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Space Forum / Astronomy / July 2008Optical activity and the detection of extraterrestrial life
One of the main characteristics of life is the presence of stereo isomers. Optical activity in other words. On Earth organic (associated with life rather than simply being carbon compounds) compounds are dextro rotary. A few fungi, just to be different are laevo rotary. This means that a sensitive test for the presence of life is the presence of optical activity. No optical activity, no life would seem to be a fair dictum. It is proposed to have a probe which will go though the ice of Europa into the liquid ocean underneath. My point is that there may be other reasons for drilling though the ice sheet, but the presence of life can be definitively ascertained by just looking at the ice - optical activity. The Europan ice sheet is only 10 million years old, so there will be stereo isomers in the ice. Let us look at something else, laser technology. The gravitational wave detector LISA has spacecraft up to 5 million km apart. This means that lasers have to be coherent over that distance. A not inconsiderable achievement. Suppose we put tube of water (melted and vacuum evaporated Europan/Martian ice) in the path of a high coherence stability laser. We start off with a plane of polarization in one direction, if there is optical activity in the water this plane of polarization will rotate. A stable/randomly moving plane of polarization will therefore rule out life, while rotation will confirm life. There is one thing which has always interested me. The weak nuclear force is asymmetric http://en.wikipedia.org/wiki/Kaon Note that the lack of symmetry is deduced by the decay of the antiparticle. Relativity is assumed to be true. Has the symmetry of life got a preferred direction. Before we say "this is very weak, chemistry can only differ by one part in 10^12" we should look at the fact that equilibrium is unstable. One one direction is preferred all life forms will adopt it. It pays to be in the "big battalions". Is a minute difference capable of tipping the scales one way? - Ian Parker :One of the main characteristics of life is the presence of stereo :isomers. Optical activity in other words. On Earth organic (associated [quoted text clipped - 4 lines] :presence of optical activity. No optical activity, no life would seem :to be a fair dictum. You can't know that. This is the logical fallacy of trying to prove a negative. It works one way (optical activity probably indicates the presence of life) but does not work the other (lack of optical activity indicates the absence of life). :It is proposed to have a probe which will go :though the ice of Europa into the liquid ocean underneath. My point is :that there may be other reasons for drilling though the ice sheet, but :the presence of life can be definitively ascertained by just looking :at the ice - optical activity. The Europan ice sheet is only 10 :million years old, so there will be stereo isomers in the ice. Water doesn't have any stereoisomers.  Signature"Ignorance is preferable to error, and he is less remote from the truth who believes nothing than he who believes what is wrong." -- Thomas Jefferson > :One of the main characteristics of life is the presence of stereo > :isomers. Optical activity in other words. On Earth organic (associated [quoted text clipped - 9 lines] > presence of life) but does not work the other (lack of optical > activity indicates the absence of life). If there is life in some Europan vent there will be vents all over the moon, and life all over. Lifge MUST have optical activity. Life consists of enzymes all of which are stereo isomers. Anything cataysed by an enzyme must therefore be optically active. Question is about concentration. The experiment I am proposing is of great sensitivity. As I sauid, there will either be life all over the moon, or no life anywhere on it. A high stability laser + vacuum distillation (see below) is enough to convince me there is no life. > :It is proposed to have a probe which will go > :though the ice of Europa into the liquid ocean underneath. My point is [quoted text clipped - 4 lines] > > Water doesn't have any stereoisomers. Water is not pure. What you would do would be to take Europan ice, melt it and vacuum distil it. Europa BTW does not have an atmosphere so you simple take the waste heat from isotope generators and leave the water container open. - Ian Parker > One of the main characteristics of life is the presence of stereo > isomers. Make that "life ON EARTH". Life elsewhere may not be stereoselective. > Optical activity in other words. On Earth organic (associated > with life rather than simply being carbon compounds) compounds are > dextro rotary. A few fungi, just to be different are laevo rotary. Fungi are not "organic compounds." They are organisms, and much of their composition includes a variety of organic compounds. The levo-rotary toxin beta-methylamino-L-alanine is found in cycad nuts and is associated with Lytico-Bodig Disease - a neurodegenerative disorder mimicing ALD: http://www.itg.be/itg/DistanceLearning/LectureNotesVandenEndenE/47_Medical_probl ems_caused_by_plantsp12.htm The apparent bias in optical activity may be accidental or a result of the stochastic processes of natural selection. > This means that a sensitive test for the presence of life is the > presence of optical activity. This does not follow. Life is not necessarily stereoselective, nor is optical activity necessarily produced only by living metabolisms. > No optical activity, no life would seem > to be a fair dictum. Wrong. Tom Davidson Richmond, VA > > One of the main characteristics of life is the presence of stereo > > isomers. [quoted text clipped - 26 lines] > > Wrong. Optical activity is an insensitive probe. What kind of rotations do we see for terrestrial biological materials over a decimeter length of fractional molar solution, sugar or protein amino acid? Commonly a few tens of degrees rotation at most. Now shrink that down to a centimeter cell at micromolar concentration. Yer gonna be screwed by things like Faraday rotation and strain polarization. A meter of capillary cell has its own problems. You could dump in some Cu(II) or whatever to coordinatively boost the epsilon and shift it near your monochromatic (diode) light source wavelength to boost rotation. All that's gonna discover is publications on systematic and absolute error sourcing. Go to the Atacama desert, the Empty Quarter, Sahara desert, Antarctic dry deserts (wear a bunny suit, booties, and face mask), set down your wiz-bang life detector, and walk away downwind. If it doesn't find any life it is crap. Uncle Al has it on good authority that India is plastered with mounds of open air human excrement (and literally plastered with cow sh.t - dried on every available wall for fuel and nachos). Being a few miles away shouldn't make a difference. SignatureUncle Al http://www.mazepath.com/uncleal/ (Toxic URL! Unsafe for children and most mammals) http://www.mazepath.com/uncleal/lajos.htm#a2 > > One of the main characteristics of life is the presence of stereo > > isomers. [quoted text clipped - 23 lines] > > No optical activity, no life would seem > > to be a fair dictum. I will accept arguments based on sensitivity. However your argument seems to me to follow the SF dictum of trying to make life as unlike ours as possible. I have read a novel about nuclear based life on a pulsar. Biochemistry is based on enzyme catalysed reactions. These are optically active for the following reason. An enzyme is chiral, it will assume a chirality for its feedstuffs. Let us suppose life not to be chiral. It would then require 2 distinct sets of enzymes. Clearly an organism with one set of enzymes will be fitter than one with 2. On sensitivity, could we concentrate particular molecules, say sugars, on Europa. Could we distill and then crystallize? - Ian Parker > > > One of the main characteristics of life is the presence of stereo > > > isomers. [quoted text clipped - 27 lines] > seems to me to follow the SF dictum of trying to make life as unlike > ours as possible. Not at all. I am merely pointing out that stereoselectivity is neither a necessary nor sufficient condition for life, and thus provides a poor diagnostic test. > I have read a novel about nuclear based life on a > pulsar. For every novel in which life is based on exotic chemistry, there is a fistful in which intelligent species from disparate planets are able to interbreed, producing hybrids like Spock or Worf. But that is totally irrelevant, as we are not discussing science fiction but xenobiology. I recall that one of the arguments for including video cameras on Mars lander missions went to the effect that "our chemical analyzers are only capable of identifying biochemicals for life as we already know it, and expect it to be, but if a silicon-based animal marches across the field of view, we will be able to recognize it as a life form regardless of its chemistry." (IIRC) > Biochemistry is based on enzyme catalysed reactions. ...as we currently know it... > These are > optically active for the following reason. An enzyme is chiral, it > will assume a chirality for its feedstuffs. Let us suppose life not to > be chiral. It would then require 2 distinct sets of enzymes. Clearly > an organism with one set of enzymes will be fitter than one with 2. That does not follow. It *does* follow that an organism with a racemic mixture of optically active components (and therefore possessing a net optical activity of 0) is more adaptable and able to exploit either enantiomer in its foodstuffs, should they be optically active, and ALL of it should it also be racemic. > On sensitivity, could we concentrate particular molecules, say sugars, > on Europa. Could we distill and then crystallize? Great sensitivity (picogram quantities) in analysis can be attained with a combined gas-chromatograph-mass spectrometer (GC-MS): http://www.ssi.shimadzu.com/products/product.cfm?product=qp2010plus If the column substrate provided in the GC is itself optically