The Moon and the Planets [NU015]

David Noel
Ben Franklin Centre for Theoretical Research
PO Box 27, Subiaco, WA 6008, Australia.

"So many worlds, so much to do, so little done, such things to be"
-- Tennyson

Nine known major planets rotate around our sun, and most of these planets themselves have other satellites rotating around them, some as big as the smaller planets. In addition the solar system has a huge number of smaller bodies, planetoids or asteroids, most of which lie in the 'Asteroid Belt' between Mars and Jupiter. These asteroids are usually assumed to be the remains of a tenth planet which broke up at some time in the past.

These other members of the solar system are of interest to us, in examining what has happened in the history of our Earth, for two main reasons. Firstly, whatever forces caused Earth expansion will almost certainly hold sway elsewhere in the Universe, and the other planets may show evidence of these forces. Secondly, as the other planets vary greatly in size and distance from the sun, they provide a range of models from which we can draw information on general planetary conditions (particularly atmospheres), which will help us explain why the present conditions on our planet are as they are.

The Sun's Family

Table 15 (mostly from [1]) gives some details of the planets of the solar system, including Earth, and of some of the major natural satellites. The most important factors to focus on are size, distance from the sun, atmosphere (if any), and escape velocity.

Table 15. Planets and major satellites of our Solar System
Surface gravity
Escape velocity
Distance from sun
Surface temperature
Mercury (0) 0.055 2 439 0.37 5.41 4.25 0.39 200 0 --
Venus (0) 0.815 6 051 0.88 5.25 10.4 0.72 470 90 CO2,N2
Earth (1) 1.000 6 378 1.00 5.50 11.2 1.00 20 1.0 N2,O2,H2O
Moon 0.013 1 738 0.16 3.35 0 0 --
Mars (2) 0.107 3 393 0.38 3.91 5.02 1.52 -40 .007 CO2,N2,Ar
Jupiter (16+) 317.9 71 398 2.54 1.24 59.6 5.20 -140 -- H2,He,CH4,NH3
Io 0.015 1 815 0.18 -120
Europa 0.008 1 569 0.13
Ganymede 0.025 2 631 0.14
Callisto 0.018 2 400 0.12
Saturn (17+) 95.18 60 330 1.15 0.62 35.5 9.54 -170 -- H2,He
Titan 0.023 2 575 1.6 N2,Ar,CH4
Uranus (15+) 14.54 26 200 1.17 1.24 21.3 19.18 -- -- H2,He
Neptune (2+) 17.07 25 225 1.18 1.61 23.3 30.06 -- -- H2,He
Triton 0.022 1 750 0.1 N2,CH4
Pluto (1) 0.022 1 145 -- 2.06 1.3 39.44 -- -- CH4
Charon - 640

Our knowledge of the other members of the solar system is continually being improved, especially by data from the American and Russian space probes, but the general picture is clear. The planets are usually divided into two groups.

The Inner Planets

The first group, the innermost four of Mercury, Venus, Earth, and Mars, are usually called the Inner Planets. All have some similarity to Earth, but also many differences. Earth is the largest, but Venus is almost as large; Mercury is the smallest. Venus has a much denser atmosphere than Earth, Mars a much thinner one, Mercury none. Earth has a single large satellite (the Moon), Mars has two tiny ones, the other two have none.

The two natural satellites of Mars are the subject of one of the strangest puzzles in scientific history. Small, irregular lumps of rock -- even the biggest, Phobos, is less than 14 km long on its largest axis -- both of them are smaller than Rottnest. Both are close in to the planet, Phobos so close that it goes round Mars in under 8 hours, less than Mars' 25-hour day, and so it rises in the West and sets in the East. These are very unusual objects.

These tiny, close-in moons were not discovered till 1877, when they were picked up by the American astronomer Asaph Hall. Such a late discovery can be understood, the satellites' small size and closeness to the planet making them undetectable until telescopes were improved enough. How, then, can we explain the relatively accurate description of these two satellites, given in Jonathon Swift's "Gulliver's Travels", published more than 150 years previously, in 1726?

Although nowadays regarded as a children's book, "Gulliver's Travels" was, in fact, a bitter political satire on the society of Swift's times. In the third voyage, Gulliver describes the island of Laputa, inhabited by scientists and able to float in the air, its position controlled by a giant natural magnet. The island is described in some detail -- its thickness (300 yards), its area (10,000 acres, about twice that of Rottnest), its drainage system -- a typical microdomain!

