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 or cosmic ray irradiation of simple carbon-containing compounds such as carbon dioxide (CO
2), methane (CH
4) or ethane (C
6), often in combination with nitrogen (N
2) or water (H
2O). Tholins are disordered polymer-like materials made of repeating chains of linked subunits and complex combinations of functional groups, typically nitriles and hydrocarbons and their degraded forms such as amines and phenyls. Tholins do not form naturally on modern-day Earth, but they are found in great abundance on the surfaces of icy bodies in the outer Solar System, and as reddish aerosols 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 the origins of life on Earth and possibly on other planets. As particles in an atmosphere, tholins scatter light, and can affect habitability.
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 like thermogravimetric analysis, astrochemists analyze the composition of these tholin mixtures, and the exact character of the individual chemicals within them.
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. 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
2O, HCHO, and H
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’ (Gk ϴὸλος, muddy; but also ϴoλòς, vault or dome), although we were tempted by the phrase ‘star-tar’.
Tholins are not one specific compound but rather are descriptive of a spectrum of molecules, including heteropolymers, 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. 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."
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!)." French researchers also use the term tholins only when describing the laboratory-produced samples as analogues. 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.
Tholins may be a major constituent of the interstellar medium. On Titan, their chemistry is initiated at high altitudes and participates in the formation of solid organic particles. 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
3 forming complex disordered macromolecular solids. Laboratory tests generated complex solids formed from exposure of N
4 gaseous mixtures to electrical discharge in cold plasma conditions, reminiscent of the famous Miller–Urey experiment conducted in 1952.
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
2) and methane (CH
4) 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. 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.
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
4) or ethane (C
6). The radiation-induced synthesis on ice are non-dependant on temperature.
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 (see Miller–Urey experiment for discussion related to this). Tholins do not exist naturally on present-day Earth due to the oxidizing properties of the free oxygen component of its atmosphere ever since the Great Oxygenation Event around 2.4 billion years ago.
Laboratory experiments 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, and has implications on the origins of life on Earth and possibly other planets. 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. Laboratory simulations found derived residues related to amino acids as well as urea, with important astrobiological implications.
On Earth, a wide variety of soil bacteria are able to use laboratory-produced tholins as their sole source of carbon. Tholins could have been the first microbial food for heterotrophic microorganisms before autotrophy evolved.
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 asteroids, and pre-planetary solar nebulae; and as a major constituent of the interstellar medium." The surfaces of comets, centaurs, and many icy moons and Kuiper-belt objects in the outer Solar System are rich in deposits of tholins.
Titan tholins are nitrogen-rich 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. 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.
Colored regions on Jupiter's satellite Europa are thought to be tholins. 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 drive the reactions. 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.
Tholins occur on the dwarf planet Pluto and are responsible for red colors as well as the blue tint of the atmosphere of Pluto. The reddish-brown cap of the north pole of Charon, the largest of five moons 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.
Tholins were detected on the dwarf planet Ceres by the Dawn mission. Most of the planet's surface is extremely rich in carbon, with approximately 20% carbon by mass in its near surface. The carbon content is more than five times higher than in carbonaceous chondrite meteorites analyzed on Earth.
Makemake exhibits methane, large amounts of ethane and tholins, as well as smaller amounts of ethylene, acetylene and high-mass alkanes may be present, most likely created by photolysis of methane by solar radiation.
Kuiper belt objects and Centaurs
The reddish color typical of tholins is characteristic of many Trans-Neptunian objects, including plutinos in the outer Solar System such as 28978 Ixion. Spectral reflectances of Centaurs also suggest the presence of tholins on their surfaces. The New Horizons exploration of the classical Kuiper belt object 486958 Arrokoth revealed reddish color at its surface, suggestive of tholins.
Comets and asteroids
Tholins were detected in situ by the Rosetta mission to comet 67P/Churyumov–Gerasimenko. Tholins are not typically characteristic of main-belt asteroids, but have been detected on the asteroid 24 Themis.
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. The HR 4796 system is approximately 220 light years from Earth.
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.
- Abiogenesis – Natural process by which life arises from non-living matter
- Asphaltene – Heavy organic molecular substances that are found in crude oil
- Hemolithin – Protein claimed to be of extraterrestrial origin
- Kerogen – Solid organic matter in sedimentary rocks
- PAH world hypothesis – Hypothesis about the origin of life
- "NASA's New Horizons Team Publishes First Kuiper Belt Flyby Science Results". NASA. 16 May 2019. Retrieved 16 May 2019.
- 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. S2CID 4261076.
- McDonald, G.D.; Whited, L.J.; DeRuiter, C.; Khare, B.N.; Patnaik, A.; Sagan, C. (1996). "Production and chemical analysis of cometary ice tholins". Icarus. 122 (1): 107–117. Bibcode:1996Icar..122..107M. doi:10.1006/icar.1996.0112.
- Sarah Hörst "What in the world(s) are tholins?", Planetary Society, July 23, 2015. Retrieved 30 Nov 2016.
- Nna-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.
