Earth – A Tale of Spheres

Seen from space, Earth can be divided into a series of layers or zones that are referred to as spheres. The outermost of these layers is the magnetosphere, which is the region around Earth that is affected by its magnetic field. This field originates from a dynamo deep inside the planet and is mostly a dipole, characterized by a North Pole and a South Pole. Importantly, Earth’s magnetic field acts as a shield against the charged particles of the solar wind and the cosmic rays from supernova explosions. Still, some of these particles make it past the shield and are focused to the polar regions along magnetic field lines. When these particles interact with gas atoms in the upper atmosphere, they can result in spectacular phenomena such as aurorae.

The atmosphere is the envelope of gas that surrounds the Earth. It consists of ~ 78 % nitrogen (N2), ~ 21 % oxygen (O2) and a number of different traces gases, such as ozone (O3), carbon dioxide (CO2), methane (CH4) and water vapor (H2O). This mixture of gases is more commonly known as air. Both the air density and the air pressure gradually decrease with altitude and as a result, almost all gas molecules are found in the lowermost 50 km of the atmosphere. The air pressure is ~ 1 bar at Earth’s surface, but less than ~ 0.01 bar at an altitude of 30 km.

Approximately 30 % of Earth’s surface consists of continents and islands, while as much as 70 % is covered with water. The total mass of water on Earth, in the form of oceans, glaciers, lakes, rivers, groundwater and the atmosphere, is known as the hydrosphere. Notably, less than 3 % of all water on the planet is fresh water, and most of this fresh water is stored in glaciers and groundwater. The ice-covered regions of Earth’s surface are also known as the cryosphere.

The solid surface and interior of the Earth are referred to as the geosphere. Earth’s surface displays dramatic changes in topography that are related to a variety of processes, but unlike the surfaces of the Moon and Mars, it features relatively few meteorite impact craters. Both the continents and the ocean floor are characterized by steep mountains, extensive plains and deep valleys. The highest mountain above sea level is Mount Everest and the deepest part of the oceans is located in the Mariana Trench.

Perhaps the most striking feature of Earth is its ability to sustain life. Living organisms occur in many magnificent forms across a wide range of habitats and ecosystems, from barren deserts to lush rainforests. All life on Earth together constitutes the biosphere.

The boundaries between these so-called spheres are not always apparent, because the spheres interact in many ways. For example, the atmosphere plays a major role in the hydrological cycle, the exchange of water among the various reservoirs of the hydrosphere. In addition, some of the most impressive natural phenomena are related to the interplay between elements of the biosphere and the geosphere.

Book reference: Marshak, S. (2007). Earth: Portrait of a Planet: Third International Student Edition. WW Norton & Company.

Image: Earth as seen from space, also known as the Blue Marble. Credit: NASA/NOAA/Reto Stöckli.

Asteroids, comets, meteors and meteorites

Not all of the dust, ice and gas swirling around in the Solar System was eventually incorporated into the planets. At present, the leftover materials of planetary formation exist in two different classes of celestial objects: asteroids and comets.

Asteroids are small bodies consisting of solid rock or metal that orbit the Sun. Some asteroids are small planetesimals that were never incorporated into planets, while others are fragments of older, larger planetesimals that collided with each other and subsequently shattered in the early days of the Solar System. At present, most asteroids are found in the asteroid belt between the orbits of Mars and Jupiter, where Jupiter’s strong gravitational pull prevents them from coalescing into larger objects. Asteroids are irregular in shape because they are too small for their own gravity to reshape them into spheres.

Comets are icy planetesimals that generally have a highly elliptical orbit. When they are closest to the Sun, comets heat up and partially evaporate to release a glowing tail of gas and dust. They have a wide range of orbital periods and originate from two distinct regions. Comets with short orbital periods (less than 200 years) originate from the Kuiper Belt, a disk-shaped region of icy fragments beyond the orbit of Neptune, while comets with longer orbital periods originate from the Oort Cloud, a spherical region of icy fragments that extends far beyond the Kuiper Belt. These distant objects become comets when they are pulled towards the inner regions of the Solar System under the influence of gravity. Comets consist of frozen volatile compounds such as water (H2O), carbon dioxide (CO2), methane (CH4) and ammonia (NH3), together with a range of organic molecules and dust.

