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Tholins (after the Greek θολός (tholós) "hazy" or "muddy"; from the ancient Greek word meaning "sepia ink") are a wide variety of organic compounds formed by solar ultraviolet irradiation or cosmic rays from simple carbon-containing compounds such as carbon dioxide (CO
), methane (CH
) or ethane (C
), often in combination with nitrogen (N
).[1] Tholins do not form naturally on modern-day Earth, but they are found in great abundance on the surface of icy bodies in the outer Solar System, and as reddish aerosols in the atmosphere of outer Solar System planets and moons.

When in the presence of water, tholins may facilitate the formation of prebiotic chemistry to take place, and has implication of the origins of life on Earth and possibly, on other planets. As particles in an atmosphere, tholins scatter light and can affect habitability.


The term "tholin" was coined by astronomer Carl Sagan and his colleague Bishun Khare to describe the difficult-to-characterize substances they obtained in his Miller-Urey-type experiments on the methane-containing gas mixtures such as those found in Titan's atmosphere.[2] 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 gases CH
, C
, NH
, H
, HCHO, and H
. 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". However, we have recently succeeded, through sequential and non-sequential pyrolysis followed by gas chromatography-mass spectrometry (GC-MS) in determining something of the composition of this material. It is clearly not a polymer—a repetition of the same monomeric unit—and some other term is needed... We propose, as a model-free descriptive term, ‘tholins’ (Gk ϴὸλος, muddy; but also ϴoλòς, vault or dome), although we were tempted by the phrase ‘star-tar’.[1][2]

Tholins are not one specific compound but rather are descriptive of a spectrum of molecules that give a reddish, organic surface covering on certain planetary surfaces. 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."[2]

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!)."[1]


The formation of tholins in the atmosphere of Titan

Tholins may be a major constituent of the interstellar medium.[2] Their key elements are carbon, nitrogen, and hydrogen. Laboratory infrared spectroscopy analysis of experimentally synthetized tholins has confirmed earlier identifications of chemical groups present, including primary amines, nitriles, and alkyl portions such as CH
forming complex disordered macromolecular solids. Laboratory tests generated complex solids formed from exposure of N
gaseous mixtures to electrical discharge in cold plasma conditions, reminiscent of the famous Miller–Urey experiment conducted in 1952.[3]

As illustrated to the right, tholins are thought to form in nature through a chain of chemical reactions known as pyrolysis and radiolysis. This begins with the dissociation and ionization of molecular nitrogen (N
) and methane (CH
) by energetic particles and solar radiation. This is followed by the formation of ethylene, ethane, acetylene, hydrogen cyanide, and other small simple molecules and small positive ions. Further reactions form benzene 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.[4] 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.[5][6]

These atmospherically-derived substances are distinct from ice tholin II, which are formed instead by irradiation (radiolysis) of clathrates of water and organic compounds such as methane (CH
) or ethane (C
).[7][8] The radiation-induced synthesis on ice are non-dependant on temperature.[7]

Biological significance

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[2][7] (see Miller-Urey experiment for discussion related to this.) Tholins do not exist naturally on present-day Earth due to the oxidizing character of the free oxygen component of its atmosphere ever since the Great Oxygenation Event around 2.4 billion years ago.[9]

Laboratory experiments[10] suggest that tholins near large pools of liquid water that might persist for thousands of years might facilitate the formation of prebiotic chemistry to take place,[11][1] and has implication of the origins of life on Earth and possibly, on other planets.[1][9] Also, as particles in the atmosphere of an exoplanet, tholins affect the light scatter and act as a screen for protecting planetary surfaces from ultraviolet radiation, affecting habitability.[1][12] Laboratory simulations found derived residues related to amino acids as well as urea, with important astrobiological implications.[9][10][13]

On Earth, a wide variety of soil bacteria are able to use tholins as their sole source of carbon. Tholins could have been the first microbial food for heterotrophic microorganisms before autotrophy evolved.[14]


Sagan and Khare note the presence of tholins through multiple locations: "as a constituent of the Earth's primitive oceans and therefore relevant to the origin of life; as a component of red aerosols in the atmospheres of the outer planets and Titan; present in comets, carbonaceous chondrites, and pre-planetary solar nebulae; and as a major constituent of the interstellar medium."[2] The surfaces of comets, centaurs, and many icy moons and Kuiper-belt objects in the outer Solar System are rich in deposits of tholins.[15]


Linear fractures on Europa's surface, likely colored by tholins.

Colored regions on Jupiter's satellite Europa are thought to be tholins.[11][16][17][18] The morphology of Europa's impact craters and ridges is suggestive of fluidized material welling up from the fractures where pyrolysis and radiolysis 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 make the reactions occur. Impurities in the water ice crust of Europa are presumed both to emerge from the interior as cryovolcanic events that resurface the body, and to accumulate from space as interplanetary dust.[11]

Surface of Titan as viewed from the Huygens probe. Tholins are suspected to be the source of the reddish color of both the surface and the atmospheric haze.


The trailing hemisphere of Saturn's moon Rhea is covered with tholins.

Titan tholins are nitrogen-rich[19][20] 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.[21] In the case of Titan, the haze and orange-red color of its atmosphere is thought to be caused by the presence of tholins.[4]

Triton, Pluto, and on Kuiper belt objects

Neptune's moon Triton is observed to have the reddish color characteristic of tholins.[19] Triton's atmosphere is mostly nitrogen, with trace amounts of methane and carbon monoxide.[22][23] Tholins also occur on the dwarf planet Pluto[24] and its moon Charon[25][26] and are responsible for their red colors as well as the blue tint of Pluto's atmosphere.[27]

In September 2016, astronomers announced that the reddish-brown cap of the north pole of Charon, the largest of five moons that orbit the dwarf planet Pluto, is composed of tholins, organic macromolecules that may be essential ingredients of life, and 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.[26]

Tholins were also detected by the Rosetta mission to comet 67P/Churyumov–Gerasimenko,[28][29] and on the plutino Ixion.[30]

Tholins beyond the Solar System

Tholins might have also been detected in the stellar system of the young star HR 4796A using the Near-Infrared Camera and Multi-Object Spectrometer (NICMOS) aboard the Hubble Space Telescope.[31] The HR 4796 system is approximately 220 light years from Earth.[32]

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 typical interstellar cloud.[7]

See also


  1. ^ a b c d e f Sarah Hörst "What in the world(s) are tholins?", Planetary Society, July 23, 2015. Retrieved 30 Nov 2016.
  2. ^ a b c d e f Sagan, Carl & Khare, Bishun (11 January 1979). "Tholins: organic chemistry of interstellar grains and gas". Nature. 277 (5692): 102–107. Bibcode:1979Natur.277..102S. doi:10.1038/277102a0. 
  3. ^ 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. doi:10.1016/j.icarus.2008.07.012. 
  4. ^ a b Waite, J.H.; Young, D.T.; Cravens, T.E.; Coates, A.J.; Crary, F.J.; Magee, B.; Westlake, J. (2007). "The process of tholin formation in Titan's upper atmosphere". Science. 316 (5826): 870–5. Bibcode:2007Sci...316..870W. doi:10.1126/science.1139727. PMID 17495166. 
  5. ^ McGuigan, M.; Sacks, R.D. (9 March 2004). "Comprehensive Two Dimensional Gas Chromatography Study of Tholin Samples Using Pyrolysis Inlet and TOF-MS Detection". Pittcon Conference & Expo. 
  6. ^ McGuigan, M.A.; Waite, J.H.; Imanaka, H.; Sacks, R.D. (2006). "Analysis of Titan tholin pyrolysis products by comprehensive two-dimensional gas chromatography-time-of-flight mass spectrometry". J. Chromatogr. 1132 (1-2): 280–288. doi:10.1016/j.chroma.2006.07.069. 
  7. ^ a b c d McDonald, G.D.; Whited, L.J.; DeRuiter, C.; Khare, B.N.; Patnaik, A.; Sagan, C. "Production and chemical analysis of cometary ice tholins". Icarus. 122 (1): 107–117. Bibcode:1996Icar..122..107M. doi:10.1006/icar.1996.0112. 
  8. ^ A spectroscopic study of the surfaces of Saturn's large satellites: H2O ice, tholins, and minor constituents (PDF). Dale P. Cruikshank, Tobias. Owen, Cristina Dalle Ore, Thomas R. Geballe, Ted L. Roush, Catherine de Bergh, Scott A. Sandford, Francois Poulet, Gretchen K. Benedix, Joshua P. Emery. Icarus, 175, pages: 268–283, 2 March 2005.
  9. ^ a b c Atmospheric Prebiotic Chemistry and Organic Hazes. Current Organic Chemistry. 2013 August; Volume: 17, pages 1710–1723. doi:10.2174/13852728113179990078
  10. ^ a b Prebiotic chemistry on Titan ? The nature of Titan's aerosols and their potential evolution at the satellite surface. Coll, P. J.; Poch, O.; Ramirez, S. I.; Buch, A.; Brassé, C.; Raulin, F. American Geophysical Union, Fall Meeting 2010, abstract #P31C-1551
  11. ^ a b c A new energy source for organic synthesis in Europa's surface ice. Jerome G. Borucki, Bishun Khare, Dale P. Cruikshank. Journal of Geophysical Research, 23 November 2002. doi: 10.1029/2002JE001841
  12. ^ "Mooning over Titan's atmosphere". SpectroscopyNOW. 15 October 2006. 
  13. ^ Prebiotic synthesis of protobiopolymers under alkaline ocean conditions. Ruiz-Bermejo, M.; Rivas, L.A.; Palacín, A.; Menor-Salván, C.; Osuna-Esteban, S. Orig Life Evol Biosph. 2011 August; Vol: 41, Number: 4, pages: 331-345. doi: 10.1007/s11084-010-9232-z
  14. ^ Stoker, C.R.; Boston, P.J.; Mancinelli, R.L.; Segal, W.; Khare, B.N.; Sagan, C. (1990). "Microbial metabolism of tholin". Icarus. 85 (1): 241–256. Bibcode:1990Icar...85..241S. doi:10.1016/0019-1035(90)90114-O. 
  15. ^ Sublimation of water ice mixed with silicates and tholins: Evolution of surface texture and reflectance spectra, with implications for comets. Olivier Pocha, Antoine Pommerol, Bernhard Jost, Nathalie Carrasco, Cyril Szopac, Nicolas Thomas. Icarus, Volume 267, 15 March 2016, Pages 154–173.
  16. ^ MISE: A Search for Organics on Europa. Whalen, Kelly; Lunine, Jonathan I.; Blaney, Diana L. American Astronomical Society, AAS Meeting #229, id.138.04. January 2017.
  17. ^ "Europa Mission to Probe Magnetic Field and Chemistry". Jet Propulsion Laboratory. 27 May 2015. Retrieved 2017-10-23. 
  18. ^ Impact Driven Chemistry on Europa's Surface. Khare, B. N.; NNa Mvondo, D.; Borucki, J. G.; Cruikshank, D. P.; Belisle, W. A.; Wilhite, P.; McKay, C. P. American Astronomical Society, DPS meeting #37, id.58.10; Bulletin of the American Astronomical Society, Vol. 37, p.753.
  19. ^ a b Gene D. McDonald, W.Reid Thompson, Michael Heinrich, Bishun N. Khare, and Carl Sagan, "Chemical Investigation of Titan and Triton Tholins", Icarus Volume 108, Issue 1, March 1994, pp 137-145; doi:10.1006/icar.1994.1046. Retrieved 30 Nov 2016.
  20. ^ S. Derenne, C. Coelho, C. Anquetil, C. Szopa, A.S. Rahman, P.F. McMillan, F. Corà, C.J. Pickard, E. Quirico, C. Bonhomme, "New insights into the structure and chemistry of Titan’s tholins via13C and 15N solid state nuclear magnetic resonance spectroscopy", Icarus, Volume 221, Issue 2, November–December 2012, pp. 844–853. Retrieved 30 Nov. 2016
  21. ^ Coustenis, Athena; Taylor, Frederic W. (2008). Titan: Exploring an Earthlike World. World Scientific. pp. 154–155. ISBN 978-981-270-501-3. 
  22. ^ [ Neptune’s Moon Triton]. Matt Williams, Universe Today. 16 October 2016.
  23. ^ Triton: In Depth. Bill Dunford, NASA Planetary Science Division.
  24. ^ "Pluto: The 'Other' Red Planet". NASA. 3 July 2015. Retrieved 2015-07-06. 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
    ) in Pluto's atmosphere, powering chemical reactions that create complex compounds called tholins.
  25. ^ Albert, P.T. (9 September 2015). "New Horizons Probes the Mystery of Charon's Red Pole". NASA. Retrieved 2015-09-09. 
  26. ^ a b Bromwich, Jonah Engel; St. Fleur, Nicholas (14 September 2016). "Why Pluto's Moon Charon Wears a Red Cap". New York Times. Retrieved 14 September 2016. 
  27. ^ Amos, Jonathan (8 October 2015). "New Horizons: Probe captures Pluto's blue hazes". BBC News. 
  28. ^ OSIRIS observations of meter-sized exposures of H2O ice at the surface of 67P/Churyumov-Gerasimenko and interpretation using laboratory experiments. A Pommerol, N. Thomas, M. R. El-Maarry, M. Pajola, et al. Astronomy and Astrphysics, Volume 583, November 2015.
  29. ^ CHO-bearing organic compounds at the surface of 67P/Churyumov-Gerasimenko revealed by Ptolemy. Science, Vol 349, Issue 6247, 31 July 2015.
  30. ^ H. Boehnhardt; et al. (2004). "Surface characterization of 28978 Ixion (2001 KX76)". Astronomy and Astrophysics Letters. 415 (2): L21–L25. Bibcode:2004A&A...415L..21B. doi:10.1051/0004-6361:20040005. 
  31. ^ Kohler, M.; Mann, I.; Li, A. (2008). "Complex organic materials in the HR 4796A disk?". The Astrophysical Journal. 686 (2): L95–L98. arXiv:0808.4113Freely accessible. Bibcode:2008ApJ...686L..95K. doi:10.1086/592961. 
  32. ^ "Red dust in disk may harbor precursors to life". Spaceflight Now. 5 January 2008. 
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