Some of these stars, the so-called low-mass stars, fizzled out, eventually burning through their fuel and collapsing into the gentle years of stellar retirement as white dwarfs. But some of the stars, more massive and within which more-elaborate onion layers of elements reside, collapsed in a violent conflagration, the gravitational forces catastrophically overwhelming the pressures of gas and thermal energy pushing outward, causing the convulsive shedding of material. In these massive explosions, or supernovas, new and heavier elements beyond iron in the periodic table were forged and strewn across the universe.
From astronomers’ early work in the nineteenth and twentieth centuries, there emerged a better appreciation of where the necessary elements for life came from. Many light elements, including carbon, were mainly fashioned within the cores of low- and high-mass stars, while many other heavy elements needed by some types of life—molybdenum and vanadium, for example—were synthesized within supernovas.
This understanding of how elements were formed was a startling advance in our perspective of how life fits within the cosmic context. With this astronomical insight, we could now know the very origins of the elements of life and thus link the physics of the universe to the atomic structure of living things. Within this growing clarity, there was also something strangely disturbing and sobering about the truly cosmic origins of our existence. It has become something of a hackneyed observation to say that we are all stardust, but perhaps that sense of triteness merely reflects how much we take for granted our modern grasp of cosmology and astronomy. The ancients would have found such a statement baffling and abstruse.
This appreciation of our understanding of the link between life and the vacuum beyond might have been the first phase of a truly astrobiological comprehension of our origins. It was within the second half of the twentieth century, running through to the modern day, that another phase opened up and allowed us to grasp the ubiquity not so much of the elements for life in general, but specifically of the universal nature of carbon chemistry.
We can turn our telescopes toward that black void, but instead of viewing it in the range of the spectrum you and I are familiar with (the visible region), we can view it in the region just beyond the red—the infrared—using sensors that can turn that data into something you and I can see. If we study the infrared data, we observe not blackness but the swirls and endless beautiful puffs and eddies of gas, giant clouds that billow with grand vastness across the night sky. Where there was blackness, we now observe material.
Much of this now-visible material is diffuse interstellar clouds. They are so named because in their interiors, the gas concentrations can reach down to about 108 molecules or ions per cubic meter. Now this might sound like quite a lot, but all around you right now, there are about 2.5 × 1025 molecules of gases per cubic meter that make up the air you breathe. The diffuse interstellar clouds contain less material than do the vacuums we can create in laboratories on Earth. Nevertheless, within these clouds, there is still enough material for some astounding chemistry.
Recalling chemistry lessons at school, you might remember that to make a chemical reaction happen, you must get a high concentration of the reactants. Add very dilute sulfuric acid onto household sugar, and nothing much happens. Excitement only mounted when you entered the classroom to see the bottle of viscous yellowish concentrated sulfuric acid on the chemistry teacher’s desk. The evil grin of the teacher gave away the black volcano of classroom distraction about to be unleashed from the dish of sugar soaked in the acid, and the acrid fumes as the sugar molecules were violently disassembled fulfilled every health and safety officer’s dream of form-filling. So, you might ask yourself, how could anything of any interest be going on in a space cloud that is more diffuse than a typical lab vacuum?
There is something that occurs in abundance in space, but is not easily found in the school classroom: radiation. Protons, electrons, gamma rays, UV radiation, and many heavy ions such as ions of iron or silicon permeate interstellar space, including our cloud. This radiation imparts energy to the meager collection of ions and molecules, but nonetheless at sufficient levels to break them apart, energize them, and drive reactions between different kinds to produce novel chemical compounds. Even though these clouds are at a chilly temperature of about −180°C, the radiation can bombard the ions and molecules to force the chemical reactions along their way.
