ALIEN Extraterrestrial life is life that does not originate from Earth. It is also called alien life, or, if it is a sentient and/or relatively complex individual, an extraterrestrial or alien (or, to avoid confusion with the legal sense of alien, a space alien). These as yet hypothetical forms of life range from simple bacteria-like organisms to beings far more complex than humans. The possibility that viruses might exist extra terrestrially has also been proposed. The development and testing of hypotheses on extraterrestrial life is known as exobiology or astrobiology, although astrobiology also considers Earth-based life in its astronomical context. Many scientists consider extraterrestrial life to be plausible, but there is no direct evidence of its existence.[2] Since the mid-20th century, there has been an ongoing search for signs of extraterrestrial life, from radios used to detect possible extraterrestrial signals, to telescopes used to search for potentially habitable extrasolar planets. It has also played a major role in works of science fiction. Several hypotheses have been proposed about the possible basis of alien life from a biochemical, evolutionary or morphological viewpoint. BIOCHEMISTRY All life on Earth is based upon 26 chemical elements. However, about 95% of this life is built upon only six of these elements: carbon, hydrogen, nitrogen, oxygen, phosphorus and sulfur, abbreviated CHNOPS. These six elements form the basic building blocks of virtually all life on Earth, while most of the remaining elements are found in only trace amounts.[26] Life on Earth requires water as the solvent in which biochemical reactions take place. Sufficient quantities of carbon and the other elements along with water may enable the formation of living organisms on other planets with a chemical make-up and temperature range similar to that of Earth. Terrestrial planets such as Earth are formed in a process that allows for the possibility of having compositions similar to Earths. The combination of carbon, hydrogen and oxygen in the chemical form of carbohydrates (e.g. sugar) can be a source of chemical energy on which life depends, and can provide structural elements for life (such as ribose, in the molecules DNA and RNA, and cellulose in plants). Plants derive energy through the conversion of light energy into chemical energy via photosynthesis. Life, as currently recognized requires carbon in both reduced (methane derivatives) and partially oxidized (carbon oxides) states. Nitrogen is needed as a reduced ammonia derivative in all proteins, sulfur as a derivative of hydrogen sulfide in some necessary proteins, and phosphorus oxidized to phosphates in genetic material and in energy transfer. Pure water is useful because it has a neutral pH due to its continued dissociation between hydroxide and hydronium ions. As a result, it can dissolve both positive metallic ions and negative non-metallic ions with equal ability. Furthermore, the fact that organic molecules can be either hydrophobic (repelled by water) or hydrophilic (soluble in water) creates the ability of organic compounds to orient themselves to form water-enclosing membranes. Additionally, the hydrogen bonds between water molecules give it an ability to store energy with evaporation, which upon condensation is released. This helps to moderate the climate, cooling the tropics and warming the poles, helping to maintain the thermodynamic stability needed for life. Carbon is fundamental to terrestrial life for its immense flexibility in creating covalent chemical bonds with a variety of non-metallic elements, principally nitrogen, oxygen and hydrogen. Carbon dioxide and water together enable the storage of solar energy in sugars and starches, such as glucose. The oxidation of glucose releases biochemical energy needed to fuel all other biochemical reactions. The ability to form organic acids (–COOH) and amine bases (–NH2) gives rise to the possibility of neutralization dehydrating reactions to build long polymer peptides and catalytic proteins from monomer amino acids. When combined with phosphates, these acids can build the information-storing molecule of inheritance, DNA, and the principal energy transfer molecule of cellular life, ATP. Due to their relative abundance and usefulness in sustaining life, many have hypothesized that life forms elsewhere in the universe would utilize these basic materials. However, other elements and solvents could provide a basis for life. Life forms based in ammonia (rather than water) have been suggested, though this solution appears less optimal than water. From a chemical perspective, life is fundamentally a self-replicating reaction, but one which could arise under a great many conditions and with various possible ingredients, though carbon-oxygen within the liquid temperature range of water seems most conducive. Suggestions have even been made that self-replicating reactions of some sort could occur within the plasma of a star, though it would be highly unconventional. Life on the surface of a neutron star, based on nuclear reactions, has also been suggested. However, communicating with such creatures would be difficult because the time scales involved are much faster. Several pre-conceived ideas about the characteristics of life outside Earth have been questioned. For example, a NASA scientist suggested that the color of photosynthesizing pigments of hypothetical life on extrasolar planets might not be green. EVOLUTION AND MORPHOLOGY In addition to the biochemical basis of extraterrestrial life, many have considered evolution and morphology. Science fiction has often depicted extraterrestrial life with humanoid or reptilian forms. Aliens have often been depicted as having light green or grey skin, with a large head, as well as four limbs—i.e. fundamentally humanoid. Other subjects, such as felines, insects, blobs, etc., have occurred in fictional representations of aliens. A division has been suggested between universal and parochial (narrowly restricted) characteristics. Universals are features which are thought to have evolved independently more than once on Earth (and thus, presumably, are not too difficult to develop) and are so intrinsically useful that species will inevitably tend towards them. The most fundamental of these is probably bilateral symmetry, but more complex (though still basic) characteristics include flight, sight, photosynthesis and limbs, all of which are thought to have evolved several times here on Earth. There is a huge variety of eyes, for example, and many of these have radically different working schematics and different visual foci: the visual spectrum, infrared, polarity and echolocation. Parochial, however, are essentially arbitrary evolutionary forms. These often have little inherent utility (or at least have a function which can be equally served by dissimilar morphology) and probably will not be replicated. Intelligent aliens could communicate through gestures, as deaf humans do, by sounds created from structures unrelated to breathing, which happens on Earth when, for instance, cicadas vibrate their wings or crickets stridulate their wings, or visually through bioluminescence or chromatophore-like structures. Attempting to define parochial features challenges many taken-for-granted notions about morphological necessity. Skeletons, which are essential to large terrestrial organisms according to the experts of the field of gravitational biology, are almost assured to be replicated elsewhere in one form or another. The assumption of radical diversity amongst putative extraterrestrials is by no means settled. While many exobiologists do stress that the enormously heterogeneous nature of life on Earth foreshadows an even greater variety in outer space, others point out that convergent evolution may dictate substantial similarities between Earth and extraterrestrial life. These two schools of thought are called divergionism and convergionism respectively. [PHOTO: 1. Alien Base Discovered On Google Mars; 2. Alien Dome In Crater On Moon, NASA PHOTO; 3. Another Proof of Alien Life on Mars - NASA Rover Curiosity Vehicle Footage 2013] [REFERENCE: en.wikipedia.org/wiki/Alien] [EDIT.: D. K. Srivastava]
Posted on: Sat, 04 Oct 2014 18:05:13 +0000