S3C2: The Hydrosphere

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a) Role of water in life

The hydrosphere can easily be defined as the aggregate of all the water present on a planet, under all its form, be it solid ice, liquid in the ocean or groundwater, or as vapour in the air. Sounds mildly interesting, though perhaps not to the extent of dedicating an entire chapter to it. Well, not so because the entire Series 2 dealt with human biology, occasionally touching on that of other species, mostly animals and plants, and even Series 1 introduced the concept of information, structure and function, and evolution, all coming together to support the emergence of complex life. And so, there is a very strong argument that water molecules are a key facilitator in this emergence, perhaps even an indispensable one. This is undoubtedly a fact for life as we know it, on our planet Earth, but there is also a genuine argument for extending this statement more broadly across the universe.

No claim is being made here, and it isn’t the primary focus of this chapter, however I will simply highlight a few properties of water molecules (H₂O) that aren’t easily matched by other molecules.

In liquid form, water responds well to intermolecular forces so that, by combining the phenomenon of surface tension and adhesion, it is subject to the passive process of capillary motion. You can observe this by dipping a sheet of paper in a water-based liquid and the sap of plants partly relies on this mechanism. And humans rely on plants, directly and indirectly, as a food source.
Another passive process called osmosis takes place in aqueous environments and is key in regulating some of our internal operations such as maintaining the electrolyte concentration level in blood plasma (refer to S2 Section 2.d). It also underpins the turgor pressure allowing plants to keep their external shape with water being attracted within the cells and creating pressure on the cell walls.
H₂O is indispensable in many chemical reactions and cascades within our body, often by way of hydrolysis where the molecule splits the chemical bonds of other reactants and its hydrogen and oxygen atoms are recombined to form simpler products. Thus, the degradation of ATP by hydrolysis liberates the energy powering our organism.
The creation of cells relies on the hydrophobicity of lipid membranes, as seen in S1 Section 4.d, and in turn this feature is the foundation of cells, often called the “fundamental unit of life”, as discussed in S1 Section 4.e.

This may need to be rephrased as “fundamental unit of complex life” although complexity is a continuum, not a binary property. There are plenty more established and purported unique, indispensable roles played by water in processes that make life as we know it possible on Earth. And just as there is an argument to say carbon is key in the development of complex life, there is an equally strong one for including water in this rarefied category. If the propensity to bind is what gives carbon its edge, for water it is its physical properties such as polarity and its chemical versatility, be it acting as a micro-environment or as a reactant.

There are also strong arguments suggesting complex life requires a mix of liquid and solid states of matter to develop, as is the case in animal bodies and those of plants and fungi. The solid parts are important to store information with high-fidelity and perform functions reliably – in either case, we don’t want molecules changing places constantly. And many homeostatic processes require a liquid environment to take place, as we have seen a few paragraphs ago.

What about gas? Well, it works well to extract certain molecules from but neither information storage nor structure nor proper function can develop on account of all those molecules bouncing around. Therefore, it is hard to imagine how life would have evolved within the mantle of the earth, let alone its core, on stars or on gaseous planets like Jupiter and Saturn – way too much kinetic energy there.

This means the range of temperature and pressure experienced on a planet is important when it comes to its ability to harbour life. This is the idea of the goldilocks zone, neither too far nor too close from a star and with an orbit that isn’t too elliptical to avoid dramatic variations, although the heat emanating from the core of a planet can serve to counterbalance its being too far away to permit life to develop at its surface, exposed to its atmosphere or outer space. Certainly, it would help but it looks like probabilities are better by being a little far like Mars than too close like Mercury.

With all that said, isn’t there another molecule with analogous properties to water? The best candidate in the list is ammonia (NH₃), it melts at around -78°C and boils around -33°C at 1 atm. Like H₂O, NH₃ is polar but the temperature range for its liquid state is only just over 44°C as compared to 100°C for water so there is less flexibility built-in. My guess is that ammonia-based life probably does exist somewhere but it may not have allowed for as much complexity to develop, compared to water.

