Professor Chandra Wickramasinghe and colleagues at the University's Centre for Astrobiology have long argued the case for panspermia - the theory that life began inside comets and then spread to habitable planets across the galaxy. A recent BBC Horizon documentary traced the development of the theory.
Now the team claims that findings from space probes sent to investigate passing comets reveal how the first organisms could have formed.
The 2005 Deep Impact mission to Comet Tempel 1 discovered a mixture of organic and clay particles inside the comet. One theory for the origins of life proposes that clay particles acted as a catalyst, converting simple organic molecules into more complex structures. The 2004 Stardust Mission to Comet Wild 2 found a range of complex hydrocarbon molecules - potential building blocks for life.
The Cardiff team suggests that radioactive elements can keep water in liquid form in comet interiors for millions of years, making them potentially ideal "incubators" for early life. They also point out that the billions of comets in our solar system and across the galaxy contain far more clay than the early Earth did. The researchers calculate the odds of life starting on Earth rather than inside a comet at one trillion trillion (10 to the power of 24) to one against.
Professor Wickramasinghe said: "The findings of the comet missions, which surprised many, strengthen the argument for panspermia. We now have a mechanism for how it could have happened. All the necessary elements - clay, organic molecules and water - are there. The longer time scale and the greater mass of comets make it overwhelmingly more likely that life began in space than on earth."
The new paper, The Origin of Life in Comets, by Professor Wickramasinghe, Professor Bill Napier and Dr Janaki Wickramasinghe is to be published shortly by the International Journal of Astrobiology.
Note: This story has been adapted from a news release issued by Cardiff University.
NEO News (now in its thirteenth year of distribution) is an informal compilation of news and opinion dealing with Near Earth Objects (NEOs) and their impacts. These opinions are the responsibility of the individual authors and do not represent the positions of NASA, the International Astronomical Union, or any other organization. To subscribe (or unsubscribe) contact dmorrison@arc.nasa.gov. For additional information, please see the website http://impact.arc.nasa.gov. If anyone wishes to copy or redistribute original material from these notes, fully or in part, please include this disclaimer.
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Bill Napier [napierwm@Cardiff.ac.uk]
Dear Benny,
Chandra Wickramasinghe, his daughter Janaki and I have just had a paper published in the International Journal of Astrobiology, proposing that life may have originated in comets. A "critique" of our paper has been produced with remarkable alacrity by David Morrison, and circulated on his NEO News bulletin. His conclusion is that Chandra and I have a "theory built on air, not solidly grounded in scientific facts".
We have put together a rebuttal, which I'm attaching. We also comment briefly on recent work in Antarctica relating to panspermia, which was mentioned not long ago in CCNet.
Thanks,
Bill
ORIGIN OF LIFE IN COMETS: A REPLY TO DAVID MORRISON
The points raised in David Morrison's critique are already rebutted in our paper and we can only assume that he has not read it properly.
All hypotheses about the origin of life are speculative, and it is true that there are several ideas about life's origins other than the clay one. However Morrison errs in assuming that clay in comets is essential to our hypothesis. The key point is that whatever mainstream mechanisms we consider -- RNA world, lipid world, clay world etc -- the conditions in which these are claimed to operate are replicated as well or better in liquid cometary interiors as on the early Earth. We used the "clay origin" hypothesis as an exemplar, not as a unique proposition. Thus we state: "Mechanisms discussed in the literature (such as the clay world of Cairns-Smith) work as well or better in liquid cometary interiors as they do in the harsh conditions of the early Earth, while in terms of total mass and surface area available, stability of environment, nutrient concentration, and the generation and protection of chirality, comets are overwhelmingly favoured". And again: "Similar considerations apply to other proposed prebiotic pathways, such as those of the PAH (Hazen 2005), lipid (Szostak et al 2001) or peptide (Carny & Gazit 2005) worlds".