- A Bit of Titan on Earth Helps in the Search for Life's Origins. Lori Stiles, University of Arizona. 19 October 2004.
- Cleaves, H. James; Neish, Catherine; Callahan, Michael P.; Parker, Eric; Fernández, Facundo M.; Dworkin, Jason P. (2014). "Amino acids generated from hydrated Titan tholins: Comparison with Miller–Urey electric discharge products". Icarus. 237: 182–189. Bibcode:2014Icar..237..182C. doi:10.1016/j.icarus.2014.04.042.
- Cruikshank, D.; et al. (New Horizons Composition Team) (January 2019). THE COLORS OF 486958 2014 MU69 ("ULTIMA THULE"): THE ROLE OF SYNTHETIC ORGANIC SOLIDS (THOLINS) (PDF). 50th Lunar and Planetary Science Conference 2019 (LPI Contrib. No. 2132).
- Dubois, David; Carrasco, Nathalie; Petrucciani, Marie; Vettier, Ludovic; Tigrine, Sarah; Pernot, Pascal (2019). "In situ investigation of neutrals involved in the formation of Titan tholins". Icarus. 317: 182–196. arXiv:1807.04569. Bibcode:2019Icar..317..182D. doi:10.1016/j.icarus.2018.07.006. S2CID 119446074.
- 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.CS1 maint: uses authors parameter (link)
- 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. S2CID 25984655.
- 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.
- 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". Journal of Chromatography A. 1132 (1–2): 280–288. doi:10.1016/j.chroma.2006.07.069. PMID 16934276.
- Cruikshank, D.; et al. (2005). "A spectroscopic study of the surfaces of Saturn's large satellites: HO ice, tholins, and minor constituents" (PDF). Icarus. 175 (1): 268–283. Bibcode:2005Icar..175..268C. doi:10.1016/j.icarus.2004.09.003.
- Trainer, Melissa (2013). "Atmospheric Prebiotic Chemistry and Organic Hazes". Current Organic Chemistry. 17 (16): 1710–1723. doi:10.2174/13852728113179990078. PMC 3796891. PMID 24143126.
- Coll, 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.
- Borucki, Jerome G.; Khare, Bishun; Cruikshank, Dale P. (2002). "A new energy source for organic synthesis in Europa's surface ice". Journal of Geophysical Research: Planets. 107 (E11): 24-1–24-5. Bibcode:2002JGRE..107.5114B. doi:10.1029/2002JE001841.
- "Mooning over Titan's atmosphere". SpectroscopyNOW. 15 October 2006.
- Ruiz-Bermejo, M.; Rivas, L. A.; Palacín, A.; Menor-Salván, C.; Osuna-Esteban, S. (2011). "Prebiotic synthesis of protobiopolymers under alkaline ocean conditions". Origins of Life and Evolution of the Biosphere. 41 (4): 331–45. Bibcode:2011OLEB...41..331R. doi:10.1007/s11084-010-9232-z. PMID 21161385. S2CID 19283373.
- 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. PMID 11538367.
- 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.
- McDonald, Gene D.; Thompson, W.Reid; Heinrich, Michael; Khare, Bishun N.; Sagan, Carl (1994). "Chemical Investigation of Titan and Triton Tholins". Icarus. 108 (1): 137–145. Bibcode:1994Icar..108..137M. doi:10.1006/icar.1994.1046. PMID 11539478.
- Derenne, S.; Coelho, C.; Anquetil, C.; Szopa, C.; Rahman, A.S.; McMillan, P.F.; Corà, F.; Pickard, C.J.; Quirico, E.; Bonhomme, C. (2012). "New insights into the structure and chemistry of Titan's tholins via 13C and 15N solid state nuclear magnetic resonance spectroscopy" (PDF). Icarus. 221 (2): 844–853. Bibcode:2012Icar..221..844D. doi:10.1016/j.icarus.2012.03.003.
- Coustenis, Athena; Taylor, Frederic W. (2008). Titan: Exploring an Earthlike World. World Scientific. pp. 154–155. ISBN 978-981-270-501-3.
- 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.
- "Europa Mission to Probe Magnetic Field and Chemistry". Jet Propulsion Laboratory. 27 May 2015. Retrieved 2017-10-23.
- 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.
- Neptune’s Moon Triton. Matt Williams, Universe Today. 16 October 2016.
- Triton: In Depth. Bill Dunford, NASA Planetary Science Division.
- "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
4) in Pluto's atmosphere, powering chemical reactions that create complex compounds called tholins.
- "NASA released an incredibly detailed photo of snow - and something else - on Pluto", Business Insider Australia, Mar. 6, 2016 (accessed 28 Feb. 2018).
- Amos, Jonathan (8 October 2015). "New Horizons: Probe captures Pluto's blue hazes". BBC News.
- Albert, P.T. (9 September 2015). "New Horizons Probes the Mystery of Charon's Red Pole". NASA. Retrieved 2015-09-09.
- 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.
