Popular Questions On The Earth

The Earth, our home planet, is the third planet from the Sun and is the only planet known to harbour life. It is the largest of the four rocky planets closest to the Sun, but only the fifth largest in the solar system.

Around 70% of the Earth is covered in water. This is mostly as oceans but also includes lakes, rivers and other fresh water. The other 30% of the Earth is land consisting of continents and islands.

The name ‘Earth’ is at least 1,000 years old and is a Germanic word simply meaning “the ground”

The Earth

Image Credit : NASA

How Old Is The Earth?

For hundreds of years scientists have struggled with this calculation.

This changed in the early 20th century when determining the age of rocks was refined by a method called radiometric dating. This can measure the age of a particular rock with great precision.

Rocks older than 3.5 billion years can be found on all continents.  The oldest, found in north-west Canada, are known to be 4.03 billion years old.

Scientists in Australia have also discovered mineral grains of tiny crystals which have been dated at 4.3 billion years old.

The material which formed the core of the Earth is regarded as the point from which to age the Earth. It isn’t known with great certainty how long this process took, but different models range from a few million up to 100 million years.

From this estimation, and by radiometric dating of rocks and minerals (from the Earth’s crust, Moon samples and meteorites) scientists have estimated that the Earth is 4.54 billion years old…give or take around 50 million years!

How Did The Earth Form?

The point from which to age the Earth, is regarded as being from when the core was formed. Before this was the proto-Earth.

The proto-Earth formed approximately 4.6 billion years ago through the accretion of a disk of dust and gas orbiting the newly formed Sun.

Accretion in astrophysics is the term to describe the gathering of particles into a massive object by gravitationally attracting more matter.

These collisions of larger and larger lumps of rock and eventually planetesimals (huge chunks of rock big enough to have their own gravitational field) formed the proto-Earth.

This process, over a period of millions of years, generated an enormous amount of heat which caused the early Earth to melt completely. In time though, the accretion slowed resulting in a gradual cooling of the Earth’s surface.

The result of this cooling was the formation of the first crust, called the primary or primordial crust. This crust was likely repeatedly destroyed by large impacts, then reformed from the magma ocean left by the impacts.

A recent study relating to the accretion theory has found that proto-Earth formed much faster than previously thought.

An analysis of meteorite dust showed that proto-Earth formed within about 5 million years which, on an astronomical scale, is extremely fast.

What Is The Earth Made Of ?

Scientists have learned a lot about the Earth’s interior from observations of seismic waves that travel through the Earth, our planets magnetic field and also Earths gravity.

The Earth is made up of three different layers: the crust, the mantle and the core. The core has two distinctive parts, a liquid outer core and a solid inner core.

Image Credit : NASA

The Crust

The crust is the relatively thin outermost layer of Earth, accounting for less than 1% of the Earths volume. The depth of the crust varies between 30 to 70 kilometres thick beneath the continents to as little as 5 kilometres thick under the oceans.

The crust is divided into huge plates that float on the ‘mantle’, which is the layer beneath. These plates are constantly in motion but only move extremely slowly (about the same rate as fingernails grow!)

Pressure can build at certain points as these plates grind together and, given time, a sudden movement occurs which releases that pressure. The result being an earthquake.

Mountain ranges also have formed as a result of two plates coming together and this process continues today. The Himalayas continue to rise by an average of 2cm a year. Plate tectonics is the theory explaining the motion of these plates.

The crust is made up of a great variety of igneous, metamorphic, and sedimentary rocks. It is also made up of several elements, with the largest percentage being Oxygen and Silicon.

There is also smaller amounts of Aluminium, Iron, Calcium, Sodium, Potassium and Magnesium.

The Mantle

Below the crust is the mantle which, at approximately 2,900 kilometres deep, is the Earth’s thickest layer.

Made up of mostly silicate rocks rich in Iron and Magnesium, it is a very hot, dense layer with the consistency of caramel.

The viscosity coupled with the intense heat causes the rock to rise, then cool, and sink back down to the core. This convection is considered to be the reason tectonic plates move on the Crust.

When this very hot semi-solid rock rises it may push through the mantle’s outermost zone which is relatively cool and rigid. If this occurs it may push though a rupture in the Crust and create a volcanic eruption.

The Core

The core is the centre of the Earth and has two distinct parts, a liquid outer core and a solid inner core.

Below the mantle the outer core is approximately 2,100 kilometres deep and, unlike the mineral rich crust and mantle, is composed almost entirely of a liquid iron-nickel alloy.

This liquid metal, at up to 5,500°C, churns in immense turbulent currents which form and sustain the Earths magnetic field.

At around three quarters the size of the Moon, the inner core has a radius of about 760 miles and is a very hot dense ball of mostly Iron.

Even though temperatures at the inner core are far past the melting point of iron, the intense pressure at the centre of the Earth is so great (at nearly 3.6 million atmospheres) that it simply prevents the iron from melting.

