I always thought that water was part of the steamy mix of rocks and gasses that formed earth around 4.5 billion years ago. But these days scientists think that Earth was too hot and its gravity too weak to hold on to much water vapor. The belief now is that most of our water came in on comets and asteroids millions of years later. The current debate (Science News Magazine, May 16) is whether the vehicles were primarily comets, from far out in the solar system, or asteroids, from closer in, and what role Jupiter and Saturn played in pulling the water-laden rocks our way.
The bombardment of extraterrestrial water is, in some ways, no surprise. All the atoms on earth have origins elsewhere. “We’re made of star-stuff,” as Carl Sagan put it. Water, though, is not an atom but a ready-to-use chemical compound. Of the key ingredients in physical life, it is the most visible, the one we are most conscious of. It makes up 60 % of us and flows into and out of us every day. And we can’t make more of it; there is only that original shipment.
To some people, this late and fortunate arrival of water may be one more sign of a master plan for life’s creation. That’s a reassuring notion. But from what I read, the arrival of so much water was essentially lucky. It could easily have happened on a much smaller scale or not at all. Thinking of our water as a lucky twist in the cosmic unfolding is cause for humility and appreciation.