Solar System History: How Was the Earth Formed?

Winter Solstice


Just how did the Earth — our home and the place where life as we know it evolved — come to be created in the first place? In some fiery furnace atop a great mountain? On some divine forge with the hammer of the gods shaping it out of pure ether? How about from a great ocean known as Chaos, where something was created out of nothing and then filled with all living creatures?

If any of those accounts sound familiar, they are some of the ancient legends that have been handed down through the years that attempt to describe how our world came to be. And interestingly enough, some of these ancient creation stories contain an element of scientific fact to them.

When it comes to how the Earth was formed, forces that can only be described as fiery, chaotic, and indeed godlike, were involved. However, in the past few centuries, research and refinements made in what is today known as Earth Sciences have allowed scientists to assemble a more empirical and scientific understanding of how our world was formed.

Basically, scientists have ascertained that several billion years ago our Solar System was nothing but a cloud of cold dust particles swirling through empty space. This cloud of gas and dust was disturbed, perhaps by the explosion of a nearby star (a supernova), and the cloud of gas and dust started to collapse as gravity pulled everything together, forming a solar nebula — a huge spinning disk. As it spun, the disk separated into rings and the furious motion made the particles white-hot.

The center of the disk accreted to become the Sun, and the particles in the outer rings turned into large fiery balls of gas and molten-liquid that cooled and condensed to take on solid form. About 4.5 billion years ago, they began to turn into the planets that we know today as Earth, Mars, Venus, Mercury, and the outer planets.

The first era in which the Earth existed is what is known as the Hadean Eon. This name comes from the Greek word “Hades” (underworld), which refers to the condition of the planet at the time. This consisted of the Earth’s surface being under a continuous bombardment by meteorites and intense volcanism, which is believed to have been severe due to the large heat flow and geothermal gradient dated to this era.

Outgassing and volcanic activity produced the primordial atmosphere, and evidence exists that liquid water existed at this time, despite the conditions on the surface. Condensing water vapor, augmented by ice delivered by comets, accumulated in the atmosphere and cooled the molten exterior of the planet to form a solid crust and produced the oceans.

It was also during this eon – roughly 4.48 billion years ago (or 70–110 million years after the start of the Solar System) – that the Earth’s only satellite, the Moon, was formed. The most common theory, known as the “giant impact hypothesis” proposes that the Moon originated after a body the size of Mars (sometimes named Theia) struck the proto-Earth a glancing blow.

It is believed that 4.4 billion years ago, a celestial body (Theia) slammed into Earth and produced the Moon. Image Credit: NASA/JPL-Caltech
It is believed that 4.4 billion years ago, a celestial body (Theia) slammed into Earth and produced the Moon. Image Credit: NASA/JPL-Caltech

The collision was enough to vaporize some of the Earth’s outer layers and melt both bodies, and a portion of the mantle material was ejected into orbit around the Earth. The ejecta in orbit around the Earth condensed, and under the influence of its own gravity, became a more spherical body: the Moon.

The Hadean Eon ended roughly 3.8 billion years ago with the onset of the Archean age. Much like the Hadean, this eon takes it name from a ancient Greek word, which in this case means “beginning” or “origin.” This refers to the fact that it was during this period that the Earth had cooled significantly and life forms began to evolve.

Most life forms today could not have survived in the Archean atmosphere, which lacked oxygen and an ozone layer. Nevertheless, it is widely understood that it was during this time that most primordial life began to take form, though some scientists argue that many lifeforms may have occurred even sooner during the late Hadean.

At the beginning of this Eon, the mantle was much hotter than it is today, possibly as high as 1600 °C (2900 °F). As a result, the planet was much more geologically active, processes like convection and plate tectonics occurred much faster, and subduction zones were more common. Nevertheless, the presence of sedimentary rock date to this period indicates an abundance of rivers and oceans.

The super-continent Pangea during the Permian period (300 - 250 million years ago). Credit: NAU Geology/Ron Blakey
The super-continent Pangea during the Permian period (300 – 250 million years ago). Credit: NAU Geology/Ron Blakey

The first larger pieces of continental crust are also dated to the late Hadean/early Achean Eons. What is left of these first small continents are called cratons, and these pieces of crust form the cores around which today’s continents grew. As the surface continually reshaped itself over the course of the ensuing eons, continents formed and broke up.

The continents migrated across the surface, occasionally combining to form a supercontinent. Roughly 750 million years ago, the earliest-known supercontinent called Rodinia began to break apart, then recombined 600 – 540 million years ago to form Pannotia, then finally Pangaea. This latest supercontinent broke apart 180 million years ago, eventually settling on the configuration that we know today. (See graphics from Geology.com here)

Since that time, a mere blip on the geological time scale, all the events that we consider to be “recent history” took place. The dinosaurs ruled and then died, mammals achieved ascendancy, hominids began to slowly evolve into the species we know as homo sapiens, and civilization emerged. And it all began with a lot of dust, fire, and some serious impacts. From this, the Sun, Moon, Earth, and life as we know it were all created.

We have written many articles about the Earth for Universe Today. Here’s an article about the temperature of Earth, and here are some facts about the planet Earth.

