Celestisle — Our Cosmic Water World
By Michael H. Kew
As a boy browsing the oceans in my parents’ world atlas, I often wondered: Amid all this accretion, atop all the strata encircling all these islands, how did our Big Blue eventuate? Where did this water come from?
I had read that my local terrestrial islands, the Coronados—which on clear days I could see near my grandparents’ house in Point Loma—were hunks of land upthrust by plate tectonics. Earth itself seemed to be an island, one of billions in the magnificent sweep of the Milky Way. The Solar System’s eight planets—eight islands, indeed—all formed inside the molecular cloud known as the Solar Nebula, a spinning stew of interstellar dust and hydrogen leftover from the proto-Sun’s birth 4.5 billion years ago in the Nebula’s scorching center. Eventually, simmering at a relatively cold 9,800°F as a sort of small, embedded Sun, Earth’s solid inner core was founded by heavy, dense elements that collided and congealed. This was followed by the fluid outer core, the lower mantle, the mostly solid upper mantle, and finally the cool, hard crust that we all know. And it is just one percent of Earth’s total volume.
Icy comets and water-rich asteroids were once abundant and became very active during the Late Heavy Bombardment some four billion years ago. The Bombardment was started, researchers think, by a strange outer-planet tango. This upset the orbital harmony of the two gas giants (Saturn, Jupiter) and the two ice giants (Neptune, Uranus) and scattered nearby comets and asteroids which soared through the Solar System to strike the small, adolescent inner planets (Mercury, Venus, Earth, Mars).
Theoretical planetologists believe Earth’s first water and proto-oceans stemmed from these impacts, which pulverized our then-dry, rocky planet. The asteroids’ minerals, full of oxygen and hydrogen, melted from the mantle’s heat; the comets’ ice melted too and eventually, with the asteroids’ water, seeped up and covered most of Earth’s crust. (Planetologists also believe the mantles contain at least 10 more oceans of water.)
Beautifully bluey-green Earth hangs in the so-called Goldilocks Zone—the Solar System’s magical sliver of space where water can remain liquid. Most astrogeologists think this water, via hydrothermal vents, was the abiogenesis of Earth’s first bacteria and prokaryotes—indeed, the basis of Earth’s first lifeforms and, eventually, of you and me.