Class of molecules formed by ultraviolet irradiation of organic compounds
Mordor Macula, a dark region onCharon's north pole. The region is stained a dark brown by deposits of tholins
Tholins (after the Greekθολός (tholós) "hazy" or "muddy";[1] from theancient Greek word meaning "sepia ink") are a wide variety oforganic compounds formed by solarultraviolet orcosmic ray irradiation of simple carbon-containing compounds such ascarbon dioxide (CO 2),methane (CH 4) orethane (C 2H 6), often in combination withnitrogen (N 2) orwater (H 2O).[2][3] Tholins are disordered polymer-like materials made of repeating chains of linked subunits and complex combinations of functional groups, typicallynitriles andhydrocarbons, and their degraded forms such asamines andphenyls. Tholins do not form naturally on modern-dayEarth, but they are found in great abundance on the surfaces of icy bodies in the outerSolar System, and as reddishaerosols in the atmospheres of outer Solar System planets and moons.
In the presence of water, tholins could be raw materials for prebiotic chemistry (i.e., the non-living chemistry that forms the basic chemicals of which life is made). Their existence has implications for theorigins of life on Earth and possibly on other planets. As particles in an atmosphere, tholins scatter light, and can affecthabitability.
Tholins may be produced in a laboratory, and are usually studied as a heterogeneous mixture of many chemicals with many different structures and properties. Using techniques likethermogravimetric analysis,astrochemists analyze the composition of these tholin mixtures, and the exact character of the individual chemicals within them.[4]
Polyacrylonitrile, one hypothesized polymeric component of tholins, mostly in chemically degraded form as polymers containingnitrile andamino groups. It is used experimentally to create tholin mixtures.[4]
The term "tholin" was coined by astronomerCarl Sagan and his colleagueBishun Khare to describe the difficult-to-characterize substances they obtained in hisMiller–Urey-type experiments on the methane-containing gas mixtures such as those found inTitan's atmosphere.[1] Their paper proposing the name "tholin" said:
For the past decade we have been producing in our laboratory a variety of complex organic solids from mixtures of the cosmically abundant gasesCH 4,C 2H 6,NH 3,H 2O, HCHO, andH 2S. The product, synthesized by ultraviolet (UV) light or spark discharge, is a brown, sometimes sticky, residue, which has been called, because of its resistance to conventional analytical chemistry, "intractable polymer". [...] We propose, as a model-free descriptive term, 'tholins' (Greek Θολός, muddy; but also Θόλος, vault or dome), although we were tempted by the phrase 'star-tar'.[3][1]
Tholins are not one specific compound but rather are descriptive of a spectrum of molecules, includingheteropolymers,[5][6] that give a reddish, organic surface covering on certain planetary surfaces. Tholins are disordered polymer-like materials made of repeating chains of linked subunits and complex combinations of functional groups.[7] Sagan and Khare note "The properties of tholins will depend on the energy source used and the initial abundances of precursors, but a general physical and chemical similarity among the various tholins is evident."[1]
Some researchers in the field prefer a narrowed definition of tholins, for example S. Hörst wrote: "Personally, I try to use the word 'tholins' only when describing the laboratory-produced samples, in part because we do not really know yet how similar the material we produce in the lab is to the material found on places like Titan or Triton (or Pluto!)."[3] French researchers also use the term tholins only when describing the laboratory-produced samples as analogues.[8] NASA scientists also prefer the word 'tholin' for the products of laboratory simulations, and use the term 'refractory residues' for actual observations on astronomical bodies.[7]
The key elements of tholins are carbon, nitrogen, and hydrogen. Laboratory infrared spectroscopy analysis of experimentally synthesized tholins has confirmed earlier identifications of chemical groups present, including primaryamines,nitriles, andalkyl portions such asCH 2/CH 3 forming complex disordered macromolecular solids. Laboratory tests generated complex solids formed from exposure ofN 2:CH 4 gaseous mixtures to electrical discharge in cold plasma conditions, reminiscent of the famousMiller–Urey experiment conducted in 1952.[9]
