A MOON THAT CREATED EARTH
MORE THAN 4.5 BILLION years ago, the inner solar system was a jumble of would-be planets and planetoids. It is thought that Earth shared its orbital zone with at least one competitor, about the size of Mars, similarly composed of a heavy metal core and outer mantle. Both planets would have mopped up smaller bodies, but eventually the two collided with each other.
The collision of Earth and Thea; courtesy NASA/JPL-Caltech
In a huge explosion, Earth was severely impacted, and the energy release caused vast amounts of both planets’ surface rocks to be vaporized or projected into space. This resulted in a ring around Earth that quite rapidly consolidated into a single body, which soon cooled and aggregated into the Moon that orbited Earth every 20 days, a mere 2,700 kilometers (1,690 miles) above Earth’s surface. The planet that struck Earth has been called Thea, after the goddess that gave birth to the moon, called Selena in Greek myth. Meanwhile, the metallic core of Thea was not absorbed by Earth’s core. Instead, significant metal deposits were embedded in the surface layers of Earth—which gave Earth’s crust a rich set of workable ore deposits named the “wedding ring.” This was significant to the later metalworking ages.
Such a massive satellite traveling around Earth caused the whole surface of the planet to deform gravitationally below the moon, but Earth was then rotating every 6 hours, so that this bulge would always be dragged ahead of the moon. Just as with tides today, but much more strongly, Earth’s rotation transferred energy to the moon, causing the satellite to accelerate and take an ever-higher orbit. This, however, did not occur before the moon had kneaded all the surface rocks in the Earth’s tropics. Subsequently, this type of lunar influence continued in an unusual way.
Around 4 billion years ago, the orbits of Jupiter and Saturn aligned to create a slingshot for solar system bodies that had not yet been incorporated into planets. This Late Great Bombardment proceeded to strike the moon rather than Earth, and this protective role of the moon probably saved Earth from damage to its nascent resources, such as the water present on its surface. The recognizable face of the moon was largely created from this bombardment, as the craters and seas of molten basalt we see today.
By three billion years ago, the moon had been accelerated to an orbital distance of 320,000 kilometers (200,000 miles), which meant that its tidal effects were no longer strong on the earth’s crust but instead, the seas of that period experienced massive tides that were hundreds of meters high. These must have been like continuous tsunamis that raced around the globe. Moreover, due to the still-great rotational speed of Earth, these tides occurred many times a day, and in addition, the early atmosphere was whipped up by the Coriolis effect, which created continuous, hurricane-speed winds. The extreme ocean tides, causing massive erosion and mineralization of the seas, formed a number of chemical scenarios that might have facilitated the creation of life in the form of primitive replicating molecules.
Even today, volcanoes and earthquakes are thought to trigger eruptions and the release of seismic energy, built up in Earth’s unique tectonic plates. These plates themselves could be an artifact, in part, of the moon’s kneading of Earth. For instance, we can see that on Mars any plate activity ceased billions of years ago as the mantle became stuck to a solidified core—probably because of its lack of a large moon in that planet’s orbit.
Though the original collision almost certainly caused the great rotation of Earth, it also created the tilt of the axis on which Earth rotates. This tilt set up on our planet the seasonal conditions that have been so important to life’s varied habitats. Yet this tilt would not have been stable without the large moon. Our moon stabilizes the tilt by shielding Earth from the small, chaotic forces our planet experiences due to the other planets. Mars is particularly vulnerable to these forces, and its tilt varies, over millions of years, by about 30 degrees. The moon, by adding a large, systematic component to the precessional forces, prevents planetary chaotic resonances from affecting the Earth.
The effect of the seasons, maintained by the moon, is joined by the extra tidal effect the moon has on the seas and oceans. These tides create an extensive area of a very important habitat: the tidal ranges found on our coastlines. These areas are very diverse biologically, and they have led to evolutionary changes as significant as the adaptation of marine animals into land animals.
In summary, life on Earth would not be possible without the moon and the special itinerary of its genesis and the gradual arrival to the conditions we find today. Perhaps this role of the moon has lead to the general recognition that life on Earth could not have evolved without a special collision that occurred at the exact distance from the sun that was capable of supporting such life. The precessional mechanism appears necessary for a planet to support distinct and stable land habitats for life, and therefore, our large moon was an essential accessory for our form of existence.