# Illustrated Astronomy/The Earth

Universidad Autónoma de Chile, pages 39–51
II
THE EARTH
​The Earth is the third planet in the Solar System counting from the Sun. It is the largest planet among the rocky ones (Mercury, Venus, and Mars), but smaller than gas giants (Jupiter, Saturn, Uranus, and Neptune) and it is located about 150 million kilometers from the Sun. This enormous distance means that the light coming from the Sun doesn’t reach the Earth immediately, but takes around eight minutes, even though light is the fastest thing that exists! Since it travels to almost 300,000 kilometers per second.
If, for any reason, the Sun disappeared or stopped shinning, we would take eight minutes to know it, and the Earth would keep orbiting along that time to continue then traveling in a straight line into space at 30 km/s since the Sun would exert no force upon it.
​On the one hand, this enormous distance is something great because if the Earth were closer, the amount of light and energy that it would receive would be much higher. Therefore, the temperature would increase, putting the development of life in danger. On the other hand, if we were further, it would be the opposite: the lack of energy and low temperatures would make the living existence on our planet difficult.
Another interesting feature is that Earth’s orbit is almost circular (is indeed an ellipse), and that makes our radiation levels neither lower nor higher. An eccentric orbit would cause us to get very close to the Sun in a short time, but in that period, life would experience significant changes: it would be fire everywhere and a lot of UV radiation which would kill bacteria, causing damage to the living creatures here on the Earth surface. The rest of the time, we would be far from the Sun, which would bring a brief ice age every year, and by the freeze of everything and the lack of both energy and light, it would be significantly hard to survive.
The term “habitable zone” or “circumstellar habitable zone” was created to describe a place as the one the Earth has in space, which means, in a stable, almost circular orbit at the exact distance of its star. This term refers to the space region around a star, where the temperature of an object or planet is between 0 °C and 100 °C, and water in liquid state is possible under normal pressure conditions.
Venus and Mars are in the inner and outer bounds of the habitable zone. However, it is needed much more than the right temperature to the existence of life. Next, we are discussing four essential conditions for habitability on Earth.
PRESENCE OF LIQUID WATER
More than 70 % of the terrestrial surface is covered in water. In fact, the Earth is the only planet in the Solar System that has the three phases of water on its surface (solid, liquid, and gas).
Some of the properties that make water an essential element for life development on Earth are: it is an excellent solvent, it provides suitable conditions to many chemical reactions, and it allows the transfer of substances between cells and their environment. Also, the heat capacity of water -that is the amount of energy required to increase one Celsius degree- is very high, which means that in comparison with other elements such as rocks or metal, water takes more time to heat, remaining the surface temperature of the planet stable with slight variations between day and night.
​ATMOSPHERE
A thin layer of air surrounds the Earth that we denominate the atmosphere as a whole, which protects us from the Sun’s UV radiation and from particles such as cosmic rays, and it also provides us oxygen to breathe. However, the current atmosphere has changed dramatically along time. Probably, the first layer of the atmosphere lasted a short time. It was mainly hydrogen and helium (yes, the same element which the Sun is made of), but being so light, and the Earth being a relatively small and low-mass planet, these elements “evaporated” and were released to space. Then, after the constant volcanic activity, the second atmosphere formed, mainly made up of methane, ammonia, water vapor, nitrogen, monoxide, and also carbon dioxide.
In that environment is where life begins. The first living organisms (bacteria) used solar light and these atmosphere elements to produce energy. They generated large quantities of free oxygen. Finally, as time goes by, these bacteria ended up changing the conditions of this layer, and today we have an atmosphere consisting of nitrogen (N2) and oxygen (O2), and other gases such as carbon dioxide, argon, neon, helium, and ozone.
It is interesting to note that though the atmosphere has changed over the last million years, life has remained as a constant, so the restrictions to its existence are not limited to an exact atmosphere like the one we have today.
​MAGNETIC FIELD
The magnetic field is a force field. The Earth has a great magnetic field that seems to be related to the terrestrial core composition, a core of molten iron. This magnetic field acts like a protective shield, deflecting the high-speed particles coming from the Sun.
All of this is beneficial to us because without this magnetic field life on Earth would be exposed to a constant charged particles rain, such as electrons (particles or beta radiation), protons, and helium cores (alpha particles), among others, putting its existence at risk.
Once particles are deflected, they speed up and enter the Earth through the polar regions, producing the most breathtaking wonder ever: The Polar Aurora (aurora borealis in the Northern Hemisphere, and aurora australis in the Southern Hemisphere).
DID YOU KNOW THAT…
…Earth’s magnetic field, as the Earth itself, also has poles, but the magnetic north pole is, in fact, near to the geographic south pole and so?
In fact, if we drew a straight line between the southern and the northern pole, it would not pass through the Earth’s center since its terrestrial magnetic field is not perfectly symmetrical.
​DID YOU KNOW THAT…
...polar auroras happen when the terrestrial magnetic field deflects the electrically charged particles coming from the Sun, and when they come closer to Earth, they accelerate by electromagnetic force and not by gravity? Once they reach the Earth, they emit different colors due to their interaction with the atmospheric gases. Yellow and green are the result of the reaction of the particles to the oxygen, while red, violet, and blue are for reactions to the nitrogen.
​• • • Both magnetic and Earth’s geographic poles are not aligned. The first ones actually change in time, which means that our compasses show where is the north or the south with a slight difference as the years pass by.
In the following figure, we can see how the northern magnetic pole has been moving in time. It seems that the mentioned changes happen due to the movement variation of the outer core, but that is research in progress.
Lastly, throughout Earth’s history, the magnetic poles have turned upside down multiple times, and we know it thanks to the rocks found in the mid-ocean ridges, structures in the sea bottom formed by magma material which elements are sensitive to magnetic fields (such as iron, for instance).
These particles and the terrestrial field of that moment line up and then get solidified, making impossible to change their position again.
On these rocks, we have found orientation changes; that’s why we know the magnetic field have been upside-down and passed through constant changes.
The magnetic north pole changes its exact position in time. Currently, it is nearby Canada and is moving to the Siberian land, located in Russia. Over the last 30 years, the geographic position of the magnetic pole north has changed more than in the previous 90 years.
​THE MOON
The Moon is primarily responsible for the Earth’s almost constant axis of rotation, preventing dramatic changes in the axial tilt. It is essential to mention that thanks to this obliquity of 23,5°, we experience the four seasons. Without it, we could experience times where we would receive sunlight only on one Earth side, producing an overheat, whereas the other half would freeze; or extreme seasons of both winter and summer in different locations.
Even though, as human beings, we have lived on this planet for thousands of years, yet there are many things we don’t understand completely. For example, though the Earth is almost a perfect sphere, slightly flatten on its poles, we still have to discover why the south pole is a little bit bulkier than the north pole.
Besides, Earth has different shapes on its surface. The largest mountain is Mount Everest, standing about 8,848 meters above sea level, located in Nepal. On the other side, the Mariana Trench, with a depth of 11,034 meters, which is the deepest trench in the world. In spite of these extreme examples, which seems to be so high or so deep for us, if we could compare Earth at a scale of a billiard ball, Earth would be much more even than the surface of a brand-new ball.
​• • • SHORT MATH EXERCISE
How could you verify if the Earth is much more even than a billiard ball?
Consider these numbers:
· The Earth has a diameter of 12,730 km, and a billiard ball 5.715 cm.
· The highest roughness allowed in a billiard ball is 0.0127 cm.
Now, compare the “roughness” equivalence of the Mariana Trench or the Mount Everest.
(Clue: you can use the “rule of three”)
The inner part of the Earth is hard to explore. One may think that the best way to know what it is inside is to dig a hole and take samples, but that is not only difficult to do but also extremely expensive.
An alternative way to study the inner part of our planet is to see how does the energy transfer from the rocks. The Earth is very active in tectonic terms. Earthquakes and volcanic eruptions happen since the plates are periodically moving, and the energy that this movement produces travel across the rocks. The displacement speed, such as the loss of the mentioned energy, it depends on the kind of rock and its density. How can we measure these waves?
There are some instruments called seismometers. We are able to study the inner part of our planet at scattering around some of them.
The deepest place the human being has ever dug is the Kola Superdeep Borehole, located in Russia that reaches about 12,262 meters of depth, and it was built for research purposes.
Even so, in spite of its dimensions, it hasn’t been able to cross completely the first layer of the Earth’s crust, which is just 35 km of thickness.
​DID YOU KNOW THAT…
...the spatial mission InSight landed on Mars on November, 26th in 2018, and its target is observing the insides of the planet? To do so, it is digging a couple of centimeters, and it is placing a seismometer that helps to understand better the Martian crust, mantle, and core, and how heat escapes onto its surface.
Based on these discoveries, we hope to understand better how rocky planets evolve.
LAYERS OF EARTH
Thanks to researches that the Danish seismologist Inge Lehmann carried out is that today we know our planet has a core, and it can be divided into two parts: inner and outer core.
The first one is solid. It has a temperature around 5,400 °C, similar to the solar surface’s, and is primarily made up of iron and nickel, while the outer core is made up of molten metal, and apparently rotates differently from the outer layers, which makes it responsible for the Earth’s magnetic field.
Then, looking from inside out is the mantle, a layer where most of the Earth’s mass is concentrated (about 67 %) and corresponds to 84 % of its total volume. In there, the most considerable part of both seismic and volcanic activity happens. In some cases, during the collision of plates, an element of the material in the upper mantle and the oceanic crust raise, allowing us to have access to rocks that belong to the mantle. These places, which are crucial to understanding the composition of the upper Earth’s mantle, are called ophiolites. We have one in Chile, and its location is in Patagonia, specifically at the Taitao Peninsula.
Finally, the outermost layer is known as the crust. It is very thin. It measures between 5 and 7 km deep, and all human and animal activities developed on it.
The most of the rocks, part of the Earth’s crust, are about 100 million years old, but it has been found some minerals 4,000 million years old, which would show that the Earth has a rigid crust almost since its formation.
​• • • AT-HOME EXPERIMENT
How do compasses work?
Doing this short experiment, you will be able to see how a magnetic field interacts with metal particles.
MATERIALS:
· 1 magnet · Steel powder (you can get it from a ball of steel wool) · 1 bowl (plastic or glass) · 1 liter of water
STEP BY STEP
1 · Scrape the steel wool and save the powder you get.
2 · Then, pour down the water in the bowl, and let the steel powder falls.
3 · See that the powder stays mostly on the top of the water (at least the tiny little pieces), and they are randomly spread.
4 · Now, pass the magnet slowly over the water without touching the steel powder.
What differences do you see with what was before the magnet passed?
The minuscule metal particles aligned because the metal dust interacts with the magnetic field that the magnet produces.
That is the basis of how a compass works: a small metal needle is aligned with the Earth’s magnetic field. The compasses stop working when a magnet passes near them since the magnet distorts the magnetic field its needle perceives.
Review questions
1 · Since the Earth was formed, its atmosphere has gone through great changes, what is that for? 2 · Can you mention and describe at least two effects the Moon produces on Earth? 3 · How can we know what is in Earth’s core? 4 · Why Earth does have a magnetic field? 5 · What is the habitable zone or the circumstellar habitable zone?