What on Earth are Tholins?

In answer to the title question, tholins aren’t on Earth! Well, not naturally on Earth, but they are important elsewhere in the solar system and, most likely, are common in other star systems. Many readers of Chem 13 News will have seen the amazing photos of the surface of Titan, the giant moon of Saturn, with its orange-brown-hazed sky (Fig. 1). Why orange-brown? The colour comes from an atmospheric suspension of a diverse group of carbon-hydrogen-nitrogen compounds called tholins.

Fig. 1. The orange sky seen from the surface of Saturn’s moon, Titan.

wikimedia.org/wikipedia/commons/b/bc/Huygens_surface_color.jpg

Although the wide-spread interest in tholins has only emerged in the last 5 years, the story began over 40 years ago. It was in the 1970s that one astronomer, Carl Sagan, became curious about the high albedo and reddish colour of Saturn’s moon Titan. Titan is a huge moon, probably once a dwarf planet captured by Saturn, with an atmosphere twice as thick as Earth’s. Titan’s atmospheric composition is quite different from that of Earth. Although both are mostly nitrogen (98% and 78% respectively), it is the minor components that are quite different with Titan having methane (1.4%) and hydrogen (0.1-0.2%).

To try to simulate the reactions in Titan’s atmosphere, Sagan and his colleagues exposed a mixture of 90% nitrogen and 10% methane to intense radiation.¹ The result of this experiment produced an amazing mixture of organic compounds: alkanes, alkenes, alkynes, aromatic hydrocarbons, aliphatic nitriles, aromatic nitriles and a wide variety of nitrogenous ring compounds. The ‘gooey’ liquid/solid particulate component had a brown-orange colour. Because of the consistency, Sagan proposed to name this mixture tholins from the Greek word, tholos, meaning ‘mud’ and ‘vault or dome’. With recent visits by probes to Titan, we now know that the tholin haze is present between altitudes of 60 and 140 km above the surface (see Fig. 2, below). Also, it seems likely that there are extensive tholin deposits beneath the hydrocarbon lakes on the surface.

Fig. 2.   The detailed structure of Titan’s atmosphere and surface, showing the condensate haze layer. wikipedia.org/wiki/Climate_of_Titan#/
media/File:Titan_atmosphere_detail_narrow.svg

In recent years, laboratories around the world have produced tholin mixtures using a variety of gas combinations and extreme radiation types. But which laboratory-produced tholin most closely matches the actual Titan mixture? The best analog seems to be when the gases are in a high-voltage electric field, producing what is called a ‘cold plasma’.² In a cold plasma, the ionized species have high translational energies while neutral molecules are moving very slowly. Such a combination of high energy ions colliding with slow-moving neutral molecules gives a unique mixture of products, mostly molecules containing unsaturated carbon-nitrogen bonds. Fig. 3 shows one common polymeric component.

Fig. 3. A polymeric component of tholins. wikimedia.org/wikipedia/commons/2/26/Structural_chemical_formula_of_tholins_ of_Titan.svg

More recently, the photos of Pluto and its companion, Charon, show massive orange deposits on the surface.³ These, too, seem to be tholins, though as they may be different in composition from Titan’s, the terms ‘Pluto tholins’ and ‘Charon tholins’ are used to differentiate them from ‘Titan tholins’.

If tholin mixtures are warmed to room temperature and water is added, the compounds hydrolyse to give amino acids, heterocyclic organo-nitrogen compounds and other building blocks of life. Thus it has been postulated that life on Earth may have started from impacts of tholin-containing comets into the warm seas of early Earth.⁴

So as we continue our chemical explorations of the solar system, expect to hear more of tholins.

Student questions

1. Why would tholins be considered a mixture, not a compound? Explain the difference.
2. Look carefully at the mixture of compounds in the atmosphere of Titan. What is one element that is common in our atmosphere that is missing in this mixture? Hint, it makes up about 21% by volume of the gas in our atmosphere.
3. Organic nitriles seem to play an important role in the chemistry of the outer planets. What is a 'nitrile'?"

References

1. B.N. Khare, et al., “The organic aerosols of Titan”, Advances in Space Research, 1984, 4(12), pages 59-68.
2. P. Coll, et al., “Can laboratory tholins mimic the chemistry producing Titan’s aerosols?” Planetary and Space Science, 2013, 77, pages 91-103.
3. W.M. Grundy, et al., “Surface compositions across Pluto and Charon,” Science, 2016, 351, page 1283.
4. M.L. Cable, et al., “Titan Tholins,” Chemical Reviews, 2012, 112, 1882-1909.