This is an old scientific literature review I wrote back during my Bachelor’s degree. Note that it is more standard to have all of the ‘discussion’ sections combined into a single discussion section.
Searching for Life on Titan
From a dense atmosphere to rainfall and liquids on its surface, Saturn’s moon Titan has it all. The only known satellite with a thick atmosphere, the large moon has a very encouraging environment that is of strong astrobiological interest. Titan is shrouded by a thick haze in which various chemical reactions take place, many involving carbon and nitrogen-containing compounds. It also has a familiar weather cycle where rainfall is composed mostly of ethane.
The moon is hypothesised to have a subsurface ocean made of an ammonia-water mixture, which is able to resist being frozen at very low temperatures. This is supported by the evidence of cryovolcanism on Titan’s surface (volcanoes that erupt non-rock materials, such as water or ammonia). Moreover, Titan is also the only satellite we know of where liquids exist on its surface. The surface, made of rock and ice, features both lakes and seas made of hydrocarbons.
The search for life has been made more exciting by the fact that recent studies have found that different types of bacteria are able to survive in conditions typical of Titan’s environment. Overall, Titan is a prime candidate in the search for life and its hydrocarbon lakes are the most accessible and practical place to look.
Keywords: prebiotic, astrobiology, exobiology, extraterrestrial, tholins, amino acids
Titan is Saturn’s largest moon, and the only other body in the Solar System apart from the earth that is known to have a thick atmosphere, solid ground and the presence of liquid on its surface in the form of lakes and seas.
Due to its favourable environment, Titan is a prime candidate in the search for life. If life is found on Titan, the implications would be profound. Finding that life can exist twice in one star system would suggest that life is common throughout the universe.
Titan is shrouded by a thick haze, making it difficult to study the moon’s properties. Indeed, we know little about the haze itself (He et al. 2012). Its atmosphere does, however, contain amines, as well as other biologically important molecules (Mora et al. 2012). A laboratory simulation of the high-energy chemistry in Titan’s upper atmosphere has also come to the conclusion that the four constituent bases of DNA would also be found (Hörst et al. 2012).
While the haze and the surface liquids have been proven without doubt, it is hypothesised that Titan has an ocean underneath its surface made out of an ammonia-water mixture (Kelly et al. 2012). One piece of evidence that supports this hypothesis is the apparent geological activity on Titan’s surface.
Titan has just the right surface temperature and atmospheric pressure to allow liquid hydrocarbons to exist on its surface, especially methane (Gilliam & McKay 2011). Titan is even known to support hydrocarbon seas (Lorenz & Hayes 2012). It is hypothesised that these liquid hydrocarbons could harbour unusual forms of life (Neubauer et al. 2011).
The scope of this literature review will be to explore the features of Titan that have astrobiological importance. We will consider which of Titan’s varied environments might be most suitable for us to continue our search for life. This review will centre on the question of whether the environmental, surface and subsurface features of Titan make it a suitable place to look for life. This article will discuss the various features that comprise Titan’s environment, discussing its atmosphere, subsurface ocean and surface liquids respectively.
Titan is the only satellite in the Solar System that possesses a thick atmosphere. The atmosphere is composed of 98% nitrogen gas (Hörst et al. 2012). There are also lesser amounts of methane gas and traces of oxygen gas. To study Titan’s atmosphere, studies simulate the conditions on Titan in the laboratory according to data from the Voyager and Cassini spacecrafts.
Tholins are a substance formed when carbon-containing molecules are exposed to UV radiation (Hörst et al. 2012). Most studies centre on the tholins and methane gas in Titan’s atmosphere. Strobel (2010) found that the levels of hydrogen gas in Titan’s atmosphere were consistent with the levels that would remain after being consumed by hypothesised life forms on Titan.
Pasek et al. (2011) researched the levels of phosphorus in Titan’s environment. Phosphorus is an element that is essential for life on earth, as depicted in Figure 1 (Raulin et al. 2010). The lack of most phosphorus compounds in Titan’s atmosphere suggested that if life were to exist on Titan, it would be very different to any life we know of. In fact, it may make use of phosphine, an obscure phosphorus-compound that is found on Titan (Naganuma & Sekine 2010).
Figure 1: Though little-studied due to its scarcity, phosphorus is an element that is essential to life (Reproduced from Raulin et al. 2010).
Studies of the high-energy chemistry in Titan’s atmosphere show that the tholins can create amino acids and the building blocks of DNA, a positive sign for the search for life (He et al. 2012; Hörst et al. 2012).
Since it is so cold on Titan, hydrocarbons replace water as the main constituent of the weather cycle. The weather on Titan was previously mistakenly thought to consist mainly of methane (Hayes et al. 2011), but recent research has shown that ethane plays the main role (Dalba et al. 2012). The weather cycle helps to shape the landscape of Titan in a familiar way to the valleys and other geological features of earth, although the erosion features on Titan are considerably milder (Black et al. 2012).
Discussion of Atmosphere
Archaea are bacteria-sized organisms that consist of only single cells. When the Huygens probe landed on Titan in early 2005, there was no visible sign of life in the atmosphere or on the surface. This means what we are interested in is bacteria or archaea.
Virtually all the papers researched for this review depicted that Titan has a rich atmosphere that is conducive towards prebiotic chemistry. However, one important study by Pasek et al. (2011) concluded that due to lack of phosphorus compounds, potential life on Titan would be very strange indeed. The concepts in prebiotic chemistry are mostly concerned with carbon-containing compounds and this new study suggests that we may be required to shift our research into lesser-studied elements such as phosphorus.
The hypothesised ammonia-water ocean underneath the surface of Titan is potentially another promising place look for life (Kelly et al. 2012). It is estimated that this potential solution is approximately 13% water (Neish et al. 2009). Water would normally be frozen at the temperatures underneath Titan’s surface, but when chemically bonded with ammonia, the freezing point of this ammonia-water liquid becomes much lower than 0°C (Neveu et al. 2013). This ocean should be highly acidic and about 20 kilometres below the surface (Marion et al. 2012). Again, testing the properties of this hypothesised ocean is done in the laboratory by mixing chemical compounds in their relevant states.
Evidence of cryovolcanism on Titan’s surface is one implication of the existence of this underground ocean (Solomonidou et al. 2013). Rather than a traditional volcano which erupts molten rock, a cryovolcano sends out very cold materials such as water and ammonia. Typically, such materials, if touched, would freeze one’s hand rather than burn it. One study estimated that the materials erupted on Titan may be between approximately -96°C and -34°C (Mitri et al. 2008). While the existence of cryovolcanoes has not been unambiguously confirmed, such geological processes on Titan’s surface would be a good sign in the search for life. The more activity that occurs, the more we can expect complex chemical reactions to take place.
Certain types of life on earth are already able to adapt to the harsh conditions in ammonia-rich solutions. Kelly et al. (2012) found that a wide variety of bacteria can live in such solutions, including pathogens, bacteria that cause disease. In temperatures as low as -80°C, these bacteria can survive several months and then adapt to be able to grow.
Hydrothermal vents release bursts of heated water from the ocean floor. Raulin et al. (2010) considers an early Titan to have an ocean comprising partly of water where there are features similar to hydrothermal vents. On earth, organisms are able to live in the harsh conditions near hydrothermal vents and it is even hypothesised that this is where life began on our planet (Raulin et al. 2010).
Discussion of Subsurface Ocean
The fact that the existence of the hypothetical subsurface ammonia-water ocean on Titan is yet to be determined makes it a less attractive place to search for life. However, the evidence is mounting, such as the apparent cryovolcanic behaviour on Titan’s surface (Mitri et al. 2008).
While we already know of certain types of bacteria on earth that can live in solutions that are largely composed of ammonia, various papers have noted that life on Titan should be fundamentally different to those we know of on earth (Tokano 2009; Pasek et al. 2011).
This appears to be inconsistent, and further studies should consider the differences between bacteria that might exist on Titan and the bacteria on earth that is able to live in ammonia solutions.
Surface Liquid Hydrocarbons
The final feature of Titan to explore in this review is the liquid methane and ethane on its surface (Gilliam & McKay 2011). Though most liquids on Titan are no bigger than a lake, three of the bodies of liquid hydrocarbons are approximately 300 kilometres across and long enough to be classified as seas (Lorenz & Hayes 2012). Known surface liquids are an attractive place to explore because they are easy to reach. This feature of Titan is another reminder of how Titan shows similar characteristics to earth.
On Titan, we find that liquid hydrocarbons behave in a similar way that water does on earth. Hydrocarbons form the weather patterns and the liquids on the surface. Water on Titan would be solidly frozen and would act as rock does on earth. Titan’s solid surface is a mixture of rock and ice. The temperature of approximately -179°C on Titan’s surface is within the acceptable interval for liquids to exist on its surface (Gilliam & McKay 2011).
Liquid hydrocarbons on Titan would not harbour carbon-based life that requires water to survive (Tokano 2009). It has long been proposed that life could exist in these liquids that breathe in hydrogen gas and release methane (McKay & Smith 2005).
Tokano (2009) suggested that sunlight could cause complex chemical reactions between the hydrocarbons and the sediment that is disbursed by Titan’s atmosphere. This would create nitrogen compounds that may be biologically important. Additionally, Tokano (2009) emphasized that lakes of hydrocarbons may be more important than seas since the smaller area that lakes cover mean that substances within the lake are closer together and hence more likely to mix. However, detecting non-hydrocarbon compounds in these lakes has been difficult in practice and experiments are generally simulated in the laboratory instead (Cordier et al. 2012).
Schulze-Makuch et al. (2011) performed a study on similar environments on earth and found that certain types of archaea and bacteria are able to survive in a liquid hydrocarbon environment. Interestingly, alternative hydrocarbon environments studied found different types of archaea and bacteria.
Discussion of Surface Liquid Hydrocarbons
Through pure accessibility, the liquid hydrocarbons on Titan’s surface would likely be the best place to look for life. What are the chances of success?
We know of life on earth (archaea and bacteria) that are able to survive conditions of liquid hydrocarbons (Schulze-Makuch et al. 2011). Unfortunately, this again appears to be inconsistent with the fact that many scientists believe we are not looking for carbon-based life forms (Tokano 2009; Pasek et al. 2011).
Nevertheless, given that liquid hydrocarbons lie on Titan’s surface this inconsistency could be easily resolved by another robotic mission to Titan. (In contrast, resolving the similar inconsistency for the hypothesised subsurface ocean is much more difficult.)
Overall, Titan is a favourable environment for astrobiological exploration due to the sheer amount of chemical reactions that are expected to take place. Of all the features of researched, the liquid hydrocarbons on Titan’s surface appear to be the most promising place to search for life.
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