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Astrobiology is a science that studies the origin, evolution, and distribution of life forms in the universe and tries to find out if there are life forms that originated outside the planet Earth.
Astrobiology involves a large number of researchers worldwide from a variety of disciplines: astronomy, biology, earth sciences, mathematics, telecommunications science and technology, as well as sociology and philosophy. The International Astronomical Union, which brings together all professional astronomers, has a section dedicated to astrobiology (Commission F3 Astrobiology), and there are astrobiology institutes at several scientific institutions, such as the NASA Astrobiology Institute, Centro de Astrobiología.
Two international scientific journals are devoted exclusively to astrobiology: Astrobiology and the International Journal of Astrobiology.
Several professional research associations gather scientists working in astrobiology; the most important are the International Society for the Study of the Origin of Life – The International Astrobiology Society (ISSOL) and the European Astrobiology Network Association (EANA), which gathers astrobiology societies operating in Europe.
Molecular compounds of organic chemistry are particularly abundant in the diffuse component of interstellar matter. About 30 percent of them consist of groups of light atoms such as hydrogen, oxygen, nitrogen, and carbon. From the simplest compounds (carbon hydride CH, hydrogen cyanide HCN, methane CH4, etc.), studies, especially with large radio telescopes, have revealed at least a hundred different complex molecules, some with 13 constituent atoms.
The most important of these, in the field of prebiotic chemistry, are hydrogen cyanide and formaldehyde; but recently, compounds similar to Orion’s chlorophyllanebulose), fullerene (a molecule containing up to 60 carbon atoms), glycine (protostellar cloud in Sagittarius B2), as well as a variety of substances classifiable among amino acids and nucleotides, the basic components of living proteins, have also been found.
Theoretically, such substances, embedded in bolides, meteorites, cometary cores, could act as disseminators of biological “foci” on impact with those celestial bodies – planets, large satellites – that are endowed with appropriate environmental conditions. What puzzles scientists, however, is the mystery of their preservation under the destructive and sterilizing bombardment of the ionizing radiation of stars and cosmic rays.
In fact, as far as the Earth is concerned, it is estimated that in the past the fallout of organic matter on the surface amounted to 20 g/cm2; it still persists because the Vega and Giotto probes have directly demonstrated that comets (Halley, Giacobini-Zinner) diffuse large quantities of organic matter (cyanogen C2N2, water, carbon monoxide CO).
Biologically important elements and molecules
The main chemical elements that underlie life as we know it are Phosphorus, Oxygen, Nitrogen, Carbon and Hydrogen, known by the acronym PONCH. Sulfur also plays an important role as a source of energy for some biological processes. Among these elements, carbon is the most important and interesting for life. No chemical element is capable of forming as many compounds as carbon, not only in number, but also in variety. One of the chemical properties of carbon is its ability to form covalent bonds, using its four valence electrons to form single, double, or triple bonds. Another important property is the ability to form carbon chains of varying lengths, with linear, branched, or ring structures, and containing double or even triple bonds. These chains have an important property; they do not break or react easily. In addition, because the bonds can be placed in a variety of ways, it is common for molecules to exist with the same number of atoms but with different structures and different properties. Such molecules are called isomers, and examples include glucose and fructose.
Among the molecules of great biological importance formed from carbon are monosaccharides. These molecules have a characteristic composition and structure: a carbon chain containing three to seven carbon atoms; one carbon atom carries the carbonyl group (C=O); all other carbon atoms bind the hydroxyl group (-OH). Monosaccharides differ in the number of carbon atoms and the position of the carbonyl group. Monosaccharides that have the carbonyl group at the second position of the chain are called ketones, aldehydes if it is at the beginning. Monosaccharides can come in two forms: linear or ring. The ring form is more stable under the conditions in which cells live and is therefore more common. Other major isomers differ in the number of carbon atoms. Hexoses are formed by six atoms, and glucose is an example. When the chains have five carbon atoms, they are called pentoses; two of them, ribose and deoxyribose, form the backbone of RNA and DNA nucleic acids. Naturally occurring sugars have a dextrorotatory configuration.
Another group of molecules that play a key role are amino acids. At the base of proteins, amino acids are composed of an amine group (-NH2) and a carboxyl group (-COOH) attached to an α-carbon atom in a levorotatory configuration and a side chain called the radical group (R group). The R group contains important functional groups that determine both the three-dimensional structure and the specific chemical properties of the amino acid. Approximately 20 amino acids used by cells for protein synthesis have been identified in nature.
Other biomolecules at the basis of life are lipids. There are several classes of lipids, but they all share the characteristic of being hydrophobic. Fats can be saturated, containing only single bonds between carbon atoms, or unsaturated, containing one or more double bonds (polyunsaturated). When three fatty acids bind to a glycerol molecule through an ester bond (C-O), a triglyceride is formed, which has the function of storing energy. When one of the fatty acids is replaced by a compound formed by a phosphate group, a phospholipid is formed. In phospholipids, the phosphate functional group has a negative charge, making this part of the molecule hydrophilic. In an aqueous environment, phospholipids tend to align so that the phosphate group, the “head,” faces the water, while the “tails,” formed by fats, tend to cluster together, forming a phospholipid bilayer. Biological membranes have this type of structure. Other classes of lipids include carotenoids, a pigment responsible for light absorption in plants, and steroids, organic compounds characterized by a ring skeleton with a few carbon atoms grouped together that have structural functions, such as cholesterol, or hormonal functions.
