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5.1: Our Home Planet

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    The most basic measurements of the Earth are its size, mass and density. The ancient Greeks knew that the Earth had a spherical shape when Aristotle observed the circular shadow of the Earth during a lunar eclipse. Eratosthenes of Cyrene obtained a first measurement of the radius of the Earth that was within 20% of the value that we can measure today. Once the radius of the Earth (\(R_{\oplus}\)) is known, think about what else can be done! For example, the distance and the radius of the moon can be estimated too, by measuring the angular radius (\(\alpha\)) of Earth during a lunar eclipse (Figure below).

    Figure \(\PageIndex{1}\): Once the radius of the Earth is known, it can be used as a measuring stick. During a lunar eclipse the angular size, alpha, can be measured. Using trigonometry, the distance to the moon and the physical radius of the moon can now be derived.

    It is possible that ancient Greeks might have also calculated the density of rocks, about 3 \(g/cm^3\). Then, assuming a constant density, they could have estimated the mass for the Earth. This would have been off by a factor of about two since the interior of the Earth is compressed and has a higher density than surface rocks. These kind of calculations are often called "order of magnitude" estimates, and factor of two is a great start when nothing is known before. With density (\(\rho\)) and radius (\(R\)), it is easy to calculate the mass (\(M\)) of a spherical body:

    \[\rho\,=\frac{M}{V}\,=\frac{M}{\frac{4}{3}\pi R^3}\]

    By the early 18th century (and with the benefit of insights from Isaac Newton about the laws of gravity) Henry Cavendish had “weighed the Earth” in his lab by cleverly measuring the acceleration of gravity. Cavendish calculated a bulk density for the Earth of 5.48 \(g\,cm^{−3}\), very close to the current value of 5.515 \(g\,cm^{−3}\).

    Order of Magnitude Estimates

    The ancient Greeks are a great role model for those who have a hard time believing that the radius, mass and density of the Earth or the distance to the moon can be determined with basic high school mathematics. Undaunted and armed with a basic understanding of elementary mathematics, they devised simple experiments that gave surprisingly accurate estimates. Before you decide that something is unknowable, pause and think about whether there is a way to make a reasonable estimate, even within a factor of 10 (an "order of magnitude). This will make you a powerful problem solver in science, business and life.

    Earth’s interior

    We can estimate whether the Earth is hollow, uniform in density, or contains a higher density core by measuring a physical property called the moment of inertia. Based on this measurement, we know that the Earth has a dense core but the Moon does not.

    Almost everything else we know about the interior of the Earth comes from seismic waves. Seismic waves carry the energy from events like earthquakes or volcanic eruptions. Seismic waves compress the elastic interior of the Earth like sound waves compress the air. The speed of seismic waves depends on the depth and density of medium. The fastest moving P-waves compress material in the direction that they travel. They are the first to arrive at seismic stations and travel through the core of the Earth. Sheering S-waves are slower and do not propagate through liquid material; they are blocked by the liquid core of the Earth. Slower still, R-waves ripple along the surface of the Earth. They arrive last at the seismic stations but have the potential to do the most damage. A visualization of seismic activity from the 2013 Pakistan earthquake (video below.... wait for it!) recorded at seismic stations in the U.S., shows the arrival of P- and S-waves and the more damaging up-down motion from surface waves.

    Seismic waves reveal the bulk density structure of the Earth, as depicted in the figure below. The core is about half the radius of the Earth and is comprised of an inner core that is about the size of the moon, surrounded by a molten iron core. The thick rocky mantle wraps around the core and extends for the other half of the Earth radius. The outer shell is a thin crust of low density rock called the lithosphere.

    Figure \(\PageIndex{2}\): The interior of the Earth contains a double core and is mostly iron and nickel. The inner core is solid and is surrounded by a molten iron layer. The rest of the interior is a low density rocky mantle. The lithosphere is the thin top crust of light rock.

    Some information about the chemical composition of the mantle comes from surface rocks, especially volcanic rock. The mantle is primarily made up of magnesium, iron, silicon, and oxygen. The layered structure of the Earth is a result of differentiation - a process where heavy elements sink into the core. Chemically, the core is believe to be made up primarily of iron, with some nickel and light elements like sulfur that dissolve easily in these molten metals. The inner core is solid because it is under so much pressure.

