Magazine article UNESCO Courier

Life from Space: Does Life on Earth Have Its Origins in Comets?

Magazine article UNESCO Courier

Life from Space: Does Life on Earth Have Its Origins in Comets?

Article excerpt

Life from space

COMETS are inextricably linked with the history of the Earth. Cometary bodies, as well as debris from them in the form of fine dust particles, have been colliding with the Earth from the very earliest days of its existence as a planet some 4,600 million years ago. It is now clear that volatile materials from comets, including water, contributed in significant measure to the primordial oceans and atmosphere.

Cometary impacts and the acquisition of cometary material did not, however, stop in distant geological time. As recently as 1908, a comet, or part of a comet, collided with the Earth in the Tunguska Valley of Siberia, exploding at a height of 8.5 kilometres in the atmosphere and devastating hundreds of square kilometres of taiga forest.

The entire Solar System today is surrounded by a spherical halo of some thousands of millions of comets located at a distance of about a tenth of a light-year (a light-year is the distance light travels in a year--about 10 million million kilometres). Passing stars cause cometary objects from this halo to become deflected into orbits that bring them to the inner regions of the Solar System at the rate of about one or two a year. The Earth is well and truly entwined within these orbits so material shed from these comets must be reaching us quite plentifully. This material becomes trapped in the Earth's upper atmosphere, adding steadily to the store of terrestrial volatile materials. Fluctuations must occur in the rate of in-fall of cometary debris over long periods of geological time. It is now widely believed that significant increases in this rate would trigger the onset of episodes of global glaciations (ice ages) such as are known to have occurred.

A typical comet, such as Halley's comet, has a mass of some 100,000 million tons and a direct hit from such an object would have dramatic consequences for our planet. Fortunately, such collisions are exceedingly rare, occurring at intervals averaging about 300 million years, coinciding approximately with the time intervals between successive bursts in the evolution of terrestrial life. Smaller comets are more numerous and could collide more frequently.

This much is more or less agreed upon by most scientists. A less popular view, advocated by the British astronomer Sir Fred Hoyle and myself, is that comets were also responsible for the importation of organic molecules that contributed to the origin of life on our planet. We have further suggested that the Earth continues to receive ready-formed living structures such as bacteria and viruses even at the present time.

From 1975 onwards, we have accumulated evidence to support our view that organic dust grains, including organic polymers (large chains of carbon-based organic molecules), exist on a vast cosmic scale. These dust grains populate the clouds of gas that exist between stars and give rise to the visual effect of dark patches and striations seen against the diffuse light of the Milky Way. In 1981, through a combination of laboratory studies, mathematical computations and astronomical observation, we arrived at the conclusion that the bulk of cosmic dust was not merely organic but distinctly biological in character(1). (1) The adjective "organic' is often commonly used to mean "living', in the sense that a plant, a tree or a human being is "living'; chemists employ it in a restricted sense to describe compounds containing carbon combined with hydrogen. All living things contain carbon and hydrogen and thus are organic, but not all things that contain carbon and hydrogen are living. "Biological' means "of or relating to life or living things'.

In a series of publications we have presented arguments to support the theory that life on Earth had its origins in comets and that evolution is controlled by the continuing input of cometary material. Because the injection of comets from the outer Solar System is sporadic, this input of cometary material is not expected to be uniform and regular in time, and so the effect on evolution will also be sporadic. …

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