{"id":138,"date":"2021-02-06T19:12:00","date_gmt":"2021-02-06T13:42:00","guid":{"rendered":"https:\/\/visdam.co.in\/en\/?p=138"},"modified":"2021-02-16T20:00:19","modified_gmt":"2021-02-16T14:30:19","slug":"history-of-earth-1","status":"publish","type":"post","link":"https:\/\/visdam.co.in\/en\/posts\/history-of-earth-1\/","title":{"rendered":"History of Earth-1"},"content":{"rendered":"\n

We propose to launch into PoSTS<\/a> beginning with History of Earth, with some reference to our Solar system and the universe. That raises a question.
Is history of Earth relevant to PoSTS<\/a>?<\/strong>
From the prehistoric times until Copernicus, earth was the static centre of the universe. The earth itself appeared infinite with unlimited resources. Every happening on earth, whether good or bad, was assigned to seasons or to providence. Concept of history of earth was inconceivable. But now we know better. Let us view from the perspective of time and space.
Time perspective<\/strong>
We now know that the Sun and our solar system, along with the earth appeared around 4.6 Ga<\/a> BP<\/a>. (20) Oldest known ancestor of our species \u2018Homo Sapiens\u2019 is a member of genus Australopithecus. The oldest specimen of Australopithecus, found in Africa is c. 4.2 Ma<\/a> old. (19) Obviously, all other aspects of society, technology and science are even more recent occurrences on the timeline. Thus, history of ancient huma<\/a>n society, its ancient S&T, occupies less than 0.1% of time frame of history of earth. Modern S&T, which bega<\/a>n barely 3 ka<\/a> BP<\/a>, occupies a tiny fraction of the Earth\u2019s timeline. So why should we study the 99.9% of the period, when topics of our interest barely occupy 0.1% of the total period or less? Such a gross view of the timeline suggests that History of Earth is irrelevant to PoSTS<\/a>. We will nega<\/a>te this view later. But, what do we see from the space perspective?
Earth as a well-provisioned ship<\/strong>
Subsequently, we also came to know that Earth is a spherical body, about 12700 km in diameter. That is not infinite, but huge from an individual perspective. With more knowledge, Copernican version of the Heliocentric universe firmly replaced the geocentric universe, with the Earth, along with its moon, orbiting around the Sun. Thus men on earth are like persons travelling on a big ship. Such a view was first articulated by political economist Henry James, in 1879, when he called the earth a \u201cwell provisioned ship sailing through space.\u201d Henry James also claimed that \u201cif a particular provision grows scarce then one would open a hatch and find a new supply about which he did not dream earlier.\u201d (17) So, Henry James also thought this huge globe of earth was an unlimited resource! With such an unlimited resource at hand, why bother about its history? However, this perception also changed within a century, thanks to the beginning of near-space travel in 1960 CE, and upheavals of the economic world, a decade later.<\/p>\n\n\n\n

Earth as a spaceship<\/strong>
Come twentieth century, and we now know that the Sun is also orbiting around a bigger star, taking the earth along into some unknown territory. The change was best articulated in the concept of \u2018spaceship earth\u2019, which was popularized by R. Buckminster Fuller, c. 1968. He was the first TSEER<\/a> to describe \u201cearth as a spaceship and the huma<\/a>nity as an astronaut restricted to its resources for survival, on an endless journey to an unknown destination, into an unknown future.\u201d A classic picture of earth rising over horizon of moon that was shot by Bill Anders from Apollo 8, visually reaffirms the idea of a spaceship earth.<\/p>\n\n\n\n

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Earth as seen from Apollo 8 Photo courtesy NASA<\/figcaption><\/figure><\/div>\n\n\n\n

