The Moon and Me (and you!)

Is the Moon a Babel Fish?

(from Analog, June 2012)

In The Hitchhiker’s Guide to the Galaxy, The Babel Fish is a leech-like entity that acts as a universal translator when stuck in someone’s ear. The book points out that the Babel fish could not possibly have developed naturally, and therefore both proves and disproves the existence of God:
Now it is such a bizarrely improbable coincidence that anything so mindbogglingly useful could evolve purely by chance that some thinkers have chosen to see it as a final and clinching proof of the non-existence of God. The argument goes something like this: “I refuse to prove that I exist,” says God, “for proof denies faith, and without faith I am nothing”. “But,” says man, “the Babel fish is a dead giveaway, isn’t it? It proves you exist and so therefore you don’t. QED.” “Oh dear,” says God, “I hadn’t thought of that,” and promptly vanishes in a puff of logic.
     Could the Moon be a Babel Fish?  I suggest that it is “such a bizarrely improbable coincidence that anything so mindbogglingly useful as the Moon could evolve purely by chance” that the best explanation for its existence is that it was put there by whoever designed the Universe.  But it is so obviously an artefact that no Designer would be so crass as to put it there in the first place.
     Well, how did the Moon get to be there?
     Our Moon is the largest, in proportion to its parent planet, of any moon of any of the eight major planets in the Solar System. To an astronomer, the similarity in sizes is so close that the Earth–Moon system is more properly regarded as a double planet. So how did such an unusual system form?    The most likely explanation is that the Earth began life as a near-identical twin to Venus, with a thick rocky crust, while another planetary object, about the size of Mars, formed nearby. The most likely place for this object to form would have been at one of two places known as Lagrangian points. These lie 60 degrees ahead or behind the Earth but in the same orbit around the Sun. They are places where the combined effect of the gravitational pull of the Sun and the gravitational pull of the Earth is to produce a kind of gravitational pothole, a place where small objects can accumulate and stick around for a long time. The Lagrangian points are used today as stable parking places for satellites, such as the Herschel infrared telescope, which need to be kept far enough away from the Earth not to suffer interference from natural or man-made radiation from our planet. A small object which is not quite at the exact Lagrangian point wobbles slightly to and fro about the point itself, like a swinging pendulum; the orbits of artificial satellites at these points have to be adjusted from time to time, using their rocket motors, to keep them in place. But if a large natural object grew up out of cosmic debris near to one of the Lagrangian points of the Earth’s orbit these oscillations would get bigger and bigger, soon becoming so extreme that the object would bash into the Earth itself. This would have happened within 50 million years of the formation of the original crust of the Earth.
    The name of the hypothesized proto-planet is Theia, after the Greek goddess who gave birth to Selene, the Moon goddess. Theia formed with the other planets of our Solar System about 4.6 billion years ago. Theia’s orbit became unstable when its mass exceeded a critical value, leading to the collision which formed the Earth–Moon double planet about 4.53 billion years ago, roughly 30–50 million years after the other rocky planets had formed.
    Such a collision would not be like two pieces of solid rock colliding and chipping pieces from one another. Astronomers refer to this collision as the “Big Splash”, and the image that conjures up accurately indicates what happened when the Earth was young and was struck a glancing blow by an object the size of Mars. So much energy of motion would have been released by the collision that the incoming object would have been completely destroyed, and the entire surface of the Earth itself would have melted. The outer layers of the incoming object would also have melted, and mixed with the molten material from the Earth’s surface, with much of it being flung off to make a ring of debris around the planet. Meanwhile, the dense, metallic core of the incoming object would have sunk through this molten outer layer and been absorbed into the core of the young Earth. The lighter material from the incoming object and from the Earth’s original surface splattered out into space in this way would have contained about ten times the present mass of the Moon; most of it escaped entirely into independent orbits around the Sun, becoming asteroids, but some was captured in a ring of material around the Earth. As the surface of the Earth cooled and formed a new, thinner crust, the material in this ring coalesced into the Moon, repeating in miniature, but far more quickly, the process by which the planets themselves formed around the Sun. Computer simulations suggest that about two percent of the original mass of Theia ended up in the ring of debris, and about half of this fused together to form the Moon. The time taken to complete the formation of the Moon would have been only about a month.