Where Did Earth’s Water Come From?

Around 70% of Earth’s surface is covered in water. It plays an essential role in life on Earth and in our daily lives, and yet where all this vast amount of water comes from is still being actively debated by scientists.

Earth is thought to have moulded from rocks that came from the inner solar system, where the fierce heat of the Sun would have boiled away any water.

A longstanding theory suggests that Earth’s water must therefore have arrived later by comets and/or asteroids from the icy outer reaches of the solar system.

It now appears unlikely that the amount of water that was needed to account for oceans was provided by comets.

This is because it has been found that the chemical fingerprint of most known comets – specifically the form of hydrogen they contain – doesn’t match what is found in Earth’s oceans.

Another theory involves the creation of the moon. It is generally regarded that the moon formed from the debris left over when a planet (named Theia) thought to be about the size of Mars, crashed into the Earth around 4.4 billion years ago.

Although the existence of Theia is hypothetical, some scientists believe that Theia may have delivered the majority of Earths water.

A planet arriving from the outer solar system is thought more likely to be water bearing, and this is where many scientists think that Theia came from.

This is partly because scientists were able to determine if a certain water rich substance found on Earth called Molybdenum came from the inner or outer solar system.

Using this knowledge, it is thought that Earth’s Molybdenum came from the outer solar system.

By using computer models to simulate various scenarios, scientists were able to narrow down the timing of when Molybdenum was delivered to the planet. It appeared that the timing coincided with the collision of Theia with Earth.

There are many scientists though who are not convinced that Theia was an outer solar system object, and that it was more likely to have been the leftovers of planet formation within the neighbourhood of Earth.

A recent theory suggests that the supposed dry rock from the inner solar system which created our planet, may have been responsible for the process of water forming on Earth after all.

Studies of a particular type of meteorite – a remnant of the early inner solar system before planets formed – has indicated that they are made of the kind of rock believed to have formed our planet.

Analysis has revealed that the chemical fingerprint of hydrogen in these meteorites matches that of rocks in the Earth’s mantle and so could have been present in vast amounts.

Like the meteorites, rocks in the Earth’s mantle also contain a lot of oxygen bound up with minerals. Theory has it that this combination of hydrogen and oxygen could have been released during volcanic activity as steam.

The early Earth had volcanic activity in abundance where huge amounts of water vapor are thought to have exploded into the atmosphere, condense, and fall back to earth as water.

What Was The First Life On Earth?

The first life on Earth is estimated to have been at least 3.5 billion years ago. It is thought that microscopic organisms existed around this time, as signals of their presence in rocks of this age have been discovered.

These signs consist of a type of carbon molecule which is known to be produced by living things.

The early Earth’s atmosphere is believed to have contained only around 1% oxygen, making it a hostile environment for life. The emergence of cyanobacteria around 3.5 billion years ago played a big part in changing this.

Cyanobacteria is a type of photosynthetic bacteria which form rock-like structures in certain shallow sea shores called Stromatolites. The layers build up slowly, and a single 1m structure could take as long as 2000 to 3000 years to form.

The cyanobacteria in stromatolites are thought to have produced and released oxygen (as a by-product of photosynthesis) in such huge amounts that over time it dramatically increased the oxygen content in the air.

Credit : Tim Bertelink

Artists Impression Of Early Earth

Stromatolites had a significant part to play as the first known organisms to photosynthesize and produce free oxygen, and are the earliest fossil evidence of life on Earth.

Despite scientists knowing approximately when life first appeared on Earth, they are still far from answering how it first appeared. How life started on Earth is one of the most profound questions in science and many theories exist on this subject.

To understand the processes which created life on Earth could guide us in the search for life on other worlds. Perhaps more importantly it would help us understand our place in the Universe.

Why Does The Earth Spin?

The Earth spins because of the way the solar system was formed. Around 4.6 billion years ago our solar system began as an immense cloud of dust and gas.

Over many millions of years the cloud collapsed due to gravity and eventually flattened into a disc which started to spin. This was due to a law of physics called the conservation of angular momentum

Some of the material within this disc clumped together to eventually form the planets within the solar system. As more material was attracted to these proto-planets by gravity, it accelerated this spinning motion.

Artists Impression Of The Early Solar System

A similar effect can be seen when an ice skater spins on the spot. When they pull their arms in during the spin you see the rate of spin increase. The Earth keeps on spinning because an object in motion will stay in motion unless an outside force acts on it.

The Moon, and to a much lesser extent the Sun, has slowed our planets rotation to the 24-hour day we are all familiar with. The Earth’s rotation is slowing at such a minute amount that it has been calculated at a mere 2 milliseconds every 100 years!

How Fast Is The Earth Moving?

This can have many answers but first you need to have a frame of reference, or in other words a set of criteria or stated values in relation to which measurements can be made.