active, then optically active analytes may be resolved and the isomers individually quantified. It has been a standard technique for years. For the highest available specificity in identifying crap analytes ("God only knows what's in it!" type stuff) a GC-MS-FTIR combination, in which the effluent from the GC column is split into two portions to be simultaneously characterized by mass spectroscopy and Fourier Transform Infra-Red spectrometry is powerful and versatile, but somewhat difficult to miniaturize for interplanetary missions. A computer with a few terabytes for the GC and MS and FTIR spectral libraries od known compounds for comparison would be very helpful as well. > - Ian Parker Tom Davidson Richmond, VA > > > > One of the main characteristics of life is the presence of stereo > > > > isomers. [quoted text clipped - 85 lines] > libraries od known compounds for comparison would be very helpful as > well. It is necessary since raecemic life would be unfit (2 sets of enzymes). It is sufficient since any non catalytic process MUST produce the racaemic form. The important evolutionary question is not "is racaemic as fit?" Clearly it is not. The question is how much of a bias is beeded to favor one form over another. Is the weak nuclear force sufficient. Clearly evolution presents us with unstable equilibrium. - Ian Parker > > > > > One of the main characteristics of life is the presence of stereo > > > > > isomers. [quoted text clipped - 88 lines] > It is necessary since raecemic life would be unfit (2 sets of > enzymes). "raecemic life would be unfit" is conclusion for which experimental support would be needed. A single counterexample would be sufficient to disprove it. > It is sufficient since any non catalytic process MUST > produce the racaemic form. "any non catalytic process MUST produce the racaemic form" is flat wrong, since there are ample counterexamples already in existence. I myself have produced separate crystals of D- and L-tartrate salts from a single solution of the racemic tartrate (1970). The challenge is to get both isomers into the same crystal - they just don't want to go. > The important evolutionary question is not "is racaemic as fit?" > Clearly it is not. That is a value judgment more properly made by the forces of natural selection, which will depend heavily upon the details of the immediate environment. > The question is how much of a bias is beeded to > favor one form over another. Very little bias is needed. Minor stochastic factors could be sufficient. A crystal can become a substrate for a catalyzed synthesis of another organic compound. If an organic compound has enantiomers, the sysnthesis of only one can be heavily favored by a catalytic substrate which itself has an enantiomeric bias. An optically active crystal thus encourages compatible optical activity with a positive feedback. > Is the weak nuclear force sufficient. Generally no. The weak force does not have the range to make its presence felt even in an adjacent atom. http://en.wikipedia.org/wiki/Weak_nuclear_force "...limits the range of the weak interaction to 10-18 meters, about 1000 times smaller than the diameter of an atomic nucleus." Longer-range forces (electromagnetic, gravitational) are required for interactions between atoms. Optical activity is an atomic/molecular scale phenomenon. The smallest optically active chemical which I can imagine is deuterated HCFC 31 (chlorofluoromethane) - CHDFCl, and that assumes isotope effects for H/D replacement are considerable. > Clearly evolution presents us with unstable equilibrium. How one can claim "equilibrium" for a system as complex, chaotic, and non-repeating as the biosphere is totally beyond my ken. At best we are witnessing a system in constant flux (not in thermodynamic equilibrium) as it strives to conform to general physical laws with massive energy inputs and localized decreases in entropy, as it self- organizes and develops the capability to willfully reconfigure itself. Tom Davidson Richmond, VA > > It is necessary since raecemic life would be unfit (2 sets of > > enzymes). [quoted text clipped - 18 lines] > selection, which will depend heavily upon the details of the immediate > environment. It is NOT a value judgement. It is based on the need to have 2 sets of enzymes. > > The question is how much of a bias is beeded to > > favor one form over another. [quoted text clipped - 6 lines] > optically active crystal thus encourages compatible optical activity > with a positive feedback. Exacltly. You can only have one type of crystal. Crystallization takes place because molecules join those already there. This is what I am getting at when I say that life must have essenrially one set of enzymes. Basically if an enzyme is symmetrical it will catalyse both equally. Enzymes are complex molecules and will only calalyse one form. > > Is the weak nuclear force sufficient. > [quoted text clipped - 10 lines] > deuterated HCFC 31 (chlorofluoromethane) - CHDFCl, and that assumes > isotope effects for H/D replacement are considerable. NO, you don't in fact need