As illustrated to the right, tholins are thought to form in nature through a chain of chemical reactions known aspyrolysis andradiolysis. This begins with thedissociation andionization of molecularnitrogen (N 2) andmethane (CH 4) by energetic particles and solar radiation. This is followed by the formation ofethylene,ethane,acetylene,hydrogen cyanide, and other small simple molecules and small positive ions. Further reactions formbenzene and other organic molecules, and their polymerization leads to the formation of an aerosol of heavier molecules, which then condense and precipitate on the planetary surface below.[10]
Tholins formed at low pressure tend to contain nitrogen atoms in the interior of their molecules, while tholins formed at high pressure are more likely to have nitrogen atoms located in terminal positions.[11][12]
Tholins may be a major constituent of theinterstellar medium.[1] On Titan, their chemistry is initiated at high altitudes and participates in the formation of solid organic particles.[8]
These atmospherically derived substances are distinct fromice tholin II, which are formed instead by irradiation (radiolysis) ofclathrates ofwater and organic compounds such as methane (CH 4) or ethane (C 2H 6).[2][13] The radiation-induced synthesis on ice are independent of temperature.[2]
Models show that, even when far from UV radiation of a star,cosmic ray doses may be fully sufficient to convert carbon-containing ice grains entirely to complex organics in less than the lifetime of the typicalinterstellar cloud.[2]
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.[1][2] (SeeMiller–Urey experiment for discussion.) Tholins do not exist naturally on present-day Earth due to the oxidizing properties of the free oxygen component of its atmosphere ever since theGreat Oxygenation Event around 2.4 billion years ago.[14]
Laboratory experiments[15] suggest that tholins near large pools of liquid water that might persist for thousands of years could facilitate the formation of prebiotic chemistry to take place,[16][3] and has implications for theorigins of life on Earth and possibly other planets.[3][14] Also, as particles in the atmosphere of anexoplanet, tholins affect the light scatter and act as a screen for protecting planetary surfaces fromultraviolet radiation, affectinghabitability.[3][17] Laboratory simulations found derived residues related toamino acids as well asurea, with importantastrobiological implications.[14][15][18]
On Earth, a wide variety ofsoil bacteria are able to use laboratory-produced tholins as their sole source of carbon. Tholins could have been the first microbial food forheterotrophic microorganisms beforeautotrophy evolved.[19][20]
The surface of Titan as viewed from theHuygens lander. Tholins are suspected to be the source of the reddish color of both the surface and the atmospheric haze.
Sagan and Khare note the presence of tholins through multiple locations: "as a constituent of the Earth's primitive oceans and therefore relevant to theorigin of life; as a component of red aerosols in the atmospheres of the outer planets and Titan; present incomets, carbonaceous chondrites asteroids, and pre-planetary solar nebulae; and as a major constituent of theinterstellar medium."[1] The surfaces of comets,centaurs, and many icy moons andKuiper-belt objects in the outer Solar System are rich in deposits of tholins.[21]
Titan tholins are nitrogen-rich[22][23] organic substances produced by the irradiation of the gaseous mixtures of nitrogen and methane found in the atmosphere and surface of Titan. Titan's atmosphere is about 97% nitrogen, 2.7±0.1% methane and the remaining trace amounts of other gases.[24] In the case of Titan, the haze and orange-red color of its atmosphere are both thought to be caused by the presence of tholins.[10][25]
Linear fractures on Europa's surface, likely colored by tholins.
Colored regions on Jupiter's satelliteEuropa are thought to be tholins.[16][26][27][28] The morphology of Europa's impact craters and ridges is suggestive of fluidized material welling up from the fractures wherepyrolysis andradiolysis take place. In order to generate colored tholins on Europa there must be a source of materials (carbon, nitrogen, and water), and a source of energy to drive the reactions. Impurities in the water ice crust of Europa are presumed both to emerge from the interior ascryovolcanic events that resurface the body, and to accumulate from space as interplanetary dust.[16]
The trailing hemisphere of Saturn's moonRhea is covered with tholins.Close-up view of Sputnik Planitia on Pluto as viewed by theNew Horizons spacecraft, showing nitrogen ice glaciers and reddish-colored tholins.