Can’t we imagine a purely solid-based form of life though? My sense is that the answer is “no”, for a couple of reasons having to do with mobility and fluidity, or lack thereof. Life requires access to nutrients, movements towards preys and away from predators with evolutionary pressure ensuring that over millions of years complex forms of life will eventually emerge. Nonetheless, mobility requires the rearranging of solid structures across fluid environments , whether in gas or liquid state. Think about muscle contraction, about the structure and content of cells, and about animal locomotion on the back of the musculoskeletal system. On a planet where everything is in solid form and this mix would not be present, it is difficult to fathom how specialized, mobile structures could develop.

The final question we may ask is whether water is ubiquitous? Actually, going by our solar system alone, the answer is that it isn’t rare. It was very much present at the surface of Mars for a while and still is today mostly as ice underground and there is a high degree of confidence on it being present on several of Jupiter and Saturn moons such as Europa and quite possibly on both Uranus and Neptune themselves.

At least, on Earth, water is ubiquitous at or near the surface and, thanks to the water cycle we’ll explore in section d), it is constantly being redistributed so that life can be found almost anywhere.

b) The Blue Planet

For the rest of this chapter, we’ll ignore the water present within living organisms and concentrate on the hydrosphere of our planet, subdividing it into 4 categories: oceans and seas, fresh water above and below ground, atmospheric moisture, and water in solid state as ice or snow. It won’t be a purely descriptive exercise though and each time we’ll try to link back to what we know of or have observed in the preceding Chapter 1 on Geology as well as to the impact on and relationship with the biosphere, defined by the Encyclopedia Britannica as a “relatively thin life-supporting stratum of Earth’s surface, extending from a few kilometres into the atmosphere to the deep-sea vents of the ocean” and “a global ecosystem composed of living organisms (biota) and the abiotic (non-living) factors from which they derive energy and nutrients.”

Seawater, with an average salinity of 3.5%, accounts for over 96% of the total volume of water with seas and oceans covering ~71% of the surface of the Earth and the largest bodies being the Pacific, Atlantic, Indian, Antarctic and Arctic Oceans, in that order. These appear blue to us, hence the origin of the name “blue planet” when the globe is seen from space. The rationale behind the primacy of blue colour when it comes to water has to do with the absorption of light with longer wavelengths, what we perceive as red, which increases with depth and the refraction of the shorter wavelength, in the blue spectrum, a phenomenon enhanced by particles floating in water, as in glacial lakes. When phytoplankton is present, their absorption of light in the red and blue wavelengths yields the colour green.

By the way, there is no formal property distinguishing seas from oceans, the former are either fully or mostly enclosed by lands or are simply a sub-region of the latter. Respective examples include the Mediterranean Sea and the Arabian Sea.

The seas and oceans represent a stupendously diverse range of ecosystems with variations across depth, temperature, proximity to shorelines, warm or cold currents, and salinity. Let’s look at a few examples:

The intertidal zone, also known as foreshore, is this part of the littoral that is submerged at high tide and above water at low tide. Clearly not every species can survive, let alone thrive in alternating aqueous and gaseous environments. This type of zone is particularly propitious to barnacles, crabs, mussels, starfishes, and sea urchins.
As depth increases, so does pressure but sunlight decreases to the point where photosynthesis is no longer possible. In oceans, by the time depth reaches 200n, the strength of electromagnetic waves (light) is only about 1% of its surface level; this upper oceanic layer is called the photic zone and is home to phytoplankton who power their growth via photosynthesis and serve as a key source of oxygen in our atmosphere, an estimated 50% in fact, and form one of the main basis for the marine food chain.
Near volcanically active spots, some fissures called hydrothermal vents see the escaping of geothermically-heated mineral-rich water. It is hypothesized primordial life may have originated in such ecosystems and this would theoretically make life possible on planets that are geologically active, even if their surface is frozen.

Liquid fresh water on the other hand accounts for less than 1% of the total water volume on Earth. And so while there is interest and diversity in the river, lakes and wetland ecosystems, the main difference compared to saltwater is its use in supporting terrestrial life, either naturally or through irrigation.

When I write “naturally”, this refers to the presence of water in the ground that plants rely on, together with other nutrients, for their metabolism – you may want to refer to S2 Section 2.f on nutrients absorption by plants. Water may spread horizontally into narrow bands around bodies of fresh water, or not so narrow bands when floods occur. At the same time, rivers can carry with them minerals-rich silt making annual crops viable and therefore creating the foundations for entire civilizations, as was the case for the annual floods along the Nile River. If this is of interest, you may want to read the opening chapters of the book The Rise and Fall of Ancient Egypt where the author draws a startling picture of the importance of the river and related landscape on the cultures of Upper and Lower Egypt.