range for one part in 10^12. The electrons come into contact with each other and the nucleus as virtual particles. Question is is t enough to create an inbalalance. Or are laevo and dextro rotary life of equal probability, the decision being made stocastically? If this is the case the two forms will exist in equal numbers in the Universe. The symmetry of Eurpan life (if it exists) is therefore of interest. > > Clearly evolution presents us with unstable equilibrium. > [quoted text clipped - 4 lines] > massive energy inputs and localized decreases in entropy, as it self- > organizes and develops the capability to willfully reconfigure itself. Equilibrium simply means that both forms are equally fit. As soon as you get more of one than the other, the big battalions are fitter. This is what I mean by instability. Chaos, in the mathematical sense, occurs when one or more eigenvalues is in unstable equilibrium. - Ian Parker > > > It is necessary since raecemic life would be unfit (2 sets of > > > enzymes). [quoted text clipped - 21 lines] > It is NOT a value judgement. It is based on the need to have 2 sets of > enzymes. In what Holy Writ or upon which mountain is such a Truth recorded by Divine Hand? Only in your preconceptions are enzymes required to be optically active. They are biologically created catalysts, not more and no less. Only because the only enzymes you already know of were created by optically active biochemistry do you see that as a necessity. > > > The question is how much of a bias is beeded to > > > favor one form over another. [quoted text clipped - 11 lines] > getting at when I say that life must have essenrially one set of > enzymes. Crystallization is not an issue with life processes, except where it must be avoided. Even enzyme-substrate interactions involve interactions between DIFFERENT molecules. Crystallization required interactions of indistinguishable molecules. The indistinguishability is an important consideration in the entropy of a process. Life is basically a process that reduces entropy locally (within the organism) through the absorbtion of energy. Crystallization releases energy. > Basically if an enzyme is symmetrical it will catalyse both > equally. Yes. > Enzymes are complex molecules and will only calalyse one > form. True only for the enzymes with which we already have experience. There is no a priori requirement for optical activity in enzymes or for optical activity in the substrates upon which the enzymes act. > > > Is the weak nuclear force sufficient. > [quoted text clipped - 14 lines] > come into contact with each other and the nucleus as virtual > particles. BULLSHIT ALERT! Read up on the Pauli exclusion principle, the Schroedinger Wave equation, and the definition of the word "virtual." The electron is not a "particle", virtual or not. It is an electron. It is convenient for physicists to draw analogies between the behaviour of the electron and the behaviours of mathematical abstractions such as "waves" and "particles." The interactions between the electrons and the nucleus, and among the electrons themselves, are purely electromagnetic. The "weak interaction" is a property of nucleons and is too short-ranged for a nucleon to affect anything else other than nucleons "adjacent" to it - i.e. a select few nucleons with which it shares the nucleus. > Question is is t enough to create an inbalalance. The answer is no. >Or are > laevo and dextro rotary life of equal probability, the decision being > made stocastically? Most likely yes. > If this is the case the two forms will exist in > equal numbers in the Universe. Globally, yes. There are purely inorganic materials in which optical activity occurs naturally. The most common is probably quartz. http://hyperphysics.phy-astr.gsu.edu/hbase/phyopt/optact.html > The symmetry of Eurpan life (if it > exists) is therefore of interest. > > > Clearly evolution presents us with unstable equilibrium. > [quoted text clipped - 6 lines] > > Equilibrium simply means that both forms are equally fit. When you are discussing a topic (i.e. chemistry) in which certain terms (such as "equilibrium") are assigned explicit and unambiguous definitions for the sake of clear communication, you are NOT free to reassign those terms to new definitions. http://www.onelook.com/?w=equilibrium&ls=a "Fit"-ness applies to organisms, not to molecules. > As soon as > you get more of one than the other, the big battalions are fitter. No. The larger set is more abundant, and therefore likely to be more readily exploited as a resource. > This is what I mean by instability. Chaos, in the mathematical sense, > occurs when one or more eigenvalues is in unstable equilibrium. Again with the re-defining terms... Keep this up and technically competent people who know the real meanings will shun you as a crank. http://mathworld.wolfram.com/Eigenvalue.html http://mathworld.wolfram.com/Chaos.html > - Ian Parker Tom Davidson Richmond, VA > > > > It is necessary since raecemic life would be unfit (2 sets of > > > > enzymes). [quoted text clipped - 148 lines] > Again