Limits to resources<\/strong>
With the oil crisis of the seventies of the last century, huma<\/a>nity realized there are limits to resources on earth, even from a global perspective. Although it is not documented anywhere, but the episode of rescuing astronauts of Apollo 13 would have reinforced the idea of limits to resources in ma<\/a>ny minds, from a personal perspective. Thus, the issue of limits to resources and our dependence on them, came to the fore. This is quite unlike the well provisioned ship envisioned by Henry James, in 1879. (17)<\/p>\n\n\n\n

Now, we do understand that earth is like a spaceship and we as astronauts are bound to this spaceship earth and to its resources. We also realize that these resources are so huge in quantity that we ma<\/a>y not know the exact quantum available. Yet, they are fundamentally limited, in so far as they are exhaustible, sooner or later, such as oil.<\/p>\n\n\n\n

Yet, importance of history of earth is not so apparent. For that, let us ponder over some other resources, basic to our technological world, as well as basic to life itself, especially their very existence on earth.<\/p>\n\n\n\n

Some questions<\/strong>
Find a list a few of such resources below along with a prima<\/a>ry question associated with its very existence on earth.
\u2022 Oxygen for breathing: How come earth acquired so much oxygen when it is so rare on rest of the planets?
\u2022 Water for drinking: What is the source of the huge amount of water available on our planet?
\u2022 Fuel oil and coal for industry: How did the earth acquire stocks of coal and oil that we do not find elsewhere in the solar system?
\u2022 Iron for technological progress: How do we happen to have such huge Iron ore deposits on our planet when iron is not so abundant on other planets?
\u2022 Minerals for technological purposes: How did wide areas near surface of earth got endowed with so ma<\/a>ny minerals, allowing us to exploit these resources, relatively easily?<\/p>\n\n\n\n

Then there are ma<\/a>ny more questions in respect of evolution of life, survival of life, phenomena like background and ma<\/a>ss extinctions, etc. We will deal with them later.<\/p>\n\n\n\n

Answers to such questions are hidden in the history of Earth. This series on History of Earth is an excursion that tries to understand our planet\u2019s past and draw lessons for the future, in the context of Society, Technology and Science. That will also reveal to us how fortunate we are in being in this part of the universe, or on our earth around this sun at this point of time.<\/p>\n\n\n\n

