Mars is a stark reminder of what our world could have become if its iron core had cooled, because without a magnetic shield a planet is left prey to the solar wind, and life, as we know it, could never flourish. The time had reached 16 minutes after midnight; the Iron Catastrophe was over. But even with the formation of Earth's core and magnetic shield, our planet remained a hostile and alien world. Volcanoes spewed clouds of noxious gases and Earth was enveloped in a suffocating atmosphere of carbon dioxide, nitrogen and steam.
With no oxygen to breathe and no ozone layer to block the lethal ultraviolet radiation, this was not a hospitable place for life, at least life as we know it. And in the midst of this hellish brew, the moon was born. Beginning when I was about 11 years old, I used to climb the stairs to the roof of this apartment building, where my family lived, here in New York City, a building prophetically named the Skyview Apartments. And with simple binoculars, just like these, I gazed up above the streetlights, beyond the buildings and into the night sky. And nothing will ever capture the excitement I felt when I first turned my binoculars on the moon.
When I saw that the moon was packed with mountains and valleys and craters, I thought I discovered an entire new world. And then I began to wonder, where did the moon come from and how did it get there? Well, little did I know that about the same time, the mystery of the moon's origin was also attracting the attention of a scientist named Bill Hartmann. And when I was a little kid I had a telescope. I used to be out there drawing craters on the moon and was very excited that I could even see these craters and mountains and so on. So it's always had a special interest for me.
And when he began his career, in the late s, he and many other planetary scientists hoped that NASA's Apollo missions would solve the mystery of how the moon formed.
Scientists calculated their age using radioactive dating. To their astonishment, they discovered that the moon was millions of years younger than Earth. And those same rocks held another secret.
Back to the Beginning
So that pretty well forced the idea that the moon has to have formed from the same basic material as the Earth. In a flash of inspiration, Hartmann and a colleague came up with a controversial new theory for the formation of the moon. BILL HARTMANN: We came up with this very simple idea that maybe as the Earth was forming at our distance from the sun, somewhere nearby, made out of the same material, was a second large body which got pretty big before it finally plowed into the Earth. This massive rock, about the size of Mars, slammed into our planet.
The energy of that impact was so great it melted both the planetesimal and Earth's outer layers; the two fused together forming a new, larger Earth. At the same time, this enormous collision ejected into orbit vast amounts of molten rock. This debris eventually coalesced to form the moon. When Hartmann first went public with this idea, in , it was considered scientific heresy. No one wanted to hear that. No on wanted to, uh, start thinking about that kind of model.
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All of us were taught, as junior geology students, that all processes in geology are slow, one sand grain at a time, erosion, and so on. And people would actually come to us and say we really shouldn't consider that model until we've exhausted all other models. And that was only after hundreds of computer simulations showed that the moon could have formed from a giant impact. Today, Hartmann's big idea is almost universally accepted. And it's been really fun to see a little idea that you had a long time ago suddenly blossom forth as a leading theory. But the repercussions of this disaster were just beginning to be felt.
The moon started out about , miles closer to Earth than it is today, and appeared many times larger in the sky. Earth was spinning much faster than today making each day less than six hours long. And with the moon so close, its gravitational pull on Earth was enormous. Earth's surface rose and fell up to feet during the cycle of the moon's phases.
Over time, Earth's rotation slowed down as the moon drifted away, a process that continues even today. And so, when the astronauts went to the moon, one of the things they did is they carried out this big device which was a reflector, a retroreflector that would beam a laser beam back in the direction that it came.
Back To The Beginning (Mario Kart Song)
In , they made their first measurement of the time it took for the laser beam to reach the moon, hit the reflector, and bounce back to Earth, a round trip of about two and a half seconds. We not only get very exact information on the orbit of the moon, but we can actually see the orbit change. The collision that created the moon was also a major stroke of luck for Earth.
That impact was so immense that it forced Earth's axis to tilt in relation to the sun, causing the familiar seasons. And without the stabilizing influence of the moon, Earth would wobble dramatically about its axis. Today, the planet would experience wild climate swings. But when did a planet that looks like the Earth we know begin to take shape?
Earth's hot molten surface took at least a billion years after the moon was created to cool and form a thick skin, its crust, or so scientists believed. But no one knew for certain because Earth is such a geologically restless place that none of the original crust survives today. Yet startling new evidence is causing a major rethinking of when Earth's crust first formed. The clues to this mystery are embedded within these rocks in Western Australia.
Here, geologists have extracted tiny crystals called zircons. About the size of sand grains, zircons are nearly as tough as diamonds. These relics of the early Earth formed when molten rock cooled into solid crust, so the age of the zircon gives you the age of the crust itself. And it was here that geologist Simon Wilde hit pay dirt when he found one crystal so old he's convinced it was formed in the Earth's original crust.
But certainly what we do know is that there was continental crust at 4. But that led to another mystery: once Earth was cool enough to form solid ground, water could collect on its surface, so when did that happen? Geologists, including Stephen Mojzsis, think the answer may lie in these same tiny zircon crystals.
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Zircons are extremely rare, so to find just a few crystals, Mojzsis had to pulverize and sift through hundreds of pounds of ancient rocks. An analysis of the chemical composition of the crystals revealed that the oldest zircons contained a high concentration of a curious ingredient. It was a type of oxygen called Oxygen, an isotope that could only be present in large quantities if the zircon crystals had grown in water.
The news that water might have been present so early in Earth's history was a bombshell. Liquid water is the key to life; every living thing requires it to survive. And eventually, water would cover nearly three quarters of the Earth's surface. In fact, all the world's oceans contain nearly one hundred million trillion gallons of it. It's an almost incomprehensible amount. So, where did it all come from? How would Earth have ended up with such vast quantities of this stuff? Well, strange as it sounds, these great oceans may have been there from the very beginning, just hidden away.
One key to the riddle was volcanoes, which, throughout Earth's infancy, pumped huge amounts of steam into the atmosphere. Then, as Earth cooled, that steam condensed into rain. Drop by drop, water collected in low-lying areas. The Earth does it right now. The main gas that comes out of Hawaiian volcanoes is water, steam. So, this is happening all the time.
Instead, Earth may have had some help. The water in our oceans might have come from outer space, delivered to the surface by massive ice-bearing comets. The evidence for these ancient impacts is impossible to find today, since the original surface of our planet has long since been eroded or destroyed.
Looking Back to the Beginning
But there's one place that preserves a record of impacts from that early era: our moon. In fact, the moon was ravaged by more than a million major impacts in its early years. Since Earth is much more massive, its gravitational pull would have attracted even more debris, resulting in possibly tens of millions of impacts. But this rain of debris left over from the formation of the solar system continues for several hundred million years.
Roughly half their mass was water.