from a largely oxygen-free environment to what we might term a free-oxygen environment.

Our terrestrial atmosphere is an exceedingly thin envelope or film surrounding Earth.  Perhaps somewhat more than 99% of our planet’s air exists within a region no higher than thirty kilometers (or approximately eighteen miles) above sea level.  Earth’s radius — the distance from its center to its surface or circumference — is 6400 kilometers (somewhat less than 4000 miles), which means that the thickness of that oxygenated region of our atmosphere is a bit less than 0.5% of Earth’s radius.

But oxygen isn’t evenly distributed within that thin envelope.  Denser and, thus, heavier gases such as oxygen, carbon dioxide, nitrogen, and water vapor hang low in the current atmosphere, mostly within about three miles of the planet’s surface.  That thin band is equivalent to approximately 0.00075 of Earth’s radius, well under one ten-thousandth.  Its outer edge is not far above our heads.

These heavier gases, especially oxygen, are essential to life.  Above roughly three miles over sea level, we humans cannot usually function very well without supplemental oxygen.

Any resident of lower altitudes who has climbed in the Colorado Rockies or the Sierra Nevadas of California, or visited the old Inca capital city of Cusco in Peru, knows the risks of nausea and lightheadedness that are encountered there.  And death awaits those who travel, unaided, very much higher.

Federal regulations require the use of supplemental oxygen by pilots who fly more than 30 minutes at cabin pressure altitudes of 12,500 feet (roughly 3.8 kilometers, slightly more than two miles) or higher. And at cabin altitudes above 14,000 feet (somewhat more than 4.25 kilometers, about 2.5 miles), pilots must use oxygen at all times.

Altogether, the gases in the atmosphere serve to insulate the earth by filtering out most cosmic radiation and, as mentioned, most of the sun’s ultraviolet radiation.  They prevent large swings in temperature.  They also burn up untold millions of meteors before those objects are able to collide with our planet.  Again, in these ways, too, they are essential to life.

It’s also fortunate that our atmosphere deflects or reflects much interstellar “noise” back into space.  Without that, radio and television broadcasts would be effectively impossible, lost in an impenetrable wall of static.  On its “underside,” though, our atmosphere partially reflects (rather than merely transmitting) radio waves, which makes television and communication by radio possible.

As the Thanksgiving holiday draws near, there is much to be thankful for—including the very air that we breathe.


Terrestrial slice
A cross-section of the Earth, not drawn to scale.
(Wikimedia Commons public domain image)


Two weeks ago, I argued that we should be grateful for Earth’s atmosphere and the air we breathe.  Today, still in the Thanksgiving spirit, I suggest gratitude for the dirt beneath our feet.

The internal structure of our planet is a series of concentric spheres.  A solid metallic “inner core” is surrounded by a liquid “outer core.”  The “outer core” is, in turn, contained within Earth’s viscous “mantle.”   And then, finally, we reach the solid “outer crust,” pretty much the planet of our daily experience.

Like the skin of an apple, Earth’s crust is very, very thin in comparison to the overall radius of our planet.  Whereas Earth’s average radius is 6,378 kilometers (3,958.8 miles), the thickness of Earth’s crust ranges from about 32 kilometers (20 miles) to only about 8 kilometers (5 miles), which means that the crust represents just 0.005 to 0.00125 of that radius.

Rather like the film that forms on a cooling cup of hot chocolate, Earth’s crust “floats,” as it were, on the soft and viscous or “plastic” mantle—much hotter and much more dense—located underneath.  (This gives new meaning to the expression “solid earth” or “terra firma.”)

Obviously, we live atop the terrestrial crust.  But even that crust is mostly inhospitable  to life.  On its surface, of course, things are (by definition) at air temperature.  However, at the bottom of the world’s deepest mine, 2.4 miles down in South Africa’s TauTona, the ambient air temperature is 55 degrees C. (131 degrees F.) and the temperature of rock surfaces is 60 degrees C. (140 degrees F.).  Without artificial air conditioning, the ambient air temperature alone would soon kill the miners.  So the lowest habitable depth on our planet is generously reckoned as about two miles down into the crust.

Deeper crustal temperatures reach approximately 870 degrees Celsius, or about 1600 degrees Fahrenheit.  To put this in perspective, 350 degrees F. will bake bread.  At 1600 degrees F., rocks begin to melt.  Immediately beneath the crust is the liquid mantle, which can reach temperatures as high as 4000 degrees C. (or nearly 7,250 degrees F).

Moreover, as my previous column noted, humans cannot usually function very well without supplemental oxygen beyond roughly three miles above sea level.  Which means that we can only live in a thin region, roughly five miles thick, within the combined area of Earth’s nearly 3960 mile radius and its surrounding 500 miles of atmosphere.

That’s a stunningly narrow range.  Humans can survive in only 5/4460 (or 0.00112108)—slightly more than a tenth of one percent—of the vertical portion of Earth’s combined mass and ambient atmosphere.

The loose, upper, “weathered” layer of Earth’s crust is called “soil.”  It’s tempting to dismiss soil as mere “dirt.”  If something or someplace is “dirty,” we want to clean it, to get rid of the dirt.  But life on Earth would be impossible without soil.  For example, it helps to filter and clean our water, plays a vital role in cycling nutrients (e.g., the carbon and nitrogen cycles), and releases important gases such as carbon dioxide into our atmosphere.  Very obviously, most plants require soil in which to grow.  They anchor themselves into the ground with their roots and thereby extract nutrients from it—and animal life (including human life) clearly depends upon such plants.

Soil is not only vital to life.  It teems with life, itself.  A teaspoon of good soil, for example, will commonly contain several hundred million bacteria.  Moreover, a typical acre of good cropland will serve as the home to more than a million earthworms.  And, of course, many animals, fungi, and bacteria rely on soil as a place in which to live.

However, the primary layer where plants and other organisms live is the topsoil, which is usually only 5-10 inches thick where it exists at all.  The formation of just an inch of topsoil can require up to 1000 years.  Below the topsoil is the subsoil, which is made up primarily of clay, iron, and organic matter.  Below the subsoil is the so-called “parent material,” mostly large rocks that have not yet been broken down—so called because the topsoil and subsoil develop from it.  And beneath the “parent material” is bedrock, a large mass of solid rock located several feet below the surface.

So human life depends upon 5-10 inches of dirt on the surface of a planet that is nearly 8000 miles in diameter.


Posted from Richmond, Virginia



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