December 11 1964 - How old is the Earth?

The question of the Earth's age has long fascinated both scientists and non-scientists. For the Christmas programme of 1964, in which I was again joined by Henry Brinton, this problem was discussed, but I have had to make revisions to the article as then printed, because new information is now at hand. We gave the value for the Earth's age as 4,500,000,000 years, which was then the official figure. Subsequently, studies have been made of the Rocks of St Paul, which lie between Africa and South America. They contain the element rubidium, which decays spontaneously, by radio­active processes, into strontium; the half-life is 46,000 million years, so that the decay is hardly rapid. From these researches, it is evident that the Rocks of St Paul are about 4,500,000,000 years old, so that the Earth itself must be still older; in the article below, I have revised the figures and given the currently accepted age. It may still be wrong to some extent, but, as I said originally, it is certainly of the right order.

The last months of 1964 were interesting to the astronomer and the space-research worker; in particular there were three attempts to send probes toward the planet Mars.*

One of the objects of the space experiments is to find out more about the Martian atmosphere, which is much thinner than that of the Earth. Mars is also short of water, and it has often been described as a planet older than the Earth. This is not strictly accurate. It may well be in a more advanced state of evolution, mainly because of its smaller size, but there seems little doubt that its absolute age is very similar to that of our own world. Most authorities agree that the planets in the Solar System were formed in the same manner, and at the same epoch.

We are by no means certain how the planets came into being.

The tidal theory made popular by Sir James Jeans, according to which the planets were pulled off the Sun by the action of a passing star, was widely favoured for some time, but has now been rejected because the mathematical objections to it are fatal. More recently, C. von Weizsacker has suggested that the Sun once passed through a cloud of gas and dust in space, collecting the material out of which the planets were gradually built up. This idea seems promising, but the whole question is still wide open. So far as the age of the Earth is concerned we are on rather firmer ground, and the figure now generally accepted is 4,750,000,000 years.

Authorities of a few hundred years ago would have found this impossible to believe. The account of the creation given in the Bible was taken literally, and it was thought that the world could not possibly go back more than a few thousand years. By adding up the ages of the patriarchs, and making similar calcu­lations which seem naive today, the seventeenth-century Arch­bishop Ussher of Armagh claimed that the Earth came into existence in the year 4004 B.C.; the final figure given was nine o'clock in the morning of 23 October. Many churchmen were satisfied with this, and the matter was generally regarded as settled.

The first serious doubts arose with the development of geolo­gical science. Toward the end of the eighteenth century James Hutton, the first great British geologist, went so far as to declare that 'in the economy of the world I can find no traces of a begin­ning, no prospect of an end'. Hutton belonged to the uniformtarian school of thought - that is to say, he explained the charac­teristics of the Earth's crust in terms of processes that were going on unceasingly, albeit very slowly. It did not take many more years for geologists to gain a fair idea of the vast periods of time required for the development of geological strata, and Ussher's date began to look painfully inadequate.

Orthodox churchmen tried to escape from this dilemma by supposing that although the Earth itself might be very old, the simultaneous creation of species would allow life to be of recent origin. However, difficulties soon arose here, too. Palaeontologists, headed by the French scientist Cuvier, showed that fossils - the remains or traces of long-dead creatures - were to be found in rocks of immense age; since the fossils could not be younger than the rocks which contained them, living things must also be dated back a long way.

A last-ditch stand was made on the assumption that Man might be a recent creation. Of course, it is true that the history of man and his ancestors is brief compared with the age of the Earth, but even so it is to be measured in hundreds of thousands of years, just as the history of mammals has to be measured in tens of millions of years. The decisive evidence was provided by a Frenchman who rejoiced in the name of Jacques Boucher de Crevecoeur de Perthes, who undertook some researches in grave pits and quarries near his home town of Abbeville and discovered flints which had unquestionably been fashioned into shape by human hands. De Perthes continued his work, and by 1838 he had become convinced that men had lived in France at a period much earlier than had been generally thought. In fact, the artefacts dated back far before the period assigned to the Biblical Flood.

It took De Perthes many years to persuade others even to look at his artefacts, but eventually he won his point, and in 1859 he had the satisfaction of having his theories vindicated and acknow­ledged before the British Association and the Royal Society.* The work of Darwin completed the change-over in thought, and the way was at last clear for a proper scientific inquiry into the Earth's age. Geology showed that the time-scale must be very long. Quite apart from strata and fossils, there were several means of proving the Earth's antiquity. For instance, a fair estimate can be made of the rate at which sodium finds its way from the land into the sea; the oceans must originally have been of fresh water, and al­though there are many complicating factors it was clear that to make the sea as salty as it is now would take millions of years. Calculations were also made of the rate at which large rivers, such as the Mississippi, were washing sediment into the ocean, which provided an estimate of the age of sedimentary rocks.

At this stage there was a conflict between the geological and the physical evidence, and Lord Kelvin tackled the problem from three directions. First, there was the question of the rate at which the Earth had cooled down (assuming, as most people then did. that it had originally formed part of the Sun). Given that it started at a certain temperature and cooled at a calculated rate, it was possible to work out the time needed for the Earth to reach its present condition.

Kelvin's second yardstick was the rate of the Earth's rotation. It was known that the length of the 'day' is slowly increasing, because of tidal forces, and for various reasons Kelvin calculated that going back 1,000,000,000 years, the Earth must have been spinning 15 per cent quicker than it is at the moment. If the crust were formed about this time, he reasoned, the Earth's globe would be much more flattened than it actually is, whereas if the Earth solidified only 100,000,000 years ago then the shape of the Earth would be approximately as measured now, with a difference between the polar and equatorial diameters of less than thirty miles. Finally, Kelvin believed that the Sun could not have existed in its present form for as much as 500,000,000 years. Originally he had attributed the Sun's radiation to the kinetic energy of meteors falling on to its surface, but when this theory had been disproved he adopted Helmholtz's suggestion that the source of solar energy lay in gravitational contraction. Kelvin concluded that the Earth must be somewhere between 20,000,000 and 100,000,000 years old. He favoured the lower limit, even though it was far too short to satisfy the geologists.

The argument between physicists and geologists was not settled until more became known about the nature of matter. With the discovery of radioactivity, for instance, the 'cooling Earth' idea ceased to be valid, and the much later investigations into the source of stellar energy showed that the Sun is much older than 500,000,000 years. The Sun radiates not because of meteoric in fall or gravitational contraction, but because of nuclear processes taking place deep inside it; hydrogen is being converted into helium, admittedly by a somewhat roundabout method, so that mass is being lost and energy produced. The mass-loss is 4,000,000 tons per second, but this does not amount to much relative to the total mass of the Sun's vast globe.

It was radioactivity, too, which gave a really reliable means of working out the age of the oldest rocks. Many rocks contain uranium, the heaviest element known to occur naturally, and it is known that uranium decays spontaneously, ending up as an isotope of lead. The rate of decay, so far as is known, is quite unaffected by environment, and is assumed to be constant. The 'half-life' of uranium-238 (that is to say, the time taken for half the original quantity to decay) is more than 4,000,000,000 years. If, therefore, uranium and uranium-lead are found together, the ratio of the amounts is a pointer to the time during which the decaying has been in progress.

On this basis, the oldest rocks seem to date back around 4,000,000,000 years. This agrees well with the geological evidence, since a figure of 1,500,000,000 years has been suggested for some sedimentary rocks while life is thought to have appeared upward of 3,000,000,000 years ago. Taking all the facts into account, it seems that the Earth began its separate existence about 4.750,000,000 years ago. This estimate is not precise, but there can be little doubt that it is of the right order.

Nobody can possibly visualize what this figure really means, but some idea may be gained by means of a scale in which the age of the Earth is taken as one day (twenty-four hours). A table may then be drawn up, as follows:

So if we take the Earth's age as being scaled down to one day, men have existed only during the last couple of seconds. In a way, the old churchmen were right in supposing mankind to be a very recent addition to the scene, but we have certainly learned a great deal since Archbishop Ussher fixed the date of the Creation so exactly as October, 4004 b.c.

Old though it is, the Earth is still only about half-way through its expectation of life. Its eventual fate will, of course, be linked with that of the Sun. Opinions have changed here, too. It used to be thought that the Sun was gradually cooling down, so that it would eventually turn into a dim red dwarf star - in which case all living things on Earth would be frozen to death. This whole theory is wrong. As the Sun ages, and uses up its hydrogen 'fuel', it will become more luminous, and in from 8,000,000,000 to 10,000,000,000 years' time it may well become a red giant, with a cooler surface but a much greater size and luminosity than the Sun we know. If so, the atmosphere of the Earth will escape, the oceans will boil, and all life here will perish - it is even pos­sible that our world will be destroyed.

The crisis lies so far ahead that speculation is rather pointless. Modern theories about stellar energy are subject to constant revision, and we cannot claim to know more than a very small part of the truth. All we can say is that if we are on the right track, then the eventual fate of all Earth creatures is likely to be a heat death rather than a cold death. If intelligent beings still survive here at that remote epoch, the only solution will be to migrate elsewhere. However this may be, we may be sure that there is no fear of any-imminent disaster. The Earth has existed for more than 4,000,000,000 years, and it will remain habitable for at least an equal period of the future. It will not last for ever, but nothing in the universe is eternal - perhaps not even the universe itself.