Boletín de la Sociedad Geológica Mexicana Volumen 67, núm. 3, 2015, p. 377-385 http://dx.doi.org/10.18268/BSGM2015v67n3a2

Methane in the Solar System

Andrés Guzmán-Marmolejo¹, Antígona Segura^2,*

¹ Posgrado en Ciencias de la Tierra, Instituto de Geofísica, Universidad Nacional Autónoma de México, Ciudad Universitaria, Coyoacán. C.P. 04510, D.F., México.
² Instituto de Ciencias Nucleares, Universidad Nacional Autónoma de México, Circuito exterior C.U. Apartado Postal 70-543. Coyoacán, C. P. 04510, D.F., México.

^* This email address is being protected from spambots. You need JavaScript enabled to view it.

Abstract

This paper reviews the distribution of methane (CH₄) in our Solar System, as well as its sources and sinks in the atmospheres of the main Solar System bodies. Methane is widely distributed in the Solar System. In general, the inner planets are methane-poor, being Earth a unique exception, whereas the outer planets have CH₄-rich atmospheres. In general, the atmospheric chemistry of this compound is dominated by the solar radiation although in O₂-rich atmospheres this compound participates in a reaction system that removes atmospheric CH₄. In our planet most of the atmospheric CH₄ is produced by lifeforms, reason why scientists have proposed that the simultaneous detection of methane signal along with oxygen (O₂) or ozone (O₃) signals in the atmospheric spectra of planets may be good evidence of life. Therefore, the study of this gas at planetary level is important for understanding the chemical reactions that control its abundance on the exoplanetary atmospheres and to classify possible inhabited planets.

Keywords: methane, biosignatures, Solar System.

Resumen

El objetivo del este trabajo es hacer una revisión sobre la distribución del metano (CH₄) dentro del Sistema Solar, así como sus fuentes y sumideros en las atmósferas de sus principales cuerpos. El CH₄ está ampliamente distribuido en el Sistema Solar; en general los planetas internos son pobres en este gas, con excepción de la Tierra, mientras que los planetas externos son ricos en él. La química atmosférica de este compuesto generalmente está dominada por la radiación solar, aunque en atmósferas ricas en O₂, este compuesto forma parte de un sistema de reacciones que eliminan al metano atmosférico. Dado que la mayor parte de CH₄ atmosférico es debido a la vida, los científicos han propuesto que su detección simultánea con oxígeno (O₂) u ozono (O₃) en el espectro de la atmósfera de los planetas podría ser una buena evidencia de vida. El estudio del CH₄ a nivel planetario es importante para entender las reacciones que controlan su abundancia en las atmósferas de los exoplanetas y clasificar los posibles planetas habitados.

Palabras clave: metano, bioseñales, sistema solar.

1. Distribution of methane in the Solar System

The Solar System was formed by the gravitational collapse of a primordial gas nebula. The center of this nebula collapsed faster than its outer edge, forming the Sun at the center and a protoplanetary disc around, latter processes formed planets from the dust (Cloutier, 2007). The temperature in the inner protoplanetary disc near the Sun was high enough to evaporate volatiles like methane, which is decomposed by photolysis and it is dragged later by the solar wind. In the outer regions of the protoplanetary disc, the low temperatures allowed that ices and volatiles could be preserved. The result is CH₄-poor terrestrial planets in the inner Solar System and CH₄-rich big planets in the outer (Cloutier, 2007).

Methane is preserved in ices called clathrates, these solids present structures that can capture methane in their interior. They play an important role in the stabilization and dispersion of molecules in the Solar System because they are present in many kinds of environments with a wide range of pressures and temperatures (Miller, 1961; Thompson et al., 1987). Here, we review the CH₄abundances in planets and small bodies of the Solar System.

2. Inner planets

Inner planets are the four closest planets to the Sun: Mercury, Venus, Earth, and Mars. They are small planets composed of silicates and iron. Volatiles in inner planet atmospheres as Mercury, Venus, Earth and Mars, are mainly the result of degassing from their interiors (Cloutier, 2007).

2.1. Mercury and Venus

Mercury has a tiny atmosphere mainly formed by He, H₂, O₂, Na, Ca, K and water vapor (Broadfoot et al., 1974; Potter and Morgan, 1985, 1986). Measurements with the Mercury Laser Altimeter ─MErcury Surface, Space ENvironment, GEoche-mistry, and Ranging (MESSENGER), confirmed the long held idea that Mercury contains impact-derived deposits of volatiles than may include organics (Neumann et al., 2013; Paige et al., 2013). These deposits are located in permanently shadowed zones of the north polar region where the regolith has temperatures similar to those of the icy Galilean satellites, allowing the cold-trapping of materials from comets and rich-volatile meteorites (Neumann et al., 2013). Methane is present in comets but thermal stability models do not predict its presence in cold-traps due to its higher volatility compared to water (Zhang and Paige, 2009). Gibson (1977) proposed a volatile cycle for Mercury, starting with the production of simple molecules (H₂, H₂O, CH₄, NH₃, etc.) by solar-wind ions implanted into the planet’s silicate surface. These chemical species would be outgassed and then cold-trapped in colder regions of the planet. Until now, no detection of methane has been reported for this planet.

Venus has a thick atmosphere mainly formed by CO₂ (96 %) and N₂ (3 %) (Niemman et al., 1980). The Pioneer spacecraft instruments detected CH₄ in Venus atmosphere (1000 – 6000 ppm) and many other gases (Oyama et al., 1980). Nevertheless, measurements of CH₃D/CH₄ ratio of 5×10^-3 caused controversy because atmospheric evolution models predicted a CH₃D/CH₄ ratio of 9×10^-2. A plausible explanation is that the CH₄ was the result of the reaction between some highly deuterated molecules in Venus atmosphere and terrestrial CH₄that contaminated the instruments (Donahue and Hodges, 1993).

Based on the detection of NH₃, HCl, and H₂O in the Venus atmosphere, along with the fact that there is a strong possibility of electrical discharge in the atmosphere as a result of thermal convective turbulence, Otroshchenko and Surkov (1974) proposed that organic compounds could be formed in the atmosphere. Their hypothesis was experimentally tested, finding CH₄and other low-mass molecules. The studies of Otroshchenko and Surkov (1974) show that presence of organic compounds in the Venus atmosphere is a strong possibility.

2.2. Earth

CH₄ levels in the atmosphere are currently around 1.6 – 1.8 ppmv, the enhanced greenhouse effect caused by a molecule of methane is about 8 times that of a molecule of CO₂ (Houghton, 2005). CH₄ is homogeneously mixed in the troposphere while in the upper atmosphere the highest concentrations are at the Ecuador (http://earthobservatory.nasa.gov). CH₄ levels have changed over the history of the Earth, before the emergence of life, CH₄sources were geological.

