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
2. Mercury
3. Venus
4. Earth (Vol. 1)
5. Earth (Vol. 2)
6. Earth (Vol. 3)
7. Earth (Vol. 4)
8. Luna
9. Mars
10. Phobos & Deimos
11. Asteroids
12. Ceres
13. Jupiter
14. Io
15. Europa
16. Ganymede
17. Callisto
18. Saturn
19. Mimas
20. Enceladus
21. Tethys, Dione, and Rhea
22. Titan
23. Iapetus
24. Rings & Minor Moons of Saturn
25. Uranus
26. Moons of Uranus
27. Neptune
28. Triton
29. The Kuiper Belt & Scattered Disk
30. Comets
31. The Stellar Neighborhood
In volume 3 of our journey to Earth, we explore the most dynamic and important surface layer of the planet - that concerning water. Our progress in examining Earth, with current volume in bold:
I. Context (Vol. 1)
II. History (Vol. 1)
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
IX. Catalog of Exploration
B. Hydrosphere
i. General Attributes
The most distinguishing feature of Earth is the simultaneous presence of large quantities of water (H2O) in solid, liquid, and gas phases, and the stable cycling of the planet's water between all three. Everywhere else in the solar system, water is either absent due to hydrogen loss (see the bottom of the Venus diary section on the atmosphere for more details), present overwhelmingly as ice, or is a small vapor component of gas giant atmospheres. In addition, there are likely hot, rocky planets around other stars where the presence of a magnetic field allows water to be kept, but largely (or exclusively) as thick, perpetual storm clouds over a relatively dry surface where any liquid bodies would mainly be at high latitudes.
The domain of a planet where water dominates is called the hydrosphere, and on Earth extends from the uppermost mantle into lower atmospheric clouds, existing as liquid trapped in rocks, standing bodies of surface liquid (oceans and seas), solid shells (ice) over high-latitude liquid bodies and land surfaces, and cloud vapor. While these three phases are all significantly represented, they don't occur equally: Water is overwhelmingly (96.7%) liquid on Earth - the gaseous phase is about 0.3% of the total hydrospheric mass, and ice is about 3%, although this varies somewhat over geologic time due to ice ages and hothouse climates. The hydrosphere is sometimes defined as being limited to liquid, with ice existing as a separate layer called the cryosphere (and vapor being just an atmospheric component), but the dynamism of water on Earth makes this distinction largely moot. We will, however, make use of the distinction in later diaries, when we come to worlds where ice actually does form a complete sphere.
If all the water on Earth were to exist as a free-floating object in space, it would be about the size of Mongolia, somewhere in scale between Pluto's co-object Charon and Saturn's moon Iapetus. Most of the water present on Earth today originated elsewhere in the solar system, impacting the planet as comets and ice-bearing asteroids. However, this is not because our world was originally dry - quite the opposite: During the early stages of its formation, Earth had many times more water than it does today, but virtually all of it was blasted away by the Theia impact that created the Moon (see Vol.1 of the Earth sub-series for more details). Subsequent, smaller impacts restored a minority of the lost water mass, as did venting of small amounts still inside the mantle.
It isn't very surprising, then, that the total amount of water on Earth is not especially great compared to other bodies in the solar system: Europa, Ganymede, Callisto, Titan, and Rhea are thought to have more water - in some cases much more. Ganymede, for instance, is thought to have a water mass nearly 39 times greater than Earth's. But in every case, the water is nearly all ice, with any liquid that may exist occurring in deep mantle layers. Large Trans-Neptunian Objects (TNOs) like Eris and Pluto may also have more water, though likely all ice frozen straight through.
ii. Molecular Importance
Due to the strong electronegativity of the oxygen atom - the intensity with which its nucleus attracts electrons - the oxygen in an H2O molecule does not have an equal relationship with its two hydrogen counterparts: Their common electrons spend a larger amount of time in the vicinity of the O nucleus than near the H nuclei, causing the former to develop a partial negative charge while the latter have a partial positive charge. This is called polarity, and water is thus a polar molecule. As a result, the hydrogen atoms of one water molecule will be somewhat electrically attracted to the oxygen of another, forming weak but significant relationships called hydrogen bonds that cause liquid water to be "sticky."
