Tholins (after the Greek θολός ( tholós) "hazy" or "muddy";[Sarah Hörst "What in the world(s) are tholins?", Planetary Society, July 23, 2015. Retrieved 30 Nov 2016.] Tholins are disordered polymer-like materials made of repeating chains of linked subunits and complex combinations of functional groups, typically and , and their degraded forms such as and Phenyl group. 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 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 Abiogenesis 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, astrochemistry analyze the composition of these tholin mixtures, and the exact character of the individual chemicals within them.
Overview
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 , , , , HCHO, and . 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'.
Tholins are not one specific compound but rather are descriptive of a spectrum of molecules, including ,[ A Bit of Titan on Earth Helps in the Search for Life's Origins. Lori Stiles, University of Arizona. 19 October 2004.]
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!)."
Formation
Artificially
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 primary
,
, and
alkyl portions such as / forming complex disordered macromolecular solids. Laboratory tests generated complex solids formed from exposure of : gaseous mixtures to electrical discharge in cold plasma conditions, reminiscent of the famous Miller–Urey experiment conducted in 1952.
Naturally
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 () and
methane () 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.
Tholins may be a major constituent of the interstellar medium.
These atmospherically derived substances are distinct from ice tholin II, which are formed instead by irradiation (radiolysis) of of water and organic compounds such as methane () or ethane ().
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.
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.
(See Miller–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 the Great Oxygenation Event around 2.4 billion years ago.
Laboratory experiments
On Earth, a wide variety of Soil biology are able to use laboratory-produced tholins as their sole source of carbon. Tholins could have been the first microbial food for microorganisms before evolved.
Occurrence
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
, 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.
Moons
Titan
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.
Europa
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
Cryovolcano events that resurface the body, and to accumulate from space as interplanetary dust.
Rhea
The extensive dark areas on the trailing hemisphere of Saturn's moon Rhea are thought to be deposited tholins.
Triton
Neptune's moon Triton is observed to have the reddish color characteristic of tholins.
Triton's atmosphere is mostly nitrogen, with trace amounts of methane and carbon monoxide.
[ Neptune's Moon Triton. Matt Williams, Universe Today. 16 October 2016.]
Dwarf planets
Pluto
Tholins occur on the
dwarf planet Pluto and are responsible for red colors
[ "NASA released an incredibly detailed photo of snow - and something else - on Pluto", Business Insider Australia, Mar. 6, 2016 (accessed 28 Feb. 2018).] 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 distance to the orbiting moon.
Ceres
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.
[ Team finds evidence for carbon-rich surface on Ceres. Southwest Research Institute. Published by PhysOrg. 10 December 2018.] The carbon content is more than five times higher than in carbonaceous chondrite meteorites analyzed on Earth.
Makemake
Makemake exhibits
methane, large amounts of
ethane and tholins, as well as smaller amounts of
ethylene,
acetylene and high-mass
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
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.
[ 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.]
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
NICMOS aboard the Hubble Space Telescope.
The HR 4796 system is approximately 220 light years from Earth.
See also
