In the now-concluding Earth leg of our journey through the solar system, we examined 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
2. Mercury
3. Venus
4. Earth (Vol. 1)
5. Earth (Vol. 2)
6. Earth (Vol. 3)
7. Earth (Vol. 4)
8. Earth (Vol. 5)
9. Earth (Vol. 6)
10. Luna
11. Mars
12. Phobos & Deimos
13. Asteroids
14. Ceres
15. Jupiter
16. Io
17. Europa
18. Ganymede
19. Callisto
20. Saturn
21. Mimas
22. Enceladus
23. Tethys, Dione, and Rhea
24. Titan
25. Iapetus
26. Rings & Minor Moons of Saturn
27. Uranus
28. Moons of Uranus
29. Neptune
30. Triton
31. The Kuiper Belt & Scattered Disk
32. Comets
33. The Stellar Neighborhood
In the sixth and final volume of the Earth sub-series, we ascend into the gaseous envelope of the Earth, the rarefied and energetic environment of its magnetosphere, and the hauntingly beautiful realm of its near and far orbit, and then address contextual issues that put planet Earth in a historical framework both past and future. I realize that past entries in the Earth sub-series have bloated to a greater extent than may have been entirely constructive, so I'll try to be expeditious here.
I. Context
II. History
III. Properties
1. Orbital and Rotational Features
2. Size and Mass Characteristics
3. Internal Structure
4. Surface
A. Geography
B. Hydrosphere
C. Biosphere
D. Anthroposphere
5. Atmosphere
6. Magnetosphere
IV. Natural & Artificial Satellites
V. Past Relevance to Humanity
VI. Modern Relevance to Humanity
VII. Future Relevance to Humanity
VIII. Future of Earth
5. Atmosphere
i. Composition.
The terrestrial atmosphere is overwhelmingly nitrogen (N2 - 78%) and oxygen (O2 - 21%), with trace gases comprising the remainder. This fact helps explain the explosion of life that occurred when life on Earth first evolved the ability to breathe oxygen: Carbon dioxide (CO2) is a comparably miniscule proportion of the atmosphere at 0.035%, and has always been lower than 1% throughout the understood history of life. Being able to respirate the second most abundant gas in the atmosphere was a major advance. Water vapor is a very small proportion of the atmosphere overall, but at the surface can comprise low single-digit percentages.
ii. Layers.
There are five significant layers of the terrestrial atmosphere. In order of increasing altitude: Troposphere (altitude limits depend on latitude, up to about 20 km) - where humans generally live and operate, including in aircraft; Stratosphere (from limit of troposphere - 51 km) - where a very few high-altitude aircraft and balloons may operate, but well above Mount Everest; Mesosphere (50/60 - 90/120 km); the Thermosphere (90/120 km ~ 600 km) encompassing a huge volume including Low Earth Orbit (LEO) where the International Space Station exists; and the Exosphere (above 600 km) largely composed of rarefied, light gases like hydrogen and helium. The following diagram gives a sense of the relative size of each layer (except Exosphere, which technically just blends into the interplanetary medium):
Some layers of the Earth's gas envelope can be seen from given angles in LEO, such as the photograph above and these additional examples:
The relationship of temperature to altitude differs within the layers - i.e., it gets colder as you rise within the troposphere, but then the relationship reverses within the stratosphere because the atmosphere begins to absorb more solar energy at greater altitude, then starts getting colder again in the mesosphere, then hotter as you rise within the thermosphere. It should be noted that temperature is a more abstract concept the lower the pressure of atmospheric gases, because a person - if they could survive exposure to the environment - would not necessarily perceive it to be "hot" even if the temperature is quite high. Temperature is just a measure of the kinetic energy of gas molecules, and if there aren't that many of them, there are far fewer opportunities for them to collide with and impart their energy to an object.
iii. Circulation.
