Friday, December 10, 2010

The History of Water and Thus, Life, In the Cosmos.

Pulsar Red, 1972

Victor Vasarely, Hungarian/French (1906-1997)

(The following diary has been cross posted at DailyKos with an amusing poll, but sans painting but avec an amusing poll. The link to the original is here: The History of Water and Thus, Life, In the Cosmos.)

By NNadir

I have not heard of a scientific discovery in recent years that quite excites the imagination as the announcement this week that scientists funded by NASA have discovered a bacteria that can substitute arsenic for phosphorous in the structure of its DNA. If true, it is one of the outstanding discoveries of the last decade and as exciting as the discovery of sulfur oxidizing bacteria and thermophilic organisms in deep sea volcanic vents.

Astrobiology is a fascinating field, one that, at least for me, has profound philosophical and well as scientific import, and brings to fore those exciting places where the two intersect.

Irrespecitve of the novel life forms we have discovered in the last half century on Earth I am still inclined to conjecture that liquid phase water, because of its remarkable properties, is still an essential precondition for life.

The paper from the primary scientific literature I will discuss in this diary is "Water and Astrobiology," published by Hawaiian scientists ...Chemie der Erde/Geochemistry 67 (2007) 253–282. The paper is not a research paper, but rather is a very interesting review article. The authors are Michael J. Mottla, Brian T. Glazera, Ralf I. Kaiser, Karen J. Meech, all at the University of Hawaii.

The authors begin the paper with a brief discussion of the origin of the elements, noting that in the Big Bang, the main element formed was hydrogen, with impurities of helium and lithium also being observed.

The authors, perhaps not being interested in Tesla electric cars that recently saved us all from climate change and simultaneously saved our primally important car CULTure - just kidding - don't discuss the fate of lithium in the universe, but it is nonetheless fun to note that lithium, like hydrogen is consumed in stars rather than created, although some lithium may be produced, like boron, in interstellar clouds in nuclear spallation reactions.

Anyway. The main point of the authors is that the third most common element in the universe today is oxygen, which is formed in stars as a by-product of the CNO cycle, that is a mechanism for fusion that is commonly found in many types of stars, particularly those responsible for supernovae, of which we are all clearly the ashes.

Before the evolution of oxygen, water was an unknown compound in the universe. Today, water is commonly distributed not only on earth, but widely in the universe as a whole. According to a paper cited by the authors, about 10% of interstellar matter, generally referred to as "dust," is water ice. Because interstellar space has a fairly constant, and sometimes intense radiation flux, unless it is shielded water can be quite unstable in deep space and often reacts with other molecules. The authors reference the website of the National Radio Astronomy Laboratory, writing that "over 130" molecules have been found in interstellar space, although as of this writing, the website actually lists
129 molecules found in interstellar and circumstellar space. Interestingly enough, glycoaldehyde, the simplest possible sugar is among these, and other protobiotic molecules such as acetic acid, a precursor of the simplest amino acid, glycine, have been observed. Molecular clouds also contain significant quantities of methanol, hydrogen cyanide, carbon monoxide, carbon dioxide, and ammonia, all of which can give rise to molecules of biological importance.

The fact that amino acids are not detected in interstellar clouds does not, in fact, imply that they are not there, but only that they are not dectectable by current technology. Amino acids are seen in some meteorites, most famously in the Murchinson meteorite which fell in Australia in 1969. Amino acids on earth have, with the exception of glycine, the property that they are asymmetric, or chiral. Chirality is the property people observe with their two hands, assuming one has two hands; their hands are mirror images of one another, but cannot be superimposed upon one another - placed on top of one another, the thumb and pinkie will be on opposite sides. All amino acids, again, except glycine, have this three dimensional property, but in living things only one of the two possible mirror images is observed, the "L" isomer which for every amino acid except cysteine is also designated as the "S" isomer. ("L" refers to a physical effect on certain kinds of light, and "S" refers to a set of formalistic rules designed to help chemists understand one another.) If both mirror images - called "enantiomers" - are present the mixture will be said to be "racemic." The origins of chirality is something of a mystery - just as the source of energy for the sun and other stars was a mystery until the mid twentieth century - since unless someone has something chiral to begin with in a chemical reactor, one will always get a mixture of both enatiomers, in other words, a racemic mixture, containing both L and D (S and R) isomers in exactly equal proportions.

