Lakes of chemically non-polar methane and ethane are present on the Titan’s surface as low as 94 K. If life requires polar liquids, phosphane (PH3) is the candidate that would be co-present with liquid ethane at locally or temporally warmed conditions, although polarity is much weaker than that of water. Nevertheless, the idea of a polar liquid in hydrocarbon lakes leads to a concept of cells consisting of phosphane-droplets in liquid ethane. Interior matrix of such cells, i.e., polar droplets, is self-separated in the non-polar milieu, and therefore cell membranes as separators may not be needed for a possible life in the Titan lakes.

Key words: chemical polarity; polar molecule; water; phosphane; methane; ethane

Titan is the only known extraterrestrial object that has substantial atmosphere and liquids on the surface (e.g., Lunine and Atreya, 2008). The liquids form lakes of hydrocarbons such as methane (CH4) and ethane (C2H6) in the north polar region of Titan (Brown et al., 2008). Internal watery oceans have also been proposed for the Jovian and Saturnian satellites, e.g., Europa, Ganymede, Callisto, and Enceladus as well as Titan (Khurana et al., 1998; Lorenz et al., 2008; Waite et al., 2009). Watery oceans have implications for existence of extraterrestrial life, as liquid water (H2O) mediates the processes to sustain life of the Earth, with chemical polarity of the molecule as the essential property. No Earthly life is maintained in milieu of non-polar hydrocarbons, and no liquid water is present on the Titan’s freezing surface at 94 K and 1.5 atm (147 kPa). Only a few polar liquids would be available on the surface. If we search in hydrides, rather than oxides, under the Titan’s reducing atmosphere, phosphane (PH3) is an excellent candidate. Dipole moment (debye, D) is a measure of polarity, and that of phosophane is approximately 0.6 D, which is lower than 1.9 D of water but higher than 0 D of methane and ethane (Fig. 1). It is noteworthy that methyl-derivatives of phosphane, i.e., mono-/di-/tri-metyl-phosphanes, show increased dipole moments as high as 1.1-1.2 D (NIST Computational Chemistry Comparison and Benchmark Database, 2006). Methyl phosphanes are regarded as responsible for coloration of the Jupiter’s Great Red Spot (Prinn and Lewis, 1975).

Phosphane remains liquid between 140 K and 185 K at 1 atm, and the range for liquid arsane (AsH₃) is 157-211K at 1 atm. The temperature ranges of liquid phosphane and arsane should be several degrees higher at 1.5 atm of the Titan’s surface. Other polar hydrides such as azane (NH₃; ammonia), oxidane (H2O; water), sulfane (H2S; hydrogen sulfide), and selane (H₂Se) are solids at the temperature range of liquid ethane (Fig. 1). The non-polar hydride, carbane (CH₄; methane) stays liquid only in a narrow temperature range (91-112 K at 1 atm), in which no polar hydride liquids are available. In contrast, liquid ethane occurs in a wide range (89-184.5 K at 1 atm), and liquid phosphane and arsane may co-exist in the warm or hot liquid ethane; however, arsane has a low dipole moment of 0.2 D, which is unlikely relevant to life-supporting polarity.

As the density of liquid ethane (~0.55 g cm-3) is higher than that of liquid methane (~0.45 g cm-3), liquid ethane would accumulate on lake bottoms and be warmed by geothermal heat in the vicinity of cryovolcanoes (Sotin et al., 2005) and on floors of impact craters, i.e., crater lakes of hydrocarbons (Artemieva and Lunine, 2005). Moreover, because methane is a major greenhouse gas on Titan, small increases in solar luminosity would lead evaporations of liquid methane and dramatic increases in surface temperature (e.g., ~120 K in ~0.5 Gyrs from the present), i.e., a climatic event known as runaway greenhouse (Lorenz et al., 1999). If so, liquid phosphane could co-exist with ethane in broader areas of a greenhouse Titan.

Phosphane has been detected in the atmospheres of Saturn and Jupiter (Bregman et al., 1975; Larson et al., 1977; Noll et al., 1989) but not yet in Titan’s atmosphere. However, given that the temperature of circum-Saturnian nebula was low sufficient to capture gaseous methane into icy planetesimals that formed Titan (Mousis et al., 2009), phosphane should also be co-captured in the icy Titan-forming planetesimals; accordingly, it can be proposed as the realistic polar media for a Titan life.

Internal ocean of Titan has been proposed and believed to consist of polar liquids such as aqueous ammonia, i.e., ammonia+water (Tobie et al., 2006). This belief is supported by recent observations of the presences of NH₃ in the internal ocean of Enceladus (Waite et al., 2009) and on the surface of Titan (Nelson et al., 2009). Aqueous ammonia may mediate life-supporting processes and thus imply the existence of life, although no life in aqueous ammonia has been observed so far on Earth. Possible life in Titan’s internal ocean is likely bound by an oily membrane, as is the case with Earthly cell life, viewed as the water-in-water type. Oily membrane of the cell, e.g., lipid bilayer, is necessary to separate inner and outer waters, corresponding to cellular matrix and environ habitat, respectively.

In contrast, life in Titan’s hydrocarbon lakes would be droplets of phophane are viewed as a type of water-in-oil, more exactly the phosphane-in-ethane type. Or, phosphane may form pools in ethane and harbor phosphane-based cells bound by oily membrane; this is a type of water-in-water-in-greater-oil. Whether life may occur as phosophane droplets or membrane-bound cells in phosphane habitat is uncertain.

For example, methyl-substituted phosphanes, e.g., tri-methyl-phosphane, may serve as amphiphilic substances and form micelles in the manner of methyl-side-out and phosphorus-side-in. These micelles have hydrophobic exterior, in an opposite sense to usual micelles, and would form structures like liposomes and lipid bilayers as usual micelles would do.

In the case of the water-in-oil (phosphane-in-ethane) droplets, no boundary membranes are needed for such self-separating polar cell matrix and non-polar environs. Membranes, if any, would function only in transporting materials and transmitting signals at the interface of polar and non-polar media. This hypothesis, therefore, presents the possibility of a new form of life in addition to the traditional view of membrane-bound lives.

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