“Water is the driving force of all nature.”
– Leonardo da Vinci
By Robert E. Wrigley
I was contemplating the significance of Leonardo’s perceptive quote one summer day as I was attempting to cut my lawn before the start of forecasted rain. Putting the mower away in the shed, I felt a sizeable rain drop land on my head. I laid down on my back to rest for a moment and gazing up into the impressively darkening clouds (cumulonimbus, "piled-up clouds or storm" in Latin), I wondered about the recent origin (the Pacific Ocean?) and travels of the water molecules tucked inside this single drop, swept along as misty vapour across mountains, forests and prairies to my exact spot.
Water first arrived on our early protoplanetary as icy coatings on dust particles during its accretion stage, drawn in by the powerful force of gravity. With the eventual cooling of molten Earth, vast water stores had already been captured in rock strata by 800 million years after the major formation of the Earth (4.6 billion years ago). During what is called the Late Heavy Bombardment Period (4.1 to 3.8 billion years ago), icy asteroids and comets struck the earth, leaving behind their massive deposits of water. Much of this water accumulated on the surface as rain in ocean basins after ground and air temperatures dropped below 100º C, and from subsequent outgassing of water vapour from countless volcanoes. Interestingly, the 'transition zone' (depths of 410-750 km) of the Earth’s mantle may still contain more water (not as liquid but bound in hydrated minerals) than is found in today's oceans!
I thought it magical that the countless water molecules in my raindrop have been falling and rising as precipitation and water vapour for at least 3.8 billion years. Now that's recycling – hydrologic recycling. We are so fortunate that Earth's orbit lies in an ideal distance from the Sun (the habitable zone), close and sufficiently warm to keep liquid water on the surface, and not so far away as to cause it to completely freeze over. The enormous weight of the Earth's atmosphere (held in place by gravity and the magnetic field) keeps most of the water from being stripped away by fierce episodes of solar winds, which is believed to have happened to the atmosphere and then water on Mars. Water molecules floating to the extremely cold (-60ºC) outer edge of the atmosphere form into solid crystals, which are then pulled back to Earth by gravity.
As water vapour rises into the atmosphere from evaporation from waterbodies, and evaporation and transpiration from the ground, plants, fungi and animal life, it cools and condenses into clouds of various named formations. For precipitation to commence requires fine particles suspended in the air to act as nuclei in order to seed the process, and remarkably, even here in our province, this can be dust blown in on air currents such as the jet stream from the Sahara and Gobi deserts, topsoil dusts blown aloft from exposed agricultural fields, carbon soot emitted from aircraft, cars and forest fires, sulphur and mercury particles from coal-burning plants, and even vast numbers of bacteria which are swept aloft by wind updrafts. These bacteria, many of which live in the soil and on plants, are in this way transported around the globe – a remarkable adaptation for dispersal. These ‘cloud-condensation nuclei’ attract water molecules in liquid form or as ice crystals, depending on the temperature.
Imagine if oxygen, carbon dioxide, nutrients and countless other substances could not dissolve in water. Life on Earth would never have originated. In fact, water is called the universal solvent because it is capable of dissolving more substances than any other liquid. Due to its polarity, with two positively charged hydrogen atoms on one side and a negatively charged oxygen atom on the other, a water molecule is capable of attracting a wide variety of other molecules.
Our ancestral line evolved in water, and of course our own bodies consist mostly of water – almost 100% as an embryo, 75% as a newborn, 55% (women) to 60% (men) as a middle-aged adult and dropping to about 50% in old age. Apparently, we start drying out as soon as we are born! We need to take in 2-3 litres of water (as liquid, in moist food, and from metabolized food) a day for good health. When I quench my thirst with a glass of cool water, I cannot help but wonder about the countless past life forms – bacteria, plants, algae, fungi, microorganisms, crustaceans, whales, worms, insects, and even other people – that these identical water molecules have been recycled through over the ages. How fortunate I felt to be living on a watery planet. We would not be here without this miraculous molecule, and yet few people have any concept or appreciation of its critical importance.
We take the presence of water for granted (at least in our country), and so it is strange to think that water will not be present on Earth forever. Astrophysicists suspect that increasing solar winds from our ageing Sun will sweep away all Earth's water back into space by 4 billion years in the future, leaving Earth a lifeless rocky planet once again. Just as our precious water resource was initially synthesized in interstellar space by chemical reactions among hydrogen atoms and oxygen-bearing molecules such as carbon monoxide, in time, some of our planet's former water may find other suitable cosmic bodies as their home (as liquid or ice). Fascinating to think about, but time to get out of the rain. As I glanced up from my gazebo to the flash of lightning, and to a powerful rumble of thunder, I could see in the distance a breathtakingly beautiful rainbow, appearing as the usual arc across the sky; actually, a circle if I could have enjoyed it from outer space.
Adapted from Robert's book; "Chasing Nature: An Ecologist's Lifetime of Adventures and Observation."