The emergence of life increased the levels of CH₄ in an atmosphere without free oxygen, where CH₄ could have lifetimes of 5000 – 10000 years and reach concentrations of 1000 ppmv (Kasting and Siefert, 2002). Then, when oxygenic photosynthesis increased O₂ levels in the atmosphere, CH₄ decreased because of a set of reactions that will be described at the end of this section. Numerical models show that, today, the thermodynamic equilibrium value for CH₄ is > 10^-35, in volume fraction, however its abundance is approximately 1.7×10^-6 (Sagan et al., 1993). CH₄ is almost totally produced by biological sources and the abiotic sources represent less than 10 % (Levine et al., 2010). The pristine ice cores store a record of CH₄ concentrations of thousands of years. Analysis of these cores show that CH₄ abundances ranged from 0.35 ppmv to 0.8 ppmv corresponding to glacial and interglacial periods (e.g., Legrand et al., 1988; Chappellaz et al., 1990; Raynaud et al., 1993; Brook et al., 1996; Petit et al., 1999; Spahni et al., 2005; Loulergue et al., 2008). CH₄ has increased its atmospheric concentration since pre-industrial time to be relatively constant around 1.7 ppmv (Dlugokencky et al., 2003). Recently, CH₄ is calling the attention of scientists studying climate change due to its capability as greenhouse gas. Today near to 50 % of CH₄ global emissions are produced by human activity (mining, industry, farming, and ranching) causing an imbalance between their sources and sinks of 30 Tg year^-1, approximately, contributing from 4 % to 9 % of greenhouse effect (Lelieveld et al., 1998; Wuebbles and Hayhoe, 2002; Houghton, 2005).

It is estimated that all CH₄ sources produce 600 Tg yr^-1, approximately. There are no chemical reactions forming CH₄ in atmospheres such as Earth (Levine et al., 1985). Here, almost all CH₄ is produced by methanogen microorganisms. Methanogens can form CH₄ by two ways: 1) using CO₂ in the reaction CO₂ + 4H₂ → CH₄ + 2H₂O (Thauer, 1998) or, 2) using organic molecules as electron terminal acceptors, for example acetic acid, methanol, or methylamine, in the reaction CH₃COOH → CH₄ + CO₂ (Fukuzaki et al., 1990). Table 1 summarizes the CH₄sources. The major biological sources of methane are wetlands, followed by digestion of animals such as ruminants and decomposition of biomass. An important source, linked to human activity, is the production of energy. Other minor sources are the animal activity such as arthropods and decomposition of sediments and bacterial activity in marine environments. While the only known abiotic source is the serpentinization process and contributes with 3 % of methane emissions.

Serpentinization takes place in hydrothermal systems similar to the Lost City, located in the middle of Atlantic Ocean. In these sites, it is commonly said that CH₄ is formed by serpentinization but in fact, CH₄ is byproduct of a Fischer–Tropsch type reaction after to serpentinization process. In the Fischer–Tropsch reaction, CO₂ is reduced by H₂ forming CH₄: CO₂ + 4H₂→ CH₄ + 2H₂O. This reaction needs metal catalysts such as Fe, Co, and Ni, and temperatures and pressures in the range of 200 ºC to 350 ºC and 20 bars to 30 bars (Schulz, 1999). The H₂ used in the Fischer–Tropsch reaction is a product of serpentinization. In serpentinization, hydrolysis of olivine minerals ((Mg,Fe)2SiO₄) form serpentine (Mg₃Si₂O₅(OH)₄), brucite (Mg(OH)₂), magnetite (Fe₃O4), and H₂:

3Fe₂SiO₄ + 2H₂O → 3SiO₂ + 2Fe₃O₄ + 2H_2(ac)

3Mg₂SiO₄ + SiO₂ + 4H₂O → 2Mg₃Si₂O₅(OH)₄

2Mg₂SiO₄ + 3H₂O → Mg₃Si₂O₅(OH)₄ + Mg(OH)₂

Serpentinization reactions are possible from 1 bar to 5 kbars, temperatures from 0 ºC to 500 ºC, and Fe²⁺ abundances from 1% to 50% (Oze and Sharma, 2005).

In contrast to the numerous CH₄ sources, there are only three sinks. Table 2 summarizes the sinks of CH₄. The major of those occur in the troposphere where the reaction of oxidation of CH₄ by hydroxyl radical (OH) leads mainly formaldehyde (CH₂O); such reaction is responsible for removing almost 90 % of atmospheric CH₄. OH radical is byproduct in photolysis of O₃ by UV-B radiation (Rohrer y Berresheim, 2006). OH radical rapidly reacts with CH₄removing it from the atmosphere:

O₂ + hv(180 - 240 nm) → O + O

O₂ + O → O₃

O₃ + hv (200 - 300 nm) → O(¹D) + O₂

O(¹D) + H₂O → 2OH

CH₄ + OH → CH₃ + H₂O

Where hv is the energy of a photon with frequency v and h is the Panck constant. The remaining CH₄ is removed trough soil oxidation, and transport to the stratosphere (Wuebbless and Hayhoe, 2002; Houweling et al., 2006; Anderson et al., 2010). After being produced, either by biological activity or serpentinization, methane may be stored in clathrates. Gas hydrates belong to a general class of inclusion compounds commonly known as clathrates. Clathrates owe their existence to the ability of H₂O molecules to assemble via hydrogen bonding and form polyhedral cavities. Molecules like methane or carbon dioxide are of an appropriate size such that they fit within cavities formed by the host material (e.g., Kvenvolden, 1993). Methane hydrates are particularly important (Mahajan et al., 2007). Within clathrates there are no chemical bond involved between the water molecules and the gas molecules other than Van der Waals forces, but the presence of guest molecules inside the ice crystals makes the structure more stable. In fact, the guest molecules stabilize the structure enough for raising the melting point of the ice to several degrees above 0 °C (Miller, 1961). There are two different reservoirs for clathrates. They can be found both within and under permafrost in arctic regions and also within a few hundred meters of the seafloor on continental slopes and in deep seas and lakes (Hester and Brewer, 2008).The permafrost is soil, sediment, or rock that is continuously frozen (temperature < 0 °C) for at least two consecutive years (Anderson et al., 2010). Permafrost is the largest CH₄ reservoir in Earth. Estimates of the global inventory of methane clathrate may be 3×10¹⁸ g of carbon (Buffett and Archer, 2004). Permafrost acts as an impermeable lid, preventing CH₄ escape through the seabed. Moreover, sub-sea permafrost is potentially more vulnerable to thawing than terrestrial permafrost. A consequence of climate warming is the partial thawing and failure of sub-sea permafrost and thus an increased permeability for gases. Shakhova et al. (2010a) estimate the total amount of carbon preserved within permafrost, only in the East Siberian Arctic Shelf (ESAS), to be ~1.4×10¹⁵ g. Shakhova et al. (2010b) estimated the annual outgassing from the shallow ESAS of 7.98 Tg CH₄. This amount is of the same magnitude as existing estimates of total methane emissions from the entire world ocean (e.g., Anderson et al., 2010).

