![\"NASA](\"https:\/\/wp-media.patheos.com\/blogs\/sites\/186\/2019\/08\/640px-Thin_Line_of_Earths_Atmosphere_and_the_Setting_Sun.jpg\")
<\/a>I published the following column in the 14 November issue of the Deseret News<\/em>:<\/p>\n As we in the United States approach the national Thanksgiving holiday for 2019, it\u2019s appropriate to consider things for which we should express our gratitude.\u00a0Obviously, of course, there\u2019s the good food that many of us will be eating.\u00a0There are the family members with whom many of us will be gathering to share it.<\/p>\n However, there is much, much more.\u00a0Indeed, our reasons for gratitude are virtually infinite.\u00a0Here, let me suggest one vital factor in our lives that we almost always take for granted:<\/p>\n The phrase \u201cthin blue line\u201d\u00a0is 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.\u00a0The 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.<\/p>\n Our atmosphere as it exists today derives (as our oceans also do) from the \u201cdegassing\u201d of the primitive semi-molten earth, supplemented by later additions belched up from volcanoes and emitted by hot springs.<\/p>\n 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.\u00a0We couldn\u2019t have survived those conditions.\u00a0However, the lighter gases (e.g., hydrogen and helium) escaped toward space.\u00a0Five hundred miles above the Earth, our \u201catmosphere,\u201d if it can still be called that, is composed of 50% helium and 50% hydrogen.<\/p>\n Somewhat later in our planet\u2019s history, living organisms developed that were capable of photosynthesis.\u00a0They 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\u2019s ultraviolet radiation.<\/p>\n Evidence for this sequence of atmospheric development can be found, to some degree at least, in Precambrian rocks and a few fossils, which show a transition from a largely oxygen-free environment to what we might term a free-oxygen environment.<\/p>\n Our terrestrial atmosphere is an exceedingly thin envelope surrounding Earth.\u00a0Perhaps somewhat more than 99% of our planet\u2019s air exists within a region no higher than approximately 18 miles above sea level.\u00a0Earth\u2019s radius \u2014 the distance from its center to its surface or circumference \u2014 somewhat less than 4,000 miles, which means that the thickness of that oxygenated region of our atmosphere is a bit less than 0.5% of Earth\u2019s radius.<\/p>\n But oxygen isn\u2019t evenly distributed even within that thin envelope.\u00a0Denser and, thus, heavier gases such as oxygen, carbon dioxide, nitrogen and water vapor hang low in the current atmosphere, mostly within about 3 miles of the planet\u2019s surface.\u00a0That thin band is equivalent to approximately 0.00075 of Earth\u2019s radius, well under one ten-thousandth.\u00a0Its outer edge is not far above our heads.<\/p>\n<\/blockquote>\n I followed those thoughts up on 28 November 2019 with the column immediately below:<\/p>\n Two weeks ago, I argued that we should be grateful for\u00a0Earth\u2019s atmosphere and the air we breathe (see \u201cThe miracle of Earth\u2019s atmosphere design and the air we breathe<\/a>\u201d published Nov. 14, on deseret.com)<\/a>. Today, still in the Thanksgiving spirit, I suggest gratitude for the dirt beneath our feet.<\/p>\n The internal\u00a0structure of our planet is a series of\u00a0concentric spheres.\u00a0A solid metallic \u201cinner core\u201d is surrounded by a liquid \u201couter core.\u201d\u00a0The \u201couter core\u201d\u00a0is, in turn, contained within Earth\u2019s viscous \u201cmantle.\u201d\u00a0And then, finally, we reach the solid \u201couter crust,\u201d pretty much the planet of our daily experience.<\/p>\n Whereas Earth\u2019s average radius is 3,958.8\u00a0miles (6,378 kilometers), the thickness of Earth\u2019s crust ranges from about 43 miles (70 kilometers) beneath continental mountains to less than 5 miles (8 kilometers) beneath the oceans.<\/p>\n Rather like the film that forms on a cooling cup of hot chocolate, Earth\u2019s crust \u201cfloats,\u201d as it were, on the solid but soft and viscous or \u201cplastic\u201d\u00a0mantle \u2014 much hotter and much more dense \u2014 located underneath.\u00a0(This gives new meaning to the expression \u201csolid earth\u201d or \u201cterra firma.\u201d)<\/p>\n Obviously, we live atop the terrestrial crust.\u00a0But even that crust is mostly inhospitable to life.\u00a0On its surface, of course, things are (by definition) at air temperature.\u00a0However, at the bottom of the world\u2019s deepest mine, 2.4 miles down in South Africa\u2019s TauTona, the ambient air temperature is 131 Fahrenheit (55 Celsius), and the temperature of rock surfaces is 140 F (60\u00a0C).\u00a0Without artificial air conditioning, that air temperature alone would soon kill the miners.\u00a0So the lowest humanly\u00a0habitable\u00a0depth on our planet is generously reckoned as about 2 miles down into the crust.<\/p>\n Deeper crustal temperatures reach approximately 1,600 F, or about 870\u00a0C.\u00a0To put this in perspective, 350\u00a0F will bake bread.\u00a0At 1,600\u00a0F, rocks begin to melt.\u00a0Immediately beneath the crust is the solid but plastic mantle, where temperatures reach as high as 7.250 F (4,000\u00a0C).<\/p>\n Moreover, as my previous column noted,\u00a0humans cannot usually function very well without supplemental oxygen beyond roughly 3\u00a0miles above sea level.\u00a0Which means that we can only live in a thin region, roughly 5 miles thick, within the combined area of Earth\u2019s nearly 3,960-mile radius and its surrounding 500 miles of atmosphere.<\/p>\n That\u2019s a stunningly narrow range.\u00a0Humans can survive in only 5\/4460 (or 0.00112108) \u2014 slightly more than a tenth of 1% \u2014 of the vertical portion of Earth\u2019s combined mass and ambient atmosphere.<\/p>\n The loose, upper, \u201cweathered\u201d layer of Earth\u2019s crust is called \u201csoil.\u201d\u00a0It\u2019s tempting to dismiss soil as mere \u201cdirt.\u201d\u00a0If something or someplace is \u201cdirty,\u201d we want to clean it, to get rid of the dirt.\u00a0But life on Earth would be impossible without soil.\u00a0For example, it\u00a0helps 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.\u00a0Very obviously, most plants require soil in which to grow.\u00a0They anchor themselves into the ground with their roots and thereby extract nutrients from it \u2014 and\u00a0animal life (including human life) clearly depends upon such plants.<\/p>\n 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.<\/p>\n 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.\u00a0The formation of just an inch of topsoil can require up to 1,000 years.\u00a0Below the topsoil is the subsoil, which is made up primarily of clay, iron and organic matter.\u00a0Below the subsoil is the so-called \u201cparent material,\u201d mostly large rocks that have not yet been completely broken down \u2014 so called because the topsoil and subsoil develop from it.\u00a0And beneath the \u201cparent material\u201d is bedrock, a large mass of solid rock located several feet below the surface.<\/p>\n So human life depends upon 5-10 inches of dirt on the surface of a planet that\u2019s nearly 8,000 miles in diameter.<\/p>\n\n
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(Wikimedia Commons public domain image)<\/figcaption><\/figure>\n\n