This property has three very important consequences: The hydrogen bonds absorb large amounts of energy, and release it slowly - thus water has an extraordinary ability to insulate and regulate temperatures. Secondly, at the boundary between a body of liquid water and the atmosphere, the fact that all of the polar attraction experienced by the top layer of molecules is coming from beneath and to the sides pulls it into a tougher, more resistant surface - a property called surface tension. And thirdly, since any other kind of molecule with a whole or partial charge can also be attracted to water, it serves as a powerful solvent - a fluid medium in which other molecules are suspended. All three properties make liquid water an ideal environment for life under terrestrial conditions - it keeps temperatures stable, is mechanically both fluid and supportive, and it can deliver all manner of gases and nutrients in its capacity as a solvent.
Humans directly experience all three properties on a daily basis. For instance: When the air is relatively dry, sweat has a strong cooling effect because the evaporating water absorbs large amounts of energy in the process. Jumping into a placid pool with your body parallel to the water results in maximum resistance, giving a painful lesson in surface tension - a belly flop. And soups, broths, flavored beverages, and mixed drinks all exhibit the solvent properties of water. As an illustration of surface tension, a paperclip composed of much denser materials than water will nonetheless float on its surface if placed gently:
As a result of the strong bonds holding the H2O molecule together, it never changes chemically except in very high-energy processes like photosynthesis, and consequently doesn't change the substances it dissolves - they are merely suspended, not altered. Because it neither gives nor takes hydrogen nuclei (protons) away from other compounds, it has a neutral pH - it is neither acid nor base, allowing it to mediate chemical reactions without participating in them. As both oxygen and hydrogen are abundant elements, and H2O is a simple molecule with strong bonds, water is ubiquitous in the universe, and likely supportive of life in a large absolute number (albeit very small relative proportion) of planetary environments.
But water's ability to absorb energy is not its only temperature-regulating property: In addition, its most abundant crystals are less dense than the arrangement of molecules in its liquid phase, causing ice to float at the surface rather than sinking to the bottom. As a result, water freezes from the outside-in: A process that hampers its own progress by making the liquid-solid boundary increasingly far from the atmosphere, thereby insulating it more the deeper it freezes. This is the exact opposite circumstance to the vast majority of materials, which freeze from the inside out - their liquid-solid boundary gets closer and closer to the freezing atmosphere, accelerating the process.
iii. Hydrological Cycle
Water circulates around the planet through the hydrological cycle - a process of phase changes and migration due to temperature changes, gravity, and fluid currents. As a result of H2O's strong ability to absorb and slowly release energy, its cycle is critical to the stability of terrestrial climates - especially in keeping day/night temperature differences manageable by cooling daytime air as it absorbs heat and then warming it at night by releasing it. Hot desert regions that lack this benefit locally experience much more radical temperature swings between night and day, as much as 50 C° (90 F°) or more under extreme conditions. In addition, the hydrological cycle balances seasonal changes by allowing heat exchange between the hemisphere in summer and that in winter, somewhat moderating summer heat and winter cold.
As a cycle, this process doesn't "begin" anywhere (unless we reverse time to a given molecule's introduction by celestial impacts), and it has many interconnected sub-processes rather than a single continuous loop. However, we can enter the cycle at the origin of its lowest-altitude layer (not shown above), where water is introduced into the uppermost mantle by subduction of oceanic crust (see Vol. 2 of the Earth sub-series for more details). As the descending slab is heated by the mantle, lower-density components such as water are heated and escape into the surrounding material. This lubricates the highest layer of mantle, the asthenosphere, and makes stable plate tectonics possible by allowing the mantle in this region to move flexibly.
Eventually the water escapes back through the crust, either due to the expansion of mid-ocean ridges, or through fissures in the crust appearing as volcanoes, hydrothermal vents, or geysers. Along the way, the water carries a number of dissolved minerals that may preferentially remain in the mantle or react to form different compounds, and on the return journey will bring a different set of materials to the surface. Some of the emitted compounds may be relatively complex, and include substances that promote organic chemistry in the hot liquid environment. An illustration of one local cycle on the seafloor:
At the boundary between the oceans and the atmosphere, surface tension strongly holds the H2O molecules within the liquid. But every once in a while, a photon - a particle of light energy - will hit the water either directly from the Sun or reflected back to the surface by greenhouse gases in the atmosphere, and will energize a water molecule sufficiently that it escapes the surface tension and jumps into the air. Most of these erstwhile molecules will soon let go of their newfound energy and sink back into the water, but a few will linger near the surface in mist and fog - little outlying "colonies" of vapor that are cool enough to stick to each other, but light enough to remain suspended in air. Most of this water will return to the ocean either by accreting to its surface or falling as a light rain.
