Extraterrestrial life based on the element phosphorus, instead of carbon, has been suggested as a remote possibility. However, although phosphorus can be conceived of as a potential biochemical building block, with phosphine (PH3) as a compatible solvent, a major stumbling is the poor variety of phosphorus hydrides. At best, it seems that phosphorus might be able to form chains with other elements such as carbon, silicon, or nitrogen. On the down side, there are problems with the notion of ammonia as a basis for life. These center principally upon the fact that the heat of vaporization of ammonia is only half that of water and its surface tension only one third as much. Consequently, the hydrogen bonds that exist between ammonia molecule are much weaker than those in water so that ammonia would be less able to concentrate non-polar molecules through a hydrophobic effect. Lacking this ability, questions hang over how well ammonia could hold prebiotic molecules together sufficiently well to allow the formation of a self-reproducing system. Boron is one of the few elements that seems to offer a plausible alternative to carbon as a basis for life elsewhere in the universe. Like carbon and silicon, boron has a strong tendency to form covalent molecular compounds. Being a group III element, however, it has one less valence electron than the number of valence orbitals, which makes its chemistry noticeably different from that of carbon. There are no direct analogs to hydrocarbons in boron chemistry because, although boron forms a lot of different structural varieties of hydride, in these the boron atoms are linked indirectly through hydrogen bridges. Boron forms bonds with nitrogen that are somewhat like the carbon-carbon bond – t... One of the biggest drawbacks to boron as a basis for life it is scarcity. On Earth, its abundance in the continental crust is only about 10 parts per million, so that any biology would seem to depend on their being present some mechanism for bringing about greater local concentrations of the element. At first sight, silicon does look like a promising organic alternative to carbon. It is common in the universe and is also a p-block element of group IV, lying directly below carbon in the periodic table of elements, so that much of its basic chemistry is similar. For instance, just as carbon combines with four hydrogen atoms to form methane, CH4, silicon yields silane, SiH4. Silicates are analogs of carbonates, silicon chloroform of chloroform, and so on. Both elements form long chains, or polymers, in which they alternate with oxygen. In the simplest case, carbon-oxygen chains yield polyacetal, a plastic used in synthetic fibers, while from a backbone of alternating atoms of silicon and oxygen come polymeric silicones. The absence of silicon-based biology, or even silicon-based prebiotic chemicals, is also suggested by astronomical evidence. Wherever astronomers have looked – in meteorites, in comets, in the atmospheres of the giant planets, in the interstellar medium, and in the outer layers of cool stars – they have found molecules of oxidized silicon (silicon dioxide and silicates) but no substances such as silanes or silicones which might be the precursors of a silicon biochemistry. Nitrogen is one of a handful of elements that have been suggested as alternatives to carbon as the basis of life elsewhere in the universe. Principally this is because it can form long chains at low temperatures with a liquid solvent such as ammonia (NH3) or hydrogen cyanide (HCN). On the other hand, a major drawback of nitrogen as a backbone for large molecular structures is that the energy of the triple bond in N2 is much greater than that for a single bond, so that notrogen-nitrogen bonds tend to revert back to elemental nitrogen. Nitrogen can, however, form longer molecules with some other elements, including carbon, phosphorus, sulfur, and boron. Nitrogen can also form hydrides, such as hydrazine. Hypothetical life that exists without material bodies. The idea has been extensively explored in science fiction (see Childhood's End) but, from the viewpoint of science itself, the concept of life based on "pure energy," such as electromagnetic radiation, poses significant difficulties. A being made only of photons, for example, could not move slower than the speed of light and would essentially exist outside of the normal time stream of the rest of the universe. Artificial life represents a possible alternative form of noncorporeal life with a basis in information. Hypothetical life that has evolved under and adapted to conditions of extreme gravity, either very high or very low. Many examples of microscopic barophiles are known and the prodigious strength-to-mass ratio of some more advanced organisms (the rhinoceros beetle, for example, can manipulate objects with up to 850 times its own body mass), suggests that even complex mobile creatures might occur on worlds where the surface gravity is significantly higher than Earth's, providing that other basic biological requirements are not compromised. However, some scientists and science fiction writers have speculated on the prospects for life where the gravitational pull is far beyond the normal planet... The Companion in Star Trek Hypothetical life based on what is sometimes called "the fourth state of matter," plasma. The possibility of such noncorporeal life is a common theme in science fiction and is often entertained, for example, in episodes of Star Trek. However, it was given more credence by the announcement in September 2003 that physicists had succeeded in created blobs of plasma that can grow, replicate, and communicate, thus fulfilling most of the traditional requirements for biological cells. Lacking inherited material they cannot be described as alive, but the researchers believe these curious spheres may offer a radical new explanation for how life began. This research raises the intriguing possibility that life throughout the universe could have a very much broader basis that normally recognized. If plasma-based life can arise naturally, places to look for it could include the outer layers or interiors of stars (see Sun, life in), planetary magnetospheres, HII regions, and even ball lightning. |
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