Because methane is also a greenhouse gas, release of even a small percentage of total deposits could have a serious effect on Earth’s atmosphere. A conservative estimate by Boswell and Collett (2011) for the global gas hydrate inventory is ~1.8×10¹⁵ g C, corresponding to a CH₄ volume of ~3.0×10¹⁵ m³ if CH₄ density is considered to be 0.717 kg m^-3. In the unlikely event that 0.1 % (1.8 Tg C) of this CH₄ were instantaneously released to the atmosphere, CH₄concentrations would increase to ~2900 ppb from the 2005 value of ~1774 ppb (IPCC, 2007).

Table 1. CH₄ sources on Earth.

Table 2. CH₄ sinks on Earth.

Sinks reported by (a) Houweling et al., 2006, (b) Anderson et al., 2010, (c) Wuebbless and Hayhoe, 2002.
Sources reported by (a) Houweling et al., 2006, (b) Anderson et al., 2010.

2.3. Mars

Mars is an especial case. Thermodynamic calculations predict CH₄ should not exist in its atmosphere (Levine et al., 2010), however a CH₄ signal was discovered by Krasnopolsky et al. (1997) using the Fourier Transform Spectrometer of the Kitt Peak National Observatory (Arizona, USA). The authors estimated 0.07 ppm of atmospheric CH₄. Later, in 2004, two groups (Krasnopolsky et al., 2004; Formisano et al., 2004) reported abundances of 0.01 ppm using the instruments on board of the Mars Express. Zahnle et al. (2011) doubt the detection of CH₄ in Mars, arguing that CH₄ abundances estimated by Krasnopolsky et al. (2004) and Formisano et al. (2004) were supported on tenuous signals slightly distinguishable from the noise, however Mumma et al. (2009) reported a clear signal of CH₄ and his calculations confirm CH₄abundances of 0.01 ppm.

In 2010, Fonti and Marzo made distribution map of methane on the Martian surface. They identify three localized sources on the Martian surface, related to probable underground water reservoirs. Their analyses suggest that CH₄abundances vary throughout seasonal cycles.

There are some hypotheses about the sources and sinks of CH₄ in Mars. For example, Krasnopolsky et al. (2004) considered that degassing from the interior of the planet is unlikely due to the lack of geologic activity. Bar-Nun and Dimitrov (2007) proposed that photolysis of H₂O in the presence of CO can generate CH₄, however Krasnopolsky (2007) argues that it is not possible due to the kinetic chemistry of Mars. Serpentinization has also been proposed (e.g. Oze and Sharma, 2005; Lyons et al., 2005; Szponar et al., 2013; Etiope et al., 2013), this hypothesis is supported by the spatial correlation of underground water reservoirs and volcanoes where serpentinization may be possible. The origin of CH₄ on Mars is still not clear, some authors have proposed biogenic sources such as methanogenesis via metabolic pathways (e.g. Weiss et al., 2000; Chapelle et al., 2002; Jakosky et al., 2003; Varnes et al., 2003; Buford, 2010). CH₄ lifetime is 340 years and methane should be uniformly mixed in the atmosphere. Heterogeneous loss of atmospheric methane is probably negligible, while the sink of CH₄ during its diffusion through the regolith may be significant. There are no processes of CH₄ formation in the atmosphere, so the photochemical loss must therefore be balanced by its sources (Krasnopolsky et al., 2004). It was thought that the main sink of CH₄ was its direct photolysis around 80 km from the surface. Other sink is the reaction between CH₄ and Martian soil, but theoretical studies calculate the collision probability between CH₄ y O of 2×10^-11. Therefore, this reaction is negligible versus its direct photolysis (Krasnopolsky et al., 2004).

3. Outer planets (Jupiter, Saturn, Uranus and Neptune)

3.1. Jupiter and Saturn

They are giant planets with atmospheres mainly constituted by H₂ (> 80 %) and He as the second more important constituent. Their composition and chemistry are relatively similar in those planets. Jupiter is the largest planet in the Solar System with 318 M. Methane is the most abundant species in the upper Jovian troposphere after hydrogen and helium, accounting for approximately 0.2 % of the molecular abundance (Taylor et al., 2005). Different calculations estimate that CH₄/H₂ ratio is from 1.9×10^-3 to 2.3×10^-3 (Hanel et al., 1979; Gautier et al., 1982; Wong et al., 2004). Methane does not condense at the temperatures found on Jupiter, and is chemically stable except in the upper atmosphere (P < 1 mbar), where it is dissociated by solar ultraviolet radiation. Higher hydrocarbons are produced from methane by photochemical processes in the upper atmosphere of Jupiter (Taylor et al., 2005). Photolysis of CH₄ is the only sink (Moses et al., 2000), but it is not an effective way to destroy it in the Jupiter’s atmosphere because the large excess of H₂ that suggests that radicals like CH₃, byproducts of the CH₄ photolysis, react with the H radical reforming CH₄ (McNesby, 1969). Saturn is the second largest planet in our solar system. Observations from the Cassini spacecraft suggest mole fractions of CH₄ of 4.7×10^-3 (Fletcher et al., 2009). The chemistry of CH₄in Saturn is similar to Jupiter.

3.2. Uranus and Neptune

The only known photochemically active volatile in the atmosphere of Uranus is methane. From observations of the Ultraviolet Spectrometer in the Voyager spacecraft, the calculated abundance for CH₄ is 10^-4 near 0.1 mbar. Other species normally present in the atmospheres of Jupiter and Saturn are not likely to be gaseous in the photolytic regime of the upper troposphere and stratosphere of Uranus due to the low tropopause temperature (Atreya et al., 1991). The stratospheric CH₄ is photolyzed forming acetylene, methyl-acetylene, ethane, and ethylene (Orton et al., 1987; Bézard et al., 1991; Schulz et al., 1999; Meadows et al., 2008). However, CH₄ photolysis is relatively inefficient on Uranus. Only 10 to 15 % of CH₄ molecules, which absorb ultraviolet photons, produce higher hydrocarbons resulting in a loss rate of 6×10⁶ CH₄ molecules cm^-2 s^-1 at the equator. For comparison, the loss rate on Jupiter is 30 % (Atreya et al., 1991).

