Life is a tenuous thing. Earth is just within Sol's habitable zone, and constantly pelted with solar radiation and cosmic rays. Rocky scraps of cosmic afterbirth constantly cross Earth’s orbit, threatening to eradicate all terrestrial life. In point of fact, it is almost certain that countless Extinction-Level Events would have sterilized the surface of our plucky planet had it not been for our constant companion and benefactor; a body which unwittingly wards away many of the ills that could befall us: the moon.
Luna is unique among the observed celestial bodies; there is no other satellite closer in size and composition to its mother-planet (if one discounts the dwarf-planet Pluto), and the Earth/moon system is the only tidally locked pair. Furthermore, it also happens to be the only moon in the solar system which is circling an intelligent civilization– a factor which may not be a mere coincidence.
It was 4.5 billion years ago last week that the young planetesimal Earth was forming from the sun's accretion disk of dust and boulders. Several other aspiring planets were building up nearby. One particularly promising young protoplanet was making some exemplary progress by loitering in Earth's Lagrange point, allowing it to share Earth's orbit by staying at a gravitationally neutral distance. As the mass of both young Earth and her smaller rival, Thiea increased, the gravitationally stable Lagrange point was insufficient to keep the worldlets apart, and the proto-worlds were drawn together. Theia, approximately Mars-sized by now, accelerated toward and slammed into Earth at an oblique angle. The heavy core of the smaller world didn't have the velocity to escape Earth, but a large swath of the lighter mantle material of both were flung into orbit. Within the year, the moon we know was well-under construction–or so goes the popular theory. No one bothered to record for us the the rate of Earth's spin before the incident, but like a glancing shot off a billiards ball, the Giant Impact certainly made sure it was spinning afterward.
In that era, the moon was much nearer Earth, and would have looked much larger–several times the size of the sun. For a long time the moon retained a molten core and the accompanying magnetic fields which left geological marks on our world. When things were almost settled down, there was an era called Late Planetary Bombardment when both Earth and its companion were pelted by impacts that blew planetary debris around, and left some of Earth's ancient geology on the moon. Over the eons, erosion has scrubbed away all evidence of that ancient time from the Earth, but some of the chunks that were blasted to the moon were preserved in a frozen, unchanged state. Ultimately these remnants of the Earth's violent youth would be found by enterprising humans, such as the infamous Genesis rock collected by the Apollo 15 astronauts.
Observations of the solar system show us that the moon's birth was rather unusual. All of the other worlds either lack satellites or have captured them from other places. Of course the moon isn't Earth's only unusual resident; its surface crawls with all manner of strange and delicate carbon-based life forms. Adherents of the Rare Earth Theory postulate that a large moon such as ours is not merely a benefit for life, but essentially a requirement.
Although our planetary neighbor Mars also technically lies within Sol's habitable zone, there is reason to speculate that life never could get a foothold there because of its axial tilt. Mars' axis can wobble from 10 degrees up to the current 25 degrees, and maybe more. This has sometimes leaned one of the poles so sharply that the ice melted, filling the meager atmosphere with water vapor that froze again on the next season. By introducing such extremes to the weather, the planet would potentially go through phases where sheets of ice were laid on the surface for epochs, then melted away when the axis tilt became more favorable. When the Phoenix Lander lands near a Martian icecap in May, we may get a chance to see evidence of this ice age cycle on the surface. While Earth has had its share of ice-ages, the gravity of the moon has acted as a gyroscope, keeping the Earth's axis steady at 23.5 degrees and sparing us the wild environmental changes Mars faced. This long-term stability has given life a chance to arise amidst a cycle of regular seasonal changes.
A case can also be made that the tides have been invaluable to the evolution of life on our world. The sun alone would cause some tides to occur, though they would be far less than those the moon creates. The surfing would suck, and for many that wouldn't be a life worth living. The higher tides afforded us by Luna have made long swaths of coastline into areas of that are regularly shifted between dry and wet. These variable areas may have been a proving ground for early sea life to reach out of the oceans and test the land for its suitability as a habitat. Areas farther from shore are only dry at the peak of low-tide, and the period of exposure to air increases as one nears shore, allowing for a subtle progression toward a waterless environment. Early life could have taken advantage of this gradual change to adapt to the wildly different demands of surviving outside the ocean.
It's not only water being tugged by the moon's gravity. Perhaps the moon helps keep Earth's core and seas warmer than they would otherwise be. Since the moon circles the Earth once a month, and the Earth is spinning a full turn at a much quicker 24 hours, the moon's gravity is creating drag, hence friction, as it pulls at Earth's surface. This causes several things to happen: first is a perpetual morphing of the crust–like the amateurish kneading of bread–that contributes a clumpy, broken mess that we call plate tectonics. The WolfmanEven Earth's rotation is slowed by virtue of the Moon's pull.
Without the moon, the Earth might rotate much faster, causing a more turbulent atmosphere, and thus unending gales of life-hostile, skirt-blowing winds. As Luna's orbit slowly creeps away from the Earth at 1.5 inches per year, her gravimetric drag will eventually slow the Earth's rotation to match the pace of the moon's orbit. One day will be 9,600 hours long, and the moon will only be visible from one hemisphere, fixed in the sky. Of course, by then the sun should be in an expanding red-giant phase, slowly engulfing its planets. The sun's coronal atmosphere could be creating drag against the moon, slowing it toward an eventual breakup as Earth's gravity tears it apart. The remnants of Luna will fall back to Mother Earth as meteorites, and while it may be a pretty show, it ought to prove bad for property values, and worse for the surf.
If the unlikely set of circumstances which brought forth our moon are as rare as they seem, perhaps ours is the only such planetary system in the entire, vast galaxy; or perhaps in our unfashionable limb of the universe. But every once in a great while, when the time is right, two protoplanets who love each other very much can touch each other in a special way, and make life together. Without that magic, astronomical ritual, we certainly would not be here.
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