So what are the requisite conditions for the various current hypotheses about life's origins, and are they met in cometary interiors? Liquid water, organics and in all probability a solid substrate --preferably a colloidal suspension which gives lots of surface area per gm -- are wanted. None of these is unreasonable for a cometary interior. We also need to generate and maintain chirality in biomolecules, easily done through the action of gamma-ray radiogenic photons inside a comet, hard to do on the surface of the Earth or in free space (Cataldo 2007).
Spitzer mid-infrared observations of the Deep Impact experiment on Tempel 1 (Lisse et al 2006) revealed the presence of carbonates and hydrated silicates, phyllosilicates (layered clay-type structures) which, in meteorites, are "usually attributed to formation by hydrothermal alteration inside a wet parent body" (Brownlee et al 2006). In addition, the mid-infrared emission (9--12 microns) from cometary material ejected by the impact is very well reproduced by a mixture of clays and organics, as we demonstrate in the paper. Perhaps even more to the point, there is a long-running literature showing that, in volatile-rich bodies such as comets, radiogenic heating would create liquid water interiors in moderately large comets. In bodies over 100 km across, this heat would be retained for tens of millions of years: this is down to simple thermodynamics, and one would have to invoke extraordinary circumstances to avoid the conclusion. We ourselves have shown that similar liquid water interiors must persist even in 10-20 km-sized comets for timescales of the order of a million years. There is as yet no evidence for hydrated minerals or aqueous processing in material recovered from Wild 2 (the Stardust mission), which probably just means that the comet was too small to retain much radiogenic heat, or that the initial "lakes" in the comet's interior have not yet been exposed. Comets come in all shapes and sizes!
Another essential component of our hypothesis is that comet systems should be reasonably common around other stars. It is this huge abundance of comets which gives us the bulk of the factor 10**24 -- roughly the volume of all the world's oceans compared to a cubic centimetre. Morrison objects that "no comets have been discovered yet around other stars". Detection of a comet around another star would be a remarkable feat indeed, but there is good circumstantial evidence for their existence and we give some relevant references. The formation of comets in protoplanetary nebulae is an intrinsic part of modern theories of planet formation, and there's an abundance of evidence for such dusty discs around other stars. It hardly seems likely that planetesimals -- including volatile-rich ones -- do not form in these discs (the formation environment of comets may even extend beyond this, to the denser regions of molecular clouds). Such huge probabilities give us also an extraordinary safety margin. Taking the clay world hypothesis (again as an exemplar), comets do not need to be "full of clay" for it to work: an extremely sparse colloidal suspension would still put the balance of probability overwhelmingly in favour of a cometary origin for life. Future work may indeed show that we are alone in having an Oort cloud etc, but until then we prefer not to adopt such a bizarrely pre-Copernican view.
Finally, Morrison quotes a press report on estimates by Bidle and colleagues that DNA would survive cosmic ray destruction for only a few hundred thousand years in space as "essentially ruling out interstellar pollination of life by comets". This was based on studies of Antarctic microbes as a function of age (the paper actually quotes a half-life of 1.1 million years). These claims are by no means secure: even in 8 million year-old samples Bidle et al (2007) find "....a metabolically active subset of the encased bacterial population or at least those capable of preferential protection of genomic DNA by an unknown mechanism" - essentially ruling out a simple half-life degradation argument. But in any event, interstellar panspermia mechanisms recently discussed (e.g. Napier 2004, 2007) require only that microorganisms survive such destruction for about 50,000 years. Survival times of a million years will do very nicely.
W.M. Napier
N.C.Wickramasinghe
J.T. Wickramasinghe
19 August 2006
Bidle, K.D. et al. 2007, Proc. Nat. Acad. Sci. 104, 13455
Brownlee, D. et al . 2006, Science 314, 1711
Cataldo, F. 2007, Internat. J. Astrobiol. 6,1
Lisse, C.M. et al. 2006, Science 313, 635
Napier, W.M. 2004, Mon. Not. R.astr. Soc. 321, 463
Napier 2007, Internat. J. Astrobiol. 6, 223-228
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