Gulliver notes that the astronomers of Laputa had much better telescopes than those known in Swift's day, and that "they have likewise discovered two lesser stars, or satellites, which revolve about Mars; whereof the innermost is distant from the centre of the primary planet exactly three of his diameters, and the outermost, five; the former revolves in the space of ten hours, and the latter in twenty-one and a half".

It seems quite beyond the bounds of chance for these very unusual objects to have been predicted, so accurately and so far ahead of time, almost as an aside in a satirical novel. On this occasion, the formulation of a suitable Proposition will be left as an exercise for the reader.

The Giant Planets

The second group, the Outer Planets, includes the so-called Gas Giants -- Jupiter, Saturn, Uranus, and Neptune -- and the outermost known planet, Pluto. The gas giants are much more massive than Earth, have enormously dense atmospheres, and multiple satellites, more of which come to light with each new flypast. Most of these satellites are small, but Jupiter has four huge ones, and Saturn and Neptune have a huge one each; two of these satellites are bigger than Mercury, the planet closest to the Sun.

Pluto is the odd man out as far as the Outer Planets are concerned. Very distant, and still not investigated in detail by any space probe, it is much more imperfectly known than the rest. In size it is similar to a large asteroid, but it has one comparatively large satellite, of half its own radius. Although it is, on average, the planet farthest from the Sun, its orbit is appreciably elliptical, and it is currently inside the orbit of Neptune. It has been suggested that it is an escaped gas-giant moon or errant asteroid, or has some other exceptional mode of formation.

For the whole solar system, the Sun is far and away the most significant source of heat and other energy, and the planets closer to the sun than Earth are much hotter, while the outer ones are much colder. This has a major effect on their atmospheres -- materials which are atmospheric gases on Earth may be liquids, or even solids, on the outermost planets.

Atmospheres of the Planets

The atmospheres of the planets differ very considerably, both in composition and in mass or density, but we will see that, with one exception, they do fit into a general pattern. The exception is Earth itself.

In NU011 it was shown that the two most important properties of an atmospheric gas molecule, from the viewpoint of this book, were its molecular weight and its temperature. When we need to tie in these properties with the conditions on a particular planet, we need also to consider the mass of the planet, the main determinant of its escape velocity.

Jupiter is easily the most massive of the planets, more than 300 times heavier than Earth. Its escape velocity is so high it is easily able to retain even hydrogen and helium, the lightest gases, and in fact almost all of its enormous atmosphere consists of these gases -- a reflection of their high relative abundance in the Universe. However, Jupiter's atmosphere also contains some readily detectable carbon and nitrogen gases such as methane and ammonia.

As Jupiter is a long way from the sun, it is also very cold. This cold, and the immense amount of its atmosphere, means that compounds which are gases in its outer atmosphere may be liquids or even solids lower down, under the huge pressures (perhaps many millions of atmospheres).

As yet we do not know where the solid surface begins, and whether this surface is of the same rocky nature as those of the inner planets, or is frozen atmosphere. The low density given for Jupiter in Table 15, 1.24 as opposed to the 5.50 for Earth, reflects the fact that this density is calculated for the whole planet, including its atmospheric components.

It seems likely that Jupiter's atmosphere is probably little changed from its primeval state, as far as composition is concerned. We can work from the assumption that the primeval atmospheres of all the planets were similar in composition to that of Jupiter now.

Proposition 15A
The primeval atmospheres of all the planets had a similar composition to that of Jupiter now

The actual present composition of Jupiter's atmosphere, at least the outer parts accessible to measurement, is about 90% hydrogen and 10% helium. The other gases present, such as compounds of nitrogen, oxygen, and carbon with hydrogen, make up less than 1% altogether.

The other three gas giants, Saturn, Uranus, and Neptune, follow the Jupiter pattern quite closely. All have high escape velocities, and as they are further out and so even colder than Jupiter, all their atmospheric gases, even the lightest ones, are moving too slowly to be lost to space. Their only difference is that some are cold enough to turn compounds which are gases in Jupiter's atmosphere into liquids or solids -- ultimately leaving only hydrogen and helium, which both have low molecular weights (2 and 4) and a very low liquefaction temperature.