Moving from the Earth outwards the first speed to consider is the movement of the earth’s surface with respect to the centre of the planet.

Taking into account the diameter of the Earth (40,070 kilometres), and how long its takes for one complete spin (23 hours and 56 minutes), it has been calculated that this spin at the equator is 460 metres per second, or about 1,000 mph.

Next to consider is how fast the Earth is travelling around the Sun.

Using our planets distance from the Sun (93 million miles), and the circumference of the near circular orbit, it has been calculated that the Earth travels around 1.6 million miles a day in orbit around the Sun.

That’s equivalent to a speed of 18.5 miles a second, or around 67,000 mph.

Not only can we find the speed at which the Earth moves around the Sun, but we can also see how fast the Sun (and the solar system as a whole) rotates around the Milky Way galaxy.

It has been estimated that the solar system, Earth and all, orbits around the galaxy at an incredible 490,000 mph!

Even at this blazingly quick speed, the Milky Way is so enormous that the solar system would still take about 230 million years to travel all the way around.

What Is The Atmosphere Made of ? 


Our planet’s atmosphere is composed of about 78% Nitrogen, 21% Oxygen and 0.9% Argon. The remaining 0.1% is composed of trace gases such as Neon, Helium, Methane and Carbon Dioxide. Water vapour also accounts for about 0.25% of the atmosphere by mass.

The Earth’s atmosphere is divided into five main layers.

Troposphere   0 to 12 km (0 to 7 miles)

The troposphere stretches from the surface of the planet to a height of approximately 12km (7 miles) and contains around 90% of the mass of all the Earth’s atmosphere.

Almost all of the water vapour in the atmosphere is within the troposhere, producing clouds and nearly all types of weather conditions. The air is warmer toward the surface and gets colder at higher altitudes.

The word troposphere was derived from the greek word ‘tropos’, meaning ‘a turn or change’ and this reflects the turbulant nature of this lar of the atmosphere.

Stratosphere   12 to 50 km (7 to 31 miles)                                                                                                                            

The next layer of the atmosphere, above the troposphere, is called the stratosphere. This layer is a lot less turbulant especially at lower reaches, where commercial airliners will take advantage of reduced drag to fly faster.

The air in the stratosphere is very dry and a great deal thinner than at sea level. About 9.9% of the total atmosphere is contained within the stratosphere.

In contrast to the troposphere, the air temperature increases with altitude, from an average of -51°C at the base of the stratosphere to an average of -15°C at the top.

This increase in temperature is a result of the absorption of the Sun’s ultraviolet radiation by the ozone layer.

Mesosphere     50 to 80 km (31 to 50 miles)

The next layer up is the mesosphere which ranges from a base of around 31 miles high to an upper limit of around 50 miles high.

The temperature in the mesosphere behaves much like that at the troposphere in that it decreases with height. The top of this layer is the coldest area of the earth’s atmosphere, where temperatures decrease to around -90°C (-130°F).

The mesosphere is the layer where meteors begin to burn up on entry.

Although the mesosphere contains less than 1% of the earths atmosphere, meteors enter at such great speed (ranging from 11 km/sec to 70 km/sec) that friction with the sparse atmosphere creates what we see as ‘shooting stars’

Thermosphere   80 to 700 km (50 to 440 miles)

The thermosphere is the layer of the Earths atmosphere where the Auroras mostly occur, toward to the north or south poles. Near the north pole it’s called the Aurora Borealis (or northern lights) and the south pole Aurora Australis (or southern lights).

Image Credit: NASA

The Southern Lights As Taken From The International Space Station

Much of the high energy UV radiation from the Sun is absorbed in the thermosphere, partly contributing to an increase in temperature, which climbs sharply and can reach levels of more than 2000°C (3630°F).

Even though the thermosphere is a thicker layer of the atmosphere than the previous three, the air density is so incredibly low that most of the thermosphere is what we normally think of as outer space.

Many satellites orbit within the thermosphere and this is possible because the air density is so extremely low that it barely creates a drag force. Engineers take this minute drag force into account when calculating orbits for the satellites.

As unlikely as it may sound, the space shuttle and the international space station both had/have an orbit of the Earth within the thermosphere!

Exosphere    700 to 10,000 km (440 to 6,200 miles)

This layer of the earths atmosphere starts at around 700km (440 miles) high. Since the exosphere gradually fades into outer space there is no clear upper boundary, but most scientists put it at around 10,000km (or 6,200 miles)

It is mainly composed of extremely low densities of hydrogen and helium, where the atoms and molecules are so far apart they can travel hundreds of kilometres without colliding with one another.

Some of these particles drift up to the higher elevations of the exosphere before eventually arcing back down into the lower atmosphere due to the pull of gravity. However, some of the faster-moving particles don’t return to Earth, and drift off into space.

Just like at the upper reaches of the Thermosphere, many satellites and the International Space Station orbit within the Exosphere.

The Earth

Image Credit : NASA

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