In Neptune, the mixing ratios of methane suggested by photochemical models is ~2 % at pressures > 0.1 bars (e.g., Baines et al., 1995), but there is evidence from remote observations that its abundance may be up to 4 % at P > 3.3 bars (Karkoschka and Tomasko, 2011). At lower pressures, methane is not homogenously distributed at all latitudes. The expected mixing ratio at the mean temperature of Neptune’s tropopause (~52 K) is ~5×10^-5 but values of (1.5 ± 0.2)×10^-3 have been derived from Herschel-PACS observations (Lellouch et al., 2010). This is consistent with the hypothesis that CH₄ leaking through the warm south polar tropopause (62 – 66 K) is globally redistributed by stratospheric motion (Fletcher et al., 2010). Voyager 2 observed Neptune’s atmosphere. Their images show that Neptune contains clouds of methane ice (Smith et al., 1989). Similar to Uranus, CH₄ is photolyzed in the stratosphere, producing hydrocarbons like acetylene and ethane (Romani and Atreya, 1989; Romani et al., 1993).

4. Methane in small bodies

4.1. Pluto

Pluto’s atmosphere is the result of the sublimation of superficial ices, in consequence, it is expected that the atmosphere is in vapor-pressure equilibrium with the surface (e.g., Young et al., 1997). Owen et al. (1993) estimated that the surface contains 1.5 % of solid CH₄. Later, Young et al. (1997) detected gaseous methane in Pluto for the first time, calculating a partial pressure of 0.072 µbar. In 2008 and 2012 this body was observed using the CRIRES instrument in the Very Large Telescope (VLT) to constrain the spatial and vertical distribution of methane in Pluto’s atmosphere (Lellouch et al., 2015). From these observations, the calculated methane-mixing ratio is 0.44 % with negligible longitudinal variations. Because Pluto has not yet been observed with any spacecraft, all its parameters have been inferred using instruments on the ground. In 2015, the mission New Horizons will be able to characterize the surface and atmosphere of Pluto and its satellite, Charon.

4.2. Triton

The Voyager 2 spacecraft observed Triton (Neptune's largest moon) in 1989 and it has been later studied using instruments on Earth’s surface and the Hubble Space Telescope (Buratti et al., 2011 and references therein). Similar to Pluto, its atmosphere is the result of the sublimation of the more volatile ices on its surface. The surface of Triton contains approximately 0.05 % CH₄ in ices (Tyler et al., 1989; Cruikshank et al., 1993) and its atmospheric mixing ratio was calculated to be 10^-4 from the Voyager observations (Tyler et al., 1989) and confirmed by the VLT/CRIRES instrument (Lellouch et al., 2011).

4.3. Titan

Titan is the largest moon of Saturn. Its bulk composition has nearly equal mass fractions of silicates and ices (Grasset et al., 2000). Titan’s atmosphere is mainly composed by N₂, with 5 % of CH₄ near to surface (Tobie et al., 2006). Methane was likely to be present in the materials that built Titan and is possible that cometary impacts were a significant source in the far past (e.g. Tobie et al., 2006; Mousis et al., 2009). Present abundances of CH₄ have not been possible to explain, because it is photochemically active in the atmosphere and requires a constant replenishment over geologic time scales (Davies et al., 2013). Liquid filled basins in the polar regions of Titan (Stofan et al., 2007; Turtle et al., 2009) composed by methane mixed with ethane (Brown et al., 2008) and a number of other organic species (Cordier et al., 2010) were identified using Cassini spacecraft observations.

In 2005, the Huygens spacecraft descended to the surface of Titan measuring in situ the CH₄ mole fraction when it descended. Huygens found that the CH₄ mole fraction is relatively constant in the stratosphere; it increases between 32 and 8 km, and remains constant near the surface (Atreya et al., 2006). Titan has pressures and temperatures near to the methane triple-point, for this reason the CH₄ can evaporate from the surface to atmosphere, where it can condense and rain, forming a CH₄cycle similar to water on Earth (Roe, 2009; Lunine, 2012).

Mathematical models based on the Voyager’s measurements suggest that the lifetime of CH₄ is from 10 to 100 millions of years (e.g. Yung et al., 1984; Lara et al., 1996; Lebonnois et al., 2001; Wilson and Atreya, 2004). In the stratosphere CH₄ is photolized to CH₃, CH₂ or CH, forming ethane, propane, and benzene (Strobel, 1974). There are not reactions to generate CH₄ in the Titan's atmosphere, so it is proposed that CH₄ may come from clathrates formed in the subnebula that originated the satellite (Mousis et al., 2002, Davies et al., 2013). Other authors proposed the activity of bacteria as a likely CH₄ source (e.g. McKay and Smith, 2005; Schulze-Makuch and Grinspoon, 2005), nevertheless there is not evidence about it (Atreya et al., 2006). Another possibility is the serpentinization (Niemann et al., 2005) but according to Mousis et al. (2009) this source of methane is not able to reproduce the deuterium over hydrogen (D/H) ratio observed at present in methane in its atmosphere.

4.4. Comets

These icy bodies have been studied with flyby missions and ground infrared and radio observations. Methane is a primary volatile in comets, this means that it is stored as ice in the cometary nucleus and released as gas into the coma. This compound has been detected in eleven comets and its abundance relative to water ranges from ~0.4 % to 2 % (Allen et al., 1988; Drapatz et al., 1987; Mumma et al., 1996; Bockelée-Morvan et al., 2000; Gibb et al., 2003; Mumma and Charnley, 2011).

5. Final comments

The study of methane is relevant to understand the process of synthesis and distribution of organic molecules during the formation of the Solar System. On potentially habitable planets around other stars its presence maybe the result of geological or biological activity. The bodies of our Solar System, especially Earth, serve as benchmarks for understanding the origin, sources and reservoirs of this compound to identify possible inhabitable worlds around other stars.

Acknowledgements

We acknowledge the support of the project PAPIIT IN119709-3.

References

Allen, M., Delitsky, M., Huntress, W., Yung, Y., Ip, W.H., 1988, Evidence for methane and ammonia in the coma of comet P/Halley: Astronomy and Astrophysics, 187, 502–512.

Anderson, B., Bartlett, K., Frolking, S., Hayhoe, K., Jenkins, J., Salas, W., 2010, Methane and Nitrous Oxide Emissions from Natural Sources: Washington, D.C., United States Environmental Protection Agency (EPA), EPA 430-R-10-001, 194 p.

Atreya, S.K., Adams, E.Y., Niemann, H.B., Demick-Montelara, J.E., Owen, T.C., Fulchignoni, M., Ferri, F., Wilson, E.H., 2006, Titan's methane cycle: Planetary and Space Science, 54, 1177–1187.

