Again, I want to state that what I am describing represents a condensed version of many astronomers' theories about the origins of the universe and various planets and so on. It is highly possible and even likely that, as scientists learn more about these sorts of things, this story will change. But for now, here's how it goes:
- Where we last left the Earth and the moon, the Earth had sustained an enormous collision with another planet, about the size of Mars, and out of this impact the moon was formed.
- Meanwhile, all kinds of wacky adjustments were still happening out in the rest of the solar system. Planets kept consolidating and getting bigger, and any time that happened, everybody else had to adjust. Scientists aren't exactly sure what triggered it, possibly a change in Jupiter's orbit, but the asteroid belt shifted. And all of a sudden, Earth and the moon were flying into the asteroid belt. You've seen this sort of thing happen in Star Wars, so you know it isn't pretty.
- Okay, so there were asteroids flying everywhere, right? Since it has larger mass than the asteriods, the Earth's gravity was pulling the asteroids to it. So the Earth was getting shelled by these asteroids, but since the moon was hanging around, too, spinning around the Earth, the moon sometimes got between the asteroids and the Earth. As a result, the moon got clobbered way more than it otherwise would have, simply because it was so close to Earth -- much closer than it is now (you may recall, it's retreating an inch and a half farther from Earth each year).
- This asteroid bombardment went on for some thousands of years. You can see evidence of this on the moon today, without even using a telescope. All those shadows are enormous craters, caused by thousands upon thousands of asteroids slamming into the moon.
This is the moon's south pole, rife with craters made by asteroids
(Photo from NASA's Clementine Mission)
- You can't see similar craters on Earth because all the volcanic and tectonic activity over the millenia have changed the surface too much, but some rocks have been found on the Earth that support this theory about the asteroids hitting the moon and the Earth.
- The moon, however, stopped changing not long after it got hit by all those asteroids. Its surface used to be molten basalt and volcanic stuff, and the channels that are still visible used to be seams of lava. But all that volcanic stuff cooled and hardened and eventually froze into the shapes we see now. The Sea of Tranquility, for example, used to be a giant sea of molten lava which filled in a crater made by who knows how many asteroids.
- This period when the asteroid shelling happened is called the Lunar Cataclysm or the Late Heavy Bombardment, and this happened about 3.9 billion years ago (long before the other asteroid or asteroids that wiped out the dinosaurs). It came on pretty suddenly, and in terms of astronomical time, it stopped fairly suddenly too.
This graph shows the decline in the postulated number of asteroids that have hit the moon since its inception. The red line represents the new theory of a spike in the number of asteroids that hit the moon during the Lunar Cataclysm.
(Graph by Barbara Cohen)
- But the interesting thing is, very soon after the asteroid insanity stopped, life on Earth started. Nobody's sure what role those asteroids played, if any. Did the asteroids bring water to Earth? Did the impact of the asteroids alter the molecular composition of surface carbons to form amino acids? Did the heat generated by the asteroids produce enough steam to form water? Nobody is really sure about this part, but people are running lots of tests to try to figure it out.
Drawing of an asteroid hitting Earth -- who knows exactly what could have resulted from such an impact?
(Drawing by Don Davis, sourced from the Australian Spaceguard Survey)
- Here is one possible theory about how the asteroids helped bring about the genesis of life on earth: Perhaps the asteroids brought a lot of ice with them, which then melted when they hit Earth. Or perhaps the steam issuing from all the molten goo started to condense and fall back to Earth as water. In any case, at some point around 3.9 million years ago, a lot of water appeared on the Earth.
- The moon, which was still very close to the Earth, was orbiting around the Earth and creating tides. But because the Earth was still spinning really fast from the impact with that other planet, and because the moon was still so close, these aren't just your everyday tides. These are enormous tides, some thousands of feet high. The waves from these things traveled over fantastic distances, then slid back out to sea, scooping all kinds of material with them into the water. Thus, the highly sought-after "primordial soup" was mixed, creating the right environment for life to generate.
- Something else important about the moon and the tides happened too. Because the moon was still really close to the Earth, its gravitational pull was sucking water toward it in a big bulge some thousands of feet high. This bulge created a drag on the Earth's spin and very slowly started making it slow down.
How the moon's gravitational pull creates tides and helps slow the Earth's spin
(Diagram from Helmer Alasken's page about the moon)
- In addition, because the Earth was spinning way faster than the moon was orbiting, the Earth was in essence spinning underneath its oceans. This means that the land under the water was creating friction, and that friction also helped slow the Earth's spin.
- When the Earth slowed its spin, the insanely violent winds calmed down, hurricanes weren't happening as often or as severely, and there was less chaos in general. This meant that the life that was burgeoning in the water could have a chance to survive without being ripped apart by searing winds or tremendous waves.
- So, in essence, the moon helped make it possible for life to thrive on Earth.
- And here's another way the moon helped. When that second planet hit Early Earth and knocked it off-kilter, that was crucial because the Earth's tilt is what ensures that no part of the planet is exposed to the sun's heat for too many months at a time. But while that rogue planet set the Earth aslant to begin with, the moon is what keeps it there.
The Earth's tilt is what gives us different seasons throughout the year
(Diagram from Howstuffworks.com)
- Without the moon's gravitational pull tugging at the Earth, the Earth would start to wobble as it spun, the same way a top starts to wobble as it slows down. If the Earth started wobbling, different parts of the planet would be exposed to sun at all kinds of chaotic times, the winds would shift in crazy ways, and the weather would get all chaotic and super-violent again, and we or maybe no life might be able to survive it.
Sources
National Geographic Society, Naked Science television series, Moon Mysteries and Birth of the Earth
NOVA Transcripts, Origin: Earth is Born and subsequent segments, September 28 and 29, 2004
David A. Kring, Arizona University and NASA/UA Space Imagery Center, The Lunar Cataclysm Hypothesis
Ron Cowen, "An early cosmic wallop for life on Earth?" Science News Online, December 2, 2000
Lori Stiles, "Asteroids Caused the Early Inner Solar System Cataclysm," University of Arizona Sci/Tech news, September 15, 2005
Lori Stiles, "Lunar Meteorite Ages Strongly Support Lunar Cataclysm," Space Daily, November 30, 2000
Strom et al, "The Origin of Planetary Impactors in the Inner Solar System," University of Arizona Faculty Reprints, April 2005
Bernard Foing, "Linking the Earth to the Moon," Astrobiology Magazine, August 3, 2006
Jeremy Bailey, Australian Centre for Astrobiology, "The Inner Solar System Cataclysm, the Origin of Life, and the Return to the Moon"
G. Jeffrey Taylor, Origin of the Earth and Moon, Planetary Science Research Discoveries, December 31, 1998
Michael Richmond, What if the Earth had no Moon? July 15, 2005
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