The Sun and Earth Form 4.6 Billion Years Ago
For the first 9.2 billion years after the Big Bang our Solar System did not exist. The formation of the sun and earth most likely began as the result of a supernova explosion of a nearby dying giant star. As this star began to run out of hydrogen fuel, it expanded into a great red giant and then collapsed on itself and exploded into a major supernova. Most of the giant star's matter would have been ejected into nearby outer space forming a dense cloud, a cosmic nebula. According to the Solar Nebula Model, generally accepted by most cosmologists, stars form in massive, dense nebula clouds mostly made up of dust and hydrogen molecules.
a) When a random dense region inside a cloud attracts enough matter, it starts to collapse under its own gravity. It forms a smaller cloud with a distinct center that is more dense than the rest of the cloud. (See a in the chart to the left.)
b) As the nebula contracts further, it spins faster and faster eventually flattening into a disk-like object. The center begins to take shape and a ring of material forms revolving around the center.
c) The center of the "proto-star" keeps growing denser and hotter until thermonuclear fusion begins. Hydrogen atoms then begin to combine to form helium atoms, which releases the energy that makes the star begin to shine. Our sun was born and began to shine about 4.6 billion years ago. The remaining material coalesces and swirls around the newborn sun at a faster and faster rate.
d) As the dust grains collide, they stick together to build larger objects (this process is called accretion, but it is not the same as black hole accretion). The gravities of these larger objects continue to grow and become strong enough to sweep up more and more of the local surrounding dust and gas molecules. Collisions between the larger items lead to the formation of big chunky objects called "planetesimals". Some planetesimals continue to collide to form planets, including earth which formed about 4.56 billion years ago. Other planetesimals form the solar system moons, asteroids, and comets. (For a more detailed formation explanation, see the Star and Planet Formationpage).
The early sun did not shine as brightly 4.6 billion years ago (bya) as it does today. The early sun was about 25% to 30% less bright than our current shining star. Without the heat of a bright sun, the earth needed some other mechanisms to ensure that it would not become just another gigantic frozen rock. Enter items such as meteors, volcanoes, and molter iron which made the earth what it is today. These phenomenon will be explained further in the sections below. Top
The Very Early Earth 4.5+ Billion Years Ago
The very early earth was entirely different than it is now. It was extremely hot, about 1,800°C. The temperatures of a thin hydrogen atmosphere and the surface of the earth were approximately the same. However, there was no molten metallic core like there is today. The core was probably about as warm as the surface and consisted of a magma ocean (liquid rock below the earth's surface) just like the rest of the planet.
The surface probably consisted of flowing lava (liquid rock on the earth's surface) with "islands" of solid materials (igneous rock). The crust had not yet formed. There was no water present and there was very little oxygen in the atmosphere. Although the earth was very hot, the surface was exposed to outer space which was only a few degrees above absolute zero. Therefore the earth was losing heat and cooling all the time, but very slowly in terms of present time frames. By 4.4 bya (billion years ago) scientists believe the earth had cooled below the boiling point of water (100°C) and some water was present. Scientists also believe the early earth was spinning much faster than today. A complete revolution, a day in our terms, was possibly only two to three hours long then, some say six to seven hours.
The earliest earthly evidence that has been preserved (at this time) is 4.4 bya. Because there is no proof of anything older, one has to classify information older than 4.4 bya as intelligent conjecture. However, since even the earliest data is so scanty (so few samples), one has to take everything older than 3.9 bya with a grain of salt. Top
The Crust, Mantle And Core Form 4.5 Billion Years Ago
As the earth cooled, it is estimated that the first crust formed on its surface roughly a hundred million years after the earth's formation, about 4.46 billion years ago (bya). (Scientific calculations suggest that only a few hundred years would be enough for the earth's entire surface to become solid.) Once the crust had formed, the heat from the interior no longer was exposed to the atmosphere but to the underside of the crust. This allowed the thin atmosphere and surface crust to cool much more quickly. Because of the many large collisions to the early earth, the crust may have been at least partially re-formed several times.
On average the crust today consists of about 10 miles of rock and loose materials. At its thickest points the crust is about 50 miles deep. Underneath the continents, the crust is about three times as thick as it is under the oceans.