Atreya, S.K., Sandel, B.R., Romani, P.N., 1991, Photochemistry and Vertical Mixing, in Bergstralh, J. (Ed.), Uranus, University of Arizona Press, Tucson, Arizona, 110–146.

Baines, K.H., Mickelson, M.E., Larson, L.E., Ferguson, D.W., 1995, The abundances of methane and ortho/para hydrogen on Uranus and Neptune: Implications of New Laboratory 4-0 H₂quadrupole line parameters: Icarus, 114, 328–340.

Bar-Nun, A., Dimitrov, V., 2007, Methane on Mars: A product of H₂O photolysis in the presence of CO: Icarus, 188, 543–545.

Bézard, B., Romani, P.N., Conrath, B.J., Maguire, W.C., 1991, Hydrocarbons in Neptune's stratosphere from Voyager infrared observations: Journal of Geophysical Research, 96, 18961–18975.

Bockelée-Morvan, D., Lis, D.C., Wink, J.E., Despois, D., Crovisier, J., Bachiller, R., Benford, D. J., Biver, N., Colom, P., Davies, J. K., Gérard, E., Germain, B., Houde, M., Mehringer, D., Moreno, R., Paubert, G., Phillips, T. G., Rauer, H, 2000, New molecules found in comet C/1995 O1 (Hale-Bopp), Investigating the link between cometary and interstellar material: Astronomy and Astrophysics, 353, 1101–1114.

Boswell, R., Collett, T.S., 2011, Current perspectives on gas hydrate resources: Energy and Environmental Science, 4, 1206–1215.

Broadfoot, A.L., Kumar, S., Belton, M.J.S., McElroy, M.B., 1974, Mercury's Atmosphere from Mariner 10: Preliminary Results: Science, 185, 166–169.

Brook, E., Sowers, T., Orchardo, J., 1996, Rapid variations in atmospheric methane concentration during the past 110000 years: Science, 273, 1087–1091.

Brown, R.H., Soderblom, L.A., Soderblom, J.M., Clark, R.N., Jaumann, R., Barnes, J.W., Sotin, C., Buratti, B., Baines, K.H., Nicholson, P.D., 2008, The identification of liquid ethane in Titan's Ontario Lacus: Nature, 454, 607–610.

Buffett, B., Archer, D., 2004, Global inventory of methane clathrate: sensitivity to changes in the deep ocean: Earth and Planetary Science Letters 227, 185–199.

Buford, P.P, 2010, Microbial life in Martian ice: A biotic origin of methane on Mars?: Planetary and Space Science, 58, 1199–1206.

Buratti, B.J., Bauer, J.M., Hicks, M.D., Hillier, J.K., Verbiscer, A., Hammel, H., Schmidt, B., Cobb, B., Herbert, B., Garsky, M., Ward, J., Foust, J., 2011, Photometry of Triton 1992–2004: Surface volatile transport and discovery of a remarkable opposition surge: Icarus, 212, 835–846.

Chapelle, F.H., O'Neill, K., Bradley, P.M., Methé, B.A., Ciufo, S.A., Knobel, L.L., Lovley, D.R., 2002, A hydrogen-based subsurface microbial community dominated by methanogens: Nature, 415, 312–315.

Chappellaz, J., Barnola, J., Raynaud, D., Korotkevich, Y., Lorius, C., 1990, Ice-core record of atmospheric methane over the past 160000 years: Nature, 345, 127–131.

Cloutier, P.A., 2007, Comparative planetology: History of water in the inner solar system: The Leading Edge, 26, 1308–1311.

Cordier, D., Mousis, O., Lunine, J.I., Lebonnois, S., Lavvas, P., Lobo, L.Q., Ferreira, A.G.M., 2010, About the Possible Role of Hydrocarbon Lakes in the Origin of Titan's Noble Gas Atmospheric Depletion: The Astrophysical Journal Letters, 721, L117–L120.

Cruikshank, D.P., Roush, T.L., Owen, T.C., Geballe, T.R., de Bergh, C., Schmitt, B., Brown, R. H., Bartholomew, M.J., 1993, Ices on the Surface of Triton: Science, 261, 742–745.

Davies, A.G., Sotin, C., Choukroun, M., Matson, D.L., Johnson, T.V., 2013, Methane Clathrate Destabilisation by Heat from Lava Flows: Implications for Supplying Titan's Atmospheric Methane in 44th Lunar and Planetary Science Conference: The Woodlands, Texas, LPI Contribution No. 1719, 1681 p.

Dlugokencky, E.J., Houweling, S., Bruhwiler, L., Masarie, K.A., Lang, P.M., Miller, J.B., Tans, P. P., 2003, Atmospheric methane levels off: Temporary pause or a new steady-state?: Geophysical Research Letters, 30, 1992, doi:10.1029/2003GL018126.

Donahue, T.M., Hodges, R.R., 1993, Venus methane and water: Geophysical Research Letters, 20, 591–594.

Drapatz, S., Larson, H.P., Davis, D.S., 1987, Search for Methane in Comet Halley: Astronomy and Astrophysics, 187, 497–501.

Etiope, G., Ehlmann, B.L., Schoell, M., 2013, Low temperature production and exhalation of methane from serpentinized rocks on Earth: A potential analog for methane production on Mars: Icarus, 224, 276–285.

Fletcher, L.N., Drossart, P., Burgdorf, M., Orton, G.S., Encrenaz, T., 2010, Neptune's atmospheric composition from AKARI infrared spectroscopy: Astronomy and Astrophysics, 514, A17, doi: 10.1051/0004-6361/200913358.

Fletcher, L.N., Orton, G.S., Teanby, N.A., Irwin, P.G.J., Bjoraker, G.L., 2009, Methane and its isotopologues on Saturn from Cassini/CIRS observations: Icarus, 199, 351–367.

Fonti, S., Marzo, G.A., 2010, Mapping the methane on Mars: Astronomy and Astrophysics, 512, A51, doi: 10.1051/0004-6361/200913178.

Formisano, V., Atreya, S., Encrenaz, T., Ignatiev, N., Giuranna, M., 2004, Detection of Methane in the Atmosphere of Mars: Science, 306, 1758–1761.

Fukuzaki, S., Nishio, N., Nagai, S., 1990, Kinetics of the methanogenic fermentation of acetate: Applied and Environmental Microbiology, 56 (10), 3158–3163.

Gautier, D., Bezard, B., Marten, A., Baluteau, J.P., Scott, N., Chedin, A., Kunde, V., Hanel, R., 1982, The C/H ratio in Jupiter from the Voyager infrared investigation: Astrophysical Journal, 257, 901–912.