Astronomers can observe these products of interstellar chemistry using spectroscopy. As light passes through a diffuse interstellar cloud, compounds will absorb some of that light. More exactly, electrons will absorb the energy and jump energy levels, essentially introducing a ghostly void, if you will, in the spectrum of the light by robbing it of particular wavelengths. By analyzing the spectrum that passes through the cloud and reaches telescopes on Earth or in space, scientists can use this absorption spectroscopy to identify what is in these clouds. Alternatively, if electrons absorb energy from light and reradiate it, they emit light, perhaps of a different wavelength, which again is a fingerprint of a particular compound. Both of these spectroscopic methods allow us to build up a family portrait of the chemical compounds that make up a cloud. Our understanding of the results of these sophisticated approaches, though, is still very much in its infancy. There are countless absorbances and emissions in the clouds, whose origins we understand only vaguely or not at all. Diffuse interstellar bands, an assortment of absorbances found in spectra from clouds, contain many signatures that we are still at a loss to explain.
Despite the mysteries that remain lurking in these clouds, what has been successfully identified is an enormous variety of simple compounds, including CO, OH, CH, CN, and CH+ ions. Now you will notice that within this short list—and there are many more compounds besides—one thing does stand out: there is a great selection of carbon-containing compounds. Carbon, produced by fusion in low- and high-mass stars and eventually flung into space to later coalesce in the clouds, reacts with many other elements to form simple compounds that contain the nascent structures of organic carbon compounds.
Things begin to get decidedly more interesting if we turn our attention toward other objects in the universe, for bigger and mightier clouds pervade the realms of the cosmos. Here and there, we can observe giant molecular clouds. These objects are gigantic, some about 150 light-years across. They can contain a thousand to ten million times the mass of our own Sun. These are the nurseries in which new stars are formed, the density of gas sufficient for eddies and swirls to congregate and initiate fusion burning of an infant orb. The density of material is much higher than in the diffuse interstellar clouds, some one trillion ions or molecules in a cubic meter, still much less than the air you breathe, but now sufficient to make chemistry that is even more interesting.
The material these clouds contain is now dense enough to shield much of the UV radiation being given off by new stars and other astrophysical objects. Although there may be less radiation to drive chemistry, it means that the compounds formed are less liable to be broken up by that same radiation. Within giant molecular clouds, we find over one hundred chemical compounds, including HCOOH, C3O, C2H5CN, CN, CH3SH, C3S, NH2CN, and many more. The picture should be becoming clearer. Within these cosmic nurseries, we go from very simple molecules containing one or two atoms to more impressive structures. The startling observations in our diffuse interstellar clouds are magnified—the complexity of carbon chemistry has increased. Giant molecular clouds are full of carbon-based chemistry!
The complexity that clouds can harbor is astonishing, and beyond compounds with just a few atoms, there are more startling structures. The six-atom rings of carbon, including benzene, can be attached together to make a family of molecules called polycyclic aromatic hydrocarbons. Intriguingly, the six-atom carbon rings can be reacted in the laboratory to form quinones, molecules involved in shuttling electrons around in life as it gathers energy from its environment. These sorts of laboratory reactions provide tantalizing demonstrations that molecules w
ithin the interstellar medium are already some way along the path to being useful precursors in the energy-yielding and metabolic pathways of living things.
These joined carbon sheets are thought to make yet more-unusual molecules. Assemble carbon rings in three dimensions, and you can create carbon balls, such as buckyballs that contain sixty carbon atoms. These C60 soccer-ball-like compounds made of twenty hexagons and twelve pentagons of carbon atoms linked together into a sphere can themselves coalesce to form layered onion-like carbon structures. They form tubes and grids of interlinked carbon atoms, exploring many possible combinations of structures.
Despite the revolution that has come from these observations in our grasp of how chemistry operates and what products it forms at the universal scale, scientists were still flummoxed by how this diversity of chemical compounds might have formed. They were especially perplexed by two important issues. To begin with, chemical compounds must be close to each other to react. This observation is underscored by the aforementioned example of sulfuric acid. Mix it with water, and the molecules of sulfuric acid become so dilute they do little exciting chemistry. Mix this diluted acid with sugar, and we end up with slightly acidic dissolved sugar, but no exciting classroom drama. As the molecular clouds are so diffuse, compared even with air, how could any chemistry possibly occur? And things get worse. The clouds are cold, very cold. We also know from our chemistry lessons that heating is a good way to get a reaction going. A small strip of magnesium metal does very little lying around on a lab bench, but place it in the Bunsen burner flame, and as long as the temperature gets up to above 473°C, it will ignite in a bright white glow. At −260°C to −230°C in the molecular cloud, the possibility of getting chemicals—or chemists—excited seems decidedly thin.
