The prospect of alien life in exotic forms on other worlds

Abstract

The nature of life on Earth provides a singular example of carbon-based, water-borne, photosynthesis-driven biology. Within our understanding of chemistry and the physical laws governing the universe, however, lies the possibility that alien life could be based on different chemistries, solvents, and energy sources from the one example provided by Terran biology. In this paper, we review some of these possibilities. Silanes may be used as functional analogs to carbon molecules in environments very different from Earth; solvents other than water may be compatible for life-supporting processes, especially in cold environments, and a variety of energy sources may be utilized, some of which have no Terran analog. We provide a detailed discussion of two possible habitats for alien life which are generally not considered as such: the lower cloud level of the Venusian atmosphere and Titan's surface environment.

A química da vida como nós não conhecemos

“…48 Dentre eles, a amônia é frequentemente o solvente mais citado. 49 A amônia, como a água, dissolve diversos compostos orgânicos. Muitas reações orgânicas preparadas em laboratório são feitas com este solvente.…”

“…Os hidrocarbonetos não polares, como metano e etano, são melhores do que água para a manutenção de reações químicas complexas, uma vez que não destroem hidroliticamente espécies orgânicas instáveis. 49 Apesar de sua importância para a química da vida terrestre, na maioria das vezes, os químicos usam um solvente diferente da água para executar suas reações, pois ela própria é reativa, apresentando tanto um oxigênio nucleofílico quanto um hidrogênio ácido em concentrações de 55 molares.…”

Tables of Rosseland mean opacities for candidate atmospheres of life hosting free-floating planets

“…A liquid environment is needed because macromolecules need to be physically stable, yet capable of structural flexibility and chemical interactivity, which in practice means that they are able to move with respect to each other but are kept in constant close proximity [15,16]. This can occur only in a liquid state.…”

“…This can occur only in a liquid state. The functions of a good solvent include: (1) an environment that allows for the stability of some chemical bonds to maintain macromolecular structure, while (2) promoting the dissolution of other chemical bonds with sufficient ease to enable frequent chemical interchange and energy transformations from one molecular state to another; (3) the ability to dissolve many solutes while enabling some macromolecules to resist dissolution, thereby, providing boundaries, surfaces, interfaces, and stereochemical stability; (4) a density sufficient to maintain critical concentrations of reactants and constrain their dispersal; (5) a medium that provides both an upper and lower limit to the temperatures and pressures at which biochemical reactions operate, thereby, funneling the evolution of metabolic pathways into a narrower range optimized for multiple interactions; and (6) a buffer against environmental fluctuations [16]. Water is the liquid environment required by life on Earth.…”

An Extensive Phase Space for the Potential Martian Biosphere

“…The apparent high-pressure limit to life beneath oceanic and continental crust seems to be a limit of observations, not of life ( Jones and Lineweaver, 2010). There is theoretical discussion that liquids other than water could support different forms of life with unique biochemistries (Irwin and Schulze-Makuch, 2001;Bains, 2004;Benner et al, 2004;Schulze-Makuch and Irwin, 2006). For the purposes of this study, however, we have assumed that liquid H 2 O is the most plausible life-sustaining fluid for Mars.…”