The crust is actually not as solid as it feels to us and it shifts very, very slowly as the continents drift. The different layers of the earth's crust are referred to as plates, and their movements towards and away from each other are called plate tectonics. The continuous shifting of the earth's plate tectonics causes periodic earthquakes.
Under the crust is the mantle that extends to a depth of approximately 1,800 miles. The mantle is made of a thick, solid, rocky substance called peridotite, which is rich in iron and magnesium. The peridotite rock is heavier than surface rock. The mantle makes up about 85% of the total weight and mass of the earth. The average temperature of the mantle is about 2,400°C. The metallic core is an additional 2,200 miles to the center making the total diameter of the earth roughly 4,000 miles.
The Metallic Core Forms
How the earth came to have the arrangement of four different layers is somewhat of a mystery. Scientists think its beginnings were very messy, the result of many small bodies made up of rock and metals crashing together shortly after the formation of the sun. (Most asteroids are made up of silicate bodies with iron-nickel cores.)
One theory is that the planet in its early years was so hot that its rocks and metals melted. The molten rocks and metals in the "magma ocean" would then have separated into distinct layers as a result of their different densities. Iron and other metals would have drifted downward towards the planet's center, while silicates remained on top.
Another theory is that even if the early earth’s temperature was not hot enough to melt the silicates, the molten iron might still have separated out by percolating through the solid silicate layer (like water percolating through coffee grinds. See the yellow illustration at the left.) The idea was that pockets of molten iron (diapirs - from a greek word meaning "to pierce") trapped in the mantle layer would tunnel through the surrounding porous rock to create channels. These channels funneled the molten iron down towards the planet's metallic center core that was slowly forming. It is believed that after about two hundred million years the central core was well formed.
As shown in the first illustration above, the metallic core is made up of two parts - an inner core and an outer core. The outer core is about 1,400 miles thick and extends to a depth of about 3200 miles beneath the surface. The outer core is made up of super heated liquid molten lava. This lava is believed to be mostly iron and nickel and its average temperature is about 4,000°C. Finally, the inner core extends another 800 miles to the center of the earth. It is believed that the inner core is a solid ball of mostly iron and some nickel under a tremendous amount of pressure and at a temperature of about 5,400°C (roughly the same as the surface of the sun). Top
The Moon Forms 4.4 Billion Years Ago
Scientists believe that shortly after the earth formed about 4.4 billion years ago (bya), it collided with a Mars sized object, called Theia, in an event known as the "Giant Impact". It was not a head on collision, but one somewhat off center which changed the earth's axis of rotation by 23.5 degrees from its natural orientation. This collision set the earth spinning at a much faster rate. Scientists estimate that a day in the life of the early earth was only about 6 hours long.
The large "impactor" shot a cloud of vaporized rock off the earth's surface and into orbit around the earth. See the artist's image to the left. Over time, the cloud cooled and condensed into a ring of small, solid bodies, which eventually coalesced to form the moon. See the artist's image to the left below.
The presence of the continuously orbiting moon has stabilized the 23.5 degree axial tilt of the earth allowing the various earthly regions to have periodic seasons that encouraged life to form and grow. (This has not been true for Mars for example.)
The "impactor" probably would have melted all of the surface of the earth. According to some estimates, the earth would have been as hot as the sun (5,400°C) for about 10,000 years afterwards. If there was water before the impact, all of it would have evaporated and been blown away.
Several Collisions? Some scientists believe the moon came into existence after several large space bodies smashed into the earth one after the other, with the final one actually forming the moon. In the process several impacts repeatedly blew off our planet’s atmosphere and water. To get to the present day "neon-to-helium ratio" of gases in the atmosphere, these scientists say earth would have had to suffer multiple huge impacts. The earth probably lost its early atmosphere multiple times and its magma surface was partially or totally melted more than once.
A Gift Of Metals. Computer modelling of the collision between the earth and the large "impactor" shows that the bulk of the mantle of the impacting object and a proportion of the earth's silicate mantle were ejected into earth orbit and coalesced to form the moon. However, the metallic core of the impactor was not ejected into orbit but instead fell into the main body of the earth. This impacting core material was a ring of precious metals deposited into the earth's silicate mantle during the collision and subsequently recycled into workable ore deposits by plate tectonic processes over time - a nice gift from Theia.
