Extraterrestrial life

Life which exists, or could exist, outside our Earth is generally known as extraterrestrial life. A distinction is also made between life (or possible life) within or outside the solar system. In spite of what is claimed in many science fiction books and films, there is no single piece of evidence for a living system outside Earth. The coming years and decades will hopefully provide clarity on the question as to whether we are really alone in the universe or not. 

There are three objects within the solar system which are the subject of research on possible extraterrestrial life, traces of life, biomolecules or their precursors  

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Experimental observations are at the heart of chemical research. Many experiments are designed specifically to answer some particular chemical question. Often, the results of these experiments are unexpected and lead to new hypotheses. New hypotheses, in turn, suggest additional experiments. The Chemistry and Life Box describes how the hypothesis of extraterrestrial life can be tested. 

Of the three extraterrestrial targets in our solar system, the Saturnian moon Titan is the least likely to provide signs of life. To quote Christopher McKay from the NASA Ames Research Center, Titan is an interesting world. For example, its organic haze layer could be an example of the prebiotic chemistry which led to life on Earth . Direct links to extraterrestrial life have not, however, yet been found, as water (one of the main preconditions for life) has not been detected on Titan, apart from traces of water vapour in the higher layers of the Titanian atmosphere (Brack, 2002). 

At the end of the day, the question of extraterrestrial life is based on two principles the Copernican principle and the mediocrity principle. The first states that the Earth is not in a central, specially favoured position. The mediocrity principle states that life on Earth is nothing special, so that life (in whatever form) could evolve in many places in the universe. 

The first concerns the origin of life on earth as we know it, of our biological world. The second considers the possibility of extraterrestrial life, within or beyond the solar system. The third question wonders why life has taken the form we know, and it has as corollary the question whether other forms of life can (and do) exist is there artificial life It also implies that one might try to set the stage and implement the steps that would allow, in a distant future, the creation of artificial forms of life. 

In the movie Contact, Dr. Elbe Arroway is scolded and told that her proposal to search for extraterrestrial life seems less like science and more like science fiction. She responds, Science fiction. You re right, it s crazy. In fact, it s even worse than that, it s nuts. You wanna hear something really nutty I heard of a couple guys who wanna build something called an airplane, you know you get people to go in, and fly around like birds. It s ridiculous, right ... Look, all I m asking is for you to just have the tiniest bit of vision. 

Photomicrograph of the ALH84001 Martian meteorite, field of view = 1.3 cm. Large, broken grains of pyroxene form this breccia. This sample created a stir when it was proposed to contain evidence for extraterrestrial life. Image from Lauretta and Kilgore (2005), with permission. 

ALHA 84001 is a unique Martian meteorite with an ancient age of 4.5 billion years. It consists mostly of orthopyroxene crystals that accumulated in basaltic magma. Its most distinctive feature, however, is the occurrence of small globules of carbonates with unusual compositions and textures. The controversial (now largely discredited) hypothesis that the carbonates contain evidence of extraterrestrial life (McKay et al., 1996) made this the most famous meteorite on Earth. ALHA 84001 has also experienced intense shock metamorphism. 

Beginnings of cosmochemistry (and geochemistry) Philosophical foundations Meteorites and microscopy Spectroscopy and the compositions of stars Solar system element abundances Isotopes and nuclear physics Space exploration and samples from other worlds New sources of extraterrestrial materials Organic matter and extraterrestrial life ... 

Dick, Steven J. 1996. The Biological Universe The Twentieth-Century Extraterrestrial Life Debate and the Limits of Science. New York Cambridge University Press. 

The National Aeronautics and Space Administration (NASA) has long given high priority to missions that ask whether extraterrestrial life might exist in the solar system and beyond. That priority reflects public interest, which was enhanced in the mid-1990s when fragments of Mars delivered to Earth as meteorites were shown to contain small structures reminiscent of microbial life. 

Sagan, C. 1973. Extraterrestrial life. Pp. 42-67 in Communication with Extraterrestrial Intelligence CETI (C. Sagan, ed.). MIT Press, Cambridge, Mass. 

Earth may be just one of many models of planets that can evolve complex life. We do not know whether it is even practical or logical to assume that planets that exist outside our perception of a habitable zone could harbor life, particularly life that we know nothing about. Our practical search for extraterrestrial life is focused on water-rich planets and moons because of the possibility that they can support Earth-like life. That does not preclude... 

Cl chondrites contain 6% of their cosmic complement of carbon, mainly in the form of organic matter. The intense controversy that once surrounded the origin of this organic matter has subsided. Most authors now agree that this material represents primitive prebiotic matter, not vestiges of extraterrestrial life. The principal questions remaining are what abiotic processes formed the organic matter, and to what extent these processes took place in locales other than the solar nebula interstellar clouds or meteorite parent bodies. 

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