Due to the Earth's 24-hour rotation cycle, the Coriolis effect has a profound influence on the large-scale manifestation of terrestrial weather patterns - an influence strongly connected to latitude. Whereas on Venus latitude is primarily concerned with the force of winds due to the strength of solar heating on the middle and upper atmosphere, on Earth there is also a strong latitude-related rotational differential force on air circulation.
In other words, the relatively quick rotation causes a difference between the forces on the Northern and Southern boundaries of an atmospheric phenomenon, causing them to spin cyclonically as if being perpetually twisted. Hence the spiral formation of hurricanes and tropical storms. It's not as rapid as with the solar system's gas giants, but is quite rapid compared to Venus with its mainly linear wind patterns - Mars, by comparison, has a rotation only one hour longer, but is a substantially smaller planet and has much less of an atmosphere, so cyclonic patterns are less evident.
iv. Visual Phenomena.
Some of the major visual phenomena of the terrestrial atmosphere include smog (light-obscuring or diffusing anthropogenic pollution)...
Fog, when water vapor is thick enough at ground level to obscure visibility...
Puffy, stately-moving white cumulus clouds, which occur at low altitudes...
Towering, Wagnerian cumulonimbus clouds, occurring when cumulus build up tremendous volume and are blown to high altitudes by conditions presaging storms:
High, wispy, ethereal cirrus clouds...
Tornadoes - violent funnels of cloud caused by the shear forces of winds both rising and falling rapidly...
Lightning, caused by the sudden discharge of an accumulated electrical potential in a cloud system...
Rainbows are arcs of spectral color caused by the refraction of sunlight through mist...
Aurorae are colorful, high-altitude phenomena caused by the impact of solar wind particles on the upper atmosphere of Earth. They occur at high latitudes, both North and South (the Borealis and Australis versions of Aurora, respectively), due to the magnetosphere's channeling of solar wind particles toward the poles. Antarctica, Canada, Alaska, Greenland, Iceland, Scandinavia, and Northern Siberia are often treated to Aurorae, while they occasionally move into lower latitudes of Southern Chile and Argentina, or the Northern US, Japan, China, Korea, and some European countries unaccustomed to the light show. Other planets with powerful magnetospheres and thick atmospheres such as Saturn and Jupiter are observed to have aurorae.
v. Significance.
The atmosphere of Earth is critical to life, as it provides the pressure envelope needed to sustain liquid water at prevailing temperatures. Without this pressure, water would boil off the surface into vapor and then be chemically disassociated into hydrogen and oxygen by solar energy, the former of which would largely escape into the interplanetary medium while the latter remained to form various acidic compounds as on Venus. The atmosphere is also critical as a temperature regulator, as it both stores and rejects solar energy, maintaining a relatively consistent environment for life on and beneath the surface.
Life capable of surviving exclusively surrounded by atmosphere rather than living in the oceans is relatively new (substantially less than a billion years old), and an incremental step that would be more difficult with less of an atmosphere. The ability of life to survive on land depends on the ability of membranes such as skin to hold water inside, and atmospheric pressure makes that possible by keeping the boiling point of water high enough that biological processes do not exceed it. But even with these conditions, we must constantly replenish our supply of water, so it is clear that we land-based animals are simply ocean-dwelling creatures who've evolved the equivalent of "spacesuits" in order to walk on land in the open air. Basically we just die a lot more slowly than a fish would, but our survival out here is still highly conditional.
vi. General wind patterns from orbit.
A few unsorted images illustrating wind patterns as seen from satellites:
Just for fun, a very beautiful and interesting view of high altitudes as seen from a balloon, courtesy of a Mitsubishi Toshiba commercial created by volunteer-driven firm JP Aerospace:
And some more adrenalized explorations of air...