If you’d like more info on Earth, check out NASA’s Solar System Exploration Guide on Earth. And here’s a link to NASA’s Earth Observatory.

We’ve also recorded an episode of Astronomy Cast all about planet Earth. Listen here, Episode 51: Earth.

Further reading: Windows to the Universe, BBC.

How Old Is The Earth?

How Old Is The Earth?

This article comes from the Universe Today archive, but was updated with this spiffy video.

How old is the Earth? Scientists think that the Earth is 4.54 billion years old. Coincidentally, this is the same age as the rest of the planets in the Solar System, as well as the Sun. Of course, it’s not a coincidence; the Sun and the planets all formed together from a diffuse cloud of hydrogen billions of years ago.

In the early Solar System, all of the planets formed in the solar nebula; the remnants left over from the formation of the Sun. Small particles of dust collected together into larger and larger objects – pebbles, rocks, boulders, etc – until there were many planetoids in the Solar System. These planetoids collided together and eventually enough came together to become Earth-sized.

At some point in the early history of Earth, a planetoid the size of Mars crashed into our planet. The resulting collision sent debris into orbit that eventually became the Moon.

How do scientists know Earth is 4.54 billion years old? It’s actually difficult to tell from the surface of the planet alone, since plate tectonics constantly reshape its surface. Older parts of the surface slide under newer plates to be recycled in the Earth’s core. The oldest rocks ever found on Earth are 4.0 – 4.2 billion years old.

Scientists assume that all the material in the Solar System formed at the same time. Various chemicals, and specifically radioactive isotopes were formed together. Since they decay in a very known rate, these isotopes can be measured to determine how long the elements have existed. And by studying different meteorites from different locations in the Solar System, scientists know that the different planets all formed at the same time.

Failed Methods for Calculating the Age of the Earth
Our current, accurate method of measuring the age of the Earth comes at the end of a long series of estimates made through history. Clever scientists discovered features about the Earth and the Sun that change over time, and then calculated how old the planet Earth is from that. Unfortunately, they were all flawed for various reasons.

  • Declining Sea Levels – Benoit de Maillet, a French anthropologist who lived from 1656-1738 and guessed (incorrectly) that fossils at high elevations meant Earth was once covered by a large ocean. This ocean had taken 2 billion years to evaporate to current sea levels. Scientists abandoned this when they realized that sea levels naturally rise and fall.
  • Cooling of the EarthWilliam ThompsonWilliam Thompson, later known as Lord Kelvin, assumed that the Earth was once a molten ball of rock with the same temperature of the Sun, and then has been cooling ever since. Based on these assumptions, Thompson calculated that the Earth took somewhere between 20 and 400 million years to cool to its current temperature. Of course, Thompson made several inaccurate assumptions, about the temperature of the Sun (it’s really 15 million degrees Kelvin at its core), the temperature of the Earth (with its molten core) and how the Sun is made of hydrogen and the Earth is made of rock and metal.
  • Cooling of the Sun – In 1856, the German physicist Hermann Ludwig Ferdinand von Helmholtz attempted to calculate the age of the Earth by the cooling of the Sun. He calculated that the Sun would have taken 22 million years to condense down to its current diameter and temperature from a diffuse cloud of gas and dust. Although this was inaccurate, Helmholtz correctly identified that the source of the Sun’s heat was driven by gravitational contraction.
  • Rock Erosion – In his book, The Origin of Species by Means of Natural Selection, Charles Darwin proposed that the erosion of chalk deposits might allow for a calculation of the minimum age of the planet. Darwin estimated that a chalk formation in the Weald region of England might have taken 300 million years to weather to its current form.
  • The Moon

  • Orbit of the Moon – George Darwin, the son of Charles Darwin, guessed that the Moon might have been formed out of the Earth, and drifted out to its current location. The fission theory proposed that the Earth’s rapid rotation caused a chunk of the planet to spin off into space. Darwin calculated that it had taken the Moon at least 56 million years to reach its current distance from Earth. We now know the Moon was probably formed when a Mars-sized object smashed into the Earth billions of years ago.
  • Salinity of the Ocean – In 1715, the famous astronomer Edmund Halley proposed that the salinity of the oceans could be used to estimate the age of the planet. Halley observed that oceans and lakes fed by streams were constantly receiving more salt, which then stuck around as the water evaporated. Over time, the water would be come saltier and saltier, allowing an estimate of how long this process has been going on. Various geologists used this method to guess that the Earth was between 80 and 150 million years old. This method was flawed because scientists didn’t realize that geologic processes are extracting salt out of the water as well.

Radiometric Dating Provides an Accurate Method to Know the Age of the Earth
In 1896, the French chemist A. Henri Becquerel discovered radioactivity, the process where materials decay into other materials, releasing energy. Geologists realized that the interior of the Earth contained a large amount of radioactive material, and this would be throwing off their calculations for the age of the Earth. Although this discovery revealed flaws in the previous methods of calculating the age of the Earth, it provided a new method: radiometric dating.