Yet chemical reactions occur, and they do so in surprising places. Scattered through the clouds are particles, interstellar dust grains that contain a core of silica or carbon-rich material surrounded by ice. The ions and molecules within a cloud can attach to these grains and concentrate. Now we have a mechanism to bring them together; otherwise, these ions and molecules would be left floating aimlessly in interstellar space. Astrochemists think that much of the chemistry in molecular clouds happens on these grains. Each grain is a factory and, more interestingly, a miniature reactor for making organic compounds.
The profusion of carbon in the universe is so great that some stars are even defined by the element. At the edges of “carbon stars” is a plethora of reactions that form, destroy, or shuffle carbon, generating an unknown level of complexity and diversity in organic chemistry. Just 390 to 490 light-years away, there is a star that can be seen to have halos, a signature of the envelopes of material expanding away at over fifty kilometers a second. Within the envelopes, over sixty types of molecules have been detected, among them linear and cyclical carbon molecules, organic chemistry literally being blown into space. Around the photosphere of the star, the region that emits light, simple compounds such as CO (carbon monoxide) and HCN (hydrogen cyanide) have been detected.
These rather cursory observations are sufficient to make some points that are probably becoming evident. The universe is not a cold, chemically infecund place where stars listlessly revolve around the center of galaxies, merely producing and spreading the basic elements of the periodic table, which the origin of life and eventually living things must assemble through unfathomable and obscure mechanisms into the organic building blocks for a reproducing, evolving entity. Everywhere throughout the universe, complex chemistry is occurring, even in the most diffuse puffs of gas, and this chemistry includes a vast pantheon of organic chemistry, generating bonds between carbon and other elements in a huge panoply of arrangements.
We see within this emerging picture a strange inevitability in the production of complex carbon chemistry. The variety of possible ways carbon can bind with other elements and produce a great catalog of molecules is not an idiosyncratic product of the temperature and pressure conditions found on Earth, a limited case of a specific planetary environment that allows carbon to take center stage in the world of complexity to build living things. In the coldest of places in the universe, carbon still does its thing, binding with the elements of the periodic table, including itself, to produce the gelateria of flavors of organic chemistry from which life on our planet is constructed. The path to the building blocks of carbon-based chemistry seems universal.
No doubt, the greatest interest from our perspective is how far this chemistry can go to assembling the molecular precursors of life. Attempts to find the basic monomers from which life is constructed, such as amino acids, sugars, and the nucleobases that make up proteins, carbohydrates, and the genetic code, respectively, have yielded mixed results. Researchers from Copenhagen University found glycolaldehyde in the interstellar medium near newly forming stars. This molecule can take part in the formose reaction, a chemical synthesis that itself can ultimately lead to sugars. Isopropyl cyanide, a compound that can be a precursor to amino acids, has been found in the interstellar medium. Formaldehyde (CH2O) is also known to exist in the interstellar medium; this compound can take part in reactions thought to lead to compounds such as amino acids.
Complex molecules may be rarer and more difficult to find in interstellar clouds than simple compounds like HCN. As observational methods improve, we will doubtless find more of these complex molecules, but the general conclusion is clear: the interstellar medium contains many carbon-based molecules that can act as precursors and intermediates of chemical syntheses to construct living things.
Equally extraordinary and compelling evidence that the basic components of life can be produced in space can be found closer to home in meteorites. Although the concentrations of the over seventy amino acids found in carbonaceous varieties of meteorites are low (about ten to sixty parts per million), the compounds’ presence nevertheless shows that the protoplanetary nebulae from which our Sun formed were propitious places for carbon chemistry.
So far there is no evidence that these amino acids have come together to form a simple protein chain. The conditions in the early Solar System seem to have been suitable for the formation of amino acids, but that is where complexity stopped. The more benign conditions of a watery environment on a planet’s surface were needed if these building blocks were to become the chained, complex molecules we associate with life.