    If you could put the Earth into a giant blender and re-mix everything, you would have an Earth-mass blob that is chemically similar to meteorites in the asteroid belt. This is because the objects in the asteroid belt are planetary building blocks that were left behind, not swept up into a planet. During the formation process, the accretion of these objects to form the Earth delivered a tremendous amount of energy that would have melted the interior core and mantle and triggered the settling of heavy elements. The friction of material falling toward the core delivered another dose of heat - a positive feedback that hastened the differentiation and layering of the Earth. A final source of internal energy that heats the Earth is radioactive decay of heavy elements. About half of the heat that exists in the interior of the Earth is left over from collisions during formation of the planet. The rest of the internal heat that exists today comes from radioactive decay.

    A global magnetic field

    The core of the Earth has a temperature similar to the surface of the Sun. The molten iron core has a turbulent fluid motion that generates electrical currents and spawns a global magnetic field that extends thousands of miles above the surface of the planet. This magnetic field plays an important role in protecting the atmosphere by deflecting charged particles that make up the solar wind (Figure below).

    Figure \(\PageIndex{3}\): The invisible global magnetic force field of the Earth protects the upper atmosphere from charged particles that are released during solar flares.

    Conducting currents in the liquid iron core tend to align, forming a dipole (N-S) magnetic field. However, there is some chaotic randomness in the fluid flows that can twist the currents. When this reaches a critical threshold, the magnetic field of the Earth will reverse. Today the north geographic pole of the Earth is a south magnetic pole but that pole wanders and has weakened by about 15% over the past 200 years. There is evidence in the geologic record from magnetic minerals that many magnetic pole reversals have occurred in the past and some scientists believe that the magnetic fields are beginning to collapse and flip again. Earth's magnetic field Life has existed on our planet for billions of years and no correlation has been found between mass extinctions and magnetic pole reversals. The risk to life during a magnetic pole reversal may be modest, but the risk to power grids that modern humans depend upon will be more substantial.

    Plate Tectonics

    An important property of Earth is that it has plate tectonics. While Venus and Mars are likely to have mantle convection, Earth is the only planet in our solar system known to have plate tectonics. Geological evidence suggests that plate tectonics began operating on the Earth about 3.8 Gya. The upper rocky layer of the Earth is divided into about twelve major tectonic plates that float on top of the convecting mantle. The distribution of these plates affect global climate because continental crust has a higher albedo than ocean water. Continental crust was thought to have a maximum extent between 1.6 - 2.7 Gya, causing ice ages and high rates for burial of organic material in the early and late Proterozoic.

    Plate tectonics provide a mechanism for circulating material between the surface of the planet and Earth's interior. Subduction zones form at boundaries where one edge of an approaching plate moves under another. These recycling zones are critical for life on our planet. They are part of a negative feedback loop that stabilizes our climate thanks to chemical interactions between surface rocks and the atmosphere. Rain removes carbon dioxide from the atmosphere and stores it as calcium carbonate in surface rocks through a weathering process. Carbon-enriched rocks are then transported into the deep mantle along the subduction zones, removing excess CO2 from the atmosphere. The flow of material goes the other way as well; some elements are transported from the mantle to the surface of the planet.

    Figure \(\PageIndex{4}\): Look at these images of the four rocky planets in our solar system. What makes Earth the only habitable - and inhabited - planet?

    Earth's Atmosphere

    Reflect upon the image in Figure above that compares Mercury, Venus, Earth and Mars. What are the properties of Earth that were fundamental to the emergence of life? Life requires an inventory of chemicals for self-assembly. Life requires energy to carry out metabolic processes. And life - as we know it - requires water. The veneer of oceans of water is one important attribute that is unique to the Earth. Water should be a ubiquitous molecule throughout the universe. However, Mercury and Venus are too close to the Sun. Any water that those planets might have had in the past has been lost. The surface of Mars suggests that it had a water-rich past, but Mars is small and would not have been able maintain a gravitational hold on it’s atmosphere.

    Earth Oceans

    One distinguishing characteristic of Earth is the oceans of water. The oceans on Earth are relatively shallow with an average depth of about 7 km. Water also cycles through the upper crust of the Earth (about 20% relative to the volume of water in the oceans). And the deep interior may contain several oceans of water.

    Earths oceans are thought to be so critical for the emergence of life on Earth that a motto at NASA is "Follow the water." Where did our oceans come from? The planetesimals that would have formed Earth should have been water-depleted as they were baked by the energy of the Sun. But, we know that the planet formation is more complex, leaving the possibility that water-rich pebbles were streaming in from the outer solar system and accreting on inner planets.