Earth's Rotation Is Slowing. The moon formed much closer to earth than it is today and the earth rotated much faster. As the earth rotates, the moon's gravity causes the oceans to rise and fall. There is a slight bit of friction between the tides and the turning earth causing the rotation to slow down just a very little bit. As earth slows, it also lets the moon creep a bit away. We can use extremely accurate atomic clocks to measure exactly how much the rotation is slowing down. One hundred years from now, a day will be about 2 milliseconds longer than today - a very small change indeed.
Evidence For Theories. How do scientists know what happened 4.4 billion years ago? Researchers have samples from volcanoes in Iceland, which have rocks that are among the oldest on earth and retain the signatures of the earth's lower mantle that is closest to the planet's core. From these remnants, they are able to piece together the chemical history of the early earth.
Scientists have also found evidence of a magma ocean from rocks brought back from the moon. See the various Apollo landing sites in the moon photo to the left. Compare any two samples from early rocks on earth and those from the moon and one will find the proportions of oxygen and titanium isotopes are identical.
However, the isotope proportions found in rock samples from outer space meteorites and those from Mars are quite different. This means that somehow the moon was formed from earthen materials. The "Giant Impact" theory is the predominant theory among knowledgeable scientists.
Was The Collision Head-On? "The collision was so vigorous, so powerful, so rich in energy that it probably mixed the whole system very thoroughly," remarked Edward Young, lead author of a January, 2016 Science paper and a professor of Planetary and Space Sciences at UCLA. Thousands of Apollo rocks have been brought back from vastly different parts of the moon. "The proof is in the oxygen isotopes: that is, all the different moon rocks share the same oxygen isotopes in the exact same ratios as on the earth, and preliminary results of tungsten isotopes show the same thing. In other words, the earth and the moon are made of the same materials" says Edward Young.
In the "glancing blow" model, the moon would primarily have contained material from Theia with some mixture from earth. But analysis of salt-heavy lunar rocks and soil show it to be virtually identical to the floor of the earth's oceans. Hence, Theia must have hit the earth head on resulting in the earth and moon being a composite mixture of each other.
The above photo is of Buzz Aldrin, one of the first two men on the moon during Apollo 11 on July 20, 1969. Top
Heavy Bombardment 4.2 to 3.8 Billion Years Ago
About 4.2 to 3.8 billion years ago (bya), a period of intense comet and asteroid bombardment is believed to have peppered all the nearby planets including the earth and our moon. Although the earth is still impacted by meteors, it was much, much more intense than we see today. During this period, a very large number of asteroids collided with numerous bodies in our inner solar system, including Mercury, Venus, and Mars.
The evidence for these collisions comes primarily from the dating of over 2,000 lunar rock samples brought back by the Apollo astronauts. Many of the samples had clearly remelted after asteroid impacts. Analysis of the moon rocks showed that the most intense impact period was the Late Heavy Bombardment (LHB), a period of 200 million years from 4.0 to 3.8 bya. The moon's numerous craters are the prime record of past impacts. Unless obliterated by a subsequent larger impact, the craters remain relatively intact because the moon has little atmosphere to weather them. It also has almost no internal activity like the volcanoes, moving tectonic plates, and earthquakes that constantly recirculate the earth's crust and remake our planet's surface.
However, while evidence on earth for the early bombardment is scarce because most of it was destroyed by the resultant liquid magma ocean, it is not zero. In 2002 in West Greenland, researchers discovered a version of the element tungsten in sedimentary rocks in amounts normally not found on earth. The tungsten is believed to be of extraterrestrial origin and was estimated to be about 3.8 bya. In 2013, researchers from Denmark also found rocks in Greenland that contained the heavy metals osmium and iridium, which should have disappeared into the earth's core if they were part of the original materials that formed the earth. These scientists have concluded that they were from meteors that blasted the earth between 4.3 and 4.1 bya.
