Discovered by Christiaan Huygens in 1655, Titan remained very difficult for astronomers to study in depth for more than three centuries, until the 2004 arrival of the Cassini-Huygens mission at the Saturn system and the amazing Huygens landing on 14 January 2005. Even though Titan has unveiled essential clues about its recent activity, others will be necessary to understand better its atmospheric cycle, as well as its potential role in the formation of organic materials essential to the appearance of the building blocks of life.
Trek to Titan (NASA/JPL-Caltech)
One could think that Saturn's uncanny moon really deserves its name. It almost seems that Titan is condemned to the same fate as the Greek Titans jailed in the great pit of Tartarus, with its thick atmosphere constantly shrouding its surface.
In 1904, Josep Comas i Sola claimed to have observed that Saturn's biggest moon actually had an atmosphere, using the limb darkening method. Much later, the unexpected discovery of methane on Titan, by Gerard Kuiper in 1944 at the McDonald Observatory in Austin (Texas), came along to support Comas i Sola's affirmations 1.
When it reached Saturn's system in 1979, NASA's Pioneer11 found that Titan was too cold to sustain life1,2, and it also detected aerosols in its upper atmosphere, which happened to be of bigger size at lower altitudes. Thanks to close-up surveys by the two Voyager spacecraft in 1980, astronomers were then able to determine that molecular nitrogen is the main constituent of Titan's atmosphere, up to 98%. They also discovered hydrocarbons, nitriles in lower quantities and methane-ethane lakes, and that Titan is...well, pretty chilly (-198°C/94K), so that water ice there is as solid as rocks on Earth, with an atmospheric pressure almost Earth-like at 1.6 bars. Titan's gravity is also similar to that of our Moon. With a 5150 km diameter, Titan is actually bigger, but lighter, than Mercury. Being the second moon in the solar system by size, it was long thought to be actually larger than Jupiter's Ganymede because of its haze, until the Voyager craft came along1,2,3.
In spite of Voyager1’s successful study of Titan’s atmosphere, which extends further from the surface than on Earth, it was desperately impossible to get any hints about its surface.
Methane dearth on Titan?
Astronomers have long speculated about the existence of immense lakes, even oceans, filled with liquid methane, so as to explain the presence of methane in Titan's upper atmosphere. This was detected by Hubble4 and ISO telescope5 findings from the mid-1990s via infrared spectrometry. Actually, radar mapping by Cassini in 2004 indirectly established that there are features corresponding to Titan's lakes6,7, essentially located at higher latitudes8 and rather sparse on a global scale. An infrared spectrometer aboard the Cassini orbiter confirmed that they are primarily made of a mixture of methane and ethane.
But that is not enough to explain how Titan’s upper atmosphere is replenished with methane9,10. Indeed, the methane cycle is not closed, and this molecule undergoes UV-photolysis, which breaks it down into radicals. These then recombine into ethane and hydrogen (H2), the latter escaping into space – a hydrogen torus was even found around Saturn! Because of that, there's no way to get the methane back. Moreover, Saturn's magnetosphere ionises methane to give either nitriles or higher-order hydrocarbons, which irreversibly polymerise into bulky compounds at lower latitudes. As these reactions have already occurred over millions of years, one or several sources should explain why Titan's upper atmosphere is still saturated with methane10, as well as the relatively limited extent of ethane lakes, in spite of UV-photolysis of methane since the appearance of Titan 4,5 billion years ago.
It seems difficult to assert that methane rainfall, following partial evaporation of lakes in lower arid regions, could counteract this depletion process, as Titan’s low gravity limits the convection phenomenon: rainstorms on Titan can't be triggered at the equator with its relative humidity of 45%, whereas such events would be possible on Earth10. Maybe, if the lakes are deep enough, part of the methane can evaporate without leading to their slow disappearance, or perhaps the crust at the equator is so smooth, similar to the Huygens probe landing site, that the methane is released into the atmosphere to flow towards Titan’s poles10,11.