The extensive dark areas on the trailing hemisphere of Saturn's moonRhea are thought to be deposited tholins.[13]
Neptune's moonTriton is observed to have the reddish color characteristic of tholins.[22] Triton's atmosphere is mostly nitrogen, with trace amounts of methane and carbon monoxide.[29][30]
Tholins occur on thedwarf planetPluto[31] and are responsible for red colors[32] as well as the blue tint of theatmosphere of Pluto.[33] The reddish-brown cap of the north pole ofCharon,[3] the largest of fivemoons of Pluto, is thought to be composed of tholins, produced from methane, nitrogen and related gases released from the atmosphere of Pluto and transferred over about 19,000 km (12,000 mi) distance to the orbiting moon.[34][35][36]
Tholins were detected on the dwarf planetCeres by theDawn mission.[37][38] Most of the planet's surface is extremely rich in carbon, with approximately 20% carbon by mass in its near surface.[39][40] The carbon content is more than five times higher than incarbonaceous chondrite meteorites analyzed on Earth.[40]
^abNna-Mvondo, Delphine; de la Fuente, José L.; Ruiz-Bermejo, Marta; Khare, Bishun; McKay, Christopher P. (September 2013). "Thermal characterization of Titan's tholins by simultaneous TG–MS, DTA, DSC analysis".Planetary and Space Science.85:279–288.Bibcode:2013P&SS...85..279N.doi:10.1016/j.pss.2013.06.025.
^Eric Quirico; Gilles Montagnac; Victoria Lees; Paul F. McMillan; Cyril Szopa; Guy Cernogora; Jean-Noël Rouzaud; Patrick Simon; Jean-Michel Bernard; Patrice Coll; Nicolas Fray; Robert D. Minardi; François Raulin; Bruno Reynard; Bernard Schmitt (November 2008). "New experimental constraints on the composition and structure of tholins".Icarus.198 (1):218–231.Bibcode:2008Icar..198..218Q.doi:10.1016/j.icarus.2008.07.012.
^abColl, P. J.; Poch, O.; Ramirez, S. I.; Buch, A.; Brassé, C.; Raulin, F. (2010). "Prebiotic chemistry on Titan ? The nature of Titan's aerosols and their potential evolution at the satellite surface".AGU Fall Meeting Abstracts.2010: P31C–1551.Bibcode:2010AGUFM.P31C1551C.
^Poch, Olivier; Pommerol, Antoine; Jost, Bernhard; Carrasco, Nathalie; Szopa, Cyril; Thomas, Nicolas (2016). "Sublimation of water ice mixed with silicates and tholins: Evolution of surface texture and reflectance spectra, with implications for comets".Icarus.267:154–173.Bibcode:2016Icar..267..154P.doi:10.1016/j.icarus.2015.12.017.S2CID56028928.
^Whalen, Kelly; Lunine, Jonathan I.;Blaney, Diana L. (2017). "MISE: A Search for Organics on Europa".American Astronomical Society Meeting Abstracts.229: 138.04.Bibcode:2017AAS...22913804W.
^Khare, B. N.; Nna Mvondo, D.; Borucki, J. G.; Cruikshank, D. P.; Belisle, W. A.; Wilhite, P.; McKay, C. P. (2005). "Impact Driven Chemistry on Europa's Surface".Bulletin of the American Astronomical Society.37: 753.Bibcode:2005DPS....37.5810K.
^"Triton".NASA Science. 21 November 2017. Retrieved14 November 2023.
^"Pluto: The 'Other' Red Planet".NASA. 3 July 2015. Retrieved2015-07-06.Experts have long thought that reddish substances are generated as a particular color of ultraviolet light from the sun, called Lyman-alpha, strikes molecules of the gas methane (CH 4) in Pluto's atmosphere, powering chemical reactions that create complex compounds called tholins.
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