Gibb, E.L., Mumma, M.J., dello Russo, N., DiSanti, M.A., Magee-Sauer, K., 2003, Methane in Oort cloud comets: Icarus, 165, 391–406.

Gibson, E.K. Jr., 1977, Production of simple molecules on the surface of Mercury: Physics of the Earth and Planetary Interiors, 15, 303–312.

Grasset, O., Sotin, C., Deschamps, F., 2000, On the internal structure and dynamics of Titan: Planetary and Space Science, 48, 617–636.

Hanel, R., Conrath, B., Flasar, M., Kunde, V., Lowman, P., Maguire, W., Pearl, J., Pirraglia, J., Samuelson, R., Gautier, D., Gierasch, P., Kumar, S., Ponnamperuma, C., 1979, Infrared observations of the Jovian system from Voyager 1: Science, 204, 972–976.

Hester, K.C., Brewer, P.G., 2008, Clathrate Hydrates in Nature: The Annual Review of Marine Science, 1, 303–327.

Houghton, J., 2005, Global warming: Reports on Progress in Physics, 68, 1343–1403.

Houweling, S., Röckmann, T., Aben, I., Keppler, F., Krol, M., Meirink, J.F., Dlugokencky, E.J., Frankenberg, C., 2006, Atmospheric constraints on global emissions of methane from plants: Geophysical Research Letters, 33, L15821, doi:10.1029/2006GL026162.

Intergovernmental Panel on Climate Change (IPCC), 2007, Climate Change 2007: The Physical Science Basis: New York, NY, Cambridge University Press, 1009 p.

Jakosky, B.M., Nealson, K.H., Bakermans, C., Ley, R.E., Mellon, M.T., 2003, Subfreezing activity of microorganisms and the potential habitability of Mars’ polar regions: Astrobiology, 3, 343–350.

Karkoschka, E., Tomasko, M.G., 2011, The haze and methane distributions on Neptune from HST–STIS spectroscopy: Icarus, 211, 780–797.

Kasting, J.F., Siefert, J.L., 2002, Life and the evolution of Earth’s atmosphere: Science, 296, 1066–1068.

Krasnopolsky, V.A., 2007, Comment on “Methane on Mars: A product of H₂O photolysis in the presence of CO” by A. Bar-Nun and V. Dimitrov: Icarus, 188, 540–542.

Krasnopolsky, V.A., Bjoraker, G.L., Mumma, M.J., Jennings, D.E., 1997, High-resolution spectroscopy of Mars at 3.7 and 8 μm: a sensitive search for H₂O₂, H₂CO, HCl, and CH₄, and detection of HDO: Journal of Geophysical Research, 102, 6525–6534.

Krasnopolsky, V.A., Maillard, J.P., Owen, T.C., 2004, Detection of methane in the Martian atmosphere: evidence for life? Icarus, 172, 537–547.

Kvenvolden, K.A., 1993, Gas hydrates-geological perspective and global change: Reviews of Geophysics, 31, 173–188.

Lara, L.M., Lellouch, E., Lopez-Moreno, J., Rodrigo, R., 1996, Vertical distribution of Titan’s atmospheric neutral constituents: Journal of Geophysical Research, 101, 23261–23283.

Lebonnois, S., Toublanc, D., Hourdin, F., Rannou, P., 2001, Seasonal variations of Titan’s atmospheric composition: Icarus 152, 384–406.

Legrand, M., Lorius, C., Barkov, N., Petrov, V., 1988, Vostok Ant-arctic ice core: atmospheric chemistry changes over the last climatic cycle (160000 years): Atmospheric Environment, 22, 317–331.

Lelieveld, J., Crutzen, P.J., Dentener, F.J., 1998, Changing concentration, lifetimes and climate forcing of atmospheric methane: Tellus B, 50, 128–150.

Lellouch, E., de Bergh, C., Sicardy, B., Forget, F., Vangvichith, M., Käufl, H.-U., 2015, Exploring the spatial, temporal, and vertical distribution of methane in Pluto's atmosphere: Icarus, 246, 268–278.

Lellouch, E., de Bergh, C., Sicardy, B., Käufl, H.-U., Smette, A., 2011, The Tenuous Atmospheres of Pluto and Triton Explored by CRIRES on the VLT: The Messenger, 145, 20–23.

Lellouch, E., Hartogh, P., Feuchtgruber, H., Vandenbussche, B., de Graauw, T., Moreno, R., Jarchow, C., Cavalié, T., Orton, G., Banaszkiewicz, M., Blecka, M.I., Bockelée-Morvan, D., Crovisier, J., Encrenaz, T., Fulton, T., Küppers, M., Lara, L.M., Lis, D.C., Medvedev, A.S., Rengel, M., Sagawa, H., Swinyard, B., Szutowicz, S., Bensch, F., Bergin, E., Billebaud, F., Biver, N., Blake, G.A., Blommaert, J.A.D.L., Cernicharo, J., Courtin, R., Davis, G.R., Decin, L., Encrenaz, P., Gonzalez, A., Jehin, E., Kidger, M., Naylor, D., Portyankina, G., Schieder, R., Sidher, S., Thomas, N., de Val-Borro, M., Verdugo, E., Waelkens, C., Walker, H., Aarts, H., Comito, C., Kawamura, J.H., Maestrini, A., Peacocke, T., Teipen, R., Tils, T., Wildeman, K., 2010, First results of Herschel-PACS observations of Neptune: Astronomy and Astrophysics, 518, L152, doi: 10.1051/0004-6361/201014600.

Levine, J.S., Rinsland, C.P., Tennille, G.M., 1985, The photochemistry of methane and carbon monoxide in the troposphere in 1950 and 1985: Nature, 318, 254–257.

Levine, J.S., Summers, M.E., Ewell, M., 2010, Methane on Mars: Thermodynamic Equilibrium and Photochemical Calculations. Astrobiology Science Conference 2010: Evolution and Life: Surviving Catastrophes and Extremes on Earth and Beyond, Contribution No. 1538, 5267 p.

Loulergue, L., Schilt, A., Spahni, R., Masson-Del-Motte, V., Blunier, T., Lemieux, B., Barnola, J. M., Raynaud, D., Stocker, T., Chappellaz, J., 2008, Orbital and millennial-scale features of atmospheric CH₄over the past 800000 years: Nature, 453, 383–386.

Lunine, J.I., 2012, The hydrologic cycle on Saturn's moon, Titan: Memorie della Societa Astronomica Italiana Supplement, 19, 75–80.

Lyons, J.R., Manning, C., Nimmo, F., 2005, Formation of methane on Mars by fluid-rock interaction in the crust: Geophysical Research Letters, 32, L13201, doi: 10.1029/2004GL022161.

