Naked Science Forum
Non Life Sciences => Chemistry => Topic started by: damocles on 13/09/2012 03:30:11
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In a thread in the Physics forum on the origins of life on Earth JP has written:
There are definitely some rough connections you can draw from the big bang to the formation of the elements for example, and from the formation of the elements to inorganic chemistry, then from inorganic chemistry to the formation of amino acids (I'm thinking of the Urey/Miller experiment here, but I know there's been other work on it), which would likely be a path to organic chemistry.
Are there still any specialist scientists (as opposed to speculative physicists) who still see the Urey/Miller experiment (and subsequent work in that tradition) as at all relevant?
Firstly, there is ample chemical and geological evidence that the atmosphere of the Earth at the time that life got started on Earth contained neither ammonia nor methane.
Some quotes from Wayne's scholarly monograph "Chemistry of Atmospheres", Chapter 9.4
Access to the detailed geological record for Earth constrains speculation about our ancient atmosphere (palaeoatmosphere) and its development ...
Strongly reducing atmospheres of CH4 and NH3 must have been very short-lived, even if they existed at all ... {NH3 is rapidly photolysed -- lifetime 2-3 yr, and subject to rainout -- lifetime 10 days; CH4 is subject to attack by OH radicals from water photolysis -- lifetime ~ 50 yr}...
No known chemical processes generate either CH4 or NH3 in the atmosphere, and hydrocarbons (including CH4) in geothermal emanations are ultimately of biological origin, according to carbon isotope measurements. ...
The rock record begins 3.8 Gyr ago with highly metamorphosed sediments at Isua in West Greenland. These rocks were formed in the presence of H2O and CO2, but without abundant CH4 ... (etc.)
Secondly, there is also considerable evidence that the earliest forms of life on Earth were thermophile microorganisms in colonies near undersea vents. It is in any case nearly impossible for life to have existed in the atmosphere in these early times because of the unattenuated solar ultraviolet radiation in the 200-300 nm wavelength region which would break C-N, C-C, and C-O bonds in polyatomic organic molecules randomly and rapidly. Lightning arcs are unlikely to occur under an electrolyte ocean.
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Are there still any specialist scientists (as opposed to speculative physicists) who still see the Urey/Miller experiment (and subsequent work in that tradition) as at all relevant?
You might want to write to these folks to let them know that they're giving play to irrelevant theories:
http://www.thenakedscientists.com/HTML/content/interviews/interview/1021/
I'd let them know, but as a speculative physicist, my opinion doesn't count!
:)
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In all seriousness, though, I am interested in hearing critiques of his ideas. It is well outside my field of study, so I don't read the journals or scholarly monographs in the field. I rely on what I gather through science journalism, and if the facts have been misrepresented regarding what's considered relevant to the field, I'd be interested to know.
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In the 1950s when the Urey/Miller experiments were actually conducted, there was a common belief among geochemists that the Earth's early atmosphere had been strongly reducing, with significant amounts of hydrogen, methane, and ammonia present. Both the evidence and the underlying theory for such a view were very shaky (How could you have a decent amount of ammonia in an atmosphere in contact with a huge body of liquid water?)
Warneck in "Chemistry of the Natural Atmosphere" (1988) pp 598-608 argues that the gas mix in early volcanic exhalations cannot have been very different to the present day mix. Volcanic emissions today provide carbon monoxide and hydrogen, but no non-biological methane. He also demonstrates that methane cannot be produced in significant quantity by chemical reaction between carbon monoxide and hydrogen in any ambient environment, although such a reaction can provide a major energy source for methanogenic microorganisms. In other words, life must precede methane, not methane precede life as Urey and Miller supposed. As an unbiassed (and admirable) scholar Warneck readily conceded that his argument does not absolutely refute Urey and Miller's assumptions and conclusions, but it does provide strong counter-indications.
There were two major important uncertainties in the 1960s and 1970s about the early (after ocean formation and before life) atmosphere:
1) Was the early atmosphere reducing or fairly neutral? There is now a strong consensus that it was fairly neutral; very mildly reducing because of the presence of trace amounts of H2 and CO, but certainly no CH4 nor NH3.
2) Was there ever much CO2 in the Earth's atmosphere? There is a consensus that there was not. Carbon dioxide was locked up in carbonate mineral deposits and dissolved oceanic bicarbonate soon after the oceans formed, and long before life emerged.
The consensus views on these two questions were greatly reinforced with the Cassini mission to Titan, which showed, unexpectedly, that the thick atmosphere of this satellite (previously thought to be largely made up of methane) was in fact 98.4% nitrogen, and only 1.4% methane, and most of the balance made up with hydrogen.
from the radio transcript linked above:
Jeff - Today many geochemists think that the atmosphere as a whole did not contain methane and hydrogen like Stanley used in his original experiment.
I think Jeff is being very disingenuous (or possibly unaware?) here. There is a strong consensus among geochemists, well supported by geological evidence, that the atmosphere as a whole contained almost no methane, and very little hydrogen.
Even if Jeff is right, his speculative system does not work because the exposed surface of the Earth is a lethal environment before the build up of atmospheric oxygen and an ozone layer. Early life, pre photosynthesis, can only have survived in the neighbourhood of ocean vents or, just possibly, in lava caves. And only at ocean vents could it have found the exploitable energy source for its continued existence. Early photosynthesizing life had another niche available -- ocean water at a depth between about 1 and 2 metres (depending on turbidity), where the lethal UV light would have been scattered away, but the vitally necessary bright visible light would have been available.
Jeff and his team had a valuable resource at their disposal -- the preserved results of an experiment that had provided inspiration for a generation of young scientists, and had had appeal for the general public as well. They needed and had specializations as organic chemists and as analytical chemists, and a modern GCMS system can detect and identify organic compounds in femtomolar quantities. I do not know their geochemical qualifications -- presumably they have an expert geochemist on the research team. Their project is one that is almost guaranteed to attract public interest and research funding. But as far as any insight into the real origins of Earthly life, it is a very very long shot.
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Perhaps we need to look further afield for the conditions...
The claim to fame of the Urey & Miller experiment was the production of amino acids.
But some of these are also found in carbonaceous chondrite meteorites http://en.wikipedia.org/wiki/Murchison_meteorite#Organic_matter
Unfortunately, these meteorites are not very robust, and most probably burn up in the atmosphere, so it's hard to find them...
It's still a large step from amino acids to anything we would recognise as life... (speaking as a speculative non-specialist...)
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-- and glycine does not survive above about 400°C, and it certainly does not survive in any coherent form in vacuum UV radiation
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I had heard Jeff discussing his work on both TNS and another science podcast around the same time (the anniversary of the original experiment, I believe) and it's nice to hear the criticisms of these experiments, which sound very much based in science.
Thanks for sharing the info, Damocles!