6. Magnetosphere
The Earth's core dynamo - the circulating metallic fluid of the Outer Core - creates a very powerful magnetic field that protects most latitudes of the Earth's surface from the solar wind and other space-based radiation. It does so by either deflecting ionized particles outward or channeling them toward the poles, where their collision with the upper atmosphere creates the aurorae. In other words, a much lower proportion of radiation reaches the planet than interacts with its magnetic field, but the polar areas get a much higher proportion of what does reach the surface than other areas. As a result, passenger aircraft that fly over the poles get a significantly higher dose of radiation than those flying over the equator.
The magnetosphere - the magnetic field encompassing the Earth - is much larger in size than the atmosphere, and protects humans even out into low orbit. However, it does not extend into the entire domain of Earth's gravitational influence, so astronauts and robotic probes traveling to the Moon or the Earth's lunar and solar Lagrange points would be unprotected by its field.
Since the magnetosphere is determined by the motions of a fluid, its features are not static over time, and not entirely stable or symmetric even now. The magnetic poles migrate, and deviate substantially from the planet's rotational poles. Also, there are parts of the planet that are less protected due to "holes" in the field caused by oddities in the field structure, such as the South Atlantic Anomaly - a region over parts of Brazil, Argentina, and Chile where particle flux is relatively high, and spacecraft generally have to plan in advance to pass through in order to avoid a greater risk of negative effects. Since no units are given in the following map, it's reasonable just to assume the scale is one of relative intensity:
The South Atlantic Anomaly is part of a large magnetospheric phenomenon known as the Van Allen radiation belts - a region of space with intense ionized particle activity due to the magnetic field. There are two belts - inner and outer - and spacecraft leaving low Earth orbit have to travel through them, exposing electronics and human crew to increased radiation for a brief period. As a result, mission planners typically try to have spacecraft traverse the belts at speed, and do so through thinner regions.
IV. Natural & Artificial Satellites
Hundreds of operational artificial satellites currently orbit the Earth, in addition to thousands of defunct ones, and countless small debris objects placed in orbit either by normal spacecraft operations or the unplanned collisions of larger objects left in space. International command centers coordinate orbits of controlled objects, and monitor those of uncontrolled objects for possible collision trajectories with valuable assets and crewed spacecraft. Since China infamously tested an anti-satellite weapon against one of its own assets, the amount of uncontrolled material in orbit has drastically increased. A vastly understated illustration of orbital debris:
The problem of debris in orbit is a significant one, and one likely to increase with time. Being in Earth orbit is already analogous to running a marathon around a track being actively used as a shooting gallery, and space assets are increasingly being designed with projectile armor in mind. This is a debris impact in a radiator panel of one of the Space Shuttles when it was still flying - the impactor was about the size of a grain of sand, but its kinetic energy caused it to create a much larger crater:
There is a theory regarding what will happen when a critical mass of orbital debris is reached, and it's called the Kessler Syndrome - a phenomenon articulated by Donald J. Kessler of NASA several decades ago as being a turning point at which a chain reaction of collisions and new collision-causing debris unfolds, making Earth orbit temporarily impractical to inhabit or occupy with expensive unmanned assets. Imagine a war where the shrapnel of every bullet fired and explosive detonated did not land, but simply circled the area at its peak velocity until it hit something, and then the shrapnel and debris from that collision also did not land, but joined it in circling endlessly. Then you have some conception of the Kessler Syndrome.
But that is still a theoretical possibility, and active efforts are underway to deal with it. Currently the largest artificial satellite by far is the International Space Station - the second largest object orbiting the Earth after the Moon. It is attended by supply and crew transport spacecraft that also qualify as artificial satellites - i.e., the defunct Space Shuttle, the soon-to-be-launched Dragon spacecraft, the Russian Soyuz, the European Automated Transfer Vehicle (ATV), and the Japanese H-II Transfer Vehicle (HTV). Unsorted images of ISS, Shuttle, ATV, and HTV (some juicy images of Dragon being integrated on the factory floor are available by clicking on the link above):
Then, of course, there's the Earth's Moon - Luna - which we'll deal with more fully in the next installment of this series. Suffice it here to review the key facts as already laid out about it in previous parts of the current sub-series: It formed from part of the proto-Earth's mantle that was blown off in a massive impact, its orbit has stabilized the Earth's rotational axis and made the planet far more amenable to life, and it is gradually getting farther away with time due to tidal friction.