In the famous Miller-Urey experiment, in which electrical discharges in a mixture of supposed prebiotic inorganic gases produced amino acids - leading to an number of theories on the origin of terrestrial life - all of the amino acids were racemic, since there were no sources of chirality in the reactor. Since life on earth is asymmetric, a fundemental question about the Miller-Urey remained. One would also expect that in outer space, where amino acids are not expected to be in a a chiral environment - such an environment is generally associated with living things - but in fact, in the Murchinson meteorite an excess of the S enantiomer (generally referred to as the L-isomer because of its effect on the rotation of plane polarized light is to rotate the light to the left) which is, in fact, the enantiomer that is observed in living things.

Initially, this lead many scientists to expect that the meteorite had been contaminated with amino acids found on earth. However, it was discovered that the isotopic signature of the nitrogen species in the meteorite, that the excess of the L-isomer, and in fact, all of the amino acids present, have a distribution of nitrogen isotopes that is more indicative of interstellar origins. A discussion of a sample of this work is found in the prestigious scientific journal Nature in a paper published in 1997. (NATURE VOL 389 ,18 SEPTEMBER 1997 pp 265-268.)

The authors of the 1997 Nature paper, M. H. Engel of the University of Oklahoma & S. A. Macko of the University of Virginia, write as follows:

Our report of non-racemic amino acids is in conflict with the
initial report of higher amino-acid D/L values for the Murchison meteorite10. However, we emphasize that our amino-acid distributions and relative abundances for all three stones are in general agreement with those initial and subsequent findings26. This would not be expected if contamination occurred. It is also important to recognize that the amino acids present in the Murchison meteorite stones may not reflect the distribution at the initial time of synthesis. Organic and inorganic reactions may have altered amino-acid distributions over a period of unknown duration when the meteorite parent body (from which Murchison was derived) probably retained an aqueous phase9. Given this caveat, it is possible that different stones may contain components that do not reflect identical alteration histories. But although previous authors have attributed an apparently slight L-excess to biological contamination10, we believe, as will be discussed below, that the stable-isotope values point to an alternative explanation. The moderate-to-extreme 15N enrichments observed for amino acids in the Murchison meteorite establish their indigenous nature and, as previously suggested, point to a possible interstellar source for their precursors if not for some of the compounds themselves. Given the current hypotheses that (1) an enantiomeric excess is likely to have been a precondition for the origin of life on Earth (for example, refs 1, 2) and (2) the Earth’s initial inventory of organic matter was derived from meteorite and comet bombardment6, the occurrence of an excess of the L-enantiomers in the Murchison meteorite provides the first evidence for a source of this chirality deemed essential for life’s origin. The origins of the excess in the L-enantiomers presently remains unclear. The excess may have resulted from the alteration of initially abiotic, racemic mixtures by a process such as preferential decomposition by exposure to circularly polarized light1. The extent to which such processes would have altered the stereochemistry of amino acids during the billion years preceding the first fossil evidence of life on Earth is unknown. But if amino-acid precursors (and perhaps some amino acids themselves) are probably interstellar9, this would certainly increase the time for exposure of organics to circularly polarized light before the formation of our Solar System.

In other words, the authors of the Nature paper would like to suggest that is possible that the materials from which life evolved may not have originated on Earth, but may have originated in space, and that without having been in space, may have been unsuitable for the evolution of life.

One possible source of intense radiation that may produce chiral radiation of great intensity, if I remember correctly - I'll check with my youngest son on this because his Junior High School Science project will have to do with Stellar Evolution, and, frankly, he knows more about the niceties of stellar nuclear physics than I do, as my interest in nuclear physics sort of stops at neutrons, and his, um, doesn't - is circularly polarized radiation from pulsars. Pulsars are the corpses of supernovae, and, as indicated earlier, we are the ashes of supernovae. The closest known pulsar, thought to be PSR J0437-4715, however is rather far away, some 400 light years away.

Is this pulsar close enough to be, um, "yo' momma?"

The radial velocity of the sun in the galaxy is thought to be on the order of 13 ms-1, which translates into 47,000 km/hr or using dumber units, 29,000 mph. (Ref: Nidever et al The Astrophysical Journal Supplement Series, 141:503–522, 2002 August) The earth is thought to be about 4.5 billion years old. I will spare you any "back of the envelope calculations" involving 244Pu/238U ratios concerning the time that may have passed between the accretion of the earth and the time of the last supernova to have provided elements for it, and limit myself to a "back of the envelope" calculation about how fast, relative to the sun, PSR J0437-4715 had to travel from the time the earth formed until the present day to be where it is. A light year is 9.4605284 × 1015 meters and a year contains 31,556,926 seconds. It follows that over a period of 4.5 billion years, that the pulsar would need to have traveled about 27 ms-1 to have traveled that far away from earth by now. This is on the same order of magnitude as the velocity of the sun and is thus reasonable, if in fact, pulsars pulse for 4.5 billion years. I'll ask my son. He might know.

