Earth History on a 24-hour Time Scale
If Earth's 4.54 billion year history is compressed into 24 hours of a day, the origin of life occurs at about 4.a.m., single-celled life forms evolve at around 2.08 p.m., dinosaurs die at about 10:24 p.m., mammals evolve around 11.39 p.m. and humans make an appearance about 2 minutes before midnight at 11.58 p.m. Our entire human history gets compressed in a time frame of less than 2 minutes!Earth is our home and perhaps the safest place in the universe. We have no idea about how lucky we are to be here. A series of serendipitous events led to origin of life on the planet. Compared to outer space, this third rock from the Sun is more than a cozy place. It is almost
tailor-made for life. Some things that make life on Earth possible are abundance of water, a magnetic field blanket (
that protects us from high energy solar radiation), an ozone layer, and the right mixture of elements in the atmosphere, especially oxygen, and a geologically stable planetary crust.
If you look at all the factors, it is almost like the universe has
conspired to create life on Earth. By this comment, no divine origin is hinted. It is just the laws of nature, molding matter and energy, into evermore complex forms. Through years of painstaking research and the collection of irrefutable evidence, we can now peep into the past, to know how our planet came to be. A brief overview of Earth's evolution, from being a part of a gargantuan gas blob, to being a habitable planet with creatures who question its origin, is presented in this article.
How was the Earth created? This question has bewildered each and every civilization that has existed on the planet. Each wanted to know how our world came to be. They came up with answers, according to their own philosophies and understanding of how the world works. Most painted a naive picture, attributing the origin of our planet, to the will of some supernatural being. What modern science found was an
impersonal universe, which works through physical laws, that shape matter and energy, to create entities of ever-evolving complexity.
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Earth photographed by Apollo 12 (Credit: NASA)
The most accurate picture of Earth's origin, backed by observational evidence, was constructed in the 20
th century, when the tools of technology that could explore this subject deeply, became available. Let us recount the story, put together through years of research in astronomy, geophysics, and geology. What follows is a hypothesis that can best describe the observed state of the solar system and our dear planet. The 'Solar Nebula' theory presented here, has been indirectly verified through the observation of extrasolar planetary systems (
planets revolving around other stars), which are still in a nascent stage of creation. Let's begin our journey into the past.
It All Started with The Big Bang!
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Image Credit: NASA
If you really want to know where it all began, you must go way back to the beginning, the origin of the universe itself, to the Big Bang. That story has been narrated in detail, in another article, titled, '
How was the universe created?'. We will fast forward from the Big Bang to a time when our galaxy had formed and a third-generation star, our Sun, was taking shape.
Gas Cloud Collapse
An interstellar molecular Hydrogen cloud began collapsing under its own gravity, to eventually create the Sun. We are talking about a gargantuan molecular cloud that may have stretched as long as 20 parsecs (
more than 65 light years). The Sun and the solar system formed from the compression of a
small fragment of this giant cloud. To know more, refer to this article detailing the
formation of the Sun. Here, we will focus on the protoplanetary disk, which surrounded this central protostar, that eventually condensed to become our solar system.
This cloud of gas and dust, a fragment of the original molecular cloud, contained remnants from previous dead stars and supernovas, that enriched it with heavy elements. As the cloud shrank in size, collapsing under gravity, it started rotating and got flattened in the process.
Now, surrounding the
infant Sun, was the 'Solar Nebula', a revolving potent gaseous cloud that eventually accreted to become our solar system. This theory was first put forward in the 18
th century by Pierre-Simon Laplace, Immanuel Kant, and Emanuel Swedenborg. With modern observational tools and research, the theory has been modified to provide a more accurate picture. Observation of protoplanetary disks around young stars confirmed the accuracy of the theory in modeling the formation of our solar system.
Returning back to our story, as the central bulge of this protoplanetary disk formed the Sun, the surrounding debris of gas and dust started condensing. Consider the whole cloud to be a huge centrifuge. It is not an exact analogy, but a close one. When a centrifuge rotates, heavier and more massive elements are thrown outwards. Similarly, heavier elements in the gas cloud were slowly thrown towards the border of the whole disk, just outside the photosphere of the T Tauri Sun (
explained further).
Protoplanetary Disk Condensation
About 4.5 billion years ago, Sun started its
nuclear fusion engine and officially became a star. In this initial phase, it was surrounded by a flat disk of debris. This phase in Sun's life is called the T Tauri phase (
because of an observed star called T Tauri, which is in this phase at present).
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A graphic illustrating formation of planets from the protoplanetary disk (Credit: NASA)
Just as clouds grow through aggregation of water molecules around a dust droplet, matter molecules and atoms in the protoplanetary disk started aggregating to form larger solid objects. These aggregates grew at the rate of a few centimeters every year (
At that stage, there was no idea of a year, as Earth itself was yet to form!).