    Other objects created out of the ring of debris may have got stuck in Lagrangian point orbits for as long as a hundred million years, before the gravitational influence of other planets shook them out of these gravitational potholes, allowing many of them to crash into the Earth or the Moon.    Evidence to support this model of how the Earth–Moon system formed comes from samples of rock brought back from the Moon. These show that it has exactly the same composition as the Earth’s crust. And seismic measurements of moonquakes made by instruments left on the lunar surface show that it has no significant metallic core; the radius of the core is certainly less than 25 per cent of the radius of the Moon, whereas the radius of the Earth’s core is about 50 per cent of the radius of the planet. The Moon’s core contributes only a few per cent of the Moon’s total mass, but the Earth’s core makes up nearly a third of the planet’s mass. Because of the lack of iron, the overall density of the Moon is much less than the density of the Earth. Earth has a mean density of 5.5 grams per cubic centimetre, but the Moon has a density of only 3.3 grams per cubic centimetre.    The age of Moon rocks even gives us a precise date for when this dramatic event happened – 4.4 billion years ago, almost as soon as the Sun had formed. There’s also more circumstantial evidence; such a glancing blow explains why the Earth rotates so rapidly, once every 24 hours, while moonless Venus rotates only once every 243 of our days. The glancing blow which formed the Moon would actually have set the Earth spinning even faster, so that it would have had a day some five hours long after the impact, and it has been slowing down ever since. The off-centre impact also gave the Earth its tilt, which is the reason why we have seasons, but the presence of such a large Moon orbiting the Earth has since acted as a gravitational stabilizer, stopping the tilt from varying very much over geological time. Incidentally, a combination of the extra iron in the Earth’s core and the rapid spin probably explains why our planet has a strong magnetic field. All of these influences may, as we shall see, have been crucial in allowing the emergence of a technological civilization on Earth.     There is one more persuasive piece of evidence that collisions like this did happen when the Solar System was young. Spaceprobes flying past Mercury have measured the strength of its gravitational pull and found that in spite of its small size it has a relatively high density. The Moon resembles the crust of the Earth without a core, but Mercury resembles the core of the Earth without a crust. The natural explanation is that a much larger object originally formed in the orbit of Mercury, but that early in the life of the Solar System it was hit, not in a glancing blow but a head-on collision, by another proto-planet. In a head-on collision, all the lighter material would have been blasted away into space, leaving only the heavy core behind.    The impact model explains why only one out of eight planets in he Solar System has a moon comparable in size to the parent planet; but it also implies that such double planets are rare.  What is certain is that the thin crust is essential for plate tectonics to take place at all in the way that we know it, among other things allowing the kind of volcanic activity that has brought to the surface of the Earth the metal-rich ores on which our technological civilization depends.  There are many other important links between tectonic activity and life, which I do not have space to go in  to here. The thin crust is a legacy of the impact that created the Moon, and another legacy of that impact is the dense, iron-rich core of the Earth, which also turns out to be essential for the development of our kind of civilization.A field of forceIn a certain kind of science fiction story, spaceships and people are often surrounded by almost magical “force fields” that protect them from attackers. It’s a nice idea, but not very practical on the scale of spaceships and people (even assuming such fields exist) because of the enormous amount of energy that would be required to produce a shield of this kind. But the whole Earth, and in particular life on the surface of the Earth, is indeed protected from certain kinds of danger from space by exactly this sort of force field, generated by swirling currents of molten metal deep in the interior of the planet. It is the Earth’s magnetic field, or magnetosphere, and although it cannot shield us from incoming asteroids, it does protect us from dangerous charged particles from space, known as cosmic rays.     The magnetic field is a result of physical currents of electrically conducting metal swirling around in the outer core and acting as a dynamo, producing electric currents which in turn generate magnetic fields. The region occupied by the magnetic field around the Earth, the magnetosphere, is actually shaped like a teardrop, because it is squashed in by a wind of charged particles from the Sun on one side, but stretches off into space on the other. On the side of the Earth facing the Sun, the boundary between the magnetosphere and the solar wind of particles, the magnetopause, lies about 10 Earth radii (more than 60,000 km) above the surface of our planet; on the other side it stretches roughly as far as the distance to the Moon, beyond 60 Earth radii.
    Electrically charged particles in the solar wind, things like protons, travel at speeds of several hundred kilometres per second most of the time, with bursts travelling at about 1500 km per second when the Sun experiences bouts of activity known as solar storms. The Earth, and the entire Solar System, is also bombarded by particles from deep space known as cosmic rays.  All of these particles could do severe damage to life if they reached the surface of the Earth – they are essentially the same as the particles produced by radioactivity or in nuclear explosions. But because they are electrically charged, they are funnelled by the Earth’s magnetic field towards the poles, where they interact with molecules of gas high in the atmosphere to produce the colourful activity of the auroras.    Even so, during solar storms the electrical activity caused by the arrival of these particles at the Earth can disrupt communications and distort the magnetic field locally to such an extent that power lines can be affected and blackouts can be caused in high-latitude countries such as Canada. An increase in intensity of the strength of solar-wind particles can also knock out satellites, including communications satellites, and pose a health hazard to any astronauts unlucky enough to be in space at the time. So how bad would the total removal of the magnetic field be? As it turns out, we know just how bad it can be – because it has happened, and more than once.    