with the re-defining terms... Keep this up and technically > competent people who know the real meanings will shun you as a crank.http://mathworld.wolfram.com/Eigenvalue.htmlhttp://mathworld.wolfram.com/Chaos.html If you have a simple calalyst like Platinum you will not get optical activity. If you have a complex substance the odds are that it will be asymmetric. Further enzymes catalyse by means of a lock and key principle. This in itself will imply optical activity. - Ian Parker > > > > > It is necessary since raecemic life would be unfit (2 sets of > > > > > enzymes). [quoted text clipped - 152 lines] > activity. If you have a complex substance the odds are that it will be > asymmetric. Coordination numbers of 4 or larger seem to be the key, although optically active polymers with coordination numbers on individual atoms no larger than 3 are theoretically possible. > Further enzymes catalyse by means of a lock and key > principle. This in itself will imply optical activity. You are still looking at only one case (life on Earth) and trying to generalize from that to all possible life. > - Ian Parker Optical activity does lead to further optical activity. The origin of optical activity is connected to stochastic processes, and there has been no laboratory demonstration of which I am aware that indicates the inevitability of optical activity in the synthesis of organic compounds in the absence of optically active ingredients or optically active catalysts or reaction substrates. The origin of optical activity in terrestrial life is the subject of considerable current investigation, and until a mechanism is identified and demonstrated in the laboratory (i.e. empirically validated) that satisfies the Law of Natural Selection, it is premature to declare that optical activity is a *necessary* consequence of biochemical evolution. It seems it did happen at least once, but our sample population is rather small at the moment (one planet - statistically of zero significance). We cannot tell at the present whether that one case was accidental or inevitable. Tom Davidson Richmond, VA > > > > > > It is necessary since raecemic life would be unfit (2 sets of > > > > > > enzymes). [quoted text clipped - 171 lines] > compounds in the absence of optically active ingredients or optically > active catalysts or reaction substrates. If you do a synthesis with ingredients, including catalysts that are not optically active, the product will not be optically active. In the same way a synthesis with optical active components implies optical activity in the product. An enzyme by its nature can only produce one stereoisomer. Let us put this another way. If life were perfectly symmetrical I would take it as evidence for Intelligent Design. You can do a simple experiment on the computer. Take sqares and append another square to an edge selected by a pseudorandom process. If you came up with something symmetrical it would be a coincidence. In fact in pattern recognition you look at perfect symmetry as the hallmark of an artificial system. Hence my remark on ID. As I think we both agree, asymmetry leads to asymmetry. > The origin of optical activity in terrestrial life is the subject of > considerable current investigation, and until a mechanism is > identified and demonstrated in the laboratory (i.e. empirically > validated) that satisfies the Law of Natural Selection, it is > premature to declare that optical activity is a *necessary* > consequence of biochemical evolution. It is Natural Selection in a caatastrophic environment. catastrophe being defined in the mathematical sense. Optical activity EVOLVES one way, basically because the big battalions are fit. It pays to have the symmetry of other life. This is so that you can gather energy. Laevo rotary fungi arose to make themselves unappetising, but in doing so they pay a price. > It seems it did happen at least once, but our sample population is > rather small at the moment (one planet - statistically of zero > significance). We cannot tell at the present whether that one case > was accidental or inevitable. It is not just based on one sample. Dextro rotation is indeed one sample. Specualions about weak nuclear forces are again one sample. The need for optical activity - left or right, is based on mathematical arguments. - Ian Parker [snip] > If you have a simple calalyst like Platinum you will not get optical > activity. If you have a complex substance the odds are that it will be > asymmetric. Further enzymes catalyse by means of a lock and key > principle. This in itself will imply optical activity. "La Coupe du Roi." Four cuts and an apple (i.e., solid sphere) is severed into two exactly congruent left handed *or* right handed pieces. By extension a solid sphere can be losslessly cut into an arbitarily large number of exactly congruent pieces all with identical handness. Summing identical chirality can thus trivially exactly cancel. A solid sphere is point group D_[(infinity)h] with a point of inversion. A solid sphere is not chiral. Racemic life would be no big