But some will be energized further, escaping into higher layers of the atmosphere as diffuse molecules that will only reform into clouds at greater, cooler altitudes. Since the vast majority of Earth's surface is ocean, most of these clouds will return to the water directly as rain, although a few will make landfall. Some small amount will be energized further and soar into the high atmosphere, forming suspended ice crystals and taking a slow journey back to the oceans. Of these molecules, a very small quantity will become so energetic, and reach such high altitudes that they're broken apart into constituent oxygen and hydrogen - some of which recombine to form other gases, some form water again, and some are ionized by solar energy (i.e., lose one or more electrons), but at this point it is no longer water.
Of the clouds that make landfall, water can reach the surface by accreting to it as dew or frost (a deposition process, where it goes directly from being suspended to accumulating on the surface without intermediate steps), raining (liquid delivery), snowing (low-density ice crystals), or hailing (high-density ice crystals), depending on air temperature and pressure conditions - which are in turn affected by altitude.
Dew either evaporates back into the air, or is absorbed by plant organisms through direct exposure or by absorption into the soil and thereby into roots. Once plants have used the water, it is excreted by transpiration through exposed surfaces - a process of controlled evaporation that allows plants to regulate water intake and output. Water not used by plants is absorbed into porous underground layers called aquifers that act as natural reservoirs. Frost melts and is used by plants, seeps into aquifers, or accumulates into thin ice sheets that may remain seasonally and is built upon by snow and hail. Rain and melted snow/ice pack from waning cold seasons will replenish aquifers, flow to the ocean by joining rivers, and also evaporate to some extent along the way.
At the highest altitudes and latitudes, snowfall accumulates into thick, more or less permanent ice sheets - glaciers - whose size and thickness wane over geologic time due to fluctuations in atmospheric chemistry and thermal circulation. Earth is currently in an accelerating interglacial epoch called the Anthropocene in which glaciers are melting more rapidly than snowfall can replenish them, due in large part to accumulating carbon dioxide and methane (CH4) in the atmosphere from human activity and its secondary consequences. Sea level and glacier size are inversely related - i.e., the thicker glaciers become, the lower sea levels are, and vice-versa - so sea levels are currently rising.
Even when glaciers accumulate, some level of melting always occurs, and on land - particularly in mountain ranges - this meltwater provides the sources of rivers. As they move downhill, rivers carve channels in the landscape, suspending minerals from rock in the water and joining together into larger bodies carrying more water. These minerals are then deposited at lower altitudes along the course of the river, building large, verdant plains beneath the mountain ranges where the rivers originate. As the water approaches the oceans, the land gets flatter and the river spreads out into a delta where the soil is relatively rich in nutrients, and biodiversity tends to be high. Where the freshwater of the outflowing river becomes saline (salty), that is where the ocean is considered to begin.
iii. Oceanic Layers
Earth's surface is 71% liquid water, and the vast majority of that occurs as the planet's single, world-encompassing ocean - although humans distinguish it into smaller oceans for geographic and navigational reasons. The landscape of the ocean floor differs in much the same way as continental crust, with mountains, valleys, and plains, although the geography is overwhelmingly dominated by the long mid-ocean ridges.
The lowest point on Earth is called Challenger Deep - the absolute bottom of the Mariana Trench in the Eastern Pacific. Its floor is up to 11 km beneath the ocean surface - deeper than Mt. Everest is tall - and pressure at the bottom is up to 1000 times greater than at sea level (more than 10 times greater than the surface of Venus). Despite this enormous pressure, the temperature of the water is only a few degrees above freezing, so it never becomes supercritical the way that Venus's atmosphere does: Water is exclusively liquid at the ocean floor. Thus far, three expeditions have been made to Challenger Deep (1960, 1995, and 2009), with only one - the 1960 voyage of the Trieste - being a manned mission. Below is a photo from inside the Trieste, and then a video of the Nereus robotic vehicle operating on the surface of Challenger Deep.