Even at present, astronomical objects continue to collide with the Earth. Any object that enters Earth’s atmosphere from space is called a meteoroid. These objects move so fast that friction with the atmosphere causes them to heat up and start evaporating, leaving a band of bright, glowing gas in their wake. These glowing streaks are known as meteors, although they are often incorrectly referred to as ‘falling stars’. Most meteoroids completely evaporate at high altitudes in the atmosphere, but objects that are large enough to survive the heat and reach the surface of the Earth are called meteorites. Most meteorites are rocky fragments of asteroids or planets, because the icy materials of comets are often too volatile to hit Earth’s surface.

Meteorites are divided into three broad classes: iron meteorites (made of iron-nickel alloy), stony meteorites (made of silicate rock) and stony iron meteorites (made of rock embedded in a metallic matrix). Some stony meteorites, which are referred to as carbonaceous chondrites, are derived from planetesimals that never experienced differentiation into a metallic core and rocky mantle. In contrast, other stony meteorites and all iron meteorites are derived from planetesimals that differentiated early in the history of the Solar System but subsequently disintegrated during collisions with other planetesimals. Most meteorites are approximately 4.54 billion years old, but carbonaceous chondrites are up to 4.56 billion years old – older than the oldest rocks on Earth! Because they represent primordial material that has remained unchanged all this time, they are of great importance to scientists that study the dawn of the Solar System.

Book reference: Marshak, S. (2007). Earth: Portrait of a Planet: Third International Student Edition. WW Norton & Company.

Image: Comet C/2014 Q2 (Lovejoy) as seen from Earth. Credit: John Vermette, Wikimedia Commons.

Planetary Differentiation and the Development of the Earth-Moon System

After their initial formation, the planetesimals and protoplanets of the Solar System were characterized by a relatively homogeneous composition. However, they soon started to heat up and differentiate. This heat was generated mainly from two sources: further collisions with other celestial objects and the decay of radioactive elements. In planetesimals and protoplanets whose temperatures rose sufficiently to cause melting, denser metallic materials sank towards the center, while lighter rocky materials remained behind at the surface. During this process of differentiation, these objects developed a layered structure consisting of a core, mantle and crust, after which they eventually cooled and became mostly solidified. Under the effects of gravity, the irregular shapes of the protoplanets were molded into spheres.

Even after differentiation had taken place, the young planets were continuously bombarded by meteorites and as a result, their surfaces became scarred with large amounts of impact craters. Approximately 4.53 billion years ago, an especially intense collision occurred between the Earth and a small planet known as Theia. This collision caused Theia and a large part of the Earth to disintegrate, melt and be blasted into space, after which a debris ring formed around the Earth. The Moon was ultimately formed from the materials in this ring of debris. Scientists know this because Moon rocks have an age of roughly 4.53 billion years and because they are very similar in composition to rocks from Earth’s mantle.

Book reference: Marshak, S. (2007). Earth: Portrait of a Planet: Third International Student Edition. WW Norton & Company.

Image: Earth as seen from the surface of the Moon by the Lunar Reconnaissance Orbiter. Credit: NASA/Goddard/Arizona State University.

The Dawn of the Solar System

The Sun and the Solar System are believed to have formed about 4.57 billion years ago, more than 9 billion years after the universe came into existence. As with any other star, the formation of the Sun started with the development of an accretion disk from a nebula. The nebula that would form the Solar System contained all of the 92 naturally occurring elements, because it had incorporated remnants of preceding generations of stars. In the swirling accretion disk, matter was therefore not only present in the form of gas, but also as ice or dust. Because these are the raw materials required to form planets, such an accretion disk is also known as a protoplanetary disk.

Over time, the ball of gas at the center of the swirling protoplanetary disk evolved into a proto-Sun, while the remaining materials developed into a series of concentric rings. Because protoplanetary disks are hotter towards their center, particles of refractory materials (dust) concentrated in the inner rings of the disk whereas particles of volatile materials (ice) concentrated in the outer rings of the disk. The materials of surrounding rings subsequently started to coalesce and form progressively larger objects. Following continuous collisions, some of these objects grew into planetesimals, solid chunks of matter that were so large that they exerted enough gravity to attract other surrounding objects. Eventually, the planetesimals that succeeded in attracting the most matter grew into protoplanets. Once these protoplanets had incorporated essentially all of the matter that was present in their orbits, they would become true planets.