whoop. A suite of amino acid and sugar racemases would do it. SignatureUncle Al http://www.mazepath.com/uncleal/ (Toxic URL! Unsafe for children and most mammals) http://www.mazepath.com/uncleal/lajos.htm#a2 > I recall that one of the arguments for including video cameras on Mars > lander missions went to the effect that "our chemical analyzers are > only capable of identifying biochemicals for life as we already know > it, and expect it to be, but if a silicon-based animal marches across > the field of view, we will be able to recognize it as a life form > regardless of its chemistry." (IIRC) I don't think it was a serious argument. Carl Sagan, in one of a huge number of interviews about the Viking landers, made a comment to the effect that the cameras could detect a "silicon-based giraffe" if such creatures were present. It seemed to me a humorous way of making the general point that a camera is a good instrument for exploring unknown worlds. (Remember, these were the first cameras ever on Mars' surface.) I'd expect the scientific justification for a camera involved areology and weather, not life detection. SignatureSteve Willner Phone 617-495-7123 swillner@cfa.harvard.edu Cambridge, MA 02138 USA (Please email your reply if you want to be sure I see it; include a valid Reply-To address to receive an acknowledgement. Commercial email may be sent to your ISP.) > In article <fe73c6b5-d8f1-420c-8cfc-6812f542d...@27g2000hsf.googlegroups.com>, > [quoted text clipped - 15 lines] > I'd expect the scientific justification for a camera involved > areology and weather, not life detection. In fact there IS such a camera on Phoenix. It is a laser which gives a full 3D dust map. - Ian Parker > One of the main characteristics of life is the presence of stereo > isomers. Optical activity in other words. On Earth organic (associated [quoted text clipped - 35 lines] > > - Ian Parker Several researchers have looked at optical activity and associated properties as an indicator of life. Another optical property that hinge on the asymmetry of the molecule (ie.e, chirality) are circular dichroism. In principle, either optical activity or circular dichroism could indicate life. The main problem with this proposal is technical. Optical activity and circular dichroism are incredibly difficult to detect outside the laboratory. The chirality effects are small, and the background created by nonbiological particles is large. As an rough example, if unpolarized light passes through a chiral molecule with circular dichroism, so that 10^6 photons pass through it, maybe there may be 1 photon more of left handed polarized light than right handed polarized light. And remember your biological molecules are a small part of the environment, maybe one in a trillion molecules in a desert surface is biological. Never the less, there are other processes that can create a larger imbalance without chiral molecules. Light scattered off of dust particles gets polarized one way toward the front, and another way in the back. Light reflecting off a dielectric surface is palane polarized in the plane if the normal and incident ray. However, it gets elliptically polarized on each side of that plane. The polarization at twilight is elliptically polarized differently from the polarization at dawn, mostly due to the scattering of nonbiological molecules. These effects are on the order of 1000 times stronger per molecule than the effects of a chiral molecule. Also remember that the microrganism has a shape which may be chiral, independent of the molecule. Quartz sand grains have a birefringence, an optical activity, and a circular dichroism too. They also come in random, usually chiral shapes. Their effects average away over millions of sand grains, but that leaves a big background of nonbiological signal. This is the problem of physicists and organic chemists. From the physicists perspective, look up Stokes parameters. Some of the Stokes parameters contain optical activity and circular dichroism in them. Look up "limits of detection", "optical activity", and "circular dichroism". You will find that these measurements are extremely difficult in the field. In the laboratory, with concentrated and processed organic chemicals, the measurement of chirality using optics is much easier. Optical activity and circular dichroism are very useful tools for certain chemical analysis. Optical activity is commonly used to measure the sugar content. However, these are in solutions where the sugar is concentrated. Unless Mars is covered in sugar cane, the measurement of optical activity will be extremely difficult. And even then, I there is a good possibility of false alarm. I briefly looked into the problem myself, in the course of my research. I am not saying that someone else couldn't make a huge background in the problem. However, I found the detection of optical activity to be very difficult and not a sensitive method of detecting bacteria. I will not discourage anyone else. In fact, if you have a good idea go for it. However, this is a well-worn path where many have