Finally, nucleic acids are a group of essential biomolecules. They are polymers specialized in the storage, transmission and use of genetic information. There are two types of nucleic acids that are essential to us: DNA (deoxyribonucleic acid) and RNA (ribonucleic acid). The monomers at the base of the chain are called nucleotides and consist of a pentose sugar, a phosphate group, and a nitrogenous base. The latter can take two chemical forms: a single ring structure, called a pyrimidine, or a double ring structure, called a purine. The constituent nitrogenous bases of DNA are cytosine and thymine, i.e. purines, guanine and adenine, i.e. pyrimidines; the sugar is deoxyribose. In contrast, RNA has uracil instead of thymine and ribose as the sugar, which has one more oxygen than deoxyribose. Nucleotides also play other roles, such as in ATP (adenine triphosphate), which acts as an energy carrier in many chemical reactions.
The study of meteorites
The study of some large meteorites found on the ground (Murchison, Orgueille, etc.) confirms the presence of at least 8 of the 20 amino acids that make up living proteins; as well as that, embedded in mineral chondrules, of some purine and pyrimidine bases of the nucleic acids RNA and DNA. All of these evolved organic formations show that, within the meteoritic mass, they have well tolerated the thermal shock associated with entry into the Earth’s atmosphere. From these “space-imported” molecules, chemists have attempted to reconstitute the three families of molecules that govern cell biology: the nucleic acids that preserve and replicate the genetic code, the proteins that enable the biological activity of the cell, and the phospholipids that support its membrane chemistry.
For example, using only two types of amino acids of extraterrestrial origin, the Orléans Center for Molecular Biophysics was able to obtain a protein molecule that, although it contains only 10 amino acids, shows a certain degree of replicability. Some hypotheses suggest that organic material dispersed by dust and meteorites could have taken root in an aqueous environment at a suitable temperature – in any case not higher than 150°C – where it could have formed protein complexes, initially based on the replication properties of RNA (ribonucleic acid) alone. These primitive organisms could have been electrically charged (Gunther Wachterrshauser’s autocatalytic organic molecules) and made capable of growing on FeS iron sulfide crystals.
The idea that life can exist outside of planet Earth is very ancient. Already in ancient Greece philosopher Democritus argued that, since everything in the universe was made from atoms, and life itself was made from atoms, then life must exist in the whole universe. Democritus’ theories were also supported by later philosophers, such as Epicurus and Lucretius, while they were criticized by others, in particular by Aristotle, who supported the immutability and perfection of the heavens composed of ether, also arguing that each element (earth, water, air and fire) tends to its own unique center of motion, and therefore can not exist other worlds besides ours.
The Aristotelian-Tolemaic view of the universe then became the accepted idea of Christianity and was not questioned until the end of the Middle Ages and the Renaissance, when Copernicus’ heliocentric theory became popular. At the end of the XVI century Giordano Bruno affirmed the existence of infinite worlds arguing that the work of God could not be limited only to one world in the universe. In the following centuries the idea of plurality of worlds was supported by many philosophers and scientists, such as Descartes, Huygens (who affirmed the possibility that other planets could host life forms), Fontenelle and Voltaire. The latter wrote the novel Micromégas, which has as its protagonists beings from Sirius and Saturn, and is considered one of the earliest examples of science fiction.
At the end of the nineteenth century Schiaparelli observed Mars and described elongated structures he called generic channels, but considered by some astronomers as artificial channels, the work of an intelligent civilization. Great supporter of this hypothesis was Lowell, who published much in this regard and described extensively these channels, spreading the idea, long persisted, that Mars was inhabited. Only in the sixties, with the first space missions to Mars, new photos and maps of Mariner probes refuted the hypothesis of the channels and described the planet as arid and desert.
In the nineteenth century Darwin introduced the idea of evolution of life from simple organisms to complex species, while in the same period the birth of spectroscopy allowed to discover that in the universe are widespread the same chemical elements present on Earth, supporting the idea that planets could form around other stars. In the same years Pasteur showed that life is generated only from other life and not from inanimate things.
The debate on the origin of life on Earth in the following years went further, arriving to extreme hypothesis such as panspermia, which makes the hypothesis that terrestrial life forms were born elsewhere and then brought to our planet, in the form of spores pushed by sunlight to interstellar space as claimed by Kelvin and Arrhenius, or were voluntarily brought to Earth by spaceships, as proposed by Crick and Orgel. The hypothesis of panspermia was denied by Haldane and Oparin, who proposed that life was generated by a combination of chemical properties from organic molecules, without the need for external intervention. The experiment performed by Miller and Urey in 1953 showed that some amino acids could be formed from simple molecules such as hydrogen, ammonia and methane.
In the twentieth century, the invention of radio receivers and later the radio telescope brought new tools to the search for extraterrestrial life. Already in the twenties Marconi tried unsuccessfully to pick up signals from Mars. Later Cocconi and Morrison proposed to search for radio signals from other stars, as evidence of the existence of intelligent civilizations. In 1961 Drake developed an equation to determine the number of civilizations able to communicate. Shortly after the first SETI projects for the search of extraterrestrial life were born and in 1974 the first radio message to other civilizations was transmitted, followed by messages placed on the probes Pioneer and Voyager. Dyson instead described the possibility that an evolved civilization can create a sphere around its star to use its energy and proposed to search for signals in the infrared.
In recent decades, the discovery of exoplanets has led to new projects, which include spectroscopic analysis of the atmosphere of these bodies in search of oxygen, ozone and chlorophyll. In the solar system, research focuses on Mars and the satellites of Jupiter and Saturn, which may harbor microscopic life forms.