Water can travel some distance and remain some time underground, in the form of subterranean aquifers. These can provide a source of water and indispensable minerals for plants and humans alike who can access it via their roots or wells, respectively.

c) The Cryosphere

Altogether, freshwater tallies around 2.5% of the total volume of water on our planet but in the previous paragraph I stated the number of 1%; this is because I referred to the liquid state and 70% of the freshwater is in solid state. This encompasses various forms such as frozen water found in the ground, snow cover, ice sheets or glaciers – what we call the cryosphere (“cryo” meant “ice” or “frost” in Ancient Greek). This state of matter is clearly much less conducive to local life with nothing or next to nothing growing at the surface to build a food chain on. And yet, all this frozen water plays an important role on the climate of our planet and in the annual regulation of the throughput of rivers fed by snowmelt.

Glaciers are perennial accumulations of ice, they slowly move due to gravity, essentially carried downhill by their own weight, not without carving a massive trail behind, taking with them rock debris and refashioning landscapes. In the popular imagination, glaciers can be found in mountain ranges, with Northern Pakistan hosting the largest number by a margin – more than 7,000 of them across the Karakoram and the Western Himalayas. However, by far the largest of these repositories, containing about 99% of glacial ice on Earth, are the continental glaciers of Antarctica and Greenland. These also bear the technical name of ice sheet, to qualify for this the glacier should cover over 50,000 km², a threshold they easily cross with 14 million km² and 1.7 million km² respectively. As they slowly slide into the ocean, they calve icebergs.

These ice sheets should not be mistaken with the freezing of sea water, called sea ice, taking place in the polar and other extremely cold regions of both hemispheres. These surfaces are even larger than the ice sheets and can aggregate up to 20 million km² in the Southern Hemisphere and about 15 million km² on average in the Northern Hemisphere. However, unlike the ice sheets, these may not be perennial and the total surface area experiences large swings, in fact up to 80% may melt in a given summer in the southern latitudes under the combined effect of the warming of the ocean temperature and exposure to solar radiation.

As for permafrost, it does not have the allure of a glacier and seldom makes the news and yet, it currently accounts for 11% of the land surface of our planet, and this number is shrinking. Permafrost is ground that remains frozen for at least two years and so, as the average temperatures ascend slowly, some of it thaws. This is problematic because a lot of these areas located in Siberia, Canada and Alaska act as carbon sinks by preventing historical dead biomass from decomposing and emanating gases with greenhouse effects such as methane and carbon dioxide.

Good segue to mention a few effects of climate change on the cryosphere as well as the feedback mechanisms the impacts to the cryosphere have on the climate change dynamics. The most dramatic consequence of the global rise in temperatures is the acceleration of the melting of the ice sheets, glaciers and overall reduction in minimum sea ice surface. This results in more water in liquid form and therefore a rise in sea levels all over the world, affecting human communities living at or near the shoreline with some entire regions facing increased risks of floods. In addition, the decrease of overall snow cover in mountainous areas has an economic impact on regions geared towards snow-related activities and on the flow of rivers fed by snowmelt.

As the surface of snow and ice on the planet surface decreases, since those have a high rate of reflection of incident solar radiation (about 80-90%), the overall surface reflectance of the planet decreases, meaning that all else being equal more solar radiation energy is being absorbed by Earth. The second significant feedback has to do with the high energy required to melt ice, a phenomenon we saw in S1 Section 2.e on phase transitions and so, the less ice there is to melt, the less heat is absorbed in this process. And finally, snow cover and ice to a lesser extent, act as strong insulators reducing the freezing of ground during the winter. This impacts groundwater drainage and potentially agricultural practices. Fortunately, the rise in temperature also leads to an increase in the emission of infrared radiation; this is called the Planck effect and it is the most important negative feedback as relates to climate change.

d) The Water cycle

As mentioned in section a), most areas at or near the surface of our planet are exposed to water, be it in the form of seas, ice, snow, rainfall, or soil moisture. If we are to distil it down, what we call the water cycle can be thought of as a 5-step process.