All that was found at the bottom were small worms and soft-shelled fish in a relatively bland, undynamic landscape. This result was unusurprising, since the Challenger Deep occupies the lowest ocean layer, the Hadalpelagic (or Hadal) Zone - one deep enough where sunlight cannot penetrate, the seawater is frigid, and little in the way of nutrients makes it to the bottom. This is in contrast to higher layers, where increasing amounts of light, and decreasing amounts of pressure allow increasingly complex forms to life to exist. A diagram illustrating ocean layers:
From the bottom up to a depth of about 200 m, the water is pitch-black - sunlight is completely blocked. This region, encompassing the vast majority of the ocean's volume, is called the aphotic zone - a region characterized by low (near-freezing) temperature, increasing pressure with depth, and the cessation of photosynthesis. Organisms that live here feed on detritus descending from above or on each other, and are evolved to survive in the cold, darkness, and high pressure: They are either blind or only capable of seeing dim bioluminescence, and their organ membranes will rupture from decompression if carried to higher ocean layers under lower pressure. Although the aphotic zone is very large and deep, the biota are very similar throughout - small invertebrates, soft-shelled mollusks, and bony, monstrous-looking fish, although wondrous displays of bioluminescence also occur. Some examples:
The temperature of the ocean is separated into two regions - the vast majority is only a few degrees above zero Celsius, comprises most of the ocean volume, and is roughly contiguous with the aphotic zone. However, beginning at about 1000 meters in depth up to about 500 meters, the effect of the Sun begins to be felt and temperature starts to rise rapidly with ascent. Beneath this point, the water is relatively calm and static, while above it the water experiences considerable mixing and convection driven by the absorption of sunlight. The boundary where this begins to occur, where the temperature rises quickly, is a thin region called the thermocline that is reflective to sonar - a fact used in whale communication to bounce signals greater distances. A diagram of ocean temperature with depth:
Beneath the thermocline, ocean temperature is almost uniformly frigid; above it, the temperature is close to that at the surface. As one would expect, the most diverse marine life tends to live above the thermocline, in the ocean layers (specifically, those in the photic zone) where sunlight is a significant influence and photosynthesis occurs - i.e., the epipelagic zone 200 meters and up.
iv. Ocean Movement
Globally, Earth's oceans move in continuous currents that cycle around the planet and are crucial to heat regulation and climate stability. While some currents occur on the surface and are driven largely by wind or the Earth's rotation, the deepest and most important currents - part of an integrated global-scale system called thermohaline circulation (or the global conveyor) - are driven by differences in temperature and salinity. These currents carry warm water from the equator to the poles near the surface, release their heat into the atmosphere, sink to a lower depth, and then bring cool water back to lower latitudes, managing global temperatures and maintaining the oceans within a close thermal range. Without this circulation, equatorial regions would become much hotter and drier while polar glaciers would expand as high-latitude temperatures decline and temperate regions shrink between the two extremes. An illustration and animation:
One scenario alarming climate scientists today is the possibility that melting freshwater from Greenland ice - as well as increased rain and snowfall from the exposed waters of an ice-free Arctic Ocean - could disrupt the thermohaline current in the North Atlantic by making cold surface water less likely to sink. This could be a big problem, because freshwater is less dense than salt water, and its thermally-driven descent in the North Atlantic is a strong driver of the global current, so if the balance of freshwater changes too greatly, less downward flow will occur, less energy will drive the current, and cool water will be carried to the equator at a lower rate. By consequence, less warm water would be carried back up to the North Atlantic from the warmer regions.
If this were to happen, climatologists think the results could be catastrophic for Europe and the Northeastern part of North America, leading to frigid conditions similar to those experienced in Russia. The fact that global warming can lead to radically colder climates on the regional level due to the disruption of complex thermal balances is one of the more difficult lessons of climate science, and one of the more troublesome threats of climate change. In addition, the disruption of the current could also impede the ability of lower latitudes to export heat Northward, making the tropical climates of the Gulf of Mexico and equatorial Atlantic wetter and more violent, spawning stronger and more frequent hurricanes.