But, um, I was talking about, um, water, wasn't I?

The authors of the Chemie der Erde paper have something interesting to say about the presence of glycoaldehyde in space, to wit:

The recent identification of HOCH2CHO in Sgr B2 is a significant milestone from an astrobiological viewpoint. Glycoaldehyde represents the first member of monosaccharide sugar – a hydroxyaldehyde diose – and denotes an important biomarker that can react to HOCH2CHO phosphates (Krishnamurthy et al., 1999) and complex sugars such as ribose. The latter is the building block of ribonucleic acid, which carries genetic information in living organisms. Carbon hydrates such as HOCH2CHO also play a role in vital chemical reactions (Weber, 1998).

(Sgr B2 refers to interstellar matter near a hot core source in the constellation Sagittarius.)

A recent idea in the scholarship on the origins of life is that of an "RNA world," an idea that, um, evolved with the discovery that RNA, just like proteins, could catalyze reactions, specifically reactions involving phosphate ester bonds that are associated with, well, um, the formation of RNA. Thomas Cech and Sydney Altman, working independently were awarded the Nobel Prize for this discovery in 1989. (Altman's catalytic RNA contained a protein segment bonded to the RNA, which is itself suggestive.)

Nucleotide bases have been found extraterrestrially, specifically in the Murchison meteorite, making the meteorite a kind of Rosetta stone of extraterrestrial formation of life. (See for instance, Earth and Planetary Science Letters 270 (2008) 130–136). These almost certainly arose from interstellar cyanides and formamides. The latter may arise from the hydration of hydrogen cyanide, a known constitutent of interstellar clouds and a known precursor to pyrimidines, although both guanine and adenine may be synthesized from hydrogen cyanide directly. (The reverse process, which is endothermic is the basis of the industrial Shawinigan Process, in which hydrogen cyanide is formed via the dehydration of formamide.) Thus the chemistry of water may be involved intimately with the formation of nucleotide bases in outer space.

Returning to the Chemie der Erde paper, the authors suggest that earth based laboratory studies suggest that much of the chemistry associated with formation of prebiotic complex molecules not in the gas phase, but on the surface of ice particles. In this case, water is is not limited to being a reactant, but also works as a surface on which chemistry can be performed. I have written many diaries in this space referring to the wonder fuel DME, dimethylether. This molecule is found in space, which is not to say that we could collect it to run diesel engines - may that be forbidden - but the interesting thing is that under simulated extraterrestrial conditions, this molecule is not formed from methanol in the gas phase, but is observed under slow warming of ice crystals containing methanol.

One could imagine - and I am merely speculating here - that water ice may orbit a large hot object, maybe even a pulsar having circularly polarized light, which warms slowly during its rotation around the neutron star. Over long periods, maybe measuring in the billions of years, sufficient chirality might arise to support the evolution of RNA and then, ultimately, life itself.

This does not mean to imply that life didn't arise on a planetary surface, the surface of Earth, or the surfaces of planets we know once had oceans and are neighbors, Venus and Mars - the case for the latter is becoming stronger all the time, even though it is by no means certain that it will ever be proved since it may not be true.

Still, it is true that the building blocks, even primitive building blocks are routinely distributed throughout the interstellar regions, near stellar regions and maybe even intergalactic regions. The earth, and hopefully many other planets may merely represent a patch of wet fertile soil in which life realize its most remarkable potential.

Be that as it may, the situation that has evolved on this planet whereby one species on the planet is placing all other species at risk, in the sense that even if they were once alive, Mars and Venus now seem to be dead, proving that planets, whole planets, can be killed. Even if that one species, us is merely creating a catastropic selective pressure on life - perhaps such selective pressure as to eliminate sentient life, the work described here seems to suggest that it is almost a certainty that life exists somewhere else.

Take solace wherever one can.