The region in vicinity of the Sun, to the point where the orbit of Mars lies today, was at a very high temperature. So volatile atoms with low atomic weights could not condense in that region. Hence, only heavy metallic compounds (
made up of elements with high melting points like Iron, Nickel, and Silicates) condensed in this region. These heavy metal compounds, which condensed in the inner circle of the protoplanetary disk, eventually formed the terrestrial planets (
Mercury, Venus, Earth, and Mars).
The aggregation continued for about 100,000 years after the Sun formed. Once this phase ceased, there were thousands of small planetoids (
about 10 km long), that pervaded the region around the Sun.
Over a period of millions of years, they coalesced through collision and condensation to form the terrestrial planets. One such planet, which was the aggregation of such small planetoids, created 150 million kilometers away from the Sun, was Earth (
rather the proto-Earth).
In the region, beyond this high temperature region, which was relatively colder, volatile molecules made up of atoms like Hydrogen, Helium, and Methane (
which had low atomic weights), condensed. These aggregates of molecules kept growing and swallowing most of the leftover Hydrogen and Helium after the Sun evolved, becoming the gas giants (
Jupiter, Saturn, Venus), as we know them today.
As the fraction of heavy elements in the whole content of our solar nebula was quite less, compared to Hydrogen and Helium, terrestrial planets like Earth are small, compared to the gas giants.
Over a period of 100 million years, most of the loose debris and planetoids were either wiped out by the solar wind or they coalesced into the large planetoids, forming a solar system, which was beginning to look like what it is today. Now let us see how the Earth was born from a
protoplanetary embryo.
The Proto-Earth Phase
About 4.54 billion years ago, through accretion of small planetoids, numbering in thousands, an entity which we call proto-Earth formed. Our planet grew through accretion of these planetoids and their collisions. This generated a tremendous amount of heat. This, combined with an increase in density, led to the melting of all heavy elements like Iron and Nickel, that were contained in the proto-Earth's composition.
As our planet was in a molten condition during this time, these heavy elements sank and got concentrated at the planet's center of mass, forming the hot metallic core. This separation of the primitive liquid metal elements, 10 million years after proto-Earth formed, is called '
Iron catastrophe'.
This event created the layered structure of our planet, with a solid metallic core, liquid metal mantle around it, with a solid crust forming later. The iron catastrophe was responsible for creating Earth's magnetic field, generated by the metallic liquid mantle, flowing around a solid core. During this period, Earth was surrounded by an atmosphere, primarily made up of Hydrogen and Helium. This thin envelope was however, blown away by the solar wind.
Giant Impact Created Moon
About 40 million years after the creation of Earth, a giant object about the size of today's Mars (
named Theia) impacted the hot and molten Earth, jettisoning a large part of molten material into space, which accreted to create our
Moon.
This giant impact hypothesis is widely accepted due to the similarity in composition of the Moon and Earth's crust. Another piece of supportive evidence is the absence of a liquid metallic core in our only natural satellite.
Earth Cooled to Form a Crust, Atmosphere, and Oceans
About 150 million years later, Earth had cooled enough to create a primitive basaltic crust, which was not same as the one we have today. Still, the magma was hotter than what it is today (
at about 1600o C). The first atmosphere was created from gases and moisture that escaped from fractured crust and volcanoes. Water was derived from millions of impacting planetoids, which were loaded with ice. They also enriched Earth with compounds like Carbon dioxide, Methane, Nitrogen, and Ammonia.
About 500 million years later, the first continental crust formed. These continents drifted on a sea of mantle, causing plate tectonics. They drifted and collided over millions of years, to create the world as we know it today. With further cooling, clouds formed on Earth, causing the first rains and creating oceans. The earlier basaltic crust was mostly lost due to the bombardment of asteroids.
Origin of Life
With the creations of oceans and enrichment of the atmosphere with gases like Oxygen, Methane, and Nitrogen, the stage for arrival of the first carbon-based life forms was set. From fossil evidence, it can be estimated that life on Earth, emerged about 3.8 to 3.5 billion years ago.
How the first unicellular life forms evolved in oceans, is still a matter of research. Replicator molecules evolved from basic chemical elements, in the oceanic ecosystem, which were the precursors of RNA (Ribonucleic Acid) and DNA (Deoxyribonucleic Acid), the genetic material that makes all organic life. Evolution from unicellular life to the arrival of the human species is a deep subject of discussion, beyond the scope of this article. (
A good reference, if you are interested in knowing more about the origin of life is 'The Selfish Gene', by Dr. Richard Dawkins.) Also, there is a debate about whether life evolved on Earth or was seeded by impacting asteroids (called the
Pan Spermia theory), that are known to be teeming with microbial life.
The future of our planet is closely tied to the fate of the Sun. About 5 billion years into the future, our star will die a slow death and expand to several times its current size, destroying our planet in the process. Considering that we figured out interplanetary travel in about a century, by then, humans (
or whatever our species will have evolved into) may have figured out interstellar travel, populating other stellar systems.
Still, many Earth mysteries remain to be solved like the reason for occurrence of ice ages, the mechanism behind creation of the terrestrial magnetic field, its reversal, and much more. If some young readers of this article are motivated to explore these mysteries, I would say that my job is well done.