Otherwise, methane sources could be inside of the moon. If the proto-satelitary disk around Saturn was cold enough, methane would have been trapped in cometesimals as clathrate hydrates. Alternatively, if the disk was warmer, primordial carbon dioxide would have reacted with Titan's water in serpentinization reactions to eventually produce methane, which would have been trapped in Titan's icy crust2,3,10. In either case, sporadic outgassings, either triggered by cryovolcanoes, crater impacts, or internal constraints due to convection occurring in the core, would eventually release methane to the surface. If enough methane is released, it can lead to episodic rainfalls at lower latitudes that carve various geological features such as dunes, dendritic or dry valleys, and channels6,12. Then, methane evaporating over 10 to 100 years, would partly replenish lakes at Titan’s poles. As for ethane, it is either stored in clathrate hydrates, or involved in aerosol formation. Such aerosols that form in Titan’s upper atmosphere lead to the formation of Titan's haze.
Tholins and potential prebiotic chemistry
Titan has a pretty rich hydrocarbon chemistry13,14. The first step in the process that leads to its thick haze involves molecular nitrogen and methane which both undergo either UV-photolysis or ionization15,16. The first type of reaction gives higher-order hydrocarbons – including ethane, acetylene, ethyne – and nitriles; the second one, positively charged ions. These molecules then polymerise, forming polyaromatic hydrocarbons (PAHs) such as benzene, naphtalene or anthracene. From these PAHs, negative organic ions are obtained19,20 that can lead to further polymerisation reactions with unsaturated macromolecules19. In the end, these macromolecules condense into tholins, contributing to the haze consistency, or precipitate on Titan’s surface.
When these tholins fall, they become trapped on the surface. Such phenomena like cryovolcanism and impact craters lead water to melt partially, so that tholins hydrolysis can occur21. It may happen that the water doesn't freeze immediately, because hydrolysed tholins can likely be found in water-ammonia mixtures on Titan, which requires much more time than water to freeze21,22. If tholins are given enough time to be hydrolysed, organic compounds of biological interest could actually be formed on Titan22,23. That is what was suggested by several experiments which have attempted to reproduce Titan’s conditions in the laboratory, and some of them have given pretty encouraging results: biochemical compounds have been obtained24,25, including urea and amino acids such as alanine, glycine, aspartic acid, and uracil. The fact is that amino acids are the building blocks of proteins, and uracil the one found in RNA, so that really makes Titan a good vantage point when it comes to the study of prebiotic chemistry.
To this end, robotic missions to Titan are proposed for the decades to come. The first is the TitanSaturnSystemMission26,27(TSSM), a NASA/ESA joint mission for 2020 aimed at Saturn's system, Enceladus and Titan, which would send a hot air balloon to circumnavigate the uncanny moon for in-depth study of its atmosphere, and TiME, a lake lander probe with the primary aim of sampling and analyzing organic compounds from Titan’s lakes. The second mission is AVIATR28 (Aerial Vehicle for In-situ and Airborne Titan Reconnaissance) with a “focus on surface geology/hydrology and lower-atmospheric structure and dynamics”.
Thanks to Huygens, a new world was unveiled to us29. Whether it will be TSSM or AVIATR sailing to Titan, it will surely make the uncanny moon unveil some more of its mysteries. Perhaps we will one day be able to get through its picturesque haze. Easily, like Star Trek's titan-esque USS Enterprise .
About the author:
Jérémy Roggy is studying chemistry at the University of Poitiers (@UnivPoitiers). He has a passion for astrochemistry and space exploration. In this article he provides us with a precise account of the adventurous discovery of Saturn's moon Titan. His article echoes the account of Mars' exploration published on the blog MySciencework by nonetheless than André Brahic, the famous French exobiologist (article in French):
You can also have a look at the portraits of astronomers and physicists on MyScienceWork the blog:
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