Proposition 15B
Saturn, Uranus, and Neptune have similar 'primeval' atmospheres to Jupiter, except that they have less of the heavier atmospheric components due to freezing or liquefying out

Information on Pluto's atmosphere is very uncertain, but it may have a very thin one consisting only of a little methane and possibly some neon. Pluto is so light that these heavier gases would only be retained at all because the intense cold would slow them right down.

Atmospheres of the Inner Planets

When we move from the Outer to the Inner planets, the picture changes completely. Each of the inner planets has a clearly-defined rocky surface, while their atmospheres have all been markedly changed from the primeval pattern, but in different ways.

Mars is the second smallest of the inner planets. Further out from the Sun than Earth, and hence colder, it has been able to retain only a residual atmosphere, less than one-hundredth of the pressure of that on Earth. This very thin atmosphere is believed to include about 95% carbon dioxide (molecular weight 44), 2.7% nitrogen (28), and 1.6% argon (40).

These figures are actually just about what we might expect. Because of the relatively small mass of Mars, and so fairly low escape velocity, all the lighter gases would be lost quite quickly. Hydrogen and helium, with molecular weights of 2 and 4, would go immediately. The next lightest gases, methane (16), ammonia (17), and water vapour (18), could hang on long enough for partial conversion into nitrogen (28) and carbon dioxide (44); anything not converted to these two heavier gases would be lost. Argon, an inert, heavy gas, would be largely retained, too heavy to be lost and too unreactive to be converted.

In contrast to Earth, Mars has an atmosphere with virtually no oxygen, which it could retain (mw=32), presumably because it has never been exposed to extensive oxygen generation through biological processes (Proposition 11E). The ratio of nitrogen to argon in Mars' atmosphere is instructive; Earth has a ratio of about 83.9, Mars has one of about 1.7. It seems clear that Mars has lost much more of its original nitrogen than Earth, purely because Mars itself is lighter, and medium-weight gases such as nitrogen are less able to be retained than heavier ones such as argon.

Proposition 15C
Mars has lost much more of its atmospheric nitrogen than Earth because of its lower escape velocity

In fact, if we assume as a first approximation that both planets once had the same nitrogen/argon ratio, and Earth has kept all its argon and nitrogen, while Mars has kept its argon but lost some of its nitrogen, the actual percentage lost turns out to be 98.7%. If all this nitrogen was restored to Mars' atmosphere, it would increase its atmospheric pressure by a factor of over 30, bringing it up to about a fifth of Earth's. This seems on the right track.

Moving now to the planet closest in to the Sun, Mercury, here we have an extremely hot, quite small planet. The complete lack of atmosphere is to be expected -- even the heaviest gases would be boiled off quite quickly.

Venus the Mysterious

Venus is the most interesting of all the planets from our present viewpoint. Until the middle of the 1900s, Venus was very much a mystery planet. It is perpetually shrouded in thick clouds, so that the surface, and how far it lay beneath the clouds, was completely unknown. Even the time it took to rotate on its axis was not known. This left the field open for suggestions that Venus was a lush tropical swamp, perhaps inhabited by dinosaurs.

Data from the Venus probes, and from radar studies, have now given us a clearer picture of what turns out to be a very harsh and hostile environment. At the surface, which is is a typical rocky one like the other inner planets, pressures average around 90 times those of Earth. There are no seas, negligible free water in fact. The very dense atmosphere consists of about 96% carbon dioxide and about 3.5% nitrogen.

Venus is only a little smaller than Earth, with a slightly lower escape velocity, 10.4 instead of 11.2. It is closer to the Sun, and much hotter -- surface temperatures around 450ºC have been recorded by the probes. This temperature is actually higher than that on the surface of Mercury, which is even closer in.

Why should this be so? This high atmospheric temperature is usually ascribed to a sort of "Greenhouse Effect", particularly because of all the carbon dioxide present. I believe this view is mistaken, not because we should expect higher temperatures at the bottom of thicker atmospheres (although this is true), but mostly because of the reflection-radiation ratio. We will throw more light on this matter when we come to look at the Greenhouse effect on Earth, in NU017.

Back now to the matter of the mass or pressure of the atmosphere on Venus. Other things being equal, we would expect Venus to have a similar atmospheric pressure to Earth, or slightly less, because of its smaller size and higher temperature. Why does it have this exceptionally thick atmosphere?