gotten lost. |:-( - Hide quoted text - - Show quoted text - > One of the main characteristics of life is the presence of stereo > isomers. Optical activity in other words. On Earth organic (associated > with life rather than simply being carbon compounds) compounds are > dextro rotary. A few fungi, just to be different are laevo rotary. > This means that a sensitive test for the presence of life is the > presence of optical activity. No optical activity, no life would seem [quoted text clipped - 4 lines] > at the ice - optical activity. The Europan ice sheet is only 10 > million years old, so there will be stereo isomers in the ice. > Let us look at something else, laser technology. The gravitational > wave detector LISA has spacecraft up to 5 million km apart. This means [quoted text clipped - 6 lines] > polarization will therefore rule out life, while rotation will confirm > life. > There is one thing which has always interested me. The weak nuclear > force is asymmetric > http://en.wikipedia.org/wiki/Kaon > Note that the lack of symmetry is deduced by the decay of the > antiparticle. Relativity is assumed to be true. Has the symmetry of [quoted text clipped - 3 lines] > life forms will adopt it. It pays to be in the "big battalions". Is a > minute difference capable of tipping the scales one way? > - Ian Parker Several researchers have looked at optical activity and associated properties as an indicator of life. Another optical property that hinge on the asymmetry of the molecule (ie.e, chirality) are circular dichroism. In principle, either optical activity or circular dichroism could indicate life. The main problem with this proposal is technical. Optical activity and circular dichroism are incredibly difficult to detect outside the laboratory. The chirality effects are small, and the background created by nonbiological particles is large. As an rough example, if unpolarized light passes through a chiral molecule with circular dichroism, so that 10^6 photons pass through it, maybe there may be 1 photon more of left handed polarized light than right handed polarized light. And remember your biological molecules are a small part of the environment, maybe one in a trillion molecules in a desert surface is biological. Never the less, there are other processes that can create a larger imbalance without chiral molecules. Light scattered off of dust particles gets polarized one way toward the front, and another way in the back. Light reflecting off a dielectric surface is palane polarized in the plane if the normal and incident ray. However, it gets elliptically polarized on each side of that plane. The polarization at twilight is elliptically polarized differently from the polarization at dawn, mostly due to the scattering of nonbiological molecules. These effects are on the order of 1000 times stronger per molecule than the effects of a chiral molecule. Also remember that the microrganism has a shape which may be chiral, independent of the molecule. Quartz sand grains have a birefringence, an optical activity, and a circular dichroism too. They also come in random, usually chiral shapes. Their effects average away over millions of sand grains, but that leaves a big background of nonbiological signal. This is the problem of physicists and organic chemists. From the physicists perspective, look up Stokes parameters. Some of the Stokes parameters contain optical activity and circular dichroism in them. Look up "limits of detection", "optical activity", and "circular dichroism". You will find that these measurements are extremely difficult in the field. In the laboratory, with concentrated and processed organic chemicals, the measurement of chirality using optics is much easier. Optical activity and circular dichroism are very useful tools for certain chemical analysis. Optical activity is commonly used to measure the sugar content. However, these are in solutions where the sugar is concentrated. Unless Mars is covered in sugar cane, the measurement of optical activity will be extremely difficult. And even then, I there is a good possibility of false alarm. I briefly looked into the problem myself, in the course of my research. I am not saying that someone else couldn't make huge progress in the problem. However, I found the detection of optical activity to be very difficult and not a sensitive method of detecting bacteria. I will not discourage anyone else. In fact, if you have a good idea go for it. However, this is a well-worn path where many have gotten lost. |:-( > Let us look at something else, laser technology. The gravitational > wave detector LISA has spacecraft up to 5 million km apart. This means > that lasers have to be coherent over that distance. No. Even if LISA worked like a classical Michelson interferometer (which it doesn't), the laser would only need to be coherent over the *difference* in the armlength. This is in LISA still significant, (up to 50 000 km), but quite smaller than the armlength. SignatureSpace - The final frontier |
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