v. Major liquid bodies
The Pacific Ocean comprises the largest and most regular surface on Earth - it should be thought of as the representative face of our planet:
Indian Ocean:
Atlantic Ocean:
Mediterranean Sea:
Black Sea:
North Sea (Summer):
Baltic Sea (Summer):
Caspian Sea:
White Sea (Summer):
Barents Sea (Summer):
Bering Sea (Summer):
Gulf of Carpentaria (Australia):
Gulf of California:
Gulf of Mexico:
Great Lakes:
Hudson Bay:
vi. Ice
As illustrated in Vol. 1 of the Earth sub-series, higher latitude results in sunlight reaching the terrestrial surface at a more acute angle and thus spreading the same amount of energy over a larger area - i.e., the light is dimmer and thus its ability to warm the surface is diminished. This is largely true of any planetary (or sub-planetary) body with respect to its parent star(s), although bodies with very thick atmospheres may not be especially affected beneath a certain atmospheric depth. On Earth, however, the atmosphere is sufficiently thin that - subject to complicating factors like ocean and wind currents - it is generally correct that the further one gets from the equator, the colder the climate becomes.
As a result, this planet currently has three large, heretofore "permanent" ice regions (I qualify their permanency because of global warming trends), all at high latitudes: The Antarctic ice sheet (South pole), the Arctic ice pack (North pole), and the Greenland ice sheet (Northern sub-Arctic). The Antarctic ice sheet - the largest of the three - more or less completely covers the continent (98%), and extends substantial distances out into the ocean:
All large ice sheets form from the long-term accumulation of snow that fails to melt as quickly as it falls, and land-based ones (the Arctic sheet is nearly all oceanic) are fractured by geography and their own (quite slow) fluid motion into independently-moving glaciers that flow downhill toward the ocean. The rate of flow varies, but a very fast-moving glacier will take over a year to move a few kilometers. A flow-rate map for the Antarctic ice sheet, with glacier fault lines depicted (the circle is centered on the South Pole):
Once the ice sheet reaches its natural limits, calving takes place - the fracturing of ice on the edge from the main body. Most of the time, calving occurs either as chunks of ice falling from a sheer face into water and floating away as small icebergs, or as a more gradual, river-like flow of ice into the ocean. However, on infrequent occasions there can be very large calving events - an entire face of a glacier can shear off and collapse into the water, causing local tsunamis; or a large expanse of ice that has extended into the ocean will simply lose contact with its parent glacier, floating away toward higher latitudes and gradually melting as it goes. Calving in action (from various global locations):
Of particular interest are Antarctica's subglacial lakes - bodies of liquid water overlain by the thick Antarctic ice sheet. Not only have these lakes been physically isolated from the external environment for millions of years, likely preserving unique genetic lines and strange ecosystems, but researchers see them as possibly being a terrestrial analog for environments that may exist beneath the global ice shell of Jupiter's moon Europa. Among these lakes, the largest - and the one receiving the most scientific attention - is Lake Vostok. Researchers have not yet penetrated the ice to the liquid layer, although they are close - and generating a great deal of controversy due to their method of drilling. An illustration of what is thought to be the structure of Lake Vostok:
The Antarctic ice holds many strange, beautiful, and desolate sights for the traveler hungry for alien vistas:
In the Northern hemisphere, about 80% of the landmass of Greenland - a giant island the size of California, Oregon, Washington, Idaho, Utah, Nevada, and Arizona combined - is under perpetual ice typically between 2 and 3 km thick. The sheer weight of the ice has actually depressed the central region of Greenland's underlying rock to beneath sea level, so if extreme global warming scenarios were realized and the entire ice sheet melted (and if this failed to cause a long-term cold snap due to thermohaline disruption, as described above) there would be large inland seas, making Greenland into a ring rather than a solid land mass. Topographic map of Greenland without ice - note that the ring of land in an ice-free scenario would actually be slightly narrower than this because sea levels would be higher:
Partly due to the shallower gravitational gradient (i.e., Greenland in general doesn't rise very high), and also to the internal depressions shown above, its glaciers are relatively slow-moving compared to Antarctica - about 200 meters per year, with increases largely in the very last legs of the journey as the ice approaches the sea. Flow map:
Some views of Greenland from orbit:
Greenland also has its share of otherworldly geography due to the ice from closer perspectives, from the air and on the ground - as air travelers can attest who've flown from North America to Europe (routes often cross the Arctic):
A brief snippet from a documentary about the melting of Greenland's ice sheet:
North of Greenland is only the Arctic Ocean, so the Northern ice cap is a flat, floating expanse - essentially a single giant iceberg, although it branches and calves extensively due to thermal, seasonal, and current properties. During winter it pervades the Canadian interior seas, is contiguous with the Northern coast of North America, extends down the coasts of Greenland, closely follows Siberia, and causes the North Atlantic to have a substantial amount of iceberg activity.