As you might expect for a magnetic field produced by swirling currents of molten metal, the Earth’s magnetic field is not steady. It varies in strength from time to time, and the exact location of the magnetic poles drifts across the surface of the Earth. A record of past magnetism is preserved in rocks that were being laid down at different times – as the molten rock sets, magnetic field gets trapped in it, so that the rock today preserves, like a fossil, an indication of both the strength and the direction of the magnetic field that existed long ago. From such evidence, the way the field has changed can be reconstructed by geologists and compared with the fossil evidence of what life was like at the time.    For reasons that are not understood, from time to time the magnetic field gradually dies away completely to nothing then builds up again, either in the same configuration as before or with the magnetic poles reversed, so that what was the north magnetic pole becomes the south magnetic pole, and vice versa. The fossil record shows that when the magnetic field dies away, many species of life on the surface of the planet go extinct. The obvious explanation is that land-dwelling species in particular are killed off by radiation from space which reaches the Earth’s surface during magnetic reversals. In fact, though, it doesn’t matter what the exact connection is. What matters is that there is a link between the absence of the magnetic field and death on the surface of our planet. Clearly, the existence of a protecting magnetosphere is an important factor in allowing life forms like us to have evolved on Earth.     So another reason why we are here is that the Earth has a strong magnetic field; and the reason it has a strong magnetic field is that it has a large metallic core, formed as a result of the impact in which the Moon was created. We have, indeed, a lot to be thankful to the Moon for.  And there are other benefits of having a large Moon.A planetary stabilizerIn terms of its diameter, the Moon is more than a quarter of the size of the Earth. It has only about one eightieth of the mass of the Earth, but this is still far larger in proportion to the mass of the planet than that of any of the other moons of the major planets of the Solar System. As a result, the gravitational influence of the Moon on the Earth is, and has been, a major influence on the development of our planet. Together with the importance of the Moon’s origin for plate tectonics, the three main influences of our companion can be summed up as the three “T”s – tectonics, tides and tilt. And even the third of these owes something to lunar gravity.    Tilt refers to the amount by which the Earth leans over in its orbit. Instead of being upright, with a line through the Earth from the North Pole to the South Pole making a right angle with the plane of the Earth’s orbit around the Sun, our planet is tilted at an angle of about 23 degrees out of the vertical. This tilt is responsible for the cycle of the seasons. The Earth always leans in the same direction in space, so as it goes round the Sun first the Sun is on the side where one hemisphere is tilted towards the Sun and it is summer in that hemisphere and winter in the opposite hemisphere, then six months later the situation is reversed. The tilt also plays a part in the rhythms of Ice Ages – more of this later.    Although the tilt of the Earth changes slightly on timescales of tens of thousands of years, it cannot vary very much, because the gravitational influence of the Moon acts as a stabilizer. If we did not have such a large Moon, or if the Moon were much farther out from the Earth, the combined influence of the Sun and Jupiter (and to a lesser extent the other planets) would tug on the Earth and make it tumble in space, so that it might suddenly switch from being nearly upright to lying completely flat in its orbit (“suddenly”, on this timescale, meaning in as little as 100,000 years). This kind of behaviour is chaotic, in the mathematical sense of the term, which means that small changes in the various forces acting on the Earth would produce large and unpredictable effects. Just such chaotic tumbling has happened on Mars, where there is no large moon and where the tilt can change suddenly by at least 45 degrees, and more slowly by as much as 60 degrees. But on Earth, the tilt has been essentially constant for at least hundreds of millions of years, and probably a lot longer.    It doesn’t take much imagination to appreciate the effect on an incipient technological civilization if the Earth suddenly rolled over on its side, with the North Pole, say, pointing directly at the Sun. The oceans and land around the equator would freeze over, and at high latitudes each hemisphere in turn would experience a sequence of searing summers followed by freezing winters. The equatorial regions would never thaw, even when the Earth was “side on” to the Sun in its orbit, because the shiny surface of ice and snow would reflect away most of the incoming solar heat. The tropics are, of course, home to the vast majority of species on Earth, most of which would become extinct. It seems that chaotic changes in tilt are normal for terrestrial planets, and this alone could be relevant for the emergence of a technological civilization, or any kind of complex life based on land, on a planet that “just happens” to have a large Moon.    As the Moon is slowly retreating from the Earth, this stabilizing influence will decline as time passes, which sets a limit on the window of opportunity in which a civilization like ours could have emerged on Earth. When the Moon formed, it was much closer to Earth, and has been steadily retreating as the energy of its orbital motion has gone into stirring up tides. At present it is moving outwards at a rate of about 4 cm per year, and within 2 billion years it will no longer be able to stabilize the Earth’s tilt.
