Seeds of Life?

I thought this extract from my old book Companion to the Cosmos might be of interest in the light of the news about the system TRAPPIST-1


interstellar chemistry Many molecules (more than 80 by the mid- 1990s) have been discovered in clouds of gas and dust between the stars. Although the nuclei of different elements are built up inside stars by the processes of nuclear fusion, under the conditions of heat and pressure inside a star atoms cannot combine together to make molecules. So all of the variety of molecules seen in interstellar space must have been produced by chemical reactions going on in the clouds of gas and dust where we detect those molecules today. The complexity of the reactions involved in this interstellar chemistry is indicated by the complexity of some of the molecules identified — several contain 10 or more atoms, and one is the amino acid glycine (NH2CH2COOH), an essential building block of life on Earth.

Many of the molecules found in interstellar space are made up from carbon, oxygen and nitrogen (the most abundant elements manufactured from hydrogen and helium inside stars), together with hydrogen itself (see CHON). Simple compounds made up of carbon and hydrogen (CH) and carbon and nitrogen (CN) were discovered at the end of the 1930s, using optical spectroscopy. But the first real progress towards an understanding of interstellar chemistry came in the 1960s, when suitable radio astronomy techniques were developed to identify the characteristic radiation of polyatomic molecules in space. The hydroxyl compound (OH), water (H2O), ammonia (NH3) and formaldehyde (H2CO) were soon identified.

In the 1970s, astronomers were surprised by the variety and complexity of organic molecules (that is, molecules that contain carbon atoms) found in space. These included ethyl alcohol (C2H5OH), which is present in one large complex of molecular clouds (known as Sgr B2) in sufficient quantities to make 1027 litres of vodka. As well as these complex molecules, interstellar clouds must also contain simple compounds such as oxygen (O2), nitrogen (N2), carbon dioxide (CO2) and hydrogen (H2), which are stable and form very easily from the basic atomic ingredients that are known to be present.

The key to interstellar chemistry is the presence of a large amount of carbon in the form of grains of graphite in these interstellar clouds. These show up from the way in which they absorb visible light from more distant stars, which can be explained by the presence of elongated grains about 0.1 millionths of a metre long, mostly made of carbon but with water ice and silicates present as well. It may seem odd to think of interstellar clouds as being laced with soot, but carbon is one of the most common products of nucleosynthesis inside stars, and there is a family of stars (known as carbon stars) which are shown by their spectra to have atmospheres relatively rich in carbon, and which vary regularly, puffing in and out with periods of a year or so, and ejecting material into interstellar space as they do so. The evidence suggests that many (if not all) stars go through a phase of such activity.

Most of the complex molecules are found in unusually dense clouds in space, where there is enough of the sooty dust to act as a shield, protecting the molecules from the strong ultraviolet radiation from nearby young stars, which would tend to break the molecules apart. These are exactly the clouds in which new young stars, and their associated planets, are forming. The molecules are probably built up by reactions that take place on th surfaces of dust grains, where atoms can “stick” and have a chance to interact with one another. The molecules later evaporate from the surfaces of the grains. All of this makes it extremely likely that new planets are “seeded” by quite complex molecules early in their existence; any molecules present in the interstellar clouds from which stars and planets form could easily be deposited on a planet by, for example, the impact of a large comet.

Interstellar chemistry involves not only interactions between material in gaseous form and the solid grains of dust in molecular clouds, but also interactions with the stars themselves. It is harder than you might think for such a cloud to collapse and fragment to form stars. When it starts to do so, gravitational energy is released in the form of heat, making the molecules in the cloud move faster and generating a pressure which resists further collapse. The cloud can only collapse further if this excess heat can be disposed of, in the form of electromagnetic radiation. This is produced by the molecules of compounds such as carbon dioxide and water vapour in the cloud. Then, when a young star begins to form, it produces copious amounts of ultraviolet radiation, which would tend to blow the cloud apart. Fortunately, though, the grains of carbon dust (or soot) in the cloud absorb the ultraviolet radiation and re-radiate it in the infrared part of the spectrum, at wavelengths which can escape much more easily into space. Carbon dust grains, and molecules produced by interstellar chemistry, are essential in the cooling processes without which the stars which edge the spiral arms of a galaxy like our own Milky Way would not form in such abundance.

In the early 1990s, astronomers found evidence of a complex molecule in the form of a ring in interstellar clouds. The evidence came from the NASA Ames Research Center, and concerned the detection of features interpreted as those of the spectrum of pyrene (C12H10), in which a dozen carbon atoms are joined together in a ring, with ten hydrogen atoms attached to it around the outside. This provided the first independent support for controversial claims made by Fred Hoyle and his colleague Chandra Wickramasinghe in the 1970s and 1980s.

Hoyle and Wickramasinghe have gone further than any other astronomers in claiming that some of the features seen in the radiation from molecular clouds can be explained in terms of very large organic molecules called polymers. These form chains of repeating units. The basic component of a polysaccharide chain, for example, is the so-called pyran ring, a hexagon made up of five carbon atoms and one oxygen atom (C5O). These rings link together to make a chain when one of the carbon atoms joins on to another oxygen atom, which itself joins on to the next pyran ring in the chain, and so on. Once formed, a pyran ring shows one of the fundamental properties of life — it acts as a template, encouraging the formation of more identical rings which join up in a growing polysaccharide chain.

Hoyle and Wickramasinghe have suggested that even more complex polymers such as cellulose may already have been directly revealed by their spectral signatures in radiation from these clouds (other astronomers dispute this interpretation of the data), and that molecules of life itself may be present in the clouds but not yet detected. Once, these ideas were derided as so heretical that just voicing them may have cost Hoyle the Nobel Prize he deserved for his work on nucleosynthesis. In fact, the team has an impressive track record (they were, for example, the originators of the idea that interstellar clouds contain grains of soot, an idea established now beyond reasonable doubt), and their ideas look far less extreme now that glycine and pyrene have both been identified in space.

At the very least, it is now difficult to escape the conclusion that when a planet like the Earth forms its atmosphere and oceans are soon laced with complex organic molecules. Since interstellar chemistry seems to be the same in molecular clouds across the Milky Way, this suggests that the complex chemistry of other planets would be similar to that of the Earth, and that where life has evolved from that complex chemistry it should be based on the same sort of compounds (including amino acids) that we are.



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