The ultimate cause of the Late Heavy Bombardment remains a mystery, but scientists have come up with several possible explanations for the triggers of the LHB. For example, there might have been a 10th and 11th planet that collided and scattered their remains. It is also possible that the outward migration of Neptune scattered comets and small planetesimals from the asteroid belt. And, the close passage of a neighborhood star could also have initiated widespread scattering of the asteroid belt. The fact is, no one really knows the source of the Late Heavy Bombardment. Top
The Early Atmosphere
The newly formed earth probably did not have a true atmosphere. Instead, gases which were dissolved in the magma and lava, escaped and probably formed a thin haze above the planet. If a thin atmosphere did exist, it almost certainly was stripped away by the Giant Impact that created the moon. In fact many of the larger impacts that occurred during the early earth would have blasted any accumulated atmospheric gas into space. Therefore, the earth probably did not have a significant atmosphere until the Late Heavy Bombardment period ended.
The main theory explaining the formation of the first atmosphere is a process called volcanic outgassing. As the interior of the earth became very hot, the heat caused some gases to surface into the atmosphere through volcanic emissions. These gases included ammonia, carbon dioxide, carbon monoxide, hydrogen, methane, nitrogen, sulfur dioxide, and water vapor. An additional few minor gases, such as argon, were added by the decay of some radioactive elements within the earth. In particular, note that there probably was only a trace of precious oxygen which makes up about 20% of today's atmosphere.
The early earth's atmosphere was much thicker than it is now. Some scientists believe earth's early atmosphere only contained ammonia, helium, hydrogen, and methane, much like the present atmosphere of Jupiter. Others believe it may have contained a large amount of carbon dioxide, as does the atmosphere of Venus. In any case, it was a very harsh and poisonous mixture of gases that any form of life would have had a hard time surviving. Top
Oceans Form About 4.2 Billion Years Ago
Water currently covers about 71% of the earth's surface. Where did all this water come from?
As the earth cooled below the boiling point of water (100°C) about 4.4 bya, the water vapor the volcanoes had been spewing eventually condensed and formed lakes and maybe even some rivers and small seas. However, scientific calculations indicate there probably was not enough volcano water to form today's huge oceans. There probably was some very early water, but it all would have been vaporized and blown away by the Giant Impact moon event mentioned above. In fact, there probably were quite a few major impacts that followed, each which would have depleted a significant amount of the early earth's water. So where did all our current water come from?
The water we have today must have originated after the Giant Impact incident. The leading theory suggests that at some point in the evolution of the Solar System, the outward drift of the orbits of Saturn and Jupiter caused meteor type material from the outer Solar System to be flung inwards into the present day asteroid belt and eventually impacting the earth during the bombardment era.
Astronomers realized that there are two obvious sources of water, comets and asteroids, because both contain ice. Either one could easily have delivered enough water to fill the oceans. Recent analyses of their chemical makeups are tipping the scale toward asteroids. Researchers reported in 2012 that the ratios of hydrogen and nitrogen isotopes in asteroids better match those that we find here on earth. However, the analyses were based on a limited number of samples and so the results are not final. Top
Volcanoes Were Everywhere
The early earth was covered with many volcanoes. For a long time, most of the volcanoes in the world were underwater. See the under water illustration to the left. Scientists have found that land volcanoes on thick early crusts were pretty much absent during the early planet times.
Underwater volcanic eruptions differ in some significant ways from above ground eruptions. The lower temperatures of underwater eruptions mean that the magma cools rather quickly and produces gases like hydrogen sulfide which would have sucked up any available oxygen.
The under water volcanically produced early atmosphere was a nasty and noxious blend that would have killed most life. Volcanic eruptions back then are thought to be just like current eruptions. (See the Early Atmosphere above for its gas contents.) The ancient earth should have had oxygen in its early atmosphere but something was converting the oxygen and removing it. Scientists believe that under water volcanoes effectively scrubbed almost all the oxygen from the atmosphere, binding it into oxygen containing minerals.
Above ground volcanoes (see the illustration to the left) release gases that stay hot and do not end up sucking all the oxygen out of the environment. When the large continents began to form much later in the planets life, about 2.5 billion years ago, more volcanoes were above ground than below. Having a mix of volcanoes dominated by land volcanoes allowed oxygen to exist in the atmosphere.
Shortly thereafter, the atmospheric oxygen levels began to rise and slowly climbed to our current level of 20%. Very early life did not depend on oxygen. After the earth’s oxygen levels rose, conditions were ripe for the development of complicated life.
For more information on the beginnings of life, see the Early Life page.