There are two main parts to the answer. The first is that it is not Venus which has the exceptional atmosphere, it is Earth. Venus is the typical planet, Earth is out of line. Both planets have sufficient mass to retain most gases of medium weight or above. It will be interesting to see if the nitrogen-argon ratio on Venus is similar to that on Earth, as we would expect. The big difference is that Venus has not experienced the massive deposition of carbon dioxide into solid forms which took place on Earth (Proposition 11J). This in itself indicates that Venus has never developed any type of life capable of driving such a carbon-extraction process.

If all the carbon dioxide were removed from its atmosphere, Venus would experience a drop in atmospheric pressure by at least 96% -- actually a bit more as carbon dioxide is heavier than nitrogen -- and this would bring it down to between 2 and 3 Earth atmospheres. Again we are on the right track.

Proposition 15D
Venus has a much higher atmospheric pressure than Earth because it has never experienced massive carbon deposition from its atmosphere

Even so, this new figure is still rather higher than we would expect -- it should be less than Earth. There is a minor point in that, as Venus has no seas the 'surface' pressure is comparable to that on Earth at the ocean bottom with the water removed, but this really makes very little difference. For the second part of the answer we will need to return to Earth itself.

The Earth and the Moon

The Earth/Moon system is actually close to being what is called a double planet. The Moon has about one eightieth of the mass of the Earth, and is roughly 400,000 km away. As a first approximation, the centre of gravity of the system therefore lies about one-eightieth of the way along a line from the centre of the Earth to the centre of the Moon.

The present radius of the Earth is about 6,400 km, so the centre of gravity lies only about 1,400 km below the surface, only one-fifth of the way down to the centre. If the centre of gravity was actually above the surface, then the Earth/Moon system would conform to the true definition of a double planet.

Now for the crunch. Some 300-400 million years ago, the Earth's radius was perhaps half what it is now, say around 3,200 km. If the Earth and the Moon had the same separation and masses as they have now, they would have formed a true double planet, with a centre of gravity some 1,800 km above the Earth's surface!

In actual fact, it appears quite likely that the Moon was once quite a lot closer to the Earth, but even so there is clearly scope for a double planet situation to have applied in the past. In a double planet, the two components tend to share atmospheres, especially if their mutual centre of gravity lies within the normal atmosphere of one component (and this is more likely if the Moon was once closer to the Earth).

Of course the centre of gravity is not the point at which the gravitational forces acting on a gas molecule between two bodies cancel out, but these two locations are linked. We can get a better feel for the situation by looking at another mind model.

Interaction between Gravity Wells

Figure 15.1 is a development of the 'gravity well' model shown for the Earth in Figure 11.1. The original model was simplified. That model, a cross-section of a funnel shape lying in a flat plain, only considered the Earth in isolation.

Fig. 15.1. Gravity wells for separated (left) and close (right) Earth-Moon systems

We can think of the gravity-well surface as being a huge flat sheet of very thin rubber. The Earth is then a heavy ball-bearing which is placed on the sheet and stretches it downwards to form the well shape.

We can extend this model by taking the Sun into account. The Sun is enormously heavier than the Earth, but is a long way away. We can think of the Sun as having its own gravity well, formed by a much heavier ball, placed a long way off on the rubber sheet. The Earth and its own tiny gravity well lies out towards the edge of the Sun's well, where its slope has become quite shallow. The Earth stays at the same distance from the Sun because it is running round the Sun, and holds its position on the sloping well-wall like a wall-of-death motorcyclist.

The first diagram in Figure 15.1 shows the gravity wells for the Earth and the Moon when they are fairly widely separated. The Moon lies high up in the Earth's gravity well and does not distort its shape much. That is close to the present situation.

The second diagram shows the Earth and the Moon much closer together. Their gravity wells are more merged together, with the rim of the Earth well 'dented' down, and so allowing atmospheric gases to overflow more easily. And, most important, the situation gives these gases greater scope to escape from the Earth-Moon system altogether, even though they may still remain within the Sun's immensely wider gravity well, elsewhere in the solar system.

In this way, at some stage of the Earth's history, a lot of its atmosphere is likely to have leaked off via the Moon. With its much smaller escape velocity, the Moon would have been unable to hold this atmosphere -- of the solar system moons, only Saturn's giant moon Titan is able to hold a significant atmosphere, and Titan is much more massive and colder than our Moon. So here is a possible explanation for the fact that Earth has a thinner atmosphere than Venus even when carbon deposition is allowed for -- Earth has a massive moon, while Venus has none.