That said, the authors note that the Earth has most likely been destroyed several times before now and sentient life had very little to do with that outcome. They write:

The problem of the origin of H2O on Earth and its distribution and history has remained one of the most intractable problems in geochemistry, because it is inextricably bound to three equally difficult problems: The origin of the Earth; its chemical differentiation into core, mantle, and crust; and the heterogeneity of its
mantle (Martin et al., 2006). The pioneering calculations of Safranov (1969) and Wetherill (1985) on orbital evolution and the dynamics of planetary accretion first showed that planets such as Earth were probably assembled stepwise. Rather than growing by gradual accretion of small fragments to a much larger body, it is now thought that the final assembly of Earth and Venus took place catastrophically, by sequential collision of a few dozen Moon- to Mars-sized planetary embryos (Canup and Righter, 2000; Chambers, 2004) which themselves had assembled by runaway growth within only a million years after the Solar nebula began to condense (Yang et al., 2007), 4.567 billion years ago (Ga) (Amelin et al., 2002). Earth was assembled in this violent fashion over a period probably no longer than 30–50 Myr following the onset of nebular condensation, based on Hf-W isotopic evidence that Earth’s core had formed by this time (Schoenberg et al., 2002; Yin et al., 2002; Kleine et al., 2002; Jacobsen, 2005, Halliday and Kleine, 2006; Halliday, 2006). The final episode in core formation is believed to be the giant impact that formed the Moon, by which Earth gained the last 10% of its mass in collision with a Mars-sized body that has been named Theia (Canup and Asphaug, 2001; Canup, 2004; Kleine et al., 2005). This new scenario has important implications for the origin and history of H2O on Earth because (1) modeling indicates that embryos that collided to form Earth could have come from considerable radial distances within the inner Solar System and thus may have contained widely variable amounts of water and other volatiles

The earth has always been a fragile place.

To wit:

There is no doubt that Earth suffered at least one and probably several episodes of massive atmospheric loss early in its history. Earth’s present component of volatiles is therefore the small remnant of what was once present. The early atmosphere could have been eroded by intense UV radiation and winds during the million-year-long T- auri phase of the young Sun as well as by large impacts. UV radiation and winds would effect the lighter H atom most, but as the likely major constituent of an early Solar nebular atmosphere, produced in additional quantities by radiative splitting of H2O (Zahnle, 2006), H streaming into space would pull heavier atoms such as Ne and molecules such as N2 with it, in the thermally induced process known as hydrodynamic escape.

For the record, as I have noted elsewhere, because of N2O release because of the use of synthetic nitrogen fertilizers, the Montreal Protocols did not make the ozone depletion problem go away. UV radiation is believed to be a mechanism for stripping rocky planets near stars of their water, since it creates equilibrium quantities of hydrogen gas. Earth's gravity is too weak to hold hydrogen gas in its atmosphere, as can be shown by appeal to Maxwell Boltzman distribution calculations. It is possible that without the addition of oxygen to our atmosphere by early plants, our water would now be long gone. Nonetheless, I very much doubt that any one here will live long enough to observe much effect on the planetary inventory of water from UV radiation. It is far more likely that some car CULTure scheme will boil off our hydrogen way faster than that might have happened if we only destroyed the ozone layer.

If, however, hydrogen does begin to escape from the earth, we can count on Jim Inhofe types to blame it on the sun.

Blame it all on the sun, or blame it all on pulsar PSR J0437-4715. Were it not for pulsar PSR J0437-4715, there might be no one here to learn from Cenk Ugyar that Barack Obama is, um, what was the polite word he used, oh yes, "stupid."

With or without a pulsar, with or without a pulse, get a life.

For the record, I came across this paper not because of a particular interest in the origins of the light elements like oxygen in the universe - although I find the paper fascinating - but because of my interest in the accretion of the heaviest element found in the protoearth, 244Pu, plutonium-244, which is known to have been present in the early history of this planet. With a half-life of only 82 million years, all of the 244Pu originally on the earth (and in many meteorites) has now all decayed essentially to thorium-232 and its daughter elements.

Having opened the paper up and having read a little of it, I have felt compelled to look further into the history of the universe, of water, of life and of us. I've now collected and read lots of papers on this stuff. It's cool, especially because it diverts my attention from more contemporary, and more depressing issues.

A discussion of 244Pu is relevant to discussions of the earth's atmosphere, geology, and the hydrosphere on this planet, because natural spontaneous fission of 244Pu has left a signature in earth's atmosphere via the distribution of Xenon isotopes in the atmosphere. If we did not, in fact, know the nuclear properties of 244Pu, we wouldn't understand terrestrial rocks, or the extraterrestrial rocks we have collected in the form of meteorites and moon samples. The xenon signature in rocks and the atmosphere, and differences between the two forms can tell us a great deal about how the earth's atmosphere formed and how it disappeared and then came back again.

