Arguments for Alien Life on the Earth
3 Sep, 2007 01:35 pm
The presence of organics that would serve as building blocks of life, and evidence for
liquid water in comets makes such objects a possible site of an origin of life. Considering the total volume of congenial sites present in comets compared with the volume of similar sites on the Earth, the odds in favor of an an origin in comets are larger by astronomical factors.
Organic molecules that could serve as the basic building blocks of life are everywhere to be found in the cosmos – in interstellar dust clouds, comets and in meteorites – but such organic molecules are far cry from life.
The precise moment of life’s first appearance on the Earth is still uncertain. Geochemical studies are tending to suggest that this moment occurred shortly after an episode of intense bombardment of comets1,2. Comets brought much of the water in the Earth’s oceans and organic molecules as well. The conventional view is that these organic molecules were transformed into a primitive living system, through a sequence of ill-understood intermediate steps, on the Earth itself. The odds against life being reached through such steps are difficult to calculate with any degree of confidence, but they are indisputably vast, ranging from being astronomical in magnitude to super-astronomical. The fact that life exists on Earth shows this hurdle of improbability was bridged, but there is no requirement for this to have happened in the context of our tiny planet, if routes of transference across the galaxy (panspermia) exist. To achieve such a grotesquely improbable result like life it would clearly pay handsomely to go to the biggest available system3.
Comets offer such a possibility. Recent studies in collaboration with Bill Napier and Janaki Wickramasinghe4 have shown that even a single comet is better placed to produce life from non-life than all the lakes on a primitive Earth. The reason is that comets have been shown to contain organics and water together with a suspension of clay particles, the water remaining liquid for a few million years due to radioactive heat sources5. In contrast, on the Earth, similar combinations of organic materials and particle surfaces have a seasonal persistence, near the margins of lakes and seas, and possibly in geothermal vents. Taking account of such differences we can argue that a single comet is over 100 times more suitable for the processes that have been discussed for life’s origins than the Earth6,7, and the totality of comets in our solar system makes an origin of life in such comets 1014 times more likely. And the solar system cannot be regarded as being unique in possessing cometary clouds. There are some 100 billion sun-like stars in the galaxy, and if all these stars have comet clouds, the odds in favour of comets being the venue for the origin of life are on the order of 1025 .
But how can life emerging within a favoured comet in some distant planetary system come to infect the Earth? An emergent chemotrophic microbial living system spreads through the volume of the comet well before the nuclear heat sources run out and the comet refreezes. Dissemination of-freeze dried microbes back into space would occur with cometary activity that on the average erodes a metre or so of the cometary body with each passage close to the sun.
Comets in our solar system were the first solid bodies to coalesce from smaller clumps of interstellar dust and water-ice that condensed in the solar nebula near the present orbital distances of Uranus and Neptune. The accumulation into 10 km-sized cometary bodies took place relatively quickly on a timescale of millions of years, and there would have been an.inevitable dispersion in their times of formation also of the order of millions of years. The comet to develop the first fully-fledged microbial ecology would become active as a result of collisions with smaller bodies and produce sporadic outbursts of bacterial cells that would be accreted by the multitude of smaller comesissmals from which other comets have still to form. It is reasonable, therefore, to expect an entire Oort cloud to become infected on a timescale of millions of years.
An Oort-type cloud, such as is present in our solar system, then spreads its legacy of life to other nascent planetary Encounters of the Oort cloud with molecular clouds of a typical mass of 105 solar masses, at a passing distance of 20 parsecs, would occur with an average frequency of once every 40 million years5,8 . Such an encounter has two effects: the Oort cloud is perturbed and a surge of comets enter the inner solar system each such surge lasting for a few million years. Comets then become active leading to the unleashing of viable microbes into the zodiacal dust cloud, and these particles are propelled by radiation pressure to reach nascent planetary systems in the passing molecular cloud. The travel time of the microbes so released from cometary deep freeze plus the timescale of re-incorporation in the new system is less than a million years, thus ensuring the survival of even relatively fragile bacterial species. Cosmic radiation poses no challenges to this process of panspermia.
References:
1. Mojzsis, S.J. et al., 1996. Nature 384, 55.
2. Mojzsis, S.J. et al., 2001. Nature, 409, 178.
3. Hoyle, F. and Wickramasinghe, N.C., 2000. Astronomical Origins of Life: Steps towards Panspermia. Kluwer Academic Press.
4. Napier, W.M., Wickramasinghe, J.T. and Wickramasinghe, N.C., 2007. Int. J. Astrobiology, in press
5. Wickramasinghe, J.T., 2007. PhD thesis, Cardiff University
6. Haldane, J.B.S., 1929. The Origin of Life. London: Chatto and Windys.
7. Cairns-Smith, A.G. 1966 J. Theor.Biol., 10, 53
8. Wickramasinghe, J.T and Napier, W.M., 2007, in preparation
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