    Tides are well understood, and they must have played a significant part in the emergence of life from the sea and on to the land. Tides on Earth are primarily produced by the gravitational pull of the Sun and the Moon – in principle there are tiny effects from other planets, but too small to be noticed. Both the Sun and the Moon cause both the oceans and the solid Earth to bulge upwards underneath them and on the opposite side of the planet (you can think of the bulge on the far side as being related to a stretching of the Earth as it is tugged towards the Moon or Sun). In between, we have low tide. On their own, lunar tides today are about twice as big as solar tides. But the two tides add together or partially cancel at different times of the month. At New Moon and Full Moon, the Moon, Earth and Sun are in a straight line and the tides add together. This brings very high tides known as spring tides (because they “spring up”; nothing to do with the season spring). At the quarter moons, the Sun, Earth and Moon form a right angle, and the solar effect cancels out half of the lunar effect, producing much less impressive high tides, known as neap tides. There are local variations caused by the shapes of coastlines, but in essence this means each place on Earth has two high tides and two low tides each day, as the Earth rotates under the Sun and Moon.    Even today, the ocean tides seem impressive, and in some ways it is even more impressive that tides in the “solid” Earth have an amplitude of about 20 cm. The ground beneath your feet literally goes up and down over this range twice a day, but you don’t notice because you are going up and down with it. The solar influence has been constant as long as the Earth has been in its present orbit. But when the Moon was closer to the Earth, the tides it raised, both in the seas and in the solid Earth, were correspondingly larger.    Simulations of the event in which the Moon formed suggest that it coalesced out of a ring of debris no more than 25,000 km above the Earth, compared with its distance of just over 384,000 km today. That’s less than a tenth of the present Earth–Moon distance. This would have raised enormous tides in the oceans, if there had been any oceans at the time, but as it was the repeated stretching and squeezing of the solid Earth, associated with solid tides more than a kilometre in height, would have generated enough heat to keep the surface molten for some time after the impact. But the enormous amount of energy released would have seen the Moon move outwards relatively quickly, and things would have settled down enough for the Earth’s crust to form (or re-form) within a million years. Even so, the heat generated by lunar tides within the Earth would have remained significant, and contributed, along with the heat from radioactivity, to the establishment of tectonic activity on Earth.    The Earth was also spinning much faster just after the impact that created the Moon – as a direct result of that impact. Tidal forces have slowed the spin of the Earth as the Moon has retreated from us. Just after the Moon formed, a day on Earth was only five hours long. At that time, instead of tides 2 metres high every 12 hours there would have been tides several kilometres high every two-and-a-half hours. But these extreme tides did not last long. The first reasonably complex forms of plant life on land emerged from the sea a little over 500 million years ago, and in a memorable numerical coincidence there were about 400 days in the year about 400 million years ago; so the emergence of complex life from the sea occurred when tidal conditions were not dramatically different from those of today. The plants, and later animals, that made the transition onto the land could do so by spreading out from the tidal zones. First they evolved the ability to survive drying out twice a day in the intervals between high tides, then some of them developed the ability to survive above the tide line altogether. This must have been a huge evolutionary advantage, giving them the ability to spread into and colonize vast areas where there were no predators. Of course, the predators soon followed! But would it all have happened so easily without the large tides associated with our large Moon?    The more we look, the more important the Moon seems for our existence.  And there is one staggering coincidence that nobody has been able to explain.  Remember that the Moon is moving outward from the Earth and has been doing so for billions of years. This ties in with one of the most curious observations in astronomy – indeed, in science – which seems to have no explanation and is utterly puzzling. Just now, the Moon is about 400 times smaller than the Sun, but the Sun is about 400 times farther away than the Moon, so that they look the same size on the sky. At the present moment of cosmic time, during an eclipse, the disc of the Moon almost exactly covers the disc of the Sun. In the past, the Moon would have looked much bigger and would have completely obscured the Sun during eclipses; in the future, the Moon will look much smaller from Earth and a ring of sunlight will be visible even during an eclipse. Nobody has been able to think of a reason why intelligent beings capable of noticing this oddity should have evolved on Earth just at the time that the coincidence was there to be noticed.  It’s like a sign hanging in the sky, drawing attention to the Moon and shouting “Hey, look at me – without me you wouldn’t be here.”  It worries me, but most people seem to accept it as just one of those things.  But it must be just a coincidence. Mustn’t it?
Dr John Gribbin
is a Visiting Fellow in Astronomy
at the University of Sussex,
and author of
The Reason Why (Allen Lane)

One comment on “The Moon and Me (and you!)

  1. Rafa says:

    Fascinating stuff.

    Also, having something relatively close to aim for and then use as a staging post to explore the rest of the solar system could be vital in our future development, especially if there’s significant water and other useful raw materials up there.

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