Letter: Life from space?

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The Independent Culture
Sir: Recent comments on the crossing of the Leonid meteor stream have all overlooked an interesting and potentially important consequence. It is now widely accepted that comets carry complex organic molecules, including amino acids, that might at the very least have been connected with the beginnings of life on this planet. There also serious discussions in progress in scientific circles of the even more radical possibility of cometary panspermia of the type we pioneered in the late 1970s.

The importance of the present crossing of the Leonids is that the source, comet Tempel-Tuttle (which has a period of 33 years), came closest to the sun on the last occasion only nine months ago, and so the Earth will be in receipt of freshly evaporated cometary particles over the next few days. Spectacular meteor showers are caused by the entry of particles of sizes typically larger than a grain of sand which burn as they plough into the Earth's high atmosphere at a speed of some 70 kilometres per second.

Besides these larger particles, the meteor stream will also contain, perhaps in comparable mass, a population of bacterial-sized particles. We have shown that particles of the size of micrococci or smaller, travelling at 70 kilometres per second, would be flash-heated to temperatures up to about 500 Kelvin for brief intervals of the order of seconds, after which they will be slowed down to reach the stratosphere. (See for instance our book Diseases from Space, JM Dent 1979.)

In several laboratory experiments it has been demonstrated that bacteria retain viability under such conditions of flash heating in a near vacuum. Laboratory experiments have also shown that bacteria that become deactivated through exposure to ultraviolet light (as might happen after nine months in orbit) are easily reactivated, through the operation of enzymes, when the source of radiation is removed. Thus the possibility of viable microorganisms from comet Tempel-Tuttle reaching the Earth cannot be ruled out.

The average daily input of cometary dust to the Earth is estimated at about 50 tonnes. A 10,000-fold increase in this quantity over a couple of days seems likely, leading to a total mass of the order of a million tonnes. If as little as one part in a thousand of this is in the form of viable microorganisms, the total number of microbes drifting down to the Earth will be a staggering 1023!

Professor CHANDRA WICKRAMASINGHE

Professor Sir FRED HOYLE

Cardiff University

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