Our universe and solar system (13.8 to 4.6 Ga<\/a> BP<\/a>)<\/strong>
Cosmologists now propose a concept of multiverse. It is a set of universes, each with different set of values of universal constants, and each unlike the other. Atoms, their bond lengths, molecular structures they form and physicochemical properties they display would vary from universe to universe. As a consequence outward appearance of inorga<\/a>nic ma<\/a>tter and orga<\/a>nic life forms could be widely different from one universe to the other. We have no way of knowing how the other universes look or behave except in our ima<\/a>gination. Physically, we will never be able to go there. We just happen to be in one of the multiverses, that is, our universe.
Cosmologists generally agree that our universe bega<\/a>n with an explosion \u2013 the big bang \u2013 c. 13.8 Ga<\/a> BP<\/a>. (21) With that explosion of a primordial cocktail of fundamental particles, simplest of atoms like hydrogen and helium were the first to form, very early on. Heavier atoms (Boron to Uranium) formed near the end of the big bang, only in trace quantities. In due course, Hydrogen and Helium ended up in giant ga<\/a>laxies that further ga<\/a>ve birth to stars. Heavier elements ended up as intra and interga<\/a>lactic dust.
Fusion reactors within these stars further generated heavy elements. The stars went through life and death cycles. As stars died they exploded and spread the rema<\/a>ining Hydrogen, product Helium and dust of heavy elements, in space. Such exploding stars are called Nova<\/a> (that explode relatively mildly) or Supernova<\/a> (that explode relatively strongly). Stronger explosions of Supernova<\/a>e send shock waves in space that leads to accretion of ma<\/a>tter and forma<\/a>tion of newer stars, and the process goes on. Despite such repeated action by billions of stars, cosmologists estima<\/a>te that 99.998% of all atoms in our present universe are those of hydrogen and helium. It was just fortunate that our solar system formed nearly 9 Ga<\/a> after the first big bang. It could acquire enough of heavy metals (from Boron to Uranium) from the intraga<\/a>lactic dust to sustain life and technology.
Our solar system is a product of explosion of one such Supernova<\/a> in our part of the Milky Way c. 4.6 Ga<\/a> BP<\/a>. The explosion sent a shockwave that compressed the interstellar ga<\/a>s and dust cloud into a dense spinning disk of ma<\/a>tter. Gravity pulled 99% of the ma<\/a>ss of the disk inward that turned into a fusion reactor, i.e. our Sun. Ma<\/a>tter that survived sun\u2019s attraction, clumped into asteroids of irregular sizes moving in all sorts of trajectories around the sun. These molten clumps collided to form bigger objects that became spherical because of gravity. We call them planets. Clumps that could not merge became asteroids, meteoroids, comets and irregular shaped moons. (22) According to one estima<\/a>te the process took about 100 Ma<\/a>. (20) Our Earth is one of the ten planets known to-date. Luckily, our earth holds a fair share of these heavy elements, ma<\/a>ny of them radioactive to give us our energetic earth.
Immediately, a question arises. Are we so unique that species like us huma<\/a>ns are unlikely elsewhere in our universe? Scientists disagree. (22) There could be ma<\/a>ny solar systems like ours in this universe that ma<\/a>y harbour life. By one estima<\/a>te, within a 1000 light years radius, there ma<\/a>y be 10 million stars comparable to our sun. A learned estima<\/a>te places the number of solar systems like ours at 0.5 million. Obviously, likelihood of finding life similar to ours near us is quite significant. We have not succeeded in finding any life in close quarters, but the data suggests that we ma<\/a>y find some clues, sooner or later.
Amongst all such solar systems and the planets that orbit around them, we seem to be very fortunate in ma<\/a>ny ways. Some of these elements of fortune were mentioned above. Let us recount such elements due to our solar system.
\u2022 Our sun does not emit a strong proportion of shorter wave electroma<\/a>gnetic radiation harmful to life. Whatever it does, earth\u2019s atmosphere is thick enough to attenuate it.
\u2022 The asteroid belt between Ma<\/a>rs and Jupiter does occasionally eject asteroids that threaten earth and the inner planets. However ma<\/a>ssive planets like Jupiter and Saturn guard us from asteroids, through their gravitational attraction.
\u2022 Earth is at an optima<\/a>l distance from the sun. Earth\u2019s average temperature of ~17\u00b0C is neither too high like Venus (~180\u00b0C) nor too low as on Ma<\/a>rs (~-50\u00b0C). Water rema<\/a>ins liquid at this temperature, fostering life at large.