The characteristics of the planets that formed depended on their distance from the proto-Sun. Small terrestrial planets (Mercury, Venus, Earth and Mars) formed in the inner rings of the young Solar System, which consisted mostly of dust, while large gaseous planets (Jupiter, Saturn, Uranus and Neptune) formed in the outer rings, which consisted primarily of gas and ice. Because of their massive size, the outer planets attracted so much additional gas and ice that they evolved into gas giants. Towards the end of planetary formation, the proto-Sun became so hot that it ignited and transformed into the true Sun. This generated a stellar wind – or solar wind – that blew away any remaining gases from the inner region of the Solar System.

Altogether, this model for the formation and evolution of the Solar System is referred to as the nebular hypothesis. It is the most widely accepted model because it is able to explain several key characteristics of the Solar System, including why all planets orbit the Sun in the same direction and why their orbits all occur in the same plane. These observations are all consistent with the formation of stars and planets from gas, ice and dust in an accretion disk rotating around a central mass.

Book reference: Marshak, S. (2007). Earth: Portrait of a Planet: Third International Student Edition. WW Norton & Company.

Image: Artists impression of the Solar System showing the Sun, the eight planets and several other celestial objects. Credit: NASA.

The Origin of the Elements

The first stars formed from nebulae that consisted entirely of atoms of the five lightest elements (H until B), which were formed during big bang nucleosynthesis. Atoms of heavier elements, such as carbon (C), oxygen (O), silicon (Si) and iron (Fe), were all formed later during the life cycles of stars in a process known as stellar nucleosynthesis. Through a progressive series of fusion reactions, stars continuously assemble heavier elements out of lighter elements.

The specific elements that may be formed during stellar nucleosynthesis depend on the mass and temperature of the stars. Stars with a low mass, like the Sun, burn slowly and are able to produce elements with an atomic number of up to 6 (C). In comparison, stars with a high mass, for instance 10 – 100 times the mass of the Sun, burn quickly and are able to produce elements with an atomic number of up to 26 (Fe). However, in order to form elements heavier than Fe, even more extreme conditions are required than those that are generally found in very massive stars. The heaviest elements are therefore mostly formed during supernova explosions at the end of a stellar life cycle.

Atoms may either be released into space during the lifetime of stars, or upon their collapse. If atoms move fast enough to overcome the gravitational pull of their stars, they may escape in streams of gas known as stellar winds. Alternatively, they are discharged in large gas clouds and supernovae during the death of stars. In space, atoms may subsequently form new nebulae or may be incorporated into existing nebulae. So, from the remnants of dying stars, successive generations of stars with an increasingly diverse elemental composition are born.

Book reference: Marshak, S. (2007). Earth: Portrait of a Planet: Third International Student Edition. WW Norton & Company.

Image: Galactic center of the Milky Way as seen by the Hubble Space Telescope, Spitzer Space Telescope and Chandra X-ray Observatory. Credit: NASA/JPL-Caltech/ESA/CXC/STScl.

The Formation of the First Stars

Approximately 200 million years after its inception, the universe consisted of massive, slowly swirling nebulae with large voids in between. Because of the effects of gravity, denser regions of these nebulae started to attract gases from their surroundings and thereby started to grow in mass. These regions pulled in progressively more matter and as they became more condensed over time, the initial swirling movement of the gases transformed into a progressively faster rotation around an axis in accretion disks. Eventually, the gravitational attraction of these spinning accretion disks grew strong enough to cause complete inward collapse of the surrounding nebulae. Gravity further moulded the inner portions of these accretion disks into dense balls and consequently large amounts of energy were transformed into heat. These hot balls of gas ultimately became the first precursors of stars, so-called protostars.

Protostars continued growing until their cores became very dense and reached a temperature of approximately 10 million degrees. Under these conditions, hydrogen nuclei joined to form helium nuclei in a series of fusion reactions that released tremendous amounts of energy. The bodies of the protostars began to light up, resulting in the formation of the first true stars approximately 400 million years after the universe was born.

