- Tholin
Tholin (after the Greek word for muddy), is a
heteropolymer formed by solarultraviolet irradiation of simpleorganic compound s such asmethane orethane . Tholins do not form naturally on modern-dayEarth , but are found in great abundance on the surface of icy bodies in the outersolar system . They usually have reddish-brown appearance.Tholins have also been detected in the solar system of an eight-million-year-old star known as HR 4796A using the Near-Infrared Camera and Multi-Object Spectrometer (NICMOS) aboard the Hubble Space Telescope. HR 4796A is 220 light years from Earth. [ [http://www.spaceflightnow.com/news/n0801/05disk/ Spaceflight Now | Breaking News | Red dust in disk may harbor precursors to life ] ]
"Triton tholin" and "Titan tholin" are
nitrogen -rich organic substances produced by the irradiation of gaseous mixtures of nitrogen and methane such as that found in those moons' atmospheres; Triton's atmosphere is 99.9% nitrogen and 0.1% methane and Titan's atmosphere is 98.4% nitrogen and the remaining 1.6% composed ofmethane and trace amounts of other gases. These atmospherically derived substances are distinct from "ice tholin", which is formed by irradiation ofclathrate s ofwater and organic compounds such as methane or ethane. Theplutino Ixion is also high in this compound.The surfaces of
comet s, centaurs, and many icy moons in the outer solar system are rich in deposits of Triton, Titan and ice tholins. The haze and orange-red color of Titan's atmosphere and Centaur-classplanetoid s is thought to be caused by the presence of tholins. Tholins may also have been detected in theprotoplanetary disk of young stars; seeHR 4796A . Some researchers have speculated that Earth may have been seeded by organic compounds early in its development by tholin-rich comets, providing the raw material necessary for life to develop; seeUrey-Miller experiment for discussion related to this issue. Tholins do not exist naturally on current-day Earth due to the oxidizing character of its atmosphere.A theoretical model explains formation of tholins by the dissociation and ionization of molecular
nitrogen andmethane by energetic particles and solar radiation, formation ofethylene ,ethane ,acetylene ,hydrogen cyanide , and other small simple molecules and small positive ions, further formation ofbenzene and other organic molecules, their polymerization and formation of aerosol of heavier molecules, which then coagulate and deposit on the planetary surface. [ [http://www.daviddarling.info/encyclopedia/T/tholin.html tholin ] ]The term "tholin" (from the Greek word meaning "muddy") was coined by famed astronomer
Carl Sagan to describe the difficult-to-characterize substances he obtained in his Urey-Miller-type experiments on the gas mixtures that are found in Titan's atmosphere. [cite journal| title =Tholins: organic chemistry of interstellar grains and gas|author =Carl Sagan & B. N. Khare| doi =10.1038/277102a0|journal= Nature |volume =277|issue= 5692|year= 1979| pages =102] It is not a specific compound but is a term generally used to describe the reddish, organic component of planetary surfaces.Tholins formed at low pressure tend to contain nitrogen atoms inside their molecules while tholins formed at high pressure display nitrogen atoms more likely located in terminal positions. [ [http://ca.pittcon.org/technical+program/tpabstra04.nsf/Agenda+Time+Slots+Web/233224B2F6F17D3785256D9E00725026?Opendocument Abstract ] ]
Tholins can act as an effective screen for
ultraviolet radiation, protecting the planetary surface from it. [ [http://www.spectroscopynow.com/coi/cda/detail.cda?chId=4&id=14793&type=Feature&page=1 Mass Spectrometry - Base Peak - The web's leading Mass Spectrometry Resource ] ]A wide variety of soil
bacteria are able to use tholins as their sole source of carbon. It is thought tholins may have been the first microbial food forheterotrophic microorganisms beforeautotrophy evolved. [ [http://adsabs.harvard.edu/abs/1990Icar...85..241S Microbial metabolism of tholin ] ]ee also
*
Kerogen
*PAH world hypothesis References
*Citation
last1 = Waite | first1 = J.H.
last2 = Young | first2 = D.T.
last3 = Cravens | first3 = T.E.
last4 = Coates | first4 = A.J.
title = The process of tholin formation in Titan's upper atmosphere
journal = Science
volume = 316
issue = 5826
pages = 870–5
date =2007-05-11
year = 2007
doi = 10.1126/science.1139727
author = Waite, J. H.
pmid = 17495166
Wikimedia Foundation. 2010.