\u2022 Ma<\/a>ss of Earth is just right to hold on to its atmosphere by the force of gravity. This atmosphere acts as a blanket for preserving heat by a greenhouse effect. Without this blanket earth\u2019s surface temperature would have been at least 30 C\u00b0 lower. This would have been unfavorable to life at large. Remember that when average temperature of earth was 8\u00b0C, huma<\/a>ns faced the coldest ice age since their arrival. This was around 20 ka<\/a>BP<\/a>, when greater than 40% of land on earth was under a cover of snow throughout the year. Environment with earth at -15\u00b0C would have been far more inhospitable for life in general and for us huma<\/a>ns in particular.
Our earth contains the right set of elements and the right conditions for a stable system of the atmosphere, lithosphere and hydrosphere. This in turn hosts a diverse biosphere \u2013 a complex eco-system of approxima<\/a>tely ten million species. That includes us, Homo sapiens sapiens.
Next we will take a chronological account of how earth came to acquire its resources. That will introduce us to ma<\/a>ny other fortunes that we enjoy.
Acquisition of earth\u2019s inventory (4.6 to 3.96 Ga<\/a> BP<\/a>)<\/strong>
As stated above, accretion of earth and the planetary system took about 100 Ma<\/a> after the explosion of supernova<\/a>. During this period earth was a growing blob of hot molten liquid. As the ma<\/a>ss grew, gravity differentiated it into a heavy metal radioactive core inside and a lighter ma<\/a>gma<\/a> layer outside. The core was ma<\/a>intained hot by this molten rock layer insulation. This upper layer cooled off to form a solidified crust, or the lithosphere.
It is practically impossible to locate rocks that constituted the ancient lithosphere. This is because the lithosphere is constantly eroded or taken up by the molten ma<\/a>gma<\/a> layer. Half-life of earth\u2019s crust is estima<\/a>ted at ~ 600 Ma<\/a>. This means after a lapse of 600 Ma<\/a> half of the rocks of today will be lost, either through erosion or dissolution into ma<\/a>gma<\/a>. At this rate, less than 5% of the rocks formed around 4 Ga<\/a> BP<\/a> will have survived natural destruction. However, 4.3 Ga<\/a> old Zircon crystals have been located in Australia. The stone in which they crystallized is long lost by erosion.
Since we have no way of locating old rocks, constitution of early earth has been inferred from extra-terrestrial bodies such as moon, other planets, asteroids and comets. These studies reveal how earth came to acquire its resources.
Acquiring rare elements<\/strong>
As cited above, early solar system contained a large amount of ma<\/a>tter that had not condensed into the sun, planets or satellites. It existed as dust, comets, meteoroids, asteroids, mini-planets moving in diverse orbits around the sun. Orbits of this sundry ma<\/a>terial would cross the orbits of any of the planets or moons. When close by, gravity of planets attracted this ma<\/a>tter, resulting in a collision. For example, a study of lunar impact crater age statistics was possible after ma<\/a>n obtained samples of lunar rocks after 1969 CE. From these studies, it is estima<\/a>ted that at the time of its forma<\/a>tion, earth received an average of more than 1 impact of a sizable meteorite (~ 50 m size) every 1000 years. Impacts due to sma<\/a>ller asteroids, meteoroids, or comets and stellar dust would be innumerable and more frequent. Over the first few hundred million years of its existence, the molten earth must have literally cleaned this debris that came in its path. In the process earth enriched her elemental inventory. Measurements of elemental abundance on earth and in our solar system bear this out. Some qualitative highlights of relative elemental abundance on earth vis-\u00e0-vis our solar system are presented below.
\u2022 In general earth retained elements that could react. Unreactive elements like Ne are lost. Although not reactive, He, Ar, Kr are found in earth\u2019s crust as these elements are generated because of radioactive decay.
\u2022 Lighter molecules like H2 and He were lost from earth\u2019s atmosphere because their molecules had velocities higher than escape velocities. We will see some details later. However, hydrogen compounds (H2O, CH4, NH3, HCN) that were acquired from comets could rema<\/a>in on earth. Incidentally, comets were a big source of water, as they contain 20 to 90% of it.
\u2022 Elements like Nitrogen, Carbon, Sulfur, Oxygen, Phosphorus, and ma<\/a>ny metals have been retained because of their reactivity. All these elements are vital to life.