Stars of the first generation were generally very massive (for example, 100 times the mass of the Sun) because of the large amounts of matter present in the young nebulae. These stars burned very hot and bright, but consequently their lifetime was also relatively short – only a few million years. When stars exhaust all of their resources, they die in a dramatic explosion and flash of light known as a supernova.

Book reference: Marshak, S. (2007). Earth: Portrait of a Planet: Third International Student Edition. WW Norton & Company.

Image: The Pillars of Creation in the Eagle Nebula as seen from the Hubble Space Telescope. Credit: NASA/ESA/Hubble Heritage Team.

The Birth of the Universe

Before the universe as we know it came into existence, all matter and energy is believed to have started out in an infinitesimally small point. For reasons yet unknown, this point exploded approximately 13.7 billion years ago in a cataclysmic event known as the big bang, after which the universe started to expand.

In the first moments of its existence, the universe was so dense and hot that it consisted entirely of energy, but already within seconds it had cooled enough for atoms of the lightest element – hydrogen (H) – to form. During the following minutes, new atomic nuclei of other light elements such as helium (He) were formed through the collision and fusion of hydrogen atoms, which is referred to as big bang nucleosynthesis. This process continued until the universe was approximately 5 minutes old, when it had expanded so much that atomic collisions became increasingly rare. At this point in time, all matter was present in a plasma state consisting of atomic nuclei scattered in a dynamic ocean of electrons.

After a few hundred thousand years, temperatures decreased to a few thousand degrees and neutral atoms with a positively charged nucleus orbited by negatively charged electrons were formed. The appearance of chemical bonds between atoms of specific elements subsequently gave rise to the first molecules. Upon further expansion and cooling, atoms and molecules accumulated into clouds of gas called nebulae. The earliest nebulae consisted only of the lightest elements, including hydrogen (74 %), helium (25 %), and trace amounts of lithium (Li), beryllium (Be) and boron (B).

Book reference: Marshak, S. (2007). Earth: Portrait of a Planet: Third International Student Edition. WW Norton & Company.

Image: Timeline of the universe from the big bang until present-day. Credit: NASA/WMAP Science Team.


Mars is the second smallest planet in the Solar System and the fourth planet from the Sun. It has a mean radius of ~ 3390 km, a mean distance to the Sun of ~ 228 million km and an orbital period of ~ 687 Earth days. Mars is a terrestrial planet with a reddish color, which is related to dust and rocks at its surface that are enriched in iron oxides. The planet is surrounded by a thin atmosphere and is characterized by impact craters, volcanoes, valleys as well as polar ice. Mars is also known for Olympus Mons, the largest volcano in the Solar System, and Valles Marineris, one of the largest canyons in the Solar System.

Information source: NASA

Image: Mars and the Valles Marineris as seen from the Viking 1 Orbiter. Source: NASA/Jet Propulsion Laboratory-Caltech.


Mercury is the smallest of the eight planets in the Solar System, with a mean radius of ~ 2440 km, a mean distance to the Sun of ~ 58 million km and an orbital period of ~ 88 Earth days. Mercury is a terrestrial planet with a surface characterized by impact craters and basaltic lava plains, similar to Earth’s Moon. Because it is the closest planet to the Sun and has almost no atmosphere, Mercury experiences the largest variations in surface temperatures of all planets in the Solar System.

Information source: NASA

Image: Mercury as seen from MESSENGER. Source: NASA/John Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington.

The Moon

The Moon is the celestial body that orbits the Earth. It has a mean radius of ~ 1737 km, a mean distance to Earth of ~ 385 thousand km and an orbital period of ~ 27.3 days. Earth’s satellite is thought to have formed approximately 4.5 billion years ago as a result of the impact between the proto-Earth and a celestial body called Theia, not long after the formation of the Solar System. The Moon has a differentiated structure and consists of a crust, mantle and core and its surface is scarred with many impact craters.

Information source: NASA

Image: Full Moon as seen from Earth. Credit: Gregory H. Revera, Wikimedia Commons.

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