However, the Arctic ice is a hollow shell mostly about 3-4 meters thick, and liquid water remains within easy reach of the surface through cracks and fissures. Arctic penguins, seals, and polar bears are able to use this fact to their advantage, diving into ice holes for food and then returning to mate or lounge on the surface. The landscape is, for the most part, flat and featureless, with only small ice crumbles and ridges - often beneath waist-height - breaking up the monotony. Where snow has accumulated enough, the surface is completely featureless, but in the Southern parts or during Summer, it becomes much more diverse due to differential melting. Map of Arctic ice thickness:
There is a quite beautiful environment under the Arctic sea ice:
Back on land, as glaciers flow they carve distinctive landforms in the underlying rock that look very different from features created by liquid water, wind erosion, or plate tectonics. This is especially true for mountain glaciers, which flow downhill relatively quickly in river-like streams due to the steepness of the gradient. For instance, in the following image of Mt. Everest from orbit, the track that looks like a road was carved by the flow of a glacier:
Retreating glaciers also leave behind remarkably smooth rock faces, huge expanses of small lakes, and fjords - narrow, highly-branching channels in coastal rock. All of these features are particularly interesting because we also see them on other worlds dominated by ice - especially Saturn's moon Titan, which also has bodies of liquid on its surface (although not water). You'll recognize overhead views of fjords from the images of Greenland's coast - features they also heavily share with Scandinavia and Northern Canada. We'll see lots of these on Titan. Examples of glacial erosion on rock:
Beneath the furthest extent of the permanent Northern polar cap, there are large regions of seasonal icing - mainly Canada, Russia, Scandinavia, and various desolate Northern places with minimal or no population (like Svalbard). Some images of seasonal ice (mouse over to see the location):
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vii. Rivers
Rivers are the most important part of the hydrosphere for complex life, so it's no surprise that human civilization arose on riverbanks and is still economically bound to them. By volume flow, Earth's main non-tributary rivers are: Amazon, Ganges, Congo, Orinoco, Yangtze, Paraná, Yenisei, Zambezi (location of Victoria Falls), Lena, Mississippi, Mekong, and the Yellow River. The Nile is further down the list, but is highly significant due to its length, history, and the fact that it is surrounded on both sides by large deserts, making obvious how powerfully a river can affect the environment.
The Amazon can be said without hyperbole to be the current epicenter of life on Earth:
The Ganges is famous for its associations with Indian culture and ritual:
The Congo feeds one of the most diverse, densest tropical rainforests on the planet, and also serves as the backdrop to one of the most politically and economically chaotic human environments:
Orinoco:
The satellite image of the Yangtze below demonstrates the important function of rivers in carrying sediments downstream and ultimately into the sea - it is this which makes river and coastal water rich in mineral nutrients for living organisms:
Paraná:
Yenisei:
Zambezi:
Lena:
Mississippi:
Mekong:
Yellow River:
The Nile is both storied and awesome in its significance, and provides a stark contrast between environments with and without water:
And, just because I thought it looked interesting, here is the Ob River in Russia:
viii. Special Note: Deserts
We can't possibly understand the effects of the terrestrial hydrosphere without at least glimpsing those regions of Earth where its influence is limited - particularly, the hot deserts where water exists solely in scattered oases or high-altitude vapor that almost never rains to the surface. So I close the active part of this entry with one of the most otherworldly scenes on the planet - a remarkable, isolated spot in the Libyan Sahara that looks like something out of a Frank Herbert novel:
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As a special treat, I ran across the video below. It is absolutely, 100% true-color and real-time - everything you see in it is exactly what you would witness out the windows of the International Space Station. The Earth passes beneath at the same rate it would if you were traveling at an orbital velocity. If you have the visual imagination to do so, try to imagine the scenes you see filling your entire field of vision - it's enough to give vertigo, I promise. Needless to say, I recommend going full screen - you may also wish to mute the music, as it quickly becomes repetitive and annoying. The viewpoint shifts between horizon shots, angled shots, and straight-down, and the environments glimpsed are awesomely diverse. Just as you think all the bases have been covered, a new and strange scene appears. You may find some of the views familiar from imagery above, but there is quite a lot that is totally new. Even though it's quite a long video, I guarantee it's fully worth the time:
