Coding Aliens: a Study on Potential Microbes on Titan
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Coding aliens: a study on potential microbes on Titan Bachelor thesis Liberal arts and sciences Major: Astronomy Student: Yeuk Hei Cheng s3141721 [email protected] Supervisor: Dr. M.A. Hofstee Co-assessor: Dr. T.C. Andringa Groningen, August 2020 Abstract: Saturn's moon Titan hosts complex atmospheric processes and extraordinary sur- face features involving hydrocarbons instead of water. It thus provides room for research on the possibility of life in a non-Earthlike environment. A review of existing research on the topic is given; as well as calculations on the possible biomass density, based on the current unex- plained methane abundance and on other models. Similarities between the results indicate that the assumptions of each model are mutually consistent. The values suggest that life on Titan according to the models should be detectable by in situ observations. 1 Contents 1 Introduction 3 2 Literature review 3 2.1 The atmosphere of Titan . .3 2.2 The lakes of Titan . .5 2.3 Definition of life . .8 2.4 Possible chemistry . .8 2.5 Possible lifeforms . 11 3 Model 11 4 Results 14 5 Discussion 15 6 Conclusion 16 7 Acknowledgements 17 8 References 17 A Calculation code 21 2 1 Introduction Titan, the sixth and largest moon of Saturn, was discovered in 1655 by Christiaan Huygens. As the only other object in the Solar System with liquid on its surface apart from Earth, it also harbours a thick atmosphere where complex chemical cycles take place. Except on Titan, the chemical cycles involve hydrocarbons, instead of water as on Earth. They result in energy-rich molecules, made in the atmosphere with the help of sunlight, to rain down along with liquid methane, onto hydrocarbon lakes and dunes [1]. This complex hydrocarbon chemistry, along with unexpected observations of acetylene and hydrogen [2,3] plus a young surface [4] and atmosphere [1], all give room to observations and predictions on geochemical or even biological processes in a cryogenic, anhydrous, and anoxic environment. The possible presence of a subsurface (ammonia-water) ocean [5] and the hazy atmosphere may also yield clues about prebiotic Earth. Research on this topic, especially conclusions that would also apply to \warm Titans", might expand the space of habitable planets beyond the Goldilocks zone, which would have implications on the Drake equation. The abundance of M-dwarf stars in the universe means that a majority of exoplanets with liquid on the surface should be more Titan-like than Earth-like [6], although the spectrum received on those planets would be different than what Titan receives from the Sun, a G-type star [7]. Thus, Titan presents a myriad of opportunities to further our understanding of how life could originate in the universe. However, the origin of methane on Titan remains an open question. The simplest explanations of a source would be outgassing1 from subsurface alkanofers2 or from clathrates3 in the crust; but the existence of these sources and the precise mechanism on Titan has yet to be confirmed [1]. In this paper, existing research is compiled to review the prospects for (microbial) life on Titan. A calculation of the biomass surface density based on the current methane abundance is also presented. It is hoped that the results would aid future missions to detect potential life on Titan. 2 Literature review 2.1 The atmosphere of Titan Despite having a mass only 2.2% of Earth's, Titan has an atmosphere of 6:25 × 1018 kg, which is 1.19 times Earth's [8]. The haze in the atmosphere creates an anti-greenhouse effect, which lowers the temperatures and thus keeps the gases below their escape velocity (see Figure1). Titan has no magnetosphere of its own to shield against solar winds, but they are less intense at Titan's distance from the Sun than at that of the inner planets. However, Titan does sometimes pass outside of Saturn's magnetosphere, and Saturn's (and thus its magnetosphere's) rotational period is faster than Titan's orbital period. These processes intensify the destructive effect of solar winds on Titan's atmosphere [9, 10]. Some gases, especially nitrogen, are thought to be replenished from Titan's interior [11, 12]. The atmosphere consists of 97% nitrogen (N2), 1.4{1.5% methane (CH4), 0.1{0.2% hydrogen (H2) and small but still significant amounts of other molecules such as ethane (C2H6), hydrogen (H2), acetylene (C2H2), and hydrogen cyanide (HCN). In particular, hydrogen and acetylene are produced with the help of solar radiation, consuming atmospheric methane in the process. As methane condenses into rain, these molecules are brought onto the surface [13], where the temperature (94 K) and pressure (1:5 atm) [8] are 1Release of previously trapped gas. 2Underground reservoir of liquid hydrocarbons (instead of water as in aquifers). 