Proposition 15E
Earth has lost atmosphere through leakage via the Moon, especially when the Earth's radius was smaller and a double-planet situation was approached

Life in the Universe

The vast rubber sheet in which the Sun's gravity well lies is part of an even vaster one extending over our whole Galaxy, and beyond that to other galaxies and to the remote ends of the Universe -- if it has ends.

A question which has fascinated people ever since the existence of other planets was known is whether life, especially intelligent life, exists elsewhere in the Universe. Many views, ranging from the serious (such as Asimov's [4]) to the crazy, have been put forward.

So far no really positive evidence has emerged, and I will not be venturing a Proposition here. But we can note that the fact that no other planet in our solar system except Earth has an atmosphere containing much free oxygen, and that I have suggested that this oxygen originates only through the action of life. All the planets have immense reserves of oxygen in their rocky cores, oxygen is the most common element there, so it is not a question of unavailability.

Earth is also the only planet in the solar system with significant surface reserves of water. Water is made up of hydrogen and oxygen, and all the planets appear to have started off with huge reserves of hydrogen. Free hydrogen is easily lost except from the more massive planets. Water vapour can be retained by the middle-range planets, but this is always subject to breakdown in the outer atmosphere through the action of cosmic rays. The resulting hydrogen would be easily lost, and the oxygen retained, or reacted with methane to form carbon dioxide in a hydrocarbon-rich atmosphere.

In Asimov's book he tries to work out the probability of intelligent life existing elsewhere in the Universe, on making various assumptions as to the mode of formation of stars and planets, and the probability of occurrence of given conditions, whether normal or unusual. It does seem that our Earth is unusual. Earth life is believed to have originated in the water, and water is not common elsewhere in the solar system.

But perhaps the most unusual feature of Earth is its relatively large moon, unique among the inner planets. We have seen how the presence of the Moon may have affected the Earth's atmosphere, and we may wonder if the Moon may have been a crucial element in the formation of life on Earth. If so, it lessens the likelihood that life has arisen elsewhere, if the Earth-Moon system is truly very unusual.

The Expanding Planets

The Earth-Moon system may or may not be unusual, but there is nothing to suggest that it is not subject to the same physical laws as the rest of the Solar System, or indeed the rest of the Universe. We can therefore expect that if the Earth has expanded in the past, the same forces will have acted on the other planets, and evidence of this may be found on these planets.

In fact evidence of global changes of this sort were looked for on the other planets back in the days when Continental Drift was coming into its own, much earlier in the century. At that time, knowledge on the surfaces of the other members of the Solar System was quite sparse. What knowledge existed came entirely from telescope observations, and the only other world within proper reach of the telescope was one face of the Moon (the Moon always keeps the same face turned to Earth).

The Moon did show apparent evidence of volcanic activity, or at least of igneous rock flows, but its most prominent features were the huge impact craters which covered much of the visible surface. Further afield, the two closest planets were Venus, completely cloud-covered and enigmatic, and Mars.

Mars lies at the aggravating limit of resolution from Earth-based telescopes. Some surface details can be made out, but it was impossible to say for certain what they represented. At the beginning of this century, the American astronomer Percival Lowell was a strong advocate of the theory that a series of lines just barely visible on Mars was a network of canals containing irrigation water used by the inhabitants.

The advent of the space probes changed all this. Actual landings of remote-controlled vehicles have been made on both Venus and Mars, and even some simple analyses of rocks made. We have photographs of these rocky surfaces. Radar scans of the surface of Venus have given increasing information on its terrain. The surface of Mars has been mapped in fair detail from orbiting spacecraft.

Tiny Mercury, very close to the sun and for this reason very difficult to observe, is still not known in any detail. It does have a number of prominent surface features, mostly impact craters. To date, however, most of the relevant evidence has come from the two closest and best-studied planets, Venus and Mars.

Expansion on Venus

As the evidence accumulates, it lends increasing support to the view that the other planets are behaving similarly to Earth. Venus, similar in size to Earth, has a clear distribution of raised 'continents' and wide, flat 'seabeds', although of course the latter do not contain any water. And the interesting thing is that the 'seabeds' cover about 70% of the planet [14] -- the same as on Earth.

At first the radar resolution was not good enough to show searched-for features similar to the mid-ocean ridges of Earth, but these have now been found. An article in Scientific American [31] describes these, crossing the Aphrodite Terra continent on the equator of Venus. It appears that a long fracture zone, very similar to our mid-ocean ridges, stretches at least two-thirds of the way across Aphrodite Terra, and possibly runs right round the planet in its equatorial zone.

Of course, in the article this evidence is looked at from the conventional viewpoint, with no regard for an expanding Venus. The interior of Venus is assumed to be heated by radioactive decay, and the 'mid-ocean' feature due to convective upwelling of hot rock. The author comments that "If new lithosphere is created near the Venusian equator, then old lithosphere must be destroyed by subduction near the poles, but so far no evidence of a subduction zone has been found". Surprise, surprise.

It seems clear that the same type of domain movement as on Earth is occurring on Venus. However, the fact that the first prominent domain-boundary features observed on Venus run in an equatorial direction, while those on Earth run mostly north-south, may indicate that the Venus feature shows an earlier stage of expansion, perhaps similar to the Tethyan Girdle which earlier existed on Earth.

Proposition 15F
Expansion of Venus is occurring in a similar way to Earth expansion, but may be at an earlier stage of development

This slightly earlier stage of development may be expected, since Venus is somewhat smaller than Earth. This will be looked at again in the next article.

Our Little Brother Mars

Although it is not the planet which is physically most comparable to Earth, Mars, the Red Planet, is perhaps the least hostile as far as human life is concerned. It is the only place we know of where a man might live with no more apparatus than a breathing mask and an insulating suit.

The Red Planet is red because of the colour of its terrain -- waterless, eroded, and swept by thin winds able to support an occasional dust storm. Almost certainly the red rocks and dust are 'rusty' iron oxide colours, as in many Earth deserts. There is ample oxygen there, trapped in the rocks, but virtually none left in the atmosphere.

The terrain of Mars is not a featureless plain, but has a huge array of structural features, including giant volcanic calderas. The highest of these, Nix Olympica, is 25 km high. But easily the most prominent topographic feature is a huge equatorial canyon, some 5000 km long and with an average depth of 6 km. Known as Valles Marineris (Figure 15.2), this great equatorial gash is the site of three out of the four largest volcanic calderas on Mars.

Fig. 15.2. The great equatorial canyon of Mars, Valles Marineris [14]

Incredibly enough, this vast canyon has been interpreted as evidence of former water erosion on Mars -- after all, that was what caused Earth's Grand Canyon, wasn't it? No matter that water cannot exist on Mars (the air pressure is too low to allow liquid water). No matter that volcanos stand in a line along this feature.

In the light of evidence already given in this book, it seems obvious that Mars is in the early stages of planetary expansion, and that the Valles Marineris rift is part of a Tethyan Girdle in process of formation round the planet's equator.

Proposition 15G
Expansion of Mars is occurring in a similar way to Earth expansion, but is at a much earlier stage of development, with an Equatorial Girdle just emerging

In Vulcan's Realm

In NU008 we looked at the formation of volcanos on Earth, and concluded that these were locally-produced phenomena caused by domain-edge rubbing, and did not stem from any inner heat of the planet. We have seen that volcanos exist elsewhere in the Solar System, off Earth, and we can ask how they fit in with the theories put forward.

The most spectacular volcanos in our Solar System do not exist on any of the planets. They occur on Io, the innermost of Jupiter's four giant moons. Images from the space probes have shown huge eruptions from Io's surface, visible as giant plumes leaping up from the edge of the moon's disc.

Researchers have accepted the obvious fact that Io's volcanos have no connection with primeval internal heat welling up from its interior, and ascribed them to the effects on the surface of gravitational forces, caused by its giant companion, Jupiter. This view could be rephrased by saying that crustal movements on Io cause its volcanos -- perhaps the same might apply on Mars, Venus, Mercury, the Moon, and even Earth?

This concludes our examination of the planets. Much remains to be discovered about them, but at present it can be said that they are known to contain nothing contradicting the ideas expressed here, and a great deal to support them. We can now move on to the reasons for the behaviour we have noted in the Earth and the other planets.

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(Full list of references at NURefs)

[1]. Encyclopedia of physical science and technology. Academic Press, 1987.
[4]. Isaac Asimov. Extraterrestrial civilizations. Pan, London, 1979.
[14]. Cambridge atlas of astronomy. Cambridge University Press, 1985.
[31]. Fractured Aphrodite. Scientific American/ Jul 1988.

NU016: The Cosmic Engines

NU014: Geoprospecting and Mineral Riches

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