The Moon And Plate Tectonics: Why We Are Alone
Sydney - July 11, 2001
The existence of a large Moon in orbit around the Earth and its implications for the origin and nature of life have been a subject of considerable discussion. With the Hartmann/Davis models for the catastrophic origin of the Moon by glancing collision, it has become clear that our Moon is a rare celestial object and that few Earth-like planets could have produced such a chance outcome during their assembly.
If, therefore, aspects of the Moon's existence strongly impact on the origin and development of life, then we may be able to explain why life (as we know it) might be rare in the Universe.
The bulk of arguments about the Moon relate to its effect on the orbital dynamics of the Earth-Moon system (which is stabilised against spin-axis inclination variations, unlike Mars), and to the tidal influence on ecosystems (developing broad coastal flats with regular currents, water-depth variations, and monthly cycles).
None of these are compelling arguments for the origin or nature of life. Instead, we look here at plate tectonics as an essential engine for maintaining the continent/ocean duality on Earth, which enabled advanced life to emerge on land and develop to a tool-using electro-mechanical civilisation (our definition of "advanced life"?).
Others have speculated that the heat pulse due to the Lunar-originating impact was the trigger to start plate tectonics but we show that this is a minor effect of timing. Instead, the enabling factor is the removal of ~70% of the primordial crust of the Earth to a position in orbit 400,000 km overhead. If that crust were returned and replaced on Earth it would fill the ocean basins with wall-to-wall continent. This would choke plate tectonics, as on Venus, and displace the oceans to flood the land to a depth of several km.
Without plate tectonics, new mountain belts could not form. Earth would be a Waterworld with occasional shield volcanoes emerging briefly above the waves. If regular catastrophic convective overturn occurred, as on Venus, life would have a precarious foothold indeed. There may be many habitable worlds, but they are likely to be Waterworlds where swimming or flying creatures might evolve significant intelligence, but would be unlikely to progress to discover fire, electricity, computers, radio, and rockets.
The Origin of the Moon
During the last decade, the science community has rallied squarely in favour of a catastrophic origin model for the Moon. Late in the Earth's accretion a large body of perhaps Mars size hit the Earth (then about 60% complete, and already with a differentiated core, mantle and crust). This in itself was not unusual. Most of the planets show signs of large late impacts in their eccentric orbits and tilted spin axes, but this one was something special.
The impact was at just the right angle to almost bounce off, but to be captured and swallowed by the Earth. The highly oblique impact struck a glancing blow to the Earth's surface and set up giant shock waves that spalled material into space. The surface of the Earth was heated and may have begun to boil --not just the Oceans, but the rock itself!
The debris that was spalled off, and the boiling clouds of rock gas formed a ring around the proto-Earth and then condensed into one or more glowing clumps of molten material that finally amalgamated into a giant Moon. When the dust settled, the nature of the Earth had been changed forever some 60-70% of its primordial crust had been blasted off into space. A giant glowing ball reared overhead, perhaps ten times closer than the Moon is today (and ten times larger, visually). Giant tides would have been raised of over one kilometre in height in the oceans that condensed back onto the Earth, and of several tens of metres in solid rock!
The heat produced by these tides absorbed energy from the spin of the Earth, and some was transmitted by tides (gravity gradients) to the Moon itself, pushing it further away from the Earth. Gradually the tides lessened and the surface of the Earth stabilised, while the Moon cooled and settled into a slower, steadier orbit. Accretion was still continuing, mopping up the last debris of the Solar System, and soon the giant scars of the formation process were concealed by new impacts.
Of key importance for the Earth, it had gained extra core material from the dense centre of the impactor, but it had lost a large amount of its crust. That crust is now in orbit nearly 400,000 km over our heads - a very strange place for a planet to keep its crust. The consequences for the Earth, and life on it, have been profound. It is no exaggeration to say that without the Moon, there would be no civilised life on Earth. In fact, there would probably be very little land on Earth, as we shall see...
Earth - the divided planet
Our planet seems familiar to us, and we accept as commonplace its gross structure and how it works. Although it took until the 1970's for plate tectonics to be accepted fully by the scientific community, that model for the way that all processes on Earth are geared together, and for how mountains are built and rebuilt over time seems totally normal to us today. We even accept without comment that we actually have continents and Oceans as distinct entities, but no other planet or Moon in our Solar System is organised like this. No other planet has plate tectonics.
Some authors suggest that plate tectonics was enabled by the heat pulse of the Moon impact, but once the first billion years have passed, events like that are irrelevant thermally. What is important is why plate tectonics continues, despite the lack of such a thermal pulse. To understand this we need to look again at the Moon, the Oceans, and the Continents.
One common misconception about the Moon is that it was somehow spat out of the Earth, and the Pacific Ocean marks the scar of its birth. We know this is dynamically impossible, and anyway the Pacific Ocean is far too young (<180 Ma) to be the birth scar of the 3.8Ga Moon (and that's just a surface age, the Moon-forming event probably occurred at ~ 4.45Ga). However, we can make use of this concept. Let's take the Moon, which is after, all, the primordial crust of the Earth. Let's tear it up into pieces like a giant ball of modelling clay, and flatten out those pieces into broad flat plates about 40 km thick - In short, let's form the Moon back into crust, and put it back on Earth.
Today, the Earth is about 70% ocean and 30% land. If we replace the missing 70% of crust, then we will entirely fill the ocean basins. The water will be displaced and will flood out over the land. If we do this to the modern Earth, only a few decent areas of land remain from the 3 km deep flood. All of Australia is gone. The Himalayas and the Andes are some of the few remaining "continents", and Iceland of course survives because it is such a profuse volcanic centre. Everything else is just a tiny pimple of a mountain (usually volcanic) adrift in a planetary ocean.
Even this is an overestimate of the land area. Those major "continents" are all intimately related to plate tectonics. They are either young active mountain belts, or intense volcanic centres built high on the spine of the mid-ocean ridge system. How would these fare on the remodelled Earth?
The secret of plate tectonics is that the Earth has gaps between the continents, and so they can move around like a sliding block puzzle. But if we replace the missing crust, there are no longer any spaces to slide into. Although tectonic forces might tug and squeeze, all they can do is make a few wrinkles here an there. That's what happens on Venus, where the crust is planetwide and ~30km thick everywhere. On Venus nothing can rift, or spread, or subduct, or collide, because there's already something there blocking the way.
If we restored the Moon to the Earth, we would block up plate tectonics. The planet would have to find other ways of losing heat - like the profuse volcanism of Venus, or the massive stacked volcanoes of Mars. Plate tectonics would stop. (Or would have never started). The oceans would flood the land, and any mountain belts would be worn away in a few hundred million years. Soon, there would be nothing left but a ball of water, with just an occasional volcanic island poking through the spindrift.
The Earth is not unique because if its oceans. Any planet in the right part of the habitable zone will have those. What is unique about the Earth is that it has LAND. If the moon had not carried away most of the crust, there would be no ocean basins, no land, and no chance for life to evolve on land.
Click For Part Two: Venus - What The Earth Would Have Been