In the next few parts of our journey through the solar system, we examine our world through the eyes of a stranger, seeing past the assumptions that blind us to the awesome complexity around us. Earth is a lot more than the birthplace and currently unique home of humanity, but a world of rich diversity where a number of cyclical processes keep a dynamic and precarious balance. One of those processes, life, may ultimately catalyze a transformation of the solar system through the continuing evolution of technological intelligence.
The progress of our adventure so far (current in bold):
1. The Sun
4. Earth (Vol. 1)
5. Earth (Vol. 2)
6. Earth (Vol. 3)
7. Earth (Vol. 4)
10. Phobos & Deimos
21. Tethys, Dione, and Rhea
24. Rings & Minor Moons of Saturn
26. Moons of Uranus
29. The Kuiper Belt & Scattered Disk
31. The Stellar Neighborhood
In Volume 2, we begin to examine the properties of Earth, including the first few layers or "spheres" of the terrestrial environment, moving outward from the center up through the rocky surface. Most photographic images will be from low Earth orbit and - as amazing and awe-inspiring as this is to realize - most of them are in true color, exactly as our eyes would see them at that altitude. Those images which are not true color are either obvious or noted. Our progress in examining Earth, with current volume in bold:
I. Context (Vol. 1)
II. History (Vol. 1)
1. Orbital and Rotational Features2. Size and Mass Characteristics3. Internal Structure4. SurfaceA. GeographyB. HydrosphereC. BiosphereD. Anthroposphere5. Atmosphere6. MagnetosphereIV. Natural & Artificial Satellites
V. Past Relevance to Humanity
VI. Modern Relevance to Humanity
VII. Future Relevance to Humanity
VIII. Future of Earth
IX. Catalog of Exploration
1. Orbital and Rotational Features
The mean distance from the Earth to the Sun defines the Astronomical Unit (AU) - one of the units used by astronomers to describe distances on planetary system scales, and equivalent to about 150 million km. Light takes about 8 minutes to cross this distance, so another way of expressing it would be that Earth is eight light-minutes from the Sun, although light-minutes are not an especially common unit. Earth's orbit around the Sun is very close to this value throughout its year, with the third lowest eccentricity of solar system planets after Venus and Neptune.
In absolute terms, its furthest distance from the Sun (aphelion) is about 5 million km further out than its closest (perihelion), which changes the intensity of sunlight reaching Earth by about 6.9% between the extreme points of the orbit. This does have an effect on terrestrial plant life, but contrary to popular misconception is not what causes the planet's seasons. The seasons are actually a result of the Earth's axial tilt of about 23.5°:
The axis remains pointed in the same direction as the Earth orbits the Sun, so at opposite times of year either the Northern or Southern hemisphere is more directly facing the Sun while the other is angled away during daytime. In the latter case, when sunlight falls at an increased angle, the same amount of light spreads over a wider area and is thus dimmer. This is also the reason that Earth's polar regions are the coldest areas of its surface, as sunlight arrives at the most acute angles.
But while the seasons are caused by axial tilt rather than orbital eccentricity, the latter does have visible effects. For instance, you may have noticed that plant life in the Southern hemisphere has a distinct appearance from that found in the North, even among closely related species at similar distances from the equator. This is because Summer in the Southern hemisphere occurs closer to perihelion, giving plants a little more sunlight than their Northern counterparts, while Northern Summer is closer to aphelion, making it somewhat dimmer. As a result, Southern plants tend toward brighter, more tropical shades of green, although the overall climates may be milder due to unrelated geographical factors.
Although Earth's axis is fixed on human timescales, it actually wobbles in small circles over periods of 25,800 terrestrial years - a behavior known as precession. However, the angle of the tilt remains constant. Over time, a consequence of precession is that the North pole will no longer be directed toward the current "North Star" Polaris, but will point toward Deneb, Thuban, and then Vega before cycling back to Polaris.
Earth comprises the dominant body in the Earth-Moon system, accounting for about 98.8% of the mass. As a result, the barycenter (center of mass) of the system is about 1,710 km beneath the terrestrial surface - a little more than a quarter of the way toward the planet's geometric center. Both Earth and the Moon orbit this point over a period of about 28 days, although the motion of the Earth in this orbit is small enough to have no visible consequences relative to objects in the sky.
Because the Earth is rotating around its own center at the same time that it orbits the barycenter of the Earth-Moon system, and at a much faster rate, the planet experiences a gravitational tug that has been gradually slowing its rotation and lengthening its day ever since the Moon formed. However, the same effect is causing the Moon to speed up in its orbit and recede, so the rate at which the terrestrial day extends is also declining. This process is called tidal acceleration. Over human history, the rate of increase in the day has been about 2 milliseconds per century. An animation showing the relationship of Earth's day to the lunar cycle:
Lunar gravity causes Earth's primarily liquid surface (the oceans) to deform into an oblong shape, with one lobe extended toward the position of the Moon and the other on the opposite side of the planet. This is experienced on land as a periodic increase or decrease in sea level - the tides. Tides are greatest when the Moon approaches the Sun relative to Earth, allowing their gravitational effects to combine in the same direction.