Mahajan, D., Taylor, C.E., Mansoori, G.A., 2007, An introduction to natural gas hydrate/clathrate: The major organic carbon reserve of the Earth: Journal of Petroleum Science and Engineering, 56, 1–8.

McKay, C.P., Smith, H.D., 2005, Possibilities for methanogenic life in liquid methane on the surface of Titan: Icarus 178, 274–276.

McNesby, J.R., 1969, The photochemistry of Jupiter above 1000 Å: Journal of the Atmospheric Sciences, 26, 594–599.

Meadows, V., Orton, G., Liang, M.C., Yung, Y., van Cleve, J., Burgdorf, M., Crisp, D., Houck, J., 2008, First Spitzer observations of Neptune: Detection of new hydrocarbons: Icarus, 197, 585–589.

Miller, S.L., 1961, The occurrence of gas hydrates in the Solar System: Proceedings of the National Academy of Sciences, 47, 1798–1808.

Moses, J.I., Bézard, B., Lellouch, E., Gladstone, G.R., Feuchtgruber, H., Allen, M., 2000, Photochemistry of Saturn's Atmosphere. I. Hydrocarbon Chemistry and Comparisons with ISO Observations: Icarus, 143, 244–298.

Mousis, O., Gautier, D., Bockelee-Morvan, D., 2002, Turbulent model of the Saturn subnebula: implications for the origin of methane in Titan’s atmosphere: Icarus, 156, 162–175.

Mousis, O., Lunine, J.I.; Pasek, M., Cordier, D., Waite, J.H., Mandt, K.E.; Lewis, W.S., Nguyen, M.-J., 2009, A primordial origin for the atmospheric methane of Saturn’s moon Titan: Icarus, 204, 749–751.

Mumma, M.J., Charnley, S.B., 2011, The chemical composition of comets—Emerging taxonomies and natal heritage: Astronomy and Astrophysics, 49 (1), 471–524.

Mumma, M.J., Disanti, M.A., dello Russo, N., Fomenkova, M., Magee-Sauer, K., Kaminski, C.D., Xie, D.X., 1996, Detection of Abundant Ethane and Methane, Along with Carbon Monoxide and Water, in Comet C/1996 B2 Hyakutake: Evidence for Interstellar Origin, Science, 272, 1310–1314.

Mumma, M.J., Villanueva, G.L., Novak, R.E., Hewagama, T., Bonev, B.P., DiSanti, M.A., Mandell, A.M., Smith, M.D., 2009, Strong Release of Methane on Mars in Norhtern Summer 2003: Science, 323, 1041–1045.

Neumann, G.A., Cavanaugh, J.F., Sun, X., Mazarico, E.M., Smith, D.E., Zuber, M.T., Mao, D., Paige, D.A., Solomon, S.C., Ernst, C.M., Barnouin, O.S., 2013, Bright and Dark Polar Deposits on Mercury: Evidence for Surface Volatiles: Science, 339, 296–300.

Niemann, H.B., Kasprzak, W.T., Hedin, A.E., Hunten, D.M., Spencer, N.W., 1980, Mass spectrometric measurements of the neutral gas composition of the thermosphere and exosphere of Venus: Journal of Geophysical Research: Space Physics, 85 (A13), 7817–7827.

Niemann, H.B., Atreya, S.K., Bauer, S.J., Carignan, G.R., Demick, J.E., Frost, R.L., Gautier, D., Haberman, J.A., Harpold, D.N., Hunten, D.M., Israel, G., Lunine, J.I., Kasprzak, W.T., Owen, T.C., Paulkovich, M., Raulin, F., Raaen, E., Way, S.H., 2005, The abundances of constituents of Titan’s atmosphere from the GCMS on the Huygens probe: Nature, 438, 779–784.

Orton, G.S., Aitken, D.K., Smith, C., Roche, P.F., Caldwell, J., Snyder, R., 1987, The spectra of Uranus and Neptune at 8-14 and 17-23 microns: Icarus, 70, 1–12.

Otroshchenko, V.A., Surkov, Y.A., 1974, The possibility of organic molecule formation in the Venus atmosphere: Origins of Life, 5, 487–490.

Owen, T.C., Roush, T.L., Cruikshank, D.P., Elliot, J.L., Young, L.A., de Bergh, C., Schmitt, B., Geballe, T.R., Brown, R.H., Bartholomew, M.J., 1993, Surface ices and the atmospheric composition of Pluto: Science, 261, 745–748.

Oyama, V.I., Carle, G.C., Woeller, F., Pollack, J.B., Reynolds, R.T., Craig, R.A., 1980, Pioneer Venus Gas Chromatography of the Lower Atmosphere of Venus: Journal of Geophysical Research, 85, 7891–7902.

Oze, C., Sharma, M., 2005, Have olivine, will gas: Serpentinization and the abiogenic production of methane on Mars: Geophysical Research Letters, 32, L10203, doi: 10.1029/2005GL022691.

Paige, D.A., Siegler, M.A., Harmon, J.K., Neumann, G.A., Mazarico, E.M., Smith, D.E., Zuber, M.T., Harju, E., Delitsky, M.L., Solomon, S.C., 2013, Thermal Stability of Volatiles in the North Polar Region of Mercury: Science, 339, 300–303.

Petit, J., Jouzel, J., Raynaud, D., Barkov, N., Barnola, J.-M., Basile, I., Bender, M., Chappellaz, J., Davis, M., Delaygue, G., Del-Motte, M., Kotlyakov, V., Legrand, M., Lipenkov, V., Lorius, C., Pepin, L., Ritz, C., Saltzman, E., Stievenard, M., 1999, Climate and atmospheric history of the past 420000 years from the Vostok ice core, Antarctica: Nature, 399, 429–436.

Potter, A., Morgan, T., 1985, Discovery of sodium in the atmosphere of Mercury: Science, 229, 651–653.

Potter, A., Morgan, T., 1986, Potassium in the atmosphere of Mercury: Icarus, 67, 336–340.

Raynaud, D., Jouzel, J., Barnola, J., Chapellaz, J., Delmas, R., Lorius, C., 1993, The ice record of greenhouse gases: Science, 259, 926–934.

Roe, H.G., 2009, Titan's Methane Weather: Annual Review of Earth and Planetary Sciences, 40, 355–382.

Rohrer, F., Berresheim, H., 2006, Strong correlation between levels of tropospheric hydroxyl radicals and solar ultraviolet radiation: Nature, 442, 184–187.

Romani, P.N., Atreya, S.K., 1989, Stratospheric aerosols from CH₄ photochemistry on Neptune: Geophysical Research Letters, 16, 941–944.

Romani, P.N., Bishop, J., Bezard, B., Atreya, S., 1993, Methane photochemistry on Neptune - Ethane and acetylene mixing ratios and haze production: Icarus, 106, 442–463.