- H. S. Shi, I. L. Lai, and W. H. Ip (2019). The Long-Term Evolution of Pluto's Atmosphere and Its Effect on Charon's Surface Tholin Formation (PDF). Pluto System After New Horizons 2019 (LPI Contrib. No. 2133).CS1 maint: uses authors parameter (link)
- "Dawn discovers evidence for organic material on Ceres (Update)". Phys.org. 16 February 2017. Retrieved 17 February 2017.
- Combe, Jean-Philippe; et al. (2019). "The surface composition of Ceres' Ezinu quadrangle analyzed by the Dawn mission". Icarus. 318: 124–146. Bibcode:2019Icar..318..124C. doi:10.1016/j.icarus.2017.12.039.
- Team finds evidence for carbon-rich surface on Ceres. Southwest Research Institute. Published by PhysOrg. 10 December 2018.
- Marchi, S.; et al. (2019). "An aqueously altered carbon-rich Ceres". Nature Astronomy. 3 (2): 140–145. Bibcode:2018NatAs.tmp..181M. doi:10.1038/s41550-018-0656-0. S2CID 135013590.
- Mike Brown; K. M. Barksume; G. L. Blake; E. L. Schaller; et al. (2007). "Methane and Ethane on the Bright Kuiper Belt Object 2005 FY9" (PDF). The Astronomical Journal. 133 (1): 284–289. Bibcode:2007AJ....133..284B. doi:10.1086/509734.
- M. E. Brown; E. L. Schaller; G. A. Blake (2015). "Irradiation products on the dwarf planet Makemake" (PDF). The Astronomical Journal. 149 (3): 105. Bibcode:2015AJ....149..105B. doi:10.1088/0004-6256/149/3/105.
- Brown, M. E.; Barkume, K. M.; Blake, G. A.; Schaller, E. L.; Rabinowitz, D. L.; Roe, H. G.; Trujillo, C. A. (2007). "Methane and Ethane on the Bright Kuiper Belt Object 2005 FY9" (PDF). The Astronomical Journal. 133 (1): 284–289. Bibcode:2007AJ....133..284B. doi:10.1086/509734.
- 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.
- Cruikshank, Dale P.; Dalle Ore, Cristina M. (2003). "Spectral Models of Kuiper Belt Objects and Centaurs" (PDF). Earth, Moon, and Planets. 92 (1–4): 315–330. Bibcode:2003EM&P...92..315C. doi:10.1023/B:MOON.0000031948.39136.7d. hdl:2060/20040012770. S2CID 189906047.
- Barkume, K. M.; Brown, M. E.; Schaller, E. L. (2008). "Near-Infrared Spectra of Centaurs and Kuiper Belt Objects" (PDF). The Astronomical Journal. 135 (1): 55–67. Bibcode:2008AJ....135...55B. CiteSeerX 10.1.1.62.5470. doi:10.1088/0004-6256/135/1/55.
- Szabó, Gy. M.; Kiss; et al. (2018). "Surface Ice and Tholins on the Extreme Centaur 2012 DR30". The Astronomical Journal. 155 (4): 170. Bibcode:2018AJ....155..170S. doi:10.3847/1538-3881/aab14e.
- NASA to Make Historic New Year's Day Flyby of Mysterious Ultima Thule. Here's What to Expect. Nola Taylor Redd, Space.com. 31 December 2018.
- Pommerol, A.; et al. (2015). "OSIRIS observations of meter-sized exposures of H2O ice at the surface of 67P/Churyumov-Gerasimenko and interpretation using laboratory experiments". Astronomy & Astrophysics. 583: A25. Bibcode:2015A&A...583A..25P. doi:10.1051/0004-6361/201525977.
- Wright, I. P.; Sheridan, S.; Barber, S. J.; Morgan, G. H.; Andrews, D. J.; Morse, A. D. (2015). "CHO-bearing organic compounds at the surface of 67P/Churyumov-Gerasimenko revealed by Ptolemy". Science. 349 (6247): aab0673. Bibcode:2015Sci...349b0673W. doi:10.1126/science.aab0673. PMID 26228155. S2CID 206637053.
- Campins, Humberto; Hargrove, K; Pinilla-Alonso, N; Howell, ES; Kelley, MS; Licandro, J; Mothé-Diniz, T; Fernández, Y; Ziffer, J (2010). "Water ice and organics on the surface of the asteroid 24 Themis". Nature. 464 (7293): 1320–1. Bibcode:2010Natur.464.1320C. doi:10.1038/nature09029. PMID 20428164. S2CID 4334032.
- Rivkin, Andrew S.; Emery, Joshua P. (2010). "Detection of ice and organics on an asteroidal surface" (PDF). Nature. 464 (7293): 1322–1323. Bibcode:2010Natur.464.1322R. doi:10.1038/nature09028. PMID 20428165. S2CID 4368093.
- Kohler, M.; Mann, I.; Li, A. (2008). "Complex organic materials in the HR 4796A disk?". The Astrophysical Journal. 686 (2): L95–L98. arXiv:0808.4113. Bibcode:2008ApJ...686L..95K. doi:10.1086/592961. S2CID 13204352.
- "Red dust in disk may harbor precursors to life". Spaceflight Now. 5 January 2008.