3A crystal lattice with another molecule enclosed within the lattice space. 3 Figure 1: Escape velocity versus temperature. Regions indicate which gases are retained. Solar System bodies are plotted using surface temperature and drawn to scale. Source: Cmglee near the triple point of methane, much like water on Earth [1], explaining the presence of methane lakes on the surface (see Section 2.2). Towards the surface, the mole fraction of methane increases to 4.9%, and that of nitrogen decreases to 95.1% [8]. Due to the photolytic processes mentioned above, the methane content in Titan's atmosphere should be short-lived [4], with the current abundance modelled to be destroyed within 2:13 × 107 years [14]. The photodissociation rate of methane was determined to be 5 × 1027 particle=sec based on INMS4 measure- ments of H2 outflow [15]. Various measurements and models indicate that the atmosphere as it is seen today should have an age between 3 × 108 to 6 × 108 years if methane is replenished, or to 109 years if it is not. However, the Copernican principle5 means it is unlikely that we are in the time window to observe the last remnants of Titan's un-replenished methane [16]. As the lakes cannot contribute enough to the measured abundance [1], one other replenishing mechanism could be outgassing from Titan's interior. However the methane could also be from methanogenic life6 on the surface, as outgassing has yet to be confirmed. Furthermore, although ethane and acetylene should accumulate on the surface, no layer of these molecules have been found [2,17]. Based on observations, it has been predicted that a downflow of hydrogen should occur on Titan [3], contrary to intuition from simple physics. There are also measurements of nonuniform distributions of H2 and other trace elements that are skewed towards the north [18, 19]. These could be explained by a downward current in the north polar region, with the northern lakes acting as a sink, but could also be a pointer to biological activity in the south consuming these molecules [17] (see Section 4The Ion and Neutral Mass Spectrometer instrument on the Cassini Orbiter 5That we are not privileged observers of the universe 6Organisms that produce methane as part of their metabolism. 4 2.2). Biological activity tend to prefer lighter isotopes of elements, since the chemical bonds would be easier to break and the energy thus easier to obtain [7,20]. This changes the isotopic ratios that are detectable in the 12 13 atmosphere. On the surface of Titan, 6C/ 6C = 92:0±0:5, which is close to the Solar System average [21] +25 and within the constraints of Saturn (89−18) and Jupiter (94±12) [15]. Thus, the observed isotopic ratios do not lend support to the presence of life on Titan. However, there is no strong isotopic effect resulting from the breakdown of N2 on Earth either, and the recycling of CH4 by organisms (see Table1) should not necessarily result in an isotopic shift [17]. Further understanding of Titan's atmosphere is needed to explain the ratio [15], as well as the unexplained downflow of hydrogen in the atmosphere and deviation in abundance of acetylene at the surface. 2.2 The lakes of Titan The hydrocarbon lakes of Titan are found at the polar regions only (Figure2). Figure 2: The polar regions on Titan. Source: [22] While clouds [4] and rain [23,24] have been observed or modelled to occur at different latitudes, only the polar regions experience more precipitation than evaporation [24]. This difference is more exaggerated for the north pole than the south, due to hotter summers in the south during the present climate cycle [25], consistent with the observation that most filled lakes are in the north [22]. These lakes contain predominantly methane and ethane, which are liquid under the surface temperature of 94 K. The exact concentrations in each lake are still up to debate [7], but recent works suggest that those 5 in the north generally have more methane than ethane [26], while the only large southern lake, Ontario Lacus, has ethane as its main component [27]. Unlike water, both methane and ethane are nonpolar solvents, which means any potential life in these liquids would need a completely different chemistry than water-based life (which depends on the ability of water to dissolve nutrients as a polar solvents). The nonpolarity combined with low temperatures would mean a low solubility for most molecules [7], which would be a challenge for life on Titan. However, carbon-based life in such an environment is not impossible, and might be more straightforward than that in Earth-like ones, considering that molecules would have no risk of hydrolysis nor thermal decomposition [17], that many known enzymes fucntion with a non-hydrophilic active site [28], and that nucleic acids may form faster in the absence of water [20].