The crash, bang, wallop theory

Cosmic collisions, a dust-shrouded Sun and an explosion of life-forms. It sounds like futuristic science fiction but it may prove to be the key to our past.

Why did life on Earth proliferate at the beginning of the Cambrian period, some 600 million years ago? Until that time, for nearly four billion years, life did not exist on land. It only existed in the oceans in very simple forms, essentially single-celled bacteria. But at the beginning of the Cambrian, in the space of a few million years, many complex life-forms developed, expressing different multi-celled body plans. This proliferation of life eventually led to the colonisation of the land, the evolution of all the forms of life we see around us today, and to our own existence. But why did it happen then?

Why did life on Earth proliferate at the beginning of the Cambrian period, some 600 million years ago? Until that time, for nearly four billion years, life did not exist on land. It only existed in the oceans in very simple forms, essentially single-celled bacteria. But at the beginning of the Cambrian, in the space of a few million years, many complex life-forms developed, expressing different multi-celled body plans. This proliferation of life eventually led to the colonisation of the land, the evolution of all the forms of life we see around us today, and to our own existence. But why did it happen then?

Over the past couple of years, many scientists have been intrigued by evidence that the Earth experienced a deep freeze at the end of the Precambrian, exactly at the time of this proliferation of life. This has become known as "Snowball Earth". It seems that the entire surface of the planet may have been covered by ice and snow at that time. This is hardly likely to be a coincidence; if the Earth experienced a snowball phase just at the time multi-celled life-forms proliferated, almost certainly the explosion of life was a result of the changing climate. The suggestion is that the freeze first of all caused a bottleneck in evolution, killing off many species, and then encouraged the proliferation of different varieties of life to cope with the changing conditions. But this only moves the question one step further away. Why did the Earth freeze up about 600 million years ago?

In a seemingly unrelated piece of work, scientists have recently discovered that there was an upsurge in the meteoritic bombardment of the Earth and the Moon at the beginning of the Cambrian. The evidence comes from tiny glass beads called tektites, which are found in lunar soil. The beads are produced when meteorites hit the surface of the Moon and melt some of the solid material into liquid glass, which splashes out around the impact crater. Because the samples of lunar soil can be dated measuring the amount of radioactive potassium and Argon-40 present in the samples, it has been possible to work out how the number of impacts has changed as the Solar System has aged.

The evidence reveals that the number of impacts dropped after the first burst of activity in which the planets formed, reaching a low point about 600 million years ago. But then, the rate at which impacts occurred suddenly increased by a factor of four. The most likely explanation is that one or more large objects broke up in the inner part of the Solar System around that time, producing a number of smaller asteroids orbiting around the Sun, many of which collided with either the Earth or the Moon.

This provides an alternative explanation of the explosion of life in the Cambrian period. Biologists have speculated that impacts on Earth could have produced their own evolutionary bottleneck, wiping out many species and creating new ecological niches. But could the impacts have been associated with the production of a big freeze on Earth?

Today, the Sun is surrounded by a ring of zodiacal dust, debris which has been produced by the break-up of comets and asteroids. Each grain of dust may be only about 10 millionths of a metre across, but altogether there are estimated to be 1,000 billion billion billion particles in the ring of zodiacal dust today. But there may well have been a lot more zodiacal dust in the past than there is today.

The total mass of the dust today is less than the amount you would get by breaking up the nucleus of an ordinary comet such as Halley's Comet. But there are much bigger cometary objects in the Solar System. Some have diameters of 200 or 300 kilometres, and if one of these broke up in the inner Solar System this zodiacal cloud of dust would contain at least 300 times as much matter as it does today.

This amount of dust would block out sunlight in sufficient quantities to reduce the amount reaching the surface of the Earth by one or two per cent. Over a timescale of decades, this would be sufficient to cool the surface of the Earth by one or two degrees Celsius - enough to plunge it into conditions like those of the little "Ice Age" of the 17th century. But if the dust persisted for centuries or millennia, the resulting cooling of the oceans could be enough to trigger full ice-age conditions. Even this, though, would not create a Snowball Earth.

There have been some dramatic impacts during the lifetime of the planet. It is now widely accepted that the Moon itself was created at the end of the epoch of planet building, more than four billion years ago, when an object at least the size of Mars struck the Earth a glancing blow. The nature of this collision is best understood from its popular name, the "Big Splash". Enough energy would have been released in the collision to melt the entire surface of both the Earth and the impacting object, splashing molten rock into space to form a ring around the Earth. Some of this material would have escaped into deep space, while some coalesced to form the Moon. If something like that had happened to the Earth 600 million years ago, we would not be here today.

However, there is a third coincidence in the story of the Solar System that happened around that time. When the surface of Venus was mapped by the probe Magellan, astronomers were surprised to discover that it has far fewer craters than they expected. By counting the number of craters on Venus and comparing this with the number of craters seen on the surface of the Moon and the surface of Mercury, they have worked out that the entire surface of Venus was molten about 600 million years ago, then re-solidified as a blank slate on which impact craters could form.

This is just what could have happened if Venus had suffered a collisions like the one in which our Moon formed, but a more direct, head-on crash. In that case the surface of the planet would have melted, but there would have been no glancing splash to form a ring of debris around the planet. Instead, the debris that splashed out from the impact would have escaped entirely from Venus, forming a belt around the Sun. If we are talking about a mass comparable to the planet Mars, this would have been sufficient to block out enough heat from the Sun to produce a super ice age on Earth - a Snowball Earth, in fact.

Although these ideas are speculative, they provide a link between three otherwise unrelated events - an explosion of life forms, a snowball Earth and cosmic collisions with Venus - which all occurred at the same time in the same part of the Solar System. The only thing that is required to make everything fit together is that an object about as big as Mars should have survived the epoch of planet formation and wandered about the inner part of the Solar System for four billion years or so before colliding with Venus. Computer simulations of how the planets formed suggest that just before Venus and Earth reached their present size, there were perhaps as many as 100 objects the size of our Moon, at least half-a-dozen objects the size of Mars, and 10 or more objects the size of the planet Mercury, all orbiting in the inner Solar System.

The two large planets, Venus and Earth, were built up from these pieces, with Mercury and Mars as the left-over fragments. On this picture, it is entirely plausible that there may have been one more left-over piece. If these ideas are correct, they are directly linking our own existence with the catastrophic impact of this last piece of planetary material with Venus 600 million years ago.

The writer is the author of 'Fire on Earth', and is a visiting fellow in astronomy at Sussex University