The first we may describe as evaporation, the physical process causing water to become airborne (again) and ascend into the atmosphere in the form of water vapour. About every 6 out of 7 water molecules undergoing this process originate from the oceans, with the remaining coming from other bodies of water, soil moisture and plant transpiration; the portmanteau term of evapotranspiration captures both the biological and non-organic aspects. If you want to understand a little more about evaporation and transpiration you may want to refer to S2 Section 3.d but essentially it comes down to surface molecules in a liquid acquiring sufficient kinetic energy to overcome inter-molecular bonding and escaping as gas.

The second phase of the cycle is the movement of all this water vapour called atmospheric circulation. It is a function of winds, convection leading hot air to rise and cool one to sink, and other weather patterns. I will wait until Chapter 3 on the atmosphere and climate to go into more details but this mention of convection and water vapour requires an explanation of the nature and formation of cloud, a key player in this step and the next. Clouds are effectively solid particles suspended in the air, what is called aerosols in the jargon. These particles are mostly tiny drops of liquid water and ice crystals. The water molecules are less dense than their other counterparts in the atmosphere, namely nitrogen and oxygen, so the water vapour rises and in so doing it experiences a lowering of both pressure and temperature until the point where the gas undergoes condensation and then potentially freezing. A cloud is nothing less than a large concentration of this erstwhile water vapour. There is a real taxonomy of clouds dividing them into 5 main forms and ten genera; for those interested I have included the link to the Wikipedia entry for “clouds” at the end of this chapter. Clouds can be found in their highest concentrations in zones of low pressure, and more consistently in the equatorial region due to the high evaporation rate as well as in latitudes where cool polar air meets tropical and sub-tropical air.

The third step is called precipitation and essentially has the reverse effect of the first one by translating water molecules back to the surface of the planet. The physics of precipitation are somewhat simpler than those of cloud formation, it occurs when enough liquid droplets coalesce to create drops that are heavy enough to reach the point where gravity overcomes air resistance. In cases where the water droplets get supercooled (below their freezing point but still in liquid state) then freeze, snowflakes can form and fall from the sky instead of rain – that part is actually a lot more technical. This step also differs from the first in that precipitations typically do not occur in the same spot as where evaporation took place, because step #2 resulted in moving masses of water vapour around the globe.

After water crashes at the surface, either it re-enters the seas and oceans, skipping step 4, or it slowly makes its way towards a final or temporary storage area under the effect of gravity. Rain will aggregate in rivers flowing into the seas or be absorbed into the ground and travel into underground aquifers or alternatively remain there as ground moisture. Some snow will be temporarily warehoused in glaciers, sometimes for several thousand years, or a snow blanket and when it melts it will follow the same path as rainfall.

The final step is the storing of water in reservoirs, be it as underground moisture, in freshwater or saltwater bodies, and into plants after biological uptake through their roots. And on the cycle continues.

e) Trivia – The Mariana Trench

To conclude this chapter, I thought it would be interesting to look at hydrosphere meets geology, at the very bottom of our oceans: the oceanic trenches and specifically the Mariana Trench, the deepest of them all.

Oceanic trenches are the result of depressions in the ocean floor in zones of plate subduction; a mechanism involving convergent tectonic plates we covered in S3 Section 1.d. The highest concentration is found around the Pacific Ocean with the crown belonging to the Mariana Trench, 200km east of the Mariana Islands named after a Spanish Queen in the 17^th century. There, the Pacific plate is forced underneath the Mariana plate and the lowest point of the trench is called Challenger Deep, about 10,920m under the surface of the ocean.

At such depth, the pressure is more than a thousand times that experienced at the surface so exploration really has been a challenge (!) and the first successful crewed descent was realized in 1960.

With all that said, Challenger Deep is actually not the point at the surface of the Earth that is closest to its centre. Indeed, Litke Deep, at 5,449m below sea level is to Challenger Deep what Chimborazo is to Mount Everest. Unsurprisingly, since Chimborazo can be found very close to the equator, Litke Deep is located near the North Pole at 82°24’ N, and is 6,351.7 kms away from the centre of our planet Vs 6,366.4 kms for Challenger Deep which is at 11°22′ N. Clearly, the Earth is not a perfect sphere by any means.

a) Role of water in life

b) The Blue Planet

c) The Cryosphere

d) The Water cycle

e) Trivia – The Mariana Trench

f) Further reading (S3C2)