4500-3800 Ma

On this date... (hide)

§In a Galaxy Far Far Away

April 29, 2009 the gamma radiation from GRB 090423, which took 13 billion years to reach earth, was detected by a NASA satellite called Swift. The infrared radiation was detected by the Gemini Observatory in Hawaii. This dying star is the oldest known object in the universe. Detecting GRB 090423 allowed scientists to focus their knowledge about the early universe. The event occurred roughly 630 million years after the Big Bang, confirming that massive stellar births (and deaths) did indeed occur in the very early universe.

§ Hadean Eon

The formation of the Earth and the simultaneous formation of the Sun and other bodies of the solar system resulted from the contraction of a solar nebula. The Earth was a hot glowing ball of white hot gases with a temperature that was millions of degrees Fahrenheit. This was caused by particles of gases being drawn together and compressed, giving off a lot of heat. The Solar System (including the Earth) formed from a large, rotating cloud of interstellar dust and gas called the solar nebula. It was composed of hydrogen and helium produced in the Big Bang, as well as heavier elements ejected by supernovas. About 4.6 billion years ago, the solar nebula began to contract, possibly due to the shock wave of a nearby supernova. Such a shock wave would have caused the nebula to gain angular momentum. As the cloud began to accelerate its rotation, gravity and inertia flattened it into a protoplanetary disk oriented perpendicularly to its axis of rotation. Most of the mass concentrated in the middle and began to heat up, but small perturbations due to collisions and the angular momentum of other large debris created the means by which protoplanets up to several kilometres in size began to form.

The infall of material, increase in rotational speed and the crush of gravity created an enormous amount of kinetic heat at the center. Its inability to transfer that energy away through any other process at a rate capable of relieving the build-up resulted in the disk's center heating up. Ultimately, nuclear fusion of hydrogen into helium began, and eventually, after contraction, a T Tauri star ignited to create the Sun. Meanwhile, as gravity caused matter to condense around the previously perturbed objects outside the gravitational grasp of the new sun, dust particles and the rest of the protoplanetary disk began separating into rings. Successively larger fragments collided with one another and became larger objects, ultimately becoming protoplanets. These included one collection approximately 150 million kilometers from the center: Earth. The planet formed about 4.54 billion years ago (within an uncertainty of 1%), and the planet was largely completed within 10–20 million years. The solar wind of the newly formed T Tauri star cleared out most of the material in the disk that had not already condensed into larger bodies.

Computer simulations have shown that planets with distances equal to the terrestrial planets in our solar system can be created from a protoplanetary disk.

§ Origin of the Earth's core and first atmosphere

The Proto-Earth grew by accretion, until the inner part of the protoplanet was hot enough to melt the heavy, siderophile metals. Due to their larger densities such (now liquid) metals began to sink to the Earth's center of mass. This so called iron catastrophe resulted in a separation of a primitive mantle and a (metallic) core only 10 million years after the Earth began to form. This produced the layered structure of Earth and also set up the formation of Earth's magnetic field.

During the accretion of material to the protoplanet, a cloud of gaseous silica must have surrounded the Earth, to condense afterwards as solid rocks on the surface. What was left surrounding the planet was an early atmosphere of light (atmophile) elements from the solar nebula, mainly hydrogen and helium, but the solar wind and Earth's heat would have driven off this atmosphere.

This changed when Earth was about 40% its present radius, and gravitational attraction retained an atmosphere which included water.

§The giant impact

A rare characteristic of our planet is its large natural satellite, the Moon. During the Apollo program, rocks from the Moon's surface were brought back to Earth. Radiometric dating of these rocks has shown the Moon to be 4527 ± 10 million years old, about 30 to 55 million years younger than other bodies in the solar system. Another special feature is the relatively low density of the Moon, which must mean it does not have a large metallic core, like all other terrestrial bodies in the solar system. In fact, the Moon has a bulk composition closely resembling the Earth's mantle and crust together, without the Earth's core. This has led to the giant impact hypothesis, the idea that the Moon was formed during a giant impact of the proto-Earth with another protoplanet. The Moon formed by accretion of the material blown off the mantles of the proto-Earth and impactor.

The impactor, sometimes named Theia, is thought to have been a little smaller than the current planet Mars. It could have formed by accretion of matter about 150 million kilometres from both the Sun and Earth, at their fourth or fifth Lagrangian point. Its orbit may have been stable at first, but destabilized as Earth's mass increased due to accretion of more and more matter. Theia swung back and forth relative to Earth until it finally collided with Earth an estimated 4.533 billion years ago.

Models show that when an impactor this size struck the proto-Earth at a low angle, a lot of material from the mantles (and proto-crusts) of the proto-Earth and the impactor was ejected into space, where much of it stayed in orbit around the Earth. This material would eventually form the Moon. However, the metallic cores of the impactor would have sunk through the Earth's mantle to fuse with the Earth's core, depleting the Moon of metallic material. The giant impact hypothesis thus explains the Moon's abnormal composition. The ejecta in orbit around the Earth could have condensed into a single body within a couple of weeks. Under the influence of its own gravity, the ejected material became a more spherical body: the Moon.

The radiometric ages show the Earth existed already for at least 10 million years before the impact, enough time to allow for differentiation of the Earth's primitive mantle and core. Then, when the impact occurred, only material from the mantle was ejected, leaving the Earth's core of heavy siderophile elements untouched.

The impact had some important consequences for the young Earth. It released a gigantic amount of energy, causing both the Earth and Moon to be completely molten. Immediately after the impact, the Earth's mantle was vigorously convecting, the surface was a large magma ocean. Due to the enormous amount of energy released, the planet's first atmosphere must have been completely blown off. The impact is also thought to have changed Earth’s axis to produce the large 23.5° axial tilt that is responsible for Earth’s seasons (a simple, ideal model of the planets’ origins would have axial tilts of 0° with no recognizable seasons). It may also have sped up Earth’s rotation.

§Origin of the oceans and atmosphere

Because the Earth lacked an atmosphere immediately after the giant impact, cooling must have been fast. Within 150 million years a solid crust with a basaltis composition must have formed. The felsic continental crust of today did not yet exist. Within the Earth, further differentiation could only begin when the mantle had at least partly solidified again. Nevertheless, during the early Archaean (about 3.0 billion years ago) the mantle was still much hotter than today, probably around 1600°C. This means its fraction that was partially molten was still much larger than today.

Steam escaped from the crust, and more gases were released by volcanoes, completing the second atmosphere. Additional water was imported by bolide collisions, probably from asteroids ejected from the outer asteroid belt under the influence of Jupiter's gravity.

The large amount of water on Earth can never have been produced by volcanism and degassing alone. It is assumed the water was derived from impacting comets that contained ice.[21] Though most comets are today in orbits further away form the Sun than Neptune, computer simulations show they were originally far more common in the inner parts of the solar system. However, most of the water on Earth was probably derived from small impacting protoplanets, objects comparable with today's small icy moons of the outer planets. Impacts of these objects can have enriched the terrestrial planets (Mercury, Venus, the Earth and Mars) with water, carbon dioxide, methane, ammonia, nitrogen and other volatiles. If all water in the Earth's oceans was derived from comets alone, a million impacting comets are required to explain the oceans. Computer simulations show this is not an unreasonable number.

As the planet cooled, clouds formed. Rain gave rise to the oceans. Recent evidence suggests the oceans may have begun forming by 4.2 billion years ago. At the start of the Archaean eon, the Earth was already covered with oceans. The new atmosphere probably contained ammonia, methane, water vapor, carbon dioxide, and nitrogen, as well as smaller amounts of other gases. Any free oxygen would have been bound by hydrogen or minerals on the surface. Volcanic activity was intense and, without an ozone layer to hinder its entry, ultraviolet radiation flooded the surface.

§ Traditional view

This eon extends in time from the formation of the planet to about 3.8 billion years ago. The name was derived from the state of the Earth at the time, what some considered to have been like Hades from the Greek meaning Hell. Because little geological evidence exists to teach us of this period it has not been further divided into eras or even further into periods.

The Hadean describes the period as the Earth formed. Largely in a molten state, denser materials such as iron sank toward the center of the planet, while the lighter silicaceous and basaltic materials "floated" toward the surface. The Earth's crust was therefore formed of the lightest of the solid elements. The early atmosphere was largely formed by volcanic action. Eruptions ejected water, methane, ammonia, hydrogen, nitrogen, and carbon dioxide.

The Earth experienced large impacts, and some believe a Mars-sized object struck the Earth and ejected the moon. But during this period the bombardment began to subside. The crust began to cool, forming rocks that are now considered the oldest rocks on the planet. As cooling took place oceans began to form through atmospheric condensation.

§ Recent view

Background

Researchers at the Australian National University developed a technique for examining zircon crystals to determine the temperature and possible environmental conditions of early Earth. The reason for using zircons is that they predate the world's oldest rocks by 400 million years, allowing investigations further back in geologic time than was before possible.

The temperature data gathered from the zircons supports the existence of a wet Earth within 200 million years of the solar system’s formation allowing it to support life far earlier than was before believed.

Research data support recent theories that Earth began a pattern of crust formation, erosion, and sediment recycling as early in its evolution as 4.35 billion years ago (4350 Ma).

In a new paper in the journal Earth and Planetary Science Letters, a team of scientists led by UW-Madison geologists Takayuki Ushikubo, Valley and Noriko Kita show that rocky continents and liquid water existed at least 4.3 billion years ago and were subjected to heavy weathering by an acrid climate.

§Creation Myths (New and incomplete)

§Japanese Creation Myth

"Of Old, Heaven and Earth were not yet separated, and the In and Yo were not yet divided. They formed a chaotic mass like an egg which was of obscurely defined limits and contained germs.

The purer and clearer part was thinly drawn out, and formed Heaven, while the heavier and grosser element settled down and became Earth.

The finer element easily became a united body, but the consolidation of the heavy and gross element was accomplished with difficulty.

Heaven was therefore formed first, and Earth was established subsequently.

Thereafter Divine Beings were produced between them. The soil of which lands were composed floated about in a manner which might be compared to the floating of a fish sporting on the surface of the water.

At this time a certain thing was produced between Heaven and Earth. It was in form like a reed-shoot. Now this became transformed into a God, and was called Kuni-toko-tachi no Mikoto. Next there was Kuni no sa-tsuchi no Kimoto and next Toyo-kumu nu no Mikoto, in all three deities." - Nihongi

§Chinese Creation Myth

"According to the third-century BC Huainanzi: 'Before Heaven and Earth had taken form all was vague and amorphous. Therefore it was called The Great Beginning. The Great Beginning produced emptiness, and emptiness produced the universe. The universe produced qi [vital force or energy], which had limits. That which was clear and light drifted up to become Heaven while that which was heavy and turbid solidified to become earth ... The combined essences of Heaven and Earth became the yin and yang.'

"A more popular, though later, version of this genesis myth describes the primordial environment as not just amorphous but 'opaque, like the inside of an egg'; and it actually was an egg to the extent that, when broken, white and yolk separated. The clear white, or yang, ascended to become Heaven and the murky yolk, or yin, descended to become Earth. Interposed between the two was the egg's incubus, a spirit called Pan Gu. Pan Gu kept his feet firmly in the earth and his head in the heavens as the two drew apart. 'Heaven was exceedingly high, Earth exceedingly deep, and Pan Gu exceedingly tall,' says the Huainanzi. Though not the creator of the universe, Pan Gu evidently served as some kind of agent in the arrangement of it. ...

"Less relevant still in Chinese tradition is the origin of man. In another version of the Pan Gu story, it is not Pan Gu's lanky adolescence which suggests a degree of personal agency in the creative process but his posthumous putrescence. In what might be called a decomposition myth, as Pan Gu lay dying, it is said that:

" '[his] breath became the wind and the clouds; his voice became the thunder; his left eye became the sun, and his right the moon; his four limbs and five torsos became the four poles and the five mountains; his blood became the rivers; his sinews became geographic features; his muscles became the soils in the field; his hair and beard became stars and planets; his skin and its hairs became grasses and trees; his teeth and bones became bronzes and jades; his essence and marrow became pearls and gemstones; his sweat became rain and lakes; and the various worms in his body, touched by the wind, became the black-haired commoners."

§ Sources

Year Index | Eoarchean Era | >>