\u2022 Earth has a large stock of combined Ma<\/a>gnesium, as it is abundant in our solar system. Its reactivity renders it easy to retain. Incidentally Chlorophyll, that enables photosynthesis and oxygen generation, is a co-ordination compound of Ma<\/a>gnesium.
\u2022 Surprisingly, Iron is not as common as Ma<\/a>gnesium in the solar system. It is also not as reactive as Ma<\/a>gnesium. Yet earth seems to have acquired it in excess of the average in the solar system. This is yet another benevolent accident! Meteorites are known to contain a high proportion of a native iron. Earth could have had a larger share of meteorite impacts in its early life. The process of forma<\/a>tion of moon (Cf. next section) could also have contributed to the excess iron. Most of this iron exists as a molten ma<\/a>gnetic alloy with Nickel, Silicon and some dissolved Sulfur and Carbon, at the core of earth. Hemoglobin, another important molecule for oxygen metabolism in advanced life forms is a co-ordination compound of Iron.
\u2022 Fortunately, Earth is one of the few radioactive planets. Most of the radioactive metals are alloyed with iron in the core of earth. Radioactive decay keeps the core charged, hot and churning. Fortuitous advantage of this phenomenon is discussed later in the context of cosmic rays.
Note that our knowledge in this area is limited. From whatever we know, it is clear we have been very fortunate in ma<\/a>ny ways. But perhaps the most fortuitous accident was forma<\/a>tion of moon.
Forma<\/a>tion of Moon<\/strong>
We have seen how the orbiting earth wiped the comets, stellar dust and asteroids in its path. Yet, a belt of asteroids and mini-planets that did not coalesce into planets survived between Ma<\/a>rs and Jupiter. It exists even today. Scientists simulating early solar system have estima<\/a>ted that the evolving solar system was very unstable, leading to a release of large bodies in diverse orbits, leading to impacts with planets. Cosmologists estima<\/a>te that, sometime between 4.5 Ga<\/a> BP<\/a> and 4.1 Ga<\/a> BP<\/a>, an oblique impact with a ma<\/a>rs size asteroid (with diameter ~50% of earth, often named Theia) explains forma<\/a>tion of the moon, its present structure, and momentum.
By 4.5 Ga<\/a>BP<\/a>, earth had differentiated into a heavy metallic iron core and the molten ma<\/a>ntle of ma<\/a>gma<\/a>, or molten rocks. Like earth, the impacting body is also estima<\/a>ted to have been differentiated into a core and ma<\/a>ntle. With the impact, core of the impacting body merged into core of earth. A sma<\/a>ller ma<\/a>ss, somewhat more than the ma<\/a>ss of moon ma<\/a>ss was ejected into space with this impact. Ma<\/a>tter that rema<\/a>ined under earth\u2019s gravitational influence, later coalesced into our moon. The ejected ma<\/a>ss, and thus moon that formed out of it, included a relatively sma<\/a>ller piece of earth\u2019s core and a larger piece of the molten ma<\/a>ntle floating on the core. Most of the impacting body, especially its core, was absorbed into earth and enriched her. The energy release of this impact exceeded trillions of hydrogen bombs. It is just fortunate that this event did not occur after emergence of life forms, since no life could have survived such an event. However the event resulted into ma<\/a>ny advantages for the future. Ma<\/a>ny of the points cited below, are still being researched. Although not fully explained, they do ma<\/a>ke sense on the basis of observed facts and first principles.
The process ga<\/a>ve a moon, which along with the sun created a dynamic but predictable system of high and low tides on the shores, with ma<\/a>ny obvious advantages, such as mixing oceans and coastal seas and a dynamic ma<\/a>rine environment favoring various ecosystems, coastal currents that help communication, or even power generation.
The process also ga<\/a>ve us highly energetic liquid iron core at the center of earth, that ga<\/a>ve a ma<\/a>gnetosphere or ma<\/a>gnetic field around earth. Advantage of ma<\/a>gnetosphere is discussed later, in the context of cosmic rays.
The process also inclination of earth axis that produced annual cycles of weather, promoting diversity of life across the globe.
This high energy impact homogenized earth\u2019s inventory, and triggered diverse reactions between and within the core and ma<\/a>ntle, under the high temperature and pressure that resulted from the impact. The process is believed to have ended by 3.96 Ga<\/a>BP<\/a>. The sum total of enriched elemental inventory of earth stabilized and rema<\/a>ined nearly unchanged thereafter.
This inventory got resolved into atmosphere, hydrosphere and lithosphere. Atmosphere was the first to form.<\/p>\n

Views: 31<\/p>","protected":false},"excerpt":{"rendered":"

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