V. Past Relevance to Humanity
Obviously, Earth is the world of our evolutionary genesis, and the world that continues to exclusively shelter and shape our development. But more broadly, its nature as a very diverse planet with a wide variety of available conditions makes it an ideal nursery for training a truly generalist intelligent species, provided we are able to learn the lessons the environment makes available to us. We have had, and continue to have, opportunities to live on perpetual ice; in underground caverns; in the rarefied air of mountains; on and under oceans; in blazing hot sandy deserts; in cold, arid plateaus; on the endless tundra; in sheltered canyons; amidst rolling hills and mind-numbing plains; and various places more specialized.
We left the nurturance of geothermal hot springs when sunlight became an available energy pathway via algal photosynthesis, then left the water when we evolved skin capable of keeping it internally long enough to survive between drinks, and we've spread to most of the Earth's environments thanks to the technological evolutions of clothing, shelter, and fire. With modern heating, cooling, recycling, and indoor climate control, we can live just about anywhere on this planet given the proper investment, and it will hopefully prove useful training for much more trying adventures off-world: Deserts and wastelands more total than the most desolate this world has to offer, and yet brimming with potential.
The circumstances of this planet are almost exactly poised on the cusp between hostility and nurturance, and between preventing and facilitating our escape. Its gravity is enough to make attaining orbit extraordinarily difficult, and yet not prohibitively impractical; its atmosphere fluid and yet not impassably violent; and its geology energetic enough to keep the atmosphere stocked with the proper gases, and yet not built up into a hellish cauldron. Personally, I subscribe to the simple anthropic explanation that such is the case because we wouldn't be here to note otherwise if it weren't - a simple, no-frills default explanation for conditions that cause far less imaginative people to gush in religious joy over how benign an actually quite cruel universe is. I think we'll find that, wherever we go, this razor's edge will follow us, because of the simple fact that on the side of a too-accommodating cosmos, we would too soon evolve beyond the point of noticing how fortunate we were, and on the negative side, die and thereby have no frame of reference from which to note the fact. But this is speculation and philosophy.
VI. Modern Relevance to Humanity
Earth is the basis for all of our science, and the theories by which we seek to understand other worlds. Its extreme environments guide our ability to model and plan for both robotic and human exploration of other solid bodies in our solar system - in particular, the Moon and Mars, but certainly not limited to them. And its more pleasant climes house the vast bulk of humanity concerned largely with the timeless march of raising families and getting by - the Home to which adventurers seek to return to by taking the long way around.
But more pertinently, it is the most massive body in the inner solar system, and utterly indispensable for gravity-assist flybys of probes (and hopefully someday human spacecraft) on trajectories to other worlds. Even more than Venus, which is another planet used extensively for gravity assist, Earth provides a very useful slingshot for spacecraft headed to various locations throughout the solar system, both closer to the Sun and further away. And it's not only the bulk of the Earth itself that is gravitationally useful - as mentioned before, it has several potentially very helpful Lagrange points, both in association with the Moon and the Sun.
VII. Future Relevance to Humanity
One of the oddities of Earth's razor's-edge duality with respect to human endeavor is that, I think, once we are largely free of practical dependence on it, we will not have nearly as much sentimental attachment to it as we who've lived on it our whole lives assume today. The way I think of it is this: How many people who don't live in that kind of climate yearn for the stifling tropical African "paradise" of human origins? Plenty are curious and find much to admire in the environment, but what proportion actually crave it or have any kind of psychological connection to it? Not many - it's just too much. Too hot, too humid, too many bugs, too many living things in general humans have to share the environment with. I think people born to space and the human-designed environments of off-world colonies will have similar attitudes toward Earth, seeing it as a cloying, disorderly swamp full of wild animals and creatures with uninterrupted natural evolution behind them.
