Scientists are generally confident that they know where water came from originally, but the origin of the water on Earth itself is more of a mystery. Indeed, the Biblical book of Genesis itself, though offering a detailed account of some other aspects of the Judeo-Christian beliefs on creation, is somewhat vague regarding where the original supply of water came from, or when it was created. Scientists now believe that water was formed by the reaction of free-floating hydrogen and oxygen created in earlier generations of stars, and have several theories about where precisely the water on the surface of our planet today had its beginnings.
– Star Stuff –
“We are made of star stuff,” noted astronomer Carl Sagan once famously declared. What he meant was that, at a chemical level, the matter which makes up the body of each human being was once formed, billions of years ago, in old, long-dead stars. The vast majority of matter takes the form of hydrogen atoms, but in the intense heat and pressure of stars’ cores, heavier atoms are created, and, once created, can combine together to form compounds.
Water would have followed a similar process of creation. H2O, or two hydrogen ions and a single oxygen ion, originated and then combined in these processes. Previous generations of stars lived, created such chemical elements, and then died; the matter ejected in their death throes then went on to coalesce into new star systems like our own, which feature a sun and planets filled with the metallic contribution of the previous stars.
– Earth’s Surface Water –
So much for where water comes from in general. The question of why we have water on the surface of the Earth, however, is somewhat more complex. We know that water is uncommon but hardly rare in our solar system: Mars once had water oceans, Venus may have, and several of Jupiter’s and Saturn’s moons actually have underground oceans today. For this reason, we can safely assume that a fair amount of water was mixed into the iron and other metals which came together in the early years of our solar system to make up the Earth, billions of years ago.
The general assumption is than in Earth’s early period, the so-called Hadean period of over 4 billion years ago, the planet’s surface was molten. Metals and water on the surface could have sunk into the planet; water, hydrogen, helium, and other gases could have drifted up through the atmosphere and been lost to space. If theories that the Moon originated from a massive, cataclysmic collision early in our history, this too would have eliminated much of the original surface material of the Earth. However, not all studies indicate that the Hadean period really was a time of lava and fire. It is possible, though not yet the generally accepted theory, that liquid water was present more or less all along.
If Earth’s original surface water was lost (or was simply never present), the next most logical theory is that it was deposited relatively early on by the enormous number of comets and asteroids which slammed into the early Earth. Distant bodies originating in the Kuiper Belt are known to carry substantial quantities of water, although those we have studied carefully, like Halley’s comet, do not carry the same type of water found in Earth’s oceans (they have too much heavy hydrogen, or deuterium). If these comets are representative of the Kuiper Belt in general, then comets can be ruled out as more than token contributors to the origins of Earth’s water. It is possible that some liquid water survived on Earth’s surface from an early date and was then added to by cometary deposits.
If these fail to account for the full quantity of water on Earth’s surface, there is a third possible source of water: life itself. The early primordial oceans were rich in sulfides, and early bacteria would have lived off this material, producing organic compounds from the reaction of hydrogen sulfide and carbon dioxide. In the process they would have released large quantities of water into the sea, and sulfur into the atmosphere. If this seems like an implausible source for more than a token amount of water itself, consider that our current planetary atmosphere is the product of an equally stunning terraforming transformation rendered by the first plants, which converted carbon dioxide into massive amounts of oxygen.