Sagan, C., Thompson, W.R., Carlson, R., Gurnett, D., Hord, C., 1993, A search for life on Earth from the Galileo spacecraft: Nature, 365, 715–721.

Schulz, B., Encrenaz, Th., Bézard, B., Romani, P.N., Lellouch, E., Atreya, S.K., 1999, Detection of C₂H₄in Neptune from ISO/PHT-S observations: Astronomy and Astrophysics, 350, L13–L17.

Schulz, H., 1999, Short history and present trends of Fischer–Tropsch synthesis: Applied Catalysis A. General, 186 (1–2), 3–12.

Schulze-Makuch, D., Grinspoon, D.H., 2005, Biologically enhanced energy and carbon cycling on Titan?: Astrobiology, 5, 560–567.

Shakhova, N., Semiletov, I., Leifer, I., Salyuk, A., Rekant, P., Kosmach, D., 2010a, Geochemical and geophysical evidence of methane release over the East Siberian Arctic Shelf: Journal of Geophysical Research, 115, C08007, doi:10.1029/2009JC005602.

Shakhova, N., Semiletov, I., Leifer, I., Salyuk, A., Yusupov, V., Kosmach, D., Gustafsson, O., 2010b, Extensive Methane Venting to the Atmosphere from Sediments of the East Siberian Arctic Shelf: Science, 327, 1246–1250.

Smith, B.A., Soderblom, L.A., Banfield, D., Barnet, C., Beebe, R.F., Bazilevskii, A.T., Bollinger, K., Boyce, J.M., Briggs, G.A., Brahic, A., 1989, Voyager 2 at Neptune - Imaging science results: Science, 246, 1422–1449.

Spahni, R., Chappellaz, J., Stocker, T.F., Loulergue, L., Hausammann, G., Kawamura, K., Flückiger, J., Schwander, J., Raynaud, D., Masson-Del-Motte, V., Jouzel, J., 2005, Atmospheric methane and nitrous oxide of the Late Pleistocene from Antarctic ice cores: Science, 310, 1317–1321.

Stofan, E.R., Elachi, C., Lunine, J.I., Lorenz, R.D., Stiles, B., Mitchell, K.L., Ostro, S., Soderblom, L., Wood, C., Zebker, H., Wall, S., Janssen, M., Kirk, R., Lopes, R., Paganelli, F., Radebaugh, J., Wye, L., Anderson, Y., Allison, M., Boehmer, R., Callahan, P., Encrenaz, P., Flamini, E., Francescetti, G., Gim, Y., Hamilton, G., Hensley, S., Johnson, W.T.K., Kelleher, K., Muhleman, D., Paillou, P., Picardi, G., Posa, F., Roth, L., Seu, R., Shaffer, S., Vetrella, S., West, R., 2007, The lakes of Titan: Nature, 445, 61–64.

Strobel, D.F., 1974, The photochemistry in the atmosphere of Titan: Icarus, 21, 466–470.

Szponar, N., Brazelton, W.J., Schrenk, M.O., Bower, D.M., Steele, A., Morrill, P.L., 2013, Geochemistry of a continental site of serpentinization, the Tablelands Ophiolite, Gros Morne National Park: A Mars analogue: Icarus, 224, 286–296.

Taylor, F.W., Atreya, S.K., Encrenaz, Th., Hunten, D.M., Irwin, P.G.J., Owen, T.C., 2005, The Composition of the Atmosphere of Jupiter, in Bagenal, F., (ed.), Jupiter: Cambridge University Press, Cambridge, 59–78.

Thauer, R.K., 1998, Biochemistry of Methanogenesis: a Tribute to Marjory Stephenson: Microbiology, 144, 2377–2406.

Thompson, W. Reid., Murray, B.G.J.P T., Khare, B.N., Sagan, C., 1987, Coloration and darkening of methane clathrate and other ices by charged particle irradiation: Applications to the outer solar system: Journal of Geophysical Research, 92, 14933–14947.

Tobie, G., Lunine, J.I., Sotin, C., 2006, Episodic outgassing as the origin of atmospheric methane on Titan: Nature, 440, 61–64.

Turtle, E.P., Perry, J.E., McEwen, A.S., Del Genio, A.D., Barbara, J., West, R.A., Dawson, D.D., Porco, C.C., 2009, Cassini imaging of Titan's high-latitude lakes, clouds, and south-polar surface changes: Geophysical Research Letters, 36, L02204, doi:10.1029/2008GL036186.

Tyler, G.L., Sweetnam, D.N., Anderson, J.D., Borutzki, S.E., Campbell, J.K., Kursinski, E.R., Levy, G.S., Lindal, G.F., Lyons, J.R., Wood, G.E., 1989, Voyager radio science observations of Neptune and Triton: Science, 246, 1466–1473.

Varnes, E.S., Jakosky, B.M., McCollom, T.M., 2003, Biological potential of Martian hydrothermal systems: Astrobiology, 3, 407–414.

Weiss, B.P., Yung, Y.L., Nealson, K.H., 2000, Atmospheric energy for subsurface life on Mars?: Proceedings of the National Academy of Sciences, USA, 97, 1395–1399.

Wilson, E.H., Atreya, S.K., 2004, Current state of modeling the photochemistry of Titan’s mutually dependent atmosphere and ionosphere: Journal of Geophysical Research, 109, E06002, doi:10.1029/2003JE002181.

Wong, M.H., Mahaffy, P.R., Atreya, S.K., Niemann, H.B., Owen, T.C., 2004, Updated Galileo probe mass spectrometer measurements of carbon, oxygen, nitrogen, and sulfur on Jupiter: Icarus, 171, 153–170.

Wuebbles, D.J., Hayhoe, K., 2002, Atmospheric methane and global change: Earth-Science Reviews, 57, 177–210.

Young, L.A., Elliot, J.L., Tokunaga, A., de Bergh, C., Owen, T., 1997, Detection of Gaseous Methane on Pluto: Icarus, 127, 258–262.

Yung, Y.L., Allen, M., Pinto, J.P., 1984, Photochemistry of the atmosphere of Titan comparison between model and observations: Astrophysical Journal Supplement Series, 55, 465–506.

Zahnle, K., Freedman, R.S., Catling, D.C., 2011, Is there methane on Mars?: Icarus, 212, 493–503.

Zhang, J.A., Paige, D.A., 2009, Cold-trapped organic compounds at the poles of the Moon and Mercury: Implications for origins: Geophysical Research Letters, 36, L16203, doi:10.1029/2009GL038614.

Manuscript received: April 30, 2014.

Corrected manuscript received: February 9, 2015.

Manuscript accepted: February 12, 2015.