Geologists discovered that radioactive materials decay into other elements at a very predictable rate. Some materials decay quickly, while others can take millions or even billions of years to fully decay. Ernest Rutherford and Frederick Soddy, working at McGill University, determined that half of any isotope of a radioactive element decays into another isotope at a set rate. For example, if you have a set amount of Thorium-232, half of it will decay over a billion years, and then half of that amount will decay in another billion years. This is the source of the term “half life”.

By measuring the half lives of radioactive isotopes, geologists were able to build a measurement ladder that let them accurately calculate the age of geologic formations, including the Earth. They used the decay of uranium into various isotopes of lead. By measuring the amount of three different isotopes of lead (Pb-206, Pb-207, and Pb-208 or Pb-204), geologists can calculate how much Uranium was originally in a sample of material.

If the Solar System formed from a common pool of matter, with uniformly distributed Pb isotopes, then all objects from that pool of matter should show similar amounts of the isotopes. Also, over time, the amounts of Pb-206 and Pb-207 will change because as these isotopes are end-products of uranium decay. This makes the amount of lead and uranium change. The higher the uranium-to-lead ratio of a rock, the more the Pb-206/Pb-204 and Pb-207/Pb-204 values will change with time. Now, supposing that the source of the Solar system was also uniformly distributed with uranium isotopes, then you can draw a data line showing a lead-to-uranium plot and, from the slope of the line, the amount of time which has passed since the pool of matter became separated into individual objects can be computed.

Bertram Boltwood applied this method of dating to 26 different samples of rocks, and discovered that they had been formed between 92 and 570 million years old, and further refinements to the technique gave ages between 250 million to 1.3 billion years.

Geologists set about exploring the Earth, seeking the oldest rock formations on the planet. The oldest surface rock is found in Canada, Australia and Africa, with ages ranging from 2.5 to 3.8 billion years. The very oldest rock was discovered in Canada in 1999, and estimated to be just over 4 billion years old.

This set a minimum age for the Earth, but thanks to geologic processes like weathering and plate tectonics, it could still be older.

Meteorites as the Final Answer to the Age of the Earth
The problem with measuring the age of rocks on Earth is that the planet is under constant geological change. Plate tectonics constantly recycle portions of the Earth, blending it up and forever hiding the oldest regions of the planet. But assuming that everything in the Solar System formed at the same time, meteorites in space have been unaffected by weathering and plate tectonics here on Earth.

Geologists used these pristine objects, such as the Canyon Diablo meteorite (the fragments of the asteroid that impacted at Barringer Crater) as a way to get at the true age of the Solar System, and therefore the Earth. By using the radiometric dating system on these meteorites, geologists have been able to determine that the Earth is 4.54 billion years old within a margin of error of about 1%.

Sources:
Understanding Science – Lord Kelvin
USGS Age of the Earth
Lord Kelvin’s Failed Scientific Clock
The Role of Radioactive Decay
Astronomy Cast Episode 51: Earth
Oldest Rock Formations Found

Astronaut Photos Create a Map of the World

If you could spend a few months — or even a few days — living aboard the ISS, what would you take pictures of? Earth, most likely, with your favorite landforms and your family’s and friends’ hometowns ranking high on the list. After a while, I’m sure plenty of other Earthly features would become photo targets — weather, aurorae, world cities at night, etc. — but ultimately, over the course of your stay in orbit, you would be able to see a trend in the pictures you take, and where you took them.

And over the span of 35 missions across more than 12 years, the graph above shows the trend of all the astronauts’ pictures. Look familiar?

Nighttime photo of the Nile delta region taken from the ISS (NASA)
Nighttime photo of the Nile delta region taken from the ISS (NASA)

Created by open-source NASA data aficionado Nate Bergey, the image above is a map made up of  over a million points (1,129,177, to be exact) each representing the global coordinates of an JSC-archived photograph taken from the ISS.

Clearly the continents are astronauts’ favored photo subjects, with the populous urban areas of North America, Europe,  Egypt and the Middle East, as well as the western and southern coasts of South America standing out.

“This makes sense, photos of clouds over an otherwise blank ocean get old after a while,” Nate Bergey wrote on his blog, open.nasa.gov. “I’m sure every astronaut has taken at least one photograph of the town they grew up in.”

Of course, the map doesn’t create an image of the entire globe. This is because the points denote actual over-ground coordinates of the Station (not necessarily what the photos themselves are of) and “the ISS stays between about 50° and -50° latitude as it orbits the Earth,” as noted by Bergey.

A map of the world with the points overlaid onto it, color-coded by mission, shows the difference:

all_iss_missions_map.preview

Bergey also notes the proliferation of purple-colored dots… these indicate the hundreds of images taken by NASA astronaut Don Pettit during Expedition 30/31, when he created incredible time-lapse videos of the Earth from the ISS.

One of many long-exposure images taken by Don Pettit aboard the ISS (NASA/JSC). See more here.
One of many long-exposure images taken by Don Pettit aboard the ISS (NASA/JSC). See more here.

With such a unique and lofty perspective of our world, it’s no wonder that astronauts spend so much time snapping photos — I can’t say I’d be able to tear myself away from the window myself! Read more about Nate Bergey’s project and how he created his map on his open.NASA blog here.