Who Built the Moon? (11 page)

Nick Hoffman’s suggestion that the creation of the Moon removed so much material from the surface of the Earth that plate tectonics could become a reality is fascinating. It is estimated that seventy per cent of the primordial crust of the Earth would be necessary in order to create the Moon. Its removal caused the remainder of the crust to spread, allowing continental drift to take place.

Whether or not this is the whole story, plate tectonics are a reality as far as the Earth is concerned and what is more, it is a phenomenon that only occurs on the Earth. In other words, no other terrestrial- type body in the solar system had continents travelling about its surface.

One of the three Earth-like planets in the solar system, apart from the Earth itself, is Mars, which is half the size and a tenth the mass of our planet. It has an atmosphere that is ninety-five per cent carbon dioxide and nearly five per cent nitrogen with a pressure at the surface that is only 1/200
^th
that of Earth. Unfortunately for any potential Martian life form, liquid water cannot exist at the ambient pressure and at the temperature of the Martian surface. On this planet, water goes directly between solid and vapour phases without becoming liquid at all.

The puzzle as to why plate tectonics have either never started or else never been maintained on Mars has not been totally explained, but there are theories.

Mars has no appreciable mountain ranges, though it does have giant volcanoes. Some geologists suggest that the absence of true mountain ranges gives one clue as to why Mars did not develop plate tectonics. Like Earth, Mars has a lithosphere. This is a region in the crust of the planet that is cooler than its interior – a little like the skin that forms on a cup of hot milk. The centre of the Earth is extremely hot, probably more so than that of Mars, but the presence of volcanoes on Mars must indicate a hot core. One difference might be that Mars has nowhere near as much water in its composition as Earth. It is thought that it is water trapped within the Earth which acts as a lubricant allowing different parts of its rocky surface to slide against each other. The limited amount of water on Mars seems to prevent the lithosphere from allowing fresh material from deep within the planet to rise to the surface in the way it is constantly doing on Earth. As a result the lithosphere has not been disturbed for aeons and has cooled gradually, getting thicker and thicker. When pressure has become so great within the body of Mars that it is powerful enough to escape, it has done so via volcanism and not along features like the mid-oceanic ridges on Earth.

The other Earth-like body, Venus, which orbits closer to the Sun than our own planet, has a surface very different to that of Mars or the Earth. In some ways Venus is more like Earth than Mars. Venus is a similar size and mass and is also compositionally quite like Earth – or at least it was once. Experts such as David Grinspoon, a research scientist at Southwest Research Institute in Boulder, Colorado, have studied Venus closely, aided by a whole series of orbital and lander space missions.

Grinspoon is not alone in believing that in its early stages of development Venus was even more like the Earth. There is no discernable water on Venus now but there are traces in the atmosphere, which most likely indicates that in its very early stages it had proportionally as much water as Earth. This is not too surprising because the planets formed at the same time and fairly close together.

Venus is not unlike Mars in many ways but its surface pressure is ninety-two times that of Earth. It is thought that Venus lost its water because of a greenhouse effect and it is now covered in dense swirling clouds of sulphuric acid. These clouds are so thick that only a small percentage of the sunlight that falls on Venus actually gets through to the planet’s surface, so even if it weren’t such a hell in other ways, it would be a very gloomy world. It might be thought that less sunlight would lead to a lower temperature but this isn’t the case. Rather, heat already at or near the surface is maintained and increased because it cannot escape through the dense carbon dioxide. This has caused a dramatic heating of the surface of Venus to a present temperature of 730°C.

Like Mars and Earth, Venus has volcanoes; in fact it has more than any other planet in the solar system. But again, like Mars, the volcanoes of Venus exist as individual entities and not as part of long mountain ranges as is the case on Earth. The volcanoes of Venus are randomly spread about its surface and many of them look very recent, even though this may not be the case. Electrical storms rage constantly through the clouds of sulphuric acid but, even so, wind erosion on Venus is limited compared to the water-rich Earth. It turns out that erosion is extremely important in terms of supplying the right chemical and nutrient balances that have made the Earth a haven for life.

The surface of Venus looks broadly similar wherever one looks and is thought to be comparatively recent in origin – something in the order of 600 to 700 million years. Venus has a generally smooth surface with some rifts and folds but everything appears to be the same age. It is generally accepted that between 600 and 700 million years ago some cataclysm took place on Venus that remodelled its whole surface. Whether this was as a result of the internal stresses within the planet is not known, but for some reason the planet’s surface appears to have literally melted or more likely was uniformly covered with volcanic basalt.

Nobody knows for certain whether a similar thing will happen again on Venus, in other words whether we are seeing only one phase of a stop-start process that is taking place, but it is considered to be a distinct possibility. Probably because of its greenhouse atmosphere Venus is deficient in water and so once again may have built up a thick lithosphere. It certainly does not display any of the characteristics of plate tectonics.

It is interesting to note that Venus has no moons, whilst Mars has two, though both of these are extremely small and can have little or no effect on their host planet. As we have seen, it is now being suggested that the very creation of such a large moon as that enjoyed by Earth was directly responsible for the start of plate tectonics, which in turn allowed life to form on the planet in the first place.

In the early stages of its existence, the Moon was very much closer to the Earth than it is today. And it is the existence of the Earth’s oceans that is primarily responsible for the gradual lengthening of the distance between the Earth and the Moon. This is a process that has been taking place for the last four billion years and which is still taking place.

One way of looking at the situation was presented by Neil F Comins, Professor of Astronomy at the University of Maine. Back in 1990 he had been struck by the comments of a colleague, to the effect that science educators are always looking at the world from the same old perspective. Comins suggested that it might be sensible to step aside and look at the world differently.