This diary is too long, and too much time has been wasted writing it.

A few more remarks.

This is an NNadir diary, and so a remark on the long half life of 244Pu and its relation to commercial nuclear energy is almost as tightly required as a poll question about lutefisk. Often you will hear people claim that nuclear energy is unacceptable because, using their words, not mine, "the waste lasts for millions of years." This rote response is illiterate in many ways not the least of which is to assume that coal wastes, for instance, are not eternal, because they do not decay, at least when one refers to coal wastes that are heavy metals. I note that the major chemical that corresponds to the most worrisome coal waste, carbon dioxide, has been present on Venus for billions of years, and it will not go away until Venus itself is swallowed by the sun.

Nuclear energy need not be perfect to be better than everything else. It only needs to be better than everything else, which it is.

It can be shown that under certain modes of handling, nuclear fuel can be used to reduce the overall radioactivity of the Earth, although I'm not entirely convinced that this would be a good thing.

It is, nonetheless, easy to accumulate all of the plutonium isotopes in spent nuclear fuel except 243,244Pu, 244Pu is the only isotope of plutonium that lasts for "millions of years,”and 243Pu lasts only a matter of hours before decaying into 243Am. Thus used nuclear fuel contains no appreciable plutonium isotopes that will last for "millions of years." Because the half-life of 243Pu is so short, and its fission cross section relatively large compared to its capture cross section, it doesn't hang around long enough to become 244Pu. Very tiny amounts are formed from the low branching ratio positron emission in the decay of 244Am (the majority decays by beta decay to 244Cm) formed via neutron capture by 243Am, but this is trivial, but truth be told, there is very little 243Am in the types of reactors operated today.

In a way, this is unfortunate because for certain reasons it would be cool to have macroscopic quantities, a few hundred of kg at least, of 244Pu, but its very unlikely that will ever happen.

I don't believe in dumping plutonium in any case. I regard plutonium, all of it, as a valuable resource. I similarly regard neptunium, amercium, curium, berkelium and californium as valuable resources. In fact, to steal a phrase, I regard these elements as the last, best hope of the human race.

In fact, in used nuclear fuel, the vast majority of radioactive substances do not persist for "millions of years" and overall, including the few that do, it is relatively easy to arrange things so that the overall radioactivity of all used fuel can be made to be lower than the overall radioactivity of natural uranium ores. Many isotopes in used nuclear fuel become non-radioactive within days, some within hours. Moreover, the radioactivity, and in most cases, the risk of radisotopes is inversely proportional to the half life of isotopes. In fact the use of nuclear energy is a way - within several centuries - of actually reducing the radioactivity of the planet as a whole. The radioactive uranium on this planet, almost in its entirely, formed in the very same stars that, as I have argued, are responsible for the existance of life itself, and indeed, the water it contains and on which it so clearly depends. Given that life evolved in the presence of radioactivity, it is reasonable to ask whether in fact life depends in subtle ways upon having radioactivity. Thus the potential risk that bothers me the most about nuclear energy is that it will reduce the overall activity, although the radioactivity of potassium and rubidium, a substantial portion of the planetary radioactivity will be totally unaffected by what happens to uranium and thorium.

Anyway, my point, which is as consistent as Cato's remarks on Carthage, although Cato was a pernicious representative of a barbarian degenerate culture, and I believe I only sound so:

Nuclear science, the science that is so widely despised by people who know nothing at all about it - just as evolutionary molecular biology is despised by people who know no evolutionary molecular biology - has proved to be essential to understand whence we came, and whence we may go, and who or what may come before or after us or who are what may even be with us now, albeit across a universe. Technology aside, these things are yet other reasons why nuclear science is beautiful and why this essential science deserves not censure, but rather deserves respect.

Have a nice evening.


Rick Maltese said...

Great Post.

Entertaining and quotable.

What's the significance of the arsenic able to sustain life. It's ironic but is there more?

Anonymous said...

I am a retired biology professor. I greatly enjoyed you essay. I formerly taught that stuff. You gave in depth coverage to the chemistry and biochemistry which support the seed from outer space theory. Your molecular biology was on target too.

I retired ten years ago and have made an indepth study of world energy issues No question nuclear fission is our hope for future generations.

I have enjoyed your diaries on Daily Kos. I missed them when you took a break. Glad that you are back.

John Tjostem


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