The amino acids in meteorites raise the obvious question of why we do not see amino acids strewn throughout interstellar space. Why the discrepancy? Maybe their low concentrations mean that they can be detected easily enough within a lump of rock in a lab, but dispersed in the interstellar medium, with all the other signals of chemistry out there, they remain elusive. Or maybe the discs of early solar systems make particularly favorable places for these types of compounds to form. A plethora of surfaces in proximity, temperature gradients, and volatiles such as water might make protoplanetary discs felicitous birth places for some of the more interesting reactions to bring forth the components of life.
Meteorites have given up their secrets as storehouses of much more than amino acids. Sugars, the building blocks of carbohydrates, and nucleobases, the letters of the genetic code, have been found, mixed in with a witch’s brew of sulfonic acids, phosphonic acids, and other compounds. For each of the four major classes of biological molecules, or chains of compounds from which life is constructed—proteins, carbohydrates, nucleic acids, and membrane lipids—their monomers, or their basic building blocks, are found in meteorites.
Significantly, we do not find complex silicon-based compounds within meteorites, a storehouse of silicon variety that would make us wonder whether a battle might be played out between the possibilities of carbon- and silicon-based life wherever these materials land. The silicon compounds in meteorites are overwhelmingly dominated by the not-very-reactive silicate compounds. Meteorites show us the universality of complex carbon chemistry.
Meteorites come from asteroids and the rocky remnants of solar system
formation, but an equally important class of objects, the comets, occupies the Oort Cloud, a spherically shaped reservoir of objects about 20,000 to 100,000 astronomical units away. Just beyond the orbit of Neptune, in the Kuiper Belt, yet another swathe of these icy bodies is to be found. Comets, like asteroids, are the leftovers of solar system formation. These objects are made up of a mixture of rock and ice, and the dark coloration of their nucleus is thought to be partly caused by organic compounds. In these objects, too, the same extraordinary story of carbon chemistry is being told.
By now, we have had a good chance to observe these frozen little worlds, using telescopes on Earth and in space, and by sending spacecraft to intercept and visit them. No mere blocks of ice, comets have been shown to contain carbon monoxide, carbon dioxide, and, from there on out, an increasingly complicated array of compounds, including methane, ethane, acetylene, formaldehyde, formic acid, isocyanic acid, and, last and especially not least, the amino acid glycine, detected on comet 67P by the Rosetta spacecraft. The presence of glycine raises some fundamental questions about whether other amino acids and the other classes of molecules needed for life exist on comets.
If these processes happened in our Solar System, unless there was something very unusual about our location—and there is no reason to suspect this—then they are happening everywhere. On the other side of our galaxy, in Andromeda and millions of light-years from here, amino acids, sugars, nucleobases, and fatty acids are raining down on planets. Organic carbon chemistry is universal, and therefore, the likelihood that life, if it has taken hold elsewhere, would be carbon based is high.
These stunning discoveries in space should not cause us to lose sight of the possibility that our own world was the pot in which some of these compounds for life were synthesized. The tendency of energetic environments to produce complex carbon compounds was elegantly demonstrated in the 1950s by Stanley Miller and Harold Urey. At a time when scientists lacked knowledge of the abundance of carbon compounds in space and meteorites, the two researchers set about to understand how chemistry might have made that momentous transition from prebiotic soup to replicating form. They carried out a superb and simple experiment in the laboratory. Within a container of gases—including methane, ammonia, and hydrogen—they circulated water vapor. They added an electric discharge using two electrodes to simulate early lightning on Earth, a bona fide Frankenstein setup. After they energized the gases and added the water to simulate the wet early Earth, what emerged was not a monster, but a brownish goo that contained amino acids. Glycine, alanine, aspartic acid, and many other compounds were synthesized. However, their experiment used gases that we now think were not so abundant in the early Earth’s atmosphere. Change the composition of the starting gases, and the yields and types of amino acids and other products are altered. Nevertheless, the general idea that, given some energy, a few simple starting gases, and some water, amino acids can be formed was seminal in establishing that the organic chemistry of life is no miracle. Indeed, to produce absolutely no complex organic chemistry when you add energy to a mixture of gases that contain some carbon atoms requires quite a bit of work.
The Equations of Life Page 23