    Ask a geologist and most will say that the oceans were outgassed from the Earth mantle. Ask an astronomer and many will suggest that the delivery of water was an afterthought to planet formation. Comets have been proposed as a possible source of Earth's oceans. In the early solar system, collisions of comets with the surface of the Earth would have been common enough to deliver several oceans of water. An important clue is the fraction of deuterium (hydrogen with a neutron) to hydrogen - the "D/H" ratio. The value in Earth's oceans is similar to the D/H measured in meteorites. However, the D/H ratio measured in comets is smaller that terrestrial or meteoritic values and this seems to argue against comets as the source for Earth's oceans. A better source for water with the correct D/H ratio would be asteroids beyond 2 times the Earth-Sun distance. The origin of Earth's oceans remains a mystery for now.

    The rise of oxygen

    The Earth is also unique in harboring an oxygen-rich atmosphere, but there are two key lines of evidence showing that it wasn’t always this way. Oxygen is the third most abundant element in the universe. It is an aggressive element that is chemically reactive with other atoms or molecules. Free oxygen would have destroyed nascent organic molecules, so the fact that prebiotic molecules survived and life exists today implies that the oxygen levels were very low on early Earth.

    The second line of evidence comes from the geologic record. Radiometric techniques are very good at dating rocks on the surface of the Earth and the oldest rocks contain minerals that reveal the chemical constituents of the early atmosphere. We know that there was some carbon dioxide in the atmosphere because rocky outcroppings that date back 3 Gya contain carbonates that form when carbon dioxide reacts with silicate rocks. Various iron oxides are also found; however, these would only formed if there was a less than a 1% trace of oxygen in the atmosphere.

    The bulk of the early Earth atmosphere probably consisted of inert neon and nitrogen. The abundance of primordial neon is higher than nitrogen, yet nitrogen atoms in our atmosphere today outnumber neon by 60,000 to 1. The Earth seems to have lost it’s endowment of neon and one theory is that the entire “first atmosphere” of the Earth was blown away during the Late Heavy Bombardment, the time when the surface of the Earth was battered by swarms of meteorites. Later, the atmosphere could have been replenished by volcanic activity, which would have been more frequent when the Earth mantle was hotter. Volcanoes spew out gases (a process called “outgassing”) that consist of sulphur, nitrogen, carbon dioxide and almost no neon, methane, ammonia or oxygen.

    The first traces of oxygen appeared when ultraviolet radiation from the Sun dissociated water molecules. The lighter hydrogen was able to escape while heavier oxygen was gravitationally bound to the planet and interacted with iron sediments. This process dehydrated the surfaces of Venus and Mars. On the surface of Venus, oxygen coupled with carbon to form a \(CO_2\)-rich atmosphere and that greenhouse gas resulted in a runaway warming of the planet that further accelerated the evaporation and dissociation of water molecules. On Mars, the remaining tenuous atmosphere is also made up of carbon dioxide. In his book "Oxygen," Nick Lane posits that it was life itself that saved Earth from loss of ocean waters.

    In the same way that the history of Earth’s atmosphere is written into the geologic record, the evolution of life is written into the genetic record. The first appearance of life dates back to between 3 - 4 Gya - shortly after the end of the Late Heavy Bombardment. These first microorganisms, called archea, were anaerobic; they thrived in the absence of oxygen, probably deep in the oceans near thermal vents where chemical gradients could be used as an energy source. Oxygen would have been poisonous for these anaerobic microbes. But natural selection is a power driver of evolutionary change and around 2.7 Gya, a mutation in the genetic code of anaerobes in the upper layer of the oceans selected for a hardy new organism called cyanobacteria. The remarkable adaptation of cyanobacteria was that they could use energy from the Sun to carry out metabolic processes through oxygenic photosynthesis. The cyanobacteria took up free CO2 and began releasing oxygen. This new evolutionary pathway was so efficient that there was an explosion in the population of cyanobacteria and they drove anaerobic microbes deep into the oceans and began to change the atmospheric composition on Earth.

    The evidence for this increase in atmospheric oxygen is also recorded in the geologic record. About 2.5 Gya, the isotopic ratio of sulfur changed, indicating that oxygen levels were starting to rise, and iron minerals showed evidence of increasing oxidation (“rusting”). The production of oxygen by cyanobacteria began to outpace the uptake by reactants in the crust of the Earth and free oxygen began to climb above the initial levels (less than 1%) in the Earth atmosphere. Free oxygen stops the loss of water because as dissociation occurs, oxygen atoms snap up the released hydrogen before it can escape. With todays levels of free oxygen (21%), a mere 3 million tons of water is lost each year. At this rate, only 1% of the ocean’s water would be lost in 4.5 billion years. We can thank cyanobacteria, which still thrive on our planet today, for producing the oxygen that we breath and for saving our oceans of water.

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