Nonetheless, I have little doubt that, just as Africa, India, and China continue to be massively relevant to humanity despite their ancient pedigrees, Earth would continue to be a place of some vague respect and at least numerical significance even after it ceases to be the most prosperous, comfortable, or stable. But once we stretch the timeline into millennia, what I imagine is not a techno-utopia or a post-apocalyptic wasteland, but an an ever-less-important place with conditions that the denizens of increasingly strange eras find less and less interesting or appealing. It has a lot of raw material, but its gravity well would make it relatively expensive to transport that material from where it naturally occurs to where the chief economic centers of the far future would likely be. Even with such ultimately inevitable developments as space elevators, I'd say it would still be much cheaper to have a shorter, less strong, more easily manageable elevator transporting material from a much smaller body.
I think Earth will become a global City similar in some ways to Isaac Asimov's vision of Trantor, but with space elevator threads sticking out into space like the wisps of a dandelion seed head. Climate change will happen, and societal disruptions will happen because of it, but it will not stop us - call it arrogance, call it hubris if you will, but I do not believe humanity is stoppable short of a truly unlikely diabolus ex machina. We will grow, adapt, and evolve through war and peace, deprivation and prosperity, tyranny and liberty, intrepid inquiry and cowardly retreat. Earth will send out wave after wave of its children, retreat into itself, then send more waves, then repeat the cycle again through an unknowable number of cycles, and occasionally it will reabsorb some of its erstwhile offspring either as refugees or invaders.
But ultimately its ability to cope and change will fade, its charms become rustic rather than urbane (in purely relative terms, of course), and knowledge of its significance to humanity - or whatever vast ecosystem of intelligences humanity has become - will be a matter of obscure curiosity rather than general understanding or interest. Eventually, after the easy pickings elsewhere in the solar system had been exhausted for whatever incomprehensible projects they'd been devoted to, Earth's raw materials might again become of interest, but I doubt there would be any more sentimental reaction to the fact than you might to stepping on a grain of sand that witnessed the first amphibian.
At first, we will try to recreate our new homes on other worlds to be as much like Earth as possible, but then people will begin to realize the creative possibilities in their hands and begin to explore the unique potential of new environments. Terraforming will make new Earths out of Mars and Venus, and perhaps some other bodies - oddly enough, it may actually be easier to terraform small, airless rocks like the Moon, because all you have to do is release gases from the rocks at a faster rate than the gas can escape its gravity: A rate much slower than any timeline human civilizations are accustomed to. But that will be messy, and increasingly less convenient than more "packaged," comfortable alternatives to recreating all the chaos and delicate instability of a natural ecosystem. Far down the line, Earth would look less like a lifestyle option than a primordial ocean, beautiful, exotic, and utterly creepy. It may be kept as a wildlife preserve or park, or it may simply be torn apart for its mass.
VIII. Future of Earth
Provided the civilizations that succeed ours in the unfathomable deeps of time don't strip-mine the joint out of existence, Earth has a well-known and predictable future history as a physical body. Due to the tidal friction with the Moon already mentioned, Earth's day will gradually slow down and in fact has been slowing down over the whole course of its history after the impact that created the Moon. About 1-2 billion years from now, as our Moon gets further and further away, its ability to regulate Earth's axial tilt will decline and the planet's axis will migrate over a wide range of orientations that will badly destabilize climate and turn the planet periodically into an iceball and a blazing global desert. Climate will gradually come to favor the latter condition as the Sun expands, with the terrestrial oceans boiled away and its atmosphere blown off, and in one of the Sun's later expansionary periods it will engulf the Earth. Our planet shares this fate with its sister Venus, and also Mercury.
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An eerily appropriate song for our homeworld:
