The most detailed information concerning comets has come from spaceprobe studies of Comet Halley in 1986. These show that nearly half their mass consists of highly complex organic molecules, similar in spectral properties to bacterial cells. The assortment of smaller organic molecules found in comets' gaseous envelopes results from the breakup of more complex organic structures.
It has recently become clear that the Earth's atmosphere could not have produced the organic building blocks needed to start life. These are now thought by a majority of planetary scientists and geochemists to have come from comets.
Between 4,500 million years ago, when the Earth was formed, and 3,800 million years ago, our planet was pummelled by comets and meteorites, giving rise to the Hadean (hellish) epoch. Towards the end of this epoch, the oceans, the atmosphere and primitive life came into being, at almost exactly the same time. Moreover, there is evidence that the carbonaceous deposits associated with the Earth's oldest rocks - laid down during this period - may have had a biological origin.
Because the conditions prevailing on the Earth at this time were far too inhospitable for life to originate, it seems reasonable to think that fully fledged microbial life may have been arriving along with the comets. Cometary life then took root on Earth at the first moment that favourable conditions evolved.
The spectacular tail of a comet, such as is now seen in Comet Hale-Bopp, is caused by the release of dust particles embedded in the frozen crust which evaporates as it approaches the Sun. However, when dust production is seen at great distances from the Sun, a different explanation is required. Comet Halley was still throwing out dust when it had receded beyond the orbit of Jupiter, and a similar release was seen in Comet Hale-Bopp in August 1995.
A few months ago I, together with Professor Sir Fred Hoyle and microbiologist Professor David Lloyd of Cardiff University, argued that this remarkable behaviour could reasonably be explained if microbiology was at work in liquid domains beneath a hard-frozen crust. Bacterial activity can release large amounts of gases, creating high-pressure pockets and causing sporadic explosions of the overlying crust - rather like the fermentation of liquor in sealed jars.
On the comet, the crust could shatter and then refreeze, leading to sporadic outbursts. When you watch the spectacular outbursts of Comet Hale-Bopp in the coming days, consider for a moment that you might be witnessing the process by which the fabric of life is transferred throughout the Universe.
Chandra Wickramasinghe is Professor of Applied Mathematics and Astronomy at the University of Wales in Cardiff. With Sir Fred Hoyle, he has written numerous book and papers dealing with the cosmic origins of life. Their latest book, Life on Mars? - the case for a cosmic heritage, is published next month by Clinical Press, Bristol.Reuse content