As a result of this conversation Comins decided to turn his attention to something we all take for granted, namely the Earth and its relationship to the Moon – but from an entirely different perspective. He set out to consider what the Earth would have been like today if it had not enjoyed the benefits of so large a Moon. He called his hypothetical world ‘Solon’ and over a period of time he wrote a series of articles about Solon that appeared in
Astronomy
magazine. He eventually published his overall observations in a book, which was entitled
Voyages to Earth that Might Have Been
.
²¹

Comins examined every aspect of the Earth and its relationship with the Moon to build a picture of a similar planet, at the same distance from the Sun and which was the same age as Earth. The only thing that was different is that the Moon did not exist, but the alterations this absence would make to the Earth were dramatic.

Nick Hoffman suggests that the very nature of the Earth’s surface would have been entirely different if the material that makes up the Moon had not been removed from the Earth’s crust. However, Comins’ starting point is to assume that the surface details of the Earth would be roughly the same as they are now.

One of the greatest differences in terms of the early, developing Earth would have been tides. Comins makes the point that a Moon ten times as close would have led to daily lunar tides that would have been a thousand times greater than they are today. Bearing in mind that it is generally accepted that the infant Earth was spinning about its centre every six hours, this means that tsunami-strength tides would have been hurtling across the Earth every three hours! Not only were these tides more frequent, but, being so very much larger, they would have crashed many hundreds of kilometres inland – and with tremendous destructive force.

The mechanism that has slowed the Earth’s spin is directly related to tides and the Moon is not the only body responsible for them because part of the ocean tides on the Earth are responsive to the Sun. But the Moon is much closer and has done far more to slow the Earth than has the more distant Sun. Comins estimates that without the Moon, the Earth day would be only eight hours in length and solar generated tides alone would be less than a third of what they are today.

The immediate implication has great ramifications on the possibility of evolving life. At present many scientists accept that DNA, the fundamental building block of all life, occurred spontaneously in Earth’s early oceans. We will have much more to say about DNA later, but for the moment we will accept the general view that it first appeared in the early oceans of the Earth, a legacy of what is known as the ‘primeval soup – a specific blend of water and chemicals upon which life depends.

The massive tides created by the infant Moon would have caused erosion on a scale quite beyond our experience today. Millions upon millions of tonnes of land would have been pulverized and swept out to sea, then widely distributed and eventually settled on the seabed. This process liberated vast amounts of minerals into the oceans – minerals that emerging life simply could not do without. Presumably a Moonless world would still have had weather patterns, including rain, so erosion would have taken place but on a tiny scale compared with what happened when the Moon was so much closer to the Earth. This means that life would have taken much longer to gain a foothold, if it had managed to do so at all.

We have no problem with the concept that life first developed and flourished in the ocean, but there had to be a time at which it migrated from its salty environs and learned to survive on dry land. It is possible that insect life took the leap first but the fish ancestors of amphibians and reptiles followed and between them they eventually gave way to all land-living animals in the world today.

Life is always evolving to harmonize with the prevailing environment and to capitalize on new niches that are not already being exploited. Around 400 million years ago one such area of potential exploitation was rock-pools. Fish are accidentally left behind in rock pools with every retreating tide, both then and now. In most cases it doesn’t matter because the next high tide will free the fish again, back into the sea. However, if a fish is isolated in a rock pool during a particularly high tide, it may have to survive for weeks before it will be liberated. Fish that found themselves in this situation would die unless they somehow managed to get back to the ocean by moving over dry land and also managing to breathe out of the water.

It seems that some fish did find ways to drag themselves across the sand, at the same time changing enough physically to take gulps of air whilst out of the water. These fish found that dry land offered some rich pickings and any animal that learned to live, even temporarily, on dry land, would be well rewarded. Gradually, and over a long period of time, fins that pushed the fish over sand became stouter until they became legs and the fish in question ceased to be fish at all.

Since the Sun also creates tides it isn’t out of the question that fish would ultimately have left the oceans, even if lunar tides had not been present. However, the waves in question would have been significantly smaller and their value in terms of depositing detritus much more limited. What is quite clear is that life would also have been very much slower in developing to a stage advanced enough to leave the oceans had it not been for the lunar tides, if it could ever have happened at all. When we take on board the prospect of an Earth with a variable obliquity, no plate tectonics and such a dizzying spin about its axis, the prognosis for life of any sort on Comins’ Solon is not good.

Fortunately for us the Moon was present and stamped its authority on the developing Earth in a number of different but equally crucial ways. It helped to create many differing habitats, which in turn engendered biodiversity. Most experts believe that it was biodiversity that led to intelligent life becoming possible. Evolution tries and retries many different models. Animals that were ideally suited to their environment flourished on the Earth, only to fall by the wayside when conditions changed and they could not adapt.

Giant reptiles, that we generically call ‘dinosaurs’, ruled the Earth for millions of years until these impressive and diverse creatures vanished from the face of the planet. Whether as a result of some cataclysm, such as a huge meteorite strike, or thanks to some other misfortune, species that had flourished for eons were wiped out astonishingly quickly, but life itself remained untouched. Such was the multiplicity of species already inhabiting the Earth that some were bound to overcome the problems that put paid to thousands of others at a stroke.

One of the animals that did survive whatever circumstances put paid to the dinosaurs was a tiny shrew-like creature that occupied the vacant niche left by the demise of the reptiles. However, it was different to the reptiles because it gave birth to live young and suckled its infants with milk created from its own body. These first mammals then evolved to diversify and spread across the planet where they have been adaptable enough to survive and flourish.