This article is about the mainstream hypothesis that the organic building-blocks of life originated in space. For the fringe theory that life permeates the universe and gave rise to life on Earth, seePanspermia.
Pseudo-panspermia (sometimes calledsoft panspermia,molecular panspermia orquasi-panspermia) is a well-supported hypothesis for a stage in theorigin of life. The theory first asserts that many of the smallorganic molecules used for life originated inspace (for example, being incorporated in thesolar nebula, from which theplanets condensed). It continues that these organic molecules were distributed to planetary surfaces, where life then emerged onEarth andperhaps on other planets. Pseudo-panspermia differs from thefringe theory ofpanspermia, which asserts that life arrived on Earth from distant planets.[1]
Some stages in theorigin of life are well-understood, such as thehabitable Earth and the abiotic synthesis of simple molecules, whether in space or on Earth. Later stages remain more speculative.[2]
Theories of the origin of life have been recorded since the 5th century BC, when the Greek philosopherAnaxagoras proposed an initial version of panspermia: life arrived on earth from the heavens.[3] In modern times, full panspermia has little support amongstmainstream scientists.[1] Pseudo-panspermia, in which molecules are formed and transported through space is, however, well-supported.[2]
Interstellar molecules are formed by chemical reactions within very sparse interstellar or circumstellar clouds of dust and gas. Usually this occurs when a molecule becomesionised, often as the result of an interaction withcosmic rays. This positively charged molecule then draws in a nearby reactant by electrostatic attraction of the neutral molecule's electrons. Molecules can also be generated by reactions between neutral atoms and molecules, although this process is generally slower.[4] The dust plays a critical role of shielding the molecules from the ionizing effect of ultraviolet radiation emitted by stars.[5] TheMurchison meteorite contains the organic moleculesuracil andxanthine,[6][7] which must therefore already have been present in the early Solar System, where they could have played a role in the origin of life.[8]
Nitriles, key molecular precursors of theRNA World scenario, are among the most abundant chemical families in the universe and have been found in molecular clouds in the center of the Milky Way, protostars of different masses, meteorites and comets, and also in the atmosphere of Titan, the largest moon of Saturn.[9][10]
Evidence for the extraterrestrial creation of organic molecules includes both their discovery in various contexts in space, and their laboratory synthesis under extraterrestrial conditions:
Organic molecules can then be distributed to planets including Earth both when the planets formed and later. If the materials from which planets formed contained organic molecules, and were not destroyed by heat or other processes, then these would be available for abiogenesis on those planets.
Later distribution is by means of bodies such ascomets andasteroids. These may fall to the planetary surface asmeteorites, releasing any molecules they are carrying as they vaporise on impact or later as they erode.
Studies of rock and dust from asteroidBennu delivered to Earth by NASA’sOSIRIS-REx have revealed molecules that, on Earth, are key to life, as well as a history of saltwater.[27]
Findings of organic molecules in meteorites include:
^abMay, Andrew (2019).Astrobiology: The Search for Life Elsewhere in the Universe. London: Icon Books.ISBN978-1-78578-342-5.OCLC999440041.Although they were part of the scientific establishment – Hoyle at Cambridge and Wickramasinghe at the University of Wales – their views on the topic were far from mainstream, and panspermia remains a fringe theory
^Brown, Laurie M.; Pais, Abraham; Pippard, A. B. (1995). "The physics of the interstellar medium".Twentieth Century Physics (2nd ed.). CRC Press. p. 1765.ISBN978-0-7503-0310-1.