The Earth-Moon system is inclined about 5° off the plane of the ecliptic (the path of Earth's solar orbit), so at most times of year the Moon passes either above or below the Sun as seen from the terrestrial surface.
However, two to five times per year, the Moon will pass in front of the Sun (solar eclipses), and at most two of them will completely obscure the Sun as seen from a narrow strip of the Earth's surface. We are at a period in the evolution of the Earth-Moon system where the apparent size of the Moon is large enough to completely cover the Sun during a total eclipse - something that will not always be the case as the Moon recedes - but small enough to be infrequent occurrences. Earlier in the history of the system, the Moon's apparent size was large enough to cause a total solar eclipse every month, and then later a partial solar eclipse every month. The shadow of a total solar eclipse on the Earth:
Time-lapse videos of an eclipse shadow on the Earth (the video cycles back and forth, so don't be confused by the path the shadow takes), and then of an Earth day:
I would also highly recommend watching this other video of Earth from space - unfortunately the uploader has disabled embedding, so you will have to watch it at Youtube. I continue to be puzzled why anyone does that, and especially bewildered what would motivate someone to upload a video that beautiful and universal and then deliberately limit its distribution. Anyway, I suggest opening it in another tab and going full-screen to get the most out of the experience.
2. Size and Mass Characteristics
Below are some size comparisons of Earth with various other solar system bodies. Because it is the largest rocky body in the solar system, I also include comparisons with the gas giants. Mouse-over the image if you don't recognize the other body:
Although the size boundary between rocky planets and gas giants is sharp in our solar system, planets around other stars have been discovered that bridge the gap - worlds only a few times larger than ours. Hopefully some day, advances in telescope technology will allow reasonably clear images of these worlds, in addition to whatever smaller planets are found in the intervening years.
Earth is the densest planet in the solar system, due in large part to its relatively large iron core - something that owes its size to the giant impact that created the Moon (see Vol. 1 of the Earth sub-series for a more detailed description). It is only slightly denser than Mercury and Venus, but about 8 times denser than Saturn - the least dense solar system planet.
As a result, if one were standing on a solid surface in the region of Saturn's atmosphere with equivalent pressure to Earth's sea level, the gravity would only be slightly higher - about 6.6% higher. In the same region of Uranus's atmosphere, the gravity would be lower than on Earth by 10.5%. This helps to illustrate the influence of density on surface gravity, and may some day allow humans to personally explore Uranus and Neptune in balloon stations (Saturn, unfortunately, emits intense radiation). The gravity at this atmospheric region of Jupiter, by contrast, would be two-and-a-half times Earth gravity, so even if radiation were not a problem, humans would likely not be able to personally explore its atmosphere. Brief periods of 2.5 g are easily tolerable to humans, but extended periods would likely damage internal organs and blood vessels.
3. Internal Structure
A. Inner Core
Earth's inner core is a solid ball mostly composed of iron, a smaller amount of nickel, and trace impurities of other elements. The inner core formed and continues to grow (at a rate of about 1 mm per year) as liquid iron and nickel in higher layers release heat and freeze into solid globules that descend to the surface of the solid region. Unlike water, metals freeze from the inside out because their solid states are denser than their liquid states, causing solidified "snow" in cooler liquid layers to continuously descend while hot liquid rises and carries away heat from the interior. In a few billion years, the entire core will be solid and Earth's magnetic field will become much weaker.
Structurally, the inner core is thought to be crystalline (i.e., having an orderly, regular structure in the same way as quartz or diamonds), with large-sized crystals oriented North-South along Earth's spin axis (not its magnetic poles). Since the freezing process is imperfect, liquid globules may be encased in solid and introduced into the inner core, possibly resulting in layering of the inner core itself based on the level of liquid impurities. Temperature is about 5,700 K - roughly that of the Sun's photosphere - and pressure is about 3.5 million times sea level atmospheric pressure.
Due to being surrounded by liquid, the inner core rotates at a different rate from the rest of the planet, although the exact rate is not well-understood - it is thought to be slightly faster. The region inside the inner core is believed to be a relatively uneventful place in comparison to the rest of the planetary interior.
B. Outer Core
At the boundary between the inner and outer core, we come to a phase transition where the pressure has decreased enough for nickel and iron to be liquid. The temperature has also decreased, but not as drastically, so the material has not cooled enough to remain solid under the lower pressure. As a result, the outer core is a 2,266 km thick metallic ocean that is able to transfer heat by convection, and also experiences currents due to the Coriolis effect caused by Earth's rotation.
Nickel and iron heated by interaction with the inner core rise while the same materials cooled by interaction with the layer above descend. Some portion of the descending material has frozen into solids and will join the inner core if it does not re-liquify prior to reaching the boundary. Most of the heat of Earth's internal structure (about 80%) comes from the radioactive decay of heavy elements like uranium, which release a tremendous amount of energy despite being a small proportion of the composition. Almost all of the remaining heat is left over from planetary formation and has not yet dissipated, although some amount is also created by tidal effects like those described above.
The terrestrial magnetic field originates in the outer core due to the dynamo effect, which is caused by the Coriolis motion of the liquid iron as well as its convection. The strength of the magnetic field in the outer core is stronger than that on the surface by a factor of about 50, but remains nonetheless powerful well into near-Earth space - a fact crucial for the evolution and long-term existence of life on this planet. Without this strong magnetic field, the solar wind would cause the planet to lose its water through the ionization of hydrogen (see the Venus diary for a more detailed description).
B. Lower & Upper Mantle
A sharp change in temperature and chemical composition occurs at the core-mantle boundary, where temperatures decline by about a thousand degrees Celsius and the geological composition becomes overwhelmingly silicon dioxide (SiO2) and magnesium oxide (MgO) - also known as rock. Iron, aluminum, calcium, sodium, and potassium oxides are also represented in significant quantities.
The mantle in general is solid, although it would be more than hot enough to melt under surface conditions - but the high pressure at mantle depths normally prevents a phase transition. What results is a thick, solid shell that quite literally floats on the liquid metal ocean of the outer core. However, the bulk of the mantle does not appear to move. Rather, at the core-mantle boundary, the heat of the outer core turns the lowermost rock into partially-melted magma, and this thin "puddle" of gooey rock at the boundary is called the D'' layer ("D double-prime layer") - the first of three low velocity zones in the mantle. These regions are not called "low velocity" due to their own motion, but because of the low speed at which they transmit a certain type of seismic wave, which was how they were detected.
At certain points on the boundary, magma from the D'' layer upwells massively and penetrates the solid mantle in huge columns called mantle plumes. They continue upward until they reach the boundary between the Lower and Upper Mantle, the Transition Zone - a region where various minerals undergo phase changes from one type of crystal to another due to decreasing pressure, causing the plume to slow down and spread out into a broad, flat "head" along the Transition Zone. Out of this flattened magma, additional, smaller plumes rise up and penetrate the Upper Magma, some of which will go on to escape the crust as volcanoes. Plumes that reach the Transition Zone are much larger and fewer in number than their Upper Mantle offspring, so they are called superplumes.
Magma plumes are regular, fixed structures, and do not appear to move around much over time. This is the cause of many volcanic island chains, including Hawaii - the crust is continuously in motion due to plate tectonics, but the plumes responsible for volcanic upwelling stay in the same place. So instead of building continents, these types of volcanoes build long, extended archipelagos and peninsulas as the crust sweeps over their location. However, not all volcanoes are associated with plumes, and not all plumes that reach the top layers of the upper mantle are associated with volcanism. Most spread out beneath the crust, but instead of forming sub-plumes they drive the crust to pull apart, resulting in seafloor spreading.
Where the spreading of these plumes meet, the now-cooler material is driven by convection to sink, and in the process helps pull subducted crust with it. This material is continually being shoved deep underground by the head-on collision of tectonic plates, and magma convection helps drive the process both by pushing the plates that cause subduction and by pulling the subducted rock along with it as it descends. Protrusions of crustal rock into the magma are called slabs, and their denser components will sink all the way back to the core-mantle boundary while lighter compounds (such as water) upwell into the boundary region between the mantle and crust. The material that sinks to the mantle-core boundary will be heated, partially melted, and eventually return upward as part of a plume.
The uppermost part of the mantle, and the last low-velocity zone before the crust, is the asthenosphere - a region of high plasticity and ductility (it can bend and stretch easily) that is important to the ability of the mantle to transmit heat into the crust. It has these properties because water in subducted crustal rocks lubricates the otherwise solid magma, making it more able to respond to heat. Venus, by contrast, lacks an asthenosphere, so its mantle just sits there, intensely hot, and is unable to efficiently release heat into the crust except through a few volcanic fissures. As a result, that planet may undergo periods where the heat builds up to catastrophic proportions and resurfaces the entire world. Earth, however, is able to gradually release its internal heat. Critical to that ability is a physical process called Rayleigh-Bénard convection - the pattern of motion exhibited by fluid areas of the mantle. However, recall that the mantle is overwhelmingly solid, and most of this convection occurs in boundary regions and plumes. A simulation:
At the outermost layer of the mantle, the rock becomes brittle and is capable of maintaining cracks and fissures for increasingly large intervals of time. All rock above this boundary constitutes the lithosphere - a layer defined by its material brittleness relative to the mantle, and that is sub-divided based on composition. At first the rock is composed of the same compounds as mantle, but above a boundary called the Mohorovičić discontinuity (Moho for short) it takes on the typical composition of crust - the furthest, thinnest, and most brittle region of terrestrial rock.
The crust is divided into oceanic and continental crust, with the latter being less dense and thereby thicker, allowing it to rise higher and be pushed up to form the land surface. Oceanic crust, being thinner and denser, is in closer contact with the influence of mantle plumes in the asthenosphere, and thus is typically where new material reaches the surface from the mantle (the seafloor spreading mentioned above). Upwelling through oceanic crust produces the mid-ocean ridges - linear, underwater mountain ranges associated with the creation of new crust and the driving apart of continents on either side. An illustration of a mid-ocean ridge, followed by video of one such region:
Where spreading crust created by one ridge collides with that from another, the denser material is forced downward (subduction), creating an oceanic trench, while the lighter material moves upward to form mountains (orogeny) - the geological origins of land continents. Orogeny occurs because lighter components of the subducting crust flow upward from the descending slab, not only exerting an upward force but also increasing its actual bulk. This process can result in continental volcanoes unaffiliated with mantle plumes. Erosion of the mountains thus formed creates the broader plains regions of continents, causing most of the land surface to occur in large blocs rather than narrow ridges. Subduction and orogeny:
The intersection of mid-ocean ridges, oceanic trenches, and transforms (where two regions of crust slide past each other) defines the boundaries of Earth's tectonic plates, which are in constant motion. A map of plate boundaries and the crustal movement that occurs at them:
Areas of the crust that are in contact with mantle plumes are called hot spots, and are typically associated with volcanic islands and archipelagos. Some of these (like the Hawaiian islands) are quite obvious, as they occur in the middle of tectonic plates, far from any crustal boundary and distant from the continental masses they form. Others are less obvious, such as the Yellowstone supervolcano in Wyoming - something that may pose a long-term threat to terrestrial humanity. A map of hot spots:
Videos of various types of volcanic activity on Earth:
Some stills of plumes from stratovolcanoes:
Inactive volcanoes on the Kamchatka peninsula of Eastern Russia:
The crust is dominated by metallic oxides, particularly silicates. Elemental composition:
As we look at the surface of the Earth, we'll spiral in from a distance. Earth has six major continental land masses, contrary to the seven that are normally taught - Europe and Asia are technically the single supercontinent Eurasia, which India also belongs to despite being on a different plate. Europe is a peninsula of the supercontinent. Below are satellite images of the continental bulk of Eurasia in winter and then summer - note there are few or no clouds in the widest-angle images, and the oceans are a uniform color because these images have to be pieced together from many satellite passes, so the transient clouds and color differences due to angle are not present (i.e., this is not what you would see with your eyes from space, nor are most of the continent-wide images):
European peninsula in summer and winter, with political borders and cartographic lines irritatingly added in the second image:
Africa - complete continental view, and then a mosaic of the Northern half with clouds represented:
Australia - and these ones are completely true to life (Update: The second image is actually a little over-saturated), so you would see this looking out the window of a spaceship from an orbit considerably higher than where astronauts and cosmonauts typically are:
Antarctica in summer and winter - these are naked-eye compatible as well (except for the subtle radial discontinuities in the first image), and contrary to appearances they are both in color:
Now we can zoom in to a lower level and begin looking at large islands and a few sub-regions of continents. These images are all true to the naked eye, as far as I know - with the exception of small red circles denoting fires or active volcanoes in a few images. Let's start with large (or at least awesome-looking) islands, like for instance Cuba and Jamaica:
The island of Hispaniola, comprising Haiti and the Dominican Republic:
Britain and Ireland:
Newfoundland, Canada - where Leif Ericcson sailed:
Now we can look at some larger areas, such as peninsulas and continental regions. Let's start off with a self-portrait: Here's a picture of me, in Southern California, beneath the smoke from periodic wildfires - but it's an old picture, so don't judge my haircut too harshly:
The Arctic National Wildlife Refuge (ANWR) in summer, Northern Alaska:
The Kamchatka peninsula:
Lake Balkhash in southeastern Kazakhstan (hey, it looked interesting):
The Aral Sea:
Novaya Zemlya in Northwestern Russia:
Lake Nasser, Egypt (frozen):
Aegean and Adriatic:
An impact crater in the Chadian Sahara:
The Algerian desert:
Spain, Morocco, Algeria, and Portugal:
Hudson Bay, Canada:
Cape Cod, Massachusetts:
Argentina, Paraguay, and